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1 Introduction
1.1 Scope and definition This package describes the technologies for using five Renewable Energy Sources (RES) which are normally considered to be both fully renewable and environmentally friendly. Two of them use direct solar radiation, but it is important to distinguish solar thermal technology (section 3) from photovoltaic effects (section 4) since their primary energy outputs are radically different. Wind (section 5) and Small Hydro (section 6) use the energy imparted to air and water through physical atmospheric effects driven by the sun, while Biomass (section 7) uses energy stored from biological absorption of solar energy. Utilisation of the first four does not normally involve any direct after-effect on the environment, while the combustion of Biomass can in principle be regulated so that its atmospheric emissions precisely compensate for the gases required in the original biological growth process. It should also be noted that energy may be derived from RES mainly either as electricity or as heat. Electric power is more valuable than heat for the simple reason that whereas electricity can be completely converted into heat, the reverse direction involves substantial energy loss unless the 'waste' heat can be usefully used. For this reason it is important to distinguish the quantities of energy output as either electric power (e.g. MWe) or thermal energy (e.g. MWth). 1.2 Structure
The solar resource is covered first (section 2). After this each of the sections about particular energy sources is self contained except for some comparisons and common references in sections 8,9. 1.3
1.1 Scope and definition ________________________________________________________ 1 1.2 Structure _________________________________________________________________ 1 1.3 Contents _________________________________________________________________ 1
2.1 Introduction ______________________________________________________________ 4 2.2 Solar irradiation ___________________________________________________________ 4 2.3 Web Links ________________________________________________________________ 5
3.1 Introduction ______________________________________________________________ 6
3.1.1 3.1.2 3.2.1 3.2.2 Background __________________________________________________________________ 6 History ______________________________________________________________________ 6 Solar Collectors _______________________________________________________________ 7 Storage of the Collected Heat_____________________________________________________ 9
3.2 Technical Principles ________________________________________________________ 7
3.3 Solar Thermal Systems Applications __________________________________________ 9
Domestic hot water production __________________________________________________________ 9
3.3.2 3.3.3 3.3.4 3.3.5 3.4.1 3.4.2 3.5.1 3.5.2 4
Solar pool heating_____________________________________________________________ Space Heating and Cooling _____________________________________________________ Special applications ___________________________________________________________ Electric Power _______________________________________________________________
10 10 12 15
3.4 Economic Implications_____________________________________________________ 16
Solar Heating systems _________________________________________________________ 17 Solar Thermal Electricity _______________________________________________________ 18 Bibliography: ________________________________________________________________ 19 Web links:___________________________________________________________________ 19
3.5 References _______________________________________________________________ 19
4.1 Introduction _____________________________________________________________ 20 4.2 History __________________________________________________________________ 20 4.3 Technical outline _________________________________________________________ 21
4.3.1 4.3.2 4.3.3 4.3.4 4.3.5 4.3.6 4.3.7 4.4.1 4.4.2 4.4.3 4.4.4 4.4.5 What is a PV system composed of? _______________________________________________ PV Cells ____________________________________________________________________ PV Modules _________________________________________________________________ Types of PV System ___________________________________________________________ Operation of solar cells_________________________________________________________ Rating ______________________________________________________________________ Market segments______________________________________________________________ Overview ___________________________________________________________________ PV module costs ______________________________________________________________ PV system costs ______________________________________________________________ Costs of solar electricity ________________________________________________________ Prices, compensation schemes and support systems___________________________________ 21 21 22 23 23 24 24 25 25 26 27 27
4.4 What is the cost of PV? ____________________________________________________ 25
4.5 Current developments _____________________________________________________ 29 4.6 The Benefits of PV Power __________________________________________________ 29
4.6.1 4.6.2 4.6.3 4.6.4 Advantages of solar electricity: __________________________________________________ Environmental considerations ___________________________________________________ Space-saving installation _______________________________________________________ Improving the electricity network ________________________________________________ 29 30 30 30
4.7 Conclusion_______________________________________________________________ 30 4.8 References _______________________________________________________________ 31
4.8.1 4.8.2 5 Web links ___________________________________________________________________ 31 Bibliography_________________________________________________________________ 31
5.1 History __________________________________________________________________ 32
5.1.1 5.1.2 5.1.3 5.2.1 5.2.2 5.2.3 5.2.4 5.3.1 5.3.2 Origins _____________________________________________________________________ 32 Key Events __________________________________________________________________ 32 Electricity generation __________________________________________________________ 32 Wind Turbines _______________________________________________________________ Wind strength ________________________________________________________________ Performance _________________________________________________________________ Electricity generation __________________________________________________________ 33 35 37 38
5.2 Technical outline _________________________________________________________ 33
5.3 Current developments _____________________________________________________ 40
Wind Farms _________________________________________________________________ 40 Offshore ____________________________________________________________________ 41
5.4 Environmental implications ________________________________________________ 42
5.4.1 5.4.2 5.4.3 5.5.1 5.5.2 5.5.3 5.6.1 5.6.2 6
Emissions ___________________________________________________________________ 42 Visual ______________________________________________________________________ 42 Noise_______________________________________________________________________ 42 Construction _________________________________________________________________ 42 Capacity credit _______________________________________________________________ 43 Unit cost ____________________________________________________________________ 43 Bibliography_________________________________________________________________ 44 Web links ___________________________________________________________________ 44
5.5 Costs and economics_______________________________________________________ 42
5.6 References _______________________________________________________________ 44
6.1 Introduction _____________________________________________________________ 45
6.1.1 6.1.2 6.1.3 6.2.1 6.2.2 6.2.3 6.2.4 6.2.5 6.3.1 6.3.2 6.3.3 6.4.1 6.4.2 7 Background _________________________________________________________________ 45 Short history _________________________________________________________________ 46 Geographical assessment _______________________________________________________ 46 From water to electricity________________________________________________________ Site selection and basic layout ___________________________________________________ Civil engineering works ________________________________________________________ Electromechanical equipment____________________________________________________ Future trends_________________________________________________________________ 47 47 49 50 55
6.2 Technical outline _________________________________________________________ 47
6.3 Implications______________________________________________________________ 56
Social and political ____________________________________________________________ 56 Environmental _______________________________________________________________ 57 Economic ___________________________________________________________________ 58 Bibliography_________________________________________________________________ 59 Web links ___________________________________________________________________ 59
6.4 References _______________________________________________________________ 59
7.1 Introduction _____________________________________________________________ 60
7.1.1 7.1.2 7.2.1 7.2.2 7.2.3 7.2.4 7.3.1 7.3.2 7.4.1 7.4.2 8 9 Scope ______________________________________________________________________ 60 History _____________________________________________________________________ 60 Basic Concepts _______________________________________________________________ Factors Affecting The Utilization Of Bioenergy _____________________________________ Nature and structure of biomass __________________________________________________ Systems for biomass combustion _________________________________________________ 60 62 65 68
7.2 Technical outline _________________________________________________________ 60
7.3 Political implications ______________________________________________________ 72
Public awareness _____________________________________________________________ 72 EU scenario _________________________________________________________________ 72 Bibliography_________________________________________________________________ 73 Web links ___________________________________________________________________ 73
7.4 References _______________________________________________________________ 73
9.1 Bibliography _____________________________________________________________ 75 9.2 Web links________________________________________________________________ 75
2 Solar Energy Resource
2.1 Introduction All our sources of energy derive ultimately from the sun. Some, such as Fossil and Nuclear, have stored that energy and concentrated it over a very long time, whereas Biomass, Hydro and Wind rely on conversion from solar energy over a short timescale. The underlying advantage of using solar energy directly is that it is free, abundant and inexhaustible. The total amount of energy radiated from the sun onto the earth's surface is more than 10 000 times the annual global energy consumption. With care, its use can also avoid degradation of the environment. Since both Solar Thermal and Solar Photovoltaic energies make use of the same source, this short section covers their common solar resource. 2.2 Solar irradiation
The sun is continuously radiating enormous amounts of energy. A fraction of that energy reaches the earth. The fraction of the energy from the sun that reaches the earth in just one day is still more than enough to cover the energy use of the world in a whole year. However, not all the energy of the sun that reaches the earth can be used effectively. Part of the sunlight is absorbed in the earth's atmosphere or reflected back into space. The intensity of sunlight reaching the earth varies with time of day and year, location, and weather conditions. The total energy on a daily or annual basis is called irradiation and indicates the strength of the sunshine, typically expressed in kWh/m² per day. To simplify calculations with irradiation data, solar energy is expressed in equivalents of one hour's bright sunlight. Bright sunlight corresponds with a radiative power of about 1,000 W/m² so one hour of bright sunlight corresponds with 1 kWh of energy per m². This is approximately the solar energy falling on a surface of one square meter perpendicular to the sun's rays on a cloudless day in summer. On a horizontal surface, this maximum value could only occur near the equator at midday.
Source: BP-Solarex
Fig 2.1 Annual Solar Radiation kWh/m
The irradiation varies with time of day but it can also vary considerably from location to location, especially in mountainous areas. Irradiation varies from an average of 1,000 kWh/m² per year for northern European countries such as Germany, to 2,000 to 2,500 kWh/m² per year for desert areas, as shown in Fig 2.1 for horizontal surfaces. These variations are caused by weather conditions and differences in the relative position of the sun (solar elevation) in the sky, which depends on the latitude of each location. Although some benefit may be obtained by tilting solar panels to face the sun when it is not directly overhead, the improvement is less than might be expected under these conditions for two reasons: • • the atmosphere has already absorbed some energy due to the increased path length of solar rays through atmospheric dust and gases diffuse radiation from the rest of the sky partly compensates for this effect
Diffuse radiation also provides some energy on cloudy days, even though the sun may not be visible, and this is included in the data above. 2.3 Web Links
Interactive maps The following websites are very useful for a view of the solar radiation in Europe depending on the period of the year. 1. Daily variation of sun shadows during several months: • • • March: http://iamest.jrc.it/pvgis/pv/animations/images/eu_shadows_0321.gif June: http://iamest.jrc.it/pvgis/pv/animations/images/eu_shadows_0611.gif December: http://iamest.jrc.it/pvgis/pv/animations/images/eu_shadows_1221.gif
2. Monthly variation of global horizontal irradiation http://iamest.jrc.it/pvgis/pv/animations/images/eu_globrad_year.gif 3. Daily variation of global horizontal irradiation • December: http://iamest.jrc.it/pvgis/pv/animations/images/eu_globrad_1221.gif • June: http://iamest.jrc.it/pvgis/pv/animations/images/eu_globrad_0621.gif © European Communities, 1995-2003
3 Solar Thermal Systems
3.1 Introduction 3.1.1 Background When we are walking outside on a sunny day we notice that the sun warms us as well as providing light. This simple observation shows the difference between solar thermal and photovoltaic (PV) energy, which converts light directly into electricity. By using the very simple principle that darker coloured materials absorb more light and consequently get hot, solar thermal technologies use black colored plates as their core heatcollecting device. Solar heating may be either passive or active. The simplest example of passive solar heating is opening the curtains in winter, which lets the sun heat the room. This type of 'natural' space heating is very important and is an essential part of modern building design, but is not covered here in any detail (see refs 3.1, 9.1). More complex passive systems may require a concentration of solar heat, such as for heating water. Solar water heating systems save money by reducing the energy costs associated with traditional water heating and are also environmentally friendly. Saving electricity or fossil fuel usually means avoiding the types of emissions associated with combustion. Active solar thermal technologies use a range of simple to complex machinery. Some solar water heating systems are unable to operate on basic water pressure and gravity principles, and require the use of electric or other pumps to move the cold water through the collector. Larger scale solar thermal projects for electricity generation are under development and require the full array of power station technology. 3.1.2 History Solar energy has been used since the beginning of humanity when the sun was used to dry skins and foods. Archaeologists have shown that glass lenses were used no later than the 7th century BCE to concentrate the light of the sun and burn small pieces of wood to start a fire. In the first modern development (1767), Horace de Saussure, a noted Swiss scientist, observed that a room or a carriage is hotter when the rays of the sun pass through glass. He built a rectangular box out of half-inch pine, insulated the inside, covered the top with glass, and placed two smaller boxes inside. When exposed to the sun, the bottom box reached 228 ºF (109 ºC) (or 9 ºC above the boiling point of water). Renowned astronomer Sir John Herschel used solar hot boxes (fig 3.1) to cook food during his expedition to Southern Africa in the 1830s. Solar thermal energy became important in parts of Africa for cooking and water distillation. Although more than a dozen inventors filed patents during 1900-1911 that improved upon the hot box, the heating and storage units always remained one and the same and both stayed exposed to the weather and the Figure 3.1 Hot boxes for cooking cold night air. Hence, water heated by the sun the night before never stayed hot enough for washing the next morning or to heat the bath. In 1909, William J. Bailey patented a revolutionary solar water heater by separating it into two parts: a
heating element exposed to the sun and an insulated storage unit tucked away in the house so families could have sun heated water day and night and early the next morning. Solar water heating thrived during the years of high energy prices arising from the oil crisis in the 1970s. Many EU companies were created to manufacture, sell and install new solar water heaters in private houses, public buildings and swimming pools, and were backed up by government sponsored research and development projects. Great hopes were built on this growing market, but by the mid-1980s the situation changed, oil prices started to fall and the public fear of energy shortage slowly died out. The solar industry suffered badly and most of the new companies disappeared. Those that managed to survive improved their products, production methods and quality controls in order to satisfy more exacting customer demands. The market stabilised, but at a rather low level. During the 1980s evacuated tube collectors were developed to improve efficiency but at greater cost. This new technology has taken an increasing share of the EU market during the last decade, particularly in Germany, Greece and Austria. 3.2 Technical Principles
3.2.1 Solar Collectors The collection of solar radiation is based on the "greenhouse effect". The solar collector is mounted on or near the house, facing south (in the northern hemisphere). A large proportion of the sun's radiation can pass through the glass or plastic glazing and strikes a lightabsorbing material. The material converts the sunlight into heat, which is prevented from escaping by the glazing because most of the resulting infra-red waves are reflected. A car parked in the sun with its windows closed illustrates this greenhouse principle. The sunlight passing through the windows or glazing strikes the upholstery and is converted into heat energy. This heat is prevented from escaping by the closed windows and the inside of the car becomes extremely hot. A proper solar collector can easily achieve 95oC. 18.104.22.168 Types
Source CRES–TRASOL CD-Rom
Figure 3.2 - Typical layout of a flat plate collector
Flat plate collectors (figure 3.2) are the most economical choice for applications requiring energy delivery at moderate temperatures, up to perhaps 100oC. They capture both beam and diffuse solar radiation, do not need to track the sun, and require little maintenance. The major
applications of these units are in solar water heating, building space heating, air conditioning and industrial process heat. In a concentrating collector, the radiant energy is optically concentrated before absorption. Light entering a large aperture area is reflected or refracted to a relatively small target or receiver (e.g. at the focus of a parabola), where it is transformed into heat energy that is then collected in a conventional way, as shown in figure 3.3. The advantage of concentrating collectors is that thermal losses are lower when the heated area is reduced. This then allows the collector to heat fluid to a higher temperature with a higher efficiency. Concentrating collectors are therefore associated with high temperatures, mainly due to the reduced losses resulting from this concentration. These losses nevertheless still increase as the difference between the fluid and ambient temperatures rises, and the energy efficiency falls as a result (see fig 3.4). Trough type concentrators heat a length of tube, whereas dish concentrators focus on a single receiver, and therefore have the smallest area and higher efficiency.
The natural configuration for an evacuated tube collector (fig 3.4) is the glass tube, which provides the structural strength to withstand the pressure differences. They are hermetically sealed and contain getters to absorb any molecules that outgas into the vacuum. These tubes are excellent for operating temperatures up to the 120-150oC range. Their basic feature is a vacuum between the transparent cover surface and the absorber surface. In this way there are no heat losses associated with conduction and convection between these surfaces.
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Figure 3.3 Concentrating collector
100% 80% 60% 40% 20% 0% 0 100 200 300 400 Temperature difference ºC
Figure 3.4 Typical efficiency of a trough type concentrating collector
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Figure 3.4 Schematic view of an evacuated tube collector
3.2.2 Storage of the Collected Heat The majority of solar heating applications require a well insulated hot water storage tank, to store about 1½ days worth of collected solar energy. Storage tanks are classified as pressurized or unpressurized. Pressurized tanks are commonly available, as all conventional gas and electric water heaters are pressurized. Any water tank connected directly to a well or municipal water system must be built to withstand pressures up to 20 bar. Pressurized tanks are readily available in 20 to 450 litre sizes and are constructed of steel with a glass lining to prevent rusting. Unpressurized tanks are used to store larger amounts of water in space heating systems. These tanks are built in sizes from 750 to 35,000 litres and larger, and are made from stainless steel, fibreglass or high temperature plastic. Hot liquid from the collectors flows through a coiled tube in the tank to transfer heat to the water. There are two basic kinds of storage used in solar-heating systems. The first, well-mixed storage is most common with water storage in space-heating systems, while the second one, stratified storage, is virtually mandatory in air-heating systems and often used for domestic hot water systems. Solar domestic water heaters generally use a tank containing an electric immersion heater element, which serves as a back-up heater when solar energy is either not available or insufficient. Water is pumped from the lower portion of the storage tank to the collector or heat exchanger, where it is heated (usually by 1.5-5 ºC) and returned to the top of the tank. A dip tube at the top or upper side of the tank feeds the heated water in below the heating element, ensuring proper hot water stratification. 3.3 Solar Thermal Systems Applications
3.3.1 Domestic hot water production Passive and active solar hot water heaters can provide households with a large proportion of their hot water needs while cutting back on home energy costs. The amount of hot water that solar energy will provide depends on the type and size of the system, the climate, and the quality of the site in terms of solar access. There are several different kinds of solar hot water system, ranging from low cost, simple thermosiphonic "batch" systems, where the water circulates naturally and the storage tank doubles as the collector, to more efficient, complex and costly forced circulation systems that use pumps and sensors/controllers to move the hot water from Figure 3.5 A solar collector heating an flat-plate or evacuated tube collectors to a separate antifreeze solution for domestic water storage tank, as shown in figure 3.5. The economics will depend on both the system chosen and the geographical location. Solar water heating can be more economical over a lifetime of several decades than electricity, fuel oil or propane gas. Savings are greatest in sunbelt areas, where an investment of several thousand dollars in a solar system can earn a return in excess of 10% per year - higher than returns on savings accounts or government bonds. Looking at the lifetime savings is important, because the initial installation cost of a solar system can be three to six times greater than any electric or propane heating system. It is the 9
daily energy savings through the use of the sun's free rays that make a solar thermal system a good investment. The initial cost of a residential system can range from 1200€ (or less) to 3000€, with the fraction of solar energy provided ranging between 50 and 85%, resulting in a cost of delivered energy that ranges between 3 and 7 c€ per kWh at a good site. 3.3.2 Solar pool heating An unheated swimming pool has a natural yearly temperature cycle that varies with climate and geography, which in most parts of the world limits outdoor swimming to just the summer months. However, a comfortable three to four month swimming season can be stretched out to five or six months when a pool heater is added, even longer in warm climates, as shown in figure 3.6.
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Figure 3.6 Solar pool heating system
While heating the pool can enhance the enjoyment of the pool, the cost of keeping all that water warm is very high. However, if proper advantage of the strong summer sun is taken, then those high heating costs can be reduced while still obtaining the benefits of a warm pool. A swimming pool requires low-temperature heat to maintain the water temperature at a minimum of 24°C and, preferably, at 27°C. The small difference between the average daytime temperature of an unheated pool and the desired temperature for swimming allows the use of very simple but efficient collectors, very similar to bare plastic hoses. Such systems require no separate storage tank, since the pool itself serves as storage. In most cases, the pool's filtration pump is used to push the water through the solar panels or plastic pipes. When adequate sunshine is available, the filtered pool water is circulated through the collector tubes, where it is heated by the solar radiation and then returned to the pool. Circulation of all water through the filter about once every 8 to 12 hours is recommended. 3.3.3 Space Heating and Cooling 188.8.131.52 Space Heating Solar space heating represents a very large potential market, although it may have limited possibilities in existing multi-storeyed buildings in densely populated urban areas. The systems use air or a liquid that is heated in solar collectors and then transported by fans or
pumps using a small amount of electricity. Air systems consist of collectors, fans, ductwork, and controls and can heat the air in a house without heat exchangers or thermal storage. Large air systems typically use thermal storage, such as a bin of small rocks. Liquid solar systems consist of collectors, storage tanks, pumps, pipes, heat exchangers (in closed loop systems) and controls. Solar systems are usually designed to provide 40-80% of the annual heating needs of a house, but 50% may represent the most economical design because extra capacity is only used on the coldest days and would otherwise be idle. Heat not provided by the solar system must come from a backup system, usually a conventional furnace. The two systems can use common duct-work and heat delivery circuits, but the backup system should always be able to supply the total heating requirement for periods of cloudy weather, when little solar heat is available.
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Figure 3.7 Solar radiant in-floor system (P-1 to P-4 denote circulating pumps or fans)
The method of heat distribution most compatible with active systems is radiant slab heating, which uses plastic, rubber or copper pipes embedded in a concrete. When solar-heated water circulates through the pipes, the floor heats up and then radiates its heat to the room. In-floor heating systems (fig 3.7) use moderate water temperatures to keep from scalding bare feet. Typical inlet water temperatures range from 38 to 51.5 ºC, which is precisely the range where active solar systems excel. Since the temperature difference between a radiant slab and the collector is greater than that for a water storage tank / collector system, the collectors are more efficient since they lose less heat to the cold outside air. It is also possible to connect the solar system to small heat exchangers in individual rooms. These small heat exchangers are available as standard plumbing units in various sizes and contain their own blowers. The fluid temperature in a solar system reaches 32.5-50oC, whereas conventional systems heat water to 70-80oC. Therefore, if baseboards or radiators are used with solar heating, the surface area of the radiators should be significantly increased. 220.127.116.11 Space cooling Owing to the coincidence between cooling demand and maximum available solar irradiation, solar air conditioning appears to be a very promising technology, which might open a rapidly 11
growing market. Solar cooling offers a solution to overheating during summer time and, in addition, reduces the summer midday peak electrical energy demand. Active solar cooling uses solar thermal energy or solar electricity to power a cooling appliance. There are various types of cooling technologies that utilize solar thermal energy. Absorption cooling is the first and oldest form of air conditioning. An absorption air conditioner does not use an electric compressor to mechanically compress the refrigerant. Instead, the absorption cooler uses a heat source, such as natural gas or a large solar collector, to evaporate the already-pressurized refrigerant from an absorbent/refrigerant mixture, as shown in fig. 3.8. This takes place in a device called the vapour generator. Although absorption coolers require electricity for pumping, the amount is small compared to that consumed by a compressor in a conventional electric air conditioner. When used with solar energy systems, absorption coolers must be adapted to operate at the normal working temperatures for solar collectors: 82 - 120°C.
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Figure 3.8 Air-conditioning with the aid of an active solar system
3.3.4 Special applications 18.104.22.168 Large scale Solar heating The energy bills for the production of domestic hot water, as well as for space heating or cooling in public buildings (offices, hotels, hospitals, schools etc.), industrial plants, large sports centres, even in entire communities as district heating (fig. 3.9), can be reduced or even eliminated with the help of a thermal solar system. There are several installations of large solar systems in such buildings in Europe producing domestic hot water and a few others contributing to thermal energy production for space heating and cooling. Most of them were financed from various National or European projects. 22.214.171.124 Desalination Solar energy can be used for desalting ocean waters at a cost which permits agricultural use of the water, as well as providing drinking water. Since many arid lands are close to the sea, desalted waters from the oceans can be used to start reclamation. After coastal strips have been reclaimed, reclamation can be pushed further inland. The most intuitive solar thermal desalination technique is to feed a distiller with solar heated water. This has been attempted several times with satisfactory results using multistage flash (MSF) plants, and less frequently with multi-effect distillation (MED) plants.
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Figure 3.9 Solar collectors used for district heating in Sweden
MSF operates by heating seawater directly to temperatures as high as 140ºC, allowing it to evaporate by pressure reduction in stages, and condensing the vapour. MED uses solar generated steam to evaporate the seawater and hence distil pure water from it. Both methods are more efficient with higher temperatures, but MED can be designed for good efficiencies at temperatures around 70ºC and provide stable outputs under varying conditions. Solar input may be by means of concentrating collectors (to generate higher temperatures) or solar ponds (see web link) or a combination of the two. Cost estimates for water from a medium scale solar powered MED plant are in the order of 1.6 €/m3. For MSF, the estimates are 1.5-2.2 €/m3, depending on the size of the application. Desalination appears to be a promising solar technology in several European and Mediterranean countries because of the coincidence between maximum water demand and maximum available solar insolation, and the lack of potable water in arid regions and islands during the summer months. 22.214.171.124 Drying of agricultural products Using solar energy to dry crops is nothing new in the tropics. Many edible and even cash crops, such as cocoa and coffee beans, have been dried for decades on racks placed in the sun. In the case of a closed heated space, in which a damp agricultural crop has been stored, two things happen: • The crops are warmed by the heat source. • The surrounding air is also heated, absorbs moisture and rises, inducing more air. Solar dryers can be divided into two categories: 1. Dryers in which the sunlight is directly employed; here the product itself absorbs most of the solar heat directly (fig. 3.10). These are further divided into three types: • Traditional drying racks in the open air. • Covered racks (protecting against dust and insects). • Drying boxes provided with insulation and absorptive material. 13
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Figure 3.10 Solar drier directly employed
2. Dryers in which the sunlight is employed indirectly (see fig. 3.11). In this method, the drying air is warmed in a space other than that where the product is stacked. The products are not then exposed to direct sunlight. Various sorts of construction are possible; this design can also be provided with powered fans in order to optimise the air circulation.
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Figure 3.11 Solar drier indirectly employed
188.8.131.52 Solar Cookers and Ovens Solar cookers and ovens (fig. 3.12) have been used for over 200 years to cook, bake and purify drinking water. There are three basic types of solar oven, namely box, parabolic reflector and multi-reflector. The solar box, which absorbs solar radiation and releases it to the food, is the least expensive and fastest to make - as low as a few dollars and using mostly recycled materials that can be assembled in a few hours - and achieves temperatures as high as 149oC. They are excellent for the slow boiling of foods, such as stews, soups and cereals. The base of the solar oven is a well-insulated wooden box, which attracts, captures, increases and maintains the sun's natural heat. 126.96.36.199 Mechanical Power
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Figure 3.12 A solar cooking device
One of the major issues of interest nowadays is the conversion of solar energy into mechanical power. Where a small scale direct drive is desired, this may be done by steam engines supplied with steam from a large cylindrical parabolic concentrator, but this is likely to be expensive. Larger scale projects will normally involve electricity generation (see below). 3.3.5 Electric Power The generation of electricity from solar thermal energy requires higher temperatures than for heating, and therefore some kind of concentrating collector is required. Three techniques have been applied: trough type parabolic collectors, parabolic dishes and a power tower. They all require an array of collectors, each of which must be capable of tracking the sun in a coordinated manner. These technologies are still at the demonstration stage, and rely mainly on work at just two research centres in the world: the Solar Electricity Generating Station (SEGS) in California and Plataforma Solar de Almeria (PSA) in Spain. 220.127.116.11 Solar farms Trough type collectors heat a tube containing liquid, which is normally then pumped to a high temperature heat exchanger to generate steam for a conventional power cycle. European companies have a lead with this technique; German and Israeli companies contributed basic components at the SEGS plants. Direct generation of steam in the absorber tubes, thus dispensing with thermo oil, has also been tested in Europe. With the participation of Siemens, the hybrid operation mode, which combines solar thermal with the combined cycle gas technology, has been investigated. A consortium of European firms is offering a parabolic trough system for application on the Greek island of Crete. 220.127.116.11 Parabolic dishes Dish collectors may be used in the same way as troughs, but an alternative is to place a Stirling engine at the focus of each receiver (see fig 3.3), which will generate electricity directly with maximum efficiency. EU (German) industry still holds a leading position 15
compared to the US industry for dish/stirling systems. Particularly the 16,000 hours operational experience from the three PSA plants has provided valuable results. 184.108.40.206 Solar towers A power tower works in a similar way to a very large dish, but the reflective surface is split up and distributed as individual mirrors over a large area of ground, each of which independently tracks the sun in order to deliver radiation to the central tower (fig 3.13). By putting into operation SOLAR TWO, the American industry will gain operational experience, particularly in the use of salt as the heat transfer medium. European companies have operated an open volumetric air receiver at PSA. Currently EU industry can offer a 30 MWe solar power tower.
source: NREL
Fig 3.13 Solar Power Tower
Economic Implications
The major obstacles encountered in the commercialisation of solar systems are economic, resulting from the high cost of the equipment needed to collect and store solar energy. Some challenges that must be overcome are inherent in the nature of solar radiation:
• •
It is relatively low in intensity, rarely exceeding 950 W/m2 so large collecting areas must be used when large amounts of energy are needed. It is intermittent because of a) the inevitable variation in solar radiation intensity from zero at sunrise to a maximum at noon and back to zero at sunset b) unpredictable interruptions because of clouds, rain, snow, hail or dust. Some means of energy storage must be provided at night and during periods of low solar irradiation.
Sun In Action is a major study (ref 3.8) carried out by ESTIF (European Solar Thermal Industry Federation), from which some edited extracts are reproduced below.
3.4.1 Solar Heating systems Solar thermal can be successfully implemented at all latitudes. Some of the strongest markets (Germany, Austria) are not situated in particularly sunny regions, whereas for instance Southern Italy is clearly lagging behind. Factors like general awareness of the environment, public support (financial, regulative, campaigns) and the quality of the products/ services offered by the industry are at least as important as climatic conditions. Even assuming long-term stable energy prices, in Europe there is a significant potential for installing solar thermal systems with payback times between 5 and 15 years, clearly shorter than the average lifetime of 20–25 years. Thus, in many situations, solar thermal is a rational long-term investment. The potential is even larger, if energy prices are assumed to be growing on the long term and if the social political and environmental costs of the use of fossil fuels are fully included. 22.214.171.124 Time of purchase A key barrier to growth is the short window of opportunity when the best conditions exist for a solar system to be installed. For technical reasons, it is significantly more effective and cheaper to install solar thermal systems during construction of new buildings or when an existing heating system or building is undergoing a major renovation. As most potential users replace their heating systems only once in ten to twenty years, this is a serious restriction to a fast market growth: if the opportunity is missed, the door is closed for a long period. 188.8.131.52 Perceived desirability by the general public Solar thermal can benefit from its positive image as it gives a chance to integrate clean and renewable energies into the daily energy supply. However, almost everywhere, solar thermal is not yet perceived as a standard option, but rather as an additional feature or a premium product. Most potential users still need to have a special motivation to consider investing in solar equipment. Promotion can be targeted at the general public, to create basic awareness and provide counter-arguments to misinformation, e.g. that solar thermal is feasible only in sunny Mediterranean countries. Other prejudices concern the investment payback time and the reliability. Particularly among architects, the aesthetic impact is often considered excessive. 126.96.36.199 Market growth for solar water heating (SWH) systems The European solar thermal markets have shown substantial growth over the past decade. On average, the glazed collector area in operation increased by 11.7% per year and the market volume (newly installed collector area) grew by 13.6% per year. Adding the roughly 1.6 million m2 of glazed collectors used for heating swimming pool water, the total in operation at the end of 2002 can be estimated at 12.3 million m2. The target set by the Campaign for TakeOff launched by the European Commission (15 million m2 by 2003) has therefore nearly been reached. The collector area in operation is highly concentrated in three countries: Germany, Greece and Austria account for more than 80% of the EU total. Relating the glazed collector area in operation to the population, the leading role of Greece (264 m2 per 1000 capita) and Austria (203 m2) becomes even more evident, compared with the EU average of 26 m2. In recent years there has been a trend towards levelling the big differences between the frontrunners and the countries lagging behind. Spain, Italy and France have been growing faster than the EU average, whereas Austria and Greece have stagnated at a high level. The German market
grew very strongly until 2001. Its breakdown in 2002 explains the contraction of the European market in that year. Glazed collector area (in m2) for 1985-2002 1985
New systems Total installed 393,415 1,320,735
360,799 2,916,102
624,067 5,532,569
1,162,128 8,760,320
1,470,185 9,862,500
1,196,540 10,755,842
18.104.22.168 Potential The technical-economical potential for solar thermal is estimated at 1.4 billion m2 (EU-15). This capacity would generate 682TWh (58.7Mtoe) of thermal energy per year. This corresponds to: • • 6% of the EU final energy consumption (EU-15); More than the final energy consumption of a country like Belgium;
• 30% of the EU oil imports from the Middle East in 1999 (EU-15). Only 1% of this potential has been realised so far, and most of this in three countries only. The potential for growth in the near future is therefore immense. 3.4.2 Solar Thermal Electricity Under current market conditions, neither solar tower systems nor parabolic dishes can be characterised as sufficiently mature for the energy market today. Even solar farms, the most developed technology, have been applied in Europe only as prototypes. Current commercial deployment is therefore zero within the EU, however it is important to consider future markets. The costs of solar thermal electricity depend to a high extent on the local level of direct solar radiation. Energy costs (for 100 MWe plants) of 0.09 €/kWh could be achieved with advanced power towers or solar farms at favourable sites. The lower limit for dish Stirling systems is about 0.10 €/kWh. The logical next development steps for the various technologies are: • Solar farms: realisation of a 100 MWe plant (at a cost of around 300M€), based on thermo-oil or direct steam generation, developed in Europe. • European solar power towers based on air receivers might be considered ready for demonstration. • Parabolic solar dishes can be demonstrated at lower costs. It is highly likely that the first large-scale deployment will involve a hybrid system, combining a solar thermal heat supply with a conventional fossil fuel based generation system.
3.5.1 Bibliography: 3.1 Smith, Peter F (2001). Architecture in a climate of change. Architectural Press, Oxford, UK 3.2 Blezinger, H, (1994). "Continuous Operation of a Dish/Stirling Field on the Plataforma Solar de Alméria", presented at the UN-ECE Workshop on Renewable Sources of Energy, May 1994. 3.3 DG XVII Expert Group (l996), 'Assessment of Solar Thermal Power Plant Technologies', European Commission, Brussels. 3.4 Grasse, W, and Jiménez, M S, (1994). "Plataforma Solar de Alméria. Test Activities in the Field of Solar Thermal, Solar Chemistry, Materials Science and Technology Transfer", presented at the 7th Symposium on Solar Thermal Concentrating Technologies, Moscow, 26-30 September 1994. 3.5 H. Klaiss, F. Staiss, and C.-J. Winter, (1991), 'Systems Comparison and Potential of Solar Thermal Installations in the Mediterranean Area', Workshop on Prospects for Solar Thermal Power Plants in the Mediterranean Region, Sophia-Antipolis, France. 3.6 Jiménez, M S (ed), (1995). "Plataforma Solar de Alméria: Annual Technical Report 1994", PSA Report Number PSA-TR03/95-MS, Alméria, June 1995. 3.7 WEC, (1993). "Renewable Energy Sources: Opportunities and Constraints 19902020", World Energy Council, London. 3.8 Sun In Action II: A Solar Thermal Market Strategy For Europe. Study by ESTIF, co-financed by the European Commission under the ALTENER program. 3.5.2 Web links: Solar water heating: Solar cooking: Concentrators: Desalination: Solar ponds: Solar thermal advantages:: http://acre.murdoch.edu.au/refiles/lowtemp/text.html http://www.californiasolarcenter.org/history_solarthermal.html http://www.solarcooking.org/ http://www.energylan.sandia.gov/sunlab/overview.htm http://www.mfa.gov.il/mfa/go.asp?MFAH0apv0 http://www.commonwealthknowledge.net/Subnetw/summdes.htm http://edugreen.teri.res.in/explore/renew/pond.htm http://184.108.40.206/unisun/solar_thermal.htm
Solar thermal power generation: http://www.solel.com/solar_power_plan.html http://europa.eu.int/comm/energy_transport/atlas/html/steint.html Solar thermal energy technology: potential for a major european industry: http://www.jrc.es/iptsreport/vol10/english/Ene1E106.htm European Solar Thermal Industry Federation: Soltherm Initiative: Other relevant sites: http://www.estif.org/
http://www.soltherm.org http://www.solardome.com/index.html http://www.solardev.com/index.html 19
4.1 Introduction Photovoltaic (PV) literally means 'light-electric'. The solar PV cell is the only developed technology which can convert solar energy directly into electricity in a single step. Conceptually, it is a solar-powered battery energised by the sun. This requires special electronic materials known as semiconductors, which are also used in computer chips, and the two technologies have advanced together although in different directions. We would therefore now expect PV to be a strongly competitive method of generating electricity, but must recognise that its merits are subject to both the availability of solar irradiation and the cost of the semiconductor materials which need to cover a large area. 4.2 History
The effect of light on the electric properties of certain materials was observed well before electricity became generally available. Early PV cells made of selenium could be used for detecting light and measuring its intensity, but their conversion efficiency was too low to produce useable electric power. This had to await the postwar development of doped silicon semiconductors, on which much of our current technological culture depends. Although silicon is still the most significant PV material, several other combinations of rarer elements have been explored and found to have useful special properties. Now known mainly for its use as an energy source for satellites, it was after the first oil price shock in the early 1970s that interest grew in the use of PV for terrestrial applications. Since then, national and international investment in R&D, demonstration and dissemination have led to important technical improvements and a drop in the price of PV modules by a factor of more than 20.
PV history at a glance
1839 1873 1877 1918 1932 1941 1951 1954 1955 1958 1963 1970 1981 1982 1984 1985 1986 1990s 2001
photovoltaic effect discovered by Alexandre Edmond Becquerel (France) PV effects in selenium first reported (Willoughby Smith, UK) first solar cell (selenium, conversion efficiency 1%) manufacture of mono-crystalline silicon (Czochralski, Poland) photovoltaic effect discovered in Cadmium Telluride (CdTe) first silicon solar cell first germanium mono-crystalline solar cell solar cell conversion efficiency reached 6% first commercial production of solar cells first satellite with PV cells first PV module start of terrestrial PV era first PV airplane PV energy started to be used in the automotive industry first thin-film PV module (hydrogenated amorphous silicon) solar cell conversion efficiency of over 20% achieved first commercial thin-film PV module high growth-rate of PV industry experimental PV-powered airplane Helios reached 30 km altitude
Source: www.pvresources.com
Technical outline
4.3.1 What is a PV system composed of? The most important parts of a photovoltaic system are the cells which form the basic building blocks, the modules or solar panels which bring together large numbers of cells into a unit, and, in some situations, the inverters used to convert the electricity generated into a form suitable for everyday use.
1. 2. 3.
solar panels Inverter Battery
4. 5.
grid appliances
Source: www.pvresources.com
Figure 4.1 Configuration of a backup solar electricity system
4.3.2 PV Cells The PV phenomenon depends on the electric field created at a junction plane between two special semiconductor materials. Photons from solar radiation can then energise electrons near this junction which allows them to be conducted into an external circuit and generate power. The vital feature of a PV cell is therefore a solid interface between two different materials, which might be two crystalline wafers or a film and a substrate. The key to success lies in finding a combination of semiconductors which both have good electrical performance and can also be economically manufactured. Source : IMEC PV cells are generally made either of thick crystalline silicon, Fig 4.2 Polycrystalline sliced from ingots or castings or from grown ribbons, or of thin Silicon Cell films, deposited in thin layers on a low cost substrate. The majority of module production (84%) has so far involved the former, whilst future plans centre on the latter. Thin film technology is eventually expected to dominate the market for PV modules on buildings because of its advantages in terms of weight, robustness and visual appearance. The main types of cell are shown in Table 4.1, demonstrating considerable momentum within solar cell R&D to meet the range of applications demanded by a growing PV market. 21
Table 4.1: Cell types and characteristics Cell Type Crystalline silicon Characteristics Mainstay of most power modules (single- or multi- crystalline) Widely available and well understood Uses technology developed for the electronics industry. Efficiency: typically 13 – 16% for commercial products, but values already exceed 25% in the laboratory Single crystal solar cells in Panel Thin Films Three main types of thin film cells are available today: - amorphous silicon (a-Si) - copper indium diselenide (CIS) - cadmium telluride (CdTe) Thin films use thin layers of photosensitive materials on a low cost substrate such as glass, stainless steel or plastic. All of these have active layers in the thickness range of a few microns, and all are manufactured by continuous processes which are capable of large volume, low cost production. They also have the advantage that their operating characteristics can be adapted by relatively simple changes to the manufacturing steps. Future possibilities Pictures from ref 4.1
a-Si solar panel
Concentrators focus light from a large area on to a small area of photovoltaic material using an optical concentrator (such as a mirror or Fresnel lens), thus reducing the quantity of PV cells required. Two main drawbacks are that they cannot make full use of diffuse sunlight and they must always be directed towards the sun with a tracking system Organic dye solar cells first developed in 1991 still have low efficiencies and show a poor long term stability, but they could become important in the longer term. Spheral solar technology uses minute silicon beads bonded to an aluminium foil matrix. This offers an important potential cost advantage because of the reduced need for silicon, but the technology is still not in full scale commercial production.
Source: "Solar Generation" EPIA/Greenpeace, December 2001
4.3.3 PV Modules Modules contain a number of photovoltaic cells which are joined together (usually by soldering) and sealed under a sheet of glass. They can be adapted in size to the proposed site, and quickly installed. They are robust, reliable and weatherproof. European research and demonstration projects have already provided performance data for 20-25 years of PV module operation without failure, and module producers now typically
Figure 4.3 Module production by technology (Europe, 2002)
Others* 2% Single Crystalline Silicon 55%
Undefined* 14% Amorphous Silicon 3% Polycrystalline Silicon 26%
*'Undefined' means the technology type was not clarified; 'others' refers to technologies other than silicon based. Source: Ref 4.7
guarantee a life of 20-25 years on their products. In Europe, PV module production remains based, predominantly, on crystalline silicon technologies, of which approximately 55 % is single-crystalline and 26 % is polycrystalline. Thin film PV products have still to make a significant impact on the PV power market 4.3.4 Types of PV System
Grid Connected systems This is the most popular type of solar PV Off-Grid (stand alone systems)
system for homes and businesses in the developed world. Connection to the local electricity network allows any excess power produced to be sold to the utility. Electricity is then imported from the network outside daylight hours. Inverters are used to convert the direct current (DC) power generated by a PV array into alternating current (AC), which is compatible with the local electricity distribution network. This is essential for grid-connected PV systems. The inverter also often includes elements to protect the system against instability in the grid connection.
Most stand-alone (off-grid) PV systems contain a battery, commonly of the lead acid type, in order to store the energy for future use. This is usually connected to the PV array via a charge controller. The charge controller protects the battery from over charge or discharge, and can also provide information about the state of the system or enable metering and prepayment for the electricity used. If AC output is needed, an inverter is required to convert the DC power from the array. Protection equipment and wiring are also required.
Source: IEA-PVPS
Figure 4.4 Grid connected PV system Hybrid System
Figure 4.5 Stand alone PV system
A solar system can be combined with another source of power - a biomass generator, a wind turbine or diesel generator - to ensure a consistent supply of electricity. A hybrid system can be grid connected, or stand alone. 4.3.5 Operation of solar cells The amount of power available from a PV device is determined by: the type and area of the material; the intensity of the sunlight (insolation); and the wavelength of the sunlight. The amount of current produced is a function of the voltage. Figure 4.6 is a typical I-V curve for a crystal silicon cell under standard testing conditions in bright sunlight. This shows a typical maximum power of about 1.5W, but this is only obtained for the right combination of current and voltage, and therefore the cells must be combined into the modules and overall
system in such a way that there is a good match with the load using the power. If there is a low voltage on the cell or the current being taken is too small, the power delivered will be low however bright the sunlight.
Source : www.aurora.crest.org
Figure 4.6 I-V curve for a typical crystal silicon cell
4.3.6 Rating The performance of PV products is specified in terms of the maximum power (Watts Peak or Wp) which can be delivered under idealised standard solar conditions and loading. Prices and installation statistics are based on this concept. 4.3.7 Market segments Off-grid industrial uses for solar electricity are mainly in the telecommunications field, especially to link remote rural areas to the rest of the country. Repeater stations for mobile telephones powered by PV or hybrid systems (PV/diesel) also have a large potential. Other applications include traffic signals, marine navigation aids, security phones, weather or pollution monitors, remote lighting, highway signs and waste water treatment plants. This market segment is fully commercial. Off-grid community or Stand-alone systems can supply both developing and industrialised worlds that have no access to grid connection. Table 4.2 shows a wide range of applications.
Agriculture Community Level Domestic Sector Healthcare water pumping electric fencing for livestock and range management water pumping, desalination and purification systems lighting for schools and other community buildings lighting, enabling studying, reading, income-producing activities and general increase in living standards TV, radio, and other small appliances water pumping lighting for wards, operating theatre and staff quarters medical equipment refrigeration for vaccines communications (telephone, radio communications systems) water pumping security lighting lighting systems, to extend business hours and increase productivity power for small equipment, such as sewing machines, freezers, grain grinders, battery charging lighting and radio in restaurants, stores and other facilities Table 4.2 Stand-alone PV applications in developing countries
Small enterprises
Grid-connected systems represent the market segment that has been growing most rapidly over the past few years and which has been the motor driving the very fast recent growth of the German and Japanese markets. These types of installations are now mostly integrated into buildings, either on roofs (fig 4.7) or in facades. This market segment is likely to continue to expand rapidly in industrialised countries and will become a Source: BP Solar competitive peak power supply within the coming Fig 4.7 Berlin Bank decade. Consumer goods and services represent the fourth main market segment which is made up of a wide range of consumer products and small electrical appliances, including watches, calculators and toys up to professional sun roofs for automobiles. Other applications include power for services such as water sprinklers, road signs, lighting and phone boxes. This is also a fully commercial market segment. 4.4 What is the cost of PV?
4.4.1 Overview High investment costs are one of the major barriers to the development of PV markets in the short to medium term, although a continued downward trend in system prices can be observed over time. PV systems, and especially grid connected PV systems integrated into buildings, are locked in a critical "chicken and egg" situation between today's system price and the future economies of scale in manufacturing. Nevertheless, the falling cost of PV cells and modules has been a crucial factor in the recent development of PV technology. An indication of the potential for increased efficiency in the production of cells has already been given, together with the likely shift in favour of cheaper thin film technologies. However, unlike most conventional electricity generators, the costs associated with a PV generator after installation (operation and maintenance) are very low. Overall, today's costs make photovoltaic generators an economically attractive choice in a large variety of applications where no mains electricity is available. 4.4.2 PV module costs The costs of manufacturing solar cells and modules have been falling steadily. On average, the price of modules has fallen by ~5% per annum over the last 20 years, and is projected to continue to fall for the next 20 years. In the scenario outlined in the EPIA/Greenpeace study "Solar Generation" – a blueprint for bringing solar electricity to 1 billion people by 2020", it is projected that the price per Wp for a new cell production plant will drop from €1.69 in 2001 to €1.12 by 2010. Between 2010 and 2020 a further price decrease is anticipated. On the basis of a 20% progress ratio, the cost of "ready to install" modules would fall by more than two thirds - from €3/Wp in 2001 to less than €1/Wp in 2020. The grid-connected market must still depend for the moment on government incentive programmes, but this situation is expected to change as the PV market becomes increasingly self-sustaining, with expanding market sizes in all sectors. As with any technology the 25
development of a learning curve leads to cost reductions. In the case of PV the cost decrease is expected to be around 20% every time the total installed capacity is doubled. The effect of sales volume on cost reduction for PV systems can be seen in figure 4.8
€ 25 € 20 € 15 € 10 €5 €0
19 80 19 81 19 82 19 83 19 85 19 87 19 89 19 91 19 93 19 95 19 97 19 99 20 01
400 320 240 160 80 0
MW Shipped
Cost per Watt (Euro/Wp)
Source: Adapted from "Solar Generation" EPIA/Greenpeace, December 2001
Figure 4.8: The falling cost of PV as shipments increase
4.4.3 PV system costs Installed PV system prices depend to an important extent on the costs of the PV modules, but also on many other factors such as the project location, application, integration mode, project size, grid connection, the number of intermediaries in the supply chain, as well as the technical specification and the applicable market stimulation measures. The current capital cost of a typical installed photovoltaic system ranges from ~ 5 €/Wp to 8 €/Wp.
Source : Winfried Hoffman, RWE Schott Solar Gmbh, Germany, 2003
Figure 4.9 Value chain for PV Systems
Figure 4.9 shows breakdowns of the costs of typical PV systems, depending on their use either in stand-alone or in grid-connected applications. 4.4.4 Costs of solar electricity
Source : European Commission - DG Tren.
Figure 4.10: Projected electricity cost in €/kWh depending on the level of solar irradiation. (Projection based on levelized electricity cost)
As a consequence of falling PV system prices, the cost per kWh of solar electricity is decreasing as well, although it is also dependent on the degree of solar irradiation. The predicted decreases in the costs of PV electricity over the next 20 years are shown in figure 4.10 for three different levels of system yield, which correspond typically to yields for good grid connected PV systems in the North, middle and South of the EU. These different levels of yield arise because the electricity yield per installed kWp of PV system capacity depends on the level of solar irradiation.
To choose the best system for you, a useful "solar wizard" exists on the web, designed by Ecofys: http://www.mysolar.com/mysolar/pv/step1a.asp
4.4.5 Prices, compensation schemes and support systems The European market is heterogeneous: both the kind of public support schemes and the level of support vary considerably from one country to another. Table 4.3 shows the main incentives and existing support schemes in EU countries. Two kinds of support scheme can be distinguished: • Support for investment: through low interest rate or investment subsidies • Support for electricity production : mainly through a guaranteed price or 'feed-in rate' (FR) 27
Table 4.3 : Incentive and support schemes in EU Countries
Country Austria Belgium Flemish region : Walloon region: Denmark Investment support Yes Yes 75% of cost of PV panels paid from a budget of €1 million in 2002. Most communes also give a grant of between €250 and €750. - 75% of total feasibility costs based on quotes provided by Ministry-approved companies; - 60% of total certification costs. 35% of turnkey costs paid for PV systems integrated into existing single family houses. Investment subsidies for demonstration projects up to 40%. Payment to selected projects by ADEME from €4.6/Wp to €6.1/Wp. Production support 0.60 €/kWh for systems up to 20kWp 0.41 €/kWh for larger systems Yes 0.15 €/kWh for systems up to 25kWp FR : Remarks
Source: EPIA investigations
FR: a limit of 15 MWp total installed capacity is included in the law which makes it ineffective. In Flanders in 2003 the budget was fully committed in the first three weeks of January.
Electricity generated by the consumer is bought back at the standard utility price. Hence a consumer can run their meter backwards when generating excess electricity. 0.15 €/kWh for: residential systems up to 5kW non-building systems up to 150 kW commercial and public buildings up to 1MW guaranteed for 20 years. 0.30 € /kWh for overseas departments. 0.48 € /kWh guaranteed for 20 years, reducing by 5% each year. Technical and administrative barriers: hundreds of pages of forms to fill Financial barriers: the 2003 subsidy budget of ADEME ran out in one month
Finland France
Germany Greece Italy
Interest reduced loans, ends 2003. A new programme is under discussion Investment subsidies in some federal states 40-50% of cost for commercial PV applications. From 0.06 €/kWh to 0.078 €/kWh for 10 years. 10000 roof programme : Up to 75% of eligible No investment cost. Up to 50%of the investment 0.45 – 0.55 €/kWh 0.25 €/kWh for local authority installations
Monopoly of public utility - Administrative barriers; - production limited e.g.to the average consumption over previous 3 years. - Max system size 20 kWp
Luxembourg Netherlands Portugal Spain Sweden United Kingdom
3.5 €/Wp and can be increased by 25%. 20-40% of eligible costs. 0.50 €/kWh up to 5 kWp, then 0.28 €/kWh 30-35% of installation cost, depending on 0.21 € /kWh over 5 kWp and up to 50 MWp region. 0.39 € /kWh up to 5 kWp. No No 50% for domestic 0.5 – 5 kWp 40% for commercial 0.5 – 100 kWp 65% for social or public buildings
Important technical and administrative barriers.
Current developments
For stand-alone power systems for industry and consumers, the main remaining Research and Technological Development (RTD) issues lie in further integration of PV and BOS components in the construction of a system. By contrast, stand-alone systems in rural contexts are usually operated far from the normal repair and maintenance infrastructure, so reliability and low maintenance are of prime importance. But since the end-users are often poor, the balance between reliability and cost is vital. Specific research areas that the European industry must focus on to respond to the needs of this potentially huge market are: • Optimisation of low cost techniques for thin-film production. • Improved storage technologies – notably an affordable alternative to the poorly suited and environmentally damaging lead-acid car batteries that are used throughout the developing world. • Development of small, low-cost, reliable solar product packages that have the potential to extend clean energy service delivery to a larger section of the population. • For grid-connected distributed generation systems (200Wp to several hundred kWp) the market can be divided into systems on houses and those on commercial or public buildings. For house based systems, where the aim is functional design at reasonable cost, RTD should focus on the use of pre-fabricated roof elements, PV kits and integration into the building process. Commercial grid-connected systems are often part of the building envelope, integrated into the façade or roof to displace other materials such as expensive marble façades, or to provide shading or ventilation. • For grid-connected central power stations with generating capacity, the primary objective is to produce clean reliable PV power. RTD issues relate to support for these large systems e.g. grid-connection issues and inverters. Some PV power stations of the 501,000kWp range have been integrated into rooftops, however, and others have a secondary function as sound barriers or weather protection. 4.6 The Benefits of PV Power
4.6.1 Advantages of solar electricity: • The fuel is free • Solar electricity technologies generate power even on cloudy days • No moving parts to wear out or break down • Minimal maintenance required to keep the system running • Modular systems can be quickly installed anywhere and easily expanded as requirements or financial resources increase • Produces no noise, harmful emissions or polluting gases • Is equally well suited for installation in high density areas of the industrialised world, as well as in remote areas of developing countries Photovoltaic power systems offer many unique benefits above and beyond simple energy delivery. This is why comparisons with conventional electricity generation - and more particularly comparison with the unit energy costs of conventional energy generation - are not always valid. If the amenity value of the energy service that PV provides, or other non-energy benefits, could be appropriately costed, it is clear that the overall economics of PV generation would be dramatically improved in numerous applications, even in some grid-connection situations.
4.6.2 Environmental considerations In spite of the above, there are nevertheless some potential hazards allied to the production of some of the more exotic thin film technologies. The two most promising options, cadmium telluride and copper indium diselenide, both incorporate small quantities of cadmium sulphide, which poses potential cadmium risks during module manufacture. Fortunately, there are well-established procedures governing the handling of such compounds, which are adhered to throughout the production process. One criticism of early PV modules was that they consumed more energy during their production than they generated during their lifetime. With modern production methods and improved operational efficiencies this allegation is no longer true. The exact energy payback is obviously dependent on the available solar resource and on the degree to which the system is operational. High levels of solar irradiation and a high utilization factor will offer more rapid energy paybacks than if there is less sun and less usage, but typically energy payback will be realized within two years. 4.6.3 Space-saving installation PV is a simple, low risk technology which can be installed virtually anywhere there is available light. This means there is a huge potential for the use of roofs or facades on public, private and industrial buildings. PV modules can be used as part of a building's envelope, providing protection from wind and rain or serving to shade the interior. During their operation such systems can also help reduce buildings' heating loads or assist in ventilation through convection. Other places where PV can be installed include the sound barriers along communication links such as motorways. To satisfy a significant part of the electricity needs of the industrialised world there is therefore no need to exploit otherwise undisturbed areas. In the UK, for example, it has been estimated that the country's total electricity demand could be satisfied by solar arrays using only 3% of the land area. 4.6.4 Improving the electricity network For power companies and their customers, PV has the advantage of providing relatively quick and modular deployment. This can offset investment in major new plant and help to strengthen the electricity network, particularly at the end of the distribution line. Since power is generated close to the point of use, such distributed generators reduce transmission losses, can improve service reliability for customers and help limit maximum demand. 4.7 Conclusion
PV power generation is now economically competitive for loads of up to a few kW in many remote sites away from the mains electricity grid, and interest is growing world-wide in the development of grid connected PV power generation. Of particular interest is the integration of PV modules into buildings, where they can act not only as power generators but as architectural elements. Module costs can be offset against the costs of the cladding which they replace, resulting in a reduction in the cost of electricity. It already supplies electricity to hundreds of thousands of people around the world, provides employment for several tens of thousands and already constitutes an annual business worth more than 1 billion Euro. Projections indicate that solar electricity could be making a major contribution to the global energy mix within a generation and represent a business worth 75 billion Euro per annum by 2020.
4.8.1 Web links 4.1 The Australian Greenhouse Office :
History Of Solar Power: General information on PV :
• • • • •
www.mysolar.com www.scolar.org.uk www.pvresources.com http://www.agores.org/SECTORS/SPV_TPV/default.htm
European Commission PV Homepage:
Technical data, market information:
• • • • www.epia.org www.iea-pvps.org www.solarbuzz.com www.solaraccess.com
Information on PV from the Joint Research Center:
The Solar PV potential anywhere in Europe can be easily calculated with the following tool:
Photo galleries: • IEA-PVPS:
http://www.oja-services.nl/iea-pvps/photos/index.htm http://www.nrel.gov/data/pix/pix.html
• •
EU Barometer on energy: Issues, Options and Technologies
EU PV Barometer for 2003: http://www.observ-er.org/comm/baro154.pdf
4.8.2 Bibliography 4.2 EPIA Roadmap, EPIA, July 2003 4.3 European Commission- DG TREN 4.4 European Commission DG Research 4.5 Photovoltaic Energy Barometer, Système Solaires n°149, june 2002 4.6 Solar Generation - EPIA- Greenpeace, December 2001 4.7 Trends in Photovoltaic applications in selected IEA countries between 1992 and 2002, IEA-PVPS, Paris, August 2003 4.8 Status of PV Research, Solar Cell Production and Market Implementation in Japan, USA and the European Union, September 2002, European Commission, DG JRC, Arnulf Jaeger-Waldau
5.1 History 5.1.1 Origins 220.127.116.11 Flour mills The first wind powered machine was for grinding (milling) flour, apparently in 7th century Persia, operating on a vertical axis. Only after five more centuries did the familiar horizontal axis windmill appear in England, its realisation probably aided by water mill technology. Around the Mediterranean, simpler mills were adopted with triangular canvas sails attached to radial arms (Cretan design). The post mill and later the tower mill gradually developed in Northern Europe, until by the 18th century the latter had become the most sophisticated automatic machine of its day. 18.104.22.168 Water pumps and sawmills
Source ref 5.1
Figure 5.1 Tower Mill
Post mills were developed to drive waterwheel type pumps in the Netherlands for draining the polders from the 16th century, and these systems were also adapted to drive reciprocating saws to cut timber. The Cretan windmill has been used mainly for irrigation, driving a simple crank operated pump. But a new wind power technology emerged in 19th century USA, where water pumps to supply farms, and later the thirsty locomotives of the railroads, could be amply supplied by the new mass production industries. These were multibladed machines on wooden or steel towers, each capable of a substantial torque to drive a large piston pump. 5.1.2 Key Events Year Earliest possible mention of a Persian windmill 644 First record of a windmill, in Sistan, Eastern Persia (vertical axis) 950 Horizontal axis windmill developed in England 1185 Chinese reference to windmills in Samarkand 1219 English post mill illustrated in a manuscript 1270 Tower mills recorded in Europe 1390 Invention of fantail (automatic yaw turbine) by Edmund Lee 1745 First scientific paper on wind rotor performance (John Smeaton) 1759 American wind pump (Halladay Standard) 1854 Electricity generated by wind (Brush, USA) 1888 Wind tunnel for wind rotor design (LaCour, Denmark) 1895 Propeller type rotor (2.5 kW, Jacobs, USA) 1925 Darrieus vertical axis rotor invented (France) 1925 Maximum energy conversion formulated (Betz limit, Germany) 1926 100 kW AC wind generator (Russia) 1931 1.25 MW generator (Smith-Putnam, USA) 1941 Wind farms in California 1980 First UK wind farm 1991 5.1.3 Electricity generation Windmills and windpumps continued to be used until displaced by electric motors during the 20th century. But by that time the wind was already being used to generate electricity. Commercial success was initially confined to local battery chargers for remote locations not
served by an electricity grid, typically with 2 or 3 bladed propeller type rotors derived from aircraft engine technology. Several projects in Russia, USA and Britain demonstrated the feasibility of increasingly large individual aero-generators, but could not compete with the decreasing costs of electricity from fossil fuels. Only after oil supplies were first threatened by the Arab-Israeli war of 1973 did the USA adopt tax credit legislation to stimulate alternatives, particularly wind turbine construction. This was mainly taken up in California, where for the first time a large number of small wind turbines were installed on a common windy site to form a wind farm. A few such wind farms effectively became development sites for manufacturers from both USA and Europe (notably Denmark which had been at the forefront of wind turbine development). Although it was initially expected that economies of scale would favour very large machines (e.g. 4 MW), in practice, as with the windpump, series production of smaller machines has been found to be more effective commercially. The new vertical axis rotor (earlier invented by Darrieus) has been well represented but has not proved as competitive as first supposed. By 1985, California had installed over 1200 MW of wind powered generators, considerably more than the whole of the rest of the world. More recently, however, incentives have waned in USA but been taken up in Europe to much greater effect. 5.2 Technical outline
5.2.1 Wind Turbines 188.8.131.52 Lift and drag Wind turbine rotors come in a wide variety of types. Each one is designed to convert the aerodynamic effects of the wind into a torque turning a shaft. The Persian windmill made the
Stonework shield
Persian Windmill
Cup Anemometer
Figure 5.2 Some Drag Type Rotors (vertical axis, plan view)
wind push a vane in the direction of the wind, using aerodynamic drag (fig 5.2), whereas propeller type machines use aerofoil shaped blades which generate lift forces rather like the wing of an aircraft. Drag may be easy to use, but is inefficient because most of the wind energy is lost in eddies from the edge of the blade or cup, whereas the lift effect from an aerofoil is capable of slowing the air down with much smaller loss effects and results in a more efficient turbine. Drag is very effectively used in the rotor of the cup anemometer for measuring wind speed, but almost all modern wind turbines use lift effect rotors to maximise their power output. For instance the rotor may be made like an aircraft propeller, with rotating blades shaped like aircraft wings attached to a central hub, but other designs such as the vertical axis Darrieus rotor (see 188.8.131.52 below) also depend on aerodynamic lift.
22.214.171.124 Horizontal axis rotors
Propeller types
Fig 5.3 Some lift type horizontal axis rotors
These rotors (fig 5.3) work the opposite way to aircraft propellers, by using the thrust of the wind to generate power; but the air velocities are normally much smaller, so the rotors are much larger for a given power. The blade speeds are also correspondingly low, but are usually several times faster than the wind, in order to maximise the lift forces and minimise the blade area required. The exception is the multibladed rotor used for wind pumps, for which high torque is more important than high power, and so a large area of blades rotating at a lower speed is suitable. In either case the blades are set at a certain 'pitch' angle (i.e. each blade is rotated about its own axis away from the plane of rotation), so that the aerodynamic forces are optimised. This angle may be fixed, in which case the rotor is 'stall regulated' since the effect of a high wind is to stall the lift effect; or it may be controlled by a mechanism like an aircraft propeller in order to benefit from every change of wind ('pitch regulated'). Typical diameters and capacities for propeller type rotors range from 50 cm (50W) all the way up to 100 m (3MW). Possible disadvantages are that they need to be turned to face the wind, and that their towers normally need to be heavy Scottish Power enough to support all necessary electrical or other machinery at the Figure 5.4 Propeller level of the rotor. type wind turbines 184.108.40.206 Vertical axis rotors These designs (fig 5.5) may be less familiar, and rather surprising. They also make use of aerodynamic 'lift' effects, but rely on the relatively small component acting forwards along the chord of the aerofoil (the wing shape is effectively 'sucked' forwards as long as the relative wind is slightly inclined to it, rather like a boat sailing very close to the wind). Their conceptual advantages are that they will accept wind from any direction and the blades are designed to withstand high speeds of rotation. In the case of the Darrieus design, the blade takes a 'Troposkien' shape rather like a skipping rope, such that the only forces are tensile along the blade. The variable geometry design uses straight blades which automatically furl themselves at high speed (as on the right hand side of fig 5.5). These rotors may be heavier than propeller types, but do not require elevated machinery and may not even need a support tower at all (Darrieus rotors only need cables to support the upper bearing).
Darrieus (Troposkien shape)
Variable geometry
Figure 5.5 Some lift type vertical axis rotors
5.2.2 Wind strength 126.96.36.199 Source and Variability Winds occur as a direct result of differential solar heating of the atmosphere. These differences depend on latitude, land and sea, time of year, time of day, etc, and when combined with the rotation of the earth and local geographical features produce an extremely complex distribution of wind strength and direction over both time and space. The major wind structures (e.g. 'trade winds') are the effect of hot air rising from the equatorial regions and cold air descending onto the polar icecaps. This creates three cells of large scale vertical circulation in each hemisphere (fig 5.6), which are then skewed horizontally by the earth's rotation (for instance, surface air travelling South to the equator is moving into a latitude where the surface is rotating eastwards faster, and will therefore blow from the North East).
Figure 5.6 Global wind systems (adapted from ref 5.1)
Regional vortex instabilities occur as a result of the upward and downward air flows at the cell edges combined with interaction between the Easterlies and Westerlies, producing both cyclones and anticyclones, which then traverse across the surface and cause our complicated weather effects. In particular, a cyclone is an inward flowing vortex (similar to that observed when emptying a bath) and produces high tangential winds which change direction as it passes. Further interaction with surface irregularities (mountains and valleys, forests, large buildings etc) result in smaller scale gusts and turbulence. It will be apparent that prediction of wind velocity and direction is difficult. Nevertheless it has been found that the statistical variations of wind speed at different sites are remarkably similar, and can usually be well described in terms of a universal distribution function (the Weibull distribution) and three experimentally determined quantities, the most important of which is the average wind speed.
126.96.36.199 Average wind speeds Wind speed is normally specified as the hourly wind speed, which is the average speed measured during one hour, during which time a large range of speed oscillations may occur. Although the hourly wind speed itself varies very considerably, this period is chosen as that giving the most consistent measure available, in the sense that there is always a good chance that two consecutive hours will yield similar values. These hourly wind speeds can then be measured at a given site over at least a year, from which data the variation can be analysed and an annual average wind speed obtained. A typical variation is shown in figure 5.7, which
30 Shut down 25 m/s
Wind Speed V m/s
13 m/s rated wind for turbine Average wind 8.4 m/s
10 5 m/s turbine cut-in wind speed
0 0 1000 2000 3000 4000 5000 6000 7000 8000 9000
Hours per year during which V exceeded
Fig 5.7 Typical Speed Duration curve
gives the number of hours in a year for which any given wind speed is exceeded. This also shows that the wind blows harder than the average for slightly less than half the 8760 hours in a year (the median wind speed is slightly lower). 220.127.116.11 Choice of site Hills and mountain passes offer higher wind speeds than normal, and were initially proposed as major sites for large turbines or wind farms. However the limited number of accessible sites of this kind and local planning objections have resulted in relatively few such developments, notable examples being a few large wind farms in windy passes in California. Assessment of the wind regime at less dramatic sites is therefore all the more important, as well as careful investigation and information of local public opinion. On a smaller scale, whereas urban sites (for either community or household use) are generally difficult to develop for reasons of planning and safety, there is a large number of isolated Source: ref 5.3 rural dwellings or communities Figure 5.8 Annual average winds where wind power is competitive (EU 1989, for sheltered terrain)
by reason of the cost of cable connection to the nearest electricity grid substation. In all cases, a good enough estimate of the annual wind speed can usually be made from national data bases or isopleth maps (e.g. fig 5.8), combined with suitable factors to account for local topography such as hills, cliffs, forests etc. A particularly favourable position is near the top of a long slope leading upwards away from the direction of the prevailing wind. Where a large investment is proposed, it may be necessary to measure wind speeeds at the site over a substantial period before the final turbine specifications are fixed. 5.2.3 Performance 22.214.171.124 Energy in the wind If the rotor blades sweep an area A, the wind passing through that area with velocity V has kinetic energy ½V2 per kg air, and the mass flow V A rate of air (density ρ kg/m3) is ρAV kg/s. The product of these is the rate of energy passing through the rotor area, ½ρAV3. This is called the 'energy in the wind', and increases with the cube of the wind velocity (e.g. see fig 5.9). It is not unfortunately possible to convert all this kinetic energy into rotor power for the following two main reasons: i) if the kinetic energy of the wind was reduced to zero, the wind would cease to blow and there would be no flow of air through the rotor! ii) any wind turbine generator must be designed to deliver its 'rated' power at a certain 'rated' wind speed, and is unable to benefit from the higher energies of stronger winds.
Figure 5.9 Typical Wind Energy Duration Curve
Energy in the wind E kW/m2
10 8 6 4 2 0 0
Shut Down conditions
13 m/s rated wind for turbine Average wind 8.4 m/s Accessible Energy Inaccessible 1000 2000 3000 4000 5000 6000
Turbine cut-in 5 m/s wind 7000 8000 9000
Hours per year during which E exceeded
220.127.116.11 Power production There is therefore some optimum resistance required of the rotor design to maximise the product of kinetic energy change of the air and its mass flow rate. It was shown by Albert Betz (1926) that this occurs ideally when the wind velocity well downstream of the rotor is one third of the undisturbed wind speed, so that the stream tube steadily expands in size as it passes through the rotor. The maximum power obtainable is then 16/27 of the energy in the wind, or about 60%. Real turbines are unlikely to exceed 50% because of aerodynamic and frictional losses, and usually convert less than 35% into electricity. This limitation is most serious in low winds, and may be used to explain why wind pumps are designed with a larger blade area than electric generators. The usual requirement for a wind
pump is that it should pump water as frequently as possible and provide a regular flow, rather than supplying a high power; this means that it must catch the smallest breath of wind even if it rotates quite slowly, but be turned away from the wind for safety when it becomes strong, and a multibladed rotor is appropriate. Conversely, an electricity generator usually needs to maximise the total electric energy produced in a year, but at any given wind speed the power available is limited by the Betz limit and not the area of blades; hence a quickly rotating propeller with 2 or 3 slim blades gives the most efficient and economical design. 5.2.4 Electricity generation 18.104.22.168 Generator rating If the wind turbine is to produce electricity, it will drive a generator via a speed increasing gearbox, all contained within a nacelle (fig 5.10). It may appear from 5.2.3 above that the highest possible wind speed should be chosen for the rated power of a generator. But this in turn requires large heavy machinery for high maximum power, which will be very inefficient at lower wind speeds. For the maximum electrical output per year it has been shown that the rated wind Figure 5.10 Typical nacelle equipment speed should only be just over twice the average annual wind speed for the site. The financial return required on an investment may however restrict the size of machine even more, and many wind farms use turbines for which the rated wind speed is only about 1.6 times the average wind. The rated power is still 4 times that at the average wind speed (because of the cubic relationship), and the turbine is idle for over a quarter of the hours in the year when the wind speed is below the 'cut-in' value and no net power can be produced (see fig 5.8). 22.214.171.124 Typical performance Because the energy in the wind increases with the cube of the wind speed, the power output of a wind turbine must also follow this trend, at least for the lower wind speeds above the cut-in value. As the rated wind speed is approached however, the way in which the power is contolled becomes important. A pitch regulated rotor may be able to deliver steadily increasing power right up to the rated value; but if the wind speed then increases further,
Source: Vestas
Fig 5.11 Electric power output for a large machine
the blade must be rotated in the hub to reduce the aerodynamic effect and prevent any further power increase. The resulting discontinuity in the power characteristic may be undesirable because a gusty wind will result in repeated oscillation of the blade angle and ineffective control. The alternative method, stall regulation, produces a smoother and more gradual transition from the steep rise onto the rated power plateau without any need to rotate the blade (for example see fig 5.11), although there may be some small performance penalty at lower wind speeds. The wind generator for the example shown actually uses pitch regulation at the higher wind speeds only, since otherwise the effect of stall regulation could be to reduce power output under these conditions. The red line also shows the effect of adding a smaller low speed generator in order to maximise the output at low wind speeds, and the consequent additional losses at higher winds. 184.108.40.206 Power utilisation and storage The maximum electrical output from a wind turbine can only be realised if its power can be applied whenever the wind chooses to blow. All energy sources have some problem of this kind, and one advantage of wind energy (see fig 5.12) is that the wind blows harder during the winter when there is a greater demand for electricity. But because the wind cannot be switched on when required, nor predicted much in advance, wind turbines almost Source Econnect Ltd, UK always need to be connected with Fig 5.12 Typical Monthly Demand and Wind output energy storage and/or other power plants. Such a combined system has basically only three control options: 1) Use wind power fully at all times and reduce output from other plants as necessary. 2) Invest in energy storage systems to absorb wind power at times of low demand. 3) Switch wind power off when demand is low or storage is full. Electricity itself cannot usually be stored on a large enough scale. The only developed forms of energy storage which can be easily exchanged with electric power are hydraulic and electrochemical. Water can be pumped uphill to a reservoir, and Hydroelectric pumped storage is a well established scheme for meeting peak demand on a large scale, while a range of batteries and fuel cell derived systems can be applied in smaller units. If in the future hydrogen becomes established as a major means of energy transport, it may supersede all other forms of storage. 126.96.36.199 Stand alone units
18.104.22.168.1 Batteries (option 2)
Isolated single dwellings or small communities may be able to store enough energy in conventional batteries and rely on a wind generator, but this will depend critically on the wind regime at the site and acceptance of a significant risk of power failure (e.g. during an extended anticyclone). These risks will be reduced if some photovoltaic modules can be included in the system.
Wind and Diesel (options 1,3)
For more dependable power at a moderate scale, a diesel engine (which can be rapidly started and stopped automatically) is often paired with a wind turbine, which then operates as a fuel saving device. The economics of this will depend on the cost of supplying diesel oil (or biodiesel), which may be substantial at a very remote site. Unless some battery capacity is also installed, a sophisticated control system may be needed to prevent power failures due to sudden wind changes, when even a diesel engine may not start up in time. 18.104.22.168 Grid connected units For the immediate future it is safe to assume that the proportion of wind power in a national generating system will be small enough to allow option 1 to be chosen. It is not so clear how the energy mix will develop later in the century.
22.214.171.124.1 Control and prediction
Satisfactory AC frequency and voltage control requires that there is sufficient standby generating capacity in the overall power system. This standby plant can then be switched in whenever there is either a sudden demand surge or a sudden plant failure. At any one time therefore, a large grid system must have a substantial 'spinning reserve' of generators ready to deliver power, and others ready to be spun up to operating speed. But in Europe, demand often peaks on cold windy winter days - just when wind turbines can produce plenty of power. For these reasons, the proportion of wind power in a large system can rise to about 20% before any special control measures are needed. Beyond this amount, better prediction of power demand and supply may become necessary. The supply of wind power depends on the weather systems which traverse the nation or continent, some prediction of which may be possible within a 24 hour period. The economic viability of a system with a large proportion of wind power may depend on the accuracy of such predictions, because otherwise regular power stations must provide more spinning reserve. However for a country of reasonable size with wind farms distributed across its length and breadth, the wind is highly unlikely to drop simultaneously everywhere, and a gradual reduction can be easily accommodated.
18.104.22.168.2 Grid storage
Even hydro pumping systems are increasingly being used as general energy storage for financial return rather than specifically to meet peak demand. Specialised fuel cell systems are being developed to allow rapid conversion of electric power into chemical fluid energy (other than hydrogen) and vice versa. Increased grid storage capacities may therefore become economic in the future, in turn allowing greater wind power penetration. 5.3 Current developments
5.3.1 Wind Farms A group of 10-50 wind turbines at a single site is known as a wind farm. They must however be spaced apart because the low speed wake from each turbine is likely to interact with another machine downstream. The shearing action of the surrounding wind will however eventually accelerate the air again, and a typical spacing of about 10 rotor diameters is needed. The total size of the farm is also limited by the larger scale effect of slowing down the wind boundary layer, even if the local geography would allow it to be
Source: National Wind Power
Figure 5.13 Novar wind farm
larger, so that a typical maximum power rating for a complete wind farm on land may be only 30 MW. Although California led the way, the most significant recent wind farm installations are in Europe (fig 5.14). Virtually all of these are on land and use horizontal axis propeller type machines, with Danish manufacturers in the forefront. Turbine sizes have been steadily increasing, and range from 250 kW up to 1MW. It is particularly notable that the leaders are Germany and Spain, which are not the countries with the greatest wind energy resource in terms of annual average wind speeds, but have large areas of land available. 5.3.2 Offshore Northern Europe is now seriously installing wind farms on the offshore continental shelf (fig 5.15), where good wind regimes and less restricted space is available. New engineering techniques have been developed for subsea foundation systems for the towers, and the extra costs can be partly offset by continuing the trend to larger machines into the range 2-3 MW, with corresponding increases in total power up to 60 MW per farm (enough to provide about 50,000 homes). Although fewer planning objections on visual or acoustic grounds are expected than on land, other hazards such as interference to radar and military aircraft must be minimised.
Figure 5.14 World Wind Power Capacity 2003 (34GW total)
Spain 15%
Germany 38%
Denmark 9% Netherlands 2% Italy 2% Sweden 1% Greece 1% Other Europe 4% India 6% USA 16%
UK 2% Other 3% China 1%
Source: National Wind Power
Figure 5.15 North Hoyle wind farm
Fig 5.16 shows that the substantial rate of wind farm installation already reached in Europe (over 6GW per year) is now expected to continue, as offshore projects begin to take over from
GW/yr Actual GW/yr Projected Total GW Actual Total GW Projected 2010 target
Fig 5.16 EU Wind Power Capacity (EWEA data)
the impressive recent land based expansion. It is also interesting to see that the target for the year 2010 set by the European Wind Energy Association (EWEA) has been increased twice, and is now 3 times that set in 1991. The current target of 75 GW in 2010 will meet one third of the EU's total Kyoto commitment and deliver half of the Renewables Directive target. 5.4 Environmental implications
5.4.1 Emissions 188.8.131.52 Energy payback The amount of energy required to construct a complete wind turbine is recovered in the electricity generated typically after about 3 months of full power operation. Wind power is one of the most competitive energy sources in this respect. 220.127.116.11 Zero greenhouse Normal operation of a wind turbine generates no CO2 or other 'greenhouse gas'. Such emissions can only normally arise from the energy input required for its original construction, with a little for installation, commissioning and maintenance. 5.4.2 Visual Public attitudes to the appearance of large wind turbines vary very widely. Planners should certainly exercise some sensitivity to their placement in the landscape, for instance to avoid their domination of the countryside in such places as national parks. Many people consider wind turbines to have a beauty of their own and to represent hope for the future of the planet, while others regard them as an intrusion of industrial machinery onto the natural skyline. Some public opinion surveys have shown that those living near a wind farm are more likely to support their development than those living further away (ref 5.4). A 1994 survey in Wales showed that an initial 41% approval of local wind farms before construction was converted into 66% afterwards (ref 5.5). Although such surveys also reveal a much wider overall range of opinion (e.g. ref 5.6), both of these suggest that the anticipation may be worse than the reality! Offshore sites are certainly expected to be less controversial in visual terms since towers will generally be several km from the shore. 5.4.3 Noise Early wind farms were said to be noisy by a few local inhabitants, and this received widespread publicity in the UK. As far as overall noise level is concerned, it has been demonstrated that wind turbines are no louder than the wind noise normally generated by trees or buildings. It is perfectly possible to stand underneath a turbine and have a normal conversation without raising your voice. Some of the noise objections referred to a persistent tone, which may have come from a gearbox; gearboxes have certainly been improved, and the nacelle is now acoustically insulated to minimise any such irritating constant frequency. Rotors in Europe are normally positioned upwind of the tower, which minimises any cyclic pulse effect, and the blades are also designed to minimise any aerodynamic swishing noise. It seems to be generally agreed that noise is no longer much of an issue. 5.5 Costs and economics
5.5.1 Construction The cost of constructing a wind turbine generator is very important because this almost entirely determines the cost of the electricity generated. As may be expected, the cost per rated kW output decreases with increasing size. A typical breakdown for a wind farm installation is shown below:
Manufacture of complete wind turbine Site works Other capital outlay
63% 29% 8%
Manufacturing cost has been coming down steadily over the last 15 years as a result of improved technology, volume production and larger unit size. The turbine cost per rated kW depends on both the turbine size and the wind speed at the site, but is typically in the range 500-700 €/kW. A total wind farm installation cost may be as low as 800 €/kW. 5.5.2 Capacity credit The economic viability of wind power depends particularly upon whether the investment can be offset by a reduced requirement for conventional power station capacity in the utility system. Such a system obviously cannot meet its power demand reliably with wind power alone, because the wind does not always blow. But a large system comprising a range of generators is not the same as a remote wind/diesel unit in which the wind turbine merely saves the cost of diesel fuel, since demand and supply are continuously fluctuating (see also Section 184.108.40.206.1 above about the separate problem of control and wind prediction). The question is then how to determine the proportion of wind power which can contribute to the useful capacity of the system without loss of reliability, or its capacity credit. Because of wind speed fluctuation, the annual electrical energy output of a wind farm is typically about 30% of the maximum possible if its rated power could be delivered continuously. This ratio is known as the load factor or capacity factor of the wind farm, and depends mainly on the the site conditions but also on the type of turbine. All types of generator (fossil fuel, nuclear etc) have load factors, which although they may be higher than 30% are always less than 100%, and depend on the possibility of failure as well as the need for maintenance. The requirement of the overall power system is that a complete loss of power should be very unlikely, or have a low enough annual probability such as 1%. It can be shown that for low levels of penetration of wind power into the grid, the capacity credit of wind energy is simply the installed capacity multiplied by the load factor. However as the level of wind penetration rises (e.g. to 15% of maximum annual demand), the capacity credit falls off and might be halved in this case. Denmark has exceeded this amount of wind power without apparent difficulty, but after that the country with the highest wind power penetration to date is Germany with less than 5%, so Europe still has a long way to go before this becomes a problem. 5.5.3 Unit cost The price of wind generated electricity depends on the plant installation cost and the cost of finance, both spread over the energy generation expected during the lifetime of the plant, with a quite small addition for operating cost (e.g. 15 €/yr per rated kW). The price per kWh therefore depends on the average wind speed at the site, the load factor and the cost of finance, of which the third tends to be more unpredictable than the first two. At a good site, a price to the client or distributing company of 3.5 c€/kWh is possible, which is highly competitive with the true costs of conventional power (i.e. if all decommissioning costs are realistically included and historic subsidies discounted). It is largely for this reason that Europe is investing in Wind power so successfully.
5.6.1 Bibliography 5.1 Le Gourieres, D: Wind Power Plants, Theory and Design. Pergamon Press, Oxford 1982 5.2 Spera, David A (Ed): Wind Turbine Technology. ASME Press, New York 1994 5.3 Troen, I. and Peterson,, E.L. (1989) European Wind Atlas, Riso, Denmark for Commission of the European Communities. 5.4 Wind Farms make good neighbours (BWEA press release, Aug 2003)
5.5 Bishop, B. and Proctor, A. (1994). Research Report. Love Them or Loathe Them? Public Attitude Towards Wind Farms in Wales. BBC Wales, Environmental and Countryside Planning Unit. http://users.aber.ac.uk/eaf1/fifth%20page.htm 5.6 A Summary of Opinion Surveys on Wind Power. magazine pp 16-33 Sept/Oct 2003 (EWEA) Wind Directions bimonthly
5.6.2 Web links BWEA (The British Wind Energy Association) http://www.bwea.org/
EWEA (European Wind Energy Association) The American Wind Energy Association Danish Wind Industry
http://www.ewea.org/ http://www.awea.org/ http://www.windpower.org/
6 Small Hydropower Systems
6.1 Introduction 6.1.1 Background The unending natural process resulting from the action of the sun's radiant energy on the earth's atmosphere, called the hydrologic cycle (figure 6.1), provides a continuous and renewable supply of water onto the earth's surface. A portion of this water returns to the sea during the cycle in the form of surface water runoff. Its potential energy is normally dissipated as the water flows downhill, but can be harnessed as a viable energy source to generate mechanical power or electricity.
Figure 6.1 The hydrologic cycle
This section deals specifically with Small Scale Hydropower systems, since large-scale hydropower plants are usually not considered as RES exploitation systems by ecologists and are covered on other INTUSER pages (LINK). Large dams have acquired a reputation for damage to ecosystems. They hood and silt in natural stream areas and deplete oxygen from the water. Their reservoirs are dead-water or slack-water pools whose water is hostile to native fish species. Downstream, they create alternating periods of no water followed by powerful surges that erode soil and vegetation. Small Hydropower Plants (SHP) are mainly "run of river", i.e. not involving significant impounding of water and therefore not requiring the construction of large dams and reservoirs, though where these exist and can be utilised easily they do help. There is no general international consensus on the definition of SHP; the upper limit varies between 2.5 and 25 MW in different countries, but a value of 10MW is becoming generally accepted and has been accepted by ESHA (the European Small Hydro Association). The definition for SHP as any hydro systems rated at 10MW or less will therefore be used here. SHP can be further subdivided into "mini hydro" (usually <500kW) and "micro hydro" (<100kW). Whichever size definition is used, SHP is one of the most environmentally benign forms of energy generation, based on the use of a non-polluting renewable resource, and requiring little interference with the surrounding environment.
6.1.2 Short history 220.127.116.11 The origins of hydropower Water power has contributed to the development of mankind since Biblical times. References to the use of waterwheels for milling, pumping, and other functions date back to 300 BC in Greece, although they were probably in use long before that time. In the years between these early uses of the waterwheel and the onset of the Industrial Revolution, running water and wind were the only available sources of mechanical power, other than that generated by animals. Improvements in power recovery from flowing water were steadily introduced as exemplified by the sophisticated waterworks designed in the 1600s for the palace of Versailles outside Paris, France. This system had a capacity equivalent to an estimated 56 kW of power. 184.108.40.206 Early developments in the field of hydro Water power systems, and eventually hydroelectric generating plants, were developed from attempts at improving the efficiency of the waterwheel. Much of the early research took place in France because at the beginning of the Industrial Revolution, France, unlike many nations did not have access to large coal deposits. The French relied heavily on their water resources to generate the energy needed for industrial expansion. In fact, waterpower is still called houille blanche, or white coal, in France. Much theoretical work was done during this period by mathematicians and engineers such as Bernard Forest de Belidor, John Smeaton, Jean Victor Ponceler, Leonhard Euler, Claude Burdin, and Benoit Fourneyron. Their work resulted in significant improvements in turbine efficiency and laid the ground work for the development of modern turbines of the Francis, Kaplan, and Pelton type. As an example of the progress achieved, it is worth mentioning that the original vertical-mid turbine designed by Belidor attained an efficiency of 15 to 20%. By the mid 1850s this rose to 60 to 70%. The Francis and Kaplan turbines, which are the modern counterparts of the Belidor turbine, now achieve efficiencies of 90 to 95%. During the 1870s electricity was introduced as a popular source of energy to the public thanks to Thomas Edison and the dynamo. Not too long after that the dynamo was coupled with the waterwheel and the technology of hydroelectricity was born. The first hydroelectric unit in the United States is reported to have been a 12.5 kW plant installed in 1882 on the Fox River at Appleton, Wisconsin. Small-scale hydropower was the most common way of generating electricity in the early 20th century. In 1924 in Switzerland, for example, nearly 7000 small scale hydropower stations were in use. The improvement of electricity distribution by means of high voltage transmission lines much reduced interest in small scale hydropower. Renewed interest in the technology of small scale hydropower started in China. Estimates say that between 1970 and 1985 nearly 76,000 small scale hydro stations have been built there! 6.1.3 Geographical assessment Hydropower is the largest and most mature application of renewable energy, with some 678,000MW of installed capacity worldwide, producing over 22% of the world's electricity (2564TWh/yr) in 1998. Of this, 27,900MW is at small-scale sites, generating 115TWh/year. In Western Europe, hydropower contributed 520TWh of electricity in 1998, or about 19% of EU electricity (avoiding thereby the emission of some 70 million tonnes of CO2 annually). Despite the large existing hydropower capacity, there is still much room for further development as most assessments assume this is only around 10% of the total world viable hydro potential.
Technical outline
6.2.1 From water to electricity Hydropower systems convert the energy of flowing water into mechanical power, and then usually into electricity. The water flows via a channel or penstock to a waterwheel or turbine where it strikes the bucket of the wheel, causing the shaft of the waterwheel or turbine to rotate. When generating electricity, the rotating shaft is connected to an alternator or generator. The electrical energy produced may be used directly, stored in batteries, or inverted to produce utility-quality electricity. The amount of power a specific hydropower site can produce depends on its head, i.e. the height H (m) through which the water falls, and the flow rate. The head determines the useful potential energy of a site (PE = m·g·H). The flow of the river is the volume of water (m3) which passes a cross section of the river per second (Q m3/s). The gross theoretical power (P kW) available is then given by: P = 9.81 × Q × H This available power will be converted by the hydro turbine into mechanical power. As the turbine and the rest of the electro-mechanical equipment have efficiencies lower than 100% (usually 90 to 95%), the electric power generated will be less than the available gross power. 6.2.2 Site selection and basic layout As adequate head and flow are necessary requirements for hydro power generation, site selection is conditioned by the existence of both requirements. Before a site is selected, a preliminary assessment study should include definition of power potential, estimation of power output, identification of physical works needed, identification of critical issues (environmental and social constraints), and a preliminary study of economic feasibility.
Source: Ref 7.7
Figure 6.2 High head small hydropower scheme
SHP schemes can be high-head or low-head depending on the geographical characteristics of the available site. In general high-head sites are less expensive to develop than low-head sites, because for the same power output the flow through the turbine and required hydraulic structures will be smaller. In a river with a steep gradient over part of its course, the level difference can be utilised by diverting all or part of the flow and returning it to the river once it has passed through the turbine. The water can be brought from the intake directly to the turbine through a pressure pipe.
A cheaper alternative is the one shown in figure 6.2. The scheme comprises a dam or a weir, a river intake, and an almost-level open canal running along the side of a river valley ending in a forebay (local open basin), from where a pressure pipe conducts the water down to the turbine in the powerhouse. If the topographical or geotechnical characteristics of the ground are unfavourable the open canal may not be the best solution. A low pressure pipe, the "penstock", may provide a more economic solution in such circumstances. In low-head schemes two configurations are possible. One uses a diversion weir with a layout very similar to the above, although the canal is usually short and the penstock short or nonexistent (figure 6.3). The other involves a dam with an integral intake and powerhouse.
Source Ref 6.1
Figure 6.3 Low head small hydropower configuration
Another possibility is to install a power plant on an existing dam built for multiple purposes (flow control, irrigation, etc.). Water enters the turbine through a penstock constructed as an integral part of the dam structure or, if the dam is not too high, through a syphon intake (figure 6.4).
Source Ref 6.1
Figure 6.4 Power plant scheme on an existing conventional dam
In the syphon solution the penstock runs over the dam before sloping down to the turbine, which can be located either on top of the dam or on the downstream side. Although the head in most syphon installations varies from 1.8 to 11 m, there are a few examples with heads as large as 30 m.
6.2.3 Civil engineering works Once the site has been selected and the basic layout decided, it is necessary to develop the scheme in detail. The following section describes the function of all the possible basic components and shows how they may be designed. Figure 6.2 shows the esential features of the civil works. 188.8.131.52 Weirs and intakes Due to the high costs involved in their construction, dams are rarely used in small-scale systems. However, provided that topographical conditions are favourable, the construction of a small gravity dam to store water during periods of low demand can be justified. In diversion schemes, a weir made of mass concrete or stone masonry, with a crest one meter or more above the river bed, will be enough to create an adequate depth of water at the intake of the canal or pipeline. The function of the intake is to conduct water into the penstock or power canal under controlled conditions. The intake serves as a transition between a stream, which can vary from a trickle to a raging torrent, and a flow of water controlled both in quality and quantity. Its design, based on geological, hydraulic, structural and economic considerations, requires special care to avoid unnecessary maintenance and operational problems that cannot be easily remedied and would have to be tolerated for the life of the project. The major criterion in the design of an intake is its orientation with respect to the stream, as a means of controlling the quantity and quality of water entering. Rivers tend to deposit sediments in the inner sides of bends. Thus, the intake should not be located on the inside of the bend, to avoid blockage of the entrance by sediment, nor to the outside, because water borne debris can impair its functioning. The ideal diversion site is a relatively straight section, stabilized by rock outcropping, in which the weir could be embedded. Trash-racks should also be placed at the intake entrance to prevent the ingress of floating debris and large stones. 220.127.116.11 Canals From the intake the water is conveyed either directly to the turbine through a pressure pipe or by a canal. In a canal the flow is a function of its cross-sectional profile, its slope, and its roughness. The application of hydraulic theory yields reasonably accurate results to manmade canals where the cross-section is regular in shape and the surface roughness of the construction materials - earth, concrete, steel or wood - is well-documented. The velocity of water in a canal should be kept above a minimum value to prevent sedimentation and aquatic plant growth, but below a maximum value to prevent erosion, especially in unlined canals. At the end of the canal, just before the entrance to the penstock, there is the forebay. Although it can be designed to provide water storage, a forebay normally provides only enough storage to provide extra volume needed during turbine start-up. It should include a spillway, a purging outlet, a trash-rack and an air vent. 184.108.40.206 Penstocks From the forebay the water is conveyed to the turbine via a pressure pipe or penstock. Penstocks can be installed over or under the ground, depending on factors such as the nature of the ground itself, the penstock material, ambient temperatures and environmental requirements. A flexible and small diameter plastic penstock for instance, can be laid on the ground, following its outline with a minimum of grade preparation. Otherwise larger penstocks should be buried, provided there is not too much rock excavation.
The sand and gravel surrounding the pipe provide good insulation, and eliminate anchor blocks and expansion joints. Buried penstocks must be carefully painted and wrapped to protect the exterior from corrosion, but further maintenance is minimal. From the environmental point of view, this solution is optimal because the ground can be returned to its original condition and the penstock does not constitute a barrier to the movement of wildlife. The materials, diameter, wall thickness and type of joint characterize a penstock: • the material is selected according to the ground conditions, accessibility, weight, and cost; mild steel, unplastified PVC and HDPE (high density polyethylene) are the materials most commonly used for penstocks in micro hydro schemes; the diameter is selected to give a moderate water velocity and hence reduce frictional losses within the penstock to an acceptable level; the wall thickness is selected to resist the maximum internal hydraulic pressure, including the transient surge pressure that will occur when the flow is rapidly increased or decreased; pipes are generally supplied in standard lengths and have to be joined together on site; many factors should be considered when choosing the best joint system for a particular scheme, such as suitability for the chosen pipe material, skill level of personnel installing the pipe, degree of flexibility required for the joint, relative costs, and ease of installation.
• •
220.127.116.11 Tailraces After passing through the turbine the water returns to the river through a short canal called the tailrace. Impulse turbines can have relatively high exit velocities, so the tailrace should be designed to ensure that the powerhouse will not be undermined. Protection with rock rip-rap or concrete aprons should be provided between the powerhouse and the stream. The design should also ensure that during relatively high flows the water in the tailrace does not rise so far that it interferes with the turbine runner. With a reaction turbine the level of the water in the tailrace influences the operation of the turbine and more specifically the onset of cavitation. This level also determines the available net head. 6.2.4 Electromechanical equipment 22.214.171.124 Hydraulic turbines A hydraulic turbine is a rotating machine that converts the potential energy of water to mechanical energy. There are two basic types of turbines, denoted as "impulse" and "reaction". The "impulse turbine" converts the potential energy of water into kinetic energy in a jet issuing from a nozzle and projected onto the runner buckets or vanes. The "reaction turbine" uses the pressure and velocity of the water to develop power within the runner, which is completely submerged, and both pressure and velocity decrease from inlet to outlet. Most existing turbines may be grouped in three categories: • Kaplan and propeller turbines. • Francis turbines. • Pelton and other impulse turbines. Kaplan and propeller turbines are axial-flow reaction turbines, generally used for low heads (under 16 m). The Kaplan turbine has adjustable runner blades and may have adjustable guide-vanes (figure 6.5). In this case it is called 'double-regulated'. If the guide-vanes are fixed it is 'single-regulated'. The Kaplan turbine has a scroll case (either in steel or reinforced
cast concrete); the flow enters radially inward and makes a right-angle turn before entering the runner in an axial direction. When the runner has fixed blades the turbine is known as a propeller turbine.
Source Ref 6.1
Figure 6.5 Kaplan turbine
Propeller turbines can have mobile or fixed guide-vanes. Unregulated propeller turbines are only used when both flow and head remain practically constant. Bulb and tubular units are derived from propeller and Kaplan turbines, where the flow enters and exits with minor changes in direction. In the bulb turbine a gearbox and generator are housed within a bulb submerged in the flow. Tubular turbines permit several arrangements, namely right-angle drive, S duct Straflo turbines, belt driven generators etc. Right-angle drives constitute a very attractive solution but are only manufactured up to a maximum of 2 MW.
Source Ref 6.1
Figure 6.6 Schematic view of a Francis turbine
Francis turbines are radial flow reaction turbines, with fixed runner blades and adjustable guide vanes, used for medium heads. The runner is composed of buckets formed of complex curves. A Francis turbine usually includes a cast iron or steel fabricated scroll casing to distribute the water around the entire perimeter of the runner, and several series of vanes to guide and regulate the flow of water into the runner. Figure 6.6 illustrates a schematic view of this type of turbine.
Pelton turbines are impulse turbines with single or multiple jets, each jet issuing through a nozzle with a needle valve to control the flow. They are used for medium and high heads. Figure 6.7 illustrates the scheme of a vertical Pelton. Certain manufacturers have developed special types of machines, with a limited range of discharge and output, which may be advantageous under certain circumstances.
Source: Energy-wise Renewables – 4, EECA, October 1997
Figure 6.7 Vertical Pelton turbine
Source: Energy-wise Renewables – 4, EECA, October 1997
Figure 6.8 (1) The crossflow (Michell) turbine, (2) Cross-section through the turbine, (3) Arrangement of crossflow turbine blades
The cross-flow turbine (figure 6.8), sometimes called the Ossberger turbine (after a company that has been making it for more than 50 years) or the Michell turbine, is used for a wide
range of heads overlapping those of Kaplan, Francis and Pelton. It is specifically suitable for a high-flow, low-head stream. The Turgo turbine, like the Pelton, is an impulse turbine but its buckets are shaped differently and the jet of water strikes the plane of its runner at an angle of 20°. Water enters the runner through one side of the runner disk and emerges from the other. The higher speed of the Turgo runner, due to its smaller diameter compared to other types, makes direct coupling of turbine and generator more likely. A Turgo may prove appropriate at medium heads where a Francis turbine might otherwise be used. However, unlike the Pelton, the water flowing through the runner produces an axial force, requiring the installation of a thrust bearing on its shaft. The selection of type, geometry and dimensions of the turbine depends primarily on the head, the discharge and the runner speed. Figure 6.9 presents the operating ranges of the different types of turbines as a function of the head and the discharge. For the same head, certain turbines are more difficult to manufacture than others and consequently they are more expensive. For instance, for low heads, a propeller turbine is cheaper than a Kaplan designed for the same rated discharge. In a medium head scheme, a cross flow turbine will be cheaper than a Francis, whose runner is more complex, although its efficiency is higher.
Source: ref 6.1
Figure 6.9 Operating ranges of different types of turbines
Regarding discharge, it must be mentioned that turbines cannot operate over the whole range from zero flow to rated discharge. As can be seen in figure 6.10, the efficiency decreases
rapidly below a certain percentage of the rated discharge. The best, in this respect, cannot be used below 1/6 and many can operate only from about 40% of rated discharge upwards.
Source: ref 6.1
Figure 6.10 Mean efficiency of different types of turbine
22.214.171.124 Gearboxes and other speed increasers When the turbine and the generator operate at the same speed and can be placed so that their shafts are in line, direct coupling is the right solution; virtually no power losses are incurred and maintenance is minimal. Turbine manufactures will recommend the type of coupling to be used, either rigid or flexible, although a flexible coupling that can tolerate certain misalignment is usually recommended. In the lowest power range, turbines usually run at less than 400 rpm, requiring a speed increaser to meet the 1000-1500 rpm of standard alternators. The speed increaser can be chosen from the types commercially available in the market: • • • • Parallel-shaft gearbox, Epicyclic gearbox, Right angle gearbox with bevel gears, Belt drives.
Gearboxes substantially increase the noise level in the powerhouse and require additional maintenance. Moreover the friction losses may amount to 2% of the output power. Flat or V shaped belts constitute the simplest and cheapest solution. 126.96.36.199 Generators Generators transform mechanical power into electricity. Although most early hydroelectric systems used DC generators to match early commercial electrical systems, nowadays only
three-phase AC generators are used in normal practice. Depending on the characteristics of the network supplied, the choice is between: • Synchronous alternators, equipped with a DC excitation system (rotating or static) associated with a voltage regulator to provide voltage, frequency and phase angle control before the generator is connected to the grid. Synchronous generators can run isolated from the grid and produce power since excitation power is not grid-dependent. Asynchronous generators, that are simple electric squirrel-cage induction motors, with no possibility of voltage regulation, which operate at a speed directly related to system frequency. They draw their excitation current from the grid, absorbing reactive energy, so they cannot generate when disconnected from the grid.
Synchronous alternators are more expensive than asynchronous generators, at least up to about 2 MW, and are used in power systems where the output of the generator represents a substantial proportion of the power system load. Asynchronous generators are used in large grids where their output is an insignificant proportion of the power system load. Their efficiency is 2 - 4% lower than that of synchronous generators over the entire operating range. 18.104.22.168 Control equipment SHP schemes are normally unattended and are operated through an automatic control system. Because every power plant is different, it is almost impossible to determine here the extent of automation that should be included in a given system. Automatic control systems can significantly reduce the cost of energy production by reducing maintenance and improving reliability, while running the turbines more efficiently and producing more kilowatts from the available water. A governor that usually controls a turbine is a combination of devices and mechanisms that detect speed deviation and convert it into a change in servomotor position. Governors can be mechanical or electronic. In the mechanical type, the speed sensor is a fly-ball mechanism that controls a hydraulic oil system to operate, through servomotors, the guide vanes or the runner blades. Electronic governors control the turbine through power amplification stages, which normally incorporate a hydraulic power unit. If the generator is connected to a large network, the network provides frequency regulation and a governor is unnecessary. On the other hand, in every country the electricity supply regulations place a statutory obligation on public electric utilities to maintain the safety and quality of electricity supply within defined limits. Switchgear is then required to control the generators and to interface them with the grid or with the isolated load. It must provide protection for the generators, main transformer and station service transformer. 6.2.5 Future trends Although small hydro technology is mature and well-established in the market, there is a case for further work to improve equipment designs, investigate different materials, improve control systems and optimise generation as part of integrated water management systems. Widening the range of head and flow, which can be accommodated at acceptable cost (particularly for small capacity and low head equipment), is a particular priority. The main technical targets as regards SHP systems are for standardisation in the areas of civil works, electromechanical equipment, control systems, grid connection arrangements, and authorisations and licences. There is a market need for cheaper equipment to exploit the extensive hydro resource available below 3m head. The following areas of technical development are foreseen:
Civil works • civil works to accommodate submersible technologies; • use of inflatable weirs to optimise generation head, without compromising flood defences; • use of stone and brick masonry (rather than concrete) to achieve environmentally attractive solutions and provide local employment; • siphon structures to improve aeration of the water and simplify turbine de-watering; • head enhancement techniques. Electromechanical equipment • new low head turbine designs e.g. helical turbines; • new construction materials e.g. plastics; • submersible generators, to reduce manufacturing costs; • small turbine packages (<5kW) for use in developing countries and in stand-alone applications; • compact multi-pole generators to avoid the need for speed increasers; • standardised, modular turbine designs for mass production; • simplification of low head designs, for manufacture in workshops in developing countries using simple machine tools and materials. Electrical equipment • application of remote control, condition monitoring and metering equipment to aid automation; • development of load control equipment and frequency converters to allow the use of unregulated turbines, particularly at low and variable head (and hence variable speed); • control systems to allow stand-alone operation of induction generators. Environmental • fish guidance systems to deflect fish from small hydro intakes and outfalls, without the energy losses associated with physical barriers; • ecological impact of run-of-river schemes. 6.3 Implications
6.3.1 Social and political With the expansion of centralised, fossil fuel generation and networked electricity distribution during this century, many sites were abandoned in some countries. However, environmental concerns have re-awakened interest in the technology in recent years, and many Governments both inside and outside the EU are offering incentives to increase small scale hydro deployment. The development of SHP has been handicapped by a general failure to receive similar support that has been given for R&D and for pilot/demonstration operation of other forms of renewable energy. This is possibly due to the following facts: • There is a widespread but mistaken perception that SHP technology is mature and fully developed and that market forces alone will be sufficient to take it forward, so it does not need any significant level of institutional encouragement or support. For this reason SHP is usually excluded from (or given a minor share in) programs designed to assist other forms of renewable energy development. In reality, there is probably more potential - at
least in the short term and on the global scale - for development and improvement of lower cost SHP than for any other form of clean energy development, yet it does need support. • Economic analysis of hydropower projects generally gives no significant credit for the exceptionally long useful life and low running costs of SHP, and the high 'upfront' costs tend to make it seem financially unattractive compared with conventional energy unless low discount rates are available. There has been a tendency to develop SHP in exactly the same way as large hydro, which leads to high design overheads and sometimes to faulty optimisation of systems so as to maximise energy capture rather than to optimise cost-effectiveness. There are many other institutional barriers, mainly resulting from the difficulties inherent in gaining permission in most countries to abstract water from rivers, and also due to perceptions that hydro plant might adversely effect fishing, boating and other riverine leisure interests (although in practice well designed hydro systems can avoid causing any serious environmental impact for fish or anything else). Last but not least, much of the responsibility for the development of SHP lies with small and medium sized enterprises lacking the lobbying capability and influence that other industries such as PV or wind farm developers have on governments.
6.3.2 Environmental SHP are in most cases 'run-of-river', which means that any dam is quite small, usually just a weir, and generally little or no water is stored. The civil works purely serve the function of regulating the level of the water at the intake to the hydro-plant. Therefore these installations do not have the same kind of adverse effect on the local environment as large hydro. However, some environmental problems can always be detected, notably where the water is extracted some distance upstream from where it is discharged back into the river. The short stretch of bypassed river can then run dry or look unsightly unless adequate compensation flow is allowed. In most cases, new hydro installations are designed to leave sufficient water bypassing the turbines - which is not difficult except in times of low flow. Another area that requires care is the need to avoid harming fish and riverine flora and fauna, but modern turbine installations are designed with this problem in mind. Some low head systems allow fish to pass through the turbine generally unscathed, but various forms of screening (either physical screens or even electrical and ultrasonic) are also used. Fish ladders, a set of small waterfalls set in a channel, are provided to ensure that migrating fish such as salmon can safely bypass the hydro-plant. Figure 6.11 illustrates a common fish-ladder with vertical slots and bottom orifices that yields very good results.
Figure 6.11 Fish Ladder
Turbines also need to be protected from all the debris that are commonly found in rivers, whether natural (e.g. leaves, branches, even tree trunks) or
man-made (supermarket trolleys, plastic fertiliser bags or general garbage); this is done using screens. A major operating cost element is cleaning these screens, especially in low head situations where large flow rates pass through. The hydro-plant operators are usually prohibited by law from returning the rubbish collected on their screens back into the river. Thus, garbage collection and disposal carried out at a hydro plant can serve to clean up a river considerably for the benefit of everyone downstream, but usually at considerable expense to the operator. There are a few other environmental impact issues relating to oxygenation of the water, disturbance of the river bed or erosion immediately downstream of the turbine draft tubes, electrical machinery noise, electrical cables, the general appearance of an installation, etc. However, all these problems can be solved by suitable design techniques and the end product is a remarkably long-lasting, reliable and potentially economical source of clean energy. 6.3.3 Economic In the last sentence, the phrase 'potentially economical' was used for good reason; paradoxically, using modern conventions for financial and economic appraisal, most new SHP installations appear to produce rather expensive electricity as the high up-front capital costs are usually written off over only 10 or 20 years (yet such systems commonly last without major replacement costs for 50 years or more). In contrast, an older hydro site where the capital investment has been written off is cheap to run as the only costs relate to occasional maintenance and replacements.
Source: Ref 6.5
Figure 6.12 Effects of hydraulic head and size on installation cost
As an example, the unit cost of owning a typical small low head hydro site in the UK might be typically €0.07/kWh during the first ten years while the capital investment is being repaid, but subsequently, because of the low running costs, the unit costs could fall to around one tenth of this level (say €0.007/kWh). Clearly the output for the first decade will be more costly than power bought from the grid in most cases, although after the capital investment has been paid off, the hydro plant power prices become exceedingly attractive. Unfortunately, most potential investors take a short-term view and are put off by the initial high costs.
Decisions to use a technology are generally driven primarily by economics, so naturally there is a need to drive down the costs of SHP. Least cost hydro is generally high head hydro (see fig 6.12), since the higher the head, the less water is required for a given amount of power - so smaller, less costly equipment is needed. Therefore, in mountainous regions even quite small streams, if used at high heads, can yield significant power levels at attractively low costs. However, high head sites tend to be in areas of low population density where the demand for electricity is small, and long transmission distances to the main centres of population can nullify the low cost advantages of remote high head systems. High head sites are also relatively rare, and most of the best ones in the developed world have already been exploited. So the greatest scope for new SHP is at low head sites, although many good high and medium head sites still await development. Unfortunately, at present most low head sites are at best only marginally attractive economically compared with fossil fuel power generation and for this reason many potential sites remain to be exploited. For example, the UK has some 20,000 disused water mill sites, all low head, which so far have not been redeveloped; in many other countries a similar situation can be observed. 6.4 References
6.4.1 Bibliography 6.1 "Layman's guidebook on how to develop a small hydro site", prepared under contract for the Commission of the European Communities, Directorate-General for Energy, by the European Small Hydropower Association (ESHA), 1997. 6.2 Nigel, S., 1997. "Motors as Generators for Micro-Hydro Power", Intermediate Technology Publications, United Kingdom. 6.3 Harvey, Α., 1991. "Micro-hydro design manual - A guide to small-scale water power schemes", Intermediate Technology Publications, United Kingdom. 6.4 Fraenkel et al, 1993. "Micro-hydro power - A guide for development workers", Intermediate Technology Publications, United Kingdom. 6.5 Pauwels, H., 1997. Communication to HIDROENERGIA'97 on the THERMIE programme of DG XVII (Energy Technology Department). 6.4.2 Web links Links To Related Small Hydro Sites: Microhydro web portal: Hydro Power Links: European Small Hydropower Association The IEA Hydropower Agreement: International Small-hydro Atlas Czech Republic: Norway: Swissenergy:
http://smallhydropower.com/links.html http://www.microhydropower.net/index.php http://home1.swipnet.se/~w-19094/hyd_link.htm http://www.esha.be/ http://www.ieahydro.org
http://web.telecom.cz/hydropower/index.html http://www.ntnu.no/ich/ http://www.nve.no/english/index.html http://www.smallhydro.ch/
Swiss association of owners of small power plants: http://www.iskb.ch/
7 Energy from Biomass
7.1 Introduction 7.1.1 Scope Biomass in a solid burnable form is treated in this section. It does not include derived liquid or gaseous biofuels, or municipal waste, both of which are covered on other INTUSER web pages (LINKS). 7.1.2 History The use of biomass for energy has a long history, and was the only controllable source of heat for primitive man, who used fire without any conscience or awareness of its energy or environmental impact. Wood and woody residues provided most of the biomass burned. In the early and middle periods of industrialization, so-called firewood was the most frequently used source of energy in houses as well as in industry. Technological progress and the accompanying growth of civilization has led to a drastic decline in the use of firewood, which has been almost exclusively replaced by fossil fuels. It was a long and continuous process, which was first interrupted by the energy crisis in the 1970s. The energy crisis is a social, economical and political phenomenon in which the demand for energy exceeds the available supply. It Figure 7.1 Biomass has been has interactions with other problems such as the providing energy since man environmental crisis, the overpopulation crisis and the discovered fire social crisis. Hence the development of alternative or renewable energy sources in general, and bioenergy in particular, must be pursued not only as a scientific or technological challenge for a viable and profitable business, but also with social and environmental responsibility. 7.2 Technical outline
7.2.1 Basic Concepts Bioenergy is solar energy stored in biomass, which may be released by combustion or converted into biofuels. Biopower means electricity generated from either of these. Biomass means living organic matter derived from plants, formed through photosynthesis. Biomass mainly comprises wood, energy crops, agricultural products (both animal and vegetable), forestry waste (forest residues) and also aquatic plants. Biomass covers a very wide range of headings, including the following: phytomass, dendromass, zoomass, waste (domestic waste and industrial waste), biodegradable materials, residue-sourced materials, recycled materials, food production residues, agricultural residues, animal residues, vegetable residues, biomass materials, innovative waste materials (poultry litter, coffee residues, mustard husks, spice waste), etc. Municipal solid waste (MSW) is not considered as biomass (even though most MSW is derived from plant matter) because it also contains a variety of potentially toxic materials, such as creosote-treated wood, batteries that contain mercury, and other hazardous products. . "Biopower" plants use only uncontaminated feedstocks, or "pure" biomass.
18.104.22.168 Photosynthesis and Biomass - the solar energy conversion route Photosynthesis is that conversion process which takes place in green plants (containing chlorophyll), which need supplies of carbon dioxide (CO2), water (H2O) and solar energy as inputs in order to deliver organic material, oxygen (O2) and water (H2O) as outputs. Simple organic substances are produced first and may then be converted into more complex organic materials, all of which constitute the gross biomass production. The organic substances are partly used for physiological processes and partly stored in the plant tissues. The quantity of stored organic material furnishes the net biomass production of a plant or of the entire vegetation. Photosynthesis is just one of the possible conversion processes which can transform solar energy into biomass, but it is no ordinary process. It is possible to synthesize a similar conversion artificially (using only water, Figure 7.2 Photosynthesis and Biomass carbon dioxide and energy as inputs and with the same mass output), but not with the same efficiency. Let us analyze and compare the energy efficiency of photosynthesis and of similar artificial conversion processes.
η1 = 1.61%
η2 ~
η1 100
Figure 7.3 Photosynthesis is the most suitable route to convert solar energy – Biomass is the most convenient form to store solar energy
For photosynthesis, the efficiency of chemical transformation is about 1.61%. In a high-tech world in which we are familiar with efficiencies approaching 99%, 1.61% seems very low! Indeed, living organisms operate biologically with processes characterized by a low energy density. But an equivalent synthetic process requires 100 times more energy for the same output of organic material! It therefore seems that photosynthesis is the most suitable route to convert solar energy into biomass, and that biomass is the most convenient form to store this converted solar energy. Hence chlorophyll, CO2, H2O and solar energy are the basic requirements of the photosynthesis process, which is responsible for biomass production. The available solar energy, the local rainfall etc. set the abiotic environment, which determines the vegetation types living in that specific physical environment. There are two categories of plants, socalled C3 type and C4 type. The efficiency of photosynthesis is greater for C4 type than for C3 type plants. Consequently, in a given abiotic environment, different plants provide different biomass quantities (net biomass production). The CO2 in the atmosphere is the essential source of Carbon for the creation of Biomass. Hence there is a balance between the absorption of CO2 in photosynthesis and its release when Biomass energy is released (usually by combustion). Biomass is therefore a CO2 neutral fuel and its use does not contribute to global warming. 184.108.40.206 Summary • • • • • Biomass is a diffuse source of energy, with low energy density. Modern technologies need concentrated energy sources with very high energy density. This is why traditional energy sources have declined and our age has seen such extraordinary growth of energy use and demand . Biomass is a diffuse resource, arising over very large areas and thus requiring large land areas with substantial collection and transport costs. Photosynthesis is the most effective route to convert solar energy into Biomass. Biomass is the most convenient form to store solar energy. The use of biomass for energy is also desirable in a more complex, holistic sense
The use of Biomass can help • to protect fossil energy sources, diminish their exploitation and prolong their availability. • to protect the environment, by reducing pollution and global warming. • to reduce unemployment in rural lands with low agricultural potential. Bioenergy is now accepted as having the potential to provide the major part of the projected renewable energy provision of the future. The realisation of this opportunity depends on many factors: scientific, technological, economic and political. 7.2.2 Factors Affecting The Utilization Of Bioenergy 184.108.40.206 Geographical factors The geographical features of a country determine the environmental potential for biomass production (e.g. see table 7.1). But the exploitation of this available biomass depends on the entire history of that country, leading to its current economic and political situation. 188.8.131.52 Socio-political factors There are three traditional basic demands for biomass: food for the population, agricultural forage and industrial raw material. The supply of these demands naturally requires a rational compromise between them. Hence biomass is only likely to be produced for energy in those countries or regions where there is
already sufficient agricultural produce to nourish the population at an acceptable level, and where concrete demand exists for the locally produced bioenergy. Sustainable crop of woody biomass Northern Europe Southern Europe 10 dry t/ha/y 15-20 dry t/ha/y 1 sq km (100 ha) produces 1000 dry t/y Power generated At low conversion efficiency At high conversion efficiency 150kW 300kW Power plant capacity per 100 sq km planted area theoretical size: 30-40 MWe feasible size: 5-10 MWe
Table 7.1 Example of biomass and bioenergy potentials for different regions
184.108.40.206 Energy and food autonomies The relative energy need of a society depends mainly on its stage of economic development, whereas its internal food supply depends on its agricultural strength. The extent to which a given country can supply its own energy and food requirements may be expressed numerically in terms of its Energy autonomy and Food autonomy. A relative plot of these two parameters (fig 7.4) gives a useful country specific indicator of the opportunity for using bioenergy. It can be seen that this indicator spreads the world's countries over a wide area. The energy autonomy 0.2 0.4 0.6 0.8 1.0 1.2
h Australia
h Malaysia
hDom 1.4 hCuba inica hHungary 1.3 hThailand 1.2 hPhilippines hBrazil hKenya hSudan 1.1 hEthiopia h h France Turkey 1.0 Spain 0.9
Italy h hJapan
Colombia USAh
The food autonomy
South Africah hZaire Burmah hChina hIndia h h hRussia Poland h Bangladesh Pakistan h Vietnamh hKorea hGermany hUK
Indonesiah Nigeriah Syriah Egypth Tunisiah Iranh Venezuelah Iraqh Norwayh
0.8 0.7 0.6 0.5
Saudi Arabiah
Figure 7.4 Food and energy autonomy as opportunity indicators for biomass projects
126.96.36.199 The structure of cultivated land The structure of cultivated land is another important factor, which determines the nature and quantity of the available biomass. Table 7.2 gives an analysis of the cultivated land structure
in Hungary. It shows that both forests and unused land areas are increasing, which could stimulate future biomass production.
Cultivated land [ % ]
Arable Garden Orchard Vineyard Meadow Pasture AGRICULTURAL AREA Forest Reed Fishpond LAND IN USE Land out of use
50,9 3,1 1,5 1,8 3,6 10,3 71,2 17,3 0,4 0,3 89,2 10,8 100
50,7 0,4 1,0 1,4 12,3 65,8 19,0 0,4 0,3 85,5 14,5 100
50,7 1,1 1,0 1,4 12,3 66,5 19,0 0,4 0,3 86,2 13,8 100
Table 7.2 Land use in Hungary
126.96.36.199 Agricultural main products and residues Agricultural products present a very high bioenergetic potential. It is interesting to observe that vegetable agricultural residues are considered to be the main agricultural source of bioenergy, yet they are only complementary products of the vegetable agricultural main products. It should not be forgotten that these main products also have great energy potential, which should also be considered in certain cases. But in either case, the ratio of main product to associated residues is an important indicator of the efficiency of the bioenergetic utilization of that agricultural plantation, and varies widely for different plantations (fig 7.5).
100% 80% 60% 40% 20% 0%
Residues Main prod.
Potato plant
Industrial plants
Figure 7.5 Proportions of main product and associated residues for different plantations
220.127.116.11 Soil quality Agricultural vegetable residues also serve another vital function. It is well known that the biologic potential of agricultural land results from a complex dynamic balance between the accumulation and decay of organic materials that derive mainly from agricultural residues. This is the reason why agricultural residues often remain on the land where they originated and/or are mechanically processed there. The conservation of the biogenetic potential of the
Leguminous plants
Vegetables in gen.
agricultural land is of great importance and could limit the use of agricultural residues as bioenergy feedstocks, but certainly does not prevent it. It should also be made clear that this balance is not assisted by the use of chemical fertiliser, which is normally derived from oil and may require almost as much fossil energy input for its manufacture as the extra bioenergy output obtained by its use. Chemical fertilisers are not therefore compatible with a sustainable energy regime, of which biomass production is considered to be an important future part. In the case of energy crops (see 18.104.22.168 below), good soil quality and fertilisation can be achieved by applying residues and wastes, often dangerous wastes which are unsuitable for food crops, such as biological domestic or industrial waste, biodegradable materials, biological residue-sourced materials, biological recycled materials, food production residues. Thus the disposal problem for such waste materials can simultaneously be solved in an environmentally friendly way. 7.2.3 Nature and structure of biomass Figure 7.6 shows how the conventional sources of biomass arise and are used.
Main products
Primary biomass
Main products
Food Industrial (non food) Export Forage Other purposes Harvest losses.
Forage Litter Burnable Root residues Remain on the land Other purposes
Timber Woody Materials
Stump residues Root residues
Figure 7.6 Nature and structure of available biomass
188.8.131.52 Agricultural residues Straw, mainly from wheat, is a very important biomass source. Wheat is the main agricultural crop in many countries over the whole of Europe. It is interesting that more than 50% of the straw remains in the field as stubble to be burned or ploughed in. This is the most controversial aspect of straw management. Some consider that both these techniques improve the field, whereas others say that they have a negative effect on the biological potential (soil
chemistry) of the field. This misused or unused straw could represent potential bioenergy. The straw harvest has three main uses: as litter (30-40%), as forage (8-10%) and as industrial raw material (2%). Corn stover (maize stalks, leaves and cobs) is the most abundant agricultural residue. Nearly 90% of this is ploughed into the field, but could generate more than three times the current residues from wood waste. The efficiency of burning maize stalks decreases with humidity. Sunflower stalks are also ploughed in, but would yield a lot of biomass. Rice husks and bagasse (sugar cane residue) are burnt in US power plants to produce electricity, and could be an important source of energy in many other parts of the world. Orchard and vineyard cuttings are traditionally recognized and appreciated as burnable materials, although they are sometimes burned or ploughed in. 126.96.36.199 Forestry products Technically, in forestry and the wood industry, woody materials (usually valuable, fig 7.7) are those which derive from the felled tree (above the cutting plane). Woody fragments are the residues from what remains below which, in a given season or economic/technological situation, it may not be viable to collect and use. Stumps and roots are hard to extract and manage. Often they remain in place, with a beneficial role in recultivating and improving the ground. If eradicated in the forest, roots and stumps are chopped up and can supply further combustibles. The woody material obtained above the cutting plane is used partly for industrial purposes, partly as firewood. Of course, thin branches and bark appear as waste material. The proportions are roughly: 20% wastes, 35% firewood, 45% industrial wood [ref 7.1]. The firewood, which is usually burned in domestic fires, is available as: traditional firewood, split firewood, woody briquettes and woody pellets.
Figure 7.7 Forestry produce
In order to minimize the environmental impact of forest exploitation and eradication, the massive probable utilization of woody biomass in the future will imply the plantation of new short rotation forests and coppices dedicated entirely to energy purposes. Otherwise forest eradication, exercised without discrimination and precaution, will lead to environmental and social crisis with predictable catastrophic consequences.
184.108.40.206 Energy crops Energy crops are crops developed and grown specifically for fuel, carefully selected to be fast growing, drought and pest resistant, and readily harvested to allow competitive prices when used as fuel. They include fast-growing trees, shrubs, and grasses. Examples under development include hybrid poplar, willow, eucalyptus and energy grasses. Energy crops can be grown on agricultural lands not needed for food, feed, or fiber. These include lands taken out of service for price control reasons and other agricultural lands that are considered marginal for food production. Compared to traditional agricultural crops, energy crops are lower maintenance and require less fertilizer and pesticide treatment. Energy grasses include switchgrass, reed canary grass, giant reed, and herbaceous lignocellulosic crops such as miscanthus. They offer many advantages over other bioenergy sources. As a convincing example, we present a Hungarian novelty in the field of energy grasses, the so-called "Szarvasi-1" energy grass (fig 7.8), a promising new species of plant obtained by hybridization and selection in the Mezőgazdasági Kutató-Fejlesztő Kht., Szarvas, Hungary. The resistance of this grass to drought, soil salinity, frost and other extremal abiotic environments is surprisingly good. The crop survives in the same field for 10-15 years continuously, can be harvested every year, and the plantation costs are only 20-25% of those for forestry. The harvesting technology is simple; and the bioimprovement effect is very strong and beneficial. The 2-2.5m social impact on regions with low-valued fields is also beneficial due to higher employment. Four years of harvesting have shown that yields of Szarvasi-1 exceed those of all other energy crops. Even when compacted into combustible pellets, prices per unit of energy are comparable with firewood and significantly less than for fossil fuels (e.g. brown coal or natural gas). 18.104.22.168 Pellet technology There are many reasons (such as humidity control) to homogenize the various bio-residues and bio-wastes before distribution and combustion. A higher density is particularly desirable. Biobriquettes and bio-pellets are the commercial forms of such homogenized, compressed and compacted biomass. The main characteristics of the so-called Epellet are given below figure 7.10. Pellet technology is expensive, but cannot be avoided if such a great variety of burnable biomass is to be adequately managed (nature, characteristics and final use). Figure 7.11 [ref 7.5] shows a few stages in conveying biomass fuel to the end-user.
Figure 7.8 Szarvasi-1 energy grass
Figure 7.10 E-pellets Length: 5-8 mm Diameter 5-10 mm 2 kg pellets replace 1 m3 natural gas
Fig 7.11 (a) Woody-biomass storage
Fig 7.11 (b) Pellet processing line
Fig 7.11 (c) Pellet loading
Fig 7.11 (d) Delivery of pellet fuel to the consumer
7.2.4 Systems for biomass combustion 18.104.22.168 Direct combustion The direct burning of biomass is the oldest and most traditional use of biomass energy. But recently in many parts of the "civilized" world, under pressure from various energy lobbies on consumption patterns, the use of firewood is perceived as a symbol of poverty! These consumption patterns are really obstacles, which endanger the spread of new, environmentally friendly, sustainable consumption patterns in general, and of renewable energy sources in particular. Today the success of firewood for domestic heating (just as an example of a renewable energy source) depends on the comfort which a fire can give and on the price and costs (including control of emissions) which its use implies. The direct combustion of biomass means essentially the burning of agricultural, forestry and wood-industrial residues. The nature and appearance of each one of them is very different, requiring a whole range techniques for collection and preparation. Modern furnaces for biomass need similar technology to those used for other solid fuels, whether in small sizes for domestic heating or for large power stations. However other technologies such as gasification and pyrolysis may be desirable at the larger scale where it is essential to maximise efficiency while minimising gaseous emissions.
188.8.131.52 Domestic scale boilers Direct wood burning stoves are widely used in houses where manual charging and ash removal are acceptable. Modern designs give good performance with low emissions. Where unattended operation for longer periods is required, some form of automatic fuel feed is required. Biomass pellets are one suitable type of fuel in this case. Figure 7.12 shows a typical boiler (B) with screw feed (C) from a pellet silo (A), and delivery (D). Operation of the feed system uses a controller (E).
Source: ref 7.5
Figure 7.12 Simple boiler with pellet silo
220.127.116.11 Power plants Although biomass may also be used for district heating systems, similar technological considerations arise for all large combustion systems, and the heat energy may be used for many combinations of heat and power. Fig 7.12 shows how many different routes may arise.
Source: ref 7.7
Figure 7.12 Overview of biomass conversion routes for power production
A typical power plant with direct combustion is shown in fig 7.14 which can be applied to use any solid fuel (the main differences arising in preparation and processing), and cofiring of biomass with coal also has potential advantages. This cycle is frequently also used with steam extraction to provide both heat and power.
Source: ref 7.7
Figure 7.14 Direct combustion steam turbine system
Internal furnace design covers a wide range, from simple fixed grates through to pulverised fuel burners. For combustion of fresh biomass, a common design is the sloping grate furnace, which allows pre-drying of the fuel in the upper part of the furnace prior to it falling under gravity onto a reciprocating grate lower in the furnace where combustion takes place. Since the grate is water- or air-cooled, it does not need an ash layer to insulate it from high temperatures, which makes it more suitable for low ash biomass fuels. 220.127.116.11 Emissions from direct biomass combustion The great variety of both biomass fuel types and boiler designs, and the large range of plant size (10kW to 50 MW), all make impossible any precise comparison between procedures, products and plant, when considering not only technical and economic performance, but also environmental impact. Table 7.5 gives a general guideline about airborne emissions. Plant size Flue gas CO O2 content [ g/m3 } [ Vol. % ] 13 11 11 4 0,25 0,25 Emission limits Dust OrganicC NOx [ mg/m3 ] [ mg/m3 ] [ mg/m3 ] 150 150 50 500 50 50 500
15kW - 100 kW 100kW - 5MW 5MW - 50MW 220.127.116.11 Fluidised Beds
Table 7.5 Harmful emissions – Plants burning straw and stalks [ref 7.1]
For larger plants, a more complex option is fluidised bed combustion, in which mixing of the fuel and air is assisted by being blown through a bed of inert dry particles such as sand. This technique allows either complete combustion or gasification of the fuel (see below). Commercial developments are categorised as either bubbling fluidised bed (BFB) combustion using low air velocities, or circulating fluidised bed (CFB) combustion in which higher velocities result in some sand being blown out of the top and recirculated (figure 7.15). CFB tends to be more expensive, but reduces NOx emissions due to lower operating temperatures.
Figure 7.15 Fluidised bed boilers
Source: ref 7.7
Fluidised bed combustors are more technically intricate with associated higher costs of design, construction and operation. As a general rule, there is a threshold in the region of 8
MWth above which they begin to gain economic advantage over fixed bed combustors. On the other hand, fluidised bed has the following advantages over fixed bed combustion: • The high thermal inertia of the bed provides conditions for stable ignition, despite variability of fuel quality. Hence it is more tolerant of a wider range of fuel characteristics. • Control of bed temperature allows a range of fuels with varying ash properties to be burnt while avoiding ash softening conditions in the bed. • Relatively low combustion temperatures mean that NOx emissions are low. • If limestone is added to the bed material then in-situ capture of SO2 is possible, though this is not necessary for biomass fuels since they are low in sulphur. BFB units are commercially offered up to 100 MWe, and CFB units up to 400-600 MWe. CFB boilers have proven feasibility to burn about 70 different fuels alone or in co-combustion mode. BFB boilers have proved their feasibility for biomass and waste fuels with similar characteristics especially in lower capacities, starting from 5 MWth with well-processed fuel. 22.214.171.124 Gasification-based combustion Biomass gasification, that is the conversion of biomass to a low- or medium-heating-value gaseous fuel, generally involves two processes. The first of them, pyrolysis, releases volatile vapours (hydrocarbon gases, CO, CO2, hydrogen, tars, and water vapour) from the fuel at temperatures below 600°C. Because biomass fuels tend to have more volatile components (70-86% on a dry basis) than coal (30%), pyrolysis plays a proportionally larger role in biomass gasification than in coal gasification. The by-products of pyrolysis that are not vaporized are known as char, and consist mainly of fixed carbon and ash. In the second gasification process, char conversion, the carbon remaining after pyrolysis undergoes the classic gasification reaction (i.e. steam + carbon) and/or combustion (carbon + oxygen). It is this latter combustion reaction that provides the heat energy required to drive the pyrolysis and char gasification reactions. Due to its high reactivity (as compared to coal and other solid fuels), all of the biomass feed, including char, is normally converted to gasification products in a single pass through a gasifier system.
Source: ref 7.7
Figure 7.16 Biomass integrated gasification combined cycle (IGCC) system (HRSG = Heat Recovery Steam Generator)
In the integrated gasification combined cycle (IGCC) system shown in figure 7.16, a high pressure, direct gasifier is depicted inside the dashed line. Gasification and production of a clean fuel gas makes a wide array of power options possible, including the use of advanced power systems with higher efficiencies than those obtained from steam turbines alone. At the low cost end of the spectrum, gasifiers coupled to a high-efficiency, simple-cycle gas turbine offer simplicity as well as efficiencies competitive to the Rankine cycle. A fluidized-bed biomass gasifier (which does not necessarily require steam) integrated with a high-efficiency gas turbine may be the best combination for simple cycle operation.
Political implications
7.3.1 Public awareness There are two misconceptions which endanger all superficial conclusions about biomass: • Biomass is readily available everywhere, and just needs to be collected and processed; • Obtaining energy from biomass is a purely technological problem, which is only a matter of effort. Both of these myths can and must be avoided by raising public awareness and disseminating better information. Biomass is an available but problematic energy source, which has the potential to make a significant contribution to future energy supplies. Its utilization requires more research and development as well as technological progress, but, first of all, changes in the outlook of people, whether scientists, businessman, decision makers or consumers. This means new and conscious production and consumer patterns, societal values, and welfare goals. 7.3.2 EU scenario Bioenergy is now accepted as having the potential to provide a major part of the projected renewable energy provision of the future. This opportunity has been well recognized, accepted and promoted in the EU countries. Some important issues selected from the European Commission's documents are given below (ref 7.6). "In an EC strategy aiming at increasing the share of RES in the EU energy supply from 6%, at present, to 12% in 2010, biomass is expected to make a major contribution. Biomass is expected in the long term to reach a potential of 20% of the current primary energy supply. An integrated approach is necessary for addressing the whole production chain (including the increase of the yield of biomass, which is covered in Key Action 5 of the Quality of Life Thematic Programme)." "Indicative RTD subjects: • Co-combustion of biomass and bio-waste in coal fired electricity plants; • Development and optimisation of technologies for the conversion of biomass into power and heat, such as efficient combustion, gasification, pyrolysis, biomass operated gas turbines, and components able to use different bio fuels. Particular attention should be given to large scale gasification. • In view of the fact that the cost of biomass is a major factor in the energy cost, options should be explored as to how this cost can be reduced; this includes the use of bio waste and biogas; co-generation and poly-generation (generation of heat, electricity and other products) should be favored;
Reduction of local and global environment degrading emissions with treatment prior to combustion, improved combustion techniques, advanced control and monitoring. Particular attention should be given to ash recirculation including metal removal, tars, NOx and particulate matter. Socio-economic and pre-normative RTD on the use of bio-energy e.g. technological, economical, socio-economic and environmental barriers."
"Bio-electricity obtained from biomass and waste is vitally important for the EU and other countries where the demand for energy is expected to grow during the coming years. In order to meet the EU's Kyoto commitments, it is anticipated that the use and trade of existing biofuels and waste derived fuels will increase significantly due to their CO2 neutral character. It is therefore of great importance to further develop and demonstrate advanced conversion technologies and effective fuel supply chains. This can be achieved in either dedicated bio-energy plants or in co-utilisation applications with fossil fuels." 7.4 References
7.4.1 Bibliography 7.1 A biomassza energetikai hasznosítása – Energiagazdálkodási kézikönyv, Energia Központ Kht., Budapest, 1998. 7.2 Thermal processing of biomass for fuels and chemicals – AV Brigwater, Renewable Bioenergy: Technologies, Risks and rewards, Institution of Mechanical Engineers Conference papers, 29-30 October 2002, London, UK. 7.3 A 'Szarvasi-1' energiafű és hasznosításának lehetséges területei - Janowszky János, Janowszky Zsolt - Mezőgazdasági Kutató-Fejlesztő Kht., Szarvas – "Komplex föld és biomassza hasznosítási lehetőségek és megoldások térségünkben" Konferencia, 2003 július 11. Nagyvárad (Oradea), Románia 7.4 "Komplex Föld és Biomassza Hasznosítási Lehetőségek és Megoldások Térségünkben" – Edőcs Ottó –"Komplex föld és biomassza hasznosítási lehetőségek és megoldások térségünkben" Konferencia, 2003 július 11. Nagyvárad (Oradea), Románia 7.5 Az energiafű lakossági felhasználási lehetőségei - Edőcs Ottó - Energiafű hasznosítási konferencia, 2003.06.24.-25. Szarvas, Magyarország 7.6 Energy, Environment and Sustainable Development - Work Programme Update: October 2000; Part B: Energy 7.7 Guidebook on the RES Power Generation Technologies: CRES, CENERG and ZREU, Leonardo da Vinci Project, contract no: EL/99/2/011015/PI/II.1.1.b/FPI, August 2001. 7.4.2 Web links European Biomass Industry Association:
Biomass Research and Development Initiative (USA):
http://bioenergy.ornl.org/ http://www.eere.energy.gov/RE/bioenergy.html
8 Conclusion
Exajoules 1500 Exa = 10 1 exajoule = 34.12 Mtce
Geo-/Oceanic Energy
Solar Energy 1000 New Biomass
2020 2002
500 Wind Energy Hydro
Strongest growing market segments
1920 1940 1960 1980 2000 2020 2040 2060
Quelle: Deutsche Shell AG
Figure 8.1 Energy market projection
There can be little doubt that all of the five RES described above will continue to develop and that the market for each will increase. Future trends have been estimated by many organisations, such as that by Shell shown in figure 8.1. The outcome of such projections does however depend on a complex list of assumptions about demand, supply and political and economic initiatives. The challenges for the future remain unchanged: • • Can new energy sources compensate for reduced availability of fossil fuels? Can Carbon Dioxide accumulation in the atmosphere be controlled in time to limit global warming to acceptable levels?
The record to date shows that renewable energy has only just started in terms of energy production, with Wind power showing some initial strength in Europe. It is however essential that the other energy sources continue to be developed in order to provide a balanced economy in terms of: • • • Continuity of supply on both daily and annual cycles National grid and locally embedded distribution of electricity Optimised energy efficiency (e.g. by use of Combined Heat and Power systems with Biomass and Municipal Waste)
Inevitably the cost of each RES will influence its rate of growth. Wind Power has now reached an economy of scale which begins to make it generally competitive for electricity generation, and Biomass (as wood and wastes) continues to be widely used for both heating and electricity. Although direct solar gain in buildings has an important role, this is not measured directly and the statistics for Solar Thermal energy use do not yet show a large impact. Further stimulation of all RES by both incentives and increased public awareness appears to be essential if the above challenges are to be met.
9 Common References
9.1 Bibliography 9.1 9.2 Boyle, Godfrey (Ed): Renewable Energy, Power for a Sustainable Future. The Open University/Oxford University Press 1996
http://www.erec-renewables.org/default.htm http://www.iea.org/techno/renew/index.htm http://www.eere.energy.gov/ http://www.nrel.gov/ http://solstice.crest.org/
Web links European Renewable Energy Council: International Energy Agency
US Department of Energy National Renewable Energy Laboratory (USA): Renewable Energy Policy Project (USA) dti (UK) renewables CRES (Greece) EC Cordis OPET network Environment game: Renewing India:
http://www.cordis.lu/opet/ http://eww.bchydro.bc.ca/environment/play/start.html http://www.renewingindia.org/index.html
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