Abstract
Abstract -Electrical energy is the highest set-up and operation cost in the agricultural greenhouse crop production in most of moderate or extreme temperature climate country. The energy consumes mostly for cooling, heating, ventilation, lighting, irrigation pumps, and the automation system. This paper proposed a theoretical approach and a conceptual design of a hybrid photovoltaic (PV) system by coupling PV panel and thermoelectric generator (TEG) modules which is to be applied in an Automated Greenhouse system project. The improved efficiency is to be compared between conventional PV generation system and by using various PV and TE module type configuration. Presented system is a conceptual design and further improvement will be considered such as combining with automated semi-transparent thin film solar panel to absorb excess radiation of sunlight during hot season. Simulation is also to be developed to validate the experimental approach.
I. INTRODUCTION
Greenhouse plant production improves crop yields quality through control of environment in terms of temperature, light, humidity, water, carbon dioxide (CO2) concentration, and air ventilation. Electrical energy consumption is necessary to control those environmental elements. Main intention for sustainable greenhouse plant production are to take advantage of favorable climate while keeping the operational cost at a minimum, minimize energy consumption and to compensate the consumed energy with renewable energy. Heating and cooling systems are two major costs involved in greenhouses production [1] . Burning fossil fuels (petrol oil, diesel, or liquid petroleum gas) for heating will increase CO2 emission, and more energy consumed by using electric heaters. Hence, the important challenge for agriculture greenhouse industry is to seek ways that improve energy efficiency combined with a reduction of the overall energy consumption and related CO2 emissions of the greenhouse.
I. INTRODUCTION
Crop yields growth greatly depends on the availability of light. Nearly 1% yield loss recorded with a 1% reduction in light [2] . Solar radiation sometimes too intense for some crop species during summer or in few high-irradiation regions. As the consequence, coating applications and sun screens widely used to restrain the intensity of radiation through the greenhouses [3] . This fact suggests that the excessive solar radiation or sunlight penetrating greenhouses could be beneficial to harvest electrical energy source for the operation of environment control system using photovoltaic (PV) system. By placing PV panels on roof top or integrating them to the greenhouse structure, the large availability of surfaces taken up by PV panels can grow crop yields products below while producing self-consumed energy on the top, which allows the optimization of the solar radiated surface [4] .
I. INTRODUCTION
Over the past decade, many researchers studied different PV greenhouse system with regards to the system generation efficiency, optimization of power consumption [5] , orientation, structure and configuration of the PV System [6] , alternative PV type and material [7] , energy and heat storage system [8] , and also combination of both electrical and thermal energy production [9] . These studies reveals that although precise design is necessary, it is possible to generate sufficient electricity for the greenhouse environment control system devices using PV systems which is suitable for crops cultivation [10] .
I. INTRODUCTION
Previous PV greenhouse studies normally use conventional monocrystalline, polycrystalline or thin-film PV modules type. However, recent PV technologies development has proposed additional alternative for solar system applications in greenhouses to improve or maximize the output efficiency. Combination of PV cells with thermoelectric generator (TEG) is widely studied and notable to have better performance than single PV. The application of a system that combines photovoltaic and thermocouple technology can be a good way of using surplus heat and increasing efficiency. A thermoelectric (TE) device can directly convert excess thermal energy into electrical energy through the temperature difference between the two faces of a PV panel. In addition, the thermal structure of a TE has no moving parts and is completely quiet, clean and more durable as a complement to PV systems, thus suitable for greenhouse application.
I. INTRODUCTION
This paper proposed the photovoltaic-thermoelectric generator (PV/TEG) hybrid concept that can utilize both light and thermal energies from the solar radiation for the energy generation to supply an automated greenhouse system. The hybrid system is expected to produce sufficient power output to supply the loads such as motor/actuator system, pump system, air-cooling system, ventilator, lighting, controller, etc; whereby in this proposal, we considered DC load as to avoid power loss on DC-AC conversion and maximum power optimization [11] .
II. PRINCIPLES OF PHOTOVLTAIC (PV), THERMOELECTRIC GENERATOR (TEG) AND PV-TEG INTEGRATION

A. PV Technology
Since 1960, PV technology development has been a great concern of the industry and researchers worldwide. Today, silicon element represents the leading PV technology due to its capability of producing high efficiency and because of the large availability of silicon material. Notably, monocrystalline (mono-Si) silicon module generate the highest efficiency between 20%-25% [12] . To reduce the high production cost in PV mono-Si modules production, two alternative typologies have been developed; 1) Using polycrystalline silicon (poly-Si) which produce lower performance in terms of efficiency (13%-16%) and, 2) Using amorphous silicon (a-Si, or thin-film) that provides the loweer efficiency, but less affected by shading, high temperatures, and less bulky. Depends on technology applied, thin-film module prototypes efficiencies have reached between 7%-13% [13] . Fig. 1 show a typical construction of a PV cell. The electrical contact to the semiconductor material is made using a metal N+ or P+ junction; which found to be the most effective method of providing an ohm contact. When the energy of absorbed light is equal or larger than the band gap energy of the semiconductor, electrons are driven from the valence band to conduction band leaving holes behind. When the produced electron-hole pairs diffuse to the interface of the Ptype and N-type materials, they get separated at the P-N junction because of the set-up electric field. Electrons (-) tend to move to negative side, and holes (+) to the positive side [14] . When an external load is connected between the front and back contact, electricity flows in the cell, generating current. The basic electrical equivalent model of the photovoltaic cell is shown in Fig. 2 [15] . The basic parameters of this model are the electrical short-circuit current Isc; the open circuit voltage Voc; the saturation current of the P-N junction of the cell, Is; and the series resistance Rs represents the internal potential drop to the contact terminals. The photocurrent Iλ proportional to the solar radiation intensity. The P-N junction solar cell is represented by a direct polarized diode. The resistance Rp represents the deformity in the P-N junction. The operating temperature effect on the electrical efficiency of a PV module can be traced to the temperature's influence upon the current I, and the voltage V, as the maximum power is given by;
A. PV Technology
Both the open-circuit voltage and the fill factor (FF) turned out to decrease significantly with temperature, as the thermally driven electrons begin to dominate the electrical properties of the semi-conductor [16] .
B. TEG Technology
The Seebeck effect were discovered in 1821 by Thomas Seebeck, where a potential difference could be generated by a circuit made from two dissimilar wires when one junction is being heated [17] . In 1834, Jean Peltier founded the Peltier effect, the opposite procedure where the passage of an electric current through a thermocouple produces heat or cooling effect depending on its direction [18] . In 1854, William Thomson revealed that if a temperature difference exists between any two points of a current-carrying conductor, heat is either absorbed or dissipated depending on the direction of current and material used [19] . It was then called the Thomson effect. These three effects are called the thermoelectric effects.
B. TEG Technology
A TE module is produced of many thermoelectric couples which connected thermally in parallel and electrically in series (Fig. 3) . The couples consist of P-type (hole carriers) and N-type (electron carriers) semiconductor materials. When the TE module connected to an external load, it works as a power generator (TEG). The temperature difference causes the electrons and holes to move from the hot to the cold side separately because of density difference (Fig. 4 ). The electrons and holes then reunited at the cold side of the module. This process creates a potential difference and drives a current through the module [20] . The TE efficiency (ηTE) of a TEG is given by;
B. TEG Technology
where Z is the figure of merit, ηC is the Carnot efficiency, TC is the temperature at the cold side and TH is the temperature at the hot side. A cooling device or element is necessary to maintain the TEG density difference at the cold side.
C. PV/TE Integration
A PV-TE integrated system can be found in many configurations where both modules can either be physically integrated or separated, requiring a specific controller [21] . The integrated panels combine these devices to optimize the performance output from both sources. Common PV/TE hybrid system normally consist of PV modules, TEG modules and a cooling system, where different elements were added or modified to optimize the output performance. The sum of power output from the PV modules and TEGs yields to the overall power output of the system. Numbers of such hybrid system can be found in transportation and telecommunication applications.
C. PV/TE Integration
A model of roof mounted PV-TE system has been developed by Van Sark [22] for the meteorological conditions in Spain and Netherlands, where the TE modules were directly mounted to the PV modules, and focus was given to the combined electrical output. Considering the typical figure of merits of 0.004 K-1 of the thermoelectric material, the researcher claimed the annual energy achieved the efficiency of 14.7-11% in this system. In conclusion, the efficiency of combined system was found better.
III. CONCEPTUAL DESIGN OF PV/TEG HYBRID SYSTEM FOR GREENHOUSE APPLICATION
Following sections of this paper proposed how to take advantage of those excess heat from a greenhouse PV roof panels for generating maximum power. For this aim, we considered a combined PV and TEG system, where the TEG is mounted directly on the back of the PV. The hot side temperature of the TEG is thus equal to the temperature of the PV, TPV=TH.TEG, while indoor temperature for the greenhouse is expected to be maintained between 25 °C ~ 28 °C, being the TEG's cold side temperature (TC.TEG). The system is exposed under direct sun, with an arbitrary concentration of the light. The proposed setup is illustrated in Fig. 5 . Series and parallel connected Bismuth Telluride (Bi2Te3) TEG modules is to be thermally mounted at bottom surface of the solar panels. A thermal distribution layer shall be mounted in between of the bottom PV surface and hot side of the TEG to act as the thermal distributor. Meanwhile, to draw more heat from the cold junction of the TE modules, aluminum heat sinks are to be attached on cold side surface of the TE array. Fig. 6 shows the cross-sectional diagram of the proposed PV/TEG configuration. An electrical model equivalent of the PV/TEG system is shown in Fig. 7 , where the two blocks represent the PV and the TE modules; the thermal energy absorbed by PV panel is converted to electricity by the TEG modules. Both elements can be considered separately, since the effects that lead to the generation of current is independent of each other. A hybrid circuit shall be developed to combine and maximize the power output from the PV and TEG arrays to feed the main functional system. To complete the system to form a fully functional system capable of producing electrical power, few other components are to be installed: 1) Maximum power point trackers (MPPT) -to draw out maximum power from the PV/TEG array. 2) Charge controller -regulates current and voltage extracted from the hybrid PV/TEG arrays to charge battery, prevents overcharging and extend the battery life. 3) Battery bank -stores energy and make the energy available at night or during low solar radiation. 4) DC Converter -converts PV/TEG input DC voltage within its operating range into a constant output voltage suitable with system load voltage. Fig. 8 . shows the block diagram of the automated solar greenhouse system to be developed. 
IV. KEY STUDIES

A. System Parameters and Load Requirement
To achieve the optimum system design, system parameters should be defined well for this hybrid energy producing. The experiments will be conducted to investigate the performance of the PV system with and without TEG. The system performance will be determined based on power output of PV and TEG systems combined and separated. For the estimation of power output, the parameters such as open circuit voltage, short circuit current, I-V characteristics of PV, power output of TE modules, Temperatures at various time or seasons of the Greenhouse system, growth effect on crop yields, etc. shall be measured using calibrated measuring instruments. Fig. 9 represent the greenhouse parameter to be taken into consideration, while Table. 1 present the estimated DC loads and daily energy consumption for the equipment in the PV/TEG Hybrid generation system. 
B. System Sizing
In this preliminary stage, the estimation size of PV/TEG array, battery bank and the charge controller can be obtained by using the following method;
Estimation of PV/TEG Array
The daily energy requirement from the PV/TEG array can be determined as following:
Estimation of PV/TEG Array
where E = daily average energy consumption, = system component′s efficiencies, assuming 0.8. where Imp.PV = Rated Current of one panel.
Estimation of PV/TEG Array
To calculate the number of series panels:
Estimation of PV/TEG Array
Hence, the total number of panels equals the series panels multiplied by the parallel ones:
Estimation of PV/TEG Array
The PV array of the system should consist of 2 x 8 seriesparallel connected configuration, equivalent to 1.6KW PV Panels. where NT is the total number of modules.
1(b). PV/TEG Hybrid
On this case, the total generated power of the PV/TEG hybrid panel is the sum of the power output of the PV panel and TEG modules [21] :
1(b). PV/TEG Hybrid
= 100 + 61 = 161
1(b). PV/TEG Hybrid
Theoretically, 10 numbers PV/TEG Hybrid panels needed to generate 1.6KW output, instead of 16 numbers normal PV panels.
Battery Bank Sizing
The days of autonomy or day without minimum solar radiation set for 3 days. Consider using 250AH, 12VDC VRLA battery with 75% maximum depth of discharge DoD, capacity of the battery bank can be calculated as follows: Where Cb = capacity of one battery
Battery Bank Sizing
The connection of the battery bank then can be figured out. The number of batteries in series equals to:
Battery Bank Sizing
Whereas, the number of parallel paths is obtained by:
Battery Bank Sizing
Hence, the number of batteries needed is 6 numbers, in three parallel branches and two series batteries.
Charge Controller Sizing
We considered using 24V, 80A type charge controller, the rated current of the controller can be measured by using below method: The objective of this project is to achieve high energy output by utilizing both light and access heat from the solar radiation. This project will help demonstrate the effectiveness of using the hybrid PV/TEG to determine the maximum value of the system and the benefit of applying it to the automated greenhouse system. The parameters given for the PV and TEG module in this project were used to derive a relationship between power, voltage and thermal effect on the system. With the integration of TEG system, the solar panel quantity is expected to be reduced, whilst maintaining the generated output power to the system and maximize the space available on the greenhouse roof. A hybrid circuit simulation is also to be developed to validate the experimental approach. 

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