Geothermal energy is one of the cleanest non-conventional energy source. A promising application of harnessed geothermal energy are ‘ground source heat pumps(GSHP)’. Ground source heat pumps can be used for both commercial applications as well as domestic applications, the key reason being the fact that they are highly energy efficient, energy saving capabilities and their environment friendliness. The use of heat pipes(a heat transfer device) furthermore improves the efficiency and effectiveness of heat pumps. Hence many studies were conducted on determining the effect of various parameters on the performance of geothermal heat pipes in heat pipe-ground source heating pumps(HP-GSHP), review on which has been presented in the article. This article also aims to provide a concise review on how changing various parameters of the refrigerant flowing in the heat pipes(Darcy Number, Reynolds Number etc) and the changing of various dimensional factors can enhance the thermal behavior, energy saving capacities and effectiveness of heat pipes and in turn the effectiveness and efficiency of Heat Pipe-Ground Source Heat Pumps(HP-GSHP).
Introduction:
The working principle of the geothermal heating systems is to transfer earth’s natural heat to provide heating to the houses. Having its origin in the Greek Language the word ‘geothermal’ literally translates to 'earth heat.’ This source of energy is found below the crust of the earth - both in shallow grounds and also miles below the surface of the earth/ground and even in magma. Tapping these reservoirs of magma or geothermal resources can be put into use for both geothermal heating and also geothermal electricity which can hence be put into use for providing electricity to run household appliances or provide the house with heating, cooling or warm water. A geothermal heating system brings into advantageous use the temperature of the earth about three meters below the surface. Since the temperature in that portion of earth is nearly constant in all seasons and in all geographical areas, geothermal heating proves to be an extremely reliable source of energy in the summer, as well as during the winter seasons. Geothermal heating can be tapped with the use of geothermal heat pumps, also called Ground Source Heat Pumps.
Working of Ground Source Heat Pumps And Heat Pipes:
In heating mode, a cold refrigerant fluid or the working fluid transports the heat absorbed from the earth to the compressor. Further, the compressor functions to increase both temperature and pressure. Next, as the hot refrigerant fluid circulates through the building of the office/home. There the heat it carries is absorbed by colder interior air in order to heat the interior. As a result of this, the formerly hot refrigerant fluid loses its heat and cools down again, it is then re-circulated into the ground, where it absorbs more heat and hence the process gets repeated.
Heat pipe: A heat pipe is a heat-transfer device that works on the combined principles of thermal conductivity and phase transition to efficiently transfer heat between two solid interfaces. At the hot interface of a heat pipe, a liquid that comes in contact with a thermally conducting solid surface absorbs heat from that surface to turn into vapor. This vapor then travels along the length of the heat pipe to the cold interface and further condenses back again into a liquid – therefore releasing the latent heat. The liquid makes a return to the hot interface through either capillary action, centrifugal force, or gravity, and this is how the cycle repeats. Due to the extremely high heat transfer coefficients for boiling and condensation, heat pipes are greatly effective conductors of heat or thermal conductors. The following diagrams show the schematic and cycle diagram of conventional GSHP(Fig 1) and of Direct Expansion Ground Source Heating Pump (Fig 2).
Heating of buildings using HP-GSHP:
Because of the high heat transfer efficiency and minimal space requirements, researchers have gradually shown keen interest in geothermal heat pipes in recent years. The geothermal heat pipe may be damaged, deformed or ruptured during its use, which would result in leakage of working fluid(refrigerant). Thus, the working fluid or refrigerant fluid must satisfy the environmental rules in order to prevent damage to the environment . As per the comparison made by Oschsner ,the property of different refrigerants are shown in Table 1. He found CO2 has a very high volumetric cooling capacity being around 6-8 times greater than various other refrigerants, but its low critical temperature and high critical pressure cause a need for the cycle to operate in the subcritical range which is suitable for geothermal heat pipes rather than heat pumps. Therefore, he developed a HP-GSHP system (Fig.3) with CO2 as the working fluid for the heat pipe, which could harness the greatest performance values and the highest operational reliability. An example of such system is for a domestic house in Freistadt. The house possesses a heated area of 160m2 and a heating load of 33W/m2. The direct-expansion heat pump (DX-GSHP) has an output of 7.8 kW, uses R407C as a refrigerant, and is equipped with a spiral tube heat exchanger. The efficiency is calculated by the COP and seasonal performance factor (SPF):
SPF= Seasonal heating capacity/Heating electricity input
Table 1- The property of conventional refrigerants
Fig 3. Direct heat expansion GSHP using CO2 heat pipes
Effect of wetting ratio,borehole diameter,material of heat pipe:
Research have found along with the working fluid, parameters like wetting ratio (wetted area in relation to the total area), material of the tube, and borehole diameter also exert significant influence on the design and performance of geothermal heat pipes.
Below is the schematic of a geothermal heat pipes(Fig 4)
Fig 4. Schematic drawing of geothermal heat pipe
Studies have found out that a reduction in the borehole diameter and increase in the refrigerants(working fluid’s) thermal conductivity is highly effective for the enhancement of performance of the heat pipe. For steel pipes, wetting is carries very low importance. Almost 96% of the fully-wetted pipe’s heat flux can be possibly transferred to the half-wetted pipe. Since ,steel possesses a high conductivity which causes good heat conduction through the perimeter of the pipe, the thermal energy from the non-wetted side can be transferred into the liquid film. While the PA tube shows extremely opposite behaviour, the heat flux of the half-wetted tube is just 60% of the fully-wetted one.
Influence of shape and structure of the heat pipe/heat exchanger :
The shape or structure of the heat exchanger also exerts a significant impact on the cost-effectiveness and thermal efficiency of the heat exchanger. One such advancement made was the use of spiral-type heat exchangers, which were studied by Michiya Suzukia, Kazuyuki Yoneyamab ,Saya Amemiyab and Motoaki Oe and the results were presented in their paper (ref. 10) . In their study a high performance polyethylene PE100, a material with high and long term durability, mostly used for U tube, is put into use for exchanger and bending of small diameters which used to be impossible to be conducted is applied to the exchanger (Fig 5). They assumed that spiral pipes made from high performance polyethylene were installed in the small-diameter boreholes drilled by a piling machine. As shown in Figure 5 , the commercial spiral heat exchanger dimensions are also specified.
Based on the accuracy of the formation of the temperature field and heat flow of the spiral type and double-U-tube heat exchangers in comparison with the actually measured results. Based on such results, they calculated the initial capital investment per unit heat exchange capacity in the spiral type and doubleU-tube heat exchangers, considering estimated construction fees and material costs. Results showed that the cost of spiral heat pipes was found to be 30% less than that of U-tube pipes.
Fig5. spiral heat pipe with dimensions
There was an observation of a significant difference in the heat exchanging capacity of the U tube and spiral heat exchangers and the results have been accommodated in the graphs(Fig6 and Fig 7 respectively) below.
Fig6. Heat exchanging capacity of double U-tube Fig7. Heat exchanging capacity of heat exchanger spiral heat exchanger
Analysis of porous medium in double tube heat exchangers:
Kamel Milani Shirvan a, Soroush Mirzakhanlari b, Soteris A. Kalogirou c, Hakan F. Oztop € d, and Mojtaba Mamourian, in their paper (ref. no.7) discussed the heat transfer rate and sensitivity of the double pipe heat exchanger filled with a porous medium. They applied the Darcy -Brinkmane-Forchheimer model to establish flow in the porous zone. They used RSM technology to establish relations between various parameters of the porous substrate filling with hot or cold working fluid. The results showed that the nusselts number enhances with Reynolds and Darcy number. An increase in the substrate thickness height decreases the mean Nusselt number and effectiveness of heat exchangers until their minimum values are at a critical height, post which they start to increase again. The heat exchanger effectiveness increases with Reynolds number and temperature difference. it was found out that Darcy number increase reduces the effectiveness of the heat exchanger. The following tables show the sensitivity analysis of responses , table2- Nusselt number and table3 – heat [image: ][image: ]exchanger effectiveness
Table 2- sensitivity analysis: Table 3 – sensitivity analysis : Nusselt number heat exchanger effectiveness
Graphical results for the same were obtained as follows:
Fig 8- variation of heat exchanger effectiveness as a function of effective parameters - (a) Re- d, (b) Re-Da, (c) Re-DT, (d) Da- d, (e) Da-DT and (f) d-DT
Conclusion:
Ground source heat pumps equipped with heat pipes have proven to be highly promising equipment for both heating and cooling purposes in both industrial and domestic applications. They are energy efficient, economically viable, long-lasting, and environmentally friendly alternatives to satisfy the present-day needs for heating and cooling applications. It can be concluded that various factors such as the nature of working fluid/refrigerant fluid (CO2, Propane, etc) increase the thermal efficiency of the ground source heating system, the higher the heat transfer rate of fluid higher is the efficiency. The effect of borehole diameter affects the wetting ratio and hence the effectiveness of the exchanger, the greater the wetting ratio, the greater will be the effectiveness. Furthermore, the spiral-shaped heat exchangers were found to be more effective than the double U- shaped heat exchangers. Also, presence of a porous subtrate affected the heat exchanger effectiveness by influencing the effect of an increase or decrease of parameters such as Darcy number, Reynolds number, Nusselt number, etc, as discussed in the paper. All these factors are put together to contribute in enhancing the effectiveness of Heat Pipe Ground Source Heat Pumps.
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