Abstract
Geothermal energy is increasingly promising for residential use due to rising energy costs and environmental awareness. This work experimentally examines the impact of pipe distribution on the heat pump's performance at varying temperatures for both the incoming water and the ground. The pipes are buried in the soil, distributing them on layers of varying depths up to a depth of 2 m, separated by insulating layers. The quantities of heat gained in the evaporator and released in the condenser are calculated to determine the coefficient of performance of the heat pump. It was found that at the same temperature, the total heat loss in the soil is larger in the case of water entering from the bent pipe towards the great depth compared to water entering from the direct pipe, but the heat gain is larger in the case of water entering from the direct one.
1 Introduction
If the lukewarm heat from the surface of the earth at a measured depth is withdrawn and employed properly, a source of sustainable and free heat would have been obtained, and this is what was later called “ground heating” that helps homeowners to heat their homes. This has been done since the forties of the last century when the American inventor Robert Weber [1] realized that he could reverse the cooling process to extract heat from the ground, this system takes advantage of the fact that the ground maintains a relatively stable temperature in temperate regions, and a few meters away from the surface of the earth, the soil temperature remains close [2] from 10 to 23 degrees Celsius, and here a type of thermal machine called the heat pump can [3] that stable temperature be used to heat homes in the winter. In addition to absorbing heat, the heat into the ground can also drain [4] and here the heat pump acts as a cooling machine in the summer, so the earth's heat throughout the year – can benefit. Over the past ten years, the ground source heat pump system has expanded at a rate of more than 10% per year in more than thirty nations because of its many benefits, including high efficiency and the use of renewable energy [5].
Heating, ventilation, and air conditioning (HVAC) systems consume approximately half of a building's overall energy usage [6], which has been a major economic cost of life for people [7]. Improved energy efficiency of this system is therefore necessary to lower the building's energy consumption as well as the people's cost of living [8, 9]. Ground Source Heat Pumps (GSHP) may replace a traditional HVAC system and save a significant amount of money and energy [10]. Using renewable energy sources to support the air conditioning system is a successful approach [11]. Given its vast global reserves and renewable nature, geothermal energy has garnered significant attention as a clean and sustainable energy source around the globe [12].
Heat pumps operating on the principle of (soil-water) are considered desirable when the external temperature is moderate, but when the external temperature drops a lot, the value of the heat pump Coefficient Of Performance (COP) (soil - water) remains constant at certain values in the summer and winter seasons [13].
An experimental investigation was carried out by Bhuiya et al. [14] to determine the impact of adding a double reverse twist band to the heat exchanger tube on fluid friction properties and heat transmission. The experimental findings demonstrated that increasing the double anti-twist band's torsion ratio will increase the heat exchanger's efficiency of heat transmission. After helical coils were inserted into a twin tube heat exchanger, Reddy and Rao [15] conducted an experimental investigation of the heat transfer and flow properties of titanium dioxide nanofluid. The heat transfer coefficient and friction coefficient of the nanofluid were both enhanced by the helical helix's insertion, according to the experimental findings. The convective heat transfer efficiency of a finned twin tube heat exchanger operating in a laminar flow was studied statistically by Syed et al. [16]. Based on the numerical results, it is possible to increase the heat transfer rate and increase the energy efficiency of the heat exchanger by introducing the tip-base angle ratio parameter. By standardizing products, including the design and construction of Standing Column Well (SCW) underground heat exchangers, Kim et al. [17] established the shape of the heat exchanger and developed a method for measuring the underground thermal conductivity of these heat exchangers in order to create a standard for the heat exchanger's design capacity.
The effects of circulation water temperature, borehole heat resistance, and underground heat conductivity on various operational and design variables of SCW underground heat exchangers are studied. Of these, it has been found that the bleed has the biggest impact on the enhancement of heat transfer for SCW underground heat exchangers [18]. The energy equilibrium using thermal response test data has also been examined [19], and several subsurface heat exchangers utilizing thermal response tests are currently being studied in Korea and abroad [20].
According to an analysis's findings [21], the temperature change of the water circulated by the underground heat exchanger tended to decrease as the bleed rate rose, and at 0%–30% bleed rate, respectively, the subterranean thermal conductivity improved from 0% to 179%. The temperature change of the underground heat exchanger circulation water initially tended to rise over time as a result of the groundwater being introduced from the underside of the heat exchanger and mixing with groundwater from the bottom after exchanging heat with the ground. But when the bleed rate rose, the rise slowed down and eventually stopped altogether if the bleed rate was more than about 10%.
To complete the research, a ground heat exchanger consisting of three layers of tubes at depths 1–1.5–2 m is studied, and then the amount of heat released and absorbed from the soil is measured [22]. And the layers of the pipes are also isolated by two layers of soil, between which there is a layer of wooden boards.
The process of distributing the pipes in layers provides us with an innovation for what is called the thermal reflective valve for the ground heat pump. Through the change in the direction of water entry into the graduated or direct pipe, it is possible to control any mode in which the pump is intended to work, the cooling, or heating mode, as experiments show that each side enters the water. Paper [23] provides a better thermal COP than the other side, according to the temperature of the water and the ground in two different heating and cooling systems. The importance of this research is reflected in improving the COP of the ground heat pump, especially in light of the high costs of energy production as well as the increase in carbon emissions, which negatively affects the environment by devising a new method for distributing the tubes in the soil to ensure an increase in the amount of heat absorbed in the heat exchanger when it works as an evaporator in the heating circuit and increases the amount of heat released from the exchanger to the soil when it works as a condenser in the cooling circuit and thus reduces consumption of electrical energy and increases the efficiency of the ground heat pump.
2 Research methods and materials
The special floor heating pipes and thermal insulation materials, in addition to temperature sensors to conduct the experiment are used, and then the experimental results are collected to make the charts.
2.1 Experiment steps
Making a hole in the soil at 2 m depth using special tools while maintaining the stability of the soil;
Extending the pipe horizontally in a spiral distribution over the soil at a depth of 2 m at the bottom of the hole, then covering the pipe with a layer of soil, then a layer of wooden panels, and finally putting a second layer of soil on those panels as it is shown in Fig. 1;
Putting a second layer of pipes with a spiral distribution over the soil at a depth of 1.5 m;
Cover the second layer of pipes with a layer of soil and a layer of wooden panels as it is own in Fig. 2.
Repeating the process for the last time at a depth of 1 m then putting the soil up to the surface;
Put temperature sensors in the hole at each layer at the three depths 1–1.5–2 m is shown in Fig. 3 to measure the soil temperature as it is shown in Fig. 4.
Extending the pipe horizontally in a spiral distribution over the soil at a depth of 2 m at the bottom of the hole, a) wooden panels and b) heat exchanger
Citation: Pollack Periodica 2025; 10.1556/606.2024.01192
a) Putting a second layer of soil on the panels and b) covering the pipes with a second layer of soil and wooden panels
Citation: Pollack Periodica 2025; 10.1556/606.2024.01192
A diagram of the distribution of pipes inside the soil
Citation: Pollack Periodica 2025; 10.1556/606.2024.01192
Putting the sensors at different points inside the hole
Citation: Pollack Periodica 2025; 10.1556/606.2024.01192
Finally, a heat exchanger embedded within the soil is obtained, and its pipes are extended in layers with a spiral distribution, separated by an insulating layer of soil and wooden panels. This heat exchanger has two directions for water to enter it, one of which is direct, reaching the bottom of the hole, and the other is bent.
3 Results and discussion of the experimental study
3.1 The effect of the water direction entering the pipes
In case of water entry from the bent pipe the difference between the water inlet and outlet temperatures is larger than in case of water entry from the direct pipe, when the ground temperature Tg is smaller than the temperature of the incoming water Ti as it is shown in Fig. 5.
The temperature difference between the inlet and outlet when Ti > Tg in case of two different directions of water entry and at three different temperatures A, B, and C for the incoming water and the ground
Citation: Pollack Periodica 2025; 10.1556/606.2024.01192
Here, when the ground temperature Tg is larger than the temperature of the incoming water Ti in case of water entry from the bent pipe, the difference between the water inlet and outlet temperatures is smaller than in case of water entry from the direct pipe as it is shown in Fig. 6.
The temperature difference between the inlet and outlet when Tg > Ti in case of two different directions of water entry and at three different temperatures A, B, and C for the incoming water and the ground
Citation: Pollack Periodica 2025; 10.1556/606.2024.01192
When the ground temperature Tg is smaller than the temperature of the incoming water Ti and for the three different temperatures for the incoming water and the ground, the total heat loss in the case of water entry from the bent pipe is larger than the total heat loss in the case of water entry from the direct pipe as it is shown in Fig. 7.
Total amount of heat absorbed by the soil when Ti > Tg in case of two different directions of water entry and at three different temperatures A, B, and C for the incoming water and the ground
Citation: Pollack Periodica 2025; 10.1556/606.2024.01192
Figure 8 shows that when the ground temperature Tg is larger than the temperature of the incoming water Ti and for the three different temperatures for the incoming water and the ground, the total heat gain in the case of water entry from the bent pipe is smaller than the total heat gain in the case of water entry from the direct pipe.
Total heat gained from the soil when Tg > Ti in case of two different directions of water entry and at three different temperatures A, B, and C for the incoming water and the ground
Citation: Pollack Periodica 2025; 10.1556/606.2024.01192
Table 1 shows the results of the experiment at different directions of water entry, where there are two outside temperatures, Tsh in the shade and Ts under the sun.
Heat loss through the 1 m 2 of the four types of walls in the transient case and the energy cost in USD in addition to the payback time in years
| Direction of water entry | Ts (C⁰) | Tsh (C⁰) | Ti (C⁰) | To (C⁰) | To - Ti | Q (W) | Tg | The flow (m3 s−1) | |||
| The depth (m) | |||||||||||
| 2 | 1 | 0.5 | |||||||||
| Ti > Tg | Bent pipe | 27 | 24.0 | 23.76 | 23.30 | −0.46 | −215.0 | 22.0 | 22.6 | 22.9 | 0.112 |
| 44 | 33.0 | 26.25 | 25.60 | −0.65 | −303.0 | 24.9 | 26.6 | 26.2 | 0.112 | ||
| 37 | 34.0 | 30.90 | 28.30 | −2.60 | −1212.0 | 25.2 | 26.0 | 27.00 | 0.112 | ||
| Direct pipe | 27 | 24.0 | 23.63 | 23.46 | −0.17 | −79.0 | 22.0 | 22.6 | 22.9 | 0.112 | |
| 44 | 33.0 | 26.25 | 25.75 | −0.50 | −233.0 | 24.9 | 25.6 | 26.2 | 0.112 | ||
| 37 | 34.0 | 30.90 | 28.56 | −2.34 | −1090.0 | 25.2 | 26.0 | 27.00 | 0.112 | ||
| Ti < Tg | Bent pipe | 37 | 31.7 | 19.00 | 21.00 | 2.00 | 932.3 | 26.5 | 26.7 | 26.9 | 0.112 |
| 29 | 27.6 | 21.05 | 22.15 | 1.10 | 514.9 | 25.2 | 26.0 | 26.00 | 0.112 | ||
| 23 | 22.1 | 23.00 | 23.40 | 0.40 | 186.8 | 25.1 | 25.0 | 25.00 | 0.112 | ||
| Direct pipe | 37 | 31.7 | 19.00 | 22.30 | 3.30 | 1538.0 | 26.5 | 26.7 | 26.9 | 0.112 | |
| 29 | 27.6 | 21.05 | 22.95 | 1.90 | 889.5 | 25.2 | 26.0 | 26.00 | 0.112 | ||
| 23 | 22.1 | 23.00 | 24.00 | 1.00 | 467.1 | 25.1 | 25.0 | 24.00 | 0.112 | ||
Figures 9 and 10 show the COP of a heat pump operating according to the principles of a heat pump and a cooling machine at two different water inlet directions and three different temperatures for both the incoming water and the ground.
The COP geothermal heat pump, which operating according to the refrigeration circuit in case of two different directions of water entry and at three different temperatures A, B, and C for the incoming water and the ground
Citation: Pollack Periodica 2025; 10.1556/606.2024.01192
The COP geothermal heat pump, which operating according to the heating circuit in case of two different directions of water entry and at three different temperatures A, B, and C for the incoming water and the ground
Citation: Pollack Periodica 2025; 10.1556/606.2024.01192
4 Conclusion
The average total heat loss in the case of water entering from the bent pipe is 33% larger than the case of water entering from the direct pipe. The average total heat gain in the case of water entering from the direct pipe is 47% larger than the case of water entering from the bent pipe. The idea of temperature gradient, which is obtained in the summer season in the case of water entering from the bent pipe, achieved the highest values of temperature differences and the amounts of heat loss from the ground heat exchanger to the soil, thus the best improvement in percentage of the COP of the heat pump in the case of cooling. But in the winter season, the case of water entering from the direct pipe achieved the highest values of temperature differences and the amounts of heat gained from the soil to the ground heat exchanger, and thus the best improvement in percentage of the COP of the heat pump in the case of heating. This ground heat pump circuit can be used in practical applications in greenhouses, poultry farms, and some residential apartments.
In the immediate future, the research will be going to continued and work on finding other geometric forms for the distribution of pipes within the soil that can achieve a better COP for the ground heat pump. In the near future, make modeling and mathematical simulations of this experiment are planned and compare the numerical results with the experimental results in order to be able to study other parameters, as the surface area of heat exchange.
Acknowledgment
The research was funded by the EKÖP-24-4-I funded from the University Research Scholarship Program 2024/2025.
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