Application of the ground cooling system for rural houses air conditioning in northeastern China

: This study developed an appropriate ground cooling system that can be used for indoor cooling during summers in rural houses in northeastern China. To apply this system, a traditional rural house was selected and a heat exchanger was designed (PE pipe, 32 m in length), manufactured, and installed according to the cooling load (2206 kJ/h) of one room of the experimental house. According to the experiment, the temperature of Room 1 equipped with the cooling system was 3°C lower than that of Room 2 without the ground cooling system during the same time period. The temperature reduction effect of this system was confirmed through this study. When the ground cooling system was used in rural houses in Northeast China, it was found that a burial depth of 1.5-2.0 m for the ground heat exchanger is appropriate for considering workability and frost line. The use of the ground cooling system for indoor cooling during summers for rural houses in the northeastern region of China would reduce not only environmental pollution and the use of fossil fuels but also ease farmers off a certain amount of financial burden. China. 15(2):


Introduction 
Recently, in many countries globally, alternative energy resources are being developed to secure future energy sources and ensure environmental protection [1,2] . Additionally, to actively adapt to changes resulting from climate change conventions, new and renewable energy sources such as solar, wind, and geothermal energy are being developed [3] .
Of these sources, geothermal energy, which is a clean source of energy with high application potential not only for agriculture but also in other fields, remains constant throughout the year [4,5] . Underground heat can be directly used. However, the generally used method is to conduct a convenient heat transfer through a heat pump system. This is beneficial for intensifying the ground heat so that the ground can be used as a heat source for heating and as a heat sink for cooling purposes [6,7] . A ground cooling system can be categorized into two general types, depending on the heat source being used. The closed-loop type involves a heat exchanger installed underground, where cooling is circulated through an internal pipe. On the other hand, the open-loop type uses ground, underground water, or external air directly. Regarding the Earth's surface, the underground temperature hardly changes below the frost line throughout the year [8,9] . The annual average underground temperature below the frost line ranges between 10°C and 18°C. Thus, it has a high potential for use as a cold heat source for ground cooling systems, besides being highly stable. In fact, many studies on such ground cooling systems are being conducted globally [10,12] .
In Japan, a specific burial depth of 50 cm and an interval location of 50-60 cm of pipes for harnessing underground heat have been suggested [13] . Puri [14] studied the appropriate number of hours required for recharging using the finite-element method and interpreted a two-dimensional stationary-state equation using the boundary-element method. He reported that the stationary-state analysis was within a reasonable error range considering long-term performance. Gao et al. [15] presented a technical route for ground heat exchangers to help realize zero energy buildings, which provides a promising solution to improve energy efficiency of buildings through an experiment in China. Lekhal et al. [16] studied thermal performance of a residential house equipped with a direct solar floor and an earth-air heat exchanger. Javadi et al. [17] analyzed the performance of ground heat exchangers with different configurations.
Kim et al. [18] studied the heat transfer characteristics of the heat exchangers according to pipe laying intervals under the ground by the numerical analysis in South Korea. He emphasized that the installation interval of heat exchanger is very important. This is because the shorter the burial interval of the pipes, the greater the heat loss through the ground surface, thereby decreasing the efficiency of the underground heat exchange.
Furthermore, Park et al. [19][20][21] reviewed the heat-exchange amount, heat-exchange efficiency, coefficient of performance, and appropriate length of buried pipes to analyze the effects of atmospheric air on a ground heat exchanger. Because most of these studies were largely theoretical or the conditions were very different everywhere, it is not enough or suitable to use the above results for air conditioning of rural houses in northeast China [22][23][24] .
This study aims to develop a ground cooling system so that a geothermal energy source can be used for indoor cooling in rural houses in northeast China during summers and to propose various conditions for a ground cooling system.

Thermal energy transport
The underground temperature on the Earth's surface hardly changes throughout the year under a certain depth below the frost line of any location. The annual average ground-temperature range below the frost line is 10°C-18°C.
Through porous media, heat energy is transferred by conduction, convection, and radiation.
In this study, the considered means of transferring heat energy included forced convection (fluid flow) and heat conduction using Darcy's law [25] . The equation can be expressed three-dimensionally as follows: where, H is the heat flux, kJ; K e is the effective thermal conductivity, kJ/(h· m·°C); T is the temperature, °C; nρ w C w is the total heat amount of underground water (fluid) stored per unit volume of the underground medium; V is the velocity vector of the underground water; x, y, and z refer to the three-dimensional directions.
In the thermal conductive-convective equation defined above, the following assumptions can be made: underground water flow was in a state of equilibrium (i.e., steadily flowing), underground water current speed was 0, and the underground temperature was in a normal state. The amount of underground heat transfer can be expressed as [26] : where, H is the heat flux, kJ; K e is the effective thermal conductivity, kJ/(h· m·°C); T is the temperature, °C; nρ w C w is the total heat amount of underground water (fluid) stored per unit volume of the underground medium; x, y, and z refer to the three-dimensional directions; V is the velocity vector of the underground water.

Cooling load
Cooling load refers to the amount of heat that needs to be removed in correspondence with the indoor heat gain to maintain a constant indoor temperature and humidity. The total cooling load (Q c ) of the experimental building was calculated by the following equations [27][28][29][30] .
s is the amount of solar heat acquired through the window, kJ/h; Q c is the amount of heat obtained through the surface of the building, kJ/h; Q i is the amount of sensible heat acquired by the infiltrated wind to the building, kJ/h; Q oa is the amount of heat generated by the human body indoors, kJ/h; Q w is the amount of acquired latent heat, kJ/h. The amount of solar heat acquired through the window (Q s ) was applied according to various types (windows, entrances, etc.), the amount of heat conduction obtained through the surface of the building (Q c ) was calculated according to the following equation: where, U is the total heat transfer coefficient, kJ/(h· m 2 ·°C); A is the total surface area, m 2 ; DETD is the design equivalent temperature difference, °C.
The amount of sensible heat acquisition (Qi) by the infiltrated wind was calculated according to the following equation: where, V is the air change rate, times/h; T o is the outdoor air temperature, °C; T i is the designed indoor temperature, °C.
Also, the amount of latent heat acquisition (Q w ) to increase the humidity of the infiltrated wind to the level of the room was calculated according to the following equation: i is the air permeability, m 3 /h; W i is the relative humidity of indoor air, %; W o is the relative humidity of outdoor air, %.

Experimental outline
This experiment was conducted for about a month, starting from August 16 th , 2018, to September 15 th , 2018, in Shejia village (testing place), Liaozhong-xian, Shenyang City, Liaoning Province, China. The latitude, longitude, and altitude of the sites were 41°41′14″, 113°44′30″, and 82 m above sea level, respectively. Figures 1 and 2 show the cross-sectional view of the cooling system, which used a ground heat exchanger, and the floor plan of the ground heat exchanger installation, respectively.
As shown in the figures, the isolated loss cooling system is a type of ground cooling system that can decrease the indoor temperature by allowing outdoor air in the northern part of the building to go through the ground heat exchanger, thus guiding more cool air to the inside, using a fan, while ventilating the indoor air in the southern direction [31] .

Experiment material
The soil at the experimental site comprised sand (77%), silt (18%), and clay (5%) and had a water uptake rate of 39%. The soil's heat conductivity was 0.654 W/(m·°C). The polyethylene (PE) pipe used in the heat exchanger had an external diameter of 60.33 mm, an internal diameter of 52.5 mm, thermal resistance of 0.066 (m· h·°C)/kJ, and allowable stress of 0.80 MPa.
The total length of the ground heat exchanger was 32 m. Figures 3 and 4 and Table 1 show the values for the wall, roof structure, and materials of the experimental house, respectively. The house was a one-story building, facing south, as shown in Figure 5, and its residents included two adults and one child. Figure 6 shows the locations of the temperature sensors.

Experimental method
To develop an appropriate ground cooling system that can utilize the cool underground temperature for rural houses in northeastern China during summers, a typical house was selected and its cooling load was calculated. A PE pipe heat exchanger suitable for the cooling load of the house was then manufactured. The total length (32 m) of the heat exchanger was determined according to the workability and heat-exchange amount, and then the exchanger was buried underground (depth of 1.5 m), as shown in Figures 7 and 8.
To calculate the heat-exchange amount and heat-exchange efficiency, and to analyze the indoor temperature changes, the internal heat exchanger and changes in humidity were installed at nine locations, as shown in Figure 6. Using automatic data acquisition equipment, temperature and humidity data were measured, as shown in Figure 9.  Figure 10 shows the changes in monthly average ground temperatures for a year according to the ground depth from the surface at the experiment site. The lowest temperatures at depths of 1 m and 2 m were 11.9°C and 11.7°C, respectively, both of which occurred at the end of March due to a time lag. The highest temperatures were 18.3°C and 17.7°C, recorded at the beginning of October. At a depth of 5 m, the lowest temperature was recorded as 10.5°C during March and the highest temperature was recorded as 16.2°C at the end of November. Figure 10 Ground temperature changes The recorded differences in monthly average ground temperatures for a year between depths of 1 m and 5 m was 2.6°C. The highest and lowest temperatures at depths of 1 m and 2 m occurred simultaneously, whereas at the 5 m depth, the highest temperature was recorded one month later and the lowest temperature was recorded 2 months earlier. Due to the time lag, the temperature difference between the underground depths of 1 m and 5 m was not very significant. In addition, even in the hottest month of August, the temperatures at depths of 1 m and 2 m were both within 0.9°C, without a significant difference in temperatures. Figure 11 shows the daily-hourly average temperature change of each point according to time. R1T is the Room 1 temperature of the room where the cooling system was installed, whereas R2T shows the Room 2 temperature of the room without any cooling system, which was used as a control specimen for R1T. ITGHE indicates the interior temperature of the ground heat exchanger, whereas OAT shows the outdoor-air temperature.

Change in temperature and relative humidity
As shown in Figure 11, the times for which the highest temperatures were recorded according to the daily time changes were between 1:00-2:00 pm outside; between 4:00-5:00 pm in Room 1; at 3:00 pm in Room 2, which was 1-2 h earlier than that in Room 1; and between 4:00-5:00 pm in the interior of the ground heat exchanger, which was the same as that in Room 1.

Figure 11 Temperature change of various points
In Rooms 1 and 2, an indoor temperature exceeding 26°C continued for 15 h and 16 h, respectively. An indoor temperature exceeding 28°C continued for 0 h for Room 1 h and 5 h for Room 2. The highest, lowest, and average temperatures inside the heat exchanger were 19.3°C, 16.4°C , and 17.4°C, respectively, and the temperature difference between the interior and exterior of the ground heat exchanger was 7.7°C.
The temperature difference between the highest and lowest temperatures of the ground heat exchanger inside was approximately 3°C, and was very small and stable compared to the outdoor temperature and the temperature in both rooms. Because of the operation of the cooling system, the temperature difference between the two rooms was maintained for 12 h (from 9:00 am to 9:00 pm) during a 24 h period, whereas the indoor temperature in Room 1 was 0.9°C lower than that in Room 2, as shown in Figure  12. Figure 13 shows the relative indoor humidity change. Here, R1RH shows the relative humidity of Room 1 equipped with a ground cooling system, whereas R2RH shows the relative humidity of Room 2 without any cooling system. As shown in Figure 13, the humidity in Room 1 indicated little change over time, whereas that in Room 2 indicated almost no change. The highest and lowest recorded humidity levels in Rooms 1 and 2 were 44.4% and 43%, and 38% and 41%, respectively, whereas the average humidity levels were 41% and 42%, respectively. The average humidity difference in the two rooms was only 1%. It was concluded that the difference in the relative humidity was due to the coolness of the ground heat exchanger inside, which initially decreased the humidity, thus causing a much lower sensible temperature in Room 1 than that in Room 2.

Effects of the buried PE pipe material on ground heat exchange
Regarding the effects of the buried PE pipe material on the ground heat-exchanging performance, the pipe material does not have a significant impact on the heat-exchange efficiency according to the experimental study conducted by Kim et al. [32] Additionally, in a ground heat-exchanger performance analysis conducted by Go et al. [33] , the effect on heat exchange from changing the pipe materials to PVC, PE, or steel was only 1.1%-2.1%, which was very minimal. According to the experimental results, the heat conductivity of the actual soil was only 0.2-1.1 W/(m·°C), which was not large, and the heat-resisting effects during the heat transfer were minimal. Thus, it was concluded that the material should be strong enough to satisfy the requirement for withstanding the soil pressure.

Cooling load
As seen in Figure 12, there was almost no time period for a resident to feel very hot. Considering the indoor cooling design temperature at 28°C, the maximum cooling load of the experimental building calculated according to Equations (5)- (8) was 2206 kJ/h.

Conclusions
The application of the ground cooling system for cooling rural houses in northeastern China was studied, and the conditions for the ground cooling system were proposed.
The following conclusions can be drawn.
(1) Considering the indoor cooling design temperature at 28°C, the maximum cooling load was calculated 2206 kJ/h.
(2) According to the temperature change analysis per ground depth, a depth of 1.5-2 m should be adequate considering the workability, efficiency and frost line in northeastern China.
(3) The average temperature of the test room (Room 1) where the ground cooling system was installed was 3°C lower than that of the control room (Room 2), and the effectiveness of the application of the ground cooling system was proven.