Experimental and numerical study on the cooling performance of a new earth-air heat exchanger(EAHE) system with a supply air static pressure chamber

. Earth-air heat exchanger (EAHE) is a low-carbon device that use renewable energy to improve the thermal environment of buildings by heating/cooling the air with the help of the thermal inertia of the soil. However, the traditional earth-air heat exchanger has the problem of large fluctuations in air temperature with the outdoor air temperature fluctuations. A system that combines an air supply chamber with an EAHE is proposed to address this problem. The outdoor air first enters the underground air supply static pressure chamber for initial cooling, and then enters the EAHE treatment. To investigate the effect of this air supply static pressure chamber on the cooling performance of EAHE, a three-dimensional numerical model was developed in ANSYS Fluent and validated with field measurement data. Then comparative studies of the EAHE system with a supply air static pressure chamber and the traditionnal EAHE system were conducted under summer conditions in Chongqing (China). The results show that the air supply static pressure chamber can reduce the EAHE outlet air temperature fluctuation by 31.98% at the maximum under the continuous operation of different air volume systems for 7 days. At the same time, the cooling capacity can be increased by 19.89% at maximum compared with the conventional EAHE cooling capacity.


Introduction
In response to global climate change, China has proposed carbon peaking and carbon neutrality goals. An important way to achieve this goal is to increase the utilization of renewable energy in buildings. Among the various types of renewable energy, geothermal energy is widely used as a system of the building at the other end. With the help of the soil temperature stability, the EAHE can be used for heating and cooling in winter and summer.
Although EAHE has been reported to have great potential for energy saving and application [5,6], there are still some important issues that need to be solved in practical applications. Because most of the existing EAHE pipes are buried at a depth of 2-4m [7], the soil temperature at this depth is easily affected by weather and other factors, resulting in large temperature variations. Therefore, the EAHE outlet temperature will increase and fluctuate more in summer due to the increase of soil temperature and the fluctuation of outdoor temperature [8]. This reduces the potential for

Experimental
set-up and numerical model

Experimental set-up
The experimental setup is designed and illustrated in Fig .1(a). The part in the dashed box is the focus of this paper's research and shown in Fig .1(b). The air supply static pressure chamber consists of two parts, above and below ground.
The above-ground part has an outdoor air inlet with dimensions of 2.8m(length)×0.8m(height).   The innovative earth-air heat exchanger system was tested in the city of Chongqing (a hotsummer/cold-winter region). The air temperature is measured using temperature and humidity measuring instrument (testo174H) with a permissible error is ±0.5°C. The air velocity is measured by a thermal anemometer (testo425) having range of 0-20m/s with an accuracy of ±0.03m/s +5% reading.

Numerical model
In this study, the air supply static pressure chamber and EAHE were divided into two parts

Governing equations
The basic governing equation in CFD can be represented by a general equation [9]: Where ߩ is density; ‫ݑ‬ is the velocity component in ݆ direction; ∅ represents the common variables of interest, i.e., three velocity components, temperature, species, turbulent kinetic energy, and its dissipation rate; ߁ ∅ is the transport coefficient dependent on ∅; and ܵ ∅ is the source term dependent on ∅.

Boundary conditions and initial condition
For the air supply static pressure chamber calculation model: The airflow inlet and outlet are adopted as the pressure outlet and velocity inlet boundary conditions respectively; the air inlet temperature is introduced on an hourly rate using the UDF; In the above equations, ߣ ௦ is the thermal  Initial conditions The initial conditions of soil temperature are related to the depth y: where ‫ݕ‬ is the depth of the subsoil; ܶ ௦ is the annual average temperature of the ground surface; ‫ܣ‬ ௦ is the annual amplitude of the surface temperature variation; ܽ ௦ is the thermal diffusion coefficient of the soil; ‫ݐ‬ ௬ is the annual period of the surface temperature wave; ߱ ௬ = ‫ݐ/ߨ2‬ ௬ is the annual fluctuating frequency; y is the annual period; and ߮ ௬ is the annual phase constant of the ground surface.

Mesh independence test
The two physical models were meshed using structured meshing in ICEM_CFD software. outlet of the air supply static pressure chamber is the EAHE air inlet, and the test results are shown in Fig. 3. It can be found that the maximum cooling of the air supply static pressure room is about 7.3 when the outdoor temperature is higher, which can indicate that the air supply static pressure room has a strong precooling ability for the outdoor air before entering EAHE.

Cooling performance of SASPC-EAHE
The SASPC-EAHE system and the traditional

Conclusions
In this study, a new EAHE system called SASPC-EAHE is proposed, which works on the principle that the outdoor air enters the buried air supply static pressure chamber for initial cooling and then enters the EAHE for heat exchange. To evaluate the performance of the system, a field test bench was built and CFD numerical simulations were performed for different operating conditions.
The experimental results show that the air supply static pressure chamber in the new EAHE system can significantly reduce the outdoor air temperature during daytime. In addition, the simulation results show that compared to the traditional EAHE system, the SASPC-EAHE can reduce the outlet air temperature fluctuation by a maximum of 31.98% and increase the cooling capacity by a maximum of 19.89% in the simulated conditions. However, it should also be noted that the performance of the supply air static pressure chamber deteriorates at higher air volumes of treated outdoor air. Therefore, subsequent studies will investigate the maximum handling capacity of the supply air static pressure chamber and the optimal matching solution with EAHE.