Structural optimization of multi-pipe earth to air heat exchanger in greenhouse

Abstract Multi-pipe parallel earth to air heat exchangers (EAHE) could be used to heat or cool air by the utilization of geothermal energy. As a renewable energy, it could decrease CO2 emissions to be carbon neutral. However, energy-usage efficiency of EAHE was low, its structure and thermal performance still need to be improved. This paper aimed to optimize the structure of the EAHE in greenhouses. A numerical model of Multi-pipe parallel EAHE was first established and verified by experimental data. The evaluation factors such as airflow division uniformity coefficient and integrated evaluation factor were proposed to compare the uniformity and thermal performance of EAHE, respectively. Finally, the influence of structure parameters on the thermal performance of EAHE was compared, including the spacing between pipes, the depth of pipes, and the angle of branch pipe entering airflow. Results showed that when the spacing between pipes and the depth of pipes were 1.2 m and 3 m, the heat exchange rate was the largest and the air distribution was more uniform. When the angle of branch pipe entering airflow was 75°, the optimal integrated performance appeared. The research results could provide references for the optimization and development of EAHE in greenhouses.