Heat transfer performance of the underground CO2 pipe in the direct expansion ground source heat pump

Under the refrigerating condition, the pipe circulating CO2 is directly buried underground. It acts as the gas cooler releasing heat from the refrigerant to the soil directly. The heat transfer performance of the CO2 gas cooler is very important to the performance of the direct expansion ground source heat pump. The heat transfer process at the gas cooler is analyzed. Based on that, the mathematic model of the buried pipe is established using energy balance method. Then the influence factors including shank spacing, thermal conductivity of soil and backfill material, inlet temperature and mass flow rate of refrigerant are studied. The results show that the heat transfer performance of the underground CO2 pipe has a tendency to increase with the factors. The results also show that thermal short circuiting exists between the shanks of CO2 gas cooler. (C) 2017 The Authors. Published by Elsevier Ltd.