Theoretical approach on the heat transfer and pressure drop in the ground heat exchanger of direct expansion geothermal heat pump system

To increase the performance of ground source heat pump (GSHP) system, a lot of researches have been conducted on the direct expansion type GSHP system. The direct expansion type has a benefit in that it can reduce the temperature difference between refrigerant temperature and ground temperature. Numerical methods are typically applied to analyze the heat transfer and pressure drop in the ground pipes. This paper takes theoretical approaches for the 2-phase heat transfer and pressure drop and suggests approximate solutions for them. The approximate solutions are function of properties and quality of refrigerant and will give guides in designing ground heat exchanger of direct expansion type GSHP.