Energy and exergy investigation of a carbon dioxide direct-expansion geothermal heat pump

In this paper, a numerical simulation model has been extended for analyzing the energy and exergy of direct-expansion geothermal heat pumps (DX-GHP) in domestic hot water and heating applications. Carbon dioxide (CO2) in transcritical cycle has been used as the refrigerant and the effects of the refrigerant pressure drop in the heat exchangers have been considered. The present numerical investigation gives the energy and exergy analysis under different operational conditions in two separate cases. In the first case, the ground heat exchanger loop length is considered to be constant and in the second case, the heating load is considered to be fix. With the aim of evaluation the system work performance, a detailed parametric investigation has been carried out on the effect of different parameters including the difference between soil temperature and the outlet temperature of the evaporator, the compressor speed, the temperature and the mass flow rate of inlet water to the gas cooler, the gas cooler length, the number of ground heat exchanger loops, and soil temperature. The results of this study include heating capacity, coefficient of performance (COP), exergy efficiency, ground heat exchanger loop length can be used for design and optimization of a DX-GHP.