Performanceof carbon dioxide direct-expansion geothermal heat pump using expander and internal heat exchanger

In this paper, the effect of expander and the internal heat exchanger is investigated in transcritical carbon dioxide direct-expansion geothermal heat pump. In this regard, a comparison is performed between four cycles. The four cycles are: (1) the cycle with expansion valve, (2) the cycle with expander, (3) the cycle with expansion valve and internal heat exchanger, (4) the cycle with expander and internal heat exchanger. The present numerical model has been investigated performance analysis of the four cycles under study in different operating conditions in two district cases. The first study includes a specific heating load, and the second study is a constant evaporator loop length (ELL). Then model evaluates characteristics including coefficient of performance (COP), evaporator loop length and heating capacity of the four cycles under study. To examine the performance of the four cycles, a parametric study is performed to investigate the effect of different parameters such as difference between soil temperature and evaporator outlet temperature, water inlet temperature, water mass flow rate, gas cooler length. The results indicate that COP associated with the expander cycle is always higher than the values related to the expansion valve cycle. The use of internal heat exchanger in a cycle including an expansion valve always leads to a slight increase in COP. An internal heat exchanger has negligible effect on COP in the cycle with an expander but reduces the evaporator loop length.