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This paper presents a steady state simulation model to analyze a direct expansion ground source heat pump that uses carbon dioxide as refrigerant in a transcritical cycle. The analysis considers pressure drop characteristic in heat exchangers of the system. The present numerical model has been developed to examine the system in different operating conditions in two discrete cases. These cases include constant evaporator loop length and specified heating capacity. Then model evaluate the system based on COP and heating capacity in the first case and COP and evaporator loop length in the second case. To evaluate the performance of the system, a parametric study is performed to investigate the effect of different parameters such as difference between soil temperature and evaporator outlet temperature, compressor speed, water inlet temperature, water mass flow rate, area ratio heat exchanger and soil temperature. In both studies, It can be concluded that in a specific temperature difference of the difference between soil temperature and the outlet temperature of the evaporator, there is a maximum value for COP. Also, increasing the compressor speed and the temperature of inlet water to the gas cooler is accompanied with the diminution of COP. Increasing the mass flow rate of inlet water to the gas cooler leads to surge in COP. The results of this study include heat capacity, coefficient of performance and ground heat exchanger loop length can be used for design and optimization of a direct expansion geothermal heat pump.

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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.