Performance comparison of a vertical direct expansion geothermal evaporator: Part I, single U-pipe using different refrigerants

Abstract

Abstract CO2 is a promising candidate for replacing synthetic refrigerants used in direct-expansion ground-source heat pumps (DX-GSHPs). The present paper, as the first part of two, presents a numerical performance comparison of a vertical single U-pipe direct expansion geothermal evaporator using different refrigerants; namely R744, R410A, R22, R407C, R12345yf, and R134a. Each refrigerant was tested through a borehole heat exchanger (BHE) performance simulation at a constant rate of heat extraction. Operating parameters were chosen so that each refrigerant is subjected to a complete evaporation cycle, and a similar pressure drop from the inlet to outlet of the borehole. Several BHE parameters such as fluid temperature, pressure, mass flow rate, required pipe dimensions, and required power for fluid circulation were evaluated for different refrigerants. Finally, the overall performance of each refrigerant was characterized by the heat exchange capacity for a unit of mass flow rate and pipe surface area. Results showed that CO2 offers unique characteristics favoring the design of compact BHEs, as it required the smallest mass flow and pipe surface area. It is also concluded that the performance of a BHE with CO2 can be significantly improved through using unconventional configurations. This will be discussed further in the second part (PART II).