Research on the energy efficiency improvement of closed loop wastewater source heat pump with direct-expansion heat exchanger

Abstract

The closed-loop wastewater source heat exchanger (WSHE) has advantages such as being non-clogging and resistant to corrosion, but their heat transfer efficiency is relatively low, therefore it is still not the mainstream form of heat exchanger of wastewater source heat pump (WSHP). A novel CO2 direct-expansion wastewater source heat exchanger (DX-WSHE) is proposed in this paper, aiming to significantly improve the heat transfer rate of the heat exchanger and the energy efficiency of the heat pump system. The direct-expansion heat transfer process is complex, large grid number and long calculation time will be required if calculated by conventional numerical simulation software. Therefore, a three-dimensional transient heat transfer model was developed based on the thermal resistance and capacity model, which offers the advantages of fast computation speed and strong flexibility. The heat transfer and energy efficiency characteristics under different parameters were studied. The coefficient of performance (COP) of the direct-expansion wastewater source heat pump (DX-WSHP) can be increased by 31.0% compared to conventional design, and the heat flux of the direct-expansion wastewater source heat exchanger can be up to 250 W/m for the standard group. Compared with conventional designs, the direct-expansion wastewater source heat pump has excellent heat transfer and energy efficiency capabilities, which is a promising building energy saving and emission reduction technology.