Performance analysis of heat pump water heating system with cascade utilization of waste heat from wastewater.

Wastewater heat pump water heater (WHPWH) can be used to recover waste heat from industrial or domestic wastewater with high efficiency. In this study, a WHPWH system was proposed and its performance was theoretical analyzed. The proposed system includes a preheating heat exchanger and a water source heat pump. The fresh water is preheated by wastewater then heated to the desired temperature by water source heat pump. Thus, the waste heat of wastewater can be cascaded recovered through preheating the fresh water and being the heat source of the heat pump. A thermodynamic model was developed to evaluate and optimize the performance of the proposed WHPWH. The energetic analysis was carried out to investigate the influences of the temperature of the wastewater, the inlet and the outlet temperatures of fresh water. The results were compared to the system which only uses wastewater to preheating or as heat source of heat pump, and showed a significantly higher waste heat utilization rate.