Performance investigation of a quasi-saturated compression high-temperature heat pump system with water-injection

Water vapor high-temperature heat pump (HTHP) combines the environmental friendliness of natural water and the energy-saving of heat pump, which fully exerts the performance advantages of water working fluid in the field of HTHPs. Injecting the atomized water or low-enthalpy gas-liquid mixture into the compression chamber can not only effectively reduce the exhaust temperature and superheat at the end of compression, but also contributes to the overall performance improvement of the system. The theoretical analysis and experimental investigation of the effect of water-injection cooling on the system performance were carried out. It was concluded that the increase in number of water-injection is beneficial to the system performance. However, when the number of water-injection exceed 5, the improvement of the system performance will be very slow. When the number of water-injection reached 20, COP increased by 12.05-19.27% compared with no water-injection. The experimental results also indicate that water-injection can be used to realize the quasi-saturated compression process, which will help decrease the discharge superheat and ensure the long-time safe operation of the water vapor HTHP. When the water-injection flow rate ensures the quasi-saturated compression process of the system, then increasing the injected water will not have a significant impact on the system performance. Under the conditions of 80 °C evaporation temperature and 125 °C condensation temperature, when the system operating condition is stable, and the water-injection flow rate is gradually reduced from 90L∙h−1, the system energy consumption is 49 kW, the heating capacity is 172 kW, and the COP is 3.5.