Research On The Thermal Performance Of High Power CO₂ Heat Pump

CO₂ trans-critical cycle heat pump system has unique advantage in the application field of heat pump water heater owing to the advantages of safety, environmental protection, etc, as well as its high exhaust temperature. At present, small power CO₂ heat pump water heater has been applied at home and abroad in a certain degree. However, due to the limitations of the power of high pressure compressor and the capacity of high pressure throttle device and high pressure gas cooling, world-wide academic researches on large power CO₂ trans-critical cycle heat pump is few. The development of high power CO₂ trans-critical cycle heat pump hot water system is of great significance to promote the engineering application of CO₂ trans critical cycle heat pump. Under this background, this paper constructs the experimental device of high power CO₂ trans critical cycle heat pump hot water system, and systemically studies the thermal performance of high power CO₂ cross critical cycle heat pump hot water system.To improve the power of high pressure compressor, the volume of high pressure throttling device and the capacity high pressure gas cooling to increase the power of CO₂ trans-critical cycle heat pump heating, parallel operation of CO₂ compressor, high-pressure plate type air cooler, triple capillary parallel throttle method are adopted, and regenerative cycle high power trans critical CO₂ heat pump water heater experiment device with nominal heat of 175kW is constructed. The effects of climatic parameters and operating parameters on the steady-state thermal performance of the CO₂ heat pump hot water system were tested and analyzed under the condition of constant temperature environment laboratory. The test results show that: compared with the other working fluid heat pump hot water system, high power trans critical cycle CO₂ heat pump hot water system has a higher heating temperature, and the heating temperature has little effect on the system performance coefficient COP. In order to meet the requirements of CO₂ heat pump system for working fluid flow and pressure drop under different working conditions throughout the year, triple capillary parallel throttle method is put forward. High power trans-critical cycle CO₂ heat pump with three capillary parallel combination throttle is simulated and calculated, and throttle performance as well as thermal performance under different temperature conditions are experimentally studied. Simulating calculation and experimental test results show that the requirements of CO₂ heat pump system for working fluid flow and pressure drop under different working conditions throughout the year are met by adopting different throttle mode and adjusting the heating temperature according to environmental temperature, capillary flow rate and pressure drop between the inlet and outlet can be adjusted between 0.138～0.736kg/s and 5.96-8.34MPa respectively. The suitable heating temperature of capillary combination throttling is 55～60℃, 70～55℃,80～55℃and 55～80℃ respectively under the conditions of low temperature, middle low temperature, middle temperature and high temperature environment. The CO₂ heat pump hot water system with capillary combined throttle trans-critical cycle has excellent high temperature performance, and the performance coefficient of the system can reach over 3.1 when the heat temperature is 90 ℃.The performance of high power trans-critical cycle CO₂ heat pump is tested and analyzed under various typical climatic conditions in mild regions. The test results of average daily operating performance of the system show that if the operating parameters are reasonably selected according to the climatic conditions, the system could have excellent thermal performance. The heating temperature of the system can be selected at 60-85 ℃, and daily average coefficient of performance COP is between 3.45-4.04 in the climate condition of the ambient temperature 4.1-27.3 ℃. In addition, the high power trans-critical cycle CO₂ heat pump is applied to the central hot water system in the cold area, and the results show that the system has good low temperature thermal performance.