Investigation On Cascade Heat Pump Water Heater With Thermal Storage System

With the development of economy and elevation of People's living level, the over-exploitation of energy, the use in low efficiency, which have resulted in environment pollutions serious day by day, have became adverse factors of restricting economic development and social progress. The technology of thermal storage, which was able to solve the unbalance of energy supply and demand in time and space, has become an effective way in rational use of energy and reducing pollution. Concerning with the traditional gas, electric water heaters,Heat pump water heater have more energy-saving effect. Thermal storage with phase change has become a new hot in energy using because of its advantages such as high energy storage density, charging with a constant temperature, easier to control and so on.By theoretical simulation and experimental analysis, this paper study the phase change thermal storage system and its characteristics of phase-change material(PCM) of a cascade heat pump water heater with PCM, when immersing the PCM heat-exchanger into the container and taking the two as a whole. This research includes the tasks below: selecting the refrigerates of high and low temperature cycles and designing the components for cascade heat pump water heater; study the properties of the applicable PCM; theoretical simulation of the phase-change thermal storage system for the cascade heat pump water heater; thermal storing and exothermic parameters experimental apparatus of the thermal storage system.PCM's performance experimental results show that Na₂CO₃·10H₂O and are the best and thickener, of carboxymethocel nucleating agent optimal proportionNa₂CO₃·10H₂O is 3%-5%. Paraffin/expanded graphite is the best PCM for this heat pump water heater.The theoretical simulation results show that there exists optimum aperture in the PCM heat-exchanger. In the heat storing and exothermic processes, there is distinct temperature stratification between the water and PCM in the water tank, while it has obvious phenomenon of natural convection in the heat storage process.The experimental study results indicate that the highest water temperature lies on the central of the water tank (top of the condensing coils) in the thermal storage process; in the exothermic process, the thermal storage material has a function of stabilizing and slowing the decreasing rate of temperature of the outlet water. The hot water supplied by the thermal storage process can completely satisfy the requirements of the common family users. Moreover, it is also found that the experimental and theoretical results anastomosed well, with an acceptable error range.