Research On The Electrically Controlled Composite Phase Change Material And The Phase Change Material-enhanced Solar Heat Pump Hot Water System

Energy is the foundation of the development of human society.The demand for energy is in a great need in recent years.The green,clean solar energy and other renewable energy are receiving more and more attention.However,there exist several problems in the wide application of solar energy,such as the temporal asymmetry.The phase change heat storage technology can effectively solve the three main problems of solar energy,namely low energy density,uneven energy distribution,and intermittency,thus enables the effective storage and utilization of solar energy.This paper aims at developing a new solar heat pump hot water system using the combination of phase change material(PCM)and electrically controlled phase change technology.First,by modifying sodium acetate trihydrate(SAT),the electrically controlled composite PCM was fabricated for engineering applications.Then,the heat mass transfer characteristics of different electrically controlled composite PCM in a single tube was investigated experimentally and theoretically.Last but not least,the operating performance of the electrically controlled phase change material-enhanced solar heat pump water heater system was investigated.Firstly,a novel electrically controlled SAT composite was fabricated.The55%SAT+1%CMC composite PCM was prepared by mixing water,carboxymethyl cellulose(CMC)and SAT together.The new material demonstrated stable supercooling capability and could be effectively triggered by electric signals to achieve a long-term low-temperature storage of heat.To improve the heat conductivity of the PCM,foamed copper and expanded graphite were used.The experimental results showed that when the pore density of foamed copper(the porosity is 0.9)was less than 40 PPI(the average pore size≥0.635 mm)and the mass fraction of expanded graphite was less than 7%,and the stable supercooling property of the composite PCM was not affected.The thermal conductivity of the composite PCM increased with the pore density of foamed copper and the mass fraction of expanded graphite.The increase could be as high as 61.1% and 88.2%,respectively.Secondly,the experimental and numerical models of the component-level phase change heat storage tube were established.The heat and mass transfer characteristics of the composite PCM in the tube were investigated experimentally and numerically.Compared to the pure55%SAT+1%CMC,the thermal conductivity of the composite PCM and the heat release rate of the single tube could be improved significantly.The heat release time greatly reduced by19.2% and 23.1%.Finally,the operating characteristics of the electrically controlled phase change hot water system was investigated,and the effect of the operating parameters on the system performance was explored.The experimental results showed that the utilization rate of solar energy was 71%,and the system COP was 5.68 during the hot water system storage process between 9:00 and 16:30.The water heating power was 6.64 k W during heat release process between 18:00 and 20:30.What’s more,the influences of the flow rate on the COP of the system were analyzed.The results showed that the system COP increased with the increase in circulating water flow rate during the heat storage process,the power increases with the increase in circulating water flow rate in the heat release process.