Study On Heat Transfer Characteristics Of The Energy Storage Heat Exchanger In Heat Pump Water Heater

With the rapid development of economy and the improvement of people's living standard, the problem of energy and environment become more and more serious. One of the important ways to realize energy conservation and sustainable development is to develop new energy-saving technology. Energy storage technique can solve the contradiction that the energy supply and demand don't match in time and space, so it's a good way to improve energy efficiency. Heat pump technique is widely applied in many fields with its advantage of energy saving and environment-friendly. Its research on the hot water for domestic is becoming more and more important. Developing heat pump water heater with enerage storage system to combine the phase change thermal storage technology and heat pump technology will have a better future.The thesis focuses on the energy storage heat exchanger in heat pump heater. The heat transfer characteristics of the energy storage heat exchanger have been deeply researched by using the methods combined with theoretical analysis, experimental testing and numerical simulation in this paper. The main research work and conclusions include the following:1,On the grounds of the operation principle of the heat pump water heater with energy storage system and the selection principle of the phase change materials (PCMs), suitable PCM is chosen. A fin-tube energy storage heat exchanger is designed based on the total heat balance of the heat pump water heater and its concrete structure parameters are determined. The theoretical analysis on the heat transfer characteristics of the energy storage heat exchanger was presented and a theoretical model of composites was used to analyze the thermal conductivity of PCM. It is found that the thermal performance improvement of PCM is the main way to improve the heat transfer performance of energy storage heat exchanger.2,The enhanced heat transfer of PCM was studied by adding nano-copper to paraffin PCM to product nanocomposite phase change material(NC-PCM). Phase change temperature and latent heat of NC-PCM were measured with Differential Scanning Calorimetry (DSC). The DSC results reveal that the melting and solidification latent heats of NC-PCM shift to lower values compared with those of pure paraffin, however, the melting and freezing temperatures were almost the same as pure paraffin. The thermal conductivity of NC-PCMs was measured by a Hotdisk Thermal Constants Analyzer. The thermal conductivity of liquid and solid NC-PCM is enhanced approximately nonlinearly with the mass fraction of the copper nanoparticle. After 100 thermal cycles, The result indicates that the NC-PCMs have a good energy storage characteristic.The experimental study on the phase change of NC-PCM was carried. The temperature-time tests were conducted in order to verify the improvement of heat transfer rate in the presence of Cu particles. The Infrared Heat Camera was used to observe the temperature distribution in melting and solidification process. The results show that the heating and cooling rates of PCMs were significantly improved upon the addition of Cu nanoparticles.3,The performance of the energy storage heat exchanger of the heat pump water is tested. The internal temperature field and outlet water temperature of energy storage exchanger were measured by simulating the heat pump water heater working conditions. In the total heat charging and discharging stage, the heat transfer is a complex non-steady state process. The temperature of the outlet water is tested. The result shows the water discharge is higher, the heat release time is shorter. The heat releasing of PCM is mostly in form of latent heat, so the temperature of the outlet water maintained at a constant temperature range in most time. This is the advantage of utilizing PCM to store energy. The calculation results show that most of energy stored in the energy storage exchanger has been released. The heat utilization efficiency of latent thermal energy storage is higher than sensible heat of water.4,Based on the designed heat storage exchanger, physical model and mathematical model are built, and Fluent 6.2 is used to simulate the heat storage situation of the heat storage system. Different structural parameters influencing the releasing process were analyzed. The results show that the fin structure enhanced the process of phase change heat transfer, and various structural parameters of heat storage exchanger have an important effect on the heat releasing rate. The result can be direction and basis for the design optimization of the heat storage system.