| With the continuous and steady development of economy in China,the market of domestic heat pump water heater(HPWH)has expanded rapidly and the penetration rate of HPWH has reached 50% especially in southern China.In order to make the system more compact,the design currently under study,however,seeks to eliminate the circulating pump by directly immersing the condenser in tank.This kind of structure avoids the energy loss caused by setting the condenser separately.This paper built an air-source HPWH which can basically achieve the annual 150L/50°C hot water to meet the annual hot water demand of a family of three.In this paper,the key issues of HPWH,the structure and performance optimization of condenser coil,and the performance of water-charging/discharging process were studied theoretically and experimentally.Firstly,a coupling model consisted of evaporator,compressor,expansion valve and water tank with immersed condenser coil was established.Based on the heat exchange between refrigerant in tube and water in tank,the overall vapor-compression cycle and water heater was coupled together.Heat transfer model of water tank was calculated by Fluent software and vapor-compression cycle was developed in MATLAB.Water temperature distribution and Nusselt number of natural convection were two main characteristics to evaluate the effect of condenser coil structure on heating performance of water tank.Taking the varied-diameter coil as an example,the influence of main structural factors including the coil pitch and the coil diameter on the performance of water tank were analyzed by coupling model.The results showed that combined properties of varied-diameter coil was better than that of fixed-diameter coil in temperature stratification and heat transfer.The simulation results also suggested that decreasing coil pitch and proper varied-diameter coil design would reduce temperature stratification and improve the heat transfer of condenser coil.At the same time,more attention should be paid to control the pressure drop and the cost of condenser coil in order to achieve optimal design.Varied-diameter coil was optimal designed.Then a prototype includes a set of air conditioner external unit using refrigerant R22 and a insulation water tank with immersed condenser coil was established in Nanjing.The prototype can produce 150L/50°C hot water and can be operated in water-charging and water-discharging mode.Under three typical operating conditions in Nanjing,the temperature of hot water in the tank under different inlet water temperatures was experimentally tested to verify the accuracy of the coupling model.The simulation results were in good agreement with the experimental values.Then,thermal insulation performance of the water tank was further tested.During the test,hot water tank was placed in natural environment to record the water temperature in each layer of the water tank.The results showed that the water tank has a good thermal insulation.After a good agreement with experimental data,the coupling model was able to reproduce water-charging and water-discharging process.Further study was performed to compare the influence of varied-diameter coil and fixed-diameter coil on temperature distribution,heat transfer characteristics and operating efficiency of HPWH system.In the water-charging process,when varied-diameter coil design was adopted,flow direction of refrigerant was changed in coil,and radius of curvature in varied-diameter coil became smaller and smaller,which lead to stronger secondary circulation.Therefore,heat transfer coefficient of the refrigerant in the tube was increased,so the average water temperature in tank was higher and maximum temperature difference in the tank altitude direction was smaller for varied-diameter coil design.According to the simulation,heat transfer coefficient and COP of varied-diameter coil were 20.00% and 10.23% higher than that of fixed-diameter coil,respectively.In waterdischarging process,heat transfer area of lower part in varied-diameter coil was larger than that of upper part so the cold water from the tank inlet could be well neutralized,preventing the water temperature from falling too fast and enhancing the protection of the hot water in upper part.It was found that difference of heat transfer coefficient and COP between these two coil designs was relatively small.However,the discharging efficiency and extraction efficiency of varied-diameter coil were 7.74% and 9.70% higher than that of fixed-diameter,respectively.Taking the heat pump water heater with varied-diameter coil as research object,operating characteristics at different inlet water temperatures and different water flow rates were compared and analyzed.In addition,performance under various operating conditions were tested by our experiments.The results showed that the lower the inlet water temperature was,the longer it took to heat the water to setting temperature.However,average heat transfer coefficient in the condenser side was larger and average COP of the entire heating process was greater.When water flow rate was smaller,the time when the hot water temperature dropped rapidly was delayed.This led to longer unit discharging-time,effective discharging-time,discharging efficiency and extraction efficiency.The air source heat pump water heater system built in this paper had a good overall performance.According to the measured data under various operating conditions,average COP of the system could be maintained above 3.68. |
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