| Photovoltaic integrated solar assisted heat pump water heater(PV-SAHPWH)is an effective device to apply renewable energy such as solar energy and reduce energy consumption in buildings.PV-SAHPWH uses photovoltaic/thermal(PV/T)evaporator to achieve a higher comprehensive utilization eff-iciency of solar energy.PV-SAHPWH has advantage of saving space and it was conductive to building integration.This paper built a PV-SAHPWH 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 PV-SAHPWH,the structure and performance optimization of PV/T tube-sheet evaporator,and the real-time variable capacity control system and control strategy were studied theoretically and experimentally.Firstly,the mathematical model of the main components of the system was established,and UL was defined based on the average temperature of the back plate of the photovoltaic collector plate.The detailed calculation formula of the heat absorption capacity of the PV/T collector was given.The PV/T collector model was suitable for PV modules using crystalline silicon battery or glass substrate thin film battery.The system simulation program was prepared using Visual Basic 6.0.Primary energy saving efficiency was used to evaluate the performance of PV/T evaporator.Taking the CuInGaSe(CIGS)PV/T evaporater as an example,the main structural factors,the inner diameter,the length and the spacing of the tubes,on the performance of the PV/T collector were analyzed by simulation program using Visual Basic 6.0.The results showed that smaller evaporation tube spacing,smaller diameter and longer length of evaporation tube will improve the thermal,electrical and primary energy saving efficiency of PV/T tube-sheet evaporator.Meanwhile,smaller diameter and longer length of evaporation tube will increase pressure drop of refrigerant through the collector.It’s necessary to control other local pressure drop to ensure the performance of the collector.A monocrystalline silicon battery PV/T collector evaporater was optimal designed.A prototype was established in Nanjing.The prototype includes 4.32m2 area of solar collectors using single silicon solar panels,1 HP variable frequency compressor using refrigerant R22 and a 150L insulation tank.The prototype can be operated in both fixed frequency and controlled variable frequency mode to produce electricity and 50℃ water simultaneously.Experiments on performance of the prototype were carried out and the model was verified based on the results of the fixed frequency operation tests and the no-load tests.Then,the real-time variable capacity control system and the corresponding control program of the PV-SAHPWH are developed by using electronic expansion valve,frequency conversion compressor,PLC,frequency converter,solar radiometer and thermocouple.The principle of control strategy was developed considering three rules:A)the operation strategy was set according to the classification of the climate conditions and operating frequency;B)the heat pump operates during the period when theoretical radiation intensity is greater than 200W/m2 in each month;C)the operating frequency of the prototype was dynamically determined according to the real-time climate conditions so that the water can be heated to 50°C within the operating period.The annual performance data at different climate condition and frequency was obtained by the verified model.Then,the control table of the prototype was set up.In some specific conditons,if the solar radiation is high and the initial temperature of the tank is lower than the ambient temperature,the evapration pressure may be higher than the condensation pressure.To solve the problem,the frequency strategy for the start-up phase was set up.Define the "transition water temperature" as the minimum tank temperature at which the system can run at the minimum operating frequency set by the strategy table.Considering the time difference between the peak of the daily ambient temperature variation and the daily solar radiation variation,effect of system startup time on net power generation was analyzed in the actual environment conditions.According to the simulation,it is helpful to reduce the energy consumption of the system and increase net power generation for the whole day if the system startup time delays in sunny day compared with the startup time determined according to the half expection heating time at noon 12 o’clock.This article’s startup time strategy is to postpone the system startup time for half an hour in sunny days.The control strategy table,the frequency strategy of the startup phase and the startup time strategy constitute the comprehensive control strategy of this paper.According to the theoretical analysis and simulation results,it is proved that increasing the frequency of the compressor isn’t conductive to increasing the net power generation for the whole day and can not realize the direct PV-drived SAHPWH under the current PV collector area and the tank volume ratio.The performance results of the prototype with proposed real-time control strategy were compared with another two variable control strategies through experiment and simulation.One compared strategy is that the frequency is determined based on the average monthly climate conditions,the other is based on the starting-up climate condition.The results show that the real-time control strategy is superior to other control strategies and can fulfil the heating task in time with less energy consumption.According to the measured data under various operating conditions,the coefficient of performance of the system preparing 150L/50℃ hot water,can be 3.06～5.61 in case of general and good solar radiation,can be 2.68 in case of cloudy day even sometimes light rain. |
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