Performance Study Of Two Photovoltaic/Thermal System With Anti-freezing Function

Photovoltaic/thermal(PV/T)system is one of the solar energy utilizaiton technologies that are being studied.It combines a solar PV module with a solar thermal collector in the same frame.By this way,the PV/T can not only enhance the electrical efficiency,but also generate thermal energy simultaneously.The use ratio of solar energy can be improved.Water is a common working medium using in the PV/T collector to transform the heat through the pipes connected to the absorber plate.However,the collector may be damaged in freezing temperature because of the volume expansion of the water during freezing.The drain back systems and anti-freezing fluid/water systems have been used to solve the problem.However,these measures are faced with other disadvantages too.They may increase the unreliability or cost of the systems.In this thesis,a PV/T with phase change material(PCM PV/T)and another PV/T with micro-channel heat pipe(MHP PV/T)have been established on the basis of the PV/T water collector.Because of the reasons that the PCM can absorb and reject a lot of heat during phase change process and the MHP uses anti-freezing fluid as working medium,they can improve the anti-freezing function of the PV/Ts.The main works of this thesis are shown as follow.1.A PCM PV/T collector and an MHP PV/T are designed and constructed.Their performance are studied by experimental works and compared with the water-pipe-base PV/T collector.The rigs can be used to test the collectors during daytime and night.2.Experimental works are proposed for both daytime mode and night mode.The results show that the average temperature of the PCM PV/T is the least because of the PCM module.However,the poor connection between the pipes and absorber plate in the MHP PV/T makes its temparture rise quickly than other two PV/T.The temperature of the PV cells of the PCM PV/T,MHP PV/T and the water-pipe-base PV/T can be up to 38.00?,56.28 ?,and 44.85 ? respectively.The daily electrical efficiencies of them are 12.1%,1 1.5%and 11.9%respectively.Because of the cooling function of the PCM model,the PCM PV/T performs better in electrical producing.However,since that part.of the heat is stored in the PCM model,the daily thermal efficiency of the PCM PV/T is lower than that of the water-pipe-base PV/T.The thermal efficiency of the three systems is 42.3%,38.6%and 44.5%respectively.As for the anti-freezing performance,the lowest and average temperature of the pipes in the PCM PV/T are-4.11? and 2.60 C.The lowest and average temperature of the pipes in the MHP PV/T are-7.44 C and-2.55 ?.The lowest and average temperature of the pipes in the water-pipe-base PV/T are-7.52 C and-3.42?.The beginning time of freezing in the PCM PV/T is the latest and the MHP PV/T uses the longest time during freezing.Both the PCM and MHP can improve the anti-freezing performance of the PV/Ts.3.A numerical simulation is established to study the performance of the PCM PV/T and the MHP PV/T in different modes.The results show that in the PCM PV/T the RMSDs of the temperature of the absorber plate,the output of the PV cells,the temperature of the PCM model,the water temperature in tanks and the temperature of the pipes are5.54%,2.41%,8.09%,3.74%and 1.23%respectively durnng daytime.As for the night part,the RMSDs of the temperature of the absorber plate,the temperature of the PCM model and the temperature of the pipes are 0.63%,0.69%and 0.59%respectively.In the MHP PV/T with daytime mode,the RMSDs of the temperature of the absorber plate,the temperature of the MHP,the temperature of the tank and the output of the PV cells are 3.98%,3.95%,2.01%and 4,92%.As for night mode in MHP PV/T,the RMSDs of the temperature of the absorber plate the temperature of the collector pipe are 1.51%and 1.58%repectively.The simulation model shows good agreement with the experimental results.4.The simulation model is used to study the influence of different factors on the performance of the PCM PV/T and the MHP PV/T.The performance of the novel PV/T systems with optimized characteristics can be improved obviously and free from freezing.A PV/T with tri-melt-temperature PCMs is proposed and studied with the simulation model.It shows that the novel PCM PV/T can improve the performance of the PV/T in summer daytime and winter night simultaneously.