| The device realizing the solar water heating is the solar water heating system.The solar water heating system is usually composed of collector,circulating pipe,pump and water tank.Among them,the thermal efficiency of the system,as well as the reliability and weatherability of the system are mainly decided by the collector.The solar collector is a kind of radiation-convection heat exchanger which converts solar energy into heat energy and transfers it to water flow to realize circulating heating water tank.There are many types of collectors:flat-plate collector,vacuum tube collector,heat pipe collector,micro-channel collector and so on.Among them,flat-plate solar collector(FPSC)has the advantages of simple structure,reliable operation,low cost,low heat flux,low working medium temperature,safety and reliability.Compared with vacuum tube solar collector,FPSC has the characteristics of high water pressure bearing capacity and large isolation area.It is also one of the best type of solar collector intergrated with building.However,the use of flat solar collectors is usually limited to the southern warm area of China,and the popularity of vacuum tube collector in the northern cold areas is far greater than that of flat-plate solar collectors.Part of the reason for this phenomenon is that compared with vacuum tube collector,flat-plate solar collector has relatively lower thermal insulation performance.The heat loss through the top,bottom and border of the collector will significantly reduce the operating thermal efficiency of flat-plate solar collector system in the daytime.Another part of the reason is that water is the most commonly used circulating working fluid in flat-plate collector,and it is easy to get freeze at low ambient temperature.The water freezing in the collector will pose the result of large pressure on the pipe wall of the collector due to the phase changing volume expansion,which can lead to irreparable damage of the collector.Meawhile,flat-plate solar collector also has the risk of overheating in summer.Overheating will cause the boiling of water flow inside the collector.It will also cause large thermal stress to the absorber of the collector,resulting in irreparable damage such as curling of the absorber and the crack of the welding line.In view of the above problems,this paper studies the heat loss and freezing mechanism of the flat-plate solar collector system operated in winter.Then,this paper proposed a kind of PCM antifreeze flat-plate solar collector system,and conducted research on the working performance of the PCM antifreeze flat-plate solar collector system.Last,this paper studies the winter operation/antifreeze performance and the thermal stress of the large size flat-plate solar collector.The work includes:A whole day operating mathematical model of flat-plate collector system(including daytime model and night cooling/freezing model)is established.In the night freezing model,the enthalpy method is used to simulate the phase changing process of working fluid in the pipse.This model can obtain the temperature change at each point in the collector and the solid-liquid interface change of working fluid inside the pipe.Besides,the heat loss during the whole night cooling and freezing process can be obtaine through the model.A prototype experiment platform of PMMA absorber-pipes collector is built.The freezing condition inside the collector can be observed directly through the pipes.The accuracy of the theoretical model is validated by comparing the temperature change measured by the experiment and the observed results of the freezing process with the simulated results.Based on this,the freezing process of flat-plate collector and various factors influencing the antifreeze performance of flat-plate collector are analyzed.Then,the collector freezing model was coupled with the heat transfer model for the heat insulation device.Using the coupled model,the cooling and freezing process of the flat-plate collector equipped with TIM transparent honeycomb partition,hollow glass cover and low-E low emissivity glass cover were simulated and calculated respectively.The results show that the main reson for the freezing of the collector is the heat loss at the top of the collector and the radiation heat loss from the glass-cover to the skydome in the process of cooling and freezing at night.Reducing the pipe space,using larger diameter pipe or manifold,and increasing the thickness of airgap can improve the night antifreeze performance of the collector.The complete frozon time of collector can be delayed by one hour by equipping the hollow glass-cover.The complete frozon time of collector can be delayed by 2-3 hours by equipping the TIM transparent honeycomb or low-E glass-cover.A kind of PCM flat-plate collector system with antifreeze characristics is proposed.The system can enhance its heat storage capacity by installing PCM module into the collector,and improve its antifreeze performance at night.A mathematical model for the PCM flat-plate collector system is established,which includes daytime operation model and night cooling/freezing model.The model includes the phase changing process of PCM module in the collector during daytime melting and night solidfication.A comparative experimental platform for PCM flat-plate collector system and traditional flat-plate collector system was designed and built.The daily operation and cooling/freezing experiments were carried out under typical winter conditions in Hefei.The daytime performance and night antifreeze performance of the system were studied and compared,and the theoretical model of PCM flat-plate collector system was validated by experimental results.Based on the model,the structure of the system was optimized.The factors influecing the performance of the PCM flat-plate collector system were analyzed by changing the PCM phase change point,phase change latent heat,the combined heat transfer coefficient of the PCM to the pipes and the PCM module thickness.The applicability of the system under different climatic factors such as solar radiation,night temperature and wind speed and sky emissivity was studied.The operation characteristics of the system in Hefei,Beijing,Xuzhou and Xian during 90 days in winter were studied.The results show that the traditional collector system begins to freeze at 21:00 and the minimum temperature of the collector at night is-6 C.The PCM-antifreeze flat-plate collector system does not get freeze during the whole night.The PCM flat-plate collector system will get freeze during the night when the ambient temperature is below-4.5℃while the critical value for the traditional collector system is 0℃.For different night ambient temperatures,there exists a value of the phase-change temperature for an optimum antifreeze performance,and the phase-change temperature should be set in the range of 2-7°C.For different night ambient temperatures,there exists a heat transfer coefficient for an optimum night antifreeze performance of the system.The thermal adhesive bonding(10-40 W/(m2·K))between the pipes and the PCM module is an adoptable method to maximize the antifreeze performance of the system.In the four cities,the winter seasonal average efficiency of the PCM system is 5%-10%lower than the traditional system.The seasonal average freezing risk index of the PCM system is 0,0.04,0.12 and 0 in the four cities,while the seasonal average freezing risk index of the traditional system is 0.23,0.89,0.82 and 0.62 in the four cities.In order to study the performance of large size collector operating in cold area,the non-uniform flow model in daytime,freezing model at night and thermal stress model were established.The model considers four types of pipe structure of large size collector:VZ,VU,HZ and HU.It can simulate the temperature change at each point in the collector and the flow velocity/pressure at each point in the pipes under steady flow.The accuracy of the model is validated by the experimental results of the 10.04 m2 large size collector system test conducted by Guangdong Five Star Company.Based on this model,the influence of structural factors on the performance of large size collector(including thermal performance,hydraulic performance and antifreeze performance)was analyzed.The thermal stress model is used to predict and analyze the thermal stress distribution characteristics of large size collector at high temperature.The results show that the efficiency of H type collector is 5-6%higher than V type,whereas the V type has a better antifreeze performance than the H type.Considering the boiling danger due to the nonuniform flow in V type collector,the H type collector is recommended.Considering the boiling danger of non-uniform flow in V type collector,H type collector is recommended.Compared with using the square shaped HZ collector,using the HZ collector with the aspect ratio of 2.5 can enhance the efficiency by 2%whereas the pressure drop will be double.Meanwhile,using the square shaped VZ collector can enhance its efficiency significantly and has little influence on the pressure drop.Consequently,for a specific aspect type of absorber strips(HZ or VZ),an aspect ratio close to 1 is recommended.Increasing the number of pipes improves both its working efficiency and antifreeze performance.The maximum thermal stress of the large collector during the working is located in the two ending of the absorber-pipe welding line.The thermal stress at both ends of the welding line is about 110-130MPa,and the thermal stress in the middle part is about 40 MPa.It is recommended that increase the number of pipes to reduce the middle thermal stress,and at the same time increase the none weld length to reduce the ending thermal stress. |
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