| There are abundant solar energy resources and good solar energy utilization conditions in Tibetan areas and the Northwest Plateau in China,but the thermal environment also has significant characteristics such as frequent alternating hot and cold and large temperature fluctuations.Under such complex dynamic environmental conditions,flat plate solar collector often have problems such as cracking and overheating.Based on this,this paper first analyzes the heat transfer process and thermal performance index of the flat plate collector,and uses the Fluent software to numerically simulate the thermal performance of the flat plate collector in the Tibetan plateau environment.A thermal performance test platform was built to test the thermal performance of flat collectors with different pipe diameters.On this basis,a phase change flat collector which can simultaneously alleviate the cracking and overheating problems was proposed.The thermal process and thermal properties of the phase change collector were studied experimentally.Based on the experiment,the phase change collector model was established by Fluent software,and the temperature variation process under Lhasa environmental conditions was numerically simulated.The structure and energy conservation relationship of the plate collector were analyzed,and the energy calculation formulas such as useful energy,heat loss and light loss in the energy transfer process were obtained.According to the heat transfer heat resistance network diagram of the collector,the heat transfer model of the flat collector was established.The heat transfer links and thermal performance evaluation indexes of the flat collector were analyzed,and the evaluation indexes of freezing damage of flat plate collector such as temperature difference of freezing damage,duration of freezing damage and degree of freezing damage were proposed.The flat plate collector model was established by using Fluent software,and the thermal performance of the flat collector in the Tibetan plateau environment was simulated.The variation of the outlet temperature,thermal efficiency and useful energy of typical typical cities in summer and winter were obtained.The effects of common plate collector diameter and pipe spacing on collector efficiency and heat loss were obtained by experimental research.The experimental results showed that reducing the tube spacing can increase the efficiency of the collector and reduce the heat loss,increasing the diameter of the pipe can increase the efficiency of the collector,but the heat loss will also be increased.On this basis,the optimal structure of the flat plate collector was selected,and a phase change collector with two different melting point phase change materials was designed to improve the frost resistance and high temperature resistance of the flat plate collector by utilizing the heat storage and release characteristics of the phase change material(PCM).The performance test was carried out and the results showed that under high temperature conditions,the high melting point PCM can store heat when the collector water temperature is high,and the heating time of the absorber plate can be extended by more than 1.6h,thus effectively slowing the temperature rise of the collector and alleviating the overheating problem.When the collector cools down,it can effectively use the PCM to store heat,slow down the cooling rate of the collector,and prolong the cooling time up to 1 h,thereby improving the thermal performance of the collector under the no radiation or low radiation condition,and the heat flux curve of the surface of the collector ws similar to the temperature curve of the collector;the low melting point PCM can solidify and release heat when the water temperature is low,thus reducing the cooling rate of the collector.When the low-melting PCM placed below the high-melting PCM and in the opposite condition,the cooling time of the absorber plate can be extended by 6.4h and 3.1h,respectively,which can effectively improve the antifreeze performance of the collector.In addition,comparing the thermal performance of the conventional collector and the PCM collector,it was found that the PCM collector is more efficient,the useful energy obtained by the collector is increased,and the position of the high and low melting PCMs has little effect on the efficiency of the collector.The simulation results of PCM flat plate collector in actual conditions in Lhasa showed that under typical summer conditions in Lhasa,the outlet temperature of the PCM collector and the conventional collector were about the same,while the temperature of the absorber plate was significantly reduced,and the overheating period can be reduced by selecting the appropriate melting point or increasing the latent heat of the high melting point PCM to improve the high temperature resistance of the collector;in the winter conditions of Lhasa,the low melting point PCM in the PCM collector can greatly reduce the duration of the freeze damage of the collector,and the reduction time can reach more than 9h,the temperature difference of the collector frost damage can also be reduce and the lowest temperature at night can be increased by about 11 °C,the degree of frost damage is greatly reduced,which can effectively improve the antifreeze performance of the collector.Through the above theoretical analysis,numerical simulation and experimental research,the evaluation index of freeze damage of flat collector was proposed and analyzed,the thermal performance of the collectors in different typical cities in summer and winter in the Tibetan Plateau environment was obtained,the influence of pipe diameter and pipe spacing on the efficiency and heat loss of flat plate collector was clarified,the endothermic exothermic process and the frost and high temperature resistance of the proposed PCM collector are mastered,which can provide a basis for relieving the freezing and cracking of the flat collector in the Tibetan plateau environment. |
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