Research On Self-circulation Antifreezing Strategy Of All-glass Evacuated Solar Collector System In Severe Cold Areas

With the continuous development and perfecting of solar energy technologies,solar water heating system has acquired good effect on actual engineering applications,with good economic and environmental benefit.Among these,evacuated tube collectors are the most widely used types,taking a percentage of 90% in the market,and all-glass evacuated tube collector was more economic compared with heat pipes and U-type evacuated collectors.Winter freezing of solar water heating system is one of the major problems that restricting its utilization in cold and severe cold areas.There are also some existing methods or strategies to resolve this problem,however,the detail implementation,anti-freezing effect,and economics are still questionable,especially in solar water heating systems in severe cold areas.Therefore,this thesis concentrates on the freezing problem of solar water heating systems,combining the theoretical analysis,experimental measurement and numerical simulation research methods to study the suitable anti-freezing strategies of solar water heating systems,and the temperature distribution and variation in solar collector.First,a novel solar anti-freezing method was proposed based on all-glass evacuated solar water heating system,to use the remanent thermal heat in solar collector for freezing protection of solar collecting loop in severe cold areas,that is,self-circulation solar anti-freezing strategy.The detail control principle and control process was presented in the thesis,and the anti-freezing effect and feasibility were validated based on experimental test-bed.Compared with other anti-freezing methods of electric tracing band and using antifreeze solution only,the operation fees and initial cost by self-circulation solar anti-freezing strategy was much lower with remarkable economic benefit,and the amount of antifreeze solution could be decreased significantly.Second,based on the experimentation platform,the heat transfer process and properity of heat losses were studied by field test.The initial fluid temperature,temperature drop in the night,and the heat losses,heat loss coefficient were all tested and analyzed.And the parameters that affecting the heat dissipation and antifreezing operation time were also discussed.The studies on heat dissipation and heat losses of solar collector could help us to know the actual heat dissipation performance and the temperature changing in it.Besides,it offered theoretical basis for further simulations,and providing data base and reference for the parameter and boundary setting.Finally,the numerical model of solar collector was established using CFD,and the temperature distribution,heat dissipation,and the possible lowest fluid temperature after heat dissipation in the night were studied.Whether it could be get freezing in different weather conditions and the possible freezing locations in the tube were also discussed.The temperature variation and its possible lowest value in different weather conditions in different regions can be estimated according to its distribution and changing regulations,which was helpful for the selection of anti-freezing methods and suitable concentration of antifreeze solution.If there is antifreezing operation,the possible temperature distribution,temperature drop,and heat dissipations can also be estimated in similar conditions on the basis of its distribution characteristics and heat dissipation properties,whi ch was contribute to the setting of anti-freezing parameters and its controlling principle,and helpful for the design of solar anti-freezing.