Research On The Thermal Performance Of Solar Heating Systems With The Variable-Outlet Tank

In order to deal with the energy crisis and alleviate global warming,more and more buildings use renewable energy,instead of fossil fuel,to meet the heating demand.Solar heating system,as a renewable energy heating system,is widely used in the world because of its eco-friendliness,energy saving,low construction cost,etc..The solar energy is collected by the solar collector in the heating system and converted into thermal energy.By natural circulation or forced circulation,the thermal energy will be transferred from the collector to the storage tank.When heat is demanded,thermal energy is supplied by the storage tank for use.As the key components of the solar heating system,solar collector and storage tank are crucial to the performance of the heating system.In this paper,based on the research gap of the existed research on solar heating system,the study aims to provide a comprehensive analysis via validated theoretical and numerical methods.The horizontal water in glass evacuated tube collector(WIGETSC),which is widely used in China,is investigated thoroughly through the numerical simulation.The characteristics and the development of the inner flow field are shown in detail.A theoretical model of the inner flow field is developed.As for the storage tank,a novel variable-volume tank design is proposed in this paper.For a retrofit from a fixed-volume tank to a variable-volume tank,the new design demands only a few updates.It saves the cost of reconstruction and helps upgrade the conventional system.Comprehensive investigations are made on the new tank for better understanding.Furthermore,theoretical models of the new tank and heating system are established.With the model,the influence of new tank on the long-term performance of solar heating system is analyzed and discussed.The main work and conclusions arising from this study are summarized as follows:The characteristics and the development of the flow field with the horizontal WIGETSC are investigated using the numerical simulation and are validated by the experimental data.The development of the inner flow field is elaborated.The influence of the secondary flow on the flow field and heat transfer is pointed out to be crucial.A theoretical model accounting for the secondary flow is developed to calculate the circulation flow rate and temperature distribution.Compared to an existing model,the proposed model shows a better accuracy in predicting the circulation flow rate and temperature distribution.The numerical simulation of the new tank shows that most of the thermal energy can be kept in the effective volume.By changing the effective volume,the new tank can operate as a variable-volume tank.The new tank distinctly improves the performance of heat storing in undesired weather condition,and effectively improves the quality of stored heat.During the charging period in a cloudy day,the average temperature of the effective volume of the new tank was 4.2 ? higher than the counterpart.Based on the understanding of the new tank,a theoretical model is developed to simulate the solar charging and discharging performances of the new tank.The effects of heat load,initial tank temperature and solar irradiance on the performance of the new tank are studied.It turns out that the new tank prevails over the conventional tank under the circumstance of daytime heat supply,low initial temperature or low irradiance.It helps extends the solar heat supply time.In the case of the initial tank temperature of15 ?,the new tank provided 13.2 MJ heat for use,while the conventional tank was incapable of providing heat.In view of the outstanding performance of the new tank in daytime heat supply,this study mainly focuses on the daytime heat supply.Two typical solar heating system,namely forced-circulation and natural-circulation solar heating system,are investigated.With the theoretical model,the influences of new tank on the long-term performance of the two heating systems are calculated and compared to the conventional tank.By reducing the system heat loss,the new tank greatly improves the long-term performance of both the forced-circulation and natural-circulation systems.For the forced-circulation system,the new tank helps reduce 19% of tank heat loss and improve 11.6% of the heat supply.For the natural-circulation system,it helps improve 11.9% of the heat supply.Furthermore,the new tank also makes the heating system less vulnerable to extreme operating conditions,which helps build a robust system.This study mainly focuses on the theoretical and numerical analysis of the solar collector and thermal storage tank.The investigation of the inner flow field and heat transfer helps better understand the mechanism of the horizontal WIGETSC.On the other hand,parametric study and long-term performance study on the new tank have been done for a thorough investigation.The new tank shows positive effect on the performance of solar heating system and contributes towards the storage tank design and preventing pipe freezing at night.This study can provide theoretical and technical guidance to enhance the applications of the solar heating technology.