A Novel Non-concentrating Solar Collector Design And System Research

Energy is a fundamental safeguard of human activities.As a clean and abundant renewable energy,efficient utilization of solar energy is an important way amid the process of energy transformation in China and the world.Among the many solar energy utilization methods,using non-concentrating solar collectors for photothermal conversion has been widely concerned worldwide.This method has the advantages of high energy conversion efficiency,low installation and maintenance cost,and wide application occasions.In the process of the vigorous development of solar energy,improving the operating temperature range and solar harvesting efficiency of the nonconcentrating solar collector is a significant way to enhance its competitiveness.Flat plate solar collector is one of the most mature and extensive products in nonconcentrating solar collectors.However,due to high external energy loss,its working temperature and heat collection efficiency are underperformed,which not only obscures its technical advantages in other aspects but also hinders its development in low and intermediate-temperature applications.On the other hand,the non-concentrating solar collector is usually used in the useroriented district energy system.Due to the unique physical characteristics of solar energy resources such as strong dispersion and annual periodic fluctuations,the matching relationship between solar energy supply and user demand must be fully considered in the regional energy system based on the non-concentrating solar collector.Thus improving the comprehensive utilization efficiency of solar energy throughout the year.For example,in the application of building space heating based on nonconcentrating solar collector,there is often a phenomenon that the solar energy resources are weak in the heating season but the user’s heating demand is high,while the solar energy resources are abundant in the non-heating season but the user’s heating demand is low.The seasonal contradiction between supply and demand of energy not only weakens the competitiveness of solar heating in the field of building heating,but also impedes the implementation of our "double carbon" goal.With the increasing proportion of solar energy in the energy structure,basic research on the above related issues is imperative.To sum up,in order to solve a series of problems existing in the non-concentrating solar collector,this paper follows the research pathway from the solar collector to the energy system.In terms of solar collector structure improvement,thermal performance enhancement,solar system design and optimization,the following research work is carried out:1.By comparing and analyzing the basic structure,energy flow,and external loss of non-concentrating solar collectors elaborately,the main energy loss sources of non-concentrating solar collectors are analyzed,and the feasible ways to improve their performance are discussed.An evacuated flat plate solar collector is developed which is expected to realize efficient solar collection in the range of low and intermediate temperatures.2.A set of intermediate-temperature solar collection test platform based on evacuated flat plate solar collector is constructed.The solar collection performance of the novel solar collector at different working temperatures is tested and analyzed in detail.Under the condition of the solar irradiance is 835.2 W/m2,the inlet temperature is 123.0℃ and the ambient temperature is 35.7℃ the solar thermal efficiency of this platform is as high as 59.67%.3.In order to further improve the solar harvesting ability of non-concentrating solar collectors in the range of low and intermediate temperatures,an evacuated flat plate solar collecting structure based on translucent aerogel material was designed and studied.The transparent aerogel material has low thermal conductivity and high transmittance in the solar radiation band(0.3～2.5 μm),while it has the absorption and reemission effect in the infrared band(>2.5μm),which can realize a strong suppression effect on the infrared thermal radiation of evacuated flat plate solar collector.Thus,ultrahigh solar thermal efficiency is achieved compared to its original structure and other recent research results.When the solar irradiance is 1000 W/m2,the ambient temperature is 25℃,and the inlet temperature is 155℃,the thermal efficiency of the evacuated aerogel flat plate solar collector is 60.47%,which is 20.68%higher(absolute value)than that of the prototype by using only the aerogel layer with a thickness of 5 mm.4.A direct steam generation evacuated flat plate solar collection system is developed by combining the two-phase flow heat transfer mode with the nonconcentrating solar collector.The two-phase flow heat transfer mode design not only reduces the average operating temperature of the solar field greatly but also improves the heat transfer coefficient between the working fluid and the absorber tubes in the solar collector,so the solar collection efficiency of the whole system has been greatly improved.In the whole range of low and intermediate temperatures,it is about 10 percent higher than the traditional high-pressure water working mode.5.Based on the characteristics of solar energy and the building space heating demand of users,the concept of seasonal residual energy utilization of solar energy is put forward to solve the seasonal energy supply and demand contradiction problem of traditional solar space heating systems.A solar annual solar thermal energy provision system based on the evacuated flat plate solar collector is constructed in this study.Through reasonable system design and optimization,the novel solar annual solar thermal energy provision system can not only improve the annual utilization rate of solar resources but also play a certain role in shaving the peak load of the power grid.Therefore,the contradiction between energy supply and demand caused by seasonal solar energy variation is avoided.The results show that the system can realize the annual effective solar energy utilization time of 2.63 times that of conventional solar building space heating projects.Its unit energy cost is 40.27€/MWh,and its carbon reduction capacity is 1340.28 kgCO2/m2.By comparing with other typical non-concentrating solar thermal energy provision systems,the novel year-round solar thermal energy provision system has achieved good performance from the perspective of energy utilization efficiency,economy,energy saving,and carbon reduction.