Asymmetric Non-tracking Solar Concentrator And Its Photothermal And Photovoltaic Performance

Solar energy plays an important role in reducing building energy consumption and achieving the goal of building energy conservation.In view of the dispersion and instability of solar energy,the solar concentrator can improve the solar radiation illumination on the absorber,reduce the area of the expensive absorber,reduce the heat loss,and obtain higher temperature and reduce the cost.At present,most solar concentrators used in buildings have small absorbable solar incident angles,or require a tracking system,with a large depth-width ratio,high machining precision and poor uniformity of spotlight.Therefore,it is meaningful to develop a solar concentrator with wide range of acceptance angle,with simple structure,high optical efficiency and better uniformity of light gathering,and it is of great significance to its further research and application.Firstly,this study proposes an asymmetric non tracking solar concentrator,which has the advantages of large acceptance angle,high optical efficiency,flat structure,and more uniform concentrating energy flow distribution than traditional concentrator.The design method is given through theoretical calculation,which consists of three steps:a)Determine the design incidence angle（₀）and the number of plane mirrors（n）according to geographic latitude and design conditions.b)Calculated the coordinate formula of the end points of each plane mirror theoretically.c)Calculate the tilt angles of the plane mirrors by optimizing the geometric concentrating ratio.Secondly,optical performance is simulated by optical simulation software TracePro.The simulation results show that the acceptance angle range of the concentrator is[₀,90°).When the incident angle is less than₀,only part of the light can be absorbed.Increasing n and₀can increase the concentrator ratio（CR）of the concentrator,and both will reduce the optical efficiency.When=0.94、=1、=0.9,the optical efficiency of the concentrator is over 85%in the incidence angle is bigger than₀,and increases with the incidence angle.For different combinations of₀ and n,the concentrators with same CR have the same optical efficiency when the incident angle is larger than₀.Decreasing n value is beneficial to obtain more uniform light intensity distribution.Then,the thermal performance of the concentrating collector is studied through experimental and modeling,and the accuracy of the steady state model is verified by the measured data.The measured results show that the efficiency equation isη=-1.8350T*+0.6930,where normalized temperature difference is based on the average temperature of working fluid.The thermal efficiency of the whole day is greatly influenced by the direct radiation.The maximum thermal efficiency can reach more than 60%,and the average thermal efficiency can reach over 47%.The simulation results of structural parameters show that the thermal efficiency is greatly influenced by the reflectivity in summer due to the average reflection times are larger,while the reflection times in winter are smaller and the reflectivity is less affected.The increase of the concentration ratio can increase the thermal efficiency,but the increment is smaller and smaller.There are two favorable values for the thickness of the air layer between the cover plate and the heat absorbing plate.It is advisable to choose 10mm or 30mm.The simulation results of the operating parameters show that the thermal efficiency increases with the increase of the solar radiation,and the increase gradually slows down,and finally tends to a stable maximum value.The lower the scattering ratio is,the higher the thermal efficiency is.The environmental wind speed has little effect on the thermal efficiency.Reducing the ambient temperature or increasing the inlet temperature of the working fluid will increase the thermal efficiency of the collector.When the flow rate is near the critical velocity,the thermal efficiency will have a larger mutation,and the thermal efficiency of turbulence will be increased by about 5%compared with laminar flow.Finally,the concentrating PVT module is fabricated,and its thermal and photovoltaic efficiency is tested by experiments.It show that the maximum photovoltaic efficiency in the test period is 7.53%,the maximum thermal efficiency is 47.35%.The average photovoltaic efficiency is 5.79%,the average thermal efficiency is 40.38%.When the PV battery operates separately,the maximum photovoltaic efficiency is 5.63%,the average photovoltaic efficiency is 3.97%,compared with PVT system,it can be about 2%lower.It can be seen that the PVT module can effectively take away the heat of photovoltaic cells and reduce the operating temperature,so as to enhance the photovoltaic efficiency.The concentrator proposed in this study has a certain potential of integrated solar building applications.The steady state heat transfer model can predict the performance of the collector quickly and accurately,and provide theoretical support for the structure design and application selection.