Study Of The Optical-thermal Conversion And Thermal Characteristics Of Solar Dish Collector Systems With Quartz Windows

Solar dish collector systems(SDCSs)are an essential component of high-grade solar energy utilization systems.SDCS senable high-density solar radiation energy to be captured over a small area of the receiver,thereby increasing energy efficiency and reducing construction costs.However,the solar radiation energy gathered by the parabolic dish concentrator will form a nonuniform and high-strength heat flux load on the surface of the receiver.This will not only reduce the working efficiency and reliability of the cavity receiver but also cause the receiver to produce high-temperature hot spots or large temperature gradients,resulting in significant thermal stress and thermal deformation,which will affect the safety and service life of the receiver.To achieve uniformity in the flux distribution and reduce the temperature and thermal stress of the receiver in SDCSs,this paper studies the focusing spot characteristics of SDCSs and the flux distribution characteristics of the cavity receiver,as well as the optical-thermal conversion characteristics and thermal mechanical coupling characteristics of key parts of an SDCS with quartz windows.The main research findings are as follows:(1)The SDCS model with quartz windows is established.Based on the Monte Carlo ray tracing method,the focusing spot characteristics of the SDCS and the flux distribution characteristics of receivers with quartz windows with six different curved surfaces,namely,biconcave,upper concave,lower concave,biconvex,upper convex and lower convex,are studied.The biconcave window had a significant effect,improving the flux distribution of the receiver and reducing the peak concentration ratio.Based on this finding,the optical properties of a cylindrical cavity receiver with four typical concave quartz windows are studied,and the effects of the concave shape of the quartz window on the flux distribution of the receiver are obtained,providing a reference basis for SDCSs to select the appropriate surfacesfor quartz windows.(2)Based on the moving accumulation ray tracing method and a genetic algorithm,an optimal design method for the geometric parameters and installation position of the concave window on the receiver is established to improve the flux uniformityand obtain excellent optical efficiency.This method is used to study 12 concave windows for SDCSs by combining four typical quadratic surfaces with three surface arrangements.The optimized concave window can significantly improve the flux uniformity of the cavity receiver in SDCSs and achieve high optical efficiency.The effects of surface optical characteristic variations,mirror slope errors and tracking errors on the optical performance of an SDCS with an optimized concave window are studied,and the feasibility of using a concave window to improve the flux uniformity is evaluated for practical engineering applications.(3)The optical-thermal coupling model of the SDCS cylindrical cavity receiver is established.The finite volume method is used to calculate the effects of typical parameters on the optical-thermal conversion of the receiver under three boundary conditions: solar direct normal irradiation,flow velocity at the inlet and temperature at the inlet of the heat transfer fluid.The effects when different materials are used in the receiver spiral tube are also obtained,providing a guiding basis for the actual operation of the receiver.(4)The thermal-mechanical coupling analysis model of the SDCS cylindrical cavity receiver is established.The thermal stress of the spiral tube,which is a key component of the cylindrical cavity receiver,is calculated.The effects of typical parameters and different materials on the thermal stress of spiral tubes are discussed.The safety performance of the receiver when different materials are used for spiral tubes is evaluated by using the failure coefficient,providing a theoretical basis for the structural design and material selection of the key components of the receiver.(5)A 3kW SDCS experimental platform is designed and built.Results from the optical and heat transfer experiments of SDCSs with quartz windows carried out on the experimental platform arecompared to simulation results.The experimental results are consistent with expectations,confirming the optical and optical-thermal conversion characteristics of SDCSs.