Study On Energy Flux Characteristics And System Heat Transfer Of Linear Fresnel Concentrating System

With the survival of mankind and the development of society,the demand for energy is increasing.The traditional fossil energy not only faces gradual exhaustion,but also causes the ecological environment to be seriously polluted and destroyed.Therefore,the search for new types of clean energy has become a research hots pot.As an inexhaustible and clean energy source,solar energy is one of the important solutions to solve the world energy crisis because of its stable source,abundant reserves and low carbon environmental protection.Concentrating Solar Power(CSP)technology plays an important role in energy supply and energy conservation.In the existing CSP technology,since the linear Fresnel Reflector(LFR)solar thermal power system uses a strip mirror concentrating device,the mirror has low wind load,compact mirror arrangement,and low cost.The advantages of flexible control and so on have become one of the hots pots in the field of solar energy research in recent years.In the LFR system,the concentrating heat collecting system is one of the key components of the whole power system.It provides energy input for the entire LFR system,and improving the light-to-heat conversion efficiency of the concentrating heat collecting system is the focus of research in the whole system.It is also one of the difficulties.In the concentrating collector system,improving the photo-thermal coupling performance is the key to improving the overall efficiency of the system.Therefore,the theoretical and experimental research on linear Fresnel concentrating collector system has important academic significance and application value.Firstly,this paper analyzes the linear Fresnel concentrating collector system and establishes a concentrating system that combines the primary mirror field with the Compound Parabolic Collector(CPC).Using the principle of unshaded mirror field optimization,the important parameters such as receiver height,mirror field width and primary mirror spacing are theoretically studied,and the primary mirror selection and arrangement parameters are determined.The geometric optical model of the linear Fresnel concentrating system was established by Matlab programming technique and ray tracing method.The 3D optical model of the concentrating system was established by Soltrace optical software.Compared with the results obtained by Matlab software,the reliability of the whole system research method was verified.Secondly,the surface energy distribution characteristics of the linear Fresnel receiver are analyzed.The results show that the distribution of radiation flux on the surface of the absorber tube is non-uniform,and the top part of the absorber tube is less distributed.Most of the energy is concentrated in the lower part of the absorber.The energy flux density in this area is at a high level,and local high heat flow may occur.This non-uniform energy flux distribution brings hidden dangers to the stable operation of the system.Therefore,an optimization method based on the principle of genetic algorithm is proposed to minimize the standard deviation of the flux density distribution on the receiver surface by changing the aiming line of the primary mirror field to optimize the flux distribution on the receiver surface and changing the aiming point of a single primary mirror.Through the optimization algorithm,79.53%optical efficiency is achieved,and the upper part of the absorber tube has a power flux distribution of40%.Therefore,the algorithm has a beneficial impact on optical efficiency and energy flow uniformity.The method is also applicable to the optimization of energy flux distribution of the LFR concentrating system with other parameters.Finally,the thermal performance of single-tube CPC cavity receiver in LFR system is studied theoretically.In the LFR collector system,radiation-conduction-convection coupled heat transfer process will be involved,so it is necessary to explore the heat transfer law and process of heat transfer working fluid under high heat flow conditions.Based on the principle of energy conservation,a one-dimensional steady-state heat transfer model of the cavity receiver is established,which accurately describes the heat loss from the vacuum tube and CPC to the external environment.The results show that when the wind speed is 3 m/s,the concentration flux is chosen as the extreme value of 10-40 kW/m~2 in the normal range,and the heat loss increases with the increase of the temperature difference between the pipe wall and the environment and the radiation flux.Selecting the extreme values of general wind speed of1 m/s and 12 m/s,and the heat loss increases with the increase of temperature difference between pipe wall and environment and wind speed.However,under the same conditions,the heat loss of cavity receivers with vacuum tubes is less than that of receivers with glass plates.Therefore,the cavity receiver with vacuum collector is a promising design,which shows great advantages in economy and thermal performance.