| Concentrated solar power（CSP）is a potential clean energy with abundant resource and wide distribution.Solar air Brayton cycle is a suitable choice for peak regulation of power grid and for distributed energy systems because of the advantages of quick start-stop performance and stable power generation.A CSP Brayton cycle system consists of a heliostat field,an air receiver,a compressor,a turbine,a recuperator,a generator,a control system,etc.In this paper,the concentrating and absorbing characteristics of solar tower concentration and air receiver are investigated.The concentrating characteristics of the heliostat field greatly influence the system efficiency and the operation safety of receiver.The indirect flux distribution test based on the CCD-Lambert target method is conducted to obtain the concentrating characteristics of heliostats focusing on a 34-meter-high aperture.A Monte Carlo ray tracing（MCRT）model is established to simulate the flux distribution on the focused plane and the cavity surface of the receiver.The test results show that the total energy on the 1m2 plane is 257.8 kW of concentrating 57 heliostats while the direct normal irradiance is 750 kW/m2.Compared to the test results,the simulation average error of the total energy is about 5.4%.The test and analysis of tower concentrating characteristics and receiver performance are conducted under real condition.The test results show that when the direct normal irradiance（DNI）suddenly decreases by 31.1%,the receiver outlet temperature changes only 0.3%.During the test,the outlet air temperature can reach around 904.2℃and the maximum power attains up to 55.6 kWth.MCRT simulation is carried out to obtain the flux distribution on the cavity surface the receiver which is used as the boundary condition for CFD simulation.The simulation error of outlet air temperature is 0.5%meanwhile the error of pressure loss is 10.1%.According to the test data and simulation results,the radiation and convection losses through the receiver aperture are the main heat loss which count up to 80%of the total loss when the temperature of outlet air is about 900℃.When the heliostat focal length is adjusted line by line,the mean intercept factor increased by 19.7%.Under 60heliostats concentrating,when the aperture diameter is decreased by 20%,the efficiency of the receiver will increase by 15.1%after heliostats were optimized.The optimal aperture size lead to the most efficiency receiver which can reach 61.7%by increasing aperture diameter by 40%under the certain concentrating characteristics.The experimental and simulation study is expected to provide a useful reference for practical design and optimization of solar Brayton system. |
