Experimental And Theoretical Investigation On The Performance Of A Novel Beam Down System With Fresnel Heliostats And Cavity Receiver

A novel beam down system with Fresnel heliostats and cavity receiver has been introduced and the mechanism of heat collecting process was researched. Both theoretical and experimental investigation were done to reveal the optical and thermal performance of the system. The specific works of this thesis are as follows:1. The mechanism of heat collecting process.The designing methods for Fresnel heliostats, secondary reflector and cavity receiver were proposed. And the solar concentrating and collecting characteristics of the system were showed by means of using Monte Carlo ray-tracing(MCRT) method. Then taking the non-uniform solar flux distribution on the receiver surface as the boundary condition for the Computing Finite Dynamics(CFD) simulation model in the Fluent evaluated conversion performance of the solar thermal process.2. Optical properties. The frame tacking angle difference is far smaller than the sub-mirror tracking angle difference, so just taking the latter one into consideration.For the same heliostat, the shading and blocking efficiency in summer is 10% higher than in winter. It shows that the larger solar altitude, the slighter effect on the shading and blocking efficiency.It is pointed that the heliostat facing south is proposed for the beam down system in order improve optical efficiency. Because the annual efficiency of H1 is higher than that of H2 and H3.3. Thermal properties. It can be found that the operational volume flow rates of 0.25L/s and 0.35L/s are suggested for the water and Therminol VP-1. As the heat transfer fluid, water can gain better performance than thermal oil at the same inlet temperature. Radiative heat loss is the main source of heat loss for conical cavity receiver. When the heating temperature is above 200℃,the radiation loss accounts for over 80% of the overall heat loss. Under the identical simulation condition, the optimal thermal efficiency with minimum heat loss can be achieved when the conical angle is 100° due to a balance between the areas of the high flux distribution and the receiver surface. Although the losing part of optical efficiency, the radiative heat loss will reduce significantly by using insulation wall to instead the CPC. Because the temperature of radiative background is higher than before. If the relative height of insulation wall is 3, the radiative heat loss will reduce about 60%,45% at the heating temperature 100℃and 300℃,respectively.4. Experimental research. Experimental study was conducted to investigate the thermal performance of the beam down solar tower system. The linear fitting curve equation is 0.583 0.315 i acdTTIh-=-. Compared with the simulation result, the relative error is-3.8%~15%.The numerical simulation model is proved to be reasonable.