Experimental And Simulation Study Of A Tube-cavity Solar Air Receiver

The solar power system includes some main parts:the concentrator, the receiver and the thermal power conversion unit. The concentrator and the receiver are collectively referred to as the heat collector. The solar receiver is an important component of the system and it is used to receive the high heat flux from the concentrator and then converts solar radiation into heat energy, which is mainly absorbed by the heat transfer fluid. The thermal performance of the receiver directly impacts on thermal efficiency and ecnomy of the whole system. Therefore, the researches on the flux distribution on the cavity internal wall and the heat losses of the receiver have a significant theory guide and application value for the heat collector system’s structure optimization.The research object of this paper is an air tube-cavity receiver applied in the downward concentrating system. The influence of different flow rates (1～5m3/h) and flow directions (fair-current and counter-current) on the thermal performance of the solar receiver was investigated. This paper analyzes the air heating process and temperature distribution characteristics inside the coiled pipe. The experimental results show that the highest outlet temperature can be up to662℃and593℃for counter-current and fair-current respectively and the highest efficiency can be53.6%when the average intensity on the entrance is120kW/m2. The flow rate has a significant influence on the outlet temperature and the thermal performance of the receiver, and the comprehensive heat transfer effect of counter-current is better than fair-current. The air temperature distribution presents the trend of rise at first and drop at the tail. The temperature drop is restrained by the flow rate.Accounting for the nonuniformity of solar distribution, the optical-thermal coupled model of the tube-cavity receiver is built and a ray-tracing model based on Monte-Carlo method is built to obtain the flux distribution on the cavity internal wall, which is used as the boundary condition for3-D FLUENT heat transfer calculation. The thermal efficiency of the receiver and the heating process of air under different conditions are calculated by the coupled model. The coupled model is verified through the comparision with the experimental results:the air heat process and the temperature distribution law are consistent with the experiments and the errors of the outlet temperature are all under8%. A thermal balance is also calculated based on the simulation results to calculate the heat losses of different parts of the receiver. The results show that the radiation heat loss from the outer surface of the receiver accounts the most and the second is the convective heat loss from the outer surface and optimization measures were put forward based on the model.The thermal performance of the receiver with different structural parameters is analyzed based on the coupled model. The conclusion were obtained as follows:the outlet temperature can be706℃and818℃when the incident flux was up to200kW/m2and300kW/m2. The outlet temperature can increase to670℃and the efficiency can increase to63.1%when the inner radius decreases to4mm. However, the pressure drop will increase sharply to56.52kPa with respect to5.37kPa when the inner radius is6mm. The decrease of turns can lead to the change of flux distribution, which is more effective to higher outlet temperature. When the number of turns is decreased to12, the outlet temperature can be highest, which is572℃, and the pressure drop decreases to4.62kPa. To further reduce the turns will lead to the decrease of the total incident radiation and the heating time inside the receiver, which leads to a drop of the outlet temperature. The incident radiation and flow rate should be selected based on the requirement of outlet temperature and the flux distribution can be optimized through the adjustment of the tube inner radius and the number of turns.