Effect Mechanism Research Of The Heat Loss Of The High-Temperature Vacuum Solar Receiver

High-temperature vacuum solar receiver is the key factor of the parabolic trough solar thermal power generation system. The efficiency of the whole system is directly decided by the efficiency of the receiver thermal conversion. The heat loss and vacuum lifetime are the determining factors in restricting the operation efficiency and economic costs of the system,and there is a direct relationship between heat loss and vacuum level and emittance.Compare with a large number of research results and experiment data abroad, the systematic and comprehensive research on the heat loss and vacuum lifetime is lacking in China.Therefore, it is necessary to research on the influencing factors of the heat loss of the receiver,reveal the effect to the thermal properties of the receiver. Establishing the design theory and method foundation for developing the low loss and long lifetime receiver.In this paper, the effect of the emittance and vacuum level to the heat loss are researched.Furthermore, the variation of the emittance with temperature and substrate roughness is studied. The determination standard of the thermal properties of the receiver is achieved. At last, the thermal cycle experiments to different properties receivers are done, and the vacuum lifetime of the receivers can be predicted though the analysis of simulated working condition.The main conclusion are as follows:（1） The emittance of the selective absorbing coating of the receiver is one of the main influencing factor on the heat loss. An increase of 0.01 in emittance causes an increase of almost 25^-30 W/m in heat loss. the receiver with 0.08 emittance at 400 o C presents a good performance, and the heat loss of it at 400 o C is 230.3 W/m. The heat loss at 400 o C has a linear correlation with the diameter of the receiver, when the vacuum level of the receiver is10^-3 Pa, the linear relational formula between the heat loss and the diameter of the absorber tube and emittance can be describe as the equation Q_H.L=（0.91+3.27Da）·12.5ε_a（2） The emittance of the receiver is related to temperature and the roughness of the absorber tube, the increasing of the emittance with increasing temperature satisfies the fitting formula ε_abs=A+B* T², and the increasing of the absorber tube roughness causes the emittance increased, means that the absorber tube should keep as smooth as possible to ensure the emittance in a low level, the maximum value of the roughness should lower than0.2 μm.（3） The convection loss is low enough to be ignored when the vacuum level of the receiver in 10^-3 Pa^-10-2 Pa. When the vacuum level of the receiver is inferior to 10^-1 Pa, the heat transfer mechanism changing of the residual gases in the vacuum annulus causes the convection loss increase rapidly. Compare with the convection loss of the receiver whose vacuum level is 10^-3Pa at 400 oC, the convection loss of the receiver with 10^-1Pa vacuum level is about 60 W/m higher.（4） The error between the calculated heat loss and the experiment heat loss of the receiver can be corrected by using the corrected coefficient which is satisfied the fitting formula K=13.61·T^-1.43. The convection heat loss should be calculated separately and be included in the total heat loss when the vacuum level of the receiver is 10^-1 Pa,and then the corrected coefficient can be used to eliminate the error. After the error has been corrected, the difference of the calculated heat loss and the experiment heat loss of the receiver is less than 10 W/m, the percentage biases are mostly less than 5%.（5） The vacuum lifetime of the receiver is related to the heat preservation time and heat cycle temperature. Long heat preservation time can reduce the outgassing amount of the receiver in the initial heat cycle, which can reduce the vacuum reduction rate of the receiver and prolong the vacuum lifetime. Meanwhile, the outgassing rate of the receiver is higher with the higher heat cycle temperature, leading the vacuum reduce faster.