Study On The Optical Property And Thermal Performance Of Solar Linear Concentrating Stytem With Cavity Absorber

The parabolic trough solar concentrator has wide geometric concentrating ratio in concentrating solar utilization. The energy for both industrial and civilian heat energy in high, medium and low temperature could be outputted by the parabolic trough solar concentrator. The parabolic trough solar concentrator is easy to scale array layout for small land area with the advantages of more convenient installation and maintenance. Consequence, the parabolic trough solar concentrator is widely commercial application in many countries. The absorber is the very important component in the parabolic trough solar concentrator and the main absorber is metal-glass evacuated tube in parabolic trough solar concentrator at present. The absorber has vacuum layer and its heat loss is reduced effectively. There is a hard technical problem for the seal between the glass and the metal in long time, the prise of the metal-glass evacuated tube is very expensive, meanwhile the metal-glass evacuated is easy to be broken. However, the cavity absorber can avoid the problem. In this thesis, the performance of parabolic trough solar concentrator with cavity absorber is studied by theory, simulation and experiment. The main works are summarized as follows:(1) Acorrding to the offsetting effect of the sun’s rays by the thickness of the concentrating mirror, the concentrating model of the parabolic trough solar concentrator is established and verified. The main energy on the focal plane of the parabolic trough solar concentrator is come from the direct reflect on the upper surface of concentrating mirror and the indirect reflect on the lower surface of concentrating mirror. The result showed that absorption rate was only 1.98% with glass thickness of 5 mm. The parabolic trough solar concentrator with the focal length of 1200 mm, and the thickness of 1 mm and the refractive index of 1.6, its half width of focal line is 25.7 mm at the point distance from the center of 1700 mm.(2) It is proved with the theory that the value of heat loss is directly affected by the distribution of solar irradiance on the surface of the absorber. The whole radiant energy is equal on the same surface of absorber. The distribution of solar irradiance on the surface of the absorber is tended to be more uniform, the lower smaller radiant heat loss from the surface of the absorber is. Whereas, the distribution of solar irradiance on the surface of the absorber is tended to be more non-uniform, the more radiant heat loss from the surface of the absorber is.(3) A double standard method is built for evaluating the optical performance of cavity absorber, including optical efficiency and standard deviation of the surface irradiance of cavity absorber. The double standard method and the orthogonal experiment are used to design the optical performance of absorber. The optimized cavity absorber with the aperture width of 70 mm, deep-width ratio of 0.80:1.00, and the focal length ratio of 98.75%, its optical efficiency and standard deviation of the surface irradiance are 89.21% and 30528, respectively. The absorber is similar to blackbody, the solar rays enter the inside of cavity and some solar rays are absorbed by the absorbing surface. The others are reflected by the absorbing surface and the reflected solar rays are not easy to leave the cavity from its aperture. The photo-thermal converted model of the cavity absorber is established. It is found that the photo-thermal converted efficiency is decreased with the increase of temperature of working fluid. The intercepted thermal efficiency is 79.66%. The theoretical and experimental thermal efficiencies are 40.91% and 38.15% respectively with temperature of working fluid at 203.7 ℃. The relative error of theoretical and experimental thermal efficiencies is 8.36%. The heat loss of cavity is increase with the increase of temperature of working fluid and its gradient is bigger and bigger.(4) The average direct radiation of 805 W/m2, the parabolic trough solar concentrator is used to product directly generate steam and the average thermal efficiency is 51.48% on typical sunny weather condition. The average direct radiation of 430 W/m2 on typical cloudy weather condition, the parabolic trough solar concentrator is used to product directly generate steam and the average thermal efficiency is 49.84%. The payback period of cavity absorber is 6.97 years. It is economically feasible that the parabolic trough solar concentrator is used to product directly generate steam.(5) In order to improve the thermal performance of the cavity absorber, the parabolic trough solar concentrator with cavity absorber was optimized. The thermal efficiency of the optimized system was 54.66% with the working medium of water, which has reached the level of the vacuum tube absorber. The heating performance comparison experiment in medium temperature of the system was carried out with the cavity absorber and the vacuum tube absorber. The results showed that thermal efficiencies of the parabolic trough solar concentrator with the cavity absorber and the vacuum tube absorber were 40.02% and 42.44% respectively at the temperature of 180-200 ℃ using thermal oil as working medium.