| The low-density solar radiation could be converted to high quality clean heat source by the use of parabolic trough concentrators(PTC), which had been widely applied not only to solar thermal power generation at a temperature as high as 400℃, but also in industrial or commercial areas such as heat process, drying and cooling within the low-medium temperature range of 80~250℃. Absorber in a PTC system is the critical component to gather concentrated lights and realize the photo-thermal conversion, and its performance has great impacts on the quality of the system. At present most of the absorbers are in the form of straight-through metal-glass evacuated tube, which could maintain low heat loss under high temperature, but require complex machining techniques, high cost, and easy to break. Cavity absorbers are put forward by researchers to conquer these shortages. In the purpose of realizing high efficient and low cost PTC thermal utilization in the low-medium temperature range, the theoretical, simulation and experimental research on the optical and thermal characteristics of PTC system using a triangular cavity absorber were carried out as follows:(1) In order to maximize the light-gathering ability of the triangular cavity absorber, the ray equations between a parabolic trough reflector and the absorber are established and solved based on the secondary reflection condition. The optimal reflector design parameters for different cavity opening angles are calculated and the results are presented. The concept of effective aperture ratio was put forward to evaluate the matching performance between reflectors and cavities. For a triangular cavity with an opening angle of 60°, the effective aperture ratio of a reflector with a focal length of 1150 mm and an aperture of 2220 mm is 94.66%, while it is only 60.91% for another reflector with a 1000 mm focal length and 3000 mm aperture. Meanwhile, a TracePro software simulation was conducted which confirmed that the effective aperture ratio factor could be used to evaluate the cavity position influence on efficiency and should be considered during the system design and optimizing process.(2) The optical efficiency differences between the evacuated tube receiver and a triangular cavity receiver are theoretically analyzed. Furthermore, the calculation expressions of the critical installation position for the optimal effective absorptivity of the cavity receiver are given, and validated through a TracePro software simulation. The simulation results show that the optical efficiency of cavity receiver can reach 89.1%, while the evacuated tube’s is 77.1%. Meanwhile, the installation position deviations in the vertical or parallel direction have similar impacts on efficiencies of both receivers, but the cavity’s efficiency curve is slightly asymmetrical, which indicates that the best vertical installation position is 5~20mm below focal length. It was also observed that mirror deflection error has greater influence on receiver efficiency, and the deflection angle should be controlled within 0.4°. Beyond this range, the descend degree of cavity receiver efficiency is less than the evacuated tube.(3) The solar PTC system was construted to test the thermal efficiencies of both the cavity absorber and the evacuated tube using a transient testing method. The optical efficiencies of the two receivers under practical circumstance were calculated and the theoretical and simulation results were confirmed by experimental test results. In addition, the thermal efficiencies of both receivers would meet at equilibrium point of 55.0%, in the case of ideal installation condition and heat collecting temperature of 168.6℃. Below this critical temperature, cavity receiver has a higher efficiency. |
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