Medium-temperature Solar Collectors with All-glass Solar Evacuated Tubes

Medium-temperature solar collectors were realized with all-glass solar evacuated tubes. The solar transmittance of envelop tubes was increased to 0.94 with porous SiO2antireflection coating deposited by sol-gel method. Solar selective coating was fabricated by dual-cathode co-sputtering, and its solar absorbance and thermal emittance were optimized to 0.95 and 0.05 (at 180 °C), respectively. Vacuum of evacuated tubes was obtained by outgassing at 450 °Cand maintained by combination use of evaporable and non-evaporable getters. Copper U-tubes, whose surface was treated for anti-oxidation, were weld to aluminum fins to transfer heat from solar selective coating to working fluid. Compound parabolic concentrators (CPC) made of smooth aluminum sheets were adopted to concentrate solar radiation, the solar reflectance of CPC surface was 0.91. The instantaneous solar thermal efficiency was measured from 25to 150 °C,the fitting polynomial of efficiency data was followed η=0.691−0.83Tm−0.0035GTm2, corresponding to the solar thermal efficiency of 50.2% at 150 °C̊C.