Numerical investigation on uniformity of heat flux for semi-gray surfaces inside a solar cavity receiver

Solar cavity receiver is quite often utilized in the solar tower power system. It is a photo-thermal conversion component, which absorbs solar radiation energy and heats the working fluid. The safety and thermal efficiency of solar cavity receiver can directly affect the efficiency of the entire power system. In this paper, a modified combined method is proposed to simulate the thermal performance of a saturated water/steam solar cavity receiver. Compared with the combined method presented previously, this modified one can describe the whole steady process of the receiver and accurately estimate the steady-state efficiency and the mass flow rate of the generated steam in the boiling tubes. Under the given boundary condition of incident solar flux, the modified method is adopted to find an optimized distribution of absorptivity in the solar wavelength band for the absorber surfaces in order to improve the uniformity of surface heat flux distribution. The simulation results show that the uniformity of surface heat flux is improved effectively and the maximum mean square deviation of the external tube temperature drops from 32.4 degrees C to 23.6 degrees C after optimizing. Based on the relatively uniform heat flux distribution, the effect of thermal emissivity of the absorber surfaces on the thermal performance of the cavity receiver is further investigated. Being different from those of parabolic trough and external cylindrical receivers, both the radiative and the convective heat loss of the present cavity receiver decrease with increasing thermal emissivity, which improves the receiver's efficiency. These results can provide a useful reference for the design and selection of surface coatings for the cavity receivers. (C) 2014 Elsevier Ltd. All rights reserved.