Investigation and optimization on spectrally selective absorption for enhanced thermal performance in porous volumetric solar receivers

Spectrally selective absorption significantly enhances the efficiency of solar-to-thermal energy conversion in diverse solar receivers including the porous volumetric solar receiver. As the working temperature of concentrating solar power system becomes elevated, the overlap between the solar spectrum and the infrared emission spectrum makes the energy transfer process complicated inside the receivers. Consequently, the influence of the spectrally selective properties on the temperature distribution and thermal performances of the porous receivers is investigated by pore-scale numerical simulation. Various factors, such as cut-off wavelength, operating temperature, and actual optical properties of molten salt are considered. These findings indicate that the optimization of cut-off wavelength should coordinate the energy loss from the porous skeletons and the fluid. The shift of cut-off wavelength to the shortwave band results in the trade-off between thermal emission reabsorption from porous media and energy losses from the fluid. The impact of the fluid's cut-off wavelength becomes evident at higher working temperature, and the thermal efficiency variations under mass flow of 4 x 10-5 and 8 x 10-5 kg/ s are 3.8 % and 1.2 %, respectively, as the cut-off wavelength changes from 500 nm to 2500 nm. Compared with the ideal porous receivers, the actual optical properties of the heat transfer fluid lead to a maximum reduction in the outlet temperature and the thermal efficiency by 19.9 K and 2.77 percentage points, respectively, at the mass flow of 4 x 10-5 kg s- 1.