Heat transfer in high-temperature volumetric solar receivers

High conversion efficiency is key to the success and competitiveness of solar thermal power plants, and can be achieved with cycles based on gas turbines and combined cycles that may reach around 30% solar-to-electricity efficiency. This requires the difficult task of heating air with solar radiation to over 1,000°C for the gas turbine inlet. Volumetric receivers made from porous ceramic structures can reach these temperatures but currently their radiation-to-heat conversion is low, typically around 70% rather than the desirable range of over 90%. Modeling of radiative and convective heat transfer in a volumetric receiver enables analysis and optimization of the various geometric and material-dependent properties to achieve a significant performance improvement. The model represents the porous medium as an effective or averaged medium with homogeneous properties. A parametric study of the volumetric receiver performance reveals several guidelines for improving the receiver design. Optimization of geometry (higher porosity and larger characteristic pore diameter) is insufficient, and improved efficiency requires a significant increase in convection heat transfer beyond the normal behavior of ceramic foams, and a reduction in thermal conductivity. Finally, spectral selectivity of the absorber material can also help in further increase of efficiency, in contrast to the common opinion that it is effective only at low temperatures.