Theoretical efficiencies of angular-selective non-concentrating solar thermal systems

This paper explores the thermodynamic limits of solar thermal systems using absorber surfaces with high angular selectivity. It shows that theoretical considerations regarding the behavior of non-concentrating solar thermal systems are not enough in themselves to rule out their use in high-temperature applications. In fact, the paper demonstrates that the maximum theoretical thermal efficiency of a non-concentrating system may equal that of a concentrating system if the monochromatic directional emittance of its absorbing surface is sufficiently restricted. It is thus confirmed that the fundamental parameter that determines the ability of a solar-energy system to operate at high temperatures is not optical concentration, but angular selectivity of emittance.