Concentration of sunlight to solar-surface levels using non-imaging optics

Abstract

THEflux at the surface of the Sun, ∼6.3 kW cm^-2, falls off with the square of distance to a value of ∼137 mW cm^-2 above the Earth's atmosphere, or typically 80–100 mW cm^-2 at the ground. In principle, the second law of thermodynamics permits an optical device to concentrate the solar flux to obtain temperatures at the Earth's surface not exceeding the Sun's surface temperature. In practice, conventional means for flux concentration fall short of this maximum because imaging optical designs are inefficient at delivering maximum concentration. Non-imaging light-gathering devices can improve on focusing designs by a factor of four or more, and approach the thermodynamic limit. We have used a non-imaging design to concentrate terrestrial sunlight by a factor of 56,000, producing an irradiance that could exceed that of the solar surface. This opens up a variety of new applications for making use of solar energy.