How much exergy one can obtain from incident solar radiation?

Abstract

A thermodynamic model is proposed to study the exergetic content of incident solar radiation reaching on the Earth’s surface which can be used to produce work through a dually cascaded thermodynamic cycle. The “topping” cycle is an ad hoc engine created by nature that connects the outer shell of the terrestrial atmosphere (which is in equilibrium with the extraterrestrial solar radiation) to the collector of a solar heat engine operating on the Earth’s surface. The work produced by the topping cycle is dissipated in form of scattering, absorption, heat, movement of air masses (wind), etc. The “bottoming” cycle is a heat engine operating between the collector and surrounding temperatures, and delivers useful work. It is shown that the maximum work extractable from this system as exergy is obtained when both cycles operate reversibly. An expression for this maximum work, which represents the exergy of incident solar radiation on the Earth’s surface, is proposed. The application of the present model is illustrated and validated by calculating the exergy of solar radiation based on some measurements. The results obtained by the present model are compared to the ones obtained through other models available in the open literature.