Overview of solar energy conversion technologies: Quantum processes and thermal processes

Abstract

The conversion of solar energy into heat, fuels or electricity is accomplished by a wide range of technologies. The sunlight may first interact with the biosphere, producing winds, ocean currents, thermal gradients, salinity gradients, moving water or plants to be harnessed by turbines, low temperature heat engines or biomass processing systems. Alternatively, systems that interact directly with incoming sunlight may provide more cost-effective sources of energy in many climates. The purpose of this paper is to provide an overview of these conversion systems and their critical interfacial problems, focusing primarily on direct quantum and thermal systems. Direct quantum conversion includes the production of fuels from photobiological, photochemical and photoelectrochemical converters, and the generation of electricity from photovoltaic systems. Key interfacial questions in these systems are related to the efficiency of charge separation and the repression of charge recombination. The electrolyte-semiconductor interface, the metal-oxide-semiconductor interfaces and the photo-oxidative stability of encapsulating layers are also important issues. On a microscopic scale the effects of grain boundaries, defects and material inhomogeneities in the semiconductor, biological or chemical complexes are also important. The thermal conversion of sunlight to useful heat, ranging from low temperature hot-water heating to process heat in excess of 1000 °C, requires the successful absorption and conversion of photons to phonons with minimum reradiation. For temperatures in excess of 100 °C, satisfactory conversion efficiencies require optical concentrators. The optical and mechanical stabilities of the absorbing and reflecting surfaces in solar thermal concentrators dominate the interfacial problems in these systems. The interaction of the working fluid (used to extract the heat from the receiver and to deliver it to the end use) with the container material of the receiver heat transfer system is also of considerable interest. Brief discussions of ocean thermal, biomass and membrane systems are also included.