Solar Energy: When, Where, How?

Abstract

Nomenclature H = enthalpy ReD —Reynolds number, based on diameter S = entropy T = temperature Wp = peak Watt AG = Gibbs free energy change 7] = efficiency I. Introduction S OLAR energy can be converted into a number of useful end-products via a wide spectrum of conversion technologies. A helpful way of categorizing technologies is to develop a morphology of solar conversion systems. Figures 13, first developed by Grosskreutz,1 illustrate a morphological breakdown of solar conversion modes by generic type. When solar radiation is absorbed by a material, two primary processes ensue: 1) vibrational excitation of atoms and molecules in the absorber produce heat which leads to endproducts via the thermoconversion path; and 2) selected quantum processes in the absorber, through electronic excitation and charge transfer, lead to useful end-products via the photoconversion path. The detailed morphology of these two paths is exhibited in Figs. 2 and 3. The salient features of each process are described below. A. The Thermoconversion Path There are five primary products associated with the thermoconversion path illustrated in Fig. 2. Of these, hot fluids, with or without concentrators, represent a direct conversion mechanism which is the most commonly known solar conversion technology. The other four are often classified as indirect conversion technologies in that solar radiation first energizes other media, such as the atmosphere (winds) and the oceans, to which man applies a conversion technology in order to extract useful power. The boundary that separates the cross-hatched area from the open area designates the cutting edge of research and development. To the left of this boundary the conversion technology is mature and is either in an advanced engineering development stage or is commercially available. Electricity and space heating represent the most familiar end-products of thermoconversion. Process heat and fuels and chemicals are more recent entries into the arena of solar thermoconversion applications. Because of its end-use matching flexibility, process heat may have a larger quad impact than electricity, whereas fuels and chemicals will add a new dimension to energy storage in solar applications, namely, spatial displacement of energy. B. The Photoconversion Path There are three primary products associated with the photoconversion path illustrated in Fig. 3. The common features to all three products are electronic excitation processes (in contradistinction to vibrational excitation for the thermoconversion path), followed by charge separation, and charge transfer mechanisms that eventually lead to electricity generation or to energy storage for fuel production. In semiconductor s, photon absorption and electronic excitation involve primarily photosensitive atoms, whereas in