The solar opportunity

Solar energy utilization poses a vexing conundrum: at present, we cannot afford to use it, but eventually we probably cannot afford not to use it. The promise rests in the unmatched size of the solar resource: more energy from the sun strikes the Earth in one hour than all of the energy consumed on the planet in an entire year (DOE 2005; Lewis and Nocera 2006). As with all energy sources, challenges for solar energy use reside in the "cost of extraction." The diffuseness of solar energy, typically providing a yearly averaged power density of about 200 W/m2 at representative midlatitudes, requires the coverage of relatively large areas with a sunlight capture system, in turn requiring very inexpensive but high-performance materials and balance of systems to be viable. Additionally, to contribute to a large fraction of a global energy system, use of solar energy requires concomitant development of an accompanying technological approach to provide tera-watt (TW)-days of reliable, robust, persistent, scalable, and cost-effective energy storage.