The Sun delivers enough energy to the Earth every day to power global society many times over. Solar energy’s potential is enticing; photovoltaic technology is improving and the cost is falling. Intermittency, lower energy return on energy investment, institutional barriers, and dependence on rare metals for manufacturing are challenges to solar PV gaining a significant share of the global energy portfolio.
Solar photovoltaic (PV) panels use the energy from the Sun to “excite” electrons into a high energy state, at which point they are converted into electricity. Most photovoltaic panels use crystalline silicon as a base material, but recent advances have led to the use of more scarce elements such as cadmium, tellurium, indium, and gallium. “Thin film” PV panels have also been developed that use less silicon than traditional PV panels. Total global installation of solar PV was roughly 40 gigawatts in 2010, distributed in more than 100 countries. The rapid expansion of manufacturing capacity, particularly in China, has caused solar PV panel prices to drop dramatically, and maturation of the industry is projected to similarly reduce “balance of system” (design, installation, etc.) costs in coming years.
Solar PV offers numerous advantages over fossil fuels for generating electricity. Greenhouse gas emissions are considerably lower over the life of the panel, even when accounting for emissions during construction. Additionally, solar energy is distributed (albeit not evenly) throughout the world, which means many remote populations can produce electricity without constructing inefficient, expensive, and habitat-disrupting long-distance-transmission infrastructure.
Like wind, however, solar energy is intermittent. Not only are there diurnal fluctuations in solar energy but cloud cover, fog, seasonal light availability, and even dust on the panels can severely affect photovoltaic electricity generation. The conversion efficiency (i.e., converting incident solar radiation into electricity) of PV panels is quite low as well, around 15 percent, although estimates vary widely and new technology is incrementally increasing efficiency. The conversion of coal to electricity, by comparison, is over two times more efficient than solar panels. New thin-film PV has been integrated into building facades and roofing, expanding the possibilities of where solar systems can be installed, although it currently has lower conversion efficiencies than conventional PV.
The countries with the fastest growth rates in solar installation are also those with the most aggressive subsidy programs. In Germany, for example, the government for a time was paying more than 60 cents per kilowatt-hour for power from small solar PV systems, which is almost ten times higher than the price of electricity in some parts of the United States. Spain had a similar program that boosted solar electricity generation there. Federal and state incentives, as well as innovative financing programs, have helped stimulate the growth in U.S. solar PV installations, and recent declines in the price of PV panels have prompted some proposed utility-scale solar themal generating stations to switch to PV.