Sunlight is Earth’s most abundant energy source and is delivered everywhere free of charge. Yet direct use of solar energy—that is, harnessing light’s energy content immediately rather than indirectly in fossil fuels or wind power—makes only a small contribution to humanity’s energy supply. In 2008, about 0.1% of the total energy supply in the United States came from solar sources. In theory, it could be much more. In practice, it will require considerable scientific and engineering progress in the two ways of converting the energy of sunlight into usable forms.
Photovoltaic systems are routinely employed to power a host of devices—from orbiting satellites to pocket calculators—and many companies make roof-sized units for homes and office buildings.
Photovoltaic (PV) systems exploit the photoelectric effect discovered more than a century ago. In certain materials, the energy of incoming light kicks electrons into motion, creating a current. Sheets of these materials are routinely employed to power a host of devices—from orbiting satellites to pocket calculators—and many companies make roof-sized units for homes and office buildings.
At the present time, however, the best commercial PV systems produce electricity at five to six times the cost of other generation methods, though if a system is installed at its point of use, which is often the case, its price may compete successfully at the retail level. PV is an intermittent source, meaning that it’s only available when the Sun is shining. Furthermore, unless PV energy is consumed immediately, it must be stored in batteries or by some other method. Adequate and cost-effective storage solutions await development. One factor favoring PV systems is that they produce maximum power close to the time of peak loads, which are driven by air-conditioning. Peak power is much more expensive than average power. With the advent of time-of-day pricing for power, PV power will grow more economical.
Sunlight can also be focused and concentrated by mirrors and the resulting energy employed to heat liquids that drive turbines to create electricity—a technique called solar thermal generation. Existing systems produce electricity at about twice the cost of fossil-fuel sources. Engineering advances will reduce the cost, but solar thermal generation is unlikely to be feasible outside regions such as the southwestern United States that receive substantial sunlight over long time periods.
Despite the challenges, the idea of drawing our energy from a source that is renewable and that does not emit greenhouse gases has powerful appeal.