and proliferation resistance of nuclear reactors, as well as widen their applications. DOE is pursuing the demonstration of one such design, a very-high-temperature reactor, through its Next Generation Nuclear Plant program, and the facility is scheduled to begin operations by 2021.


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 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 (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. In addition, 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, which is technologically feasible, PV power would be much closer to being economical.

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