Solar energy requires no additional fuel to run and is pollution free. Sunlight can be captured as usable heat or converted into electricity using solar, or photoelectric, cells or through synchronized mirrors known as heliostats that track the sun’s movement across the sky. Scientists have also developed methods for using solar power to replace a gas-powered engine by heating hydrogen gas in a tank, which expands to drive pistons and power a generator.

Drawbacks of solar energy include high initial cost, and the need for large spaces. Also, for most solar energy alternatives, productivity is subject to the whims of air pollution and weather, which can block sunlight.