Humans have been harnessing solar energy for thousands of years—to grow crops, stay warm, and dry foods. Photovoltaic (PV), cells are made from silicon or other materials that transform sunlight directly into electricity. Solar farms can generate power for thousands of homes, using mirrors to concentrate sunlight across acres of solar cells. Solar energy systems don’t produce air pollutants or greenhouse gases.
We’ve come a long way from old-fashioned windmills. Today, turbines as tall as skyscrapers with turbines nearly as wide in diameter—stand at attention around the world. Wind energy turns a turbine’s blades, which feeds an electric generator and produces electricity.
Hydropower is the largest renewable energy source for electricity globally, though wind energy is soon expected to take over the lead. Hydropower relies on water—typically fast-moving water in a large river or rapidly descending water from a high point—and converts the force of that water into electricity by spinning a generator’s turbine blades.
Biomass is organic material that comes from plants and animals, and includes crops, waste wood, and trees. When biomass is burned, the chemical energy is released as heat and can generate electricity with a steam turbine.
Tidal and wave energy is still in a developmental phase, but the ocean will always be ruled by the moon’s gravity, which makes harnessing its power an attractive option. Some tidal energy approaches may harm wildlife, such as tidal barrages, which work much like dams and are located in an ocean bay or lagoon. Like tidal power, wave power relies on dam-like structures or ocean floor–anchored devices on or just below the water’s surface.