As a reminder, three original sources—the earth, the moon, and the sun—ultimately provide all the energy we consume. The earth provides nuclear fuels and geothermal resources. The moon induces tidal energy. The sun provides the rest, including the renewables such as wind, solar, hydro, and bioenergy. The sun also provides the fossil fuels, which are forms of bioenergy converted under millennia of geological compression and heat into oil, gas, and coal.
Of the forms of renewable energy, hydroelectric energy ranks as one of the most important sources historically and remains so today. Hydroelectric turbines harvest energy from water flowing downhill to generate electricity. Sailboats employ wind as a form of mechanical energy, and modern turbines convert wind into electrical power. Solar energy can be used for direct heating or for electrical power. The earth provides geothermal energy for passive heating, passive cooling, and power generation. Bioenergy includes crops, trees, and plants, as well as organic waste materials, such as cow dung, old tires, municipal solid waste, agricultural waste, and landfill gas. Producers usually combust bioenergy sources in their original form to generate power, heat, or motion, though sometimes the combustion occurs after conversion to a liquid fuel, like ethanol. Energy from the ocean includes the forms that can be harnessed from waves, tides, ocean thermal energy conversion (OTEC), and salinity gradients where freshwater and saltwater mix, such as at the mouth of a river.
Renewable energy sources replenish continually or annually. Wind, solar, and hydro can never deplete, as meteorological, astronomical, and geological forces are not anticipated to end on any timeline that is relevant to human planning purposes. However, renewable energy and sustainable energy are not synonymous. Demand and mismanagement can deplete renewable energy sources faster than they replenish. Overuse can deplete bioenergy resources faster than they grow back, as demonstrated by deforestation. Rate of consumption also affects geological resources. For example, geothermal resources sometimes play out. Hydroelectric dams can silt up over time, which means they might not be sustainable even though they produce renewable energy. Interestingly, fossil fuels can replenish because of ongoing growth and sedimentation of organic matter, but renewal occurs so slowly that human society treats fossil fuel reserves as a fixed asset that will only deplete with time.
Renewable energy is typically very land intensive. Hydroelectric dams and their reservoirs can require many square miles of land. Wind farms can consist of hundreds of turbines each requiring dedicated space. Solar photovoltaic harvesting stations and concentrated solar power are often constructed in large arrays. Bioenergy requires biomass grown fairly traditionally in vast fields. However, many facilities and sites of generating renewable power are compatible for dual use. Wind farms can also be used for agriculture, ranching, or other land intensive activities. Rooftop solar panels can be used in urban areas with buildings that serve a different purpose. Hydroelectric reservoirs can also be used for flood control, storage, irrigation, and recreation. Balancing tradeoffs between land intensity and zero-emissions operation is one of the key challenges of renewable energy.