As we all know, the primary energy sources today are fossil fuels—they supply approximately 90% of the energy consumed by people. All other sources are considered alternative energy and are divided into renewable energy and nonrenewable energy. Nonrenewable alternative energy sources include nuclear energy and deep-earth geothermal energy (the energy from the Earth’s geological processes). This kind of geothermal energy is considered nonrenewable for the most part because heat can be extracted from Earth faster than it is naturally replenished— that is, output exceeds input. Nuclear energy is nonrenewable because it requires a mineral fuel mined from Earth. The renewable energy sources are solar; freshwater (hydro); wind; ocean; low-density, near-surface geothermal; and biofuels.
Low-density, near-surface geothermal is simply solar energy stored by soil and rock near the sur- face. It is widespread and easily obtained and is renewed by the sun. Biofuels are made from biomass (crops, wood, and so forth). Renewable energy sources are often discussed as a group because they all derive from the sun’s energy. We consider them renewable because they are regenerated by the sun within a time period useful to people. The total energy we may be able to extract from alternative energy sources is enormous. For example, the estimated recoverable energy from solar energy is about 75 times as much as all the people of the world use each year.
The estimated recoverable energy from wind alone is comparable to current global energy consumption. stored in all known reserves of coal, oil, and natural gas on Earth. Solar energy is absorbed at Earth’s surface at an average rate of 90,000 terawatts (1 TW equals 1012 W), which is about 7,000 times the total global demand for energy.1 In the United States, on average, 13% of the sun’s original energy entering the atmosphere arrives at the surface (equivalent to approximately 177 W/m2 , or about 16 W/ft2 , on a continuous basis).