Geothermal Energy
Geothermal energy is heat energy that originates within Earth itself. The temperature at the core of our planet is 4,200°C (7,592°F), and heat flows outward to the cooler surface, where it can produce dramatic displays such as volcanoes, geysers, and hot springs, or be used to heat buildings, generate electricity, or perform other useful functions. This outward flow of heat is continually being maintained from within by the decay of radioactive elements such as uranium, thorium, and radium, which occur naturally in Earth. Because of its origin in radioactivity, geothermal energy can actually be thought of as being a form of natural nuclear energy.
The U.S. Department of Energy has estimated that the total usable geothermal energy resource in Earth's crust to a depth of 10 kilometers is about 100 million exajoules, which is 300,000 times the world's annual energy consumption. Unfortunately, only a tiny fraction of this energy is extractable at a price that is competitive in today's energy market.
In most areas of the world, geothermal energy is very diffuse—the average rate of geothermal heat transfer to Earth's surface is only about 0.06 watt per square meter. This is very small compared to, say, the solar radiation absorbed at the surface, which provides a global average of 110 watts per square meter.
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