Seite - 134 - in Water, Energy, and Environment - A Primer

8.6 GEOTHERMALENERGY Geothermal energy is heat from theEarth and has been used by mankind for bathing and heating for centuries. The first power plant to use geothermal heat came online in 1904 in Larderello, Italy and is still generating electricity. Geothermal is an extremely large energy resource, andone that is still in the early stages of realizing its potential. 8.6.1 Sourcesofgeothermalenergy Geothermal energy derives largely, but not exclusively, from radioactive decay of uranium, thorium and potassium in the Earth’s core. Lesser amounts of core heating derive from heat released when iron cools and solidifies at the Earth’s central core, mineral phase changes, friction heating associated with Earth’s tides, and even impact collisions with matter from space. This heat convects and conducts up to the Earth’s thin crust (which comprises just 1% of the Earth’s mass) through various pathways, and manifests itself as hot water and steam, hot rock, warm earth, magma and volcanic eruptions. We can think of the crust as a blanket on the rest of the planet (see Figure 8.14). Geothermalheathasbeen flowing fromthecenterof theEarth formore than 4.5 billion years andwill continue as long as the Earth exists – about another 5 billion years. Since this flow is essentially limitless, geothermalmay be considered a renewable energy source. Also, it is available 24/7 and is thus a baseload energy source. Temperaturesclose to theEarth’scenterareaboutashotas the Sun’s surface, and geologists estimate that the rate at which energy flows from the Earth’s interior is of the order of 44 TW (terrawatts, i.e., millions ofmegawatts). The replenishment rate from radioactive decay is estimated to be about 30 TW. To put this number in perspective, today’s global installed electrical generating capacity is just over 5TW. Water,Energy, andEnvironment–APrimer134