Temperatures in the Earth generally
increase downward at 68°F–212°F (20°C–100°C) per kilometer,
following the geothermal gradient for the region. Some
regions near active volcanic vents have even higher geothermal
gradients and are typified by abundant fumaroles and
hot springs. These systems are usually set up when rising
magma heats groundwater in cracks and pore spaces in
rocks, and this heated water rises to the surface. Water from
the sides of the system then moves in to replace the water
that rose into hot springs, fumaroles, and geysers, and a natural
hydrothermal circulation system is set up. The best natural
hydrothermal systems are found in places where there are
porous rocks and a heat source such as young magma.
Geothermal energy may be tapped by drilling wells, frequently
up to several kilometers deep, into the natural
geothermal systems. Geothermal wells that penetrate these
systems commonly encounter water and less commonly
steam at temperatures exceeding 572°F (300°C). Since water
boils at 212°F (100°C) at atmospheric pressure, and higher
temperatures at higher pressures (300°C at a one-kilometer
depth), the water can be induced to boil by reducing the
pressure by bringing it toward the surface in pipes. For a
geothermal well to be efficient, the temperatures at depth
should be greater than 392°F (200°C). Turbines attached to
generators are attached to the tops of wells, requiring about
two kilograms of steam per second to generate each
megawatt of electricity.
Some countries are using geothermal energy to produce
large amounts of electricity or heated water, with China,
Hungary, Iceland, Italy, Japan, Mexico, and New Zealand
leading the list. Still, the use of geothermal energy amounts to
a very small but growing amount of the total electrical energy
used by industrialized nations around the world.
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