All of the U.S. geothermal power plants
are in California, Nevada, Utah, and
Hawaii. Altogether about 2800 MW of
geothermal electric capacity is produced
annually in this country.
Geothermal Direct Use
If you’ve ever soaked in a natural hot spring,
you’re one of millions of people around the
world who has enjoyed the direct use of geothermal energy. Direct-use applications
require geothermal temperatures between
about 70° to 302°F—lower than those
required for electricity generation. The
United States already has about 1,300 geothermal direct-use systems in operation.
In a direct-use system, a well is drilled
into a geothermal reservoir, which provides a steady stream of hot water. Some
systems use the water directly, but most
pump the water through what’s called a
heat exchanger. The heat exchanger keeps
the water separate from a working fluid
(usually water or a mixture of water and
antifreeze), which is heated by the geothermal water. The working fluid then
flows through piping, distributing the
heat directly for its intended use.
The heated water or fluid can be used in a
building to replace the traditional heat
source—often natural gas—of a boiler, furnace, and hot water heater. Some cities
and towns actually have large direct-use
heating systems—called district heating—
that provide many buildings with heat.
Geothermal direct use is also used in agriculture—such as for fish farms and to heat
greenhouses—and for industrial food processing (vegetable dehydration).
Geothermal Heat Pumps
While air temperatures can vary widely
through the seasons, the temperatures of
the shallow ground only range from 50°to
70°F depending on latitude. GHPs draw
on this relatively stable temperature as a
source for heating buildings in the winter
and keeping them cool in the summer.
Through underground piping, a GHPdis charges heat from inside a building into
the ground in the summer, much like a
refrigerator uses electricity to keep its interior cool while releasing heat into your
kitchen. In the winter, this process is
reversed; the GHPextracts heat from the
ground and releases it into a building.
Because GHPs actually move heat between
homes and the earth, instead of burning
fuels, they operate very cleanly and efficiently. In fact, GHPs are at least three times
more efficient than even the most energyefficient furnaces on the market today.