Environmental Engineering Reference
In-Depth Information
7
Geothermal
Geothermal energy is heat from inside the earth. It is accessed by drilling water or steam wells in
a process similar to drilling for oil or natural gas. Geothermal energy is an enormous, underused
heat and power resource that emits little or no greenhouse gases, has average system availability
of 95 percent, and is a wholly domestic resource.
Geothermal resources range from shallow ground to hot water and rock several miles below
the earth's surface and even farther down to extremely hot molten rock called magma. Wells a
mile or more deep can be drilled into underground reservoirs to tap steam and very hot water,
which can be brought to the surface for use in a variety of applications, but there are currently no
commercial applications of magma heat (USDOE 2011).
Where available, hot water near the earth's surface can be piped directly into facilities and used
to heat buildings, grow plants in greenhouses, dehydrate onions and garlic, heat water for fish
farming, and pasteurize milk (Lienau and Lunis 1991). Some cities pipe hot water under roads
and sidewalks to melt snow. District heating applications use networks of piped hot water to heat
buildings in entire communities (USDOE 2011).
The most promising and widespread applications of geothermal energy in the United States involve
heat pumps using shallow ground energy to heat and cool buildings. Almost everywhere, the upper
ten feet of the earth's surface maintains a nearly constant temperature between 50 and 60°F (10
and 16°C). A geothermal heat pump system consists of a loop of pipes buried in shallow ground
or wells drilled near a building; a heat exchanger; and additional pipes for transferring heat into
buildings. In winter, heat from the relatively warmer ground goes through a heat exchanger into
the house. In summer, hot air from the house is pulled through the heat exchanger into relatively
cooler ground. Heat removed during the summer can also be used to heat domestic water. There
is a bright future for direct use of geothermal heat pumps in heating and cooling homes and busi-
nesses in any location (UCS 2009).
Underground piping loops are made of durable material that allows heat to pass through effi-
ciently. This is important so the pipe does not retard exchange of heat between the earth and fluid
in the loop. Loop manufacturers typically use high-density polyethylene, a tough plastic, and some
offer up to fifty-year warranties. When installers connect sections of pipe, they heat-fuse the joints,
making them stronger than the pipe itself. Fluid in the loop is water or an environmentally safe
antifreeze solution that circulates through the pipes in a closed system. The length of a ground loop
depends on a number of factors, including the type of loop configuration used, a home's heating
and cooling load, soil conditions, local climate, and landscaping. Large homes with large space
conditioning requirements generally need larger loops than smaller homes. Homes in climates where
temperatures are extreme also generally require larger loops. Geothermal heat pump systems that
are too large waste energy and do not provide proper humidity control (GeoExchange 2011).
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