Geothermal Systems
Geothermal heating relies on an energy exchange between the air within the
building being heated and the ground. Below ten feet the earth's temperature is
fairly constant, generally around ~10ºC (~50ºF). During the summer
when the ambient temperature of the building exceeds that of the ground heat
pumps are used to pump heat from the building in to the transfer medium
(typically water with small amounts of ethanol or glycol) and is subsequently
pumped through narrow pipes into the ground so that the heat can be dissipated
in the earth. When the ambient temperature falls below the ground temperature
the process works in reverse. Heat pumps extract heat from the ground and use it
to heat the building.
Geothermal Heat Pumps
Even in regions without large high
temperature geothermal resources, a geothermal heat pump can still provide space
heating and air conditioning. Like a refrigerator or air conditioner, these
systems use a heat pump to force the transfer of heat from the ground to the
application. In theory, heat can be extracted from any source, no matter how
cold, but a warmer source allows higher efficiency. A ground-source heat pump
uses the shallow ground or ground water (typically starting at 10–12 °C,
50–54 °F) as a source of heat, thus taking advantage of its seasonally
moderate temperatures. In contrast, an air-source heat pump draws heat from the
colder outside air and thus requires more energy. Closed loop geothermal heat
pumps circulate a carrier fluid (usually a water/antifreeze mix) through pipes
buried in the ground. As the fluid circulates underground it absorbs heat from
the ground and, on its return, the now warmer fluid passes through the heat pump
which uses electricity to extract the heat from the fluid. The re-chilled fluid
is sent back through the ground thus continuing the cycle. The heat extracted
and that generated by the heat pump appliance as a by-product is used to heat
the house. The addition of the ground heating loop in the energy equation means
that more heat is generated than if electricity alone had been used directly for
heating. Switching the direction of heat flow, the same system can be used to
circulate the cooled water through the house for cooling in the summer months.
The heat is exhausted to the same relatively cool soil (or groundwater) rather
than delivering it to the hot outside air as an air conditioner does. As a
result, the heat is pumped across a smaller temperature difference and this
leads to higher efficiency and lower energy use.
Economics
Geothermal energy is a type of renewable energy
that encourages conservation of natural resources. According to the U.S.
Environmental Protection Agency, geo-exchange systems save homeowners 30-70
percent in heating costs, and 20-50 percent in cooling costs, compared to
conventional systems. Geo-exchange systems also save money because they require
much less maintenance. In addition to being highly reliable they are built to
last for decades.
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