Environmental Engineering Reference
In-Depth Information
fruit” of geothermal energy potential (EERE, 2012). Enhanced geothermal systems
offer the ability to extend the use of geothermal resources to large areas of the west-
ern United States, as well as into new geographic areas of the entire country. More
than 100,000 MWe (megawatt electrical) of economically viable capacity may be
available in the continental United States, representing a 40-fold increase over cur-
rent geothermal power generating capacity. This potential is about 10% of the overall
U.S. electrical capacity today, and such systems represent a domestic energy source
that is clean, reliable, and proven.
The EGS concept is to extract heat by creating a subsurface fracture system, a
reservoir, to which water can be added from injection wells, which are drilled into
hot basement rock that has limited permeability and fluid content. This type of geo-
thermal resource is sometimes referred to as “hot, dry rock” and represents an enor-
mous potential energy resource. Creating an enhanced, or engineered, geothermal
system requires improving the natural permeability of rock. Rocks are permeable
due to minute fractures and pore spaces between mineral grains. Water is injected at
sufficient pressure to ensure fracturing; the water is heated by contact with the rock
and returns to the surface through production wells, as in naturally occurring hydro-
thermal systems. Additional production wells are drilled to extract heat from large
volumes of rock mass to meet power generation requirements. A previously unused
but large energy resource is now available for clean, geothermal power generation.
GEOTHERMAL HEAT PUMPS
Geothermal heat pumps (GHPs), sometimes referred to as geoexchange, earth-cou-
pled, ground-source, or water-source heat pumps, have been in use since the later
1940s. Geothermal heat pumps use the constant temperature of the Earth as the
exchange medium instead of the outside air temperature. This allows the system to
reach fairly high efficiencies (300 to 600%) on the coldest of winter nights, compared
to 175 to 250% for air-source heat pumps on cool days. Geothermal heat pumps are
used for space heating and cooling, as well as water heating. Their great advantage
is that they work by concentrating naturally existing heat, rather than by produc-
ing heat through the combustion of fossil fuels. The system includes three principal
components (see
Figure 6.6
):
•
Geothermal Earth connection subsystems
—Using the Earth as a heat source
(sink), a series of pipes, commonly called a
loop
, is buried in the ground near
the building to be conditioned. The loop can be buried either vertically or
horizontally. It circulates a fluid (water, or a mixture of water and antifreeze)
that absorbs heat from, or relinquishes heat to, the surrounding soil, depend-
ing on whether the ambient air is colder or warmer than the soil.
•
Geothermal heat pump subsystem
—For heating, a geothermal heat pump
removes the heat from the fluid in the Earth connection, concentrates it, and
then transfers it to the building. For cooling, the process is reversed.
•
Geothermal heat distribution subsystem
—Conventional ductwork is gen-
erally used to distribute heated or cooled air from the geothermal heat
pump throughout the building.
