Environmental Engineering Reference
In-Depth Information
electricity about double that of coal, which sells for around 5 cents
per kilowatt-hour.
Heat Pumps
Even without a nearby hot spot, the ground can be tapped to reduce the
need for fossil fuels. All of the soil in the United States is suitable for a
technology known as geothermal heat pumps, which transfer heat from
the ground to homes in the winter and reverse direction to provide
cooling in the summer. Heat pumps have the potential to provide nearly
all households with heat and hot water.
Installing a heat pump has a higher initial cost than a natural gas
furnace—about $7,500 for an average-size home. But monthly savings
could total between 25 and 50 percent of current utility bills, with the
system paying for itself in two to ten years. In 2004 a heat pump system
was built into a new building in Washington for the Department of
Agriculture. The building is heated and cooled for less than ten cents per
square foot. About 1 million heat pumps have been installed, and roughly
50,000 to 60,000 more are being added annually.
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Engineered Geothermal Systems
Geothermal systems do not need to be restricted to areas where shallow
groundwaters are naturally heated. An MIT study in 2007 concluded
that converting geothermal heat into electricity by pouring water onto
hot rocks underground and using the steam generated to turn turbines
is arguably the most promising alternative and renewable source of green
energy on the planet. The process is called
engineered geothermal systems
(EGS). The researchers calculated that there is more than enough extract-
able hydrothermal energy available to generate the entire 27 trillion
kilowatt-hours of energy consumed in the United States in 2005. In fact,
a conservative estimate of the energy extractable from hot rocks less than
30,000 feet beneath American soil suggests that this almost completely
untapped energy resource could support U.S. energy consumption at its
present rate for more than two thousand years. The resource is techni-
cally available for exploitation right now. Simply pump cold water into
one borehole, and when it hits the hot rocks, it creates a network of
fractures, which allow the water to travel horizontally to a second hole,
where steam is allowed to escape and drive a turbine generator. The
spent steam is condensed and recycled back down to the hot rocks again,
making the water consumption insignifi cant. Once a geothermal plant is
built, there are no fuel costs, so production cost is less than for a coal
