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Geothermal Energy August 23, 2009

Posted by federalist in Energy.
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While researching the best way to take advantage of the 2009-2010 tax credit for household energy upgrades I looked at geothermal heating/cooling systems.  Currently the most energy-efficient residential climate control system is a heat pump, which is essentially two-way air conditioner: It doesn’t expend energy to generate heat, but rather to extract latent heat from air in one location and to pump it to another.  During the summer it pumps from inside a house to the outside, like a conventional air conditioner, and during the winter it pumps heat from outside air back indoors.  The problem with an outdoor heat pump is that it’s always working against the weather: During hot weather it’s trying to move heat from a hot location (inside) to an even hotter location (outside).  During cold winters it’s even harder to extract heat from freezing outside air to raise indoor temperatures to a comfortable level.  Geothermal heat pumps solve this problem by exploiting the fact that subterranean temperatures are a nearly constant 50 degrees year-round.  By putting the heat exchanger underground the heat pump runs much more efficiently because it doesn’t have to fight the weather.

Geothermal heat pumps are very nifty and efficient, but even with the government’s 30% tax credit they are still not economical for conventionally-sized residences.  Drilling underground heat exchange loops will typically exceed $10,000, and the efficiency gains don’t justify that expense given current energy prices.

But geothermal heat sinks aren’t the only advancing technology.  A study by the U.S. Geological Survey suggests that geothermal power generation may be able to satisfy a large portion of our energy demand.  Existing geothermal power plants depend on unique geological formations where high temperatures can be found in permeable rock within 2 miles of the earth’s surface.  Such sites are limited and only rarely economical to exploit.  Newer approaches, dubbed “Enhanced Geothermal Systems” (EGS), would tap up to 4 miles below the surface and take advantage of the higher temperatures and pressures to drive power plants with greater capacity and service life.  EGS appear to be practical over much wider geographic areas.  The USGS report, the first comprehensive geothermal resource assessment in thirty years, suggests that EGS could add on the order of half a terawatt of generating capacity to the domestic power grid.  (Current U.S. generating capacity is roughly one terawatt.)

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