Seite - 138 - in Water, Energy, and Environment - A Primer

Estimated global potential varies from 0.04 to 2 TW, depending on the depth of drilling and level of investment (wells as deep as 6 miles are now common in the petroleum industry). Research on EGS is underway in several countries, includingAustralia, France, Portugal, theUK, and theUS. 8.6.3.2 Ground-sourceheatpumps Air-source heat pumps, well known and widely used, deliver heat that isdrawnfromoutsideair intoahouseorotherbuilding. The problemwith such heat pumps is threefold: they sit outside and are exposed to the weather, they use electric-powered air conditioning, and when heat is required and the outside air gets cold enough the heat pumps are effectively electric heaters. This increases electricity demand on the grid and can cause brownouts, as happened during a winter cold snap on the US East Coast a number of years ago. Ground-source heat pumps (GHPs), also known as geothermal or geo-exchange heat pumps, in contrast, exchange energy with the ground (or undergroundwater aquifers), taking advantage of the fact that a few feet below the Earth’s surface the ground temperature remains relatively constant.Dependingon latitude, these ground temperaturescan range from45°F(7°C) to75°F(21°C). This enables a GHP, which can be located inside a building where it is shielded fromweather effects andmore rapid aging, to provide cooling in the summer and heating in the winter – basically a reversible refrigeration cycle (see Figure 8.17). If so equipped, theheatpumpcanalsosupplythehousewithhotwater. OtheradvantagesofGHPsaretheirquieteroperationcompared with air-source heat pumps, reduced peak demands on utility grids, and reduced consumer energy costs. ‘System life is estimated at 12–15 years for the inside components and 50+ years for the ground loop (51).’ The downside for consumers is the need to drill holes for the heat exchange with the ground: either deep vertical holes with Water,Energy, andEnvironment–APrimer138