Geothermal energy has been around for so long that one would think it would be the most common way to heat and cool buildings, even in the coldest climates. But, for a lengthy list of largely economic and political rationalizations, we continue to choose fossil fuels to heat and cool our buildings, rather than use the steady (and free) temperature of the earth.
Geothermal systems generally fall into two types: vertical and horizontal. In a vertical system, water pipes are buried underground (50’-400’ deep); in a horizontal system, a loop field of pipes is used that may only be six or eight feet below the surface.
In either system, as the water—typically mixed with an anti-freeze like glycol—circulates through the underground pipes it absorbs heat from the ground. The warmed water is then pumped to the surface and circulated through radiators in buildings for heating; a heat exchanger may be needed to boost the temperature. By switching the direction of the exchanger’s heat flow, excess heat in summer can be stored in the ground or in groundwater.
The Southampton Geothermal Heating Company in Southampton, England operates a combined and heat power (CHP) system that is supplemented with a geothermal plant and conventional boilers. Annually, the system cuts carbon dioxide emissions by about 10,000 tonnes.
In all, the plant supplies about 70 GWh of energy each year to several city-owned buildings (civic centre, city hall and office buildings), a hospital and university, several hotels, a shopping centre, public and private sector housing, and the local BBC TV studio.