Geothermal vs Ground Source Heat Pumps

This entry is about an important point of terminology--and a short discussion of each.

Geothermal energy is a way of extracting heat from the earth.  Usually, this is heat from a volcano or geyser or other feature of the earth's surface that brings very hot material close enough to the surface that it's practical to extract energy from it, and either use it directly or turn it to electricity with a steam turbine or other mechanism.  The number of spots where mother nature has been cooperative enough to make this practical are relatively few--consequently, geothermal energy isn't viable in a majority of places.  There is a vast amount of heat down there, far more than we could ever use.  But there have been many attempts to drill down to it, and so far, all have failed.  It's only practical where geology has made an opening for us.

There is a significantly different technology which is often misleadingly called geothermal.  A more accurate name would be a Ground Source Heat Pump.  This technology takes advantage of the fact that large masses of soil, stone or especially water tend collect and store heat, and remain stable year round, at whatever the average temperature is.   This tends to be about 50F or 10C, although many factors affect this.   By passing liquid through it, a heat pump can extract the energy from this and use it to provide heating in the winter and cooling in the summer.   This has enormous efficiency advantages: to cool a space to 70F when it's 90 outside and the ground is 50 takes no energy at all, except for whatever is needed to power the pumps and heat exchanger, while heating above 50 when it's cold outside takes a lot less energy than heating above (say) 10F, a temperature at which heat pumps are very inefficient.   All it takes is some plumbing, buried deep enough and spread out enough to take advantage.

 

 

Al Gore was an early adopter of a Ground Source Heat Pump system in his Tennessee home, and spoke of it often during his presidential campaign, usually calling it Geothermal. 

 

Geothermal energy is one of the very few sources of energy that is not really just a way of extracting solar energy that's been stored up over time.  It's actually heat that's either left over from the formation of earth, compression by gravity, or nuclear reactions (both fission and fusion) occurring deep within the earth's core.  Ground Source, as well as wind, photovoltaic, fossil fuels, etc., are ultimately ways of using the energy from the sun that's stored up by the earth somehow.   e.g.: chlorophyll turns  CO2 from the air into cellulose and other burnable materials, which can be burned to get energy--either by burning it directly, or waiting until heat and pressure from the earth has turned it into fossil fuels over millions of years.  E.g. wind is air movement caused by the differential heating of the surface of the earth by the sun.