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This question may be about semantics as much as sustainability, but let's see...

When I think of geothermal energy, I think of hot rocks (or hot aquifers) that are kept warm by the internal volcanic activity of the planet.

Some people use the same term to refer to ground-source heat pumps, which are usually installed at a shallow depth where I imagine most of the energy comes from the ground being kept warm by the sun.

Are ground source heat pumps a form a geothermal energy, or is this term being misused?

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  • The hot rocks are kept warm by nuclear fission in the Earth's core.
    – andy256
    Apr 7, 2015 at 10:08
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    @andy256 It's not so much nuclear fission that heats up the earth's core as it is nuclear decay of radioactive elements.
    – Johnny
    Apr 8, 2015 at 3:08
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    @Johnny To most people what you say is correct. Decay is a lot slower, and spontaneous. Fission is usually triggered by neutrons. Technically, decay is fission, but anyone except a physicist would say that was splitting ... er hairs :-)
    – andy256
    Apr 8, 2015 at 7:16
  • "Geothermal" means the heat comes from below using heat energy derived from the sun (or some other star) a very long while ago. | If the heat energy extracted from the ground is due to solar insolation within the last year (and usually in the last days to weeks) then it is incorrect to term the heat source "Geothermal".
    – Russell McMahon
    Apr 11, 2015 at 11:44

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Language is a very versatile thing; and the meaning of words is a social convention, nothing more. So if two people agree in their conversation to call a ground-source heat pump geothermal, then it is, for them.

However, from what I've seen, it is more commonly the case that people call it geothermal as a mistake. Ground-source heat pumps are indeed solar energy collectors, unless you're somewhere like Iceland where the geothermal energy gives significant warming to the ground very close to the surface.

So yes, it is possible to have geothermal heat pumps. But almost all the cases I've seen where that name's been used, that's not what's going on. It's that someone's misunderstood where the heat has come from.

As a quick crude check, take the temperature of the area from where the heat will be extracted. at the depth it will be extracted. If that's about the same temperature as the year-round average surface temperature, then the heat has come from the sun. If it's more than 10 Kelvin (degrees Celsius) warmer than that, then it's geothermal.

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    I would go much further than 'misunderstood'. I think it is deliberately deceptive false advertising on the part of people trying to sell ground-source heat pumps as geothermal heating, and I wish the FTC or someone would take them to court over it.
    – jamesqf
    Apr 7, 2015 at 19:13
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Yes, it is geothermal energy. From Wikipedia: Geothermal energy is thermal energy generated and stored in the Earth.

The entire point of in-ground heatpumps is to treat the ground as a large heatsink that's unaffected by weather, seasons, and the day-night cycle. These systems use pipes buried below the depth where the ground ever freezes, just like utility companies do for their water lines. They can give or take heat without noticeably effecting the surrounding earth.

Ground temperature at this depth is not based on the sun, and that's the entire point.

The one thing you're probably confused about is that, no, these systems don't generate energy. They aren't an energy source. They're just a very efficient heat exchange system. They're more efficient than normal heatpumps because the air temperature is variable while the ground temperature maintains a moderate temperature year-round.

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    "Ground temperature at this depth is not based on the sun, and that's the entire point." I am skeptical of this - I suspect that at the depths that most heat pumps use, most of the heat energy comes from above rather than below.
    – Flyto
    Apr 10, 2015 at 9:08
  • @Simon: Visit Toronto where the air sometimes hits -20C in the winter. Notice that their in-ground water pipes don't freeze. The heat isn't coming from above.
    – Sophit
    Apr 10, 2015 at 20:26

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