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This page at energy.gov says the following about geothermal (or ground-source) heat pumps.

Geothermal heat pumps (GHPs), sometimes referred to as GeoExchange, earth-coupled, ground- source, or water-source heat pumps, have been in use since the late 1940s. They use the constant temperature of the earth as the exchange medium instead of the outside air temperature. This allows the system to reach fairly high efficiencies (300% to 600%) on the coldest winter nights, compared to 175% to 250% for air-source heat pumps on cool days.

In normal parlance (as in the fields of physics or engineering), a device's efficiency cannot be greater than 100%. Therefore, I am at a loss to understand what energy.gov is trying to tell me here. What do the numbers mean?

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There are several ways to measure a Heat Pump's performance, which doesn't help. There are several different flavours of Seasonal Performance Factor, SPF, which incorporate differing elements.

The COP - coefficient of performance - ignores all these complexities, and simply looks at how much heat is moved; and how much energy was required to move it. That's all a heat pump does: it moves heat around. A ground-source heat pump moves heat from the earth into a building. And we can move a lot of heat around, with a small amount of energy. So when we divide the amount of heat by the amount of energy required to move it, that number can come out bigger than 100%. For a ground-source heat pump delivering low-grade heat, you might expect COPs of 3-5, i.e. 300-500%.

There is a theoretical limit on the COP, which is determined by ratio of the absolute temperature of the body that you are taking it from, to the difference between that and the absolute temperature that you are raising the other body to: this is the Carnot limit.

That's completely different to what, say, an electrical resistance heater does: that simply converts electricity into heat, and does indeed have a maximum theoretical efficiency of 100%.

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