[kragen]

Ground-source heat pumps are irrelevant to geothermal energy sources, and it's unfortunate that the article mentioned them. Ground-source heat pumps are just storing heat from the air during the summer and retrieving it during the winter.

[dns_snek]

Hmm, that doesn't sound right. Many homes don't use these heat pumps for cooling in the summer because getting convectors and plumbing installed is just as expensive as getting a separate AC, and then you have some redundancy in the system too.

I'm sure that battery effect is a factor but it must be a relatively minor one.

[kragen]

It's not minor at all. For a ground-source heat pump to work at all, you need to sink the pipes deep enough that the soil temperature is pretty stable year-round, which means the heat it can exchange with the aboveground air is minor or insignificant. So the vast majority of heat flux into or out of that soil is due to the heat exchange fluid circulating in the pipes.

After a few years of pumping heat out of the ground below the frost line during the winter, they'll freeze the ground solid and stop working (and possibly destroy the foundation of the house in the process, since often the pipes are installed in trenches around the house).

The only exception is if they're one of these few borderline systems that drill so deep they really are bringing up fresh energy from the depths, like some of the systems mentioned in the article.

[adrianN]

If it can be used to heat houses in winter it’s not irrelevant.

[kragen]

Insulation, adobe construction, and vigorous exercise can all "heat houses in winter" in the same way as ground-source heat pumps, but none of them can be meaningfully compared with hydroelectric or nuclear power except in a specific situation. How much insulation is enough to charge your cellphone or run a load of laundry? The question is nonsense.

[adrianN]

There are lots of houses getting the majority of their heat from ground source heat pumps. Not so many that are heated with vigorous exercise.

[kragen]

Ground-source heat pumps (with a few exceptions and borderline cases like those mentioned in the article) are not sources of heat, so you can only "get heat from" them in the short term; as with a battery or an interest-free checking account, though you may be able to temporarily run a debit balance, in the long run you can only get out what you put in.

This is a fundamental difference from energy sources.

Energy storage is an important complement to energy sources, especially renewables, and can substitute for energy sources to a limited extent, but confusing them is a fatal error.

[adrianN]

Ground source heat pumps as used for heating buildings source their energy from the sun and work very well. They are outcompeted by air source heat pumps despite better efficiency because digging is expensive.

[kragen]

As I understand it, no, they do not source their energy from the sun. You have to bury the pipes below the frost line. In temperate climates, the frost line is the place where the earth is so well insulated from the surface by the thickness of earth above it that heat conduction is insufficient to freeze it throughout the entire winter. The sun only heats that surface by radiation.

See for example https://igshpa.org/igshpa-blog/cold-climate-ground-source-he...:

> Unlike air-source heat pumps that struggle to extract heat from frigid air, ground source heat pumps tap into a remarkably stable heat source: the earth itself. Below the frost line, ground temperatures remain relatively constant year-round, typically between 50-60°F (10-15°C). This consistency makes ground source heat pumps highly efficient even during the coldest months.

If you were building a ground-source heat pump to heat your house in the summer, you could get away with burying the pipes at a much shallower depth and in effect converting the earth into a low-temperature passive solar collector. But generally people want to heat their houses in the winter instead.