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Geothermal systems are used in regions across the U.S. for home use, but industrial geothermal plants are only used in areas where subteranian temperatures are very high. Why is this?

https://en.wikipedia.org/wiki/Geothermal_energy_in_the_United_States

https://www.energy.gov/eere/geothermal/geothermal-maps

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  • $\begingroup$ Industrial wants more energy... $\endgroup$ – Solar Mike Dec 22 '18 at 6:11
  • $\begingroup$ This is primarily an economics question, more than an engineering question $\endgroup$ – Jonathan R Swift Dec 22 '18 at 8:35
  • $\begingroup$ Making electricity with a home geothermal system! That sounds too good to be true. I have a friend who draws cold water from a well and uses it to cool his house. The water then returns to the lake. I wonder if there is some way to make some electricity too?? $\endgroup$ – Doc Smith Dec 26 '18 at 18:33
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Because they're two completely different systems that have, in some circles, accidentally been given the same names.

The things that are sometimes (particularly in the USA) called home geothermal systems are ground-source heat pumps. The energy for these comes from sunlight on the ground over the year. Electricity is used to move heat from a source temperature that is broadly equal to the average ambient air temperature over the year (let's say 10-20 degrees C), to 30-90 degrees C to provide space- and water- heating.

Industrial geothermal systems are heat exchangers. The energy for these comes from the much hotter temperatures underground, that arises from the heat of magma, heating rocks above it. So in this case, the source temperature is 300 degrees celsius or higher. That's high enough to provide space and water heating without a heat-pump. It's also high enough (relative to ambient temperatures) to drive a heat-engine, which is pretty much the opposite of a heat pump. This heat engine is then used to generate electricity.

(A heat pump uses work to generate a temperature difference. A heat engine is the inverse of this: it uses a temperature difference to generate work.)

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For a home system, it makes sense if you can generate energy for less than you'd pay an energy company. If this is the case, the system will pay for itself after a number of years. This works, because the cost per unit of energy that is charged to the consumer by the companies is relatively the high. If it takes an extra year to pay off because you've got a cooler subterranean temperature, that's not a huge reason not to install it.

For an industrial system, the cost per unit needs to be much lower. A difference in temperature gradient of just a few degrees could affect operational efficiency significantly, and drastically alter the potential profit margin. Building an enormous generating plant on site A will cost much the same as site B, (compare this to a consumer who is already tied to their house - it wouldn't make sense to move!), so as a result, geothermal plants are concentrated in the areas where they will provide the greatest financial benefit to their owners.

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  • $\begingroup$ Why the downvote, friend? Feedback appreciated. I'm in Europe where true domestic geothermal is a thing - the question is primarily about why industrial plants are limited in their location, which I hope this answer addresses $\endgroup$ – Jonathan R Swift Dec 22 '18 at 15:49
  • $\begingroup$ I didn't downvote, but the question references domestic geothermal "across the U.S.". The accepted answer explains that these are heating systems, not generating systems. I've never heard of generating electricity with a home geothermal system, which you address in your answer. Are there actually such systems in Europe? It seems like the efficiency would be far too low to ever justify the cost, except in those same areas where industrial plants are located. $\endgroup$ – Mark Dec 23 '18 at 17:09

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