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Comment by megaman821 | Hacker News Reader

I wonder if geothermal can offer mild battery storage by using excess energy to pre-heat the water.

Yes, though its not geothermal preciously - https://en.wikipedia.org/wiki/Drake_Landing_Solar_Community

> The Drake Landing Solar Community (DLSC) is a planned community in Okotoks, Alberta, Canada, equipped with a central solar heating system and other energy efficient technologies.

> In 2012 the installation achieved a world record solar fraction of 97%; that is, providing that amount of the community's heating requirements with solar energy over a one-year time span.

> In 2015–2016 season the installation achieved a solar fraction of 100%. This was achieved by the borehole thermal storage system (BTES) finally reaching high temperature after years of charging, as well as improving control methods, operating pumps at lower speed most of the time, reducing extra energy need as well using weather forecasts to optimize transfer of heat between different storage tanks and loops. During some other years, auxiliary gas heaters are used for a small fraction of the year to provide heat to a district loop. The systems operate at coefficient of performance of 30.

https://www.dlsc.ca

https://www.sciencedirect.com/topics/engineering/drake-landi...

http://proceedings.ises.org/paper/swc2017/swc2017-0033-Mesqu...

I don't know about directly as a battery, but there is potential for energy storage. If you have geothermal HVAC at home, it can (sometimes) be use to heat water in a water heater. Dumping the extra heat into it. And I have heard stuff about using excess energy in the grid to put extra heat into home water tanks as well, so demand is reduced later. It is not getting converted back to electricity, but still a store for energy.

I don’t think thermodynamics lets that work. You can’t move heat from something cold to something hot so unless you are preheating your water to a temperature higher than your deep borehole rocks, all you’re doing is putting the first N joules of heat energy into the water you’re going to pump into your steam turbines using excess grid energy instead of geothermal energy.

Of course you can move heat from something hot to something cold, it’s called a heat pump. Though yeah I get what you meant. There are heat pumps that have entered production that are designed to generate the kind of industrial levels of heat required to heat the water to the level required to add heat to such reservoirs. Whether it makes sense, I can’t say. But it certainly makes sense for geothermal well used for district heating (not electricity). It’s already being done.

Funny thought: if you use a heat pump to store heat energy in summer to be used in winter.. and your reservoir can retain the heat efficiently (think the heavily insulated sand reservoir used in a project in Finland for instance)… could that make the battery effectively more than 100% efficient?

"Lisa! In this house we obey the laws of thermodynamics!" - Homer

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