If you have free energy the obvious thing to use it for is carbon capture.
It'd take a far amount of math to figure out if that tips it over though I don't feel like tackling haha.
Water needs a lot of energy to cool or heat, concentrated at a district, you could easily absorb a lot of energy at negative prices.
The point is all this stuff costs money to build, money to maintain, so if you don't use it most of the time the time you do needs to be incredibly valuable.
They are about as close to an ideal load as one could imagine.
And capacity is easily expanded with a water tower. You can scale the total thermal energy stored and the efficiency of that storage by simply building a very large water tower. You don't even need special insulation because the water insulates itself.
> You'd pay a lot of money for a rarely used asset
Assuming you've converted over to district heating and cooling, it'd be frequently used as climate control for surrounding buildings.
> that has to be replaced by something else most of the time
What? No. District heating is the most efficient way to provide climate control, bar none. The only thing that needs to happen is setting minimum and maximum temps. Maintain the minimum temp and when negative power price events hit heat to the maximum or cool to the minimum.
And if you want to get super clever, part of your storage can be sand which can store huge amounts of energy.
> because people want their heating to be reliable.
District heating/cooling is as reliable as plumbing. That's because that's effectively all that it is. It's super reliable. If the incoming water is 60C or 90C, it doesn't make too much of a difference in terms of heating a home.
It's a proven tech. Many universities use it because it significantly reduces the heating and cooling cost for their buildings. My own city uses district heating in the downtown to great effect.
Heating water, cooling water, pumping water, charging batteries, running power hungry machines.
It's half century old tech and usually the only thing missing is a financial incentive to do so.