[WalterBright]

> It is incredibly inefficient

Please elaborate how heating a rock, putting it in an insulated box, and taking it out of that box later to release it's heat to the air is "incredibly inefficient" compared to heating the air directly.

[bryans]

Because it takes 3-5x as much energy to heat rock than water, and at least twice as much as many other common materials. Sorry to say, even air is more efficient depending on the scale of time you're trying to solve for, though its ability to retain that heat is directly limited by the properties of the insulated box -- like a house, for example.

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

https://theengineeringmindset.com/specific-heat-capacity-of-...

https://en.wikipedia.org/wiki/Thermal_energy_storage#Heat_st...

You're going to be tempted to reference the last link as evidence in your favor, but it's very much the opposite. It's saying the advantage concrete has is its ability to be heated to higher temperatures than water. Except it doesn't get to break the laws of physics and still requires 3-5x as much energy input, which is why it's really only practical for large scale operations that can safely heat the concrete to extreme temperatures, using massive amounts of electricity that would otherwise be wasted due to low grid demand. They are still losing at least 75% efficiency in that process, but it's slightly better than losing 100%, as long as you pretend there aren't any environmental impacts of producing all that extra concrete.

You'll notice the first installation referenced in this section actually uses 1,000 cubic feet of additional reinforced concrete and an entire home's worth of additional electricity to supply a single home with 50% of its heating and hot water. That's a second foundation's worth of concrete, for perspective.

And moreover, as we've already established, this concept isn't new at all. If it were legitimately more efficient and more practical than alternatives, every home would already be using its foundation as heat storage. But they don't, because it's not.

[labawi]

To quote, your comment's ideas are bad and ignorant. Not because they are incorrect, but because you blindly insist on them. The reasoning is valid merely for a rock/concrete oven or a vacation house.

I suggest you read up on laws of conservation. If you want to be less than 100% efficient, you have to lose energy somehow, somewhere.

I don't know where you're getting efficiency numbers from, but quoting from your reference, storage in Sorø will double as electricity storage while beating your numbers on electricity alone.

"A similar system is scheduled for Sorø, Denmark, with 41–58% of the stored 18 MWh heat returned for the town's district heating, and 30–41% returned as electricity."

BTW, when you switch rocks for concrete, of course it's expensive and makes no sense - people don't add tons of concrete for thermal storage. Though they do use it, if it's there, and add rocks, brick walls, water tanks, phase change materials etc, if they want more.