I've proposed several times on HN a "box of rocks" at the house level to store heat/cold when electricity is cheap, and release heat/cold when electricity is expensive. It was ridiculed every time.
I should have patented it :-)
I should have patented it :-)
Storing heat in rocks or sand doesn't make sense from a thermal efficiency standpoint unless the temperature is high and that isn't economically viable unless you have large volume storage - typically district or precinct level.
A tank of water works, too, but tanks rust, mildew, and leak.
https://www.stiesdal.com/storage/
for some time, and water tanks are simpler.
When you set up a water tank, you just pump water into it to get the storage medium, and when you need to extract heat, you can pump out actual storage medium and do the heat exchange directly. With a rock storage, you either need pipes in them or move air through the rocks, which requires a much beefier overall system since it's harder to extract the heat. Does that make sense?
Rusting tanks sounds like a problem with either badly engineered mismatched piping, or badly engineered tanks. Ideally, you'd have a well-engineered tank as a closed system with a heat exchange unit next to it.
By the way, I live in a town with a wide district heating system, and there's no storage at all in the individual houses here, only a small heat exchange unit. They are building a 200.000 cubic meter water storage system in the other end, though. I don't know about this particular project, but there are other projects where the water storage is large enough and insulated well enough that they actually do store cheap solar heat from the summer through the winter - and it's cheaper than using gas, even before the current price spikes.
With the box of rocks, yes you'll need to duct hot air in to heat them, and duct cool air in to extract the heat and blow it through the house. I don't know that this needs a beefy system, if you've got central air there is already most of the duct work in place.
Same why a flea can jump >10x its height, and a human cannot (square growth of strength, cubic growth of weight).
Of course, some engineering would have to go into optimizing the size of the rocks and the surface area / mass for the expected use.
Basically, if you only have a small amount of sand, it’s harder to keep it warm for longer (more surface area for the mass, so more insulation required), and harder to deal with effectively (fan needs to blow much harder when it’s only warm, much less when it’s hot), etc.
The equation for the thermal resistance of a cylindrical system is explained in https://youtu.be/6x-jdCGWuHI?t=801
The thermal resistance is the sum of 3 (or more) terms, and while the outer terms are indeed proportional to surface area, I'm not sure if the physical constants of the system aren't designed so that the middle heat conductive terms of resistance R=ln(r2/r1)/(2pikL) is the dominant factor.
Some make themselves sick from the smoke (inadequate chimney), more than one burned down their shelter.
(I have no idea how to hunt & fish, but I bet I could build an adequate shelter and fireplace.)
Doesn't mean the idea can't be implemented better, of course.
In fact I do think there's a lot of low-hanging fruit that remains to be explored in energy production.