It also actively prevents heat from transferring into the sand, and stores far less heat than the sand itself. If you really want mass heated to 600C to be stretched as long as possible, you need as little air in the mass as possible. That's why in residential mass storage, the preference goes in order of:
- rock / gravel mix (lowest preference unless portability is a requirement)
- clay / cob mix (super fine particles, little to no air)
- water
where water is held separately from the heat source and some form of heat exchanger is used. Aside from the mess caused by leaks, accidental pressurization turns water thermal mass storage into a bit of a bomb (much like a vastly oversized pressure cooker) so it's really only used in outdoor wood-fired boilers. Also, the temp is usually capped at 180F, so nowhere near what these guys are getting.
> It also actively prevents heat from transferring into the sand
So it's slower to charge. And the charge speed is presumably adjustable by circulating more air (or moving it through faster). It's unclear whether this is a problem for its intended application. It may charge fast enough.
> and stores far less heat than the sand itself.
That would only be relevant if you were comparing an equal volume. The same mass of clay and sand should store roughly the same amount of energy, it's just that the sand one would be bigger to accommodate more air. And being bigger isn't even a drawback here, since bigger means proportionally less surface area to volume that you're losing heat through.
> That's why in residential mass storage, ...
"Residential" could imply that you don't want a giant sand silo in the middle of some housing units, so I agree there. You would need to put this somewhere that size doesn't matter much.
Clay weighs significantly more than sand. For the same volume of material, clay will store significantly more thermal energy. The big question is- is the hot air on a closed loop or an open loop? I don't know of too many fans comfortable cycling 600C air, which means that you need an open loop of air (fresh air > fan > heater > sand > exhaust). If the sand isn't drawing heat off of the hot air fast enough, the heat won't actually "sink" in through the sand. If we are talking a closed loop of air (fan > heater > sand > fan) then the sand will get fully heated.
> “Residential" could imply that you don't want a giant sand silo in the middle of some housing units, so I agree there. You would need to put this somewhere that size doesn't matter much.
Can’t you build that silo underground? If so, size wouldn’t matter much. You could have a playground, communal garden or, if you must, parking spaces on top of it, so it wouldn’t really use any area.
Digging out, building retaining walls and waterproofing (you really don't want ground water or rain water getting into your 600 degree C sand) and so forth make it much cheaper and more practical to build above ground.
That seems like a benefit if you're looking to stretch release of energy over many hours?