Huh, it intuitively seems like piping working fluid to the sand ought to be easier than moving the sand to the working fluid. Is the moving-sand approach fundamentally desirable thing (I can't imagine why) or is it just a simplification for the proof of concept?

>> Is the moving-sand approach fundamentally desirable thing (I can't imagine why) or is it just a simplification for the proof of concept?

I suspect this is about operating temperatures. If you run pipes through the thermal mass then you will be slowly heating/cooling the entire mass. That means the temp will be constantly changing and would basically never be at optimum. But by withdrawing small amounts of sand to be cooled/heated separately, the bulk can remain at an optimum. Only the removed sand is cooled. So your tank of "hot" sand remains at the same temperature until the last bit of hot sand is gone, rather than it slowly cooling as you withdraw heat from the bulk. That no doubt makes thermal transfer more efficient and predictable.

I suspect it's because the thermal conductivity of sand isn't that great. If you've got your working fluid running through pipes embedded in hot sand, the system is likely bottlenecked on getting the heat energy from the main body of the mass through the cooler sand closest to the pipes.

Do it the other way and the enormous surface area is working for you, so you can presumably get the energy out arbitrarily fast (or, at least, that's no longer the bottleneck).

The thought occurs that if that is indeed the problem, you could attack it by mixing a metal in with the sand. You'd still get the thermal mass, but conductivity would no longer be a problem as long as it had been in a liquid phase at least once.

The problem isn't so much the sand, it's all the air gaps between the grains of sand.

Adding metal won't help any- once it gets to a liquid phase, it'll sink down, or if it doesn't stay liquid long enough, will just trap most of the air in it.

Edit: you'll also want to keep oxygen out of the environment in the liquid phase. Depending on the metal you use and the exact makeup of the sand, you'll wind up with some materials that are often used in refractory cement- aluminum, oxygen, and silicon will do the trick and absolutely ruin the effectiveness of your heat battery.

That's what I was getting at. Fill the air gaps with metal.

Storing & moving fine-grained bulk dry stuff is really old, cheap, & reliable technology - think of grain silos.

Yes, and under certain conditions sand is a fluid. Remember fluid !== liquid. Gases, liquids, and loose material can all be fuilds.

Piping the working fluid to the sand means your pipes have to increase in size and cost as you add more sand. Dropping the sand into a heat exchanger means the heat exchanger doesn't increase in cost with the volume of sand.

Nah it's nuts to mobilise the sand as a working fluid. The more mature design option seems to be refractory brick for heat energy storage, with piping running through it where you can raise steam. Running a steam turbine generator is generally seen as uneconomic due to the very low round-trip efficiency, and heat storage usually follows the principle of "if you store it as heat, use it as heat".

Sand-sourcing issues are typically related to types of sand required for special applications, such as in concrete or silicon wafer production that needs very specific types or shapes of the grains. This kind of application would probably work with “regular” sand that’s unsuitable for those special purposes, and also happens to be the most common type of sand.

This seems like a really cool technology that also doesn't need much engineering to make it viable. Why isn't it already widespread?

there is a DIY community around it and because it is so simple to make I just dont see any need to rely on commercial solutions. Literally store sand somewhere and heat it, use it for days or months depending on the setup.