these guys are doing an industrial sized one, i would like to see a residential one that i can put excess solar into it, and draw down on it in winter.
Agreed. I think it would be hard to retain heat across seasons (if that's what you mean), but it would be pretty easy to create something to work intra-day. I've seen examples using stone walls, but I think you could do something smarter and more responsive to actual inside temperature. Combine that with active ventillation and you've got a pretty neat setup at a low cost.
I really like this idea for home heating, It could be a few 200L barrels filled with sand/water. I think it would take a little bit of smart ducting/piping to keep temperatures comfortable in some locations though.