Here is a decent alternative for heat storage for off grid use. There are some contraints on storing heat, including size of the storage area, cost, efficiency, longevity, and personal preference. I know one sweet Lady that runs a 55 gallon drum stove and cuts her own wood for central heat (backed up by propane she never uses) that runs the wood heat over a gravel bed. It takes up a lot of room, but it is a permanent, maintenance free, solution. I have less space, and want to run Solar as primary heat input, so I am looking at this option. I intend to capture wood heat in the same salt bed.
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From @smithlabs