There are a few threads that discuss using sand as a battery. It probably is as effective as the pebble style, but an 800 cubic foot mass would have a time warming up to past 200 without a monster stove. My 8" batch rocket runs through a cob mass of 2x4x8 (64 cubic feet) and heats up to 115 max when burned once per day. I suppose you could burn throughout the day, but I think the heat might not be able to transmit quick enough to allow adequate storage without a huge loss of efficiency going out the chimney.
Possibly overkill on size: If warmed up to max temp, my stove will keep my 900 square feet quite warm for over a week in sub zero temps. Plus, being partially bermed, your geothermal gain might prevent freezing.
