Many years ago, I examined a PhD thesis from a student who had looked into the possibilities of using a heat store in a greenhouse in East Anglia. His calculations suggested it really wasn't worthwhile, but if you want to try it, then water would be the best storage medium. How you get any surplus heat into the store is another question; you really need a decent sized absorber (think solar water heating panles). One other possibility might be to fill the tank with largish pebbles, and arrange a fan to pull warm air through the pebble bed. But it would probably not be worth the effort.

Dick Morris