How to make a $2 sand battery heater
Sand stores an enormous amount of heat energy. A 50lb bag of sand costs $4 at the hardware store, you only need a few lbs.
Here are two versons, 1)a stainless steel pet bowl heated by two tealight candles, filled with sand or 2) a clay pot filled with sand one inch from the top, inverted over a clay plate, heated by two tealight candles
Here is the kicker:
Put your 50lb bag of sand where direct sunlight will heat it, so you can put hot sand in the bowl or pot before lighting candles or 2) microwave the sand for one minute max in a micro safe pot.
The sand will radiate heat long after the candles have gone out, we are looking at 3 hours of tealight candles and two+ hours after if not more (guess).
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The idea I was working with was based around a fire pit, or a burn barrel, where you heat the pot outside over an open fire, then carefully transport the sand container back inside.
Designed for people who may not have a fire place (although placing a sand battery on a wood stove would extend your firewood supply), but could easily make a fire in their backyards with scrap wood.
Another design I’ve got is a sand container, where the wood stove exhaust pipe passes through it.
So the sand is heated by the waste heat, instead of being funneled outside where it’s wasted.