The best idea I had for wood heat for my greenhouse, which I didn't have the money for, was to run pex tubing through the gravel floor and use a wood stove to heat the water that ran through it. There's a product I looked at that is a hollow metal element to install in a wood stove so that it will heat the water inside. It is possible to circulate water without an electric pump if the stove is slightly below the system, like an old-fashioned boiler.
The best solar idea I saw is called active solar. Instead of water-filled tubing through the gravel floor, air ducts pull air down into the rock to heat it during the day. At night, the fans reverse, and pull air back up out of the warm rock.
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