My husband is building a greenhouse for some banana trees and for starting garden plants in the spring. Looking for a quick solution to heating the greenhouse so the banana trees don't die. My husband had considered a water pool heated somehow to put off heat, plus with windows on the south and east walls. West and north walls will be insulated.
Once the greenhouse is built, it's a little harder to retrofit Jerome's soil battery ideas, but not impossible.
He insulates the soil beneath the greenhouse from the rest of the soil. He installs a bunch of perforated plastic drain pipe in that soil and pumps hot moist air into the soil all summer long. This "charges" the battery. That's why the soil under the greenhouse has to be thermally isolated from the soil around the greenhouse, it would just bleed off all that precious stored heat if you didn't. Then as the temperatures cool off, the fan or fans transfer the heat and humidity from the earth battery back to the greenhouse.
He shoots for a 90% reduction in the need for electric/fossil fuel/wood added heat. So there may be a few really cold nights where you need to add supplemental heat in addition to what the earth battery can supply. Of course, genius that he is, he put in a wood fired sauna in side the green house, and on those super cold nights in colorado, he just has to have a session in the sauna to save the fig trees and other mediteranean plant.