When looking for affordable options for our greenhouse tables, we came upon 55gal plastic barrels within our local waste stream – some of them used to contain certified organic rice or date syrup (from Pakistan!) or liquid dairy feed additives. Not only were the barrels a good height for our work in the greenhouse, they also became a major source of thermal mass for our fringe season heating needs. Our 55 barrels hold about 27oo gallons of water (or 11 tons)!
On a sunny day, the water within the barrels can get about 20 degrees above the ambient greenhouse temperature and then slowly releases that heat throughout the night, significantly reducing our supplementary heating needs (from our LP heater) during the spring months. We see it as a great way to harness some of the sun’s energy in an extremely low-tech way.