Thats my nickname for this setup, “Thermal Batteries”. The barrels and smaller jugs are filled with water, and painted. Because the jugs are painted black, they absorb the suns rays easier then other colored jugs. This allows them to take in more of the heating affect of the rays.
Water also holds heat better then air, since water is denser. I have begun to appreciate this affect when checking on the greenhouse. The barrels of water are warmer then the air around them. It was about 20 F today, and the barrels had no sings of freezing.
Im guessing I will need a few more large barrels, and lots of smaller jugs to raise the tempurate to be consistently above 40.
I’ve noticed a temperature difference in jugs sprayed flat black compared to glossy black. The glossy paint must cause the light to bounce away, as the flat black containers were noticably warmer.