Pop Can Solar Heater. Cold Frame Plans

A pop can solar heater? This opens doors if it works. I just ordered 16 packets of new seeds and only have space left for 3 empty flats. With no greenhouse, whats a girl to do? So I’ve been on a mission to find a way to heat a space big enough to get some annual seeds started.
This video got my gears turning.

Now thats good. But how do you keep it heated overnight? Here’s the concept of a solar heat from greenmethods.com

A solar greenhouse is heated by collecting and storing heat energy throughout the day, then releasing it slowly at night, basically moderating the climate. To use the sun’s energy, a solar greenhouse needs to collect, insulate, and store energy. Aside from fans to circulate air, it is a passive system.

He suggests water for heat storage, and gives us a calculation to go on. 2 gallons per square feet. Not sure how this works, since were looking at cubic feet for ground and air temperature in a contained unit, but its enough to go on.
So I started thinking about how to heat a small space, just to hold flats… and it came to me.

If my wonderful drawing skills dont convey the idea well enough, its a basic 4′x4′ frame. Pop cans on the outside, covered (airtight!) with a layer of plastic (instead of glass). Milk jugs on the inside to collect and store heat. Im still considering the materials to use. Its usually constructed the first time with whatever scrap I have here, for the testing prototype, and modified until it works.
Will be testing this plan in the coming weeks to see if it keeps a small coldframe at adequate temperatures to put annuals outside before our last frost date.

Pop Can Solar Heater. Cold Frame Plans