I got it and understand all that, Elaine. With an outside temperature of 40 F, and the sun being low in the sky (relative to summer), there simply won't be much heat generated in my greenhouses. And the further north one goes, the greater the angle of the sun and thus the heat generation will be even more limited.
Keep in mind that my greenhouses are also covered with solar blanket material, and though that material is supposed to transmit the infrared but not the UV range of light, there still is just not that much heating going on. My greenhouses will never see 80 F with 40 F outside. I have followed this for many years. But let's say those jugs do heat up to 80 F (which they won't). How long is that heat retained/radiated? Will that temperature be even 50 F at midnight? There will be at least six more hours of sub-freezing temperature outside. Heck, there might be twice that time of sub-freezing temperatures.
I guess what I am saying is that when push comes to shove, the little heat I would get from all those heat-sinks is far outweighed by the space I would lose. That space is at critical mass during the fall winter months, with my hundreds of tropical plants (600+).
