Burying the barrel is clearly the trick here, as it helps conserve heat and also draws on the much higher temperature of the soil to keep the water from cooling too fast. I'm not clear, though, on why the need for the half barrel cover unless it is just to add a little extra insulation to the setup.
I wonder how much better this would work if you had an aquarium water heater in the barrel all night long - I'd think it wouldn't cost much and would yield more heat.
