I don't think this would work very well for a greenhouse. Am I correct that the cans are empty? If so, they do not retain any heat at night and only work during the day.
So, I have no idea what works way up north, but from what I have seen, if you totally close up a greenhouse during the day, it can get 135f by afternoon because of the greenhouse effect. A closed greenhouse should heat up all you need during the day, its heating a greenhouse at night, for me, is the big concern. Now, heating a bunch of cans filled with liquid would provide heating at night, but 1 cubic foot of water only provides about 65 BTUs per hour so you would need piles of cans of water to make a big difference in heat for night use.
