Has anyone ever built a greenhouse made out of 55 gallon polyethalenne
barrels?

I was thinking of building a greenhouse with the north (and east & west
possibly) side built of 55 gallon plastic barrels. The barrels are
black and could serve as a great heat store. The problem is how to
secure them so that if I stack them (2 high = 6 feet) they are
structurally solid. I'd like them to serve as the outside wall (no
other siding necessary). But the space where the barrels meet on there
edge will need some sort of insulation that would be exposed to the
outside. Maybe I do need some kind of insulation on the outside so
that the barrels will not get cold.

I was thinking of making the greenhouse 20' wide (20' exposed to the
sun). So the north and south would be 20' (one side of barrels, one
side of glass.)

Any ideas on using barrels would be greatly appreciated.
Has anyone tried this? What problems did you encounter?