Crazy idea... need some opinions

[OGrubber]

.... what do you guys think?? would it work?

I totally agree with everyone's safety concerns, but, even if that wasn't an issue, your idea wouldn't create enough heat to actually heat your greenhouse. Heating a greenhouse is basically like heating the great outdoors.

I wouldn't waste my money on one of those solar systems either. Afterall, your greenhouse skin is as good as a solar panel if not better for generating heat on a sunny day. You only need a collector system to complete what you already have.

You don't mention what your target overnight temps should be. If you are only concerned with keeping the nightime ambient temps above freezing, then there are numerous passive options you can utilize that will return the daytime heat [if collected] into and through the night. Stone and water are probably the easiest to incorporate, though a tad bulky if you have a small greenhouse.

A concrete or large stone gravel floor is a good start. 50 gal barrels painted black and filled with water will also give off heat even as the water freezes to ice. If you are lucky enough to have a south facing brick or stone wall, then placing the length of the greenhouse against that is a huge advantage. A little research could find you something that would fit your situation and needs.