Sunroom Greenhouse Conversion Fantasy

So.... I have a sunroom... a crappy one with a metal frame, screened in with cheap dark vinyl-y sheets over the screening to keep out sun or all the cold in the winter and leaks a bit when it rains.

I just had a slight mad epiphany and fantasy that maybe... MAYBE I could convert it somehow into a GREENHOUSE.

Problems though.... it's north facing and there is no good heat source, nor would I or my husband be interested in jacking up our energy bill for a project such as this and changing the crummy screening/vinyl to glass is doubtful as what's the point since the frame is no good.

BUT, I've been musing... could there be a potentially natural method to warm it? Like the huge pile of compost that leans next to the Growing Power greenhouses?

Except I can't really/don't have room/don't think husband or neighbors or authorities would really allow me to do this and keep a hot compost pile next to a dwelling.

Um, what about a sort of compost thing inside the sunroom? Leak proof? Smack dab in the center to dissipate heat and not touch any sides with ickiness?

How much waste would be needed to heat a say, 12x12 space enough for plants to be happy with a somewhat drafty door? (Ventilation, I say!)

Is there a physicist in the blogosphere to help me here?

Could I manage a compost pile in there without it smelling things up too badly?
Or cause a ridiculous invasion of fruit flies?

Grrr.... Hope.... fading fast.... knowing that this probably a very very very bad idea, but so tempting.

Some how, some way may there be a greenhouse in my future using some sort of creativity, luck and hard work on my end here without doing something too mad.