Genuine question. When I look up options for heating greenhouses its either propane, kerosene or electricity. So if you dont want to use fossil energy for any reason, why not use peltier elements which would boil down to resistive heating and moving heat from the outside to the inside?
Is it because they dont last in the weather? Or because they require a DC source? (couldnt you simply move the PSU inside the greenhouse and use any conversion losses as additional heating?) Or maybe its because you would have to possibly cut a hole in your greenhouse walls so that the peltiers cold side is actually on the outside?
You’re not going to be able to heat a greenhouse in temperatures where frost is an issue with only 100w (~341btu) of energy.
that number was just an example, of course you could scale things up as needed (multiple elements with multiple heatsinks for example)