low cost heat for a cold green house
You certainly can produce some heat for the greenhouse using candles covered by successively larger clay pots (not ceramic). Some people use tea candles or others that burn for multiple hours. This may be enough heat to prevent freezing depending on greenhouse size and numbers of "systems" used. But keep in mind that this involves having the potential danger of open flame(s) in your greenhouse and potentially very hot clay pot surfaces that could cause burns.
Here in Lincoln County you can grow many crops (like tomatoes, peppers) in the greenhouse or cloche during the warmer season without additional heat. You can lengthen the growing season here with cloches and greenhouses.http://extension.oregonstate.edu/gardening/lengthen-growing-season-building-coldframe-or-cloche
Here is a link to building a cloche. https://catalog.extension.oregonstate.edu/ec1627/html
You could also insulate your greenhouse (especially the north-facing wall) or add solar heat-gathering vessels such as black-painted barrels or bottles (You can add a little bleach to retard algae growth). You could also put black mulch material down on walkways in the greenhouse.