Most everybody who has a greenhouse or is contemplating one has a plan for freezing nights: electric or propane heaters, water barrels and solar mass for heat mediation and a back-up plan if power fails or gas supplies run out.
A frequent question on garden forums is how to heat one's greenhouse using sunlight. Unless there's a grid of solar collection panels and a bank of batteries for energy storage and superlative insulation, it isn't going to happen during freezing temperatures. The best affordable practices use supplemental heat and conservation of energy.
What is going to happen on sunny days is heat buildup in the greenhouse. We can store a little heat using containers of water. Brick, stone and concrete in the floor absorbs heat that radiates back when the sun goes down. Even clay pots of soil hold some heat.
At noon here, the outside temperature was 46º and the greenhouse was 78.º
I did the obvious thing and opened the doors.
If you want to dig further into the science of heating and cooling a greenhouse, you could search for words like enthalpy, convection, mass transfer, phase change materials and radiation. I tend to garden indoors with a minimum of sophistication.
I've experimented a little with water jugs in front of the heaters -- we use 2 electric heaters set on low. (Electric space heaters generally all use the same amount of wattage, 1500 watts when placed on high, and 750 watts when using the space heater on low. We use separate circuits for each heater. Running them on low prolongs the life of the wiring.)
My theory is that if the fans blow directly on gallon jugs of water, some energy exchange goes on there instead of all the heat rising directly to the ceiling. If the water heats up a bit, then it is more slowly released, delaying the time the heaters will come on again. I have not been inspired to go out there and time all this, it's still a theory unproven but it makes me happy to think that it might mediate the heat loss just a little. I put several jugs about 3 feet in front of the heaters.
I also undertook a sunlight trial with capping some jugs, leaving the tops of others open and covering a few with a black trash bag. Interesting to me is that the water does not heat up appreciably except where the sun shines directly on the jug but the air space in the top of the jug does get very warm. I may try half full jugs and other air spaces to see how that works, considering air as a fluid.
Do what works and uses the least energy. I have not addressed using free or inexpensive materials like bubble wrap to insulate north walls and polyester fleece to protect sensitive individual plants.
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