November 15, 2014

The Greenhouse by Night Is not the Greenhouse of Daylight Hours

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.

Ventilation Maintenance by way of Rutgers University
Greenhouse Heat Info from Auburn University
Low Tech Tips for Greenhouse Cooling 
Humidity and Temperature via U Mass 

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.


Alison said...

I think next year I'm going to use bubble wrap. Right now with our below freezing temps, at night I'm putting a propane heater in with the electric one. It uses two 1 lb. canisters, and lasts just long enough overnight to keep the temp in the 50s. After that it goes back down to the mid-40s, and once the sun comes out, the temp goes back up. I might try using water jugs and see if that helps. Thanks for the tips on words to Google.

Janie Jurkiewicz said...

I think the brick is cool irrespective of utility but the greenhouse does look awesome at night.....can't wait to see what mine looks like after the polycarbontate gets put the barrels...have one in there because I used to not have plumbing but discovered that it does keep things warm.....curious as to why they are white (ie black body radiation).

Jean Campbell said...

The barrels are white because they were soap barrels. I didn't paint them because they are covered and not exposed to sun -- the back side of the greenhouse is shaded early in the morning.

outlawgardener said...

Your greenhouse looks beautiful lit up at night, like a big luminaria. Thanks for the tips! I've been keeping some 5 gallon buckets of water near my space heater (It's an oil-filled radiator which the box said would be better for constant even heat than the other kinds of heaters they make. With the built in heaters and the space heater set on medium, the night temps have been staying around 50 degrees. The temp has been much more consistent since I covered the garage door with shower curtains. Thanks for catching that for me.

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