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Heat, Thermodynamics, and Greenhouses

March 24th, 2012 · No Comments

I have always been interested in thermodynamics. I was born in Florida and lived there till I was 6 years old. I remember the adults saying that it was so hot you could fry an egg on the pavement. My mom wouldn’t let me have an egg, but I did fill a coffee mug with water and placed it out on the pavement. It never boiled. But the pavement was too hot to walk on with bare feet. I guess that was my first introduction to the principles of thermal mass and latent heat vs sensible heat. I also remember trying to sleep without air conditioning, waking up to a cockroach crawling across my belly, my dad’s red and white dodge work van (with a sliding door) and the pain and embarrassment of fire ants being removed from my bare butt by my teacher in the girls restroom.

Then we moved to Edmonton, Alberta.  And I learned about snow and ice and cold.   I remember enjoying hot showers, registers blowing hot air, and how nice the hot water heater felt against my back.  I also learned that wet gloves and boots get cold immediately.  That if your feet and head are warm and dry, the rest of your body is probably comfortable.  Also that after a month of winter temperatures below zero F (-20 C), when it warmed up to 40F ( 5C) it felt warm.  Other than trying to stay warm in winter,  my interest in heat waned and I spent most of the time in Alberta reading and feeling sorry for myself that I had such a boring life.

When I was 11, my parents decided to move to a religious community in Northern British Columbia.    The community was similar to a Hutterite community without the beards.   For the geographically challenged, British Columbia (or BC) is the Canadian province just north of Washington.  BC has a huge range of climates from rainforest on the west coast to fruit growing areas to muskeg, taiga, and tundra in the north and mountains.  Where we lived had a climate similar to Montana or interior Alaska.

The community had no electricity, telephones, running water, or natural gas.  A barrel stove provided heat.  We used propane lights, coleman lanterns, and kerosine lamps for lights.    I had to fill the woodbox.  I hated green poplar because it was very difficult to get burning and then didn’t produce much heat.  And it was heavy and slippery.  Spruce was better.  Dry spruce the best.  My mom hated the ashes.   The walls of our house were made of round logs with fiberglass chinking between the logs.  My bed was beside a window.  And even though my parents spent the extra money for double paned windows, the windows would ice up in winter and make a glacer down the inside of the log wall.  I woke up several mornings with my blankets frozen to the wall.    My brother and I had matching afghans that my grandma had crochetted for us.  I learned that I stayed much warmer if I put the afghan under my wool blanket rather than on top.  The holes in the crochetting captured air if the afghan was sandwitched between a sheet and another blanket.  Later we got some military surplus down sleeping bags that we unzipped to make comforters.  Without any baffles, all the goose down would settle into the portion of the bag hanging off the bed.  And if my foot hit the zipper in the middle of the night, it was like an electric shock….

We had two wood -framed greenhouses covered in yellowed fiberglass.  Both had a woodstove at either end and had side and roof vents.  In the spring, we would hang a plastic partition to conserve heat at one end.   It was a bit of an art learning to adjust the damper and the draft on the barrel stove  — and even stacking the wood and selecting the diameter of logs  — to get a moderate fire that would last 5 hours.  The men and boys took turns doing “nightwatch” or “the fires” which meant waking up at least  at midnight and 5:00am to stoke the fires.  On really cold  nights (around -40) the lucky guy would stay up all night shoving wood in stoves. 

I may continue this, but I woke up thinking that trying to heat a greenhouse in Alaska winter would be like putting a potted plant and a blow dryer in a large ziplock bag and then tossing the plastic bag in the freezer overnight.    Will the plant survive until morning?  Likely there will be a rime of frost on the inside of the bag.  The heater may have cooked or dried out the plant.   If any of the leaves are touching the bag, they will be frozen.    Assuming the plant survived the night, now pull it out of the freezer and aim a heat lamp at it.  I think you’re getting the idea why most hobby greenhouses in Alaska have become storage sheds. 

I have a lot more to say on the subject but I need to go transplant 1300 strawberry plants and make sure the greenhouse is warm enough to keep them alive.  Thursday I built a small greenhouse inside my greenhouse and ran hot water pipes underneath the beds.  I was worried that the hot water pipes would be too hot and damage the roots so I put a capilary mat over the pipes (basically two layers of plastic with a poly fiber blanket between).    Unfortunately, at 5F, the air temperature in the mini greenhouse dropped to 25F.  The plastic film on top had a delta T of 20F

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