Greenhouse solar heating


The ultimate source of heat for my greenhouse is the sun.  Even if the outside temperature is less than 10C, inside, due to the energy input from the sun, I’m seeing up to 40C on sunny days and almost 30C on overcast days.  However, my water reservoir, which I pump through a geothermal bank 6 feet under the ground and through my soil bed, is only seeing a maximum of 15C on those hot days.  As nights have been dropping below freezing (-4C), it drops about 5C to 10C when I wake up.  That’s not acceptable.

The issue is that the air in the greenhouse can only heat the water if it comes in contact with the reservoir container.  Further, waiting for the air to heat up by the sun, and then the air to heat up the water isn’t ideal.  Air is a poor conductor of heat and has relatively low energy storage capacity.  To improve this situation, we need to increase the surface area of the water system.  We could put in a bigger reservoir but space is a scarce resource inside my greenhouse.  A larger reservoir will increase the surface area, but only a little bit of that will receive direct photon penetration from the sun.  What we really need is a solar heater.

I have a 4 ft x 6 ft area above my components.  I could mount a board of some sort up there are coil some water tubes to increase the surface area.  I also have 2 old CPU radiators with fans.  This will increase the effective surface area of the water’s contact with the air.

Over a couple weekends, I took the 200 ft of black poly tubing and made two spiral loops almost 2 feet in diameter on a 8 ft by 4 ft Styrofoam board I picked up at home depot.  In between the two spiral loops, I placed the radiators in series.  To hold down the tubes, I used automotive-grade 3M double-sided tape in an “X” pattern.  On the top, I used gorilla tape, but this wasn’t effective.  Right before installing, I used 10 gauge wire to lock it down in case the 3M tape fails.

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I mounted the setup at a 30 degree angle near the top of my greenhouse facing south.  This is not ideal.  In my area, a 70 degree angle would be better, but space is limited, so this will have to do.

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Before I installed it, I tested it on the ground at a 70 degree angle.  Over the space of an hour or two, the water heated up from 13C to 19C.  It was a sunny day.  This is about the same as pouring two good size pots of boiling water inside the reservoir.

I’ll continue tracking the performance, but today I’m optimistic this will help.  The running cost is pretty cheap too at only 28 Watts (12V @2.4A max).  I’ve coded up an algorithm in my automation controller to only turn on the pump if the air is hotter than the reservoir.  This is good for the winter time, but this rule probably won’t work will in the summer.  In fact, I may have to add a rule to turn on the solar heating system to help cool the greenhouse.  We’ll see.

What about the cost?  Well, this addition was relatively cheap.  If you want the cheapest solution, this might be it.

Cost breakdown:

Total: $104

Bonus:

Bonus Total: $119

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