Thinking Outside the Box, Inside the Greenhouse

indoor-solar-collectorsMost of us are familiar with the expression about learning to think outside the box, a metaphor for using creative or unconventional thinking in problem solving. Having spent 20 years working as an engineer, then a sculptor, then a poet, and then an engineer once again, I have become comfortable with this type of thought process. When I started working on developing a solar collector to boost the ability of my greenhouse to weather the bone chilling subzero nights of the Colorado high country I didn’t consider the fact that I would be literally forced to think back inside the box, in this case, inside the box known as my greenhouse.

When we consider the placement of solar collectors we would generally not think about placing them inside a structure. Panels live on rooftops or mounted to poles that track the sun like giant silicone sunflowers. Putting the panels inside the greenhouse, while somewhat unconventional, made the most sense in this case. When asked why he robbed banks, the infamous bank robber “Willie” Sutton said, “because that’s where the money is.” In this case, why mount the panels inside the greenhouse? Because that’s where the heat is! Even with ventilation, hot air can become trapped at the peak of the ceiling, so why not harvest that wasted heat and store it for later use?

With an eye on economy, ease of construction, and installation I built two collectors and installed them near the ceiling running the length of the greenhouse. The collectors are basically a loop of continuous PEX tubing, perhaps a hundred feet long, snaking through a long shallow rectangular box painted black and covered with a sheet of polycarbonate, that’s it!

pex-tubingThe PEX is connected to a low speed 200 GPH submersible pump in the thermal mass storage water tank. As the water circulates through the loops it’s heated by the sun. The pump is controlled by a thermostatic sensor located in the solar collector. The end result is the water in the tank stores the additional heat that in turn is released back into the greenhouse as needed.

PEX tubing is used commonly in the installation of domestic water systems and would be one of the main parts of the collector. I set the budget at $400 to build the complete system. This would include the 2 solar panels, valves, pumps and thermostatic control. Again, with an eye on economy, I bought my building materials from one of the big box stores and shopped online for the electronic components. While I consider this a DIY project, I would strongly recommend that you have a qualified electrician install the electrical components. When you’re mixing water with electricity in any form you need to use a professional or you’re likely to end up as they say, “Graveyard Dead!” Get the point?

assembled-collectorIn building the collector I first cut a sheet of OSB to serve as the back of the panel. My panels are 18” wide and 12 feet long. Then I attached to the OSB a sheet of metal flashing 18” wide to the full length of the panel. At this stage I spray painted the panel flat black. Next I ran the loops of PEX tubing and attached them in place with clamps, see the attached photo. Be sure to use one continuous piece of tubing with more than enough on both ends to reach your shut off valves. While connectors can be used with the tubing, they are expensive. I learned this lesson the hard way by nicking one of the tubes. The fitting cost me over ten dollars – ouch!

Next I framed in the collector box, attaching 1″ x 2″ boards on the long side edges and purlins on the ends to support the corrugated polycarbonate covering. I then spray painted the box again, this time to cover the tubing. I used a polycarbonate covering made by SunTuf. While I’m sure there are other ones out there, I like SunTuf because I can buy it at a big box store and I have been pleased with its performance in the past. Whatever product you use, follow the manufactures recommendation for installation. Once again, I learned another lesson the hard way. This time, by over tightening the screws on a large solar hot air collector array that I built four years ago, in no time it started cracking. It’s a perfectly fine product, it just happened to have a fool for an installer. Why is it that my mistakes always seem to cost me money! At this point the collector was lifted into place and the polycarbonate was installed. The trailing end of the PEX tubing ends are then run to the shutoff valves. The panel is now complete.

shut-off-valvesThe trick in using a small economical pump with the panels is using a big powerful one to charge the water in the system for the first time. Pumps are rated at how high they can lift water. The higher you need to lift the water the more power you need. As my tanks sit 8 feet below the top of the solar panels I used a beefy 12V RV pump to purge the air from the system. Once that had been completed I connected the line to its baby brother, a pump no bigger than my fist that only requires 9 watts of power to operate. During this last winter the pump used a total of $1.61 in power and the single panel that I was recording the data on added 2.6 million BTU’s of heat to the greenhouse. Not a bad return on a $200 per panel investment. One of the beauties of the system was the fact that the only maintenance it required was for me to add 1/4 cup of bleach to the water tank at the beginning of each month to keep the water clean. Even a slacker like me can handle that!


For a free PDF copy of my last poetry book “To Soar” just drop me an email at SoaringRavenLab@gmail.com.

Thanks to Russ Erganbright for participating in the [Grow] Network Writing Contest. We have over $1,500 in prizes lined up for the current writing contest, with more to come. Here is a list of the current pot of prizes:

– A 21.5 quart pressure canner from All American, a $380 value
– A Survival Still emergency water purification still, a $279 value
– 1 year of free membership in the [Grow] Network Core Community, a $240 value
– A copy of The Summer of Survival Complete Collection from Life Changes Be Ready, a $127 value
– 2 copies of the complete Home Grown Food Summit, valued at $67 each
– 3 free 3 month memberships in the [Grow] Network Core Community, valued at $60 each
– The complete 2014 Grow Your Own Food Summit interview series, a $47 value
– 4 copies of the Grow Your Own Groceries DVD video set, valued at $42 each
– A Bug Out Seed Kit from the Sustainable Seed Company, a $40 value
– 4 copies of the Alternatives To Dentists DVD video, valued at $32 each

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  • Joe says:

    Thanks for the article. I appreciate that you share your mistakes as well as your successes, since mistakes are generally the more costly lesson. That was also an excellent tip on filling the system with a larger temporary pump, then using the smaller, more economical one for circulation.

    Stony Creek Permaculture Farm

  • Freyda Black says:

    An excellent explanation of building the collector. However, could you add an addendum on what you use as the thermal mass storage tank, or suggestions for such a tank (metal vs. plastic) and the size. Best would be an explanation of the way to calculate how to size thermal mass storage to size of collector/size of greenhouse. I would love to add a greenhouse on the S side of my house and this would solve the problem of cost of warming it in the winter. Thanks!

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