Simply Solar Greenhouses

Cut Greenhouse Heating Costs with Water

Posted October 24th, 2007
by Mike Cherim - PermaLink

This is really worth doing, unless you’re happy with your current greenhouse fuel bill.

The Green Spot started life back in ‘91 and through it I grew culinary herbs for a living. That was how I learned about good bugs and jumped tracks. I had a solar-type greenhouse that I had built myself. I had a ton of strapping left over from a construction project so using glue and screws I laminated “beams” and I made a frame. It had an insulated vertical back wall, insulated vertical side walls, a short translucent double vertical front to create more head room, and a clear double front-facing roof pitched at a dramatic 51-52° — I forget. It served me extremely well, requiring vvery little maintenance. The best part of using this type of greenhouse was the huge fuel savings.

I had one tiny portable heater in my greenhouse which rarely ran, yet I happily grew Italian basil throughout the winter, and that first winter was brutal as I recall. The reason I was able to do this was the solar greenhouse. You see, solar greenhouses have a couple of noteworthy features: One is a sharply-pitched roof that not only sheds snow well, but also allows direct access to the low winter sun, yet reflects the overhead sun of summer keeping the interior cooler; The second feature is a heat or energy collection/release system. I used water.

How it Works

A solar greenhouse is heated by collecting and storing heat energy throughout the day, then releasing it slowly at night, basically moderating the climate. To use the sun’s energy, a solar greenhouse needs to collect, insulate, and store energy. Aside from fans to circulate air, it is a passive system.


A large mass is what’s used for storage. This mass can be rocks, open water pools, stored water, and more. What depends on what’s available to you. As far as how much, the more the better. For rock storage I don’t know the exact amounts but I think you’d be looking at a five foot thick rock back-wall. The amount of water storage (pooled or containerized, exposed to the sun or not), since that’s what I used, is about two gallons per square foot as a rule-of-thumb. I stored this water in containers under my benches — they held up my benches, in fact (see illustration, inset).

I used greenhouse space that’s often wasted, found a good multi-purpose bench making method, I grew healthy plants year-‘round, and saved a bundle on my greenhouse heating bill. Since you probably don’t have a solar greenhouse and probably aren’t about to rush out and build one, you may be wondering what all this means to you.

Putting it to Use

You may not have a “solar” greenhouse, but you do have a solar greenhouse. Wait, maybe I’m assuming too much. I’ll ask: does your greenhouse get warm on sunny days? If the answer is yes, your greenhouse is solar, in part. Your floor collects heat energy by day and releases it by night. All masses store energy, but you don’t enhance this. You also insulate, with inflated double poly, for example, but it’s not ideal. And you collect solar energy through your covering. The only problem is you collect it better in the summer than you do in the winter due to the sun’s angle and your roof style. Especially if you have a North-South orientation as you should with a typical greenhouse.

Okay, so you have a quasi-solar greenhouse, you’re just not taking advantage of it or enhancing its inherent properties. That doesn’t mean you shouldn’t get some benefit from your greenhouse’s natural solar collection. You can do this by adding storage mass. I’d suggest water because it can be a lot easier to deal with, more portable, and more adaptable. Plus I feel it is superior to other masses and provides a more dramatic effect. I’d find room for this water under your benches if you can.

This leaves one issue. What to use for storage. I would recommend “juice drums” since they are what’s available to me. Other vessels could be used, though. Use your imagination, check junk yards, search the web, make some phone calls, take your fingers for a walk. The “juice drums” sold locally are around 40-gallons I think (but there are various sizes including what looks like a 20 gallon, see photo inset), and made of super heavy duty plastic with tight fitting lids or screw caps made for a pump. The juice drums sold around here are originally from Kraft’s “Very Fine” juice but any concentrate for juices, punch, etc, are shipped in such containers. So, look around, make some calls, etc.

Want more? I know of one greenhouse that used to grow hydroponic basil in the same water used to rear Tilapia fish which were sold to food markets. It wasn’t a solar greenhouse (the greenhouse they chose in fact was a mistake), but the water collection was beneficial beyond its two primary purposes of sustaining fish and plants. Water storage can also be underground if you want even more of an affect or if you lack space — or if you grow in the ground. Get clever, screw on your thinking cap, and use your imagination. Then share your ideas here if you have some.

This is really worth doing, unless you’re happy with your current greenhouse fuel bill. You’re not going to heat your conventional greenhouse by adding storage as you would in a purpose-built solar greenhouse, but you will offset temperature fluctuations, better support your plants during temporary power outages, and supplement your current heating system maybe saving yourself a tidy sum of money.

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