Build a Solar Stock Tank

When winter rolls around, our reliable solar stock tank will save you money watering your animals and time and effort chopping ice.


| October/November 2010



solar stock tank diagram

Inset shows front corners from the top of the collector box.


LEN CHURCHILL

Typical stock tanks have a large, exposed water surface that loses heat. They also have highly conductive, single-wall sides and bottoms that are in direct contact with cold air or cold ground. If your goal is to design a tank to maximize heat loss, you couldn’t do much better than a typical galvanized or plastic stock tank. By using this easy-to-build solar stock tank, you probably won’t need an electric tank heater to keep your livestock waterer from freezing this winter.  It will provide ice-free water in all but the most extreme winter weather. We used it through two frigid Montana winters, and if it works here, it should work just about anywhere.

The Passive Solar Tank

The livestock waterer is built around a standard galvanized metal stock tank, which is surrounded by a well-insulated enclosure. The south wall of the enclosure is a double-glazed solar collector. Using two layers of corrugated plastic (double glazing) reduces overnight heat loss from the tank. The metal tank wall is just behind the collector glazing and acts as the collector absorber. The sun shines through the glazing onto the tank wall, which heats up the tank wall and the water. The tank wall is painted black to absorb heat from the sun efficiently. The waterer has a lid with an opening just large enough for animals to drink through, and this limits the heat loss from the water surface.

While this design is simple, it’s also efficient.

  • It’s well-insulated. Heat loss is far less than from a bare metal or plastic tank.
  • The solar collector adds heat on sunny days, and the double glazing reduces heat loss from the collector.
  • Solar collectors are more efficient if they operate at a low temperature. The water only needs to be heated to just above 32 degrees Fahrenheit. This allows the absorber (tank wall) to run cool, which reduces heat loss out of the glazing. If the tank wall needed to be hot for the system to work, a lower percentage of the heat collected would be transferred to the water.
  • The water provides thermal mass so all of the available sun energy can be effectively stored.
  • The lid further reduces heat loss from the water surface.

We measured the energy consumption of an electric heater in the old tank (a bare, galvanized tank) at 8.5 kilowatt hours per full day during mild winter weather, with low night temperatures about 15 to 30 degrees and daytime highs about 25 to 40 degrees. We estimate the energy use for “real” winter weather is about 30 kilowatt hours per day, which would cost about $3 and result in about 50 pounds of carbon dioxide emissions per day!

The solar-heated livestock waterer rarely requires an electric heater. If you get a little ice on the water overnight, it will usually melt later in the day.

A couple of times during winter — when we had several days of temperatures below 10 degrees and not much sun — we needed an electric tank heater to keep the water from freezing. Even in these extreme cases, though, the power used by the heater is greatly reduced, because it’s working less to maintain temperature.

barb
1/8/2016 6:35:55 PM

I built one using an old bath tub and a couple modifications, worked so good I built a second one, https://www.facebook.com/barb.thompson1/media_set?set=a.10153711073556206.1073741860.719486205&type=3&hc_location=ufi


christy
11/12/2014 12:54:05 PM

I was wondering why surrounding the tank with metal corrugated panels would'nt work to draw in more heat. I may have to give it a try-


lisa
9/15/2014 3:39:40 PM

After reading this article a couple years ago, we built three units. One had the wavy plastic solar panel that they describe. I didn't like it because we had to do so much caulking, etc. to seal up the unit. The other two we used clear plexi-glass instead. Two were used for cattle and one for horses. The horses finally cracked the wavy stuff last year. The plexi-glass has held up fine with the cattle. Other differences - we used Rubbermaid 100 gallon tanks - grey plastic - because that's what we had. They have worked fine. We get up to 25 below here in Western Colorado, and have only had to plug in the tanks a few times over the winter. We also built the boxes oversized which allowed up to line them with insulation on three sides - use the hard foam like stuff - cuts easy to size with a razor knife. Lastly, we painted the plywood sides and lid black to help absorb the heat. I have been very happy with these units - they have held up well and I sure like the low electric bill!


garyr
2/19/2014 9:20:55 AM

Hi Alaska, The chest freezer sounds like a good idea. I'm surprised you had trouble with the Suntuf -- its polycarbonate (the football helmet stuff). I've used hundreds of sqft of it for several projects and never been able to break or crack it. Another good option which works particularly well in cold climates is the twinwall polycarbonate glazing that most greenhouses use now. It gives you a very impact resistant double glazed solution. Gary


alaska herrins
1/7/2014 10:07:16 AM

I have that clear suntuf plastic on my greenhouse and it randomly shatters like glass when it is cold. Maybe a bad batch, but the stuff I put on the uppers of my barn are also fragile. An errant hoof is going to go right through that. Solar gain in the winter here is nil anyway. I am in Alaska where below zero is the norm for winters and I have been using an old 15 cubic foot chest freezer for the last couple winters with a foam cover that has a drinking hole cut in it. I have only used a 250 watt submersible heater for the last 30 years and it is plenty and cheap. I ran a Kill-a-Watt on it last time we were in the 20 below range and it was 38 cents a day @ 11 cents a kwh. Not too bad and it does not run when it warms up a little. A bonus is in the summer the white tank with its insulated sides doesn't heat up and grow green like the old brown Rubbermaid tank did. And the smooth painted steel cleans fairly easily. You can get these free on Craigs List all the time.


garyr
1/1/2014 11:55:02 AM

Hi, Robert, there is one person who tried using heavy canvas to extend the drinking opening vertically downward, and its working fine for him. Explained here: http://www.builditsolar.com/Projects/WaterHeating/water_heating.htm#Animals on the 2nd link. Kevin, I think painting the outside of the tank a dark color would help the performance just a bit -- we just used the paint we had on hand. Dan, there have been a lot of people asking about rubber tanks. My only concern is that if the tank is mostly empty (so the water is not cooling the tank wall), and there is lots of sun, the part of the tank wall above the waterline will get very warm -- maybe warm enough to damage the tank wall. But, no one has reported back yet either way on this. I don't think that the lower conductivity of the rubber tank wall would effect the efficiency too much as the thermal resistance of the water film next to the tank wall is in series with the tank wall resistance, and it probably of the same order. Jean, wondering where you live? The tank works well here in SW MT and I've heard from people up in Alberta who are having good results. But, there are times when you get multiple cloudy and very cold days where some ice will need to be broken (at least around here). Gary


robert dupuy
12/31/2013 8:09:01 AM

What if you were to create a rectangular surround that went down into the water for the area that the animal drinks from. This should allow for even less heat to release overnight?


kevink
12/31/2013 7:58:16 AM

I wonder why they do not recommend painting the exterior of the tank flat black. Dan, Rubber is a fair insulator and would not allow the water to heat up as much. Jean, Even if the system does not eliminate freezing it should still dramatically reduce power consumption from the back-up electric heater. Then you have to decide if the remaining power draw is enough to justify any additional cost of the Suntank. It looks like the Suntank uses the same principle.


harold
12/27/2013 10:31:31 PM

What do you do to keep the pipe that fills the tank from freezing.My son's farm is in Minnesota and the frost line can go down to six feet.


dan richardson
11/18/2011 3:40:00 PM

Why the use of galvanized tank instead of the black rubber ones?


jean nelson
10/5/2011 8:44:17 PM

Our winters here are really cold and this do-it-yourself-unit did not work, I ended up buying a suntank from pine ranch products.It has worked like a charm. thanks to pine ranch products my watering needs are over.






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