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.
The horses seemed to take the new tank in stride. We thought they might have a problem putting their heads into the fairly small opening, but they took to it with no problems.
Solar Stock Tank Materials
The tank we used for this project was 6 feet long by 2 feet wide by 2 feet high, with round ends. These tanks cost about $100, but you probably already have one if you have livestock. The prices below are estimates for new materials. If you have a scrap pile, much of them could be free. You could also check your local Habitat for Humanity ReStore for inexpensive materials.
If you would be spending $3 per day for an electric tank heater, the solar-heated tank will pay for itself in just two months! The total cost of the solar stock tank was about $183.
3 sheets 3/4-inch exterior plywood, $75
6 8-foot 2-by-4 studs, $14
40 square feet 1 1/2-inch rigid insulation board, $28
2 sheets polycarbonate glazing, $42
Glazing closure strips, $5
Black and mis-mixed paint, $5
1 can polyurethane foam insulation, $4
Caulk, glue and screws, $10
How to Build the Livestock Waterer
The insulated tank enclosure is easy to build. It uses standard 2-by-4 lumber and plywood for the enclosure, tough but inexpensive corrugated polycarbonate glazing for the collector, and standard polystyrene insulation board. All of these materials are available at a hardware or lumber store.
1. Build the Frame. The box that encloses the galvanized stock tank is framed with 2 x 4s. Be sure to build the frame large enough to allow for the insulation’s thickness. The frame should be high enough that the lid fits snugly down on the tank’s upper rim to prevent air infiltration. Allow for the height of the insulation under the tank. Use glue — plus nails or screws — for all joints.
2. Cut and Attach the Sides to the Box. Attach three-quarter-inch exterior plywood to the sides and back of the box. Leave the front open for the collector glazing.
3. Cut and Fit the Top and Bottom of the Box. Cut out the top and bottom plywood panels for the tank. Install the bottom at this point. Set aside the top to be installed as the last step. The top has a hole to allow livestock access to the water. Paint and seal the entire box. Caulk all the seams to prevent air infiltration.
4. Install Insulation Board. Cut out and fit the insulation board to the bottom and sides of the tank. We used 1 1/2-inch insulation board between the frame 2 x 4s, plus a second layer of 2-inch insulation board inside the frame,but this may be overkill. If you choose to use the extra layer of insulation, be sure to make the tank enclosure large enough to accommodate the insulation.
5. Install the Glazing. Fit two layers of corrugated polycarbonate glazing to the front opening of the box. Suntuf corrugated polycarbonate is tough, highly transparent, and tolerant of high temperatures. Many hardware stores keep it in stock. Seal the open ends of the corrugations with the foam strips Suntuf sells for this purpose.
6. Install the Galvanized Tank and the Cover. Paint the south wall of the tank black and let it dry. Then place the galvanized tank in the box. This would be a good time to measure for a drain fitting for the tank that lines up with the drain plug on the tank. Stuff the open areas between the foam board and the tank with fiberglass, crumpled newspaper or expanding foam insulation to prevent air circulation. Fasten the cover down using only screws so it can be easily removed. Make certain the lid fits snugly against the tank’s upper rim and the frame. If you want to use an electric tank heater, create a space for the cord to fit through the lid or insulated box. Paint the outside of the box. I found a gallon of mis-mixed paint for $5.
More detailed construction information is available at Build It Solar.
Using the Tank
Some hints on how to get the most out of the solar livestock waterer:
- Orient the tank so the glazed wall faces roughly south (southwest to southeast is OK). Avoid locating it where it will be blocked from the sun by trees or buildings between about 9 a.m. and 3 p.m.
- In cold weather, water will be less likely to freeze if you keep the tank fairly full.
- Install the tank on a level spot. If you set the tank on a base of washed gravel or treated 4-by-4s, it should last a long time.
- In summer, you will probably want to remove the tank from the enclosure, or at least cover the solar collector and take off the lid. Otherwise, the warmed water will promote algae growth.
- You can use the tank enclosure as a miniature greenhouse to grow your tomatoes or other warm-weather plants.
- You could also connect the corners of the insulated box with bolts so it can be disassembled for summer storage. When you reassemble the box, be careful not to leave any gaps in the corners where air could leak through.
The current design of the tank works well, but there are always ways to make improvements.
Glazing Protection. Although the horses don’t seem to be interested in damaging the glazing and the polycarbonate is pretty tough, you may want to protect the glazing if you think your stock will damage it. A hog panel over the collector glazing should do the job.
A Better Lid. A lid design that does a better job of protecting the tank water surface from exposure to cold — and is stock-proof — would be a good improvement. This is the major remaining heat loss, and reducing it would probably eliminate the need for any supplemental heat even under extreme conditions. If you visit the stock tank frequently, using a cover over the drinking opening at night during very cold weather would probably work well.
Insulation Levels. The 3 1/2 inches of foam board insulation on the bottom, sides and back may be overkill. With this much insulation, most of the calculated heat loss is through the collector glazing and water surface. Cutting the insulation down to 2 inches would save money and probably not degrade the performance much.
Eliminate the Solar Collector. If the location for the tank gets little or no sunlight, just use insulation all the way around the tank. This should still reduce the ice formation significantly. The water has a lot of thermal mass and needs to cool from about 50 degrees (typical temperature of water straight from the hydrant) down to 32 degrees before ice will start forming. If the heat loss rate is reduced by using insulation, it will take a while for the water to cool enough to form significant ice.
Use the Tank With a Heater. The tank can be used with a thermostatically controlled electric tank heater in extremely cold or cloudy weather. The tank insulation will greatly reduce the power consumption of the heater, but for most places, the heater isn’t necessary.
If you have questions or ideas on how the tank could be improved (such as improvements to the lid design), join the online discussion.