Solar Stock Tank Heater

This is a first cut at a stock watering tank that is  kept from freezing by a combination of insulation and solar heating.


The idea for a simple and cheap non-freezing stock tank came out of some discussions on the Homesteading Today Alternate Energy Forum.   This is a first rough cut at such a tank.


Around here, people with horses and other livestock typically use galvanized metal stock tanks for watering.  In the winter to keep them from freezing, most people use electric stock tank heaters like the one at the right.

 These heaters typically include a thermostat that turns the heater on when the temperature gets down toward freezing.  The one at the right consumes 1500 watts when its on.   In cold climates, running these heaters can add $100 to your electric bill during the winter.

This seems like a pretty expensive and greenhouse gas intensive way of preventing water freezing.  The tanks typically have no insulation, are in direct contact with the air and the ground, and have a large water surface area that losses a lot of heat through evaporation.

The idea is to see if there is a relatively simple and cheap and easily built stock tank design that does not need a tank heater.

Typical evil electric tank heater




This prototype developed into this final version -->

See also the Water Heating Section on Heating Water for Animals...


First Cut Prototype

The basic goals of the prototype are to: 1) reduce heat loss from the tank to the surroundings, and 2) capture enough solar gain to offset the remaining heat loss.  Ideally the tank will not need an electric heater at all to keep the water from freezing.


The tank pictured below is a first cut prototype that was made strictly from scrap lumber and stuff I had left over from other projects -- total cost was zero dollars -- its just to see if the idea works -- not to look nice.


The tank is a plywood box that is open on top for drinking access.  The inside of the plywood box is lined with R13 rigid foam board insulation on east, west, and north sides as well as the bottom.  The south side of the tank is an un-insulated sheet of plywood that also acts as the absorber for the solar collector.



The box is lined with a waterproof membrane to hold water.  In my case I used some left over house wrap, but EPDM pond liner would be a much better choice. 


The water surface that is open to heat loss is reduced with a floating insulation board lid that has hole in it to allow for drinking.  The drinking hole should be as small as the livestock will live with.


The south side of the tank is designed to collect some solar energy and transfer it into the tank water.  The piece of plywood that forms the south wall of the tank is painted black on the sun side to absorb heat.  The water side of the absorber wall is not insulated, so when it heats up, it transfers some of the collected heat into the water.   Just in front of the south wall is a layer of SunTuf corrugated polycarbonate glazing to reduce heat loss from the absorber.  


This collector configuration has the advantage of being very simple and cheap to build (about $1 per sqft of collector area).  On the down side, it will have fairly high heat loss at night.  The total insulation on the south (collector) side being only about R2 or 3 -- of course this is still much, much better than a bare galvanized tank wall. 


With a little more work, the collector could be turned into a thermosyphon design that would have a little less gain during the day, but also much less loss at night and on cloudy days.


Cross section through the solar stock tank


Sun shines through the glazing (blue), and is absorbed by the south wall of the tank.
The sun side of the south tank wall is painted black to absorb heat.
Since the south wall of the tank is not insulated, much of the absorbed heat is transferred through
the wall and into the water.
The other walls of the tank and the bottom are well insulated to reduce heat loss.
A floating lid of some type with a drinking hold should be used to reduce heat loss from
the water surface.





The tank walls are made from plywood and framed with 2X4's.  The south wall is glazed to form a solar collector.




Water in back box, collector in front opening

Scrap R-Board insulation on bottom and the
east, west, and north sides.
The insulation is 2 inch Atlas R-Board. 
About R13 total.



Stock tank sitting on its back with collector
glazing installed.  The south wall of the tank
(behind the glazing) is painted black to
absorb sunlight.

Looking at the collector side.
The south wall of the tank and the
absorber are the same sheet of plywood.




Sitting in the yard.


Filled up 3/4 full.  The house wrap liner is not a good choice, but it is what I had on hand -- EPDM pond liner would be much better.
The pole sticking through the insulation board lid has two temperature sensors mounted on it -- one near the water surface and one about 2/3 of the way to the bottom of the tank.  The floating insulation lid should have a larger hole for drinking, so I'm cheating a bit, but there is an about 1 inch open area all the way around the floating lid. 




The plot below shows the prototype tanks performance over a three day period. 


Prototype solar heated stock tank over a 3 day period starting Feb 5, 2008.


The red trace is the air temperature in the solar collector itself (sensor shaded from sun).

The two black traces are tank water temperature (high and low)

The green trace is outside ambient temperature

While there were a few sun breaks in this time period, the weather was basically lousy with a lot more wind than we usually have, and limited sun.


The water temperature starts out at about 48F, and gradually trends down to about 41 F at by the third day.   There was no ice on the surface at all.  No new water was added during this period.


Outside temperatures were highs in the low 30's to high 20's and lows down to around 16F.  

Even with this limited amount of sun, the tank temperature is able to recover a 2 or 3F degrees during the sunny periods when the collector temperatures go up 100F or more.  I would like to see what it can do in a fully sunny day.


While the tank temperature is still trending down toward freezing, the rate of drop is low, and will get even lower as the difference between ambient and tank temperature is reduced. 


Other factors that would effect performance:

Adding new water to the tank from time to time would tend to raise its temperature, and offset some of the losses.  Adding 1/4 of the tank capacity in new 50F water will raise the tank temperature 3 or 4F.  This alone would offset a lot of one days drop under the weather conditions shown above.


The weather will certainly effect performance.  Long strings of cloudy and very cold days will really put the insulation to the test, since there will not be any solar gain to offset losses.  Around here, that is uncommon, as the coldest weather usually comes with clear skies.


The lid configuration will make a difference.  Having a large water surface exposed will result in rapid drop in tank temperature.  Some kind of floating lid with minimal sized open drinking area would be good.

Of course, there is nothing to keep you from using both the solar insulated tank and have the electric heater for backup.  That way, if you get a long string of really bad weather, the electric tank will eventually come on and keep the tank from freezing, but the run time on the heater will be a small fraction of what it would be on a conventional tank.


Performance Update: Feb 15, 2008

The plot below shows temperatures for the next few days.



The tank water temp as stayed in the 40'sF,  and there has been no surface ice. 


The first couple days were warmer than average with highs up to the low 40's and lows around freezing.


The last couple days were colder with more sun on the 15th.   Lows down to about 10F.


On sunny days, the collector temperature goes up to the 120+F, and the tank water temperature warms up by a good 10F.


So, this is a total of 10 days with no ice and the tank water temperature well above freezing. 

I am tempted to say that the concept works pretty well thermally, at least under these weather conditions.

The performance would, of course, be even better if new replacement water was being added from time to time.


Some issues of practically include:

Some comments/suggestions from Homesteading Today Forum on the tank:

Use two nested metal tanks with foam in place insulation between them.   For some climates this might be enough without any solar gain, and it would be durable.   Solar gain could be included in this design by omitting the insulation on the south side, and cutting in and glazing a solar window in the south wall of the outer tank, then painting the south wall of the inner tank black.


Black tires floated in large stock tanks will absorb enough heat to melt holes in the ice for drinking.   This is probably climate and weather dependant, but might be a nice simple solution.


One person reported that the Cobett ( ) earth heated stock waters work well.  These fit in a hole that goes down several feet, and use earth heat to keep the above ground waterer from freezing.


There was a question on whether the solar heated tank could be scaled to different sizes -- I think it could, but small tanks probably present a bit more of a challenge because of their greater ratio of surface (loss) area to tank volume.


More comments here:

If you have any comments/suggestions, I'd like to hear them, Gary...


Performance Update: March 2, 2008


The plot below shows a period of 17 days ending on March 2, 2008.


I was away most of the time, but it looks like first few days had some some sun, and the tank steadily gained temperature each day.

Collector temps up in the 140F's and water temperature rising about 13F during the day, and losing about 10F through the night.  The tank worked its way up to about 65F.  Night air temperature lows were around 12F, and daytime highs were as high as 50F.

The last few days, there was not much sun, and the tank temps feel to about 45F -- always well above freezing.  This was a very stormy, windy period with several inches of snow.


Again, the tank seems to do pretty well thermally, and would do even better if new water were being added each day.



Other Possibilities

It seems like just adding insulation to any stock tank would greatly reduce the energy needed to keep it from freezing.


Limiting the open water surface area to the minimum required for drinking should also help a lot.


The solar collector does definitely add heat to the tank, even on these more cloudy than not days we have been having.   The efficiency of the collector could probably be improved.  The plywood tank wall that acts as both tank wall and absorber has an R value of about 0.6+  -- a metal wall would transfer heat into the tank better.


Perhaps a way could be worked out to build this concept around a standard metal stock tank.    One long wall of the tank could be painted black and glazed for collection, and the remaining walls and bottom could be insulated.  This coupled with a floating insulation lid with drinking hole should make for an efficient tank.


Any ideas?   Let me know ...


Has anyone kept track of how many KWH per day an electric stock tank heater uses in cold weather?

Here is one quote from a stock tank heater user:


Stock Tank Heaters
Many of us are familiar with the standard heating element that goes directly into the tank. They usually come with a thermostat to turn it off and on with the rise and fall of the temperature. From first hand experience I can tell you this is not the most economical method. Last year our electric bill, for just one tank, hit $100 a month four months in a row. In addition you’ve got concerns about refilling the stock tank, keeping it clean and keeping the electricity away from the livestock. We’ve all heard horror stories about a horse biting down on the electrical cord. However with proper planning, these electrical devices can keep your livestock in water all winter long. For safe operation, make sure you have a power supply with a third wire ground. Cost for submersible electric heaters run between $30 and $60.



Gary Feb 8, 2008

August 31, 2008 -- added sketch