Batch Solar Water Heater from Stock Tank

A nice, simple, inexpensive solar batch water heater that uses a galvanized stock watering tank for the outer case.

Thanks very much to Ron for providing this!


Note: Ron has made a number of improvements to the design.  So be sure to see the new features and updates here...

Ron also has a new book that you might want to take a look at...

Opalescence book

Description and pictures from Ron:

Picture shows the outer batch heater enclosure made from a stock watering tank -- the stock tank is not very expensive, and should last for many years.
The reflector increases solar radiation on the tank without increasing losses -- a nice feature.  Ron has a way to tilt the entire assembly so that the glazing faces more directly at the sun.

solarbatch water heater from stock tank


Design and Construction:
I was going to build a similar batch collector to that on the """" which is a nice design, then I thought that I just wanted something lighter, stronger, easier and cheaper. First I obtained the 30 gal. core of a gas hot water heater (with some difficulty as almost no one - appliance store or plumbing co. - wanted to let a used one go, even when I offered to pay for it). When I finally obtained a leak-free one from a very nice guy named Gary at a hardware store I stripped it and painted it flat black. Here is a paint available specifically made for this job "" .


View from end showing the 2X4 cradle
that the tank rests on.

Note how much the mirror increases the collection area.

Additional overview pictures of the collector -- click pictures to see full size

Next I bought a cattle/horse water trough for $99 at a local feed store. Inside I screwed a simple 2" X 4" frame to hold it in place and to keep if from touching the sides or bottom of the trough. The core fits perfectly inside it. I then drilled three holes with a hole saw which were just the right size for the piping (hot, cold and TPR valve) to go through, added rubber grommets to seal. After that I put on some weather proof vinyl weather stripping on the rim and on top of that two pieces of Plexiglas which I bought from Home Depot for $43 each for a 2' X 4' piece. On top of those I put some metal hold down strips that I attached with screws and wingnuts after drilling some holes through the rim of the tank (and making sure to aim the drill so that the lower hole went outside the trough). Since the tank does not have perfectly straight sides I had to bend these strips to match before attaching. I discovered that it is easier to first drill the holes in the hold-down strips and then one matching hole in the rim of the trough closest to the center on each side. After that attach the strips to the trough then drill the rest on the tank to insure that the holes through strip, glazing, weather-stripping and tank all line up. As it was slightly bowed inwards at the middle I stuck a piece of rebar at the top center, held on with some moldable metal epoxy putty.
Also since the Plexiglas itself can bow downwards some as it begins to get hot I placed a thinnish, flat metal bar just under the middle where the two pieces meet, across the width of the trough, on top of the rim for support. Over these I have a piece of pipe wrap tape. 
I added some door mirrors (that I bought from a large discount store for $5.88 each) that I made adjustable so that I can point the sun's rays directly at the core. They are attached to a piece of 3/4" plywood with construction adhesive. As this board is somewhat heavy, rather than use wood screws to attach the hinges to the 3/4" plywood I came in from the mirror side with flathead machine screws that I countersunk some to make flush with the wood surface so that I could bolt them down onto the hinges with washers and nuts for added strength. The mirrors can also lay down flat on the top of the Plexiglas in the event of a storm. (As an anecdote to the amplifying effects of mirrors, in the process of working on the collector, as I was walking in the area I suddenly felt a blast of heat and light hit my face from a mirror that had been propped against the side of the house about 30' away reflecting the sun!)

I was getting about 185 to 190 degrees F then. I later added some foil and Reflectex inside attached with foil tape and made a few other changes as outlined below.  After these changes the temps went up to about 210 degrees. But again, it is summer and we are having hot days. It remains to be seen how well it does at other times of the year.
I'm going to connect a water filter to the inlet to keep sediment out. I also have a shut-off on this line so that if I need to cut water pressure to work on it, or possibly turn it off in the winter I can do so without having to turn off the water to the entire house. Depending on if I do need to turn it off in winter I will put another cut-off between the hot water outlet and the house so water from the normal house water heater doesn't flow back into the tank.


Mirror support from back

Mirror hinged down -- this might be helpful
in mid-summer to reduce gain, if the
water is getting too hot.

Flex hoses penetrating side
of tank and Reflective insulation.

Center support bar for glazing.

Construction detail pictures -- click on picture to see full size

Locating and Aiming the Heater:
Since the best location to put it was on the side of the house that was unfortunately away from the existing house hot water heater (but fortunately closer to the shower) I had to do some rerouting of the water pipes in the cramped space underneath (which was not fun). The area is in a corner between the house and a shed. Another mixed blessing of this location is that while the area holds heat as it blocks cooling wind, the house also blocks the sun until about 9:00 AM with full exposure only beginning at about 10:30 (I've noticed that the peak temp period inside the trough, at least in summer, is between 2:00 and 3:00 PM).  Anyway, I then tied in a direct line from the tank to the shower.
To aim the heater, I found geographical south with declination taken into consideration, and turned it that way. Then, to maximize the light potential, I made the entire unit adjustable so as to be able to point it optimally towards the sun at any given time of the year. This I felt I needed to do since unlike the by-example collector mine is not low in front of the tank which affords more solar rays, a disadvantage of my method.
I put three of the biggest hinges I could find on one side between the 1/2" plywood board the trough is attached to and a 3/4" plywood board under it. The whole thing is on concrete blocks to keep the unit off the ground from water and cold. On the other side I cut out a piece of the 3/4" board just big enough to fit an about 2' long 2" X 4" board. This piece is not attached. When I want to tilt the unit I place the board under the trough there and under that I put a scissors car jack and begin cranking. It goes up and tilts quite nicely. Right now I have it pointed at about 63 degrees from southern horizontal, about where the sun is angled at currently relative to my location. Probably every couple of weeks as winter comes on I'll go out and tilt it some more. To be flexible I had to change the PVC I had attached outside the trough (cold & hot piping) to some flexible metal washing machine hoses. These I wrapped in foam insulation, as are the pipes under the house. I also added the foil and the Reflectex insulation at this time.


Front side view of hinges that allow
collector to tilt.

Back side view of collector
tilt hinges.


Tilt adjustment arrangement

Pictures showing the collector tilt arrangement -- click for full size

What's gone wrong so far is 1) condensation inside on the two ends which I think was because it was apparently not sealed as well there as on the sides and because a tiny leak had developed at the PVC fitting which is connected to the TPR valve inside the trough - I found a small pool of water inside. 2) A PVC fitting that was glued outside the trough with PVC cement developed tiny pinhole leaks as it couldn't hold up to the heat. I've replaced this with a threaded metal fitting and plumber's putty. 3) I cracked the mirror a bit on one corner and fixed it with aluminum tape. While it doesn't affect the working it was a bummer.

If I did it again I'd probably do a few things different. First I'd use one length of polycarbonate glazing as Gary mentions below instead of two pieces of Plexiglas. One issue to be aware of with Plexiglas is that it can expand and contract some with temperatures beginning at a little less than 200 degrees F. If you can afford it double paned is best.

Then, instead of a fragile mirror I'd use a piece of aluminum flashing that is available locally for about $20.00 for a 20" X 10' foot roll (it's about seven inches wider than the door mirrors I used which would also bring in more light, and cover the top better when laid down) then cut to size. The flashing I'm told can be polished with a buffer and some compound to a mirror-like finish. They say it will tarnish with time and will need to be re-polished but that can be reduced by waxing it - though I don't know how well the wax will hold up to heat. You can also buy a piece of polished metal through a metal shop but it's more expensive, or look in a scrap yard. Or the piece of aluminized Mylar that Gary recommends.
I like the metal tank because being built to be able to hold water it is leak-free and thus can hold up to pressure well. If there is a leak it wouldn't be a disaster as it is meant to hold water. Additionally, it has a screw-in drain plug at the bottom center. Also being galvanized it should be rust resistant. I think that the metal holds heat better, but maybe I'm wrong about that. But metal is just stronger. I am hoping that this will be a sturdy unit that once I am done I can forget about for years afterwards.
Lastly, I'd like to thank a few people who contributed and whose help I appreciate. First the creator of the byexample batch version which was an inspiration. Next is Gary at the hardware store (and the woman at the heating and repair company whose name I've lost) who, bucking the stinginess of the majority, paid it forward by giving us the tanks. Next is my brother Rod, who lent me his drill, bit set and back as we hefted this baby around. Then there's my family which patiently put up with cold water while I was working with on it. Last, but not least, the other Gary (not the same Gary) at this site, Build It Solar, who provided me this page on his great solar website and gave me some further ideas.

Oh, and of course I don't want to forget Big Oil whose rank greed, avarice and polluting product were my original impetus.

To all of you, thanks,

Ron Rayborne

You can address email questions to:  zephyray AT gmail dot com  (replace AT with @ and dot with a period)


Galvanized stock watering tanks
at the AG store

Oven thermometer resting on the storage tank.
Reading about 210F.

A couple more pictures -- click on picture to see full size


Finding good tanks to use for batch heaters can be a problem -- here is Ron's experience:

I found the tank that I am using at the hardware store in a town near us.  We called all the plumbing shops and appliance stores and were getting pretty discouraged.  Then I talked to a guy in the plumbing department at the hardware and he had one and was more than willing to let us have it.  He wrote "Save for Ron - leaks" on it to discourage other people from taking it when he set it outside for us, it doesn't leak.  He said, "when you get it patented remember me".  A little later we got call from the secretary of a plumbing outfit that I had called earlier saying that she also had one that was basically brand new but was returned by a customer for other reasons.  It's a 40 gal. and is now resting behind our shed.  I found that the biggest supply of tanks was at the dump but they refused to let any go, some sort of regulation they say :-\



Some thoughts on potential changes:

This is a nice design that is very simple to build.  Using the galvanized stock tank for the outer casing provides a ready built enclosure that should have a good long life.   Using the external reflector adds to the solar gain without adding losses, and helps to make up for the relatively small glazed area.


Here are some thoughts on potential changes:

- I would think about using the rigid foam insulation board that comes with a reflective coating to insulate and reflect sun inside the stock tank.  This must be polyisocyanurate insulation (1) to hold up to the the temperatures.  This would  be fairly easy to do, and would reduce the heat loss from the batch storage tank.


- Increase the size of the reflector to the same same size as the top opening of the stock tank, and allow it to hinge down so that it can cover the stock tank top opening at night.  The back surface of the reflector could be insulated with rigid foam board to reduce heat loss at night.  As long as one is diligent about raising and lowering the reflector on cold nights, this should greatly extend the useful season of the collector.
The reflector surface could be made from aluminized Mylar, which is light weight and will hold up for several years exposed to the weather.


While batch solar water heaters have a long list of good features, they do tend to have the disadvantage that they lose a lot of heat overnight.  This is primarily due to the losses out the glazed face.  Both of these suggested improvements would reduce heat loss and temperature drop at night.  I believe that having a larger adjustable reflector would probably eliminate any need to change the tilt on the stock tank -- it could be set to a fixed tilt, and the reflector angle adjusted every month or so.


Some concerns (that may or may not be real):

One caution is that this heater is on the small side for a family -- it would be good to look at the sizing guidelines in the Bainbridge book here...  and see if you might need a larger tank and enclosure for your situation.

The flex washer hoses appear to be doing OK, but may not hold up to the very high temperatures inside the enclosure in the long term -- or, they may do fine :)   Copper pipes for inside the enclosure plumbing would be a safer choice.


The Plexiglas for the glazing seems to be doing OK, but a polycarbonate glazing sheet would take higher temperatures and be tougher.  The twinwall polycarbonate glazing used on a lot of greenhouses might be ideal, as it would also reduce heat loss out the glazing somewhat.

Anyway, a great job with some unique ideas -- thanks very much to Ron!




Note (1) Polyisocyanurate rigid insulation board is carried by most lumber yards, but they may not know it by that name.  It usually has aluminum foil face sheets, and is usually a tan, fine grained foam.  It will say Polyisocyanurate somewhere on the sheet.  Atlas and Firestone are two common brands.  The typical blue, pink, or white polystyrene insulation boards will melt inside of a batch collector -- I have proven this experimentally :)
It costs only slightly more than polystyrene.


Gary September 2, 2008, September 18, 2008