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Thursday, November 22, 2007

A Tracking Solar Water Heater

I saw this in Mechanics Illustrated or Popular Science about 30 years ago. I have long forgotten the name of the inventors but I think they were a couple of Australians. It so impressed me that I still remember it. It was a tracking solar water heater. You might ask why bother. A flat panel makes lots of hot water so why go to all the trouble. Well, to start with it is "cool". What tecno junkie wouldn't rather take 5 minutes to open a can with an electrical can opener instead of opening it in 30 seconds with a hand opener. Besides, it can provide a lot more heat from a given day than a panel collector. You also get to fiddle around with it and tweak the operation while a flat panel just sits there and heats water.

Where I live on the South Island of New Zealand, in the summer the sun comes up almost 30 degrees south of our line of latitude so our north facing roof with its solar panel doesn't get the sun until about 8:30AM. The same applies in the evening. We lose around 4 hours of potential water heating each day. Even when the sun does start to illuminate the panels, it is at a pretty oblique angle so it is not very effective for another hour or so. The tracking system I remember uses a parabolic trough to focus the sun on a heat-pipe* located at its focus. As all you thermodynamic adepts will know, you can get more energy out of a heat engine with a greater delta T, even though you have only the same input of energy. So how does it work. This is going to be hard without pictures but I'll do my best.

*Note - the heat pipe in the link talks about a wick on the inner lining of the heat pipe.  When heat is being transferred from 'down' to 'up', a wick is not needed.  The working fluid simply trickles back down the tube.

The first part of the system is a heat pipe. For any of you who haven't come across these amazing devices, a heat pipe is a pipe (as you would expect) which is sealed at both ends and has some working fluid in it. All the air is excluded, often by the simple expedient of sealing the bottom end, boiling the fluid until all the air is pushed out and then sealing the top end. The amount of working fluid used must be carefully regulated so that if all the fluid is in the gas phase, the pressure is comfortably under the bursting pressure of the pipe. Once you have done the necessary calculation you can check your heat pipe by weighing it before and after introducing the working fluid.  The heat pipe is located at the focus of the parabolic trough mirror and the mirror and its pipe is set at the correct angle for the latitude of the location. The upper end of the heat-pipe goes up into the water tank where the vapor is condensed and dribbles back to the hot end of the heat pipe.

Heat pipes have huge heat transfer capabilities. Using water as our working fluid, for instance, since all the air has been eliminated from the pipe, water will already boil at near its freezing point. (I have trouble getting my head around this one too) All it needs to start transferring heat is that the upper end be cooler than the lower end. It takes the "latent heat of evaporation" to vaporize the water regardless of what temperature it is boiling at and this heat is given out at the top end as it condenses. Since there are no air molecules in the tube to slow the passage of  the water molecules  on their way up to the top, they travel just as fast as the condensation allows.* The transfer is for all practical purposes, instantaneous.

*molecules travel at approximately the speed of sound.  The reason a smell, for instance, takes time to go across a room is that the odor molecules have to bump their way through the intervening air molecules.

Parabolic mirror
The heat collector is a silvered trough with a parabolic cross section. Your high school math student can tell you how to cut the pieces to product a parabola. Essentially it is made according to a formula such as Y = AX2. You can put in different values for "A" to change the width of the parabola.  The heat pipe is set at the focus of the parabola. Hence, if we can get the trough to point to the sun, all this heat energy will be focused on the heat pipe with its dribbling fluid coming down from the upper condensation end to be evaporated and sent back up again. The method of getting the parabola to point is the truly elegant part of this system.

The Detector Pipes 
We need some way of detecting if the parabolic mirror is facing the right way.  For this they used two thin walled, black painted aluminium pipes which were put under the parabolic reflector such that if the sun is directly shining on the parabolic mirror, both are in the shade.  As the sun moves, one of them receives the rays from the sun.  The pipe heats up and the air inside heats up.  A thin tube takes this pressure from each aluminium pipe to the detector.  The detector consists of two hemispheres each with a rubber membrane with the membranes pushed together.  The arm of a dumping valve is sandwitched between the rubber membranes.

Dumping Valve
On the net, a dumping valve is a valve to limit the manifold pressure of racing cars. However what I know as a dumping valve is as follows. Picture a valve, connected to the municipal water system and to a tank. There is a lever on the valve like there used to be on the old urinals. When the lever is pushed one way, it allows water from the municipal pipe into the tank. When the lever is pushed the other way, it dumps water from the tank through a third port. Now we are almost ready to start.  I have no idea where one gets such a valve but this is what the inventors used so they must be available somewhere.

Rotating the Reflector
We now have to be able to rotate the reflector around the heat pipe. To do this we attach a long piece of wood/aluminum tubing etc. Across the top of the parabola sticking out on one side. If we grabbed the outer end of this lever, we could swing the parabola either way. Instead we attach the shaft of a piston to the outer end. For the sake of the illustration think of a simple tubular bicycle pump with its handle attached to the outer end of this lever. Now if we attach the pump end of the bicycle pump to something solid and let in some air or water pressure in where the air usually come out, the handle will be pushed out and turn the parabolic trough.

Using the pressure from the detector pipes
Now we have to use the air pressure from the detector pipes. A thin tube comes out of each detector pipe. Remember these were the pipes located just in the shade of the trough collector. Each tube leads into a hemisphere. Think of a toilet float cut in half but with the cut end closed hermetically with a rubber membrane. If you push air into the half ball, the rubber membrane will bulge out. Take the two hemispheres, one connected to each detector pipe and put them together, face to face (rubber membrane to rubber membrane) and enclose the lever from the dumping valve between them. Now all we have to do is to attach the dumping valve to the municipal water system and the outlet to the "bicycle pump". As soon as one of the detector pipes gets a bit of sunshine on it, the air will expand, push the membrane of its hemisphere and push the lever of the dumping valve. Water will flow into the 'bicycle pump" and push the piston which will turn the parabolic collector. So far so good but we are still missing one part which you will probably have wondered about. How do we get the trough to swing back next day. This is where the spring comes in.

The spring
A long extending spring is also attached to the end of the lever which is attached across the top of the trough. It is attached so that it opposes the motion of the "bicycle pump". As soon as the sun comes up the next morning, the other detector pipe warms up and pushes the lever of the dumping valve the other way and dumps water from the "bicycle pump". The spring causes the solar collector to swings smoothly back to face the sun and the cycle starts again.

If I remember the story, and it has been some years, the inventors of this system tried photo cells to regulate the tracking of the mirror but found that the mirror "hunted". that is, it swung back and forth. Apparently with this system, the action is smooth and positive. Of added benefit, it is always problematic having electricity with water. This system is what a physicist or mathematician would call elegant. A system with beautiful simplicity. Except for the dumping valve, it is well within the capability of the home handy man. Does anyone out there have more information on this system and especially the names of the inventors. They deserve to be mentioned.

If anyone builds one of these, let me know and I will include your web site in this blog.


Knowing Me said...

Hi there,

Interesting read. I just wondered did you find out any more information on this device? It seems to be the sort of thing we are considering for our own "backyard" solar heat system here in Hungary.


William Hughes-Games said...

No, this is all I have. I read it more than 40 years ago but it impressed me so much that it stuck in my mind.

polysun solar simulator software said...


you really described well on these devises.Is it possible to make use of sun energy for residential and commercial purposes. If yes then how much initial cost is required for them.

polysun solar simulator software said...


When you look at the solar energy solution, you will realize that it uses absolutely no fuel other than the sun's light. Moreover, it does not release into the atmosphere anything harmful.

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Solar panel installers Glasgow said...

Thank you for sharing such a nice information on solar water heater. Solar panels are the things of next generation. It has become necessary for all to have solar panels at homes. It is the only way to save and generate energy through the ultimate source "The Sun".

nebo rod said...

beautifull decription,
I once saw the working unit at a ag show many years ago.somewhere on the darling downs Australia I think. I'm pretty sure your right in saying it was two brothers that invented it. it could have been as long as 25 years ago that I saw it. I have always wondered why such a brilliant idear has not come into common use.
I have been looking for years on and off to see if any one took up the design . as you say elegant.
glad I stumbled across this page , I can now rest easy that I didn't imagine it all.
good luch in finding out more.

solar hydronic heating said...

I believe that to be able to improve our environment and economy, we need to pursue maximising energy efficiency thru technology.

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