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Name: James E. T.
Status: Educator
Grade: 6-8
Location: WA
Country: United States
Date: January 2008


Question:
My students and I are going to build a solar water heater. The (outside) heat exchanger inside a glazed box will be made of copper tubing. In some pool heaters I have seen, they are constructed with a 1" horizontal tube at the top and bottom with multiple 3/8" tubes connecting the two. Question: I am proposing we construct ours with 3/4" horizontal tubing with multiple 1/2"copper tubing. Is there some optimal correlation between the size of the horizontal large tubing and the supplying smaller tubing.



Replies:
Jim,

This sounds like a great project. I think that the best way to go about designing and fabricating your solar water heater is the following; maximize the surface area to volume ratio and make it out of a material with a very low emissivity (typically black).

If you consider the surface area to volume ratio comment. You will only have heat transfer from the heated (black) surface of the material to a small film thickness of each of your smaller tubules. To put it another way, the more volume of water you are able to exchange / circulate over a given area WHILE still minimizing pressure drop and minimizing cost (probably do not use copper) the faster you will exploit solar energy.

If I were designing this I would use drip tubing / spaghetti tubing (relatively small diameter). Rather than making it a rectangular manifold, I would spiral it in a way that maximizes its exposure to the Sun while taking up minimal volume. If I were to use 100 feet of this tubing then I would expect to have a horrible pressure drop acrosse such a large length and small diameter ... so this is why I would run multiple manifolds of this design in parallel thereby reducing the overall head pressure.

Great question and good luck,
Darin Wagner


Hi James,

What really matters more than anything else is to maximize the surface area of the tubing that is exposed to the sun. A large internal diameter of the smaller vertical tubes is relatively pointless for flow purposes, since the speed of water flow in these tubes is very slow anyway. Because of the very slow slow speed, resistance to flow is insignificant. The header pipes at the top and bottom should be large enough to handle the combined flow of all the vertical tubes, but since the flow caused by convection is so very slow, the size of the headers is not a serious consideration, and they can be selected for ease of connection and other practical criteria. That said, connecting many 1/2" vertical tubes into a single 3/4" heater tube, would seem not to make much sense, since the differences in their areas is only about 2 to 1.

A series of larger 1/2" vertical tubes may expose more surface area to the sun than the same number of 3/8" tubes, but the result will be a lot more weight and (as a result of their much greater internal water volume) the time to heat the water will be significantly increased. More sensible, would be to use more 3/8" tubes, which will result in increased surface area, and as a result of the much lower thermal mass (a lot less internal water, and a lot less copper as well), response time will be faster. Remember that the internal volume to fill with water, will increase as the square of the pipe's diameter, but the area exposed to the sun only increases linearly with diameter. To get maximum exposure to the sun using minimum water volume, many small tubes are better than fewer larger ones.

Regards,

Bob Wilson


It does not make much difference. Water can remove so much more heat from the box than the box will capture, that almost any reasonable design should work well. But optimization is certainly possible, and the best diameter for the connecting tubes will depend on how many you have, and on what you want to optimize.

I would start with the cross sectional area of the inlet tube being the same as the total cross sectional area of the connecting tubes, and adjust the design from there depending on what you are trying to achieve.

If you want to get the most heat out, without regard for the temperature of the exiting water, then you want the tubes to remain as cold as possible. You can achieve this with a high flow rate, and this calls for fewer and smaller-diameter connecting tubes. But if you want to maximize the temperature of the exiting water, then you want a slow flow rate. More and larger-diameter connecting tubes will give you that.

Also, the design depends on how the connecting tubes will be heated by the sun. You might, for example, have a metal plate in the box which absorbs light and conducts heat to the connecting tubes. In this case, you can go with a smaller number of connecting tubes -- the more conductive the plate, and the better the thermal connection between the plate and the tubes, the more widely spaced the tubes can be. If you do not have a collecting plate, then the tubes must either collect the heat, or be heated by the air in the box. In either case, you want to maximize the number and surface area of the tubes.

In purely practical terms, you always want to minimize cost and the possibility of leaks, and this argues for less tubing and fewer tubing connections.

Tim Mooney



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