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Optimizing a Solar Water Heater
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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