why not use black pvc pipe?

The DIY concentrating parabolic solar heater uses copper or stainless steel pipe for the collector tubes. A metal is preferred for the collector because of its thermal conductivity. This concentrator uses relatively little metal compared to it's effective solar size (aperture, effective area). Metal is an expensive component. The 95% rest of the collector can be made from what is common or that which makes sense locally. I used wood.

The collector tubes span an eight and a half foot gap between the support rails. It is important that the collector tubes do not sag. If they do sag, they will not be in the focus of the parabolic reflector and will not receive concentrated sunlight.

I'm often asked if it is possible to use black PVC pipe for the collector tubes. PVC is a lot cheaper than metal and besides, it is already black, a great light absorbing color. The metal tubes need a coating of flat black paint or black heat shrink to make them effective absorbers.

In this picture (click to enlarge) you can see a comparison of black PVC pipe to stainless steel, with one of the solar heaters in the background. The sag in the PVC pipe is readily visible. The tubes in the picture are empty. Normally the 1-1/2 inch pipe used in this installation contains about 10 pounds of water over the length of the span, which makes the sag greater! ABS pipe has a similar problem, although not as severe.

I did a sag test last year using copper pipe (the DWV type, with the thinnest wall) suspended over the span, filled with water, and I found that the sag was less than a millimeter.

It is important to keep the pipe in the focus of the parabollic reflector and it is not possible to do this with the PVC unless it is somehow supported mid-span.

The metal tubes while more expensive are significantly stiffer than the PVC and they span the length of the collector and remain accurately positioned at the focus with no problems.

The second problem with the PVC is what happens to it if the water drains from the system and the PVC receives the focussed heat from the reflector without the cooling effects of the water. In dry tests, the collector tube reaches over 300 degrees F. I did this test with a black painted copper pipe, not PVC. The PVC would likely sag more, or burn. The metal tubes are unaffected by this heat. ABS platic pipe is better at higher temperatures, but the material is white and would require a coating to improve thermal absorption.

Finally, the PVC pipe does not have as good thermal conductivity as metal. Not only is plastic not as good a conductor of heat, but the wall thickness is significantly more than the metal for a given pipe size, further impeding solar heat transfer to the water.

I prefer metal pipe over plastic for the collector tubes for all the above reasons.

a parabolic workshop light

I had made up a number of four foot parabolic solar reflectors for prototyping the solar heater project.

By supporting a single four foot florescent tube at the focus of one of these reflectors, I made a very effective workshop light, shown here illuminating my garage workbench.

The same ribs are used as in the longer, eight foot reflectors but with shorter side rails.

I haven't done a scientific test against a two tube fixture without a reflector, but I'm very pleased with the even and bright lighting of the bench surface and delighted to be using half of the electricity of a two tube fixture which I might have otherwise used without the reflector.

shop lighting reading list
a parabolic workshop light (this article)
led household bulbs
exploring efficient workshop lighting alternatives
work light led retrofit
testing fluorescent light fixtures - the test jig
testing fluorescent light fixtures - the test method (video)
testing fluorescent fixtures - 40 watt
efficient workshop lighting 2
updated bench lighting

the end of another season

It's the end of another wonderful summer here in southern Ontario. For the first time, we've managed to heat the pool without burning any propane. (The solar tracking parabolic heater is behind the fence in the picture, click to enlarge). In fact, last month the propane storage tank was removed since we were being charged rental by the propane supplier. I would have liked to have kept it here as another potential energency fuel source, but realistically have no use for it. Our propane pool heater sits idle and disconnected.

This is fine while the sun shines.

Unfortunately, the weather here has been overcast, cloudy, rainy and cool throughout the end of August and most of September.

The sun comes out only infrequently and it is seldom for long enough to even bother turning on the pump and the heater tracking. Most of the time, the weather forecast has looked like this:

So the idea of a solar heater as a "season extender" depends on the weather (and your location).

Without sun, the pool tends to follow the ambient temperature and it is now cool enough that no-one wants to swim in it anymore. It really is time to start thinking about putting it away for the winter.

I consider that the project is a success and that our home made solar parabolic heater contributes meaningfully to pool heat and helps in a small way to prevent some global warming. However if we lived in a sunnier climate, the results would be even more meaningful.

For much of July, the pool temperature was in the 82-85 degree range hitting a peak of about 86 in the middle of July. For next year, I plan to contue the building effort to fill the frame with collectors. I currently have six operating. The existing frame will hold 12 reflectors. This will double the potential heat output.

I am working on the plans. Progress has been slower than I had hoped because it has been a busy summer with other work.

Thanks for your interest.