Solar test Jig

I have been working on a test jig to allow me to mount and compare different solar thermal collectors of a standard 48x24" size in a tracking configuration.

I finally rolled it out of the garage today. The motor drive and the collectors are not yet mounted but the frame is pretty much finished.

This jig will allow me to compare two different collectors. The jig will have two independent insulated water circulation systems represented in the picture by the two coolers which will hold two same volume batches of water and two immersion pumps. By measuring and comparing the outlet temperatures, I will be able to quantify which collector is better at converting sunlight into heated water.

I will provide more details as the project progresses.

You can click on any of the pictures on my blog to see enlarged versions. The blue in the first picture is a tarp over the woodpile in the background, nothing to do with the solar test jig.

Thank you for your interest in my work.

Index - Comparing concentrator to flat plate solar collector

Compare to flat plate collector?

Peter from Queensland wrote:

I have an engineering background & must tell you that your website is perhaps the most professional in content & presentation, well done.

My interest is in a domestic solar water heating array for general purpose home use.

We are Queensland, Australia (Brisbane are) based, & so in ideal conditions for solar deployment much of the time.

I note the success of your array used as a swimming pool heater & wonder if you can now comment about the relative merits of this or similar designs compared to the more conventional DIY copper tube/aluminium fin collectors and about roof mounting.

Any comment, advice, or guidance you care to make would be greatly appreciated.

Thanks, Peter

Thank you for your note and for your kind comments.

I am the first to admit that mine is rather an unconventional design for a home built project. Although I feel that I have achieved good success in my intended application, there is no question that it is a more complex project and therefore not for everyone. Nevertheless hundreds of my plan book have been sold and my heater works well for me and it is a source of personal pride.

Many pursue the flat plate fin/tube design and some are quite nicely done. Probably the best place to look for these is at www.builditsolar.com. Gary has done an excellent job fostering the community of solar builders.

I have had many good discussions with Gary and some of his readers and they have made great suggestions. Probably the bottom line is that for heating water to the relatively lower temperatures of DHW or pool heating, a moving parabola is overkill and too complex for most people. I have several other applications in mind for a parabolic heater where the advantages will be better utilized but I do not for a minute regret the effort that I put into my project. Gary features my design on his page for concentrating heaters.

My advice in the book is to not place my collectors on a roof. Two reasons for this: as they are home built, easy access is desirable since you may need to work on them and the second reason is that wind load may be more of a problem in a more exposed location, like on a roof. A flat plate system is definitely better if mounting must be on a roof.

Whatever you decide you need, I wish you luck with your project. My book is inexpensive and dreadfully thorough and you might consider it as a bit of background reading for your research?

Thank you,

George Plhak

Index - Comparing concentrator to flat plate solar collector

about efficiency, mirrors and the composition of sunlight

Peter wrote:

I am really impressed with your solar project. I live in France and like yourself thoroughly enjoy my background in engineering. Some questions:

Your troughs are very effective (near on 100% efficiency). How is this possible?

Sunlight at zenith provides an irradiance of just over 1 kilowatt per square meter at sea level. Of this energy, 527 watts is infrared radiation, 445 watts is visible light, and 32 watts is ultraviolet radiation. [ref:- http://en.wikipedia.org/wiki/Infrared]. What are the implications of this spectrum for solar heating?

What type of mirror material are you using? There appears to be a choice of polycarbonate mirror or acrylic mirror plexiglass mirror. Which do you prefer and have you found any deterioration of the mirror finish over the 5 years of your project. I believe the products had a patchy start in development with UV deterioration problems.

Hello Peter and thank you for your note and kind comments. Also for the picture of your experiments.

I have detailed my performance measurements here:
performance-of-tracking-solar-parabola

The very high apparent efficiency puzzled me a great deal. Measurement accuracy certainly plays a role and since I have a very high flow rate, I have a relatively low heat rise and small temperature differences are difficult to measure accurately. I have a better flow gage to use this year.

I have discussed the efficiency with "experts" and I now understand that the smaller the heat rise, the higher the apparent efficiency. Even though the heat transfered is the same, as the flow rate is increased, the temperature differential (between inlet and outlet) falls and the efficiency rises. One important aspect to this is that with low temp rise, losses due to conduction, convection and radiation are all reduced so that part helps to explain somewhat but it still seems a paradox.

Of course, my use of the parabolic heater (to heat a swimming pool) is also a bit of a paradox and many people have trouble wrapping their minds around this use for a device that is conventionally intended for high temperatures and low flows. But heat is heat and my need was to heat a pool. I have other efforts going on that with explore higher heat applications using the same reflector and tracking and like you I will be using evacuated tube collectors.

You are correct in recognizing the distribution of the sun's spectrum. The UV portion is virtually useless for heating. The infrared/visible distribution however is interesting. For me the implication was that a mirror type reflective material did not even have to be used if one was content with the almost 50% of the sun's energy that was in the infrared. Try using a piece of galvanized steel and you will get quite impressive results for a lot less cost. In the developing world, this might be the preferred way to go particularly since in many countries the solar resource is so much better than ours and cost is always a major factor. For a historical perspective on how this solar spectrum was first discovered try searching "herschel's experiment infrared".

There is quite a bit of reading for you at my blog. I thought I had discussed the many various types of reflective materials that I had tried or considered but could not find that this morning. In the plan book I do go on for quite a few pages on that subject. There is quite a bit here in my blog about my current preferred reflector material - acrylic mirror and yes it is UV safe.

Given the reasonable cost and your apparent interest, would you consider buying my book for it's research value? Maybe you won't do it the same way, but you'd have 90+ pages of my experience and hundreds of photos and drawings to stimulate your thoughts?

Thank you and good luck with your efforts.

George Plhak