Tuesday, April 26, 2011

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:

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