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

Making steam and tracking questions

Tom wrote:

I just bought your doc and have skimmed over it quickly tonight. Very impressed. The ‘errata’ and the QandA links at the end don’t work however and I am interested in them…?

I am interested in using such an array (small initially, but scalable upwards) to generate steam, then a steam engine, then a generator etc. First of all, have you done any research on this? Secondly, what is your opinion of using oil and a closed loop system instead of water etc? And a heat exchanger etc.?

I was curious about trying to manipulate 2 axis instead of just one. Do you remember the game where you have a steel ball and 2 knobs that basically balance the entire wooden box on a point and slightly manipulate the balance to make the ball go around the maze? Do you see this concept as having any value in implementing it in this design compared to the gain you get in heat generated?

Hello Tom and thank you for your interest in my plans.

I have sold several hundred so far and you are the FIRST to point out that these links on the last page do not work! Thank you! Both pages are on the web and the text of the links is correct if you type them into your browser. Here are working ones for your convenience:

http://www.ffwdm.com/solar/errata.htm
http://www.ffwdm.com/solar/FAQ.htm

About making electricity from steam, this is indeed the "holy grail" project. I am working on this as are hundreds of other people. There is much on the web but after five years working on this and searching, I don't think that I have found a practical system detailed in such a way that it could be replicated easily by others. People talk about all sorts of things but as far as a robust, working, practical approach that can be built at an affordable cost and with simple tools, I have not seen it. It would have to be closed loop to be practical. Would it need a heat exchanger? Possibly not. Is high pressure steam dangerous? Certainly, especially if there is steam storage of any size included in the system.

About two axis tracking, one of the big advantages of a trough is that, if it is long enough, the effect of the other axis can be ignored. You can see that the larger commercial systems in the Mojave Desert make use of this principle.  A dish is another matter and all of those require two axis tracking.

Good luck with your project and let me know how you are doing with it.

Best regards,
George Plhak

Array orientation: N-S or E-W?

In your book, you mention N-S orientation vs E-W orientation.  Is it correct that with an E-W orientation, the tracking is not as critical as N-S and could be adjusted manually each morning to track the seasonal changes in the sun's angle?

Best regards,

John

I am not sure that I would agree that the tracking is less critical. I think that the main issue with orientation is the efficiency over an entire day.

Near solar noon it does not make much difference which way the array is oriented. The sun is highest in the sky at this time and it's rays fall very near perpendicular to the array in either orientation.

When it makes a difference is at higher latitudes (like mine) or if you have a wide unobstructed sky view and you want to get more out of the extremes of the day (morning and afternoon). In these cases, N-S orientation is better, more efficient.

Because home made collectors are not infinitely long (like the ones in the Mojave) we should expect a certain amount of "spill" at the ends if the array is not north-south. If you imagine an E-W array in the morning, the sunlight hitting the reflector at the end nearest the sun will focus some distance down the collector, not at the very end. The part of the collector that does not receive focused light is "wasted" during that time. At the other end of the collector, because of the sun low in the sky, the focused light at that far end does not hit the collector and is also "wasted".

Whether the controller and drive motor works less or not is to me a less important issue since I have adjusted the controller to a very low duty rate so it does not work very hard anyway, even though my array is mostly N-S.

In either case, the controller must accurately position the tilt of the array towards the sun.

I hope this makes sense?

George