Wednesday, December 14, 2011

Reader questions and answers

Hello Carlos and thank you for your interest in my book. My answers are below:

On 12/14/2011 08:06 PM, carlos e wrote:
Hello Mr George,how you doing?
I see that you have a great job on this things,congratulations.
Thank you.
I brougth from you the plans to make my own unit...but I need to get some awnsers first.
Did you have some problem with acrilyc durabitity?
At first yes but it was a problem of my own causing. I let the concentrated light that was not properly focused at the collector tube hit the back of the sheet in front and the sheets were deformed by back heating. This damage occurred when the unit was OFF and not following the sun under control of the sensor. This was solved by painting the backs of the acrylic sheets white and when not using the array, parking it horizontally. The current acrylic is on the third year of use here. It needs a good cleaning in the spring, but it is fine. You can read more about that here:
Did you tested some others interesting materials?
I did. In the book on page 30-33 I talk about MIRO IV polished aluminum, Galvanized steel (no joke), aluminum roof flashing, reflective film and acrylic mirror. I did not try polished stainless steel since I could not find it locally but others like this material. I would like to review the situation again since many different kind of materials can be used.
I am thinking about making some examples and sell it to some people but you know,I am a serious person I dont want to sell a bunch of problems to my case that they need to change the reflector in a short time!
I suggest you make one for yourself first to get experience and to adjust for local materials and situation. You should do your own life testing. Maybe your acrylic is different from mine but we don't know that until you use it for a while.
How long is your device working?
I started six years ago. The current "production" array which heats the swimming pool is essentially the same for three years. I am now replacing some worn parts and poor manufacturing (my own fault).
You did not refer in your plans,the size of your pool,how much litters or gallons do you heat with your unit?
Can you hit wich temperature increase in one day?
The pool size is about 50,000 gal, I talk about that here:
what about trying 2 axis track in your seems to be easy to do,do you think it will get some important increase of power?
Please tell me how to do it easily. The seasonable adjustment seems like a small benefit. The big operators in the desert do not do this. But reader Andrew Gray wants to try to do this also:
And about the ribs,do you think for example 10mm acrilyc it will be good to use instead of wood?
probably it will last more and no need to repair
I think it will depend on your conditions. Here there is winter cold (not good for most plastics) and wind but we do not get hurricanes, nor really heavy hail like they do in Texas. The wood highway sign material (MDO) is turning out to be very durable here but the edges must be sealed. The baltic plywood is my favorite, to be cut with a water-jet. But please try the acrylic. Check that it is UV stable. Use a lightweight material, like the wood laminate or plastic or you will be re-balancing the reflectors.
thank you so much.

ps. about mylar film can you share your experience with me too?
I did not use it. I thought it would be difficult to apply smoothly in small quantities and I was concerned about the durability. Others have used it with success. The manufacturer of Reflectech film recommends a purpose built "application machine" but I can't afford one of those nor could most of us, I thought.
Sorry if I am making a lot of questions but you seems to be a helpfull person and I really need your help!
My inglish is not very good,sorry for that...
Your English is GREAT! Thank you for your questions.


Saturday, December 10, 2011

Summary/Conclusion - DIY Flat Plate vs. Concentrator Performance

Two types of solar thermal collectors are in common use. The parabolic trough concentrator (example mounted on the right side of the solar test jig in the picture), is used in large thermal plants in the desert but is not often home built like the "flat plate" type collector (example at the left) which has avid user/builder groups promoting and advancing it's design and use.

Both make great Do It Yourself (DIY) projects that can be adapted to local needs. Both work well. How does their performance compare? I decided to find out. I was surprised that my two fairly carefully researched and constructed models were fairly close in performance. But there were some big differences.

Both types of collectors can be built by anyone who is handy with common shop tools. Most parts and materials are from home building supply centers. I used easy-to-follow plans from the internet. I built the collectors myself in my garage near Toronto and then did a series of measurements (See INDEX link below) to assess the heat gathered by each under the same conditions. Both types worked well. Both were challenging DIY projects. A solar heat source, even if home built, is a significant investment of time and dollars. For longevity and best performance, a project of other than the smallest size should be carefully done and perhaps a few models built to check concepts and presumptions. These are two of my models.

I am not yet attempting to make electricity. For now, I want to efficiently capture the sun's free heat and channel it into a flowing liquid - water in this case.

So which performs better: the flat plate or the concentrator?

As a DIY project, the result may depend on your building skills and perseverance but possibly more specifically on whether you choose to rotate your flat plate toward the sun with a tracker. This is more complex but it may be worthwhile. This is the only case where my flat plate out-performed my concentrator, but it was close, within a degree. In the other tests, the concentrator did better.

To be sure, the concentrator can have a real advantage in situations where a higher temperature is required. In this respect, it is better to choose the system with the higher stagnation temperature. Here the parabola truly shines. Over 636°F / 336°C reached in with the concentrator vs 200°F / 93°C with the flat plate (no flow). But with the same flow rate, the amount of heat captured by each is comparable.

In terms of collecting the heat that falls on each under the same conditions my conclusions are:

Configuration and link to specific testAdvantage?
Uninsulated, stationary flat plateConcentrator
Uninsulated, both trackConcentrator
Insulated, stationary flat plateConcentrator
Insulated, both trackFlat Plate

In each case, the detailed test description and measurements are shown at the links.

Gary Reysa of in his similar comparative tests of alternate flat plate designs suggests a metric of "the higher temperature wins". The graphs in my tests clearly show which collector achieves the highest temperature in each of the tests. This is the basis for the table above. Other interesting stuff happens during the tests: one starts heating earlier in the day and one cools down faster. The graphs show these effects also and are discussed in some detail for each test.

As an alternative presentation, I calculated (integrated) the area between each temperature measurement and the ambient temperature and summed them (the number of degree-rise-minutes over the whole test). The collector with the larger number I called the winner at 100% and I divided the other number by it to calculate how it compared to the winner. See below for how that data looks. Where both collectors are insulated and both track the sun, over the day, the concentrator yields only 93.4% of the energy gained by the flat plate. Not many people track their solar thermal panels. Maybe they should? It does add complexity however.

Degree-rise-minutes/day over ambient normalized to 100%
Test methodConcentratorFlat Plate
Uninsulated, stationary flat plate100?*
Uninsulated, both track10078.9
Insulated, stationary flat plate10088.7
Insulated, both track93.4100

* I can't do the calculation for the uninsulated, flat plate stationary test since I did not record the ambient but the concentrator is certainly a 100 (the higher heat from the concentrator over the day can be seen from the graph).

There are other considerations than the raw performance which will influence your approach. These can be debated and some overcome, but not others (where I live):

Other comparative considerations
Material and labor cost,
Perceived difficulty to manufacture/maintain,
Features of the design: the concentrator does not require a rotating fluid joint and uses less material, specifically less metals,
Unreliable results in winter due to unclear sun, the reflector must be clear of snow and ice and the sun must be visible (no clouds), blockage of the mechanism with ice would be a problem, cleaning of the mirrors or the glazing of the flat plate, wind loading and etc.

These factors are not discussed here, only relative heating performance under a hopefully clear blue sky. Likely you are not in Canada and the cold weather concerns might not trouble you. You may have other challenges where you are?

Hopefully the results I have posted here will encourage the exchange of more concepts and experimental results. Your own situation, your solar resource, your skill level and the materials you have available will determine what is best for you. Hopefully I can show what was possible in my situation.

A major area for continued work for me will be improvement of the insulated concentrator collector (this was my first attempt) and the achievement of higher temperatures, including steam generation, something that is not possible with a flat plate collector.

Your comments and suggestions are welcome. Thank you for your interest.

George Plhak
george (at) ffwdm (dot) com

Thank you once again to Gary Reysa for the loan of his HOBO data logger and for his support and encouragement. See his writeup on these tests here.

If you would like to read the full report, you can begin here:
Index - Comparing concentrator to flat plate solar collector