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: http://georgesworkshop.blogspot.com/2009/07/mirror-distortion-is-due-to-back.html
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 friends..in 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?
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.
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:
In each case, the detailed test description and measurements are shown at the links.
Gary Reysa of www.builditsolar.com 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%
* 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.
My interest is in using the glass evacuated tubes not as a component of a commercial collector, but as an insulated collector for a DIY parabolic trough concentrating collector. With the addition of a rotatable tracking parabolic trough behind a single evacuated tube, it is possible to increase the heat capture significantly, up to 8-15x safely. In this manner, it would be possible to do things that are not possible with a flat plate or an evacuated tube system - making steam for example. In a recent demonstration I showed that temperatures of over 630°F are possible on an experimental DIY basis.
Please see Wikipedia for a backgrounder on evacuated tube solar collectors or Google "evacuated tube solar" or "solar vacuum tubes". Much has been written about this method of solar heating that seems to be popular elsewhere than North America.
(click any picture to enlarge)
Searching the web reveals numerous manufacturers and truly, there are many. Evacuated tube collectors are widely produced in large volumes, mostly in China and India. Here are some that I found which give some specific information on tube specs and various details about the tubes.
These are not my recommendations, just suggested links for further information:
First, a video from Apricus showing manufacture of evacuated tubes in their Chinese plant:
Links to other manufacturers websites:
Haining Jixang Solar Energy Co. Ltd. Haining, Zhejiang. Schroll down the page for the specs. Note the diameters and lengths OD 47mm and 58mm, lengths of 1500 and 1800mm - you will be seeing these again. Select "heat pipe" on the left menu and you will see the specs for the heat pipe. Note Transfer power:≥150W. Check out the wide variety of their systems.
Haining Qiruite Photoelectric Co. Ltd. (Qirui) Haining, Zhejiang. A bit hard to understand in the English, but the same sizes can be seen about half way down the page. Do they make a 70mm dia? The length looks like a mistake.
Jiaxing Jinyi Solar Energy Technology Co., Ltd. (Jinyi) Jiaxing City, Zhejiang. Produce standard (JVN series) evacuated tubes with IDs of 37mm, 47mm, 58mm, lengths 500mm 800mm 1500mm 1800mm 1900mm 2000mm 2100mm. Have advanced (JVT series) three coating tubes.
Zhejiang Qianjiangcho Luminous Energy Co. Haining, Zhejiang. Seem to be redoing their web page, seems less complete than when I last visited and got specific sizes and lengths. Have three different types of coating systems available in their "vacuum tubes". ID's 37mm, 47mm, 58mm all +/- 0.7mm, corresponding ODs 47mm, 58mm, 70mm, standard lengths 1200, 1500, 1700-1800 and 2000mm all +/-5mm.
Patrick Ward of Fossil Freedom, Denver, CO? sells recycled US Government surplus evacuated tubes 45.75"/ 1160mm long.
If you like the idea of an evacuated tube which is open at both ends with expansion bellows fitted: Dezhou Mingnuo New Energy Co., Ltd., Dezhou City, China offers a 4000mm receiver tube of this type.UPDATE: The link no longer works. I have written to the company to get info."Sun Island" Haining Chaoda Solar Collector Tubes, Ltd is another manufacturer. These do not have bellows, or an absorptive coating. Open both end tubes are rare and you would have to import a case lot. The open at one end tubes are much more commonly available, at least in North America. I searched "open both ends solar evacuated tube" to find these.
The (open one end) tubes that I have measure ID 43.5mm OD 58mm and 71 inches (1803mm) including the pinch off but not including the heat pipe. This seems to be a size common to at least three of the manufacturers listed above, so we could assume this size might be also available from others? I cannot find a specific international specification for the tubes alone. If anyone knows this spec, please let me know.
For me, the length of 1800mm (5.9') is a convenient size for my standard parabolic reflector with a 4 foot length. About a foot of evacuated tube sticks out both ends of the concentrator which helps to suspend it in place at the focus. Fluid and electrical connections are made to the internal absorber at the open end. The diameter is critical for the max size of absorber I can insert into the evacuated tube. 43-47mm seems to be a fairly standard size.
I can only give pictures of the construction of the evacuated tubes that I have at hand but others I have seen, although different in some of the details, are similar. Here I am removing the "guts" of the tube by pulling on the bulb of the heat pipe. With this particular tube, the innards pulled out readily. With others, the heat pipe (the copper tube assembly in the center) comes out of the aluminum absorber leaving it behind in the evacuated tube. With those, I had to pick away at the fiber glass "bung" until I could grab the absorber with pliers and pull it out.
Here is an end view of the aluminum absorber. The heat pipe is held in the center of the evacuated tube, supported down it's entire length by this formed sheet aluminum part with also improves heat coupling from the inside wall of the evacuated tube to the heat pipe.
In this view, I have shown the bulb end of the heat pipe. The heat pipe dia is 0.317" / 8.04mm. The bulb dia is 0.943" / 23.97mm and has a length of approximately 3.905" / 100mm. The fiberglass bung can also be seen.
Here is the other end of the heat pipe, the aluminum absorber support and the evacuated tube. The evacuated tube has a shiny metallic "getter" on the inside which will be familiar to readers old enough to have worked with electronic vacuum tubes. The getter is a metallic coating which removes impurities in the vacuum after the tube has been sealed. It is also a diagnostic for the quality of the vacuum since it will turn to a powdery white appearance as it is depleted. If the vacuum is good the getter will be bright and shiny like this one.
The bung which seals the top of the evacuated tube is a powdery brittle compressed bit of what looks to be fiberglass? Hopefully it is not asbestos? It basically flakes apart when you remove it so is not reusable. Winding fiberglass pipe wrap into a cylinder was found to be a useful replacement for the bung and also able to accommodate an inlet and an outlet tube as was done for the test of an insulated concentrator.
So where do you get yourself some evacuated tubes for your own experiments? I don't think that you will yet find these at your local home center unless you are in the southwest USA or Mexico.
You could begin by writing to the companies listed above and any others you find and asking them about dealer/installers in your area. Look at your local sources for solar domestic water heating. You might be interested in a complete system, but for the purposes of a DIY project, we only really need a source for a few tubes, initially at least.
The tubes are relatively inexpensive. A solar dealer installer will typically offer this information readily if you ask what happens if a tube breaks (what if my kid throws a baseball through one? or a hail storm breaks some?), replacements will be required. The installer should keep a supply of spare evacuated tubes for this possibility and he/she will probably tell you something like "don't worry the tubes are only $X dollars each". I have heard $10-25. Find out what sizes the dealer carries in stock and have a look if you can at the heat pipe construction and coupling to the glass tube. Find out if they use a non-freezing fluid or water with some chemicals. Use the information you get from the dealer or several to determine the price you are willing to pay in your area and the most convenient supplier and then ask to buy a few tubes. You will get raised eyebrows perhaps, but you should get your tubes at a reasonable price. We don't really need the heat pipe unless you want to use it that way.
The tubes are certainly breakable and long and thin so getting them locally is a plus unless you want to buy a box lot as I did and deal with shipping costs.
CAUTION: Evacuated tubes are certainly breakable and it happens all of a sudden. They are pretty robust but like all glass, are delicate to certain types of shock. Being under a vacuum, they IMPLODE rather than EXPLODE but the effect is certainly sudden, makes a mess and is quite probably hazardous with all the VERY SHARP large and small shards of glass which result. After I broke this one, I started to wear safety glasses when handling evacuated tubes. Be careful!
