Thursday, September 29, 2011
Flat plate thermal collectors are not normally rotated to face normal to the sun but it is commonly accepted that if tracking can be used, it will increase the effectiveness of the solar heating or PV. Since the mechanism was available (and necessary) for the concentrator, I decided to try rotating the flat plate collector.
At the beginning of this test I saw that the concentrator was partially shading the flat plate collector (see the first picture above). In spite of this partial shading, the flat plate started heating quickly.
As can be seen in the first picture above (best if you click it to see an enlarged view), the surface of the mirror was covered with water drops from the rain we had last night. This would tend to scatter the sun's light at the beginning of the test. I did not clean the mirror.
As is normal at this time of year here, the day was mostly sunny with clouds. There was almost no wind. By mid-afternoon however, the cloud cover was solid and at the end of the test, it had started to rain.
You will see that I added a channel for the ambient air temperature. That sensor is out of the sun under the deck, about four feet off the ground, not touching anything but air.
I will be busy for a few days switching over to the insulated configurations for both the flat plate and the concentrator but I hope to have results from the next set of tests sometime next week.
Perhaps I should change the working fluid to 50/50 water/glycol in anticipation of winter? Brrr.
Tuesday, September 27, 2011
You can click on any picture here to see a full size enlargement.
I built the two collectors and mounted them in a test jig (described here) which can be oriented and tilted to face the sun. The collectors are connected to identical small insulated reservoirs which contain the same volume of water. During the test, the water is pumped from the reservoir through the collector and back to the reservoir. As the test progresses, the water in the reservoir heats up. My test is modeled after the technique described by Gary Reysa at BuildItSolar. As Gary says "The collector with the better performance heats the water in its reservoir to a higher temperature, and the difference in final temperatures in the two reservoirs is an indicator of how much better one collector performed than the other."
How to build a Tracking Parabolic Solar Collector. The sun's energy reflects from a mirror bent in the shape of a parabola and is concentrated onto a single copper collector pipe which is positioned at the focus. The water flowing in the pipe is heated by conduction with the copper.
For this test, I used a half length four foot long reflector. It is in all ways the same as the full size version except for the length. The shorter version was chosen to make the test jig more manageable. The test jig incorporates the motor drive and solar sensor as described in the book. The motor drive can control the positioning of both of the collectors in the jig although in this test, I fixed the flat plate collector stationary to match the way it is traditionally used, in a fixed position.
I purchased the preformed aluminum plates from Tom Sullivan of U.P. Solar Solutions. The base and the sides of the box are Baltic plywood. The copper pipe was joined with conventional solder fittings. The aluminum absorber plates got a bead of silicon caulking prior to being crimped onto the copper pipe using Tom's excellent modified vise grip pliers and then stapled in place onto the base. The entire surface of the aluminum then received three coats of flat black rust paint (chromate based).
I incorporated insulation (1" polystyrene) on the back and sides of the flat plate but did not include a cover for this test. A cover will be included in a later test of "insulated collectors".
One of my goals was to make the solar size (the aperture or the effective area that "sees" the sun) of the two collectors the same so that the results could be compared directly. I didn't get that quite right:
|Parabolic concentrator||19.25 x 48.0||924||0.596|
|Flat plate||20.625 x 46.0||948||0.612|
So my flat plate collector has an actual effective area about 2.6% greater than my concentrator. For future tests, I may mask 24 sq. inches of the flat plate to make the two truly equivalent. For this test, I have ignored the difference. Later I also did not make corrections, the test results are as recorded.
The solid line is the parabolic concentrating collector, the dashed line is the flat plate. The temperatures measured are the temperature of the water exiting each reservoir at the inlet to each collector. Throughout much of the chart, the concentrating collector temperature exceeds the flat plate by as much as 11 degrees.
This was my first test in a series that I plan to do. The weather looks overcast for most of the rest of the week. At the first opportunity, I will repeat this test, but with the flat plate ganged to the concentrator, so that they will both rotate to face the sun.
Subsequent tests are planned with insulation added to both collectors.
Thank you for your interest.
Friday, September 23, 2011
my plans to Illustrator outlines that import directly into his sister's computer controlled router. Here is how the parts nested on the CAM router. He says that each of these hangers took about 4 minutes to complete.
Thanks for your email! I'm looking forward to another progress update.
I wanted to point out that I have .dxf files of the ribs and hangers that I will send to anyone who buys the plans and requests these.
Friday, September 16, 2011
This Dayton 2L008 from Grainger priced at US$58.30 has worked well here for five years in my own solar collector. Probably it has operated for several hundred hours in all sorts of temperature and humidity conditions.
It is not essential to use this particular gearmotor. There are all sorts of gearmotors. I wrote about alternative sources here.
Granger has branches and representatives worldwide. Here in Canada, Grainger is represented by Acklands-Grainger. I called them yesterday to check price and availability and was told that I could have one tomorrow but the price was C$97.92! Wow! I thought the C$ and the US$ are about the same? I ordered one and I will argue with them about their rip-off Canadian pricing tomorrow when I pick it up. I need to have a spare new one. Then I turned back to looking at the one that had failed.
Given the relatively high cost of the Dayton 2L008 gearmotor, it is comforting to know that it is sturdily built and gives good service in the DIY Tracking Parabolic Trough Solar Heater. Even better to know that it can be repaired if necessary.
An update: Acklands-Grainger gave me a "break" on the price of the Dayton 2L008 I bought today - C$82.04. Still pretty high I think. The best thing, if you are buying one of these in Canada, or anywhere non-US, is to take in a copy of the Grainger webpage for 2L008 showing the US price or make it clear on the phone that you know the US price. Put them on the spot.
Another update 2L008 is part of a series of 12 volt gearmotors from Dayton with varying gear ratios and RPMs. In case the information in the post is useful to others using one of the the series, I have listed them here for the search robots: 2L003, 2L004, 2L005, 2L006, 2L007, 2L009, 2L010, 2L011. Page 110 of Grainger Catalog 402 about this series is here.
Tuesday, September 13, 2011
I wanted to create a sun screen to partially shade the deck. The deck is 20 feet wide so the span would have required a fairly massive structure if made from wood. I looked at metal trusses of the kind used on portable stages and exhibits but was discouraged by the high cost.
Since the advent of cable and satellite TV, many homes have antenna towers that are no longer being used. Former antenna installers now have another business of tower removal. Many of these surplus towers end up as scrap which is a shame since they are potentially very useful strong structural trusses as used here as a support for shade cloth over our deck.
I got my tower sections from the yellow pages by calling a local antenna installer. They were cheap, about $100 for the four ten foot sections. I specified that they all had to be the same or at least identical pairs (so that they could be fastened together and for appearance purposes). I didn't want the top section, the one that narrows down to hold the antenna mast. I bolted the sections together into two 20 foot spans, sanded and painted them with primer and two finish coats of white rust paint.
For the awning, I first tried brightly colored nylon cloth. Although light in weight and pleasantly colorful, the cloth panels acted like a sail in strong wind and the buffeting noise and the thrashing they took was alarming. I took the nylon panels down.
One day while visiting a garden center, I noticed the shade cloth that was suspended above the plants. This seemed like an ideal material and the open weave I thought would not catch the wind so well.
After looking around locally for sources of supply without success, I ordered shade cloth from greenhousemegastore. I got the 60% black. My order arrived quickly and was well made. Cost was about $100.
We are very pleased with the result. The old televison towers are incredibly strong. I can do a chin up in the center of the span and they don't sag. The shade cloth provides welcome relief in the hot sun and isn't bothered by the wind. I take the shade cloth down in the fall and put it back up in the spring.
So I have saved four old tv tower sections from the landfill and for very little effort and a cost of about $275 I given them something useful to do.
new use for old television antenna towers 2
Monday, September 12, 2011
Small springs are very useful parts. I've always had trouble sourcing them. It seems that unless I am buying hundreds of the same type, there aren't many places that will sell one or two small springs in an assortment of types and sizes.
Recently, before taking a computer printer to the local recycling depot, I spent a pleasant hour or so taking it apart and salvaging the springs it contained. I was able to retreive several dozen in a delightful assortment of sizes and types.
This one was a Canon printer, I forget which model, but all printers contain an interesting and potentially useful variety of springs, both compression and expansion types. They are not at all difficult to remove, particularly if you are about to scrap the printer, you don't care about damaging it.
I keep my small spring stock in four small bins divided as compression or expansion and small and large making it a bit easier for me to find just the right spring for the next project.
Plus I am helping the recycle effort in a very small way by separating the printer into plastic, metal and electronic parts before I dispose of it.
Saturday, September 10, 2011
Each line corresponds to one of the sensors. While it appears that there are only three lines, two of the sensors were virtually the same. The bottom line is sensors 2 and 3 while the top line is sensor 1 and the second line down is sensor 4.
From this first effort, I could see that the temperature resolution (the smallest measurement increment) of HOBO was 0.05 degree F. Those are the steps in the graph. It was early in the morning and the room air was slowly warming. The sensors weren't all giving the same reading even though I had them taped together and had allowed them to stabilize without touching them for about a half hour. I had expected there to be some +/- tolerance between the sensors and between the four inputs. Also, temperature measurments in air are notoriously difficult to do accurately because air is not a great heat conductor and stratification and movement of the air could also cause differences between the sensors even though they were taped together.
I took off the masking tape and bound the sensors together with a tie wrap thinking that this would provide tighter thermal coupling while tolerating the boiling and freezing water better than the masking tape.
I then prepared an ice water bath on the kitchen counter and a pot of boiling water on the stove. Our ice crusher normally used for making margaritas came in handy.
I set the HOBO to recording, disconnected it from the computer and carried it up to the kitchen. I first dipped it in the ice water thinking that this was closer to room temperature. If I did the hot test first, the ice would have to cool the sensors from boiling.
One of the nice things about HOBOware is that the graphs are interactive. You can click and zoom in on a particular area of interest or you can select one of the sensors to highlight it to make it stand out from the others. You can even click a particular data value in the table in the top part of the screen and it's location in the graph is highlighted.
Here is the area of the graph in the cold bath where the temperature stabilizes. Sensor 1 and 4 are the two at the top of the graph, 2 and 3 cluster at the bottom. I am not using distilled water so I am not sure what the actual melting temperature of ice made from our mineral rich tap water is supposed to be (probably a bit higher than 32F) but the important thing for me might be that the four sensors are within 0.15 F degree of one another which is remarkably good I think.
So I am now somewhat familiar with HOBO and HOBOware and I am looking forward to applying it to the measurments that I will make. As I will be primarily concerned about temperature differences, the absolute accuracy of any one measurement will not be as important as the differences between pairs of measurements.
I may repeat the hot water - cold water bath tests in future but with distilled water. But first I would have to make some distilled water ice cubes.
Many thanks, Gary!
PS - The title of this post is a play on a recent Canadian low budget thriller Hobo with a shotgun about a homeless vigilante who blows away crooked cops, pedophile Santas, and other scumbags with his trusty pump-action shotgun. The HOBO about which I am writing above is a much gentler sort of beast. Sorry - I couldn't resist!