Improved sensor housing

For some time, I have been looking for a small clear acrylic dome to make a sensor housing that has a semi-true spherical viewing window.

I have not completed the installation of this new version into my system but I present it here in case you would like to build your sensor housing in this way.

Although the previous version, based on the Leviton housing is satisfactory, there are three advantages to this improved approach:

● The lettering does not have to be removed

● The spherical window does not have edges which can distort the view when the sun is low.

● The Leviton housing does not seal well without the addition of a gasket under the hinged plastic cover. This new housing is completely sealed.

Above is the prototype. The clear plastic acrylic dome is cemented into the lid of a Carlon round plastic electrical box and provides a very clear window over a full 180° viewing angle. This is important to allow the sensor to see the sun rising when it is fully heeled over 100°in the opposite direction.

The clear plastic dome I found at a craft supply chain here in the local area called Michaels http://www.michaels.com/ product number 82676963087 called “PLATC BALL CLEAR” (sic) on my sales receipt, cost $2.29. You might have to get a sales person to help you find it in a Michael’s store. They are usually huge.

They have several sizes in stock but the 80 mm seems to fit the Carlon electrical box best. If you search “Darice 1105-97” you will find the importer http://www.darice.com/ and several sources of supply in the USA.

The Darice globe is (I'm guessing) sold as a holiday ornament, but it makes a fine solar radome.

The package actually includes two halves, a male and a female. I decided to use the male (picture above).

I used a fly cutter in a drill press so that I could very precisely fit the diameter of the lip on the Darice dome.

First, I set up the circle cutter to the approximate diameter I needed and did a test hole in a piece of scrap. I tested the fit of the dome and adjusted the cutter slightly.

When the circle diameter was correct, the Darice dome fit into the hole beautifully, just up to the lip.

I then set up for the cut in the Carlon box cover.

Note that the cover is firmly fixed to a piece of scrap and the scrap is firmly fixed to the drill press table. I am very cautious with fly cutters and drilling a hole this size in a piece of PVC was a first for me. I wanted to make sure that it didn’t get loose.

With a very very light feed on the cut, it went very smoothly and the hole produced was very clean and a perfect fit for the Darice dome.

There is a little hanger loop on the Darice dome that you can just see in the back of the picture below. I cut that off and smoothed the cut with a sharp knife and a file.

I then glued the dome into the lid. I used a clear PVC cement so that it wouldn’t show. You could use the regular grey PVC cement. I used a Q Tip to carefully spread glue all around the lip of the globe and also ran some around the inside of the circular hole in the cover.

I left it to dry for a couple hours.

Not bad!

I will fit a plate into the box to hold the LED3X with a similar pivoting support as used with the Leviton cover and mount the cinch terminal strip in the bottom of the Carlon Box.

What about durabilty and maintenance?

Someone asked:

Maybe you could address how it stands up to rain, wind, dust, and even (ack) snow? Any problems with birds yet? What is your maintenance cycle like?

I've had this system here in southern Canada (43N latitude) for four years now, used for summer heating of a swimming pool. I had expected problems with the cleanliness of the reflectors but my experience has been very good so far. Addressing your points:

About wind: we haven't had extremes here (tornados or hurricanes) but some very strong winds indeed in the last four years and there has been no damage at all from that cause. The profile of the array is low. There are springs at the link arms to cushion the relentless buffeting from the wind. I am recommending that folks park the array horizontally during the OFF season (all winter long around here).

About dust: I had been concerned about dust from the dirt road out front but it has not been a problem since we get rain pretty regularly and that tends to clean the reflectors effectively. The acrylic mirrors are back silvered, so the reflective surface holds up pretty well. I have plans for a "cleaner" wand that will plug into the cross pipes for those times when the rain does not come.

About snow: I have been trying to dissuade folks that want to use this system in climates with snow since it just is not practical. Besides, in spite of the popular hype about solar, we don't have enough clear sun days here in the winter anyway. That's about 200 lbs of snow on each reflector in the picture (click to enlarge). It gets even heavier when the snow is wet. The reflectors suffer no damage at all from the winter.

About birds: No problems at all. I know what you mean, but I have been pleasantly surprised. The deck is another story, particularly when mulberries are in season. The birds around here LOVE mulberries and they leave droppings everywhere.

About maintenance cycle: none really, except for that time in the spring when the plants pollinate. The pollen is really STICKY. This is usually before pool season so it is just a temporary annoyance. Perhaps you noticed the holes in the hangers that let the water run out? That helps to keep the reflectors clean.

Use another material for the ribs?

Someone asked:

"Have you had anyone build this from machined aluminum other than wood? Obviously wood is the most economical solution for the average home builder but I have access to a machine shop ..."

I wrote:

Thanks for your interest.

The plans just became available two days ago so I don't think anyone has built anything yet except for me.

People have asked about aluminum and molded plastic ribs.

With metal ribs, I would be concerned about altering the balance, making the reflectors heavier on the bottom than with the wood. Because the reflectors are more or less balanced, they are easy to rotate and the force required to move them is very small. Aluminum is heavier than wood. I'd imagine that with the greater strength of aluminum, you could trim away most of the material where the big holes are and still have the strength while keeping the center of mass at the collector? I'd keep the 1/2" or so thickness since that spreads the contact area where the rib touches the reflector sheet and gives strength where the bolts attach. You could thread holes in the aluminum or plastic and do away with the cross dowels. What about galvanic corrosion between aluminum ribs and the copper collectors?

I'd love to have a source that I could send people to for the ribs. There is no doubt that these are the most time consuming part of the project. The rest is relatively straightforward.

For those of us without a machine shop, wood is the most economical material and with a bit of time on a band saw and a router, you can make quite serviceable ribs. Using MDO (weather sealed sign material) all but eliminates the finishing step for wood. Perhaps you could make one prototype reflector this way first to study the way it's put together and how it behaves and then work on improvements before you make the whole array?

Regards,
George Plhak

what size of solar pool heater do I need?

Someone asked:

Do you have any guidelines to figure how many collectors you may need for a given size pool?

Thank you for a very good question which I did not address in the plans.

There is no specific formula for sizing a solar heater to a pool that I know of.

I relied myself on the advice given by the installers of solar pool heaters which is somewhat vague. They say (and they are talking to pool owners in sunnier climes than mine) that you should have from 1/3 to 1/2 of your pool's area in the area of solar collecting surface. Perhaps for my Canadian location I should have planned for a solar area EQUAL to that of my pool!

The propane pool heater that I retired was rated at 150,000 BTU/hr. I have measured the output of my array as almost 50,000 BTU/hr. I could assume that the 150,000 BTU heater was the recommendation of installers in this area. In that sense, I don't have enough.

I have a 50,000 gallon pool and built a solar array with a measured output of 50,000 BTU/hr. One BTU is the heat required to raise one pound of water one degree F, One gallon weighs about eight pounds, so on a great eight hour day, I would expect my pool water temperature to rise by about one degree from the solar heater. It actually goes up about three-four degrees but that is because the pool itself gathers heat from the sun. I use a pool cover and hopefully don't loose all of the heat buildup overnight.

Other considerations:

How much heater area can you afford to build or have time to build? How much area do you have available for the heater? How hot do you want the pool to get? How much sun is available? (last year here was particularly disappointing in that regard)

I have a good amount of space behind the pool to expand. I built one bank of 13 collectors with one motor drive. If I was to add more, I would build another array parallel to the first with another motor drive since I think that the current size works well but is probably at the limit for one motor drive.

Getting the pool too warm leads to increased problems with algae and increased chemical usage. Personally I don't like the water warmer than about 82F and I like the water clean with no chemicals unless there is a problem. The copper collector tubes seem to help control the algae.

A hot tub would be another matter.

I hope this helps rather than confuses the issue.

Plan availability?

Allen J*** wrote:

"How are those plans coming along? Don't mean to be a bother just anxious to see them.
Thanks
Allen"

Hello Allen,

No bother at all. Your interest is most welcome.

The plans are finished: 90 pages in color, 205 illustrations and pictures, the two large patterns and the hyper-linked document. In February I made another motor drive from the plans just to check that my dimensions were all correct. I've sent out copies to 12 reviewers a week ago and I am waiting for some reactions/suggestions/comments to come back. I am working on the web page to set up a description, sample pages and the ordering info. I've registered a copyright and got an ISBN and a bar code for the back cover.

I hope to have everything finished this weekend.

The snow has melted here. I wandered out back to check the system and everything seems just like it was last fall. The acrylic mirrors are perfectly smooth and bright. The collectors get very hot (way too hot to touch) as the sun passes overheat but the swimming pool is a solid block of ice and the underground pipes are still frozen so I can't circulate the water.

George


I've assembled some of the comments from the readers of this blog and the project page:
www.ffwdm.com/solar/comments.htm

The plan manual is finished

The plan manual is FINISHED and is out with reviewers. It consists of an 90 page detailed writeup with color photos and illustrations, diagrams and schematics and two large full-size templates. The graphics are hyper-linked to the actual full size drawings and photos (205 in total) that are included.

I am making arrangements for a print version and a downloadable version and expect to have everything ready by mid-late next week. If you have sent me an email previously, I will be letting you know with a personal email when you will be able to obtain your copy.

Thank you for your interest!

If you haven't already seen them, I have posted a number of time-lapse videos of the system in operation at:
http://www.youtube.com/user/georgeplhak