Performance questions and answers

Ahmad from Kuwait wrote:

Dear George,
I really consider myself lucky to find your book on the internet while looking for information about more secrets of the design of solar heating systems.
I already constructed a system of my own design few months ago, which is still in operation for the heating of my indoor swimming pool (18000 Galons), but I was hoping to find information about sun tracking systems, when I came across your wonderful book, which I purchased a soft copy of.
I have questions in order to clarify some issues, as follows:
1- What is the atmospheric temperature that you have in the project site?
2- What is the initial temperature of your swimming pool water?
3- How long it took to heat up the water in the pool to the temperature of 28 degrees celsius?

Hello Ahmad,

Thank you for your kind comments and for purchasing my book (info about book)

About your questions, it sounds like you are trying to understand performance. I think my conditions here near Toronto, Canada are not like yours in Kuwait! My pool is outside, it is 50000 gal and the weather here must be the worst to attempt solar heating.

Our average summer temperature is about 28C although we have highs of 35C. The temperature can drop significantly at night, to 15C so this tends to cool the pool. Only twice this summer I had a pool temperature at the end of the day of 28C. Many days in the summer here are overcast with cloud cover and near useless for solar heating although the air temperature might be warm. We get rain periodically here in the summer which tends to cool things down. You get the idea.

I suggest that the best way to predict what might be possible in your situation is the actual heating measurements I made.  These are detailed in the section Performance Measurement on page 88. (this blog posting was the basis for that chapter)

In particular, the heating value of 3705 BTU/hr (1086 kWhr) for one collector might be the most valuable for predicting what you might expect there since it normalizes all the differences between our pools and climates and is the actual heat output of one collector at an instant in time when the sun is shining in a clear sky.

Thanks for your interest,
George Plhak 

Make larger collectors?

Rob from Australia wrote:

I’ve purchased your excellent plans.   

I have a question as to how I might go about increasing the solar energy yield per collector by increasing the mirror width – say by 1/6^th to 1/4 again) I’d thus need to draw a new parabola – hints on that would be great (I’m not an engineer).

This will probably also involve increasing focal distance to the tube. Any quick pointers on how to do this within your design and engineering scope would be very much appreciated!

 Hello Rob and thank you.

Of course, you could make these any size you wish. You don't need to be an engineer. The formula for a parabola is on page 9 of the plan book and it includes the focal length.

Some of the practical considerations of making a larger parabola are:

1) The standard sizes of building materials, most notably the reflector sheets. Here in North America, materials are sold in 4 foot by 8 foot sheets. This led me to one half of a standard sheet cut lengthwise into 2x8. I suppose that it would be tempting to make a collector using a full sheet?

2) Larger reflectors catch a lot more sunlight and could be dangerous. Even with the 2x8 foot reflectors that I use, if the water is not moving, it will boil in about 20 minutes. Focussed light from larger reflectors hitting parts of the structure might cause damage. That is why I paint the ribs and hangers white. I have unintentionally melted plastic electrical fittings at the sensor box because of focused light from my 2x8 foot reflectors.

3) Larger reflectors would be more difficult to make accurately and the focus would be more critical.

4) Larger reflectors would tend to act as a larger "sail" for the wind. There are days when wind speeds here can reach 80-100 knots so I am careful to park the array horizontally if I remember to do this. If I don't, the current size of reflector has survived several of these windstorms. I am not sure about larger reflectors.

5) Larger reflectors would hold a lot more snow and snow is heavy. This is not a problem for you in Australia but it certainly is a problem here in Canada. When parked horizontally for the winter, I have estimated that each reflector potentially holds about 300 pounds of soggy wet snow. My current array has survived three winters without distortion or damage. I am not sure that larger reflectors would be as strong.

6) It is important to the operation of the array that the reflectors be more or less balanced around the collector tube. This is why only a very small motor is needed to move the entire array. I made several prototypes to ensure that I had correct balance and if I was to make a larger version, I would certainly do the same.

7) The current size is easily handled by one person. At 2x8 foot, one reflector weighs about 30 pounds. Although I occasionally had some help, I pretty much assembled the array by myself. This would not be possible with larger reflectors.

8) With the current size, the spacing between the ribs is 16 inches and I have not had to use any cross bracing. With a larger reflector I would think that the ribs might need to be cross braced in some way because of their greater length. This could be confirmed once a prototype was built.

For the above reasons, if I needed more power, I would simply build more of the 2x8 foot design and run them in series. With regulation of the flow rate and good strong sunlight, it would be possible to achieve any temperature rise that is desired up to and including steam.

Best regards,

George Plhak

sources for gear motors

I've had an interesting ongoing email conversation with a person in Mexico who bought my plans and was very interested in building a solar parabolic heating system for himself. He had been having trouble getting the specific gear motor that I recommend in the plans.

In any DIY (Do It Yourself) project sometimes it is necessary to improvise.

It occurred to me to suggest to him that he look for a discarded cordless screwdriver or drill. Often people throw these away when the batteries no longer hold a charge. Both of these products contain a planetary drive gear reducer and a DC motor, just what we need to build the drive mechanism for a tracking solar parabolic collector. In both of the tools I look at here, the gear drive has metal gears on the output side so they should be able to handle lots of torque.

Here (click to enlarge) is a 1970s vintage Skil Supertwist cordless screwdriver. This is a great screwdriver, still working today after replacement of the NiCd batteries. You can see at the left side the 3 volt reversable DC motor and the metal gear box case. The driver end is a standard 1/4" hex socket which should be easy to adapt to the rest of the mechanism. In the case of my plans, this would drive the end of the drive screw in the linear to angular motor drive.

It would also be necessary to compensate for the lower operating voltage of this motor. That could be as simple as a resistor in series with the motor.

Here is the inside of an old Black and Decker cordless drill which you can't get the batteries for anymore. Someone threw it away. This has a very nice 7 volt DC motor and a very substantial gearbox reducer. As an added bonus, there is a variable speed circuit in the trigger grip which could be used to set any desired speed. Another bonus is the very nice tool-less Jacobs chuck which could simply be tightened onto the end of the drive screw. I couldn't figure out how to remove the torque limiter but it wouldn't be necessary to remove it. It could simply be left in the "drilling" position where the torque limiter doesn't act.

The whole reason for the gear motor is of course, the very slow speed of the sun across the sky. If the solar tracker drive operates too fast, the array will overshoot the position of the sun, go back, overshoot again, go back etc, etc. So the drive would be unstable and it would in effect, oscillate which is not desirable since it causes unnecessary wear to the whole mechanism and uses more power than it needs to.

So if you can't find a suitable gear motor, look around at your old cordless tools, or visit the local re-cycling center.

My friend in Mexico liked my suggestion and is now looking for a suitable tool to re-cycle for his solar project.

Progress Report

It is the end of another summer of free solar heat for our pool.

The DIY Parabolic Solar Heater has worked well again this year. A plan book showing how to build the project yourself has been published and has been selling well - thank you for your support! This project is entirely self funded and your purchase of the plan book helps to support my work.

I can report that the problem of back heating has been completely solved. Last year I had reported that under certain conditions, notably when the array was not powered and was tilted off vertical, at some point in the day, the semi-focussed rays of the sun would land on the backs of the reflectors. This back heating caused them to permanently distort.

The fix turned out to be to paint the backs of the reflector sheets with bright white exterior grade latex paint. Three coats of latex were applied to new reflector sheets last fall and all the sheets in the array were replaced with these back painted reflectors. Almost one year later, there is absolutely no distortion of the reflectors - none whatsoever. The white paint reflects and dissipates the heat. Normally the acrylic mirrors used for the reflectors are received from the manufacturer with a gray coating on the back of the mirror.

Another small improvement was to make the cross pipe safety straps adjustable. This was done so that they could be adjusted tight after any maintenance that involved removing the cross pipes. Adjustable stainless steel radiator hose clamps were cut and flattened and joined to the strap material with nylon bolts.

An adjustable safety strap is shown installed on a cross pipe. All the safety straps in the array have been replaced with these improved versions. There have been no blowouts this summer from boiling during stagnation conditions (when the main water pump was not running and the sun was focussed on the collector tubes).

I am working on several other improvements and two other applications for the solar parabolic array. More about these shortly.

Thanks once again for your interest and support. 

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