Saturday, September 11, 2010

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 purchasing)

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 

Wednesday, September 08, 2010

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 1/6th 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

Monday, September 06, 2010

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.

Sunday, September 05, 2010

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.