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

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