Thursday, September 06, 2012

diy solar reflector squeegee from a car wiper

I get questions about cleaning my parabolic solar concentrators.

Concentrating collectors might have a mystique about them that because they use mirrors, these mirrors have to be perfectly clean. This is not possible in the real world and not really necessary I have found. As with a flat plate type collector, the reduction of output due to atmospheric grime might be in the order of 10% if the equipment is really dirty.

Adding a transparent cover over a parabolic collector does not solve the problem since the grime accumulates on the cover instead. Adding a cover means that some energy is always lost by the cover, even if it is clean, since the cover will absorb some of the heat and re-radiate it so that heat does not make it to the collector.

Here in my northern climate (Toronto Canada area) rain water cleans my solar concentrator reasonably well. My array is at an angle to the ground and most of the grime just slides away with the rain runoff. I have not cleaned them myself until now.

To maintain peak efficiency ALL types of solar collectors benefit from manual cleaning, even if only once a year, a concentrator is no different. Some atmospheric deposited grime does not wash off with the rainwater and a mild mechanical cleaning is required. If you are in a dry climate, you might need to manually clean your solar collectors more often.

If the concentrator mirror is on the backside of the sheet as it is with a plastic mirror, a squeegee is perfect.

This is about my DIY tool for cleaning concentrating parabolic solar collectors similar to my DIY design very easily. Like using a regular straight squeegee on a flat plate collector.

Recently I noticed the shape formed by the windshield wiper on my car when I pulled it away from the glass. It was almost perfect - a backwards parabola of almost the right size. But the rubber blade was on the wrong side!

click any pic to enlarge

I disassembled a standard auto wiper to invert the blade and mounted it on a pole. As you will see in the video below, it is now almost a perfect fit with my parabolic collectors.

Above is a closeup to show the swivel pin which holds the wiper to the curved arm. I looked at four types of car wiper blades to check for curvature and length. A friend came up with this rather ingenious method of fastening the blade to the arm. thank you Peter! Simple yet very tough, it allows the blade just the right degree of support and a small amount of give. Details will be in the upcoming book.

If you want to make one of these cleaners, you will need to look at the type of wiper blades you have available.

I tried OEM blades (my drive is a Ford product) and universal replacement blades I bought from an auto supply. I found that by removing the end caps (twisting and pulling) each blade could be easily disassembled. With all but one of them, the blade could be reversed.

If you are in Canada, the one I finished with and show in these pictures is the "Horizon" by rain-x available in various lengths from Canadian Tire.

I ignored the packet of rain-x that they suggest I apply to the glass prior to wiping. I didn't think it was necessary. I used a 22 inch blade for my 24 inch collectors. I am surprised that this product is sold here in inch lengths with no metric marking at all on the package? Canada is metric I think

Getting the center pivot onto the other side proved the most challenging. Of the four types of wipers I looked at, they all had removable end caps but all differed in their details. All but one could be reversed. One had too much curvature. One was too short. It was difficult to remove the center pivot and involved gently bending metal fingers and breaking a spot weld but I did it.

This cleaner works very well. The reflector must be wet before using it. You can do it after a rainfall or use the morning dew. Otherwise you will need to mist with a water spray to the point that water droplets start running down the surface.

This is the parabolic squeegee in action. The pole it is attached to is the common expandable type sold in the home centers for applying paint. The arm is bent in an arc to miss the collector tube. Only light pressure is needed to keep it in contact with almost the entire reflector surface. You can make another two passes to catch the very edges of the reflector sheet if you want to do that.

I don't see many people on their roofs cleaning their collectors - maybe that's a business idea for someone?

DIY concentrating solar reflector squeegee from a car wiper from George Plhak on Vimeo.

Thank you for your interest
George Plhak

[to the gen2 intro and reading list]


Hasta Purnomo said...

hi mr george, i am doing solar concentrator project and looking many ways into produces suitable curve that meet my needs, cheaply and easily and then i am thinking about car windshield, and wonder if it was parabolic or not. I couldnt find literature anywhere but your post are nearest that i could find cause your "car wiper squeege" and if iam not mistaken i think it was parabolic too, what's your thought?

George Plhak said...

Hasta, You are asking if a car windshield is parabolic? Probably not, but your idea is interesting. So I understand, you would take the glass out of the car, metalize the inside and turn it toward the sun? Certainly the shape is rugged, ready made, weatherproof and identical if all from the same model. The problem would be to measure how they focus, onto what shape or volume. That's where you'd put the collector. And how to mount to allow solar following.
Good luck with exploring!