Road Test - Do Mirrors Help Solar Panels?

Submitted: Saturday, Apr 17, 2010 at 20:39
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In a thread during the week about solar panels fitted with mirrors for sale on ebay I promised to do a test this weekend. Here are the results.

The Myth:

Adding a mirror to a solar system increases the power available.

My Setup:

A Solarex SX50U Panel (poly crystalline)
P.max 50W
V.oc 21.0
I.sc 3.23

2 x 35AH Gell Cell batteries wired in parallel at about 80% SOC.

The weather was warm and bright with passing high level cloud (shadows still slightly visible).

The panel was wired directly to batteries without a regulator. I put a clamp ammeter over the positive lead and a multimeter set up across the battery terminals.

The area between outer boundaries of the cells on the solar panel measured 400 square cm. The only mirror I could pinch (from my wife’s dressing table) was 221 square cm.

The panel was set up normal to the incoming sunlight and everything left out for an hour to heat up (panel disconnected from the batteries!). The mirror was adjusted to throw its full reflection onto the panel. The panel surface temp at the start of the testing was 38.2 C measured with an infrared thermometer:
Image Could Not Be Found

To make the transition from mirror OFF to mirror ON I used a tarp I could flick off the mirror quickly while photographing the meters:
Image Could Not Be Found

The Results.

Under light cloud:

No Mirror:
Image Could Not Be Found

With Mirror:
Image Could Not Be Found


Full Sun:

No Mirror:
Image Could Not Be Found

Mirror:
Image Could Not Be Found

So in summary the results were:

No mirror under light cloud: 40.0W (14.09V x 2.84A etc)
With mirror under full cloud:48.0W
Improvement 19.9%

No mirror in full sun: 52.97W
With mirror in full sun: 58.2W
Improvement 9.9%

I repeated this several times both under cloud and full sun with similar results.
While I have some doubts about the absolute power reading in full sun (52.97W from an unassisted 50W panel albeit in ideal conditions) the deltas are interesting. Given my mirror was only 55% of the panel size and hence only added illumination to just over half the cells the increase in power is significant – especially under light cloud.

So:

Myth Plausible.

As you would expect.

I assume a bigger mirror matched to the panel size would give a better result. That reflective mylar sheet could be a good starting point for a light weight reflector.

For me this is all academic as I already have a 160W folding monocrystaline panel that keeps my Waeco and other electrical stuff ticking over quite nicely with my 70AH of gell cells. And yes I do normally use a regulator.

Buit it is interesting.

OK, helmet on, sandbags up, flak vest on, prepared for incoming fire.

Cheers

Pete
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Reply By: Ianw - Saturday, Apr 17, 2010 at 20:52

Saturday, Apr 17, 2010 at 20:52
OK I believe you. Sounds about right . But what I want to know is -- What is the UNI T clamp meter like? I am thinking of getting one $65 on Ebay. How is the accuracy at low amp levels (on DC) The specs say + or - 3% I think; sound about right?

Thanks

Ian
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Follow Up By: Ianw - Saturday, Apr 17, 2010 at 20:56

Saturday, Apr 17, 2010 at 20:56
Sorry accuracy is stated at + or - 2% +3

Ian
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Follow Up By: Pete Jackman (SA) - Saturday, Apr 17, 2010 at 21:15

Saturday, Apr 17, 2010 at 21:15
The manual quotes: DC current over the 40A range resolution 0.01A, accuracy +/- 2%

It is a great unit. Mike Harding tested them and found them accurate his report is in thread 61382:

Clamp Meter Test

I bought one on his recommendation.

Cheers

Pete
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Follow Up By: Ianw - Saturday, Apr 17, 2010 at 21:45

Saturday, Apr 17, 2010 at 21:45
Is it a UT 203 or a UT 204? About $10 difference in price.
Ian
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Follow Up By: Pete Jackman (SA) - Saturday, Apr 17, 2010 at 22:08

Saturday, Apr 17, 2010 at 22:08
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Follow Up By: Ianw - Saturday, Apr 17, 2010 at 22:12

Saturday, Apr 17, 2010 at 22:12
Thanks heaps, Pete
I have just ordered one!

Ian

Sorry for the hijack
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Follow Up By: wazzaaaa - Sunday, Apr 18, 2010 at 08:34

Sunday, Apr 18, 2010 at 08:34
Thanks as well I also bought one for $27.38 it says it's UT 203 so I hope it is the same one.

link

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Follow Up By: wazzaaaa - Sunday, Apr 18, 2010 at 08:41

Sunday, Apr 18, 2010 at 08:41
Sorry should be $40.18 Delivered not $27.38

Wazzaaaa
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Follow Up By: Ianw - Sunday, Apr 18, 2010 at 11:47

Sunday, Apr 18, 2010 at 11:47
I decided to pay a little more and got the UT 204 model. $55 delivered. This is the same as the UT203 but is a True RMS version.

Ian
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Reply By: Mike DiD - Saturday, Apr 17, 2010 at 21:50

Saturday, Apr 17, 2010 at 21:50
What happens if you one quarter of your panel is in the shade - you sure get a lot less than 3/4 of the current.

The mirror you used is smaller than the panel, so the extra sunlight will have the same limited effect.
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Follow Up By: Pete Jackman (SA) - Saturday, Apr 17, 2010 at 22:07

Saturday, Apr 17, 2010 at 22:07
I agree Mike, that was why I was interested in the performance gain it did provide. And that gain was repeatable so it was genuine. Not having a bigger mirror was a handicap.

Adding mirrors must have a limit as the additional light would increase the panel temp which reduces efficiency.

Regards

Pete
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Reply By: Bob of KAOS - Saturday, Apr 17, 2010 at 22:00

Saturday, Apr 17, 2010 at 22:00
Pete

Thanks for an interesting thread.

I can't help feeling that the reflected light could be used more efficiently.

Method 1: use two mirrors. Align the panel to be perp to suns rays, align mirror 1 in same plane, align mirror 2 to capture reflected light and reflect it back onto panel. Will need to realign as sun moves.

Method 2: align two or more mirrors almost perp to suns rays, align panel to face away from sun but capture reflected rays from all of the panels (like a rough parabola with the panel at the focus). By having several mirrors in a roughly parabolic array you'd get good illumination of the panel as the sun transits across the sky- no need for moving parts.

As the panel is already working near maximum efficiency in full sun conditions, the reflective method would be cheaper in cloudy situations or higher latitudes than buying extra panels.
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Follow Up By: Bob of KAOS - Saturday, Apr 17, 2010 at 22:25

Saturday, Apr 17, 2010 at 22:25
Image Could Not Be Found
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Follow Up By: equinox - Saturday, Apr 17, 2010 at 23:05

Saturday, Apr 17, 2010 at 23:05
I'll grab the patent on that one if you don't mind :)

Looking for adventure.
In whatever comes our way.

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Reply By: BluePrint Industries Pty Ltd - Saturday, Apr 17, 2010 at 22:12

Saturday, Apr 17, 2010 at 22:12
Hi Pete,

Very good results. I think in effect what you created could be classified as a SOLA concentrator. Sort of similar to the SOLA ovens you can build. Instead of the breakable mirror you could use polished stainless steel or aluminimum.

The problem you would have if the Sola Panel was stationary then the mirrow could eventually cause shadow on the panel and be costing you sunlight instead of getting extra.

If you were adjusting the angle regularly it would be ok tho.

Something to consider tho if your trying to get a bit more from existing panels without a lot of expence.

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Reply By: Member - Rod N (QLD) - Sunday, Apr 18, 2010 at 06:22

Sunday, Apr 18, 2010 at 06:22
I note that the panel was on an angle. How would this be used 'on the road'? As mentioned would a tracking system to keep an optimum angle give better results? Obviously there are many variables to consider to achieve an economical optimum system.
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Reply By: Member - Carl- Sunday, Apr 18, 2010 at 07:39

Sunday, Apr 18, 2010 at 07:39
One point you did not mention was, are you the one with ginger red hair or are you bald and wear a black beret atop of that moustache?
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Follow Up By: Pete Jackman (SA) - Sunday, Apr 18, 2010 at 15:52

Sunday, Apr 18, 2010 at 15:52
I do have a moustache and a bald spot but no beret yet!
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Reply By: Richard W (NSW) - Sunday, Apr 18, 2010 at 08:15

Sunday, Apr 18, 2010 at 08:15
Pete,
Thanks for this info.
FWIW I bought a cheap folding Solar Panel of Ebay sold as a 60W panel but have only ever been able to get 40W out of it ie. approx 2ampsX20V unregulated. Initially I could't even get that from it in full sun.
When I phoned the supplier they told me that the watts they quoted were with a mirror mounted behind the solar panel. They said it made a difference as the photoelectric cells can produce power from both sides. Not sure if they were pulling my leg or not and I'm no techo on this stuff.
I tried sitting the solar panel on the silver surface of a windscreen sunshade so it relected the sun onto the back of the solar panel. This did seem to make a difference so I'm looking around for a robust mirror surface I can mount behind the solar panel. I guess this might cuse heat issues.
I have found this sized solar panel is only enough to trickle charge the auxilliary battery but will not run the fridge indefinitly however it does prolong the battery charge without running the vehicle motor and charging the auxilliary through the isolator. Not sure if my technical jargon is correct.
AnswerID: 413359

Reply By: Member - John and Val - Sunday, Apr 18, 2010 at 08:31

Sunday, Apr 18, 2010 at 08:31
Pete,

Very interesting results. You clearly got a significant increase. The panel consists of a number of cells in series, so since the mirror is smaller than the panel some cells were missing out on the reflected light. With a larger mirror I would expect even better results.

If that's the case, then a mirror needs to be larger than the panel so that it doesn't need frequent moving to follow the sun.

Heating will no doubt be an issue if the panel gets too much sunlight, but how about adding a waterjacket behind it to harvest domestic hot water while keeping the panel cooler?? Not much use on the road, but maybe a possibility at home.

Cheers

John
J and V
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Reply By: brushmarx - Sunday, Apr 18, 2010 at 09:10

Sunday, Apr 18, 2010 at 09:10
Interesting process and findings, but unless the chair supporting the solar panel is out of a doll house, wouldn't the solar panel be a bit larger than 400 square cm?
Eight inches is 20 centimetres, so a 8" x 8" panel be 400 square cm?

It probably doesn't matter because it's all relevant, except for the output in comparison to the size, and I've always been told that size doesn't matter.

And for what its worth, is there any improvement from the reflection from the blue tarp?
Obviously a Chinese or Ebay tarp would not reflect as well as an Australian made tarp, but the solar panel seems to have a reflected blue image.

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Follow Up By: Pete Jackman (SA) - Sunday, Apr 18, 2010 at 15:58

Sunday, Apr 18, 2010 at 15:58
You got me. I spent some time double checking all the electrical figures and stuffed up the silmpe area conversions

.400 square metres is not 400, but 4000 sq cm which makes the mirror 2250 sq cm. DOH!

The blue tarp made no improvement over the concrete pavers.
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Reply By: Member - Carl- Sunday, Apr 18, 2010 at 20:00

Sunday, Apr 18, 2010 at 20:00
Pete,

You spoke that you measured the ambient temperature for your experiment but did not mention the increase in panel temp when using the mirror. Do you happen to have this detail? Would be interested to know.

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Follow Up By: Pete Jackman (SA) - Monday, Apr 19, 2010 at 02:15

Monday, Apr 19, 2010 at 02:15
Sorry I forgot to measure the panel before I put the mirror on it. The temp above is with the mirror.

cheers

Pete
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Reply By: Steve63 - Monday, Apr 19, 2010 at 11:34

Monday, Apr 19, 2010 at 11:34
Solar concentrators have been around for fair while (thousands of years). It is no myth. It is still a reasearch field to a degree where combined with solar panels. You are increasing the energy delivered to the panel by reflecting more light on to it. The output power is dependent on the energy falling on to it and the incident angle. The angle issue can be solved in a few ways, either by tracking the sun (Have a look at the Cape Laveque solar farm if you are up in the Kimberly) or by decreasing the dependence on angle like a polycrystal panel. Other ingenious ideas have been to scribe tiny v shaped lines on the surface of the panel. This increases the efficiency of the panel as light is reflected from one side of the v to the other. There are limits to what you can do as increasing the temperature of the silicon causes other effects that limit output. Having said that there is a company in Melbourne looking at parabolic reflectors that concentrate the light onto a small panel. If I remember correctly they get kW's out of a 30 x 30 cm square panel.Steve
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