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Increasing Output of Tracking Solar Panels

03/06/2019 12:48 PM

My panels are on a tracker. The backsides of the panels are white. If I were to paint the backsides black to dissipate heat, would that increase the output of the panels? How would one calculate the increase? I'd like to try it this summer and see how the result matches the calculation...if there is any significant increase.

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#1

Re: increasing output of tracking solar panels

03/06/2019 12:59 PM

Are these panels pv or thermal?

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#2

Re: increasing output of tracking solar panels

03/06/2019 1:32 PM

I would say no. Even if it did result in a small increase in heat dissipation it would be negligible compared to the heat input of the primary source (the sun) against all the surfaces of the panel (including any reflected sun coming off the roof and hitting the back of the panel).

This is my discussion with a colleague all over again, who was adamant that the reason people in movies push the hand gun forward as they fired was to increase the speed of the bullet exiting the barrel.

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#3

Re: increasing output of tracking solar panels

03/06/2019 2:14 PM

Theoretically, there should be some, but I'm guessing it's not significant. Solar panel efficiency decreases by 0.258% per degree C, and the increased radiation from a black surface should mean the temperature would be lower. The amount of solar energy at the earth's surface is a maximum of about 1000 Watts per square meter. If your solar panel is 20% efficient, 800 Watts per square meter is converted to heat.

Black body radiation from a heated object is proportional to the fourth power of temperature, per the Stefan-Boltzmann law.

In general, if your solar panel is temperature T and the shaded area is temperature Tc, the amount of power removed by radiation would be:

where e is the emissivity, which would be increased by black paint and P is the power absorbed from the sunlight (~800 W/m2). So if you solve for T for values of e, you can then compute loss of efficiency (0.258%/deg K).

Heat transfer can be by radiation, conduction, and convection. The above calculation is for radiation only. Conduction probably isn't significant, but convection may be as efficient or more efficient than radiation, depending on openings for air flow and prevailing wind.

Convection, of course, would not be influenced by painting the back side black, so your actual gain in efficiency would be less than the calculation for radiation alone.

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#4
In reply to #3

Re: increasing output of tracking solar panels

03/06/2019 3:54 PM

Very nice, Rixter. Panel temperature coefficients vary a bit between panels; my panels are -0.41% per °C. So if I could reduce the panel temperature by, say, 10°C (18°F), the panel output would increase by about 4%. My array is 2500 watts nominal, so we're talking about 100 watts. If I could get that for 8 hours, it'd be 0.8 kWh, which is worth about 10¢. A month of that and I could almost have a StarBucks coffee. Not worth the effort even if I could manage that temp decrease. Back to my lonely drawing board.

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#5
In reply to #4

Re: increasing output of tracking solar panels

03/06/2019 4:44 PM

How's the daily local WIND velocities and directions...under the panels...between your roof and the panels?

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#6
In reply to #5

Re: increasing output of tracking solar panels

03/06/2019 6:24 PM

Panels are on a tracker, not the roof.

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#7
In reply to #3

Re: increasing output of tracking solar panels

03/06/2019 8:21 PM

I have looked on this on the fractal design. The challenge in PV is overheating. Its always on material challenge.

But I say, a cooler PV is way efficient than the hotter one.

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#10
In reply to #3

Re: increasing output of tracking solar panels

03/07/2019 12:17 PM

Convection would probably be influenced (slightly) since the paint would not be PERFECTLY conductive. Perhaps not significant but would probably negate the slight gain from radiation.

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#8

Re: Increasing Output of Tracking Solar Panels

03/07/2019 4:48 AM

Black surfaces absorb more heat as well as radiating more heat. It depends on the surface below your panels and how much heat will be reflected back onto the rear of the panel. If the panels sit above concrete or baked earth I would expect any benefit to be cancelled out. If they sit above a grassed area that collects dew overnight you might see a greater improvement. Clearing away any walls, fences or high foliage that is currently impeding air flow under the panels is likely to produce better results.

Because no two years have the same cumulative sunlight hours or intensity you cannot directly compare the coming summer to previous years. To get meaningful results you need to paint alternate panels and log daily the output from each panel. That is unlikely to be practical but fixing (they usually come with adhesive backing) a thermochromic liquid crystal thermometer (cost approx £1.75, $2.3, €2.0 each) to the rear of each panel will allow you to log temperatures and extrapolate increases or reductions in performance. Less accurate, higher cost but more time effective is to monitor just two adjacent panels, one white and one painted black with remote WiFi temperature data loggers (cost approx £50,,$69, €55 each, software inc.) taking readings 4/5 times per day. If you conduct an initial test by only converting one panel rear to black you should know after a month if it is worth converting the remaining panels or if the test panel needs to be stripped back to the original white.

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#9

Re: Increasing Output of Tracking Solar Panels

03/07/2019 5:36 AM

Painting the back side of solar panel should reduce the temperature since black body emits radiation, if the ambient temperature is lower . Of course it also absorbs if the ambient temperature is higher.

in this instance, temperature of PV panels is bound to be higher with the sun heating the dark surface , as well as intrinsic resistance of the circuits.

To know whether the output increased or not, you need a DC watt hour meter.

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#11
In reply to #9

Re: Increasing Output of Tracking Solar Panels

03/07/2019 2:03 PM

Thanks, GS. I use the Enphase system, which records the output of each panel and puts the data in the cloud so I can see it.

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#12

Re: Increasing Output of Tracking Solar Panels

03/08/2019 3:23 AM

I think the law of diminishing returns is at play here.
Solar panels ,good.
Tracking solar panels, a bit better.
Other mods, prob' not worth the expense/time/effort.
Maybe a reflector to put more light onto the panels would be more effective?
Del

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#13

Re: Increasing Output of Tracking Solar Panels

03/08/2019 3:33 PM

Possibly could get better results by thermostatically controlled misting of the backside of the panels ....anyway it would be an interesting experiment...

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70AARCuda (1); Del the cat (1); Gadepalli Subrahmanyam (1); gutmonarch (1); hoo8975 (1); jack of all trades (1); jhhassociates (1); Rixter (1); SolarEagle (2); SSCpal (3)

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