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Author Topic: Is mid-day best for solar power?  (Read 1831 times)

Offline thedoc

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Is mid-day best for solar power?
« on: 19/07/2015 18:50:01 »
Robert Browell asked the Naked Scientists:
   
Why when the sun is at it's maximum height, do solar panels give the most power?

What do you think?
« Last Edit: 19/07/2015 18:50:01 by _system »


 

Offline chiralSPO

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Re: Is mid-day best for solar power?
« Reply #1 on: 19/07/2015 19:03:47 »
It isn't necessarily when the sun is at the maximum height. There are two real issues: 1) how directly the sunlight hits the solar panel; and 2) how much of the atmosphere the sunlight has gone through.

For the first issue, this has to do with what fraction of the sunlight gets reflected, and how much gets absorbed. If the light comes in at a 90 angle, it has the best chance to get absorbed, and the further away from this ideal angle, the more light gets reflected (wasted). This is why most solar panels are built at an angle rather than being perfectly horizontal (especially at higher latitudes, where the sun is never directly overhead).

The second issue is less critical except right after dawn and right before sunset, when the sun is very close to the horizon. In these cases, the sunlight has  a much longer path through the atmosphere than when it is higher up. This results in some losses in the intensity of the light (mostly due to scattering, but some absorption).

So to answer the title question: the solar panels can capture the most power when they are pointed directly at the sun when the sun is at the highest point in the sky. For solar panels that do not track the sun, it is slightly more complex to optimize the efficiency, and it may actually be best to have the sun hitting the panel most directly during periods when it is not quite at the highest point (especially because the sun's path through the sky is different throughout the year...)
 
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Offline alancalverd

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Re: Is mid-day best for solar power?
« Reply #2 on: 19/07/2015 20:01:25 »
Just to make things complicated, midday - 2 pm is often the time of maximum convective cloud cover. Additionally, fixed solar panels are generally angled for maximum annual energy, so they may not be optimally angled for midday sun in midsummer or midwinter.

All that said, of course if you could track the sun and trhere were no clouds, midday would indeed give the maximum intensity.
 

Offline wolfekeeper

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Re: Is mid-day best for solar power?
« Reply #3 on: 23/07/2015 04:38:14 »
Actually some solar panels are angled somewhat West. The total energy and peak power produced is reduced, but it gives its peak power in the evening, when people are actually using it, which can reduce costs, and improve the economics. It's not all about the average power, you want the peak power when you're using it, if it's reasonably possible to arrange that.
 

Offline evan_au

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Re: Is mid-day best for solar power?
« Reply #4 on: 23/07/2015 12:24:56 »
If you have a solar cell of 1m2, it will intercept the most sunlight (ie 1 m2) if the solar panel is at 90 to the incoming sunlight.

If the solar cell is at (say) 45 from the incoming sunlight, it will intercept only 0.7m2 of sunlight, so you have just lost 30% of your potential solar cell output.

In general, the solar cell output is proportional to sin(θ), where θ is the angle at which the sunlight strikes the solar cell (all other things being equal).

Depending on how you mount your solar cells, midday may not be the time where θ is closest to 90.
 

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Re: Is mid-day best for solar power?
« Reply #4 on: 23/07/2015 12:24:56 »

 

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