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Solar Panels: Improving performance at low sun angles

  1. Jun 24, 2013 #1
    I have a sailboat with 2 solar panels installed. They work well, except at the end of the season when the days are shorter, the nights are longer and the sun is lower, so I built a mechanism to tilt and rotate the panels so that they could be pointed directly at the sun. With low sun angles I was expecting a big improvement in charging amps, but I’m not seeing this.
    For example, one morning (June 14th) the sun was low and there were very strong shadows. I estimate the sun's angle to be about 25° and I set the 2 panels at about 65° so that, as near as I could tell the sun was shining directly onto the panels. The batteries (all wired in parallel) were at 12.5V, so could definitely accept a charge. There was no load on the batteries at the time. Given the low angle of the sun I expected a big improvement in charging amps. However, with the panels horizontal, I was getting about 4 amps, and after pointing them directly at the sun I could not notice any improvement – I was still getting 4 amps. Very disappointing. I don’t have any quantitative data to hand, but I believe I’ve seen an improvement when the sun is higher, say 70° and I point directly towards the sun. So I’m wondering if anyone can suggest why I’m not seeing a big improvement when the sun is low.
    I have a thought, but may be way off track. Intuitively, I would think that the biggest improvement would be seen when the sun is low and the panels are adjusted to point directly towards the sun, but I’m not seeing this. So I’m wondering what frequency of light the solar panels are converting to energy. Sunsets are red because more light at the blue end of the spectrum is absorbed by the atmosphere and re-radiated as diffuse light. So red light reaches our eyeballs directly (directionally) but the blue light is diffuse and arrives from all directions. If solar panels work more from light at the blue end of the spectrum, then in the evenings and mornings, the ‘useful’ light reaching the panels would be diffuse as opposed to directional, and not greatly affected by the angle of the panels.
    Any other thoughts?
  2. jcsd
  3. Jun 27, 2013 #2


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    Gold Member

    It could be that, as you say, at that at that time of year with the low sun the diffuse is as strong as the direct.
  4. Jul 3, 2013 #3
    Refraction vs scattering

    Hi Ronald,

    Imagine shining a light through a prism onto the surface of a sphere and adjust it so that the yellow and red parts of the spectrum shine on to the sphere at about the latitude where you live on Earth and consider the angle of the incident light against the surface of the sphere and match it with the angle you describe - 25 deg or so. You will find that the blue light is nowhere near you. So your problem is not so much angle of incidence of the sunlight on your solar panels as much as the light reaching the solar panels contains few photons with high enough energy to overcome the work function of the material in your PV panels.
  5. Jul 3, 2013 #4
    That's not why the sun appears red at sunset, the blue light is not refracted somewhere else (where?), it is scattered.

    If that were the dominant effect angling the panel so it collects 2.4x as many photons would produce 2.4x as much power (up to the maximum power output of the panel), which is what was expected.

    Unless there is some external effect such as reflection of sunlight onto the panels when horizontal, or even illumination from another source, I can only think that it is as you say that diffuse radiation is dominant. However is it more likely to be diffuse infra-red from cloud cover rather than scattered blue light?
    Last edited: Jul 3, 2013
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