Article 710: Stand-Alone Systems

[ggunn]

Most? If the wikipedia article is correct that the single junction theoretical efficiency limit is 33%, and if commercially available panels are 20% efficient (is that right, and are those single junction panels?), and if the photon creating a hole-charge pair is the way that PV turns light into electrical energy, then it would have to be less than half. Certainly premature recombination would make sense as the primary mechanism for the efficiency discrepancy between the theoretical limit and the currently available panels.

Cheers, Wayne

Most of the generation of electron-hole pairs occurs in the bulk semiconductor, not at or near the junction, so they immediately recombine. Quick question - have you studied this formally or are you just google searching wikipedia articles? I majored in the semiconductor device block of my EE department; everything I am saying is from memory, and as I said, it was a long time ago. I just remembered "avalanching", though, so you might look that up.

 

[pv_n00b]

The high-level physics answer is that at some point a photon does not have enough energy to move an electron across the PN junction gap in the cell because so many electrons have already jumped the gap and are just hanging around with nowhere to go. So it just hits an electron and transfers energy but the electron never jumps the gap, and you have a hot electron with nowhere to go. That heat will be dissipated to the environment when the electron recombines with the donor atom. It's only a small number of photon electron interactions that result in a successful movement of an electron across the gap, which is why module efficiency is only around 20% or less. Most photons hitting a module don't do anything but heat up the module anyway. So when you add the heating from the 20% of the photons that would have moved an electron across the gap to the total module heat load it's not really that much of a temperature increase.

 

[wwhitney]

So when you add the heating from the 20% of the photons that would have moved an electron across the gap to the total module heat load it's not really that much of a temperature increase.

If the module is otherwise a blackbody, isn't it a 25% increase in solar heating? 80% -> 100%. And if the module is reflecting some of the incoming sunlight, and the absorption fraction jumps by 20%, that would be more than a 25% increase in solar heating.

Quick question - have you studied this formally or are you just google searching wikipedia articles?

My formal education is in mathematics, my physics knowledge is all informal. Speaking of wikipedia, I was perusing this article:

https://en.wikipedia.org/wiki/Shockley%E2%80%93Queisser_limit

If I understand it correctly, the ~30% maximum theoretical efficiency of a single junction PV cell in the sun's spectrum is the product of 3 factors: a ~45% factor representing the solar spectrum's deviation from monochromatic light at the band-gap energy; an ~75% factor representing the competition for hole-charge pairs created recombining via the external circuit instead of internally; and an ~ 85% factor described in the "impedance matching" section which I don't fully grasp but I think is related to not being able to produce maximum current at maximum voltage simultaneously.

So doesn't the ~75% factor mean that when 4 hole-charge pairs are created, 3 of provide external energy, and 1 just recombines internally? That would mean that when the PV panel is at Mpp, most of the generated hole-charge pairs do useful work.

Cheers, Wayne