Article 710: Stand-Alone Systems

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tallgirl said:
The electrons only move when the circuit is complete.
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Sure, but I think what jaggedben is talking about is the conservation of energy and the power balance of the PV panel.

When the panel temperature is not changing (and not undergoing a phase change), then it must hold that: incoming sunlight power = reflected/scattered/transmitted light power + PV power leaving the panel + net heat losses through conduction/convection/radiation. [Unless there is an energy flow I'm overlooking?] So if you compare the steady-state cases where the panel is producing at MPP, vs illuminated but not producing power, the heat losses must be higher in the latter case, i.e. the temperature has to be higher, or the panel must be somehow reflecting/scattering/transmitting more light.

Of course, PV panels are only ~20% efficient, so the effect may not be large. It would be interesting to see some real world temperature measurements of side by side PV strings in the same conditions, with one operating and the other not.

Cheers, Wayne
 
tallgirl said:
The electrons only move when the circuit is completely.

A solar cell is essentially a diode and photons create movement in the electrons which can only go one way, in the usual fashion with diodes. They don't "have" to go anywhere.
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The electrons are going to stagnate, but the energy still has to go somewhere. If electrons can't carry it out through the electrical current, it has to find an alternative destination. All other destinations than thermal energy can reasonably be ruled out. The silicon cells still keep their chemical identity; the cells don't change color, so reflecting more sunlight is ruled out; there is no change in position or speed, so mechanical energy is ruled out; and there is no nuclear reaction inside them. Unless we've unwittingly discovered a source of the cosmologists' mysterious dark energy on our own planet for the first time, thermal energy is the most likely explanation.

You may have interpreted the term "negligible" in the post you linked out of context. That poster also didn't quantify term, so it is hard to understand the intent. Most likely, that poster just meant the temperature rise will be negligible relative to the amount of temperature rise that would cause degradation. Rather than negligible as not measurable.
 
I'm pretty sure that what happens in an open circuit is that in the presence of incoming photons, electron-hole pairs keep dissociating and migrating away from the P-N junction until the resultant electric field from the concentration of electrons and holes on either side of the junction achieves equilibrium with the built in opposing field from the junction itself, and Voc is reached. The rates of dissociation and recombination of electron-hole pairs becomes balanced because there is no longer a net electric field to sweep them away from the junction, i.e. from each other, so they recombine as quickly as they split.
 
wwhitney said:
Sure, but I think what jaggedben is talking about is the conservation of energy and the power balance of the PV panel.

When the panel temperature is not changing (and not undergoing a phase change), then it must hold that: incoming sunlight power = reflected/scattered/transmitted light power + PV power leaving the panel + net heat losses through conduction/convection/radiation. [Unless there is an energy flow I'm overlooking?] So if you compare the steady-state cases where the panel is producing at MPP, vs illuminated but not producing power, the heat losses must be higher in the latter case, i.e. the temperature has to be higher, or the panel must be somehow reflecting/scattering/transmitting more light.

Of course, PV panels are only ~20% efficient, so the effect may not be large. It would be interesting to see some real world temperature measurements of side by side PV strings in the same conditions, with one operating and the other not.

Cheers, Wayne
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I have that data somewhere. I've never seen a significant back-of-panel temperature rise on an array floating a battery (so, Vout close to Voc) compared to one running at Vmpp.

As an aside, the efficiency isn't due to internal losses, like if the shunt resistance were just lower. At 100% efficiency they'd reach absolutely 0K because they'd be absorbing IR and turning it into electricity ...
 
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tallgirl said:
At 100% efficiency they'd reach absolutely 0K because they'd be absorbing IR and turning it into electricity ...
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I disagree with this statement.

At 100% efficiency, they would sustain an equilibrium temperature equal to the ambient temperature, rather than cool off to absolute zero. There are still other modes of heat transfer than radiation heat transfer, that will make them much more likely to achieve a temperature near 300 K, given a 300 K ambient temperature. Conduction through the framing, and convection with the air, will both add heat to the modules, if they were to get colder than 300K, and balance any lack of any radiation heat transfer they would ordinarily get.

Perhaps in space, a hypothetical 100% efficient panel could achieve equilibrium with the ~3K cosmic background, even in sunlight. If it were to get colder than 3K, it would have to be capable of converting microwaves into its electrical output as well.
 
Carultch said:
I disagree with this statement.

At 100% efficiency, they would sustain an equilibrium temperature equal to the ambient temperature, rather than cool off to absolute zero. There are still other modes of heat transfer than radiation heat transfer, that will make them much more likely to achieve a temperature near 300 K, given a 300 K ambient temperature. Conduction through the framing, and convection with the air, will both add heat to the modules, if they were to get colder than 300K, and balance any lack of any radiation heat transfer they would ordinarily get.

Perhaps in space, a hypothetical 100% efficient panel could achieve equilibrium with the ~3K cosmic background, even in sunlight. If it were to get colder than 3K, it would have to be capable of converting microwaves into its electrical output as well.
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At 100% efficiency they'd convert 100% of the EM spectrum received to electricity.

That's the definition of "100% efficiency".
 
tallgirl said:
At 100% efficiency they'd convert 100% of the EM spectrum received to electricity.

That's the definition of "100% efficiency".
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Yes. And suddenly, the problem comes in to focus, as this would violate the second law of thermodynamics for such a panel to even exist. Unlike a motor that could be 100% efficient in the ideal case, a PV module is ultimately part of a heat engine that would be limited by Carnot efficiency like all other heat engines. There are other theoretical limits on efficiency before this, from the details of the semiconductor technology, so it is a long way before the Carnot limit ever will be close to being realized.

For a 300K panel lit by the 5800K sun, the maximum possible efficiency a module could have by this limit, would be 94.8%.
For a 3K panel lit only by the 3K cosmic background, you couldn't produce any power. Emissivity and absoptivity are a package deal at any given frequency (Kirchhoff's radiation law), and the panel would radiate as much to the surroundings as it would absorb from them.
 
Carultch said:
Yes. And suddenly, the problem comes in to focus, as this would violate the second law of thermodynamics for such a panel to even exist. Unlike a motor that could be 100% efficient in the ideal case, a PV module is ultimately part of a heat engine that would be limited by Carnot efficiency like all other heat engines. There are other theoretical limits on efficiency before this, from the details of the semiconductor technology, so it is a long way before the Carnot limit ever will be close to being realized.

For a 300K panel lit by the 5800K sun, the maximum possible efficiency a module could have by this limit, would be 94.8%.
For a 3K panel lit only by the 3K cosmic background, you couldn't produce any power. Emissivity and absoptivity are a package deal at any given frequency (Kirchhoff's radiation law), and the panel would radiate as much to the surroundings as it would absorb from them.
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They aren't Carnot engines, though.

If they were made from pure material having a precise band-gap voltage, and the light source emitted photons with that precise energy, and only that precise energy, the theoretically limit would be whatever internal resistance there were.

In the case of that 5,800*K sun, the photons are emitted over some distribution of energies. Any photon with too low an energy does nothing. Any photon with too high an energy has that excess energy converted to heat.
 
tallgirl said:
They aren't Carnot engines, though.
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Neither is any engine we build in practice. It is still a fundamental limit of the maximum possible efficiency of anything used for converting heat into a more useful form of energy like electricity or work, regardless of the details of the process.
 
Carultch said:
Neither is any engine we build in practice. It is still a fundamental limit of the maximum possible efficiency of anything used for converting heat into a more useful form of energy like electricity or work, regardless of the details of the process.
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It's not converting "heat", though. The PV process isn't based on thermodynamics. The sun's temperature only provides the range of energies for photons, it doesn't act like a high temperature heat reservoir in a Carnot cycle engine.
 
tallgirl said:
It's not converting "heat", though. The PV process isn't based on thermodynamics. The sun's temperature only provides the range of energies for photons, it doesn't act like a high temperature heat reservoir in a Carnot cycle engine.
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Thermal energy is the reason the sun is emitting its radiation. The thermonuclear sources inside the sun generate heat, and the surface emits light based on a black body radiation spectrum. The nuclear reactions in its core don't directly produce the light you see, as there are heat transfer processes that bring the energy to its outer layers. The black body radiation of its photosphere is what produces its light. Thermodynamics still governs the destination of the energy.

If your point were correct that this process could bypass thermodynamic limits, then it would be possible to have radiation heat transfer and photovotaics as the working principle of an engine, and get around the limitations of using a working fluid.
 
wwhitney said:
At which point, their energy is converted to heat, presumably.

Cheers, Wayne
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I don't know about that. Of what energy do you speak? Electrically, it's neutral.

Dissociation and recombination of electron-hole pairs is happening all the time in the bulk of the semiconductor from which a solar module is made when it is exposed to light; it's only the dissociations that happen near the P-N junction that result in electrons and holes that do not immediately recombine when current is flowing.

I used to know how to deal with the math for this stuff, but I am way too long out of school to talk about it in other than general terms.
 
Carultch said:
Thermal energy is the reason the sun is emitting its radiation. The thermonuclear sources inside the sun generate heat, and the surface emits light based on a black body radiation spectrum. The nuclear reactions in its core don't directly produce the light you see, as there are heat transfer processes that bring the energy to its outer layers. The black body radiation of its photosphere is what produces its light. Thermodynamics still governs the destination of the energy.

If your point were correct that this process could bypass thermodynamic limits, then it would be possible to have radiation heat transfer and photovotaics as the working principle of an engine, and get around the limitations of using a working fluid.
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Well, that's a very loose definition of "thermal". The proton-proton chain reaction in our local star isn't a "thermal" process in the classical sense of "thermal energy". The photons which are emitted as black body radiation, in the sense of a glowing piece of iron, say. They are propagated from the interior as the gamma rays which are produced are re-radiated as lower energy photons in greater number.

"Thermal" usually refers to the vibration of atoms, not fusion and photon multiplication as happens with stars.

 
ggunn said:
I don't know about that. Of what energy do you speak? Electrically, it's neutral.
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But creating a hole - charge pair presumably stores potential energy in the separation of the charges, which is presumably released as heat when they annihilate.

All I know for sure is that I believe in conservation of energy, so if less electrical energy is coming off the roof, there's more heat or light energy to account for, and I suspect that it's heat energy that stays on the roof.

Cheers, Wayne
 
Carultch said:
If your point were correct that this process could bypass thermodynamic limits, then it would be possible to have radiation heat transfer and photovotaics as the working principle of an engine, and get around the limitations of using a working fluid.
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I guess the photon spectrum is perhaps the equivalent of the "working fluid"? What wkipedia has to say on the topic:


Cheers, Wayne
 
wwhitney said:
But creating a hole - charge pair presumably stores potential energy in the separation of the charges, which is presumably released as heat when they annihilate.
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Even if that is true, the stored energy has to come from somewhere and the recombination releases it. Net zero. Energy is conserved.

Even in a producing solar module most of the electron-hole pairs generated by incoming photons immediately recombine.
 
ggunn said:
Even if that is true, the stored energy has to come from somewhere and the recombination releases it. Net zero. Energy is conserved.
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Sure, but the two cases:

Incoming photon (of sufficient energy) creates hole-charge pair -- charge travels through external circuit and releases most (all?) its potential energy there, returns to recombine with the hole, at low potential energy (no?) -- little (no?) energy released on the roof.

Vs

Incoming photon (of sufficient energy) creates hole-charge pair -- pair recombines on the roof -- all potential energy is released on the roof as heat, the equivalent of the panel just absorbing the photon.

Cheers, Wayne
 
ggunn said:
Even in a producing solar module most of the electron-hole pairs generated by incoming photons immediately recombine.
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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
 
As I said, most of the electron-hole pairs created in a PV module simply recombine without passing through the external circuit. As to the question of whether modules at Voc get hotter than those at Vmp (producing power), I have no idea, but like anything else sitting in the sun PV modules get hot. Most of that is from infrared radiation, though, which does not participate in the production of electricity. I would expect the effect to be minimal if existent.
 
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