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Can a solar panel power plant, e.g. 1 MWp, just disconnect from the power grid and "switch off" the solar panels, or is there some kind of issue with that?

As far as I know, solar panels don't change their reflectivity, so when switched off they probably heat up a bit more.

Compare a MOSFET: Switching is what they do all day.

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    \$\begingroup\$ Solar panels are such relatively poor efficiency that the heating experiences without load is not a big deal on Earth. \$\endgroup\$
    – Kuba hasn't forgotten Monica
    Commented Aug 6 at 17:06
  • \$\begingroup\$ @Kubahasn'tforgottenMonica Yes, relatively... but if the cooling concept might not tolerate the change. E.g. 20%, that is more than 2% and might lead to an increase of e.g. 20°C in the panel which in turn causes faster degradation - something like that. \$\endgroup\$
    – zonksoft
    Commented Aug 6 at 17:13
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    \$\begingroup\$ Turning it off will slightly increase the terminal voltage and thus leakage losses. But it will greatly reduce conduction losses. So the net effect could be that it even cools down when you switch the inverter off. \$\endgroup\$
    – tobalt
    Commented Aug 6 at 17:34
  • \$\begingroup\$ @tobalt Losses in the wiring are a good point, thank you \$\endgroup\$
    – zonksoft
    Commented Aug 6 at 18:37
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    \$\begingroup\$ Losses plus delivered power (including power delivered to external wiring) must be the same as power received from solar radiation. There is no possible way for switch off to make the cell cooler: reduced conduction loss must be retained in the cell. \$\endgroup\$
    – david
    Commented Aug 7 at 2:18

3 Answers 3

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Yes, essentially you can just disconnect all the solar panels (you can try this if you have a panel) and the solar panel goes to the open circuit condition where no power is generated. So where does the energy go? It goes into heating the panel through internal resistance.

enter image description here
Source: https://www.pveducation.org/pvcdrom/solar-cell-operation/iv-curve

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    \$\begingroup\$ That’s just basic thermal engineering. I’m not sure who would “fail to see” it but that’s a given. Anyone who designs with panels will take it into account. It’s a much bigger deal in vacuum and at high altitudes. And of course any panel running hotter will degrade faster. Whether this degradation is a big deal or not is very much application dependent. \$\endgroup\$
    – Kuba hasn't forgotten Monica
    Commented Aug 6 at 17:29
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    \$\begingroup\$ Open circuit will likely increase the longevity than decrease it since it prevents any sort of thermal hot-spotting caused by defects or shading (although shading is unlikely in a solar plant). The extra ~20% of energy available when not converted to useful electrical energy simply evenly warms the panel. \$\endgroup\$
    – horta
    Commented Aug 6 at 21:11
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    \$\begingroup\$ @Uwe You're thinking of series resistance, but a PV cell is a diode. In an open circuit condition, the photocurrent is shunted through the diode junction. It acts as a shunt resistance. Energy is conserved: any light energy absorbed by the cell but not delivered to the load heats the cell. \$\endgroup\$
    – John Doty
    Commented Aug 6 at 22:38
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    \$\begingroup\$ @JohnDoty I guess you are an expert on that matter. But could you slightly elaborate why you believe the PV cell will produce the same power when in an open circuit condition? In the open circuit, the conduction band's lower levels will be filled with with carriers, so further photoabsorption becomes less likely, no? I.e. the light leads to a population inversion. I.e. wouldn't an open-circuited cell have a higher (in energy) IR absorption edge than a short-circuited cell and thus absorb less power? \$\endgroup\$
    – tobalt
    Commented Aug 7 at 7:48
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    \$\begingroup\$ @tobalt The population of the conduction band never comes anywhere near saturation. Instead, negative charge builds up on the N side of the junction, while positive charge builds up on the P side until the junction potential becomes too weak to separate the charges against thermal diffusion. Electrons and holes mingle and recombine. OK, a small fraction of recombinations are radiative, but for Si that's negligible in practice. Most of them just make heat. \$\endgroup\$
    – John Doty
    Commented Aug 7 at 13:52
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Indeed, any energy received from the sun must convert into heat if it doesn't travel out through wires. The amount of light the solar cell absorbs does not change when it is disconnected.

The efficiency of solar cells is about 20%. Thermal conductivity is close to linear with the temperature difference between ambient and the panels. Thus if the panel is normally e.g. 20 Kelvin warmer than ambient temperature, its temperature would increase by 4 Kelvin when disconnected.

This temperature difference will slightly speed up the degradation of the cells. It is however likely that the capital costs and lost income from a disconnected plant far exceed the depreciation of the cells.

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  • \$\begingroup\$ Have they gotten to 20% now? I remember 10%. \$\endgroup\$
    – user20574
    Commented Aug 7 at 20:48
  • \$\begingroup\$ I think 21% is current mainstream. \$\endgroup\$
    – zonksoft
    Commented Aug 7 at 23:09
  • \$\begingroup\$ @StackExchangeSupportsIsrael, the best laboratory cells are sitting at 47.6% right now. 20% is typical for commercial cells, where you need to balance the cost of the cells against their efficiency. \$\endgroup\$
    – Mark
    Commented Aug 8 at 2:06
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In and of themselves, sure. Note that, as current-limited devices, "switching" could also include a short circuit (in which case the generated photocurrent is looped back through the PV array itself, raising its temperature modestly). An open circuit is generally preferred for various reasons.

There may be extenuating reasons to taper or delay the turn-off of some megawatts. Depending on distribution line length and resistance, availability of power in the general area, what rules the regional utility has for connecting large loads or sources, there may be undesired over/under-shoot of voltage at turn-off, or the transient response of the grid is otherwise impacted (may cause phase and frequency shifts as well as change in voltage, affecting other customers, generators, etc.).

1MW I think is on the low side of where this might be a concern, but, more just to say, at some point, for a load/source large enough, some coordination is needed, between the main generators on the grid, and the load in question. Large industrial clients, generating stations, cross-region links (im/ex-porting power), etc. are examples where such communication is beneficial. So, perhaps 1MW arrays, no, but a whole farm of them, totaling some 100s MW, likely.

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  • \$\begingroup\$ Yes, those are all very good points and go in the direction I was thinking of. Could you help me out with the short circuit+current-limited device? I've never heard that one. \$\endgroup\$
    – zonksoft
    Commented Aug 6 at 21:58
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    \$\begingroup\$ The general concept is the series-parallel voltage/current transformation theorem. In precisely the same way a voltage source can be open-circuited to set its current (and therefore power) to zero, a current source can be short-circuited to set its voltage (and therefore power) to zero. CCS don't arise very often in nature, but keeping outputs shorted can be useful for setting bench power supplies, for example (where otherwise hot-plugging the live output to a circuit will cause a capacitive discharge as it adjusts to CC mode). A solar panel happens to have a somewhat CC-CV characteristic. \$\endgroup\$
    – Tim Williams
    Commented Aug 6 at 22:39
  • \$\begingroup\$ Ah I understand: switching it of literally translates to short-circuiting it. \$\endgroup\$
    – zonksoft
    Commented Aug 7 at 10:10
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    \$\begingroup\$ @zonksoft You can open-circuit it or short-circuit it. Either "switches it off". \$\endgroup\$
    – John Doty
    Commented Aug 7 at 15:11
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    \$\begingroup\$ Short circuiting is actually used in some solar systems to turn off the generator. For example, many old analog "solar lead acid charge controller" modules mostly consist of a diode from the panel to the battery, and a FET shorting the panel if the battery is fully charged. The advantage of shorting the panel is the prevention of excessive open-circuit voltage. \$\endgroup\$
    – Michael Karcher
    Commented Aug 7 at 20:27

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