I found a similar question here on this site, but my question is slightly different. If a solar panel is exposed to sunlight but is not plugged in to anything--dc load, inverter, etc--where does that electricity go? In this example, I'm assuming the solar exposure is still freeing electrons, but where do they go?
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1$\begingroup$ possible duplicate of Where does the electricity, generated by a solar panel, go if you don't use the electricity? $\endgroup$– Tobias KienzlerCommented Jul 18, 2013 at 7:35
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$\begingroup$ that one's accepted answer focuses on a different angle: when the panel is plugged into a network and there is no draw, here it focuses on a fully disconnected panel $\endgroup$– ratchet freakCommented Jul 18, 2013 at 10:47
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2 Answers
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where does that electricity go?
The photons from the sun have energy and momentum, but not "electricity".
Essentially, a photon (solar or otherwise) striking the solar panel can create an electron-hole pair (EHP) and, if the EHP is within or near the depletion zone, the pair will be separated by the built-in electric field.
This results in a separation of charge and with that, a voltage across the panel. If there is an external circuit connected, there will be an electric current through the circuit.
But, if there is no external circuit through which an electric current can circulate, the electric field due to the separated charge opposes further separation of charge; the generated electron-hole pairs simply recombine rather than separate.
Electric power is the product of voltage and current. If there is no external circuit, there can be no current and thus no electric power can be delivered by the panel, i.e., the "electricity" is never developed and thus, there is no need to consider "where it goes".
answered Jul 18, 2013 at 0:37
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$\begingroup$ So, if a disconnected solar panel has been sitting in the sun for a while, and a load is connected, the temperature of the panel should drop slightly? $\endgroup$– DJohnMCommented Jul 18, 2013 at 0:44
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4$\begingroup$ @User58220 It should due to the heat balance. The equilibrium temperature of the connected panel should be slightly lower than the disconnected panel, because those hole recombinations introduce heat. But the efficiency is only something like 12% to begin with. So the temperature difference isn't like the difference between a white and black surface. More like the difference between a dark and slightly darker surface. $\endgroup$ Commented Jul 18, 2013 at 0:56
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$\begingroup$ To say that the photon does not have "electricity" will be incorrect, as the photon does have oscillating, regenerating electric and magnetic fields. $\endgroup$ Commented Jan 2, 2015 at 15:46
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1$\begingroup$ @AlanZ2223, photons have energy and momentum but not 'electricity', i.e., photons are not electrically charged. Moreover, a photon does not "have oscillating, regenerating electric and magnetic fields". $\endgroup$ Commented Jan 2, 2015 at 17:17
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Well assuming you are talking about a basic silicon diode solar panel, sans electronics, then the diode junctions will generate their usual open circuit Voltage, which for silicon, is around 650-700 mVolts per junction. But no current will flow, since it isn't connected to anything.
If you short circuited the panel, the terminal Voltage will be near zero, but a current will flow, linearly proportional to the solar irradiance of the cells. The generated power in both cases is essentially zero, since either I or V is zero.
At the optimum load resistance, the panel Voltage will be about 0.5 Volts per cell, so you would need more than 25 cells in series to charge a 12 Volt car battery. That half Volt point is about where most silicon cells generate the maximum power in the load resistance