The first installment of the “How Solar Power Works” series got some great responses, but one of the questions we got was so good I felt like we had to write up the answer. So, before we get on to part 2, I’d like to answer Tim’s excellent question, “Why doesn’t the silicon in PV modules run out of electrons? There seems to be an endless supply of electrons that are freed up when sunlight hits the panels.”
So good, right?
The question focuses on the photoelectric effect, which I ham-fistedly described as follows:
That “photovoltaic” or “photoelectric effect” happens on an atomic level. That means that when a super-tiny photon hits an atom in just the right way, the atom absorbs a photon and releases (or, “sheds”) an electron. The movement of electrons is— you guessed it!— electricity.
What Tim caught makes sense. If there are a limited amount of electrons “orbiting” an atom— which we all learned in Ms. Wentworth’s 8th grade physical science class, right?— wouldn’t the PV cell eventually run out of electrons to shed?
The short answer is: no.
The longer answer can be answered two ways. The first way is to explain that there are “free electrons” roaming around out there, which aren’t tethered to an atomic nucleus. When an atom sheds an electron, which is negatively charged, the electron-less atom is left with a positive charge. And, because “opposites attract” is a very real thing, that positively charged atom (called a “cation”) will then attract a free electron in order to return to its most stable state: electrical neutrality.
Now, “because: free electrons” might be an acceptable answer for most people— and, don’t get me wrong, it’s a real answer— it’s not the best answer to that question.
The best answer has to do with circuits. Specifically, closed circuits.
“So,” you might be asking, “what’s a circuit?”
In simple terms, a circuit is a complete path around which electricity can flow. It must include a source of electricity (ex.: a battery or, as in our case, a PV cell), and material (a “conductor”) that will allow electricity to pass through them easily. The conductor is then attached to the positive and negative ends of the power supply (terminals) and electrons can begin to flow. Every electrical circuit you’ve ever seen (that works) is built just like this: as a loop.
Here’s two examples of a simple from the Encyclopedia Brittanica (Who knew that was still a thing!?).
When the loop is “open” (light switch off), there’s no electricity flowing. When the loop is “closed” (light switch on), electricity flows. Lightbulbs light. Speakers speak. Automobiles automobilate (?). Etc.
Keep all that in mind as I apply it next to the concept of solar power. As we discussed in part 1, what we call “solar power” is really negatively charged electrons being knocked out of an atom by a passing particle of light (or, “photon”), but that electron doesn’t just zoom off into space (usually). It travels through the conductor in the same way that electrons in a battery do.
As I put it to Tim, “to be useful, the solar cells have to be part of a larger loop (circuit) of electricity. Think of a battery circuit with a + and -, the electrons flow ‘out’ of the negative side and ‘in’ to the positive side, so one electron leaving the atom leaves it positively charged, so it will attract a free electron (which is negatively charged) which effectively fills the ‘hole’ left by the moving one. Then a photon comes in and knocks it out, etc., etc. The electrons stay in the circuit, and the energy from the sun’s light pushes them along.”
If that makes sense to you: huzzah!
If you still need some visual aids, this graphic, below, from Mammoth Memory, does a great job illustrating the movement of a single electron through a circuit. As you look at it, try to remember that it’s the same battery on both ends of the graphic.
Got it? Great! Now we can move on to part 2, for realz.
Original content from CleanTechnica; image courtesy National Science Foundation.
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