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David Nightingale: "Where does it go?"

Electrical wiring at the outskirts of Helsinki, Finland.
Aatu Liimatta
/
Wikimedia Commons

My friend has just hooked up his new rooftop solar panels, and been delighted to see his meter run backwards. "But," he said to me "where does the electricity go?"

Good question. Where indeed.

Another friend answered succinctly: "into the wires."

Let's back up to a similar question. A small town maintains a high-up water tank, which is regularly filled from a steady source, and this filling costs money. For a steady pressure the water level in the tank should be held constant. Some neighbors who may have regular privately drilled wells and long hoses may help out sometimes, and when they contribute then the town wouldn't have to buy as much from its main external source -- a massive reservoir or lake (say). For this tiny help the neighbors are given refunds on their water bills.

Thus the town tank's water level is maintained. In this case the answer to "Where does it go?" is trivial: into the town tank -- which they maintain at that fixed height, or pressure.

Or, a similar example might be your car battery. Perhaps it's gone down to 9 or 10 volts say, and needs charging. A helpful neighbor offers to connect his own standard 12 volt car battery, (+) to (+) of course and (-) to (-) , for a short while, and this will bring the first one up towards par. Commonly known as 'getting a jump'. But where does it go? Into my battery. Again, an obvious answer.

Gasoline? Obviously into the fuel tank. Food? Into the stomach. And so on.

But for the solar energy where's the equivalent tank? I would answer it this way: The utility company has quite a few generators running, but it may or may not have any method of storage.

The implication here is that if there are plenty of generators that a utility can fire up, there's no actual need for storage -- in principle. In reality, storage of electricity is a perpetual research topic. One could pump water to the top of a hill and store it in a reservoir, as was suggested at Storm King mountain near West Point in the 60s, until needed and let it out later to drive a turbine or generator. Or, compressed air could be stored in disused mines; or, electricity can be stored in huge batteries, as is done in Fairbanks, Alaska, where temperatures can get down to (-) 50C -- dangerous for things like hospitals and so on. So in 2003 Fairbanks built a football-field-sized bank of batteries in a warehouse-like building. These Ni-Cad batteries are always kept charged, and if there is a complete power outage the batteries provide the whole ~2500 square mile Fairbanks region with roughly 10 minutes of power -- enough to bring back-up generators on line.

But in principle one does not need storage. The utility's constantly running generators are producing a constant voltage -- analogous to the town's fixed water level. That voltage (or pressure in the wires) varies only negligibly. Thus, solar-installed neighbors may contribute, each household having circuitry that will properly match the line voltage, and on sunny days the utility can power down some of its generators, obviously saving oil, coal or uranium.

Finally, where do my friend's electrons go? In principle, they pass into the local wires, to mix and match that pressure, as we said.

So, my friend, that's my non-technical answer, based only on physical principles, to your intriguing question!   I have intentionally omitted any discussion of AC vs DC, or how household inverters produce their current in sync, and what happens in the very unlikely event that householders overproduce.

Dr. David Nightingale is Professor Emeritus of Physics at the State University of New York at NewPaltz and is the co-author of the text, A Short Course in General Relativity.

The views expressed by commentators are solely those of the authors. They do not necessarily reflect the views of this station or its management.

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