The recent power blackout that hit several states and parts of Canada got me to thinking about the US electric power system.

It is my understanding that there is no practical way for huge amounts of electric power to be stored. I think it has to be used as it is generated.

I know that power plants do not directly supply the end users. They feed electric power into a continent wide system of transmision lines called a grid. The end users take from the grid.

Do the generators respond to demand, producing more power when more is demanded by users and less when demand goes down?

Might there be monstrous storage batteries or capacitors throughout the system to smooth out the way the system responds to changing demand?

Does anybody here know how this works?

Originally posted by Dinosaur
The recent power blackout that hit several states and parts of Canada got me to thinking about the US electric power system.

It is my understanding that there is no practical way for huge amounts of electric power to be stored. I think it has to be used as it is generated.


Actually, there are ways that work. "Pumped storage" where you pump water up to a reservior works pretty well, though you do have significant energy losses. A similar method was to pump compressed air into a deep underground geological structure. I don't recall how the heat of compression was to be recovered and used.


I know that power plants do not directly supply the end users. They feed electric power into a continent wide system of transmision lines called a grid. The end users take from the grid.

Do the generators respond to demand, producing more power when more is demanded by users and less when demand goes down?


Absolutely. Also, they often can operate at higher output (eg, spend longer times between maintenance) as well which is more costly due to higher maintenance costs. A large generator that is only on/off isn't very useful from a power production point of view, unless it happens to have some other characteristic (like it is cheap).


Might there be monstrous storage batteries or capacitors throughout the system to smooth out the way the system responds to changing demand?


Capacitors are a popular way (combined with the inherent inductance of the transmission lines) to smooth (and incidentally store) power. Batteries are simply too expensive and too inefficient to serve as useful storage devices for large scale power storage. A hypothetical device for storing power was a superconducting toroidal electromagnet. The toroidal (donut) shape keeps the magnetic fields inside the toroid. Obviously, enough superconducting material (and enough cooling systems) is a pretty steep upfront cost here. So I don't think it's been attempted.

Something like this would probably have been useful to smooth out the voltage oscillations that occured in the Northeast blackout a few days ago. It perhaps could have bought time (on the order of a few seconds mind you) for either an orderly shutdown or perhaps a lockout of the offending parts of the grid. OTOH, the blackout might have occured anyway.

Finally, since I mentioned superconducting infrastructure, it appears that large scale superconducting transmission lines may be near on the horizon. I heard that the cost for a superconducting line (using nitrogen cooled transmission lines and high temperature superconductors) was on the order of ten to a hundred times (factor of ten was favored) more expensive than the regular wire transmission using current technology. The latter was apparently around $500,000 per km, while superconductors would be more like $5 million to $50 million per km for the same capacity (but with much lower electricity losses per km BTW). Take these numbers with a big grain of salt.

You can usually predict when more power is required (ei on very hot and very cold days), so usually power production is regulated at the source. When the grid requires less power, they just burn less coal, close the valve on the hydro-electric dam, etc. There is no practical way to store energy.

And because we're all on a grid spanning multiple countries, electricity can be shifted around. For example, if New York is lacking electricity while Pensylvania has a slight surplus, New York will buy some off Pensylvania.

The "old" rule was that a power grid had to have enough spinning reserve to accommodate the loss of the largest generating facility on line. The new plants are 1,000 megawatts and more, thus requiring substantial on-line reserves. The first unit that went off line was 680 megawatts, then as transmission lines tripped several more plants went off-line and the remaining plants could not sustain the load.
They cut off to prevent destroying their generators.

People do not realize that once a generator shutsdown it cannot be restarted without outside power. Some generators can be taken off-line without completely shutting down, they are operational but cannot be put back on-line until the load is reduced. All four of the nuclear plants on the grid shutdown and they take a long time to restart (they also need a lot of outside power to restart).

Capacitors you see installed on power lines are for phase correction.