Electric power distribution reserve margin!

Now there's an exciting title. But at least now you know what to skip if you have a low tolerance for technical stuff.

An electric power distributor's reserve margin is the difference between how much power it can deliver at full capacity, and the amount of power you consumers use. Lower values impair reliability, higher values tie up capital needlessly. When margins are smaller than the capacities of individual power plants, watch out - one plant trip means there's not enough juice to go around, and suddenly you can't blog.

Various activist groups such as Ralph Nader's Citizens Utility Boards would argue that reserve margins are too high. That was a favorite approach against nuclear power plants in Illinois in the 80's and early 90's - CUB always claimed that the plants' power was not needed, and thus the utilities were not entitled to receive returns on their huge investments via rate increases. CUB's position was extreme, but the question is fair enough - how much "headroom" do utilities need to provide reliable electric power on demand?

Well, let's consider a power generator you're probably very familiar with - an automobile.

When you drive a car your power output is limited by what's available from your engine. For that reason engines are oversized significantly relative to typical driving conditions, so you can handle such things as a headwind, a hill, or extra passengers.

The wind resistance dominates at higher velocities, and the power needed to overcome it increases roughly with the square of the car's velocity. What follows is a rough estimate of the impact of this - as the EPA says, your mileage may vary.

Let's consider a simple example - no headwind, no towing, driver only, no luggage, no hill, constant efficiency, straight road, and negligible power demand for other vehicle components (fan, airconditioning, alternator, water pump, automatic transmission...). Suppose you drive 70 MPH instead of 55. You have increased your velocity 27%, but you've increased the power required to overcome wind resistance by 62%.

Is that sufficient? Well, consider a headwind of 5 MPH. Now your power demand due to wind resistance will be at the 70 MPH rate when you're actually going 65 MPH. Will you tolerate slowing down, or will you insist on going 70? Now it feels like 75 MPH, and you need about 86% more power to overcome air resistance than you did at 55 MPH. Never mind trailers, hills, extra passengers, dirty air filters, poorly tuned engines, high altitude, underinflated tires.....

These numbers are somewhat exaggerated for various reasons that take me even more afield from the point, which is this - published margin numbers might sound a lot better than they actually are. The bottom line is that according to people who actually run power grids, as opposed to Nader groups that bitch for a living, is that capacity margins should be on the order of 20%.

In much of the country the peak power consumption season is coming. Things might get particularly interesting in California as Gray Davis fights to escape his own legacy in the gubernatorial election. The vulnerability of New York has been mentioned earlier. And we can't rule out the possibility of more rolling blackouts in Florida.

Inevitably there will be talk of building more plants or more transmission capacity. Hopefully the above will help put it in context.