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Energy 101 – 8.2 Electric Power Plant Capacity Factor

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Hello, back to Energy 101.
And today we are looking at electric power plant capacity factor.
This is another major characteristic we need to study, and be aware of when we’re
dealing with utilization of electricity from the grid.
And essentially, I don’t know, 99% or more, of our electricity comes from the
grid. That’s the, basically a pool that has many
power plants feeding into it. Renewable energy photovoltaics, renewable
energy, wind generators hydroelectric dams, nuclear coal plants, gas plants,
they all feed into the same grid. And then at the other end, we pull out
some electricity and the, the power plants are turned on and off, and up and down, to
meet our demand. So but the demand varies.
And so, what does the demand that’s placed on the grid, that the utility servicing
the grid, and managing the grid, has to deal with.
Well time of day is an obvious one. It, the peak demands, peak electricity
usage is during the day. And the lowest is at night.
Two, three o’clock in the morning, most factories are closed down, office
buildings are closed down, homes are closed down, people are asleep, and so the
electricity usage is low. Temperatures are low, by the way, and
therefore even air conditioners are, lower than the daytime, in the South, for
instance, summertime. So the air conditioners are unloaded even.
But they all, it also changes by day of week, that you might not think of at
first, because weekdays, Monday through Wednesday, have a higher demand, and have
a different profile than weekends, Saturday and Sunday.
So, the, utilities have to be prepared to operate differently on Saturdays and
Sundays, and weekend days, than during the weekdays, Monday through Friday.
But they also vary may, in another major way, and that is over a longer term, by
seasons. The Winter and Summer, have a higher
demand than the Fall and the Spring. And of course the reason for that is, the
Fall and the Spring have a low heating need for the space heating, when the
temperatures are, are 70 degrees, or the are fairly comfortable outside, that we
have to provide less space heating for our buildings, and our homes, than we do in
the Winter. And in the Summers, we have to provide air
conditioning, in the Summers, for space conditioning.
Fall and the Spring, we don’t have to supply as much air conditioning.
So, you have variations, short term variations, from hour to hour in the day,
daily variations from day of week, and seasonal variations with Winter, Summer,
Fall, Spring. So the su, utility managing the grid, has
to deal with all of these ups and downs, and be able to meet the highest demand
that people ask for, when they turn something on, and they also have to
throttle things down, when the dam, demand is low, because there’s no electrical
storage system. Electrical storage is very expensive and
it isn’t commercially viable at this point.
Hopefully it will be in the future, but electricity can, is, can only be generated
if people use it. So everything’s gotta match up.
Let’s, talk, look at this a little more closely, and before we do that, we need to
look at some definitions. Power capacity of a power plant, or a
fleet of power plants, either way, is the maximum steady power production
capability. Maximum being the running wide open as
they’re designed to do. It’s not like the car trying to run it
wide open at a 120 miles per hour, and you probably can’t do that very long before
you damage something in a lot of cars. But these are, this is industrial
equipment that is designed to operate with maxium power, rated power, in a, steady
state, for days, or months on end. That’s the maximum, capacity, or what we
call the power capacity. Now that’s like speed on your speedometer.
In other words, it’s measured in Mega Watts, which we denote by MW, and we or
Kilo Watts, which is denoted by Kilo Watts.
Of course, a Mega Watt is a million Watts, and Kilo Watt is a 1000 Watts.
And that’s the rate at which we’re using the energy.
Like miles per hour on your, in your car. And it’s the, we denote the maximum
capacity, that the power plant, or the fleet of power plants, if it’s to look at
all of them feeding into the grid, we denote that by MW, capacity, subcapacity,
or Kilo Watts of capacity, just depending on whether you want which units you want.
Ones just a thousand different than the other.
So that’s the power capacity that, and so and, that needs to be of course higher, a
little bit higher, than the maximum demand that the system will ever see.
Otherwise we have blackouts and brownouts, and equipment gets overloaded and the
system falls apart. But, another definition, that we’d like to
look at, is the electrical energy production.
That’s comparable, using the speed analogy on, in a car, to the miles analogy.
If the power that we’re using, times the time we use it, that gives us the energy
used, that determines how much fuel we’ve got to put in and how long we have to run
the plant, and how hard we have to run the plant.
So in other words, it’s the electrical energy used typically, we always say, over
one hour. So it’s measured in Mega Watt hours, MW
hours, or Kilo Watt hours, KW hours. And that gives us the amount of energy
that is used over some period of time. And for, pow, Capacity factor, we
generally look at a whole year. And so the, the annual load factors
determined by the looking at the annual electrical energy production over years,
denoted by Mega Watt hours per year, or Kilo Watt hours per year.
Typically Mega Watt hours per year, because there are a lot of Kilo Watt hours
that are produced over a year, even by one power plant.
So those are the basic definitions. Moving, further on definitions, what are
the, the capacity factor, which is the whole point here, in discussing, is to
find, as the Mega Watt hours that is produced in one year, and this is for one
power plant, if you looked at the capacity factor of a power plant, or through a
whole fleet of power plants feeding into the grid.
You can do it nationally, you can do it for just a, a region or one utility that
and, one part of the grid that has, is under control by one utility.
Then that is divided by the Mega Watts capacity, that, that, that the power,
power, plants are capable of, pro, producing, if they run wide open, times
8760 hours. Now whats the 8760 hours?
Well, 807, 8760 hours, is the number of hours in a year.
So, in other words, if the plant, the denominator, is the number of Mega Watt
hours that the power plant, or fleet of power plants, could produce if they ran
wide open, to, at rated maximum capacity, for every hour of the year.
So that’s what the denominator is. The numerator is, of course, the actual
amount that is produced. So that, the number is always less than
one, due to the fact that the capacity has got to be equal to the maximum demand put
on the system, over the entire year. And that many period, most periods of the
year, and hours of the year, the demand is going to be less than that.
So, it’s not producing, fully, every hour of the year, so the numerator was going to
be less than the denominator. So, it measures electricity produced as a
fraction of the electrical energy it could have produced by running maximum, output
every hour of the year. So that’s the definition of the capacity
factor. By the way, let me note here, that there’s
also a term that you, some of you might be familiar with, called the load factor.
And, in the load factor definition, the, there’s a small difference, hopefully
small. In most cases the meh, denominator is not
Mega Watt capacity, but the peak load seen throughout the year.
And the power plant capacity needs to be slightly greater than the maximum load at
the system, or power plant ever sees during the year in order to stay 10%
higher. In order to maintain control of the, and
be able to keep from going unstable, for stability purposes.
So some nuances there. But capacity factor, and load factor, are
very similar, except the capacity factor will always be a little bit lower than the
load factor, because the used Mega Watt peak demand during the year Rather than
the Mega Watt capacity of the system. So, let’s look at an example there.
Well, let’s take a nuclear power plant. Nuclear power plants, I picked that one
because that one has the, largest, highest, capacity factor.
And take a large, two unit, typically, they’re built in thousand Mega Watt units,
and if you take a two unit nuclear plant, that has the capability of producing 2,000
Mega Watts on a continuous basis, that’s the capacity.
And let’s assume that they, you measure the actual output over, of that plant over
the year, and it was 15 million Mega Watt hours.
That’s the actual energy is produced. But it could produce how much?
It could produce 2,000 Mega Watts times, times 8,760 hours.
That’s the, what it could have done, but it actually did 15 million.
Well if you run the numbers out I believe you get an 86% capacity factor.
Well that’s a high capacity factor, but in fact, nuclear plants typically run about
90%. And there’s several reasons for that but
that’s, well number one it shows the reliability, the maintianance.
They’re down for maintenance, around 1% of the time.
And that’s scheduled maintenance. You hopefully don’t have unscheduled
maintenance, particularly in a nuclear plant, but you may have.
As any plant may have some unscheduled maintenance where something goes wrong,
and that you weren’t prepared for and didn’t expect.
If you expected it, you replaced that part, or did the maintenance during the
annual maintenance period, when you take the plant down for maintenance.
So nuclear plants are pretty well base-loaded.
The fuel cost is very low compared to fossil fuels.
And therefore they can run them if you’re, they already got them built and they want
to run them rather than running a coal fired or gas plant, where they have to buy
fuel for it. So, but if you look at the US fleet as a
whole, if you take the entire capacity, of all power plants in the US, and add them
together, and divide it and divide that into the annual Mega Watt hours that are
produced and used in the country, then you come out with a 40 to 50% capacity factor.
Now, what that means is, and this is my whole point here, is that only about less
than half of the US power plant capacity is used at any one point in time.
On, on the, on the average, excuse me, on the average.
Over the average of a year, we’re utilizing half of the, or less, of the
power plant capacity. Sometimes, it’s 20, 30 percent.
Other times, or a few hours, normally, it’s, like, 90%.
But over average, so we had, it was only 40 to 50.
So we got a lot of excess capacity, except during the peak hours in the year.
Which normally occurs during the hottest afternoons, late afternoons, in the
summer. When there’s a very large air conditioning
demand. That’s generally when your annual peak
will occur. So, let’s look at power plant
dispatchings. So, how do you, how does the utility that
has this fleet of powerplants that they can turn up and down, like throttle, like
your accelerator on your car, which ones do they throttle back, and which ones do
they turn on and turn off when they don’t, don’t need them?
You got a big choice there. So which half do you use, and which half
on average do you throttle down, or turn off.
Well you turn on generating facilities that are the cheapest to operate, in terms
of dollars per, per Mega Watt hour. In other words the, in, which, you already
paid for the capital costs, and so you got to amortize the capital costs over the
year, whether it’s running or not. That’s the fixed cost, it’s fixed.
The capital cost is fixed, so the, the incremental costs in dispatching, if, if
it gets about what the plant costs to build, because you’ve already got money
invested, and so you now deciding whether to run it, or run plant, power plant one,
or power plant two. And, so in both cases you crank, you look
at the dollars per Mega Watt hour to turn it on, versus let it sit there and do
nothing and, and that’s the, basically the fuel cost.
So, typically the plants that are turned on first, and turned off last, will be the
ones with the lowest fuel cost. Well, it’s pretty obvious that the ones
with the lowest fuel costs are the renewable energy, because that’s energy’s
free. So, the wind, solar and hydro is used
anytime it’s available. And that’s, I mean, all you gotta do is,
is look at the reality, and that is typically what happens.
There are a few isolated cases where there’s some grid limitations, and some
contracts, that happened out in the northwest a couple summers ago, I believe,
where the grid became overloaded. And coming from a certain region, and due
to contractual obligations, they actually turned off some wind farms, but that’s a
very unusual situation. Basically in that same category, though,
is nuclear, because, not only is the fuel cost from nuclear very low, but also
they’re much more difficult to turn on and off.
You have to turn, turn them up and down very slowly.
It takes days to crank up a nuclear plant, and turn it, turn it down, and turn it
off. So, you have to consider that.
Another factor, by the way, and these are second and third order effects, but the
other factors that come into play is water usage.
In some cases, the minimum, they got minimum water flows that the plant uses,
and maximum water flows and things. And depending on what’s going on in the
river, where they’re dumping their, their heat, if they are, then that can come into
play. But basically, the renewable energy in the
nuclear run all the time. It’s what we call base load.
Next come coal. Because coal has normally been cheaper
than gas. And next comes natural gas plants.
Now that’s recently, in this year, that has changed for the first time, in my 35,
40 year history. That natural gas plants have been cheaper,
or as cheap to run as coal plants. And so they have actually shut down some
coal plants before shutting down gas plants.
But, they, that probably won’t last too long, because gas prices are already
headed back up, and coal is fairly stable at this point.
So, we expect it, that the old rule that gas plants, or the natural gas plants are
the first ones to turn off or turn down, will, will be the, is has been, generally
in the past, except in some unique periods or in situations, and probably will
continue over, over the, future. So that’s how we dispatch, our, the
plants. The coal an, the note, the coal and gas
have switched depending on relative price, and need to note that, because, you can,
you can find examples right now that, that is not the case.
But in general, it’s not, that, that is the sequence used.
And that, that covers the dispatching. And the one reason that I wanted to do
that is regarding, where do we get our incremental load from?
And it’s a major characteristic of electrical power that’s fed into the grid,
and we’re going to discuss the, emissions from electric cars, and so you need, you
need to know something about where that electricity will come from as you put in,
put on one car additional electric car for recharging onto the grid, or put a million
of them if you got a, trying to determine how much impact a tax credit will give for
our subsidy to get a lot of, a million new electric cars out there on the system and
whatever. So, that gives us the background, and one
more important characteristic of the electrical power plants, that we operate,
and utilize, and depend on every day. Thank you.

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