Capacity factor: Bigger is better?

Every now and then the nuclear lobby makes a post that 'nuclear has the best capacity factor of all'.

Well this isn't really something to brag about.

Don't be fooled with capacity factors

Capacity factor is 'average power generated' divided by 'power printed on the name plate'. This is an accountancy number. If you operate a plant, you can check its financial performance.

You can't use it to compare technologies and power plants that are operated in a different way.

It has absolutely nothing to do with 'how good' something is. In this regard it is just as usefull as comparing bank account numbers. A higher account number doesn't mean it holds more money.

Because of characteristics of nuclear power plants, they have a high capacity number. But that's more an indication of their limitations than benefits.

Why is the high capacity factor for nuclear bad?

The cost for building, operating and decommissioning a nuclear plant have almost no relation with the amount of energy it produces. See the end of this article for a more in depth discussion on this.

Having a high capacity factor means that it is running at full power for most of its lifetime. Electricity demand changes over a day, weekdays and seasons. Electricity demand is much lower on a sunny summer sunday afternoon then it is on a cold and dark monday morning.

So the high capacity factor means that the power plant is running full-out most of the time and isn't adapting to demand. This means something else is making up the difference between the nuclear generated power and demand.

This used to be fossils. Where nuclear cost are fixed for lifetime, fossil cost are mostly related to burned fuel. In this case you can have a 'base-load' made by a nuclear plant that is running flat-out and fossil plants that you turn up and down by demand. Operating nuclear & fossils this way results in lower cost and CO2 emissions for (nuclear) baseload and higher cost and CO2 for flexible demand.

So what a high capacity factor actually says is that nuclear is very inflexible and needs fossils to compensate for this.

But nuclear can do load following as well...

Again this is another argument made by the nuclear lobby. And is is partly true. It can technically do a poor job on this (5). But worse.. if you turn down the power, you turn down the capacity factor. And then accountancy comes in...

Nuclear energy has a median cost of 69 USD/MWh (1).

Let's do some calculations. Lets take a nuclear plant with 3GW and an operational life of 40 years, total cost over this time is 67b. Remember, for these costs it doesn't really matter if you produce electricity or not.

If this NPP is operated 'flat out' with some exceptions for refuelling and maintenance it can get to the capacity factor of 92%. In this case it produced 967TWh over its lifespan at a cost of 67b. This is 69.3 USD/MWh.

Now let's run the same plant in load following mode. Having no fossils to back it up. This means we need to be able to produce peak-demand with nuclear. And turn this down during low demand. This results in an average capacity factor over its lifetime of about 40%. This means it will only generate 420TWh, at the same 67b cost. This means that price jumps to 159 USD/MWh.

Why nuclear industry thinks high capacity factor is good

This is why the nuclear industry thinks high capacity factor is good. For a nuclear power plant a high capacity factor means high profits. Electricity price is determined by supply and demand. Not by a nuclear company.

The more electricty a plant produces, the more revenue is made. A high capacity factor means you produce a lot of electricity. So you make a lot of money.

A low capacity factor means your cost keep on running but you can't sell electricity so you don't make money. In this case you lose a lot of money.

The nuclear lobby is so much in love with high capacity factor that they even change their definition to make it higher in their annual performance report:

"When calculating capacity factors, those reactors that do not generate any electricity during the calendar year are not included.

For reactors that start-up or shut down during a calendar year the capacity factor for that year is calculated based on the electricity output that would have been generated where they to operate at 100% output for the fraction of the year in which they were in an operable status." (2)

This also has a historical background. Nuclear energy is complex. In the early days it wasn't a reliable source. They wanted it to run it full-power, all the time. But they couldn't. So capacity factor became a performance indicator to judge how well 'their' plant was operating. A high capacity factor meant 'they' did a good job. For example the US was only able to realize a stable performance this century (2).

Why a low capacity factor isn't bad

Gas, coal and oil power plants are the most polluting way of generating electricity. For this reason you would like to use them as little as possible. If you have any other source available, you turn their production and thus pollution and capacity factor down. This has nothing to do with how much they are able to generate. Just with how much you want them to generate. This capacity factor is mostly determined by demand.

Wind turbines harvest their energy from wind. The nameplate power of a wind turbine is the power generated at optimum windspeed. Less wind is less power. More wind is less power. Wind power is clean so we would like to use this as much as possible. However we can't control the wind so the capacity factor for wind is mostly determined by supply.

Solar has an even lower capacity factor. Not because it generates almost no power. This is because its nameplate power is how much it can generate under ideal circumstances. That is in full sunshine at the perfect angle at the equator at 12:00 exactly. If you use them on another location or average power over a full day you get less average power. But that is also what you would expect. For a PV installation in the netherlands you can expect a power factor of ~12% over the entire year. In summer it wil be higher, in winter it will be lower. During nights it will be 0%. On sunny days it will be 80%. The nice thing about sun is that it is very predictable. But again this capacity factor is determined by supply.

And as discussed, for nuclear it is determined by economics.

Why nuclear cost is not related to produced electricity

A NPP is expensive to build: it uses a lot of material, labour and it takes a very long time to build (interest). Nuclear energy is extremely dangerous and for this reason many safety systems are required. With these systems installed nuclear can be safe, but it adds cost.

Once it is taken into operation, a staff of highly skilled personel is required to operate it. Again all of the safety systems must be maintained and a lot of inspections are required. Besides the danger of something happening with the nuclear aspects, a NPP also presents a large part of the electricity supply of a region. If it fails unexpectedly, it can result in a blackout so it must be reliable (4). For these reasons cost of operation is high.

And once it is decommissioned the cost keep going. The reactor became radioactive during use. It first need to 'cool down' before it can be demolished. This takes decades. During this time the building must be maintained and secured. The construction needs to be very strong. If something goes wrong in the nuclear reaction, the reactor produces hydrogen. The big dome isn't meant to keep planes out. It is meant to contain a possible hydrogen explosing from the inside. So this dome is made from super-strong concrete, not easy to take down. So not cheap to demolish.

And then there are some cost that are usually not even included... It takes a long time to build a NPP. A NPP generates a lot of electricity. That means that during construction this energy must be generated in a different way. This means using existing fossil plants. Nuclear energy produces radioactive waste. This waste needs to be handled for tens to thousands of years, depending on the type. As ways to do this are still not clear, these costs are not included...

With this knowledge look at some capacity factor data

Link (3) returns a great source with capacity factors for several electricity sources. This gives some hard backing to the article above.

Hydro & gas are very flexible. You can just turn a valve to get the power you need. Just like a tap or gas fornuce at home. That's why capacity factor is steady at ~50%.

You can't control wind or solar. Their 'nameplate power' is for ideal conditions. You can't make these ideal conditions. By checking (historical) weather data you know what you can expect. There will be some seasonal effect, but all is very predictable. For wind you can expect ~30%. For solar ~20%.

Nuclear.. well it only makes economical sense to run it flat-out, all the time. And you can see this.. it has a capacity factor >92% most of the time. You really can't afford to turn this down. The 8% it isn't running is due to refueling and disturbances.

And another way to look at it. Below is a screendump of electricitymaps.com, Sweden feb 2023. Sweden has a mix of nuclear, wind and hydro. As you can see, nuclear is running at a flat (full-power) rate. Wind is unstable. And hydro is used to fill the gap between these two sources and demand.

(1) https://www.iea.org/reports/projected-costs-of-generating-electricity-2020

(2) https://www.world-nuclear.org/getmedia/9dafaf70-20c2-4c3f-ab80-f5024883d9da/World-Nuclear-Performance-Report-2022.pdf.aspx

(3) https://www.eia.gov/electricity/annual/html/epa_04_08_b.html

(4) https://nl.wikipedia.org/wiki/Stroomuitval#Belgi%C3%AB

(5) https://www.oecd-nea.org/upload/docs/application/pdf/2021-12/technical_and_economic_aspects_of_load_following_with_nuclear_power_plants.pdf