I was just going to explain briefly about what ‘capacity factor’ was but while I am on the subject I decided to make a couple of other points.
What is ‘Capacity Factor’
It is the amount of energy produced by a power plant divided by maximum possible. For example if a plant is theoretically able to produce 100,000 MWh but only produces 50,000 its capacity factor would be 50%.
Low capacity factors are not necessarily bad as long as you know what the capacity factor is going to be and you can take that into account. People will invest in a power plant because they hope to get some of the profits from the electricity that it will eventually produce. If you expect a capacity factor of 30% and it is 30% then your investment was wise and you should make money if you have did your sums correctly. However, if you expect a capacity factor of 90% and you only get 45% then your in trouble.
It is also important how and when the changes in capacity occur – are they predictable or unexpected, how much power is actually lost etc.
A Couple Of Concepts
First of all let us have a look at capacity factors in a relatively abstract way to understand a couple of concepts.
A 50% capacity factor can be made up in numerous ways:
Above is two plots giving an overall 50% capacity factor. The blue line is for a power plant is for an unpredictable plant and the red is for a predictable variation. Of course a real world situation would look quite different but the difference between predictable variation and unpredictable output is very important.
For the purposes of ensuring electricity supply we are not interested in the changes in the capacity factors but the change in the electricity produced. Fifty percent of the output of a large 1000MW plant is a lot more than that of a 1MW plant. To illustrate this I have plotted a simulation of the output of 100 plants in blue and 5 plants in red both with a 50% chance of failure in a given time period and with the same combined power output. 
As you can see the variability in the large number of smaller plants (blue) is a lot less than that of a small number of larger plants (red).
Dealing With Variation
How a power grid deals with variations in the supply from different power plants depends on how and when the variation occurs. Power for the grid in the UK is committed one hour before it actually supplied. For wind the wind speed forecast is reasonably predictable on that time scale. Also wind turbines are reasonably reliable (i.e. their failure rate is low) and since there are more individual turbines compared with one large coal, gas or nuclear plant. This means that the change in the overall supply is not so affected by the failure of one plant as seen in the graph above.
If wind speeds are predicted to be low then other generation can be started to account for this.
The failure of large thermal plants such as coal, gas or nuclear have a much bigger affect. To ensure continued supply in the event of such an occurrence there needs to be much more ‘spinning reserve’ – i.e. power plants that are up and running that can ramp up their output in seconds to compensate for the loss.
The cost and affect of the different types of reserves needed are dealt with in some of the references given in this post.
No power plant runs at 100% capacity factor for any reasonable length of time. So let us have a look at the causes.
Causes
- No demand
- Refueling
- Loss of energy supply
- Failure of the power plant
- Regulatory requirements
No Demand
Some plants (usually gas and hydroelectric) only work at times of peak demand and therefore have a low capacity factor simply because they are not needed.
Refueling
Some nuclear power plants need to be shut down for refueling. This is usually planned well in advance and also allows time for essential maintenance to be carried out.
Loss of energy supply
This can affect all plants. There may be no wind or sun for wind turbines or solar. Hydroelectric plants may not be able to operate if there is not enough water. However, it can apply to all power plants for example the cutting off of gas supplies by Russia to the Ukraine1 in 2009. There are also questions about the availability of Uranium in the future2 (I have written a bit about supply here but will go into it in more detail in a future post).
Failure of the power plant
Power plants can fail for various reasons. If you are lucky then you may have some notice that something is wrong and shut down the plant when necessary to carry out repairs. At other times the plant will break down unexpectedly.
Regulatory Requirements
An example of this is the shut down of nuclear plants in Japan and Germany after Fukushima. However, there have been many times when one or more nuclear plants have been shut due to regulatory requirements. For example a large proportion of nuclear plants in South Korea have been shut down due to faked safety documentation and parts3.
1 New cold war in Europe as Russia turns off gas supplies, Independent 7 January 2011 (http://www.independent.co.uk/news/world/europe/new-cold-war-in-europe-as-russia-turns-off-gas-supplies-1230036.html
2 The coming nuclear energy crunch, Guardian, 2 July 2013 (http://www.theguardian.com/environment/earth-insight/2013/jul/02/nuclear-energy-crunch-uranium-peak-blackouts)
3 South Korea nuclear reactor hit by auto shutdown; six units now off, Reuters, 28 Nov 2013 (http://www.reuters.com/article/2013/11/28/us-nuclear-korea-idUSBRE9AR07020131128)
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