Here's an interesting little detail from the National Grid report on capacity margins that I wrote about a few weeks back.
It seems we have windfarms with a nameplate capacity of 7.6GW. National Grid obviously then have to derate this capacity for planning purposes. As we all know, it's perfectly normal for the whole of the UK to simultaneously experience very low windspeeds (or no wind at all), and this has been known to happen even in the depths of winter, for example the very cold winters of 2009/10 and 2010/11. I had therefore assumed that the Grid would have to plan on the basis that they might get nothing from windfarms at all, but in fact they do nothing of the sort. According to Table 16, the grid assumes that they will get 23% of nameplate capacity, or some 1.7GW.
The explanation in the text makes no sense to me at all:
173. For wind power stations, Equivalent Firm Capacity (EFC) is used. This is a measure of how much 100% reliable generation would be required to replace the installed wind generation whilst maintaining the same level of system security.
174. This wind modelling approach combines the risk from wind variability with the other conventional risks, e.g. high demand or low availability of conventional generation. When the installed wind is a small proportion of total generation the main risk to system security comes from the conventional risks and the EFC is very close to the mean load factor for wind generation. In the coming years, as the installed wind capacity grows as a proportion of total generation, the risk that the variability of the wind will affect system security grows as well as wind’s contribution to system security, which will have the affect of decreasing EFC.
This footnote also appears:
Equivalent Firm Capacity is a model output from a Monte Carlo simulation. For further information explaining the theory behind the EFC, see paragraph 173 and http://pio.sagepub.com/content/226/1/33.
Perhaps there is something in that paper that can justify what appears on the face of it to be a rather reckless assumption, but the abstract doesn't fill me with hope:
This paper describes a new probability theory of the capacity value of additional generation in electrical power systems. A closed-form expression for the effective load carrying capability or equivalent firm capacity of a small additional capacity is derived. This depends on the mean and variance of the distribution of available additional generation capacity, and the shape of the distribution of the difference between available existing capacity and demand, near zero margin. The theory extends naturally to the case where the pre-existing background and additional resource are not statistically independent.
The theory may be used to explain and confirm the generality of various well-known properties of capacity value results, as is illustrated using Great Britain examples. Of particular note is the common observation that if the distribution for demand is shifted so as to increase the calculated risk, then the capacity value of additional generation increases. The new theory demonstrates that this is not true in general, but rather is a consequence of the shape near zero margin of the probability distribution of the margin of existing generating capacity over demand.
Incorporating an assumption that we would get 1.7GW from windfarms, the net forecast margin was 2.3GW. Without it then, the margin would be 0.6GW, or 1%. And as noted at the time the National Grid report was published, the figures that were published excluded the loss of the Heysham and Hartlepool nuclear reactors. In other words the true margin may have been negative.
Put that in your pipe and smoke it.
I do hope they are unmothballing a lot of power plants.