I was having a back and forth with a reader about wind power and how much fossil fuel capacity must be kept on standby to support grid reliability with wind. Here are some excerpts of what I wrote:
Forget all of the studies for a moment. I used to operate power plants. Any traditional capacity (fossil fuel, nuclear) except perhaps gas turbines takes on the order of a day or more to start up - if you don't take that long, the thermal stresses alone will blow the whole place up. During the whole startup and shutdown, and through any "standby" time, the plant is burning fuel. Since we don't have a good wind energy storage system, some percentage of wind capacity must be backed up with hot standby, because it can disappear in an instant. We are learning now, contrary to earlier assumptions, that wind speeds can be correlated pretty highly over wide geographies, meaning that spreading the wind turbines out does not necessarily do a lot to reduce the standby needs. And since plant startups take time, even gas turbines take some time to get running, the percentage of wind power that required hot backup is pretty high -- I would love to find this percentage.
I found at least one source for such a percentage, which posits that for England, the percentage of hot backup needed is as high as 80%: http://www.ref.org.uk/Files/ref.for.decc.28.10.09.i.pdf
I quote from page 6-7:
On any view, including the square root rule of thumb referred to above, the result, imposed for purposes of maintaining adequate response and reserve requirements, implies that a high degree of conventional (dispatchable) plant capacity is retained in the system to support wind generation. Thus, for 25 GW of installed wind capacity only 5 GW of conventional plant can be replaced leaving 20 GW in the role of standby capacity (also known as "Spare" or "Shadow Capacity").3
So 80% of the expected production from wind has to be backed up with hot spares burning fossil fuels. They go on to say that the percentage of required spare capacity may be lower if the grid area is substantially larger, but not a lot lower. I had not considered hydro power, but apparently that can be used to provide some quick response to wind production changes. The report also talks about diesel generators for standby since they can be started up quickly, but these are seriously inefficient devices. Despite the report's conclusion that the situation might be a bit better on the continent with a larger and more diverse grid, a report of the largest German utility seems to argue that German experience may actually be worse:
As wind power capacity rises, the lower availability of the wind farms determines the reliability of the system as a whole to an ever increasing extent. Consequently the greater reliability of traditional power stations becomes increasingly eclipsed.
As a result, the relative contribution of wind power to the guaranteed capacity of our supply system up to the year 2020 will fall continuously to around 4% (FIGURE 7). In concrete terms, this means that in 2020, with a forecast wind power capacity of over 48,000MW (Source: dena grid study), 2,000MW of traditional power production can be replaced by these wind farms.
It is hard to tell, because 48,000 MW is the nameplate capacity which is virtually meaningless, but my guess is that they are not doing better than 80%.