Storm Desmond - the problem with too much wind energy in one graph

Forecasted Wind energy output for 6th December, the day after Storm Desmond, shows a loss in wind generation of 91%

As I write, Storm Desmond is raging outside my window and wind energy output is providing almost 50% of demand. It is coming at a price, however, with ESB Networks reporting that about 12,000 customers had to go without power today due to damaged cables. But electricity is not just required on a Saturday, it is required every day. According to Eirgrid forecasters, wind energy output is forecast to drop by as much as 91% tomorrow, from nearly 2,000MW to 160MW over 16 hours. (a drop in capacity factor from 87% to just 7%)

This explains why wind has such a low capacity credit. Let's look at what Eirgrid had to say in 2009 :

However, the benefits [of wind energy] tends towards saturation as wind penetration levels increase. This is because there is a significant risk of there being very low or very high wind speeds simultaneously across the country. This will result in all wind farms producing practically no output for a number of hours (note that turbines switch off during very high winds for safety reasons). In contrast, the forced outage probabilities for all thermal and hydro units are assumed to be independent of each other. Therefore, the probability of these units failing simultaneously is negligible [Eirgrid 2009].

What they mean is that a gas power plant (CCGT) of 400MW might drop out but will do so independently of another gas plant. So that would be a loss of 400MW which standby reserve would adequately replace. But wind farms do not act independently of each other.  Instead, when one drops out, chances are the rest of them will aswell. So now you have a huge hole in generating output of about 1,900 MW, equivalent to about 50% of tomorrow's demand. So you need to have about 5 large gas plants ready to step in. Starting these type of plant up from "cold" is not a good solution as engineers have stated that this is 20 times more damaging to plant than "warm" starts. It is also very expensive and high emitting in pollutants. So gas plant are kept running on low load behind the wind and will then step in tomorrow to pick up the load as wind slackens off. 

Eirgrid are lucky in that the wind is forecast to decline over a period of 16 hours. What would happen if they are wrong and it falls off at a quicker rate ? Then they may need more fast acting plant like diesel or open gas cycle turbines. These are more polluting than efficient CCGT type plant and will result in higher emissions, thereby negating some of the benefits of having all this wind generation. 

Can we close down any of our power stations and replace it with a huge fleet of wind turbines ? Can we really make a transition to a more sustainable "green" based energy supply ? 

As you can see from the above, the answer is no, not with wind farms.