You can twist figures to prove anything. This is the case in this article. In fact the people that actualy run the elecxtrical grid know that wind is very reliable and predictable. If one of a hundred windmills breaks down, you only lose one percent of the wind power which may in turn only be 5 or 10 percent in total of the current in the line.
The national weather service keeps up on high and low pressures and wind patterns and knows if the wind is going to die down in so many hours and they inform the grid operator well in advance.
Blow a coal plant critical component and you have a catastrophic emergency shutdown, with no warning. that can easily cause a 2 or 3 month shutdown of the whole plant, which does happen. Only thing worse is cracking a pipe in a nuclear plant. That brings a much larger catastrophic shutdown and it might take 6 months to fix, re-inspect and re-certify the plant.
Coal and nuclear plants also have scheduled periodic months long shutdowns for inspection and maintenance and re-certification.
Here is a source for more info. www dot awea dot org/utility/reliability dot html.
excerpt;
Is wind less “reliable” than conventional generation?
No. Conventional resources occasionally shut down with no notice, and these “forced outages” require operating reserves. For example, a power system that has 1000 Megawatt nuclear or coal plants will typically keep 1000 Megawatts of other generation available, to be ready to quickly supply electricity if a plant unexpectedly shuts down. The power system can still be operated perfectly reliably in this fashion. Thus, “reliability” is not specific to any single generation facility, rather it is measured on a system-wide basis.
As noted by Jon Brekke, Vice President of Member Services for Great River Energy, a utility that operates in Minnesota and Wisconsin:
“Wind energy is a valuable part of our diverse and growing energy portfolio. When partnered with other traditional generation resources, wind energy is an effective way to provide reliable, clean and affordable power to our member cooperatives. Geographic diversity of wind energy helps even out the variability of wind energy in the regional market. In addition, wind farms are typically made up of many individual turbines which reduce the impact of outages. For instance, there are 67 1.5 -MW turbines at our Trimont Wind Farm, so if one is down for maintenance only 1.5% of the total wind farm’s generating capacity is lost.”
Every 10MW windmill needs 10MW of backup, as they all fail to generate power at the same time when the wind dies (or gets too strong). It's extremely unlikely that all coal and nukes fail at once, so you only need to have a 1-5 ratio or so of backup to frontline power.
You cannot honestly compare infrequent major outages of coal/nuke with the frequent no-wind outages of windmills. The honest comparison to a pipe break at a nuke plant, would be a tornado blowing down the windmills. And that's likely to become a common occurrence since the windmills are being built in the tornado region from Texas to the Dakotas.
There simply is nothing in the operation of coal and nuke plants that matches the vagaries of the windmill. Coal and nuke plants run at high use levels (e.g. 90%), not the 9% of windmills.