First of all, wind power can only ever form part of a country’s electric supply. Too much and it destabilises the grid.

Second, you have to maintain a substitute for the days when the wind is too high or too low. At all times the supply must match the demands.

Denmark solves both these problems by supplying power to and receiving power from Scandinavia, which has a large proportion of hydro electricity.

Hydro electricity is theoretically a good compliment because you can use the wind power to pump water up into the reservoirs. It then can be released on demand to match consumption.

However that option isn’t always available elsewhere. Consequently alternate conventional sources must be maintained to fill the gaps. Conventional power stations work by heating water to make steam which drives the generators. That isn’t an instant process. Consequently, when wind is blowing, the conventional systems have to be run in parallel so that they can step in at a moments notice. If such a station is coal or gas fired then it produces CO2 whilst the wind is blowing as well as when it stops. Consequently the achieved reduction in CO2 is very little.

To which we must add the CO2 cost of the windmill manufacture.

They are not a good deal except in a narrow range of applications.

http://www.absenergyresearch.com/energy-market-research-reports/renewable-energy-market-research-reports/wind-power/reports/wind-power-report-2007