Just how useful are wind turbines? This might still be an open question according to the popular press. As with everything, it's easy to get carried away debating very technical detail when it's the basic ideas that are important. We need to back up.
Conversations about the utility of wind turbines sometimes begin with a criticism of the low "efficiency" of wind turbines. This is a misunderstanding.
Over the period of a year, a particular kind of wind turbine, operating in a particular place, might produce 20% of its nameplate power rating. So, if the machine is rated as 5 MW by the manufacturer, then it produces 1 MW, on average, at that site (1).
This is entirely down to the fact that the average wind speed at a particular site is usually lower than the wind speed at which the turbine was rated by the manufacturer: the wind blows differently in different places. The rated wind speed is usually at the high end of typical wind speeds (2). The machine will deliver 5 MW if the wind blows at the rated wind speed but, averaged over a year, it will deliver 1 MW in the winds at our imagined site.
It'd be neat if manufacturers agreed on a standard wind speed for the machine's nameplate power rating, but in the end it doesn't matter what you call a wind turbine! It's only important what it does in a particular place. If a machine that's called the "Acme 5 MW" actually delivers 1 MW, on average, when placed at your site, then it's a 1 MW machine for you. And this has almost nothing to do with how well the machine works; it's only to do with the wind speeds at the site.
The thing is, that for well-known manufacturers and machines, all of this is well understood. So, if Acme is the world's top turbine manufacturer, you can be sure that I'll know that an "Acme 5 MW" will deliver 1 MW at my site before I write the cheque. All commercial wind developers and grid operators know! Wind farms are only built after the wind speeds at a site are thoroughly understood.
All fine and good. Now maybe it's not so obvious, but it's important to point out that I implied earlier that somehow the wind turbine's not so good. This is because of how I used the words "efficiency" and "only", as in, "the 5 MW machine only delivers 1 MW so it's 20% efficient". The implication is that the machine's supposed to generate 5 MW, but we're only getting 1 MW on average, that we're wasting 4 MW.
Well, this simply isn't true. The developer will know that the Acme 5 MW will deliver 1 MW at the site: the machine should be close to 100% efficient against expectations (3).
In addition, it's not sensible to talk about wind turbine efficiency in the same way that we talk about the efficiency of an oil-fired power plant. No fuel is consumed by a wind turbine: wind energy is free energy aside from capital costs and maintenance. Unlike the fossil fuels.
All this is important because sometimes critics say that if you buy an Acme 5 MW and its efficiency or, more correctly, its capacity factor, at your site is 20%, you'll need one Acme 5 MW machine plus 4 MW of conventional non-wind power to "make up the difference" to 5 MW. There is no "difference" to make up. If I want 5 MW of additional capacity and I want to use wind, and I stubbornly want to put the wind turbines at a site where I'll get 1 MW from an Acme 5 MW machine, then I can buy five Acme 5 MW machines. I don't need the conventional power plant. To say otherwise is simply wrong-headed.
A recent report from a conservative think-tank uses numbers based on this kind of reasoning to conclude that wind causes the release of more carbon dioxide than anything else. You know, you need to "make up" the "missing" 80% of the power with coal (4).
Another, related, criticism of wind often starts with the entirely sound idea that wind needs 100% back-up for days when the wind's not blowing at all. This is a sensible idea. If you build a wind turbine that delivers 1 MW, then perhaps you need 1 MW of non-wind power generation in case the wind's not blowing (5).
The thing is, though, that wind turbines, in general, don't need new back-up power plants. The power plants already exist! You just start up some of these power plants when you need them, when the wind drops. Additionally, across a large area, the wind hardly ever stops blowing all at once.
And it's all relatively easy to do. We can forecast wind pretty well in the very short-term. There's more than enough time to take action if necessary. As the electricity grids become more distributed and interconnected, as we move away from centralised power systems, this will become even easier to manage.
The key point is that wind turbines reduce the need for operating existing fossil-fuel power plants: they save fuel and reduce carbon dioxide emissions.
Wind will never be the only energy technology, but it will be an important part of the future energy landscape. It works.
Finally, to add that wind has its problems, of course. Not everybody likes how wind turbines look and, mostly for this reason, not everybody wants a wind turbine in their neighbourhood. This is reasonable and there's much to debate here.
1 MW is enough power for about 2000 European homes.
Paul Gipe has a brief discussion about the motivations for using higher wind speeds for rating wind turbines.
In a way, it's a bit like automobiles. You can buy, say, an Acme 200 km/h sports car, but you're not expecting to drive at 200 km/h everywhere. When you're in a zone where speed is restricted to 30 km/h, you're not "wasting" the 170 km/h of potential. It's virtual.
There's some additional discussion in the original analysis by Le Pair about the reduced efficiency of placing coal power plants in a stand-by mode and the associated wasteful burning of a dirty fuel. In my opinion, this is not significant. The calculations in this analysis assume that there's an 82.5% deficit of power to be "made up" due to the supposed inefficiency of wind turbines. As discussed above, this is not very sensible. Sure, if you're building new coal power plants to create 82.5% of your "wind energy", then it's going to create a lot of carbon dioxide!
It's possible to extend this idea in different ways. For example, why build expensive wind turbines at all if you need to build all this new conventional plant anyway? And you probably need to keep all the conventional power plants, probably coal-fired, running on stand-by just in case the wind drops. And the efficiencies of coal power plants running on stand-by are awful and so you're just creating even more carbon dioxide. Let's build nuclear power plants and frack as much gas as we need!