Mark Harnett asked:
When wind turbines create electricity they reduce energy from the air. How much does the wind slow down after passing through the turbine? Can you put an efficiency number on a turbine like the binders on solar panels
Caroline Steel had a breeze answering this question...
Caroline - So the short answer is yes you can. When the wind turns a blade of a wind turbine it does in fact lose speed because of the sheer effort it takes to turn those blades. Now this isn’t something we really need to worry about. I know there’s been some speculation that this could change climate and this would have vastly terrible implications but, realistically, with the number of wind turbines we have at the moment, that decrease in wind speed is actually, completely, negligibly, unimportantly, small. Unless you’re in a wind farm, in which case you don’t want to put wind turbines downwind of wind turbines.
But, anyway, we can work out an efficiency for them by looking at how much energy they get out of the wind, so how much does the wind slow down versus how much energy they can output. And it actually turns out that wind turbines are relatively inefficient because there’s a lot of friction between the blades and the rotor in the middle. So they have an efficiency of… it changes hugely but about forty per cent would be an okay guess. So, yes, in comparison to other energy formats wind turbines are fairly inefficient but, luckily, we’ve got a lot of wind so it doesn’t really matter.
Chris - But a solar panel’s only what twenty/twenty-five percent efficient?
Caroline - Yeah. So solar panels are fairly inefficient as well. That’s something we have to live with when we’re using forces to power things really.
I'm wondering, how much power is left over the surface area behind the turbine ? This shows the magnitude of interaction between the turbine and the environment. If I had to guess I would say something like 90%.
Wikipedia has clear description of Betz's Law
Yes, they are much more efficient. Typically the modern turbines operate at around 40% wind power converted to mechanical power.