Friday
Jan212011
by 
Bishop Hill
Bishop Hill BBC on the atmosphere
Jan 21, 2011 BBC Climate: other BBC radio is running a three-part series on the atmosphere (H/T Phillip Bratby), presented by Gabrielle Walker, a former climate change editor at Nature. The most recent episode apparently features Chris Rapley, the boss of the Science Museum, discussing radiative physics. Phillip isn't sure he's got his facts right.
Reader Comments (19)
Usual childish BBC rubbish I'm afraid. I do wish they would drop their ageism tactics and get some well rounded, intelligent presenters for this sort of thing or at least some who can ask the best questions.
Phillip was merely being polite in his statement, your grace.
The Irish term is "eejit".
In other news, there appears to be a new problem with the greens' beloved
birdshredderwind farms -- Wind turbines placed too close together (The Register)Rapley is now at UCL. His statement comes about 4 minutes in when he said words to the effect
Rapley is supposed to be a physicist, but he obviously wasn’t taught about the most fundamental law of physics.
There is other garbage, but that is the worst example.
Jeroen:
It has been known for a long time that putting turbines too close together has three effects:
1 Power output at downwind turbines is reduced
2 Increased turbulence causes increased fatigue at downwind turbines
3 Increased turbulence causes increased noise from downwind turbines
Is this wind turbine R&D being done on the tax payers dime (or 10p)?
Is this wind turbine R&D being done on the tax payers dime (or 10p)?
Yes, but it costs billions of pounds. Which, even in terms of American Billions, is a lot. But I fear it may be in terms of British billions.
Question for Philip Bratby:
Hi Philip. I have what must seem a pretty naive question about wind farms.
When wave generators are set out at sea, the water behind the generator string is pretty calm because of the amount of energy taken from the water. So, the same must be true of wind passing through a wind farm. The down-wind air must be pretty calm. My question is, are any studies made of the effects of lack of wind turbulence on the environment down-wind when, pre-turbine, the environment - and the weather - was shaped by the turbulence?
Philf
No, the turbine blades create a lot of downstream turbulence, just like downstream of an aeroplane wing. A turbine blade is just an aerofoil.
There are lots of videos of trubulence downstream of turbines. There is a superb photo of the turbulence downstream of the Horns Rev offshire wind farm at http://www.popsci.com/technology/article/2010-01/wind-turbines-leave-clouds-and-energy-inefficiency-their-wake
The down-wind air must be pretty calm. My question is, are any studies made of the effects of lack of wind turbulence on the environment down-wind when, pre-turbine, the environment - and the weather - was shaped by the turbulence?
The downwind side of a wind turbine is turbulent with gusty wind, yes some energy has been taken from it so there is a wind shadow but worse is the higher directional and speed variations which buffet the next turbine if it is too close. Effect is well known and extends to effect of hedges and trees.
You're mixing up turbulence with vortex shedding. Turbulent flow only happens when you exceed the critical Reynolds Number of c. 2100. In the same was as the vortices from a 747 can cause a light plane to flip, so it is that they will cause unusual loading on subsequent windmill aerofoils which will operate at lower efficiency.
If we are talking about turbulent or laminar flow, then yes. It is the large vortices, which can propagate many kilometres, which increase fatigue loading.
Thank you Philip (and others) for your reply. I understand now. However, I still have a small niggle: as the wind must have imparted energy into the turbine, the turbulence created by the aerofoils cannot, surely, have the same energy as the original wind, otherwise the turbine would have been creating energy (my physics is rusty, but IIRC you can't create energy from nothing). So, there may be turbulence but it does not have the potential energy of the original wind flow, does it?
Philf
A wind turbine will remove about 30% of the kinetic energy of the wind. So that well downstream of the turbine, when the mooth flow is re-established, the wind speed will be about sq root of 0.7 = 0.84 of the upstream speed (with no mixing from adjacent flow).
Well perhaps the BBC's flagship (by wind of course) science programme Horizon should settle any doubts about this and other related issues.
http://www.bbc.co.uk/programmes/b00y4yql
I nearly chocked on my cornflakes when I heard the trailer on the Today programme this morning.
I appreciate the quick turnaround Philip. That's cleared up a lot for me. And I still hate the darned things!
When I saw your response I had a quick flashback to my days of studying AC power theory so when I saw the 0.7 value I thought you were going to say there is an RMS value in the wind ;-) (Would have been a little poetic of nature to do that.....)
Jeroen B. @ Jan 21, 2011 at 3:06 PM
Excellent photo.
Take a gander at the change in albedo. Too many more of these things and we'll be wearing winter woolies here in Fiji, too.
Thanks for the link.
Oops, Jeroen B, my bad. Should be Phillip Bratby @ 4:24
(Don'tcha hate that?)
@Everyone: I'm happy I provoked a bit of good discussion
@Dave: no harm done ;-) I'll happily submerge to lurk again :)