5 ms·

Let’s say I have ten units of wind coming onshore. What would that change to if they built a wind farm right offshore?

by bilsbie 3y ago

bandyaboot 3y ago
That’s not even close to answerable without many more variables specified.
- jacquesm 3y ago
  Not in a quantitative way anyway, but in a qualitative way it is actually trivial because the wind at sea level behaves fairly consistently once you know its speed. Betz basically says you won't be extracting more than 59.3% of the power in the moving air so the remainder will always be available just behind the turbine. Close to the ground wind isn't laminar so what you'll see is that the 'rolling' air will draw in new air from higher layers just behind the machine, a short way inland you'll have a hard time proving there is a windfarm upwind other than through sound and visuals. There is a lot of air moving around (if you could visualize it try thinking of 1 kg bricks spaced out 1 meter in every direction, and all of them are moving except for when there is absolutely no wind) and the tiny little layer that we siphon off some power from is utterly insignificant compared to the whole. https://en.wikipedia.org/wiki/Betz%27s_law https://en.wikipedia.org/wiki/Betz%27s_law
  - bandyaboot 3y ago
    Very interesting. Thanks.
talldatethrow 3y ago
I live near a ton of windmills in the hills 50 miles east of SF. In highschool we would go to them, although technically illegal. You stand on the ground, with the windmill churning away 100 feet above your head. It would obviously still be windy as hell near the ground. I have a feeling that if you had 10 units of wind, and placed windmills in a linear line for a linear mile as tightly as you could... Just a small distance away you would still have 9.99 units of wind. The density of wind capture devices relative to the overall mass of wind in the general area is so low I don't think you could make a big difference. If anything, I'd worry more about solar farms that cause massive heat soak by placing black solar panels near ground level, where instead it would be light color dry dirt and dry greens that are a lot more reflective.
jacquesm 3y ago
Nothing would change from about a mile behind the line of windmills, but in the immediate wake of the machines you'll have a calming effect, though flow will be more turbulent than laminar. Spiral shaped vortices coming off the back of the rotors would eventually dissipate in the air that gets added from above. Note that windshear is such that any kind of disturbance in an otherwise open field would create a wake that eventually dissipates because wind rolls across the terrain. You can see similar dissipation effects in the wake of an ocean going vessel but it dissipates far faster because air has less mass, and on the ocean there is less flow to have that kind of windshear effect (but then from the water interacting with the bottom, which usually happens much further down).
- elric 3y ago
  Read a pretty dishartening article recently, about how offshore turbines reduce water oxygenation, and in turn probably have a negative effect on sea life. I guess there really is no free lunch.
  - jacquesm 3y ago
    I've seen that but it was quite speculative, did you see something that showed experimental results?
adhesive_wombat 3y ago
Modern turbines can extract at most about 50% of the energy passing though the disc. The physical limit is the Betz limit, just a shade under 60%. Even if you could arrange them just so (which you can't, because circles don't tessellate and wind directions vary), you'd only be able to get that from the wind at the height of the discs, from a single rank. Mixing from air that went over (or under) the turbine entirely would restore quite a bit of the wind after some distance. The biggest turbine in the world, installed about a month ago off the Fujian coast, has a disc 250m across, centred about 150m from the sea, so it's still only a small fraction of the depth of moving air. By one estimate there's about 250TW of "extractable" power (after which you can add turbine capacity but the whole ensemble doesn't produce more power, and the global wind speeds after saturation would be 50% lower). Humanity uses roughly 10TW, so even if we used wind for everything, it would only be a small fraction of the energy in the moving air. That paper also has calculations of the effects of a single wind turbine averaged across a 5km area (tl;dr: sub-0.5% downstream wind speed change) https://web.stanford.edu/group/efmh/jacobson/Articles/I/SatWindPotential.pdf https://web.stanford.edu/group/efmh/jacobson/Articles/I/SatW...
- seanmcdirmid 3y ago
  Rather than plant the gobi with trees (which need lots of water), could China just put in a bunch of wind turbines to prevent erosion? Like, is a wind turbine better than a tree at blocking the wind (well, it also doesn’t have a root system to keep dirt in place). In eastern WA, we have special trees planted to block wind (mostly to reduce erosion), I wonder if those could be turbines instead? (Although, as long as water isn’t a problem, we should probably still use trees)
  - adhesive_wombat 3y ago
    I don't think the primary aim of the project is erosion control, but the Gobi desert is already the site of the largest on-or-off-shore wind farm in the world[1], by many multiples, with a planned 20GW nameplate capacity, 10GW online now (next on the list being 2GW at Markbygden in Sweden). I suppose if there is anywhere in the world where wind power installations can cause a significant reduction in wind speeds, it would be there. [1]: https://en.wikipedia.org/wiki/Gansu_Wind_Farm https://en.wikipedia.org/wiki/Gansu_Wind_Farm

Let’s say I have ten units of wind coming onshore. What would that change to if they built a wind farm right offshore?

bandyaboot 3y ago

That’s not even close to answerable without many more variables specified.

jacquesm 3y ago
Not in a quantitative way anyway, but in a qualitative way it is actually trivial because the wind at sea level behaves fairly consistently once you know its speed. Betz basically says you won't be extracting more than 59.3% of the power in the moving air so the remainder will always be available just behind the turbine. Close to the ground wind isn't laminar so what you'll see is that the 'rolling' air will draw in new air from higher layers just behind the machine, a short way inland you'll have a hard time proving there is a windfarm upwind other than through sound and visuals. There is a lot of air moving around (if you could visualize it try thinking of 1 kg bricks spaced out 1 meter in every direction, and all of them are moving except for when there is absolutely no wind) and the tiny little layer that we siphon off some power from is utterly insignificant compared to the whole. https://en.wikipedia.org/wiki/Betz%27s_law https://en.wikipedia.org/wiki/Betz%27s_law
- bandyaboot 3y ago
  Very interesting. Thanks.

talldatethrow 3y ago

I live near a ton of windmills in the hills 50 miles east of SF. In highschool we would go to them, although technically illegal. You stand on the ground, with the windmill churning away 100 feet above your head. It would obviously still be windy as hell near the ground. I have a feeling that if you had 10 units of wind, and placed windmills in a linear line for a linear mile as tightly as you could... Just a small distance away you would still have 9.99 units of wind. The density of wind capture devices relative to the overall mass of wind in the general area is so low I don't think you could make a big difference. If anything, I'd worry more about solar farms that cause massive heat soak by placing black solar panels near ground level, where instead it would be light color dry dirt and dry greens that are a lot more reflective.

jacquesm 3y ago

Nothing would change from about a mile behind the line of windmills, but in the immediate wake of the machines you'll have a calming effect, though flow will be more turbulent than laminar. Spiral shaped vortices coming off the back of the rotors would eventually dissipate in the air that gets added from above. Note that windshear is such that any kind of disturbance in an otherwise open field would create a wake that eventually dissipates because wind rolls across the terrain. You can see similar dissipation effects in the wake of an ocean going vessel but it dissipates far faster because air has less mass, and on the ocean there is less flow to have that kind of windshear effect (but then from the water interacting with the bottom, which usually happens much further down).

elric 3y ago
Read a pretty dishartening article recently, about how offshore turbines reduce water oxygenation, and in turn probably have a negative effect on sea life. I guess there really is no free lunch.
- jacquesm 3y ago
  I've seen that but it was quite speculative, did you see something that showed experimental results?

adhesive_wombat 3y ago

Modern turbines can extract at most about 50% of the energy passing though the disc. The physical limit is the Betz limit, just a shade under 60%. Even if you could arrange them just so (which you can't, because circles don't tessellate and wind directions vary), you'd only be able to get that from the wind at the height of the discs, from a single rank. Mixing from air that went over (or under) the turbine entirely would restore quite a bit of the wind after some distance. The biggest turbine in the world, installed about a month ago off the Fujian coast, has a disc 250m across, centred about 150m from the sea, so it's still only a small fraction of the depth of moving air. By one estimate there's about 250TW of "extractable" power (after which you can add turbine capacity but the whole ensemble doesn't produce more power, and the global wind speeds after saturation would be 50% lower). Humanity uses roughly 10TW, so even if we used wind for everything, it would only be a small fraction of the energy in the moving air. That paper also has calculations of the effects of a single wind turbine averaged across a 5km area (tl;dr: sub-0.5% downstream wind speed change) https://web.stanford.edu/group/efmh/jacobson/Articles/I/SatWindPotential.pdf https://web.stanford.edu/group/efmh/jacobson/Articles/I/SatW...

seanmcdirmid 3y ago
Rather than plant the gobi with trees (which need lots of water), could China just put in a bunch of wind turbines to prevent erosion? Like, is a wind turbine better than a tree at blocking the wind (well, it also doesn’t have a root system to keep dirt in place). In eastern WA, we have special trees planted to block wind (mostly to reduce erosion), I wonder if those could be turbines instead? (Although, as long as water isn’t a problem, we should probably still use trees)
- adhesive_wombat 3y ago
  I don't think the primary aim of the project is erosion control, but the Gobi desert is already the site of the largest on-or-off-shore wind farm in the world[1], by many multiples, with a planned 20GW nameplate capacity, 10GW online now (next on the list being 2GW at Markbygden in Sweden). I suppose if there is anywhere in the world where wind power installations can cause a significant reduction in wind speeds, it would be there. [1]: https://en.wikipedia.org/wiki/Gansu_Wind_Farm https://en.wikipedia.org/wiki/Gansu_Wind_Farm