Keep in mind there is very little energy available at lower wind speeds and the complexity needed to try and capture it is not cost effective...
So after I read your comment, I immediately typed out this reply:
Ok, I'm mindful of the fact that there is less potential energy in a slower wind. Let's also assume for the sake of this discussion that costs and complexity are not relevant to our interests for now. I'm wondering if an industrial scale generator provides enough rotational resistance to make it impossible to dynamically gear up in lower wind speeds? It seems that, for example, if you had a constant 8m/s wind, the power output capabilities at that time would not be limited by torque, as I would think that the rotor would have plenty of torque available to turn the generator at a much higher speed if the gearing was different (i.e. that the generator didn't inherently provide so much resistance as to slow or stop the rotor).
However, before I submitted it I had this lingering feeling that what I'm suggesting is trying to break the law of conservation of energy. I went back to the Betz Limit Wikipedia article, and somehow from there ended up here:
https://www.windynation.com/jzv/inf/how-much-power-will-wind-turbine-produce which outlines the basic "power in wind" formula which ultimately led me to
https://en.wikipedia.org/wiki/Wind_turbine_aerodynamics. I found some power output numbers here:
http://www.wind-power-program.com/large_turbines.htm, specifically this image
, and applied this wind power formula to these numbers after looking up the specs on the Vestas V90 turbine (these graph numbers are supposedly from the V90).
What I learned is that this turbine leaves
very little on the table, even in low wind. If someone has some more reliable data for an industrial scale turbine, that would be awesome, but for the sake of hopefully providing some insight to other newbies like myself, lets take a look at the numbers we are given.
Wind power = 0.5 * air density * swept area * wind velocity
3Because the Vestas V90 in question was offshore, we will use standard air density at sea level and at 15C which is 1.225kg/m
3The V90 has a swept area of 6362m
2The graph looks like the V90 was logged at producing about 900kW at 8m/s wind speed, so lets figure the theoretical amount of power the wind contains.
P = 0.5 * 1.225 * 6362 * 8
3P = 1,995,123W or P = 1,995kW of potential wind power in 6362m
2 of 8m/s wind.
Looks like the turbine is missing an awful lot, right? Well, this is neglecting the Betz Limit, which we must take into account. According to Betz, of this 1,995kW, we are only able to physically capture 1,177kW. Once again, the Vestas V90 produced ~900kW at this wind speed, and when we divide total potential by output (1995/900), we get 45.11%. According to that webpage, this fellow says "The peak efficiency is close to 45% and the design achieves this efficiency in the range from 8 to 10 metres per second," thus the estimate of 900kW must have been close.
What I learned was that modern turbines already do one hell of a job capturing energy in low speed wind. I don't know if other turbines have similar outputs as the V90 in the 8-10m/s range, but if they do, there is very little energy that isn't being captured.
To go back to my idea of altering the gear ratio at low wind speed, I now feel that this would have an insignificant effect on total turbine output. Furthermore, I now believe that altering the gear ratio in low wind and expecting the rotor to not encounter an additional amount of resistance (of a sum that didn't allow power output to increase) is a bit crazy. The only thing I was right about was that what I was suggesting was trying to neglect the law of conservation of energy. The generator
must provide some amount of resistance that prevents the rotor from turning faster, and trying to maintain the same rotor speed and alter gearing such that you ended up turning the generator faster would seemingly be making an attempt to create more power from nothing.
If anyone that is more familiar with this topic than I am could provide some more insight, I would greatly appreciate it. You fellows have been tremendously helpful thus far, and once again, thank you so much.