Max, want to take it up on a separate thread?
Yes, I thiink so as it busts the initial discussion and has nothing to do at all with that Classic which this topic is about...
A 4 meter rotor, facing 20m/sec wind, with "see" up to 60 kiloWatts. Naturally if it is overspeeding, its CP will not be high, but you cannot count on this. It is also a dynamic situation, not a static steady-state. That means that "all bets are off", and by that I also mean the "Betz is off", too. You can experience brief moments of capturing more power than 59% as long as you admit that the flow will collapse a moment later. So your generator, with the shorting switch closed, can impose a 37 kW load on the rotor, leaving 23kW for the mechanical brake to absorb.
I really cannot follow these assumptions and there is no logical explaination for that and hearing that, Betz in his grave would probably start spinning up as well ;-)
Yes, you can have moments when the wind decreases quicky and the kinetic energy stored in thje spinning prop is "used" thjat the "cp" is higher that 59 percent...but that really has nothing to do with cp and blade efficiency and in sch a situation we do not have a problem anyway. My calculations were based on a steady wind of 20 m/s. Of course in a real life situation it will not be steady, there might be gusts of 25 or 30 m/s and situations where there is only 15 m/s.
With those blades that are not twisted and have a constant width things are different thatn with these high performance blades anyway. People feel those ones are primitiv as they seem to be verry smple and do not look as much "alien" as others do....but thats wrong. If you compare them in a wind tunnel, of course they lose but in real life situations with gusts or even turbulences, they perform really well, sometimes even better than these high tech blades. There is a number of reasons for that but thats another story. In verry high winds that we cannot use anymore anyway, thats where they really can be a great contribution to safety. I already mentioned that some posts back. And it is not an assumption but a fact that they will lose cp and stall automaticly. I could do the math on that and tell you exactly what their maximum speed under load and off cload would be but that would be offtopic as well, plese just take it as a fact.
Those blades might "see" 60 KW but they can in no way convert this 60 KW to shaft power so we do not have to worry about that at all. Anyway, gusts usually come verry fast (which is why they are gusts...) and the blades will not be able to pick up speed quickly. Of course there wil be a rise in power but it will not be as much as it could be assuming that new higer wind speed and that optimum cp. The tip speed ratio reduces and the cp drops down...quickly.
In a real world situation this means that during a gust in the first place the power output will remain constand and then slowly start rising. As it does so, the TSR will get closer to optimum again, cp rises and the rise in power output will become more and more quickly. In the meantime there is plenty of time for a control mechanism to react in whatever way.
If it is the other way raound and the prop is overspeeding, the cp curve will decrease even more quicky until it is so all the winds energy is used to just keep the think spinning. In such a situation, profiles withe big wide tips will produce a lot more drag and losses that high tech blades and mister Reynolds has done a good job putting up a model to calculate for such things. At some point the prop is just not able to spin up more. If you design the blades to survive those forces, you are on the safe side anyway...without furling and shorting and there are some turbines working with this effect doing quite well.
If the blades run away the alternator when shorted will have a lot more power as well as the open voltage rises too. Chris for sure will never have any problems shorting his turbine, even with 2 phases he should be able to stop that beast in 20 m/s and again there still is furling which will help again as soon as the alternator is loaded again
an auto brake can stop a certain size of turbine, in a certain strength of wind, but no greater.
Of course there are limitations. Don't get me wrong with this, I do not consider such a brake to be my primary safety system! That would be furling and if the turbine runs away or does funny things, I will short it, just as chris does. For me such a disc brake system is to used in case of alternator failure or disfunction- a backup for my primary safety system.
Some guys here seem to be really convinced that their alternators are the most reliable and robust technical device on earth. Ok, regarding chris' alternator that might even be true ;-) but that one really has low resistance and spinns verry fast so that those high speed magnet plates provide plenty cooling to the stator.
Anyway...here on the discussion board and also in my own forum and on thebackshed I have seen manny burned stators and not all of them lock the turbine when they break...and not all of those burnouts happend in winds of 20 or 30 m/s
When I see the configuration of some of those alternators, especially for the big turbines with 17 or 20 feet in diameter...that sometimes really scares me. If I see those stator resistances often beeing far too high and people proudly stating things like " this is supposed to be a low wind turbine, I do not need the top end and high winds and my furling sets in early... so to get an early cut in and good low wind performance I kept stator resistance high for a good match"
And often those are the people also stating things like " yesterday my turbine run away a few times in winds of 20 mph just like the other week...something is not right with my furling...I'll try to fix that within the next weeks" and have burnouts and wonder why...
These things do not only happen at winds of 30 m/s ! 12 or 15 m/s second over a longer period of time, especially for battery bound systems with high losses inside the stator windings is enough to kill most of those alternators in case furling ( this black science ) dies not do its job properly....
This is where an alternator independant brake can definitively help....providing that you have one installed.
I started thinking about it when a friend from spain talked to me about hat happend to him. In high winds of about 14-16 m/s something funny happend to his alternator in his 3 m machine. Some kind of short circuit occured inside his stator and the machine didn't run away but an incredible inbalace occured and the whole tower started shaking like hell. He tried to stop the turbine by shorting the alternator but it did not work and he was scared of approaching the machine and the sometines fully furled machine would again and again turn around facing the wind. An hour later he lost his first blade which shook losse due to those heavy vibrations and then the whole tower collapsed smashing his green house. Luckily nobody was hurt. He didn't have a break on that turbine...now he has one on all 5 of them. So thats a real life story where a disc brake would most likely have cleared the situation and turbine, tower and green house would have survived.
I am sure there are a lot of folks out there with similar experiences...
So for me such a braking system is a safety feature I would not like to miss on turbines of that size, not only to provide safety to the turbine but also for the people living around it...
Max
