good posting sparweb.... and good information for people to consider. a couple comments.
Let's take, for example, an alternator built by Dan B, who was kind enough to test it and publish the results: http://www.fieldlines.com/story/2007/2/13/14052/0763
'Now for the math:
Charging current = 750W / 56V = 13.4 Amps
Stator resistance loss = [3/4] * (13.4A)^2 * 3 Ohm = 404 Watts'
Like Flux said recently - we can probably assume the resistance to be even higher in an AC situation so it's likely worse than that.
'Rectifiers: (assume 0.7V drop across each) = 2 * 0.7V * 13.4A = 19 Watts'
Yes - that makes sense - and this is more of a problem at lower voltage.
'(friction in trailer hubs is not negligible)'
Hub: (assume 12 inch-pounds torque) = (1 ft-Lb)(300RPM)/ 5252 * 746 = 43 Watts'
I think it is damned near negligible (especially if you don't use a seal). I can put 1/4 pound of weight 4" from the center of the alternator and it turns the alternator. Much much less than 1 foot pound of torque required to turn the alternator. My guess is it's more like 5 - 10 Watts there.
Total power that must be put into the alternator:
750W + 404W + 19W + 43W = 1216 Watts
The generator, at this point, is 62% efficient. Looking at the power curves that DanB worked out, this is in the middle of the working range.'
I would say that for that alternator, 750W output is about 'rated' - occasional peaks will be higher but its no good for more tha 750W without burning out.
' I'm sure it would work well in a stronger wind, and it will also produce power in lighter winds. '
Its much muchmore efficient in higher winds.
' If you care, for some reason, about efficiency, then take heart that it will be more efficient at slower speeds. But, it is also putting out less power, so it doesn't matter.'
Most of the energy from a wind power system (especially a battery charging system) comes in at lower winds when the machine is actually quite efficient.
'How can the efficiency of a system be improved?
Making better blades that have a 35% power coefficient will increase the power output by 5% in the same winds.'
Right - not really a big deal in my mind.... at some point it becomes easier to build a larger machine than it does to split hairs over a tiny improvement in blade efficiency with fancy airfoils etc... There is always a compromise to be made somewhere, I suppose everyone finds their own.
' Whittling down the resistance and friction losses by 15% (somehow) will only improve power output by 2%.'
Yes - and unless you some sort of MPPT system or other power electronics involved - the machine is matched to the load by resistance and you must have a certain amount of inefficiency in order to keep the blades out of stall. I can build a much more efficient alternator - but to keep it working well with the blades, I have to add inefficiency somewhere else in the line - or else... I've have a very efficient alternator and the blades would be forced to such a low speed they'd be quite inefficient.
'What if Dan's system ran at 24Volts, or worse, 12Volts? The power output would be seriously reduced.'
Not really - slightly (assuming we wind a stator that's appropriate for the voltage). If you scale up the line appropriately to deal with the greater current of a lower voltage system then all things should be equal except for rectifier losses. at 12V - rectifier losses are about 10% and there's no getting around it.
' Efficiency would also plummet. '
Not if you wind the stator correctly.
' If you want to have a more "efficient" system, then kick its voltage up an increment.'
Or use larger conductors. I hate 12V systems - it becomes impractical in my mind to have much more than a 10' diameter turbine at 12V. But - you can do it and the efficiency isn't that much worse.
'My working wind turbine is a motor conversion. Conversions suffer from eddy current losses in the stator, which sucks an extra 50 watts out of my system that can't be turned into useful power. That's unfortunate, but the thing is rugged and won't cook its epoxy.'
If you build an efficient enough machine and furl at the right time 'cooking the epoxy' is not a concern. All different kinds of wind turbines burn out -axial flux machines with potted stators do have more trouble getting rid of heat but again - it can work fine if you do things right. Stators burn out when people build alternators too small/too inefficient and it's not just an axial flux problem. You see it more with axial flux machines around here because that's what most people have built... I've seen plenty of commercial wind turbines burnout and if there were hundreds of 'motor conversions' out there you'd see them burning out all the time too. Not because the basic idea behind the alternator is a problem - just because somebody didn't get something right... bad match of alternator to blades - bad furling system.... burned out rectifier... lots of things cause it. We've made over 50 wind turbines and I've seen about 5 burnouts. Some were due to bad design - one was due to 'user error' (running heaters directly with out good control) - one was due to a burned out rectifier so it ran single phase/burned out.
' I'm currently running at 12 Volts, myself, but when I plug in the new batteries, I will be working with 24Volts (maybe 48V in the future).'
Good plan!~
'The bottom line: make a good alternator - and make GREAT blades, and you will be happy with your windmill! All the other little things are small in comparison, when you've built a good system.'
Agreed. But don't get to thinking that the most efficient alternator is the best thing - there are compromised to make all around this stuff. Lately I've been trying to build more efficient machines that tend to stall blades and then tuning the system with resistance down stream. Doing so doesn't gain anything for efficiency but it pretty much solves the problem of a hot stator... at the cost of a bit more magnetic material.
Good posting and it is fun to think about efficiency - with simple systems that are matched to the load with resistance you can have very good efficiency in low winds but forget about it high winds... it's impossible. It's also fairly unimportant - again, most energy comes in low winds. If you can do OK there then odds are in higher winds you have more power than you want/need anyhow. At least that's my experience.