Generator effiency 'peakiness' vs RPM

[riverpower]

Hi,

   I'm studying the possibility of a tidal river turbine.  For this question, let's assume I have found the optimal TSR of the turbine, that I can adjust the output load any way I want, I need an AC output (need to send the electricity over 250 feet) and don't really care about the frequency of the output (assume I'll drive a resistance heater).

   My river changes speed over a 6 hour time window.   Let's say that I want to generate power with a turbine RPM ranging from 20 RPM to 60 RPM (a factor of three).  Clearly I'll need some sort of gearing (any recommended high efficiency gearing??) around 30 to 1.  

   While I'm clearly no generator expert, I'll try to ask an intelligent question.

   First, if I assume a fixed gear ratio, then the generator will be spinning from 600 RPM up to 1800 RPM.   Does anyone have a rough plot of generator effiency vs RPM (I'm sure it depends upon the type of generator)?  

   Second, does anyone build a gearbox with an adjustable gearing ratio (basically a 2 or 3 speed transmission).  I REALLY don't want to go there.

   I've found plenty of graphs of overall efficiency of turbine systems, but none of just the generator.  Since I can control the load, this means I can control the load on the turbine, but I need to learn more about the generator.

   I want to run the turbine at it's peak effiency, but don't want to get killed by the generator.

Thanks,

Dave

[Chagrin]

My thinking is that when you start grasping for efficiency over all RPM ranges you start putting yourself into situations where things just aren't economically wise. I see a lot of people trying to do similar with wind turbines and variable pitch blades -- and it's no doubt an interesting pursuit -- but usually a lot less time and energy could be put into building a larger or second turbine that would provide the same or more power.

Over a range of 600 to 1800 RPM for a PM generator or alternator there should be too little efficiency difference to even consider. For your water wheel, however, things are probably going to be a lot different. If you were going to variably-adjust anything  it should be the pitch of your wheel's (or propeller's) vanes.

[wpowokal]

Dave the experts will answer your questions but until then, perhaps you could tell us the power range you are concidering.

Do you want to build your own or use a comercial generator, if you build a duel rotor as often seen here you can make it as efficient as you want, how many dollars do you have. Principal losses in duel rotors is in winding resistance (current squared x resistance) so if you have big magnets and heavy gauge wire your efficency will be high. You can easily produce your own graph once you have a known generator output and winding gauge.

You second question also depends on power range, motor bike gear box small car gear box. Much depends on information not given, but for a fixed ground installation you may not need gearing if you make your own generator. A large rotor diamiter may achieve what you want.

allan down under

[riverpower]

Hi,

   I KNEW I forgot some info.  I'm considering using either a darrieus vertical turbine, or a propellor underwater turbine.  

    For my current thinking, a 200 W generator would match up with the turbine. In order to get enough total power, I'd use a few of these turbines + generators (if I tried making each turbine larger, the RPM would be even lower).

   I'm willing to make my own, and I've read up on making PM generators.  It doesn't look easy, but if that will give me what I need, I can do it

   If generators efficiency doesn't drop too much over the speed range I'm talking, then that makes things easier (just let it run at the different RPM's,and don't worry about it).  I was concerned if the generator was run far from it's expected 1800 RPM, that I'd lose too much that way.

   This setup will sit on a float in a river, moored to the riverbank.

Thanks,

Dave

[wdyasq]

Tale a look at the Gorlov turbines.

Ron

[Norm]

If you've got enough power to overcome the drag

of belt drive you could use the variable speed

from a snowmobile.