low revving generator for water wheel 2

[waterwheeler]

I'm building a small water wheel and want to generate electricity. Not fussy about voltage or frequency but given limitations of site(2m head, perhaps 20 l/sec flow, so about theoretically about 0.5 Kw) it must be highly efficient. Everyone seems to agree that conventional generators are not suitable for water wheels because of the gearing necessary - expensive, noisy, inefficient and subject to heavy wear. There do not seem to be any very low revving generators available. It's outside my field so any information would be welcome.


Are there such low revving generators (say 100 rpm)?
If there are, where can I get one?
If not why not; is there a good reason why such a generator cannot work?
If it's possible but just unavailable, how can I make one?
Alternatively there may be a completely different way to turn low revvs efficiently into heat some 20m away?

 

[waross]

You may be able to use an automobile alternator. If you drive it with a timing belt or a micro-vee belt you may be able to get a 10:1 speed increase with out too much inefficiency.
A custom built generator for 100 RPM will probably be more inefficient than a belt drive.
respectfully

 

[MikeHalloran]

The poly-v belt drive will work just fine, if you keep the water away from the belt. You can get the pulleys for a ~2.5:1 speedup from a single automobile. Take the belt tensioner, too. If you want more speedup than that, use a bicycle wheel as the driving pulley.





Mike Halloran
Pembroke Pines, FL, USA

 

[davidbeach]

Big hydro units are built for 100 rpm or less, but it would take a huge amount of material to build a 500W, 100 rpm generator. For 60Hz output, a 100 rpm generator would need to be a 72 pole machine. Maybe a DC generator would be a better fit in this application.

 

[MacRae]

Check this place out. These guys brew their own power heads for renewable energy. Water wheel, windmill, etc. Suffice to say, a lot of low RPM stuff. Look through the posts. They are a very knowledgable bunch.

http://www.fieldlines.com

Ian


Ian Rines
Harris Corporation
Palm Bay,FL

 

[waterwheeler]

My hunch is that high revving generators are the general rule because modern power sources are high revving. High speed bearings and gear trains running 24/7 are likely to be noisy inefficient and subject to heavy wear. In this application I must have quiet, efficient generation from a very low shaft speed. I believe some windmills use permanent magnet generators and although a water wheel is considerably slower, I wonder if a large diameter version using the wheel itself taking advantage of the high peripheral speed could be the answer. The alternating frequency would probably be of no importance.

Anyone tried this?
If such a solution not possible, how efficient can a car alternator be at achievable revs?
My experiece with v-belts in this environment is that they get wet and slip; is there a belt dressing that would cure this?

 

[MikeHalloran]

A large diameter generator is going to be a full custom deal. You can buy a thousand junkyard alternators for what it will cost you.

A generator's output is very strongly affected by the air gap in its magnetic circuit. A minimum limit to that is imposed by the runout of the rotor and its bearings. You'd probably end up with an air gap of >6mm. An alternator's air gap is maybe a tenth of that.

Efficiency is a secondary issue. You want to maximize output, of course, but so long as there's enough flow available, you really shouldn't care how much water has to hit the wheel to achieve the output you get.

No belt dressing is going to offset the influence of water. Use timing belts. Here's a trick; you don't need a giant timing belt pulley on the wheel; just a big smooth pulley with 2..4 teeth. The limit on how big a drive pulley you can use is the length of the belt you can buy; find that first.

Consider a jackshaft and two belt drives. If you make the jackshaft long-ish, you can locate the alternator(s) remote from the waterwheel and just use the timing belt in the wet area. At the other end of the jackshaft, use the poly-v drive belts so you don't have to change the alternator pulleys.




Mike Halloran
Pembroke Pines, FL, USA

 

[itsmoked]

Mike is right. A big slow generator is the wrong approach!

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[waterwheeler]

Assuming I use a car alternator, what can I expect in terms of efficiency? And what sort of power can I expect at what sort of revs. Water is delivered to site via 60m of 6" plastic pipe, so I can't increase flow rate much above 20 l/sec without sacrificing head. Also this flow rate is the maximum that I can take off all year round and still retain some flow in stream.

 

[LionelHutz]

Auto alternators aren't very efficient. The general concensus among the small scale wind power crowd is that an unmodified car alternator is about the worst thing to use.

I would consider building your own permanent magnet 3-phase generator. Look at

http://www.otherpower.com

to start.

Peter

 

[waross]

The efficiency of an automobile alternator drops as the speed drops. At low speeds, 100% of the output may be required just to energise the field. At even lower speeds, the energy to supply the field may be more than the total output of the alternator. This is probably why automobile alternators are shunned by wind power specialists.
As the speed increases, the output increases and the field demands drop.
With a 5-fold speed increase over the speed at which the alternator is just able to supply its own field, the field current will be one fifth. The field losses will be 1/5^2, or just 4% of the losses at the lower speed. As a percentage of the output, the percentage will be even lower.
If you are able to maintain your speed at design level, the losses will come off of the top.
That is, a 500 watt alternator will be able to produce 500 watts output plus the small field losses.
In a wind power application, the losses come off the bottom. That is, in light wind conditions, the speed is slow and the losses are high. There will be a significant loss of production during light wind/slow speed conditions.
There have been alternators developed for wind applications. I remember seeing a design similar to automotive alternators in which the field was supplied by a permanent magnet rather than an electromagnet. Try Googling.
For your application, the efficiency may not be a serioius issue. There are significant differences between wind power and water power.
respectfully

 

[itsmoked]

Automotive alternators are fine if driven at speeds higher than 2,000RPM. The faster the better.(Below about 11,000RPM)

The nice part is that you can get field control.

http://www.microhydropower.com/staffpubs/staff7.htm

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[MikeHalloran]

A generic car alternator should start producing serious power above ~1000rpm. They can't be terribly inefficient, given their rudimentary cooling system.

First guess: set it up for ~2000rpm at the alternator.

If the waterwheel approaches the pipe discharge velocity, then a ~1.5m dia waterwheel would spin at ~13.6 rpm at 20l/s. You lose about half a meter of head through your 6" pipe at that flow.

So, you need a pretty good speedup. If the last stage is poly-v belts and pulleys from a car at ratio ~2.5:1, you still need a speedup of 58.5 between that jackshaft and the waterwheel. Which probably means two jackshafts, ~8:1 speedup each.

... which is probably why most folks don't do it that way. Just too many moving parts.


;------

A Google search on | hydro alternator | produced a lot of relevant hits:

- Commercial Pelton turbines that need 20' of head but drive an alternator directly.

- Some Pelton wheels ready to bolt to an alternator shaft.

- Something called a Turgo Stream Engine that claims operation at 2m/6' head, turbine available separately to bolt to an alternator directly.

- The ElectroVent hydro alternator.

- Brushless alternators.

- Much discussion of efficiency by alternator brand and type.

- Photos good enough for you to duplicate nearly anything that's been done, so you can make your own tradeoff of time vs. money.









Mike Halloran
Pembroke Pines, FL, USA