Generator for microhydroelectric power generation station?

[tsgrue]

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People That Know Electricity,

I am working on the design for a new microhydroelectric power generation station. The station will be located at an existing pond in a small, urban watershed. The pond has an existing dam with 9+/- feet of available head. The base streamflow is 3+/- cfs and the maximum flow that will be utilized is 23+/- cfs. This translates to roughly 1.5kW base power potential and 14kW maximum power potential.

The proposed system schematic is as follows:

Reservoir --> Waterwheel --> Generator --> Inverter --> Grid

The most suitable power converter in terms of overall performance (cost, efficiency, maintenance) is a breastshot waterwheel (18+/- ft diameter). Breastshot waterwheels are most efficient when converting potential energy into 'mechanical rotational energy' (as opposed to kinetic energy to 'mechanical rotational energy') – in practical terms, this means the waterwheel is most efficient at slow rotational speeds. Ideally, I'd like to have a base waterwheel rotation of 1 RPM, which would roughly result in a maximum rotation speed of 7 RPM. Assuming I could achieve a 40:1 gear ratio, this would be a base generator shaft speed of 40RPM and a maximum generator shaft speed of 280 RPM. As this is an urban watershed, the flow (and thus power output) is highly variable. (I could go beyond the 23+/- cfs input, but I am limiting it to this due to various civil/mechanical reasons).

With all that, I have the following questions:

1) Is the "[DC] Generator --> [DC-to-AC] Inverter" portion of the system schematic what I should be looking at or is there a better system? I've seen various systems, but this seems to be the most straightforward, efficient, and (assuming I can find the generator) least expensive and most viable option.

2) Does anyone have a suggestion for the generator I should use? I've looked around, but haven't found much. I am guessing that I will need to find a high torque / low speed (multi-pole) DC motor used for mixing or grinding operations and run it as a generator.

I've searched the posts and haven't found anything covering this...

Any help is greatly appreciated!

.



tsgrue: site engineering, stormwater
management, landscape design, ecosystem
rehabilitation, mathematical simulation

http://hhwq.blogspot.com

 

[Skogsgurra]

The most common generator (if you are grid tied) is the simple induction machine aka squirrel cage induction motor. It is best used in applications where speed is constant and torque is dependent on water flow.

If I understand your breast wheel correctly, it does indeed produce a torque that increases with available water.

Simple, good efficiency, low cost.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[waross]

I would look into installing a very large diameter drive wheel on the side of the water wheel and using an automotive generator with a very small diameter drive shiv. Use a toothed timing belt.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

Bill those top out at about 1.4kW though.

18ft to a 2in pulley would be 108 ratio.

You'd want the alternator to turn about 8,000rpm.

8000/108 = 74rpm. Still too fast for the desired ~4rpm.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[MikeHalloran]

You'd want the first pass to be roller chain because you can't get a timing belt that's big enough. They are built up like a tire and cured on a toothed drum, then pushed off and sliced to width. They can't be effectively spliced. The big sprocket could be a wheel with a plain rib, and a tooth every couple of feet.

The necessary jackshaft could drive an arbitrary number of alternators with poly-v belts, located remote from the water spray.

You might need two jackshafts to get the ratio.

And a fairly serious frame to deal with all the tensions.

1 rpm is pretty slow, and 9 ft is not a lot of head. The jackshafts and such add a lot of complexity and cost to a low power system.

OR

To reduce the number of very large and expensive wheels to be fabricated, you might make the first pass a car tire pressed against the rim of the waterwheel, at the top. You'll need a good rain tire to get wet traction, and a pretty decent weight on the tire's axle to get enough normal force. From there you can use chains or timing belts to the jackshafts as appropriate.

OR

Build a written-pole generator into the rim of the waterwheel. Maybe the patent holders could be interested. The presence and uncertain nature of the water presents a serious challenge, as does a development cycle in both time and money.










Mike Halloran
Pembroke Pines, FL, USA

 

[ScottyUK]

A 6-pole induction machine would run at a base speed of about 1000 rpm, 8-pole at about 750 rpm. Either of those would be realistically compatible with a 7 rpm output from the prime mover using a chain drive to inmitially raise the shaft speed and a belt drive to the motor once the shaft speed is within a useable range. I suspect gearing sized for maximum output would be too lossy at low loads to be a practical proposition. Induction machines are about as cheap as you can get for a given output and are simple, long-lived, and robust.

The induction machine would work well with a regenerative drive. A regen-capable drive at (say) 15kW rating is not a common product although there are certainly options.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[waross]

The older automotive alternators were capable of over 100 Volts at high speed. Two or three automotive alternators from the auto-wreckers running over-voltage will certainly be the most cost effective.
I have heard descriptions of old flour mills converted into dwellings with the water wheel restored. Many used suitable gearing to spin an induction motor/generator as Gunnar describes. The controls are fairly simple and an inverter is not needed.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[sreid]

Adding to your original plan.

Water Wheel 30 to 300 RPM

1500 Watts = 2 HP

35 lb-ft at 30 RPM

3.5 lb-ft at 300 RPM

Generators like high RPM-10 to 1 step up transmission. Two small automotive gearboxes in series in low gear, driven "Backwards."

Generator: Brushless Permanent Magnet Motor runing at 300 to 3000 RPM.

Three phase full wave rectify gives 30 to 300 VDC. Current is 50 Amps at 30 VDC.

Boost paower supply converts 30 to 300 VDC to 300 VDC.

Finally, a commercially available Inverter to get AC Power. Possibly a Delta Electronics unit.

http://www.solar-inverter.com/cps/rde/xchg/SID-2A49FAE7-6645F03F/des-solar-na/hs.xsl/293.htm

 

[potteryshard]

Please forgive me for not being able to remember the proper name for it, but I recall from reading my Grandfather's electrical manual (Hawkins, 1917) that alternators can be constructed so that both the magnets creating the field and the coils generating the output are both stationary. The rotor consists of soft iron segments which 'steer' the magnetic field to one coil or another as it turns.

For this water wheel application, I envision a segmented stainless steel (non-mnagnetic) ring in either a radial or an axial configuration. Welded to this ring are steel teeth. This assembly is attached to the periphery of the wheel, and rotates with it.

Engaging with this rotor is a generator package that resembles a disc brake caliper; it does not need to wrap entirely around the circumference, and probably would need to be only a couple of feet long. It contains the entire magnetic circuit.

Permanent magnets are arranged along one side of the 'caliper', and the generator coils along the other side of the rotor gap. The 'backbone' of the caliper wraps the magnetic circuit back around to the permanent magnets. The coils and associated cores are arranged at the same pitch as the rotor teeth; the permanent magnets are of consistent orientation, and create what would be a continous magnetic 'curtain' field.

The rotor teeth concentrate this field into a area of a coil width, and 'steer' it to successive coils.


This type of alternator has some drawbacks; it requires more copper as half the coils are idle at any given time, and it is a bit more lossy than the usual, but the extra losses in the alternator seem unlikely to exceed the heavy losses requisite with high-ratio step up gearing. And, the mechanical simplicity could be very appealing; the caliper contains all electrics, and can be readily potted. The rotor carries no circumferential magnetic flux, and so can be constructed of segments.

Now if I could only remember what this type of alternator is supposed to be called...

 

[LionelHutz]

The first thing that came to mind was that achieving efficient operation over such a wide water flow and power ratio would be difficult. The transmission to handle 14kW might use a fair percentage of the available base 1.5kW which could otherwise be used to generate useable power.

After a few of the other posts, I was thinking that you could install some strong permanent magnets around the rim of the wheel with a "caliper" type 3-phase generator encompassing a portion of this rim of permanent magnets. Rectify the output to produce a DC output which would be proportional to the wheel speed. Then, it might be possible to use a solar inverter or a Windyboy inverter to produce grid power.

 

[ScottyUK]

I am amazed at the determination to custom build specialist electrical machines and then try to deal with their problems! Some interesting ideas for sure. If cost and reliability are important then commercially available machines are likely to be the best solution.

So far we haven't had a synchronous machine and a torque converter to drive it suggested, but I'll throw that in with the other 'interesting' ideas!


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[MikeHalloran]

Don't forget the oil cooler for the torque converter.


Mike Halloran
Pembroke Pines, FL, USA

 

[LionelHutz]

OK, I'll be blunt and say that an automotive generator is completely ill-suited for a slow speed application such as this. They require a fairly high rpm and they are not efficient.

Not far behind would be any other dc generator that includes brushes simply due to the maintenance required.

The simplest way to achieve grid tied is an induction generator and to forget about allowing the wheel speed to vary. You could also do a 2-speed motor if you really wanted to allow different speeds.

A purpose built permanent magnet generator would also work very well.

If you look at wind power the solutions used there would be similar for the same reasons (relatively low speed, varying speed, varying power levels).

 

[tsgrue]

.

People That Know Electricity,

I appreciate all your comments and expertise. It is very beneficial, to say the least. A few bits of information:

1) It is possible to design and install a waterwheel which would have constant speed over the input flow ranges, but in practice it would be rather difficult (given the complexity of the fluid phenomena involved) and very expensive (given the relative precision required and limited skill set in machining such a beast). Active governing would work, but that would unnecessarily reduce efficiency if a direct generator solution can be found. So, a ‘fixed speed’ waterwheel would be outside the range of cost-effectiveness for this project, though potentially feasible for higher power projects or projects with a much higher likelihood of being reproduced elsewhere.

2) If a suitable brushed DC motor with pretty good efficiency could be found, the maintenance would likely be acceptable.

3) Off-the-shelf combinations of components are very much preferred, though a 'custom' generator would fit the bill if such could be had for a reasonable price. I am clueless as to what such a custom generator would cost.

4) I've looked at wind power solutions. The small turbines are too high rotational speed and the large turbines are too low rotational speed (and way high power and cost).

5) I am thinking that two or more lower power generators/motors connected along a single shaft might be a viable solution. These seem to be more readily available, though I haven't seen many with shafts that run all the way through the equipment - so each would probably have to be driven by gears off a main shaft.

Any additional guidance is more than welcome.

Thanks again!

.



tsgrue: site engineering, stormwater
management, landscape design, ecosystem
rehabilitation, mathematical simulation

http://hhwq.blogspot.com

 

[rasevskii]

For Potteryshard:

That was an "Inductor" type of AC generator made by the Stanley Electric Co. in Pittsfield Mass., up to around ? 1910 or so. It had a large statioery DC field coil all round the outside of the machine between two stators, the stator coils
were in slots in the cores and in non-magnetic tubes between the stators. There were no slip rings or brushes. It was also built as a synchronous motor typically driving a DC generator (MG set) in substations.

It was used in many hydro plants in California, one unit still exists at a small plant known as Centerville Power House (P. G. & E.) about a 900Kw unit.

Google "Electra Power House" (shut down around 1950)and you will see historic photos of that plant which had five Inductor generators installed about 1902, which transmitted power to San Francisco at 60KV, over a hundred miles...

Yes, I am a historian, anybody else like that out there??

rasevskii

 

[burnt2x]

Have you tried thinking about using a crossflow turbine? Please Google "crossflow turbine" or "banki" for details!

If it's a low-head application, you could achieve a 240 rpm and utilize a jackshaft to bring speed to 1800rpm.
Example:
turbine-side gear = 62 teeth,
jackshaft = 21-52 teeth; and
driven induction motor -generator shaft = 21 teeth
Final generator rpm = 240 X 62/17 X 52/17 = 1800rpm!

As mentioned, employing a jackshaft brings the generator away from the water spray! And most of all, you can find a lot of 1800rpm motors. Hope this helps.

 

[waross]

If you use a synchronous or induction generator with a greater capacity than the prime mover, it will load the water wheel enough to limit the speed without any other controls.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rasevskii]

tsgrue:

Have a look at the website:

http://www.hydrowatt.de

for examples of delivered systems by that company involving classic waterwheels and small turbines. (I am not connected with them). The German you can translate using babelfish if needed.

rasevskii

 

[potteryshard]

Rasevskii: I was thinking that it was called either an inductor or a reluctor alternator. I do remember that hetero- and homopolar versions were discussed... The one I proposed was homopolar.

Lionelhutz: A flying magnet type of design was my first thought as well, but two problems arise... The permanent magnets are the most expensive part of the build, and it would use a lot. Also, the first bit of scrap steel that came along would bring the whole rig to a crunchiing stop.

You are right about the one-off wind turbine generators serving as a model. Where commercial products are not readily available that provide a good match to the input parameters, the clever builder steps outside the box and finds a way to roll his own. Some of the home-built wind generators are extremely simple yet effective.

ScottyUK: Certainly a home-brew generator will give up some efficiency points to a commercial product. I would argue that it would be beneficial to give up those efficiency points in a direct-drive generator than in a step-up gear train as it would result in much reduced system complexity.

Tsgrue: A breastshot wheel uses the weight of the falling water to load the downhill buckets to create a torque. Can the buckets be deepened to contain most of the peak flow? It seems that this would increase torque with greater flow more than increase speed. This more constant speed would in turn seem to work better with an induction generator.

Overall folks, it seems to me that if the intent of this exercise is to generate the most electricity, some form of turbine might be better as it would be inherently of much higher speed. If the intent is for aesthetics, the wheel is more attractive, but I wouldn't plan on getting large amounts of electricity out of it.

To make use of the wheels energy via a commercial product is going to require step up gearing on the order of 1:1800. Intuitively, I'd think one would be hard pressed to attain 15% efficiency on the gear train. I would also think that a great deal of changing final ratios to get the wheel speed tuned to the just over synchronous induction generator speed required.

I would also worry about torque pulsations... The impact of the flowing water against the wheel buckets will create periodic torque variation. That pulsation, multiplied by the huge step up ratio would seem to spin up the generator which could then overrun the slop in the gear train slamming the inertial load of the high-speed generator back and forth against the drive train slop.

One could always use the wheel to winch a series of heavily weighted magnets to the top of a tower whereupon the magnets would be released to fall through coils, and the resulting electricity stored in a battery. Non-turbine waterwheels are inherently low speed devices. Electrical generators are inherently high-speed devices. To get them to play together is going to require extraordinary effort.

 

[OperaHouse]

Always amazed at some of the cheap and dirty solutions out there. I saw an article where a guy used a 4HP hot tub pump to generate about 900W of electricity from the existing induction motor. Having rebuilt my hot tub pumps many times, I wouldn't have expected such good results from the impeller.