Generator for microhydroelectric power generation station?

[tsgrue]

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Experienced Hydro Folks,

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.

For a variety of reasons, the preferred mechanical power converter for this project is a breastshot waterwheel (18+/- ft diameter). As you may know, 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).

There are two proposed system scenarios, indicated as follows:

A: Reservoir --> Waterwheel --> Generator --> Grid

B: Reservoir --> Waterwheel --> Generator --> Inverter --> Grid

I am comfortable with the civil and mechanical aspects of the system and I'm trying to work out some of the electrical aspects. So, with all that, I have the following questions:

1) In scenario A, I am looking at employing an asynchronous generator, letting the grid regulate the AC generation. I have not been able to find an asynchronous generator that will work in this combination of power and rotation speeds. Does anyone know if such are available or, if not, is there a suitable induction motor that could be run as an asynchronous generator for this?

2) In scenario B, I have not been able to find a DC generator that will work in this combination of power and rotation speeds. 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. Does anyone know if any suitable motors or generators are available in this operation range?

3) I've looked at wind power generator solutions. The small turbine generators are too high rotational speed and the large turbine generators are too high power and cost. 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. Does anyone know cost estimates for that?

4) Given availability and price limitations, would two or more lower power generators/motors connected along a single shaft be a viable solution?

Thanks very much!

tsgrue

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[hydrosun]

I've never heard of any generator running at 40 rpm.  Usually more gears  would be needed to increase the rpm. It is one of the reasons that higher speed turbines with a propeller are used at this head. Even an over shot wheel would be much faster, being half the diameter of the breastshot wheel you are proposing.  Using a water wheel you will need to gear up and/or rewind a generator. I had a special stator rewind for a Harris hydro and it cost less than $200. But that is much smaller than what you are proposing. To produce any suitable voltage is going to need a lot of copper to generate at the rpm you are proposing. That may be why you are seeing such high cost of the larger wind generators, the rpm is lower than with smaller propellers.
  A lower voltage would be easier to attain with low rpm. That would then need an inverter to grid intertie.  A UL listed inverter might make the code compliant intertie easier.  But an inverter to handle the higher output would be expensive.
I don't understand what you mean by base rotational rpm of one and maximum speed of 7rpm. Generally the rpm is independent of flow rate. It depends on diameter of wheel and water speed(Head)  How did you calculate the rpm?  My charts show nozzle velocity of 24 feet per sec at 9 feet head. Are you planning on arranging to only use the gravity force of filling the buckets and ignoring the force of falling water?  Are you calculating how long it takes to fill each bucket at varied flow rate?
It makes sense to more than one generator on the output shaft to be able to handle the  range of flows.
Chris

[WoodWaterWheel]

As a water wheel builder I can tell you that you'll probably be happier spinning your 16' breastshot water wheel at 6-8rpm both for the looks and minimizing the gearing required.       Use a online hydro calculator with a .55 system efficiency to give you your sites electric potential.      Since I sell these things and don't want to break this sites rules so I can't refer you to my site's online calculator but there are several out there.        Then find a low rpm alternator that will make the electricity your site can make at a low rpm.       Then figure the gearing needed to get the alternator up to the speed needed to make it's output match your site @ 6-8rpm.         
A final thought.       Overshot wheels are more efficient than breast shot wheels at converting the water's potential power into usable energy so you will do better with a smaller more efficient wheel spinning faster like 8-10rpm for both a more efficient wheel and less friction robbing gearing.      Or just run with a low head turbine.      I'm developing a few low head turbines myself because as much as I like watching a water wheel turn if you just want the electricity a low head turbine is more cost effective.
Spencer