ASK THE EXPERTS: Matching Hydro Runner & Alternator

Intermediate
Finding the sweet spot for your microhydro generator

I’m designing a microhydro system using a Fisher & Paykel washing machine motor converted to operate as an alternator. The alternator has a 250-millimeter diameter and the magnets are on the outer edge of the alternator. Since a Pelton wheel’s force occurs at the outside of the wheel (via the buckets), should the wheel be the same size as the alternator? If the Pelton wheel was larger, should it have a more efficient drive force on the smaller-diameter alternator? If so, what is the optimum ratio of the wheel-to-alternator diameter?

Grant Keeley • via e-mail

There is no direct relationship between motor diameter and turbine runner diameter, but there are several important factors that must be considered to optimize the turbine.

After factoring in losses in the pipe, we can estimate the net head, or pressure, which determines the velocity of the water striking the runner. As head increases, velocity increases. Quadrupling the head, for example, results in doubling the velocity. The Pelton buckets will capture the most energy if they run at just under half the velocity of the water jet. This, in turn, determines the revolutions per minute (rpm) at which you will get the most output, based purely on water pressure and runner circumference. For example, 100 feet of head (43 psi) on a 9-inch-diameter Pelton runner will hit the runner’s sweet spot between 800 and 900 rpm. The peak is fairly flat, so there is no need to be precise.

In theory,

rpm = {[square root (head, in feet)] ÷ diameter, in inches} × 920

In reality, due to friction losses, the peak performance will be somewhat lower.

Design the motor/alternator to generate the expected power at this rpm. If the power converted is too low at the critical speed, it will overspeed; if you try to produce more power than the turbine can give, you will stall it, and run below the optimal rpm. These deviations can be observed from the spray pattern coming off the runner.

Your biggest design decision is choosing a motor winding that can deliver the power you expect at the voltage you plan to use, and at the rpm the site requires. A Fisher & Paykel “Smart Drive” motor has many coils that can be rewired in series or parallel to tweak the output in relation to voltage and rpm. You can do this yourself, or buy a modified Smart Drive stator from PowerSpout (powerspout.com). The company also provides a free, downloadable design guide (bit.ly/S-DriveDesign). The output voltage will depend on this stator winding, and it will increase in proportion to the rpm. The turbine’s voltage on load must match the battery’s voltage on charge when the turbine runs at an rpm close to the sweet spot. Bear in mind that the load current reduces the motor/alternator’s voltage (by up to 43% at maximum power), so figuring out the best stator to use may take several steps of calculation.

Even with a well-chosen stator, it’s unlikely that this will achieve the rpm you’re aiming for, but it may be close enough to meet your needs. If you want to increase the rpm, adjust the magnet rotor so that it induces less voltage at a given speed. Turbine speed will rise accordingly.

You can also use an MPPT controller (designed for PV systems) to fine-tune the hydro turbine’s rpm. This is ideal for use with variable-flow sites at which the pressure or the voltage changes a lot due to long pipes and long wire runs. Choose an operating voltage that is above the battery voltage, but bear in mind that there is a big difference between this voltage and the runaway “open-circuit” voltage that can occur when no power is being used. Open-circuit voltage may be three times higher and—if the system isn’t properly designed—may destroy your controller.

Building a home-scale microhydro system is very satisfying, and it can contribute a lot of energy. You will need to be smart and resourceful to create a good intake and install pipes, wires, and the electrical system. Building your own turbine is doable, but there are small companies that can help you with friendly advice and provide a turbine package that is sure to work properly on your site. Several of them advertise in Home Power. Good luck with your project.

Hugh Piggott • Scoraig, Scotland

Comments (1)

udos46's picture

This week I started up a pico Pelton
by my client with a brushless servo motor used as gemneratore with no electrical modifications
the three-phase output is rectified and smoothed with an electrolytic capacitor to 450 volts (100 .. 470 uF)

to empty the turbine had 150 volts DC:
with a classic car / motorcycle battery charger CTEK type (which accepts DC voltages up to 320 volts) the turbine voltage settles to 90 volts, while using a MPPT solar, goes to search for the best point doi yield and finds it to 75 volts

increasing by 25% the power supplied to the batteries

best regards
udos46

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