Matching Impeller diameter, head, and generator RPM in a Pump as Turbine Micro Hydro

Mike from Vermont said "...there is an existing dam which is in good shape (built by the state many decades ago), with  about 40’ of head, and at least 7 cfs of flow the majority of the year.  ...Based on this VERY limited information, and based on your experiences, do you think a pump could be a good option or should I stick with the crossflow? 

Mike, I think a split case pump to handle 4500G/min at 40' of head might be big, expensive to buy new and not as efficient as a as a crossflow. So unless you can pick one up for scrap iron price I'd stick with a cross flow. 

But keep your options open while you work on the intake structures, penstock, powerhouse and valving. When I started building 8 years ago I thought to just try a used PaT, Pumps as Turbine on 200' of head and ~1000Gal/min, half expecting having to go to Francis or Turgo turbines at 10X the cost. But the PaTs have worked out really well and reliably, only a few % less efficient, and much cheaper, widely available, and easy to repair / find parts. I control water flow to each turbine with a motorized butterfly valve very effectively. There is a bit of noisy turbulent flow at very restricted flow settings but that never occurs during normal operation, and would not be a problem with lower head pressures at all as long as the valve is sized properly.

(Rough rules of thumb, if the smallest port (pressure side) on PaT is 6" use an 8" motorized butterfly valve and a minimum of 14" diameter of penstock pipe if it is not too long.
I don't advertise this, but if you have read this far and email me numbers for your head, flow, pipe length and material, and number of degrees of bends if any, then I can give you better numbers for all the outputs and PaT specs just for some nice comments and feedback here.)

So, Mike, if you do go for a PaT you will need to match the pump impeller diameter to the available head to match the RPM to the generator. This dictates that you will be looking for a minimum impeller diameter and pump I/O ports that can still handle 5000Gal/min. So I estimate you'll need at least a 8"X10"X7"dia split case pump to get the full load speed up to around 900 RPM (The exact relationships you can find here and here on my blog. ) That means you are looking for a 30 - 60 HP, 8pole, 900 RPM, 3 phase, motor to use, direct coupled, as a generator. I doubt you can find motor and pump together, so you'll have to look for them individually. If you find a 10x12x larger impeller that might work also, if you cut the impeller diameter down to match the head.

Another option, to get a good impedance match for maximum power transfer, is to belt couple the generator to the PaT and tune the generator RPM with suitable pulley diameter ratios. This will incur a small % loss in the belt drive and have a slightly increased maintenance cost. 

The definitive test to see how well your equipment is matched up to your head and flow for maximum power is that your PaT and MaG with no (electrical) load should spin at around 2X the full load RPM.

FL RPM in the case of the 8 pole MaG is around 910 rpm, so 1820 just free wheeling. Adjust the impeller diameter downward to get the no load speed up close to 1800.

For a video overview see:  http://energyindependence-rob.blogspot.com/2010/07/overview.html

Happy Hydro

Rob