MOTHER's MICROHYDROELECTRIC PLANT REVISITED

Image Gallery MOM'S microhydroplant has been installed for almost five years now, and we've learned a number of lessons about the practical application of very small waterpower systems. In that time, however, the crossflow turbine has been a tireless performer.

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If you're interested in building a home-scale or larger AC hjydroelectric plant, this account of our successful—experiences may help save you a lot of time and money.

It's been more than four years since we reported to you on the construction of a small hydroelectric system at Eco-Village (see issue 66, page 106). In the meantime, we've been working steadily on refining the design to improve its performance and make it more practical. And, as our system has matured, we've learned some lessons . . . ones you can "go to school on" to make your own system work (and cost) right the first time.

TURBINE

You may remember that the heart of our plant is a homemade crossflow (or Mitchell-Banki) turbine that's 12 inches in diameter and 18 inches long. The device was made by slicing 72° sections from 4" Schedule 40 steel pipe and welding 20 of the arcs into end plates made of 1/4" mild steel. It still runs essentially unchanged: We removed it at one point and trued its circumference on a lathe, in hopes of gaining power from the tighter fit between turbine and nozzle. Unfortunately, reducing the clearance to 1/32" produced no detectable effect on the runner's output.

In its nearly five years of operation, our crossflow has spun away without a hitch. Even though debris from the lake has slightly bent one of the blades, the tweak hasn't noticeably affected the turbine's performance.

Maintenance has consisted solely of an annual greasing of the SCB pillow-block bearings that the 1-7/16" shaft rides on.

Reliable as it's been, however, we have wished that the turbine had been designed differently in the first place. Our crossflow's nozzle was sized to deliver 3.5 cubic feet per second (cfs, or about 1,500 gallons per minute) of water, and we've confirmed that discharge rate with a weir, a flow-measuring device. The problem is that we don't have a dependable 3.5-cfs supply from the 11-acre (54-acre-foot) Eco-Village lake. The system design was based on an Army Corps of Engineers estimated flow rate for Transylvania County, North Carolina, of 3.8 cfs per square mile. That was a mistake.

Since the Eco-Village plant went on-line, a detailed assessment of microhydropower potential in western North Carolina (sponsored by the North Carolina Alternative Energy Corporation, or NCAEC) has produced much more specific—and therefore accurate—flow figures for the region. As it happens, the Eco-Village's elevation is somewhat lower than the average for Transylvania County. And in the Southern Appalachian region, flow per square mile increases with altitude. The NCAEC report suggests that the mean flow at Eco-Village should be about 2.6 cfs per square mile, a number that jibes well with our experience.

What does all this mean? Well, during much of the year we simply can't run our turbine full-time without draining the lake. And, because we're adamant about maintaining stable ecological conditions in the lake and downstream, we have to keep a close eye on the lake level while the plant is operating.

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