MOTHER's MICROHYDROELECTRIC PLANT REVISITED

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Changing the turbine discharge will be a difficult undertaking, because the assembly is cast into reinforced concrete. Nonetheless, we plan to install an adjustable nozzle. To date, all the homebuilt crossflow turbines that we know of have been able to release only a fixed amount of water. Now, however, there are three different variable-flow nozzle designs in development by members of the Appalachian Micro-Hydro Association, all three of which are within the construction capabilities of an individual with a well-equipped home shop. Two of these turbines have already been built, and one is installed and undergoing testing (in nearby Murphy) as this is written.

A nozzle capable of varying water discharge could increase the annual output of our hydroplant considerably. If, for example, the nozzle were designed for flows of between 1.5 and 4.5 cfs, the system could take full advantage of periods of heavy rainfall and then operate at lower output during dry spells (instead of shutting down). You can be sure that we'll report on at least one of the variable designs if and when it's proved successful.

GENERATION

The 10-kilowatt Kato alternator that we initially installed was far too large for the actual output of the system. Most alternators achieve their best efficiency at 80 to 90% of their rated capacity, and we were a far cry from that. Besides that, the $3,000 unit was far more expensive than necessary.

In the spring of 1981 we replaced the Kato with a Winpower 2.5-kilowatt, 120-volt AC unit, which we bought for $750 off the shelf from a local electrical supplier. Our prior efforts to scrounge a replacement unit had been unsuccessful: We found that used and rebuilt alternators were available in larger sizes, but that a secondhand 1,800-RPM alternator smaller than 5 kw was a rare bird indeed. Consequently, we decided to buy a new unit. In four years the Winpower has failed only once—when a mouse nibbled through a rectifier wire. The repair cost less than $50.

The turbine and generator are linked by a pair of three-groove pulleys with two B-width belts. The lower one (on the turbine) is 20.5 inches in diameter, and the upper sheave is adjustable to fine-tune speed. We've found that, with our 12.5 feet of net head, the turbine runs most efficiently at 275 to 300 RPM, which corresponds to an upper diameter of about 3-1/3 inches. According to standard engineering texts, a single-step speed increase of this ratio is too high for optimum efficiency. However, when we tried installing a jack shaft to make the speed increase in two stages, the system's output declined.

CONTROLS

With an independent, alternating-current (standard-household-power) system such as ours, it's important that generator speed remain constant to maintain steady voltage and cycles. Our last report described a mechanical speed control that sensed generator speed and adjusted two variable pulleys to keep RPM constant. For several reasons, we've discarded that system in favor of another.

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