If you're generating power from a flowing water source with a moderate head, a Pelton wheel is a good choice.
Though one of man's oldest prime movers, a water wheel generator is still a fascinating piece of machinery. Perhaps this is because it appears comprehensible at a glance (although an efficient wheel is actually a product of subtle and inconspicuous design refinements), and because it seems to be a way of getting power for nothing. The wheel we describe here, with accompanying technical diagram, was especially designed for this series on harnessing small streams, and will reward a careful craftsman by delivering years of constant service. It's particularly suited for an installation having a moderate head (25' to 60') and relatively small flow (0.45 to 0.75 cubic feet per second). Subsequent installments will describe the construction of wheels suited for lesser heads of water and other varied conditions.
The wheel is an impulse wheel, driven by the impulses produced as water strikes revolving blades or buckets. In a perfectly designed wheel, the water strikes at high speed, exhausts its energy in driving the wheel to which the bucket is attached, and then falls free of the wheel.
Known as a Pelton wheel, this type developed from the "hurdy gurdy", a paddle wheel used in California by the forty-niners. The hurdy-gurdy was a wheel that rotated in a vertical plane, had flat vanes fixed around its circumference, and was driven by the force of water striking the vanes. It was not an efficient machine, but it was simple to construct. Then an engineer named Lester Pelton substituted a cup-shaped, divided bucket for each of the vanes, and by that step added a high degree of efficiency to the wheel's other virtues.
No single wheel will meet all operating requirements, but some will perform under a reasonably wide range of conditions. The following table indicates the r.p.m. and horsepower output that will be delivered by this wheel under given conditions of head and flow. The latter is measured in cubic feet per second:
- Flow: 0.43; RPM: 350; HP: 1.0
- Flow: 0.51; RPM: 390; HP: 1.3
- Flow: 0.59; RPM: 450; HP: 2.0
- Flow: 0.66; RPM: 500; HP: 2.8
- Flow: 0.73; RPM: 550; HP: 3.75
Thus, if a survey of your stream indicates a head and flow close to these values, our Pelton wheel will fit neatly into your plans.
Strictly speaking a water wheel is an engine powered by water, just as an automobile engine is powered by gasoline. The important power-producing elements of the wheel are the buckets and the nozzle, and considerable care should be exercised to see that these parts are made correctly. The nozzle meters the correct amount of water to the wheel, and forms and directs the jet against the buckets. Both the inside diameter and the location of the nozzle with respect to the wheel are very important, since the jet must impinge upon each bucket at the correct wheel radius or lever arm. It must also be divided equally by the center ridge of each bucket.
The function of the bucket is to convert the energy of the jet, represented by its high speed, into mechanical energy at the wheel shaft. To do this it must slow the water from its high speed in the jet to practically zero speed when it drops into the tail water. Maximum efficiency with this wheel will be obtained if the buckets have the proper form and size. This shape acts to slow the jet by turning it smoothly through 180 deg. The surface of each bucket must be as smooth as possible. A mirror finish is desirable on the inside, and even the back of each bucket should be ground and polished to minimize spray.
Important also is the correct orientation of the bucket to the jet. When the full jet strikes, the bucket should be perpendicular to it. Both the nozzle and the buckets will wear under the action of the high-speed water, at a rate determined by the silt content and should therefore be made easily removable for replacement.
Above all, buckets must be uniform. If you can get access to a metal-cutting handsaw, cut the blanks according to a single pattern. This pattern can be shaped so as to form the end bevels automatically when the blanks are bent, and the bending itself can be done in a jig or hammering form. This jig may be made of a piece of pipe of about 2" outside diameter mounted in hardwood endplates. Also provide a holding fixture that will slide in the table groove of the handsaw to assure that the slots for the end lugs are cut and spaced uniformly. A holding jig should also be made to line up the lugs and buckets for welding. On completion, balance the wheel by laying weld beads along the backs of any light buckets. Beads should be laid carefully and ground smooth.
Ball bearings may be employed, but are not necessary since the wheel turns at comparatively low speeds. If the builder prefers to use plain bearings, it will simplify machining the shaft, which should present a shoulder to the inside of the bearing so that the wheel may be positioned. If plain bearings are employed, babbitted linings are satisfactory, provided provision is made for proper lubrication.
One vital job that the foundation must do is hold the wheel and the nozzle in correct relative positions. It should be placed on firm ground or piling so that it will not settle unevenly, and must, of course, take advantage of all the head possible. The penstock from the dam should have easy access to the nozzle, and the tail water easy escape to the stream. If possible use 4" or larger pipe for the penstock and lay it out to hold frictional losses to a minimum. The width of the foundation is such as to allow the water to fly clear of the buckets. The removable cover over the upper half of the wheel may fit more closely, since no water sprays from the buckets through this half of the revolution.
The foundation may be made of such materials as timbers in a framework, masonry, or concrete, so long as it fulfills the above requirements. The wheel and the machinery being driven may then be housed in any suitable, inexpensive shed.
It's not wise to dispense with a gate valve, which is used to cut off or to throttle the water supply to the wheel. Since a gate valve cannot be operated rapidly, it is the best type, eliminating the risk of dangerous water hammer in the penstock. It is also well suited for throttling because fine adjustment is obtainable through the long operating screw. In throttling, the gate valve should be used together with a tachometer or revolution counter connected to the wheel shaft to secure the optimum speed and horsepower for the stream condition and load. Either fasten a tachometer permanently to the shaft, or keep a revolution counter handy in the wheel shed.
Generally the head and volume of water flowing to the wheel will remain constant, resulting in a constant output. If the machinery driven by the wheel has a level power demand there will be little need for constant adjustment of the valve.
The requisite piping, pipe fittings, steel sheet and rod bolts and nuts, and gaskets are available at building-supply houses houses or steel distributors. Machine screws, lock nuts bearings, and the like may be purchased from good-sized hardware distributors or mail-order houses.
One final point to keep in mind in making your calculations: head is defined as the vertical distance between the water surface behind the dam and the tail-water surface at the wheel. For an impulse wheel, however, which cannot operate submerged, the available head is measured from headwater to the center line of the nozzle. There is only 5" difference between the two definitions, but this can make some difference in output when working with the moderate heads for which this wheel is designed.
While many details can be altered, the reader should beware of any that will affect operating characteristics. Thus stainless steel buckets and antifriction bearings would improve performance, involving only some extra work in building the wheel. Changes in the nozzle diameter, wheel radius, or effective head, however, should be undertaken only after careful consideration of the probable effect on performance.
Reprinted courtesy of Popular Science Monthly, Popular Science Publishing Co. Inc. 1947