WIND POWER.
It is asa source of energy, to be classified with heat, weight of liquids, electricity, etc., that air in motion (as in a windmill) has a place as a prime mover.
Prime movers, or receivers of power, are those pieces or combinations of pieces of mechanism which receive motion and force directly from some natural source of energy. The point where the mechanism belonging to the prime mover ends and that belonging to the train for modifying the force and motion begins may be held to include all pieces which regulate or assist in regulating the transmission of energy from the source of energy.
The useful work of the prime moveris the energy exerted by it upon that piece which it directly moves; and the ratio which this bears to the energy exerted by the source of energy is the efficiency of the prime mover.
In all prime movers the loss of energy may be divided into two parts, one being the unavoidable effect of the circumstances under which the machine necessarily works in the case under consideration; the other the effect of causes which are, or may be, capable of indefinite diminution by practical improvements. Those two parts may be denominated asnecessary loss and waste.
The efficiency which a prime mover would have under given circumstances if the waste of energy were altogether prevented, and the loss reduced to necessary loss alone, is calledthe maximum or the theoretical efficiencyunder the given circumstances.
In windmills, the air, being in motion, presses against, and moves four or five radiating vanes or sails, whose surfaces are approximately helical or screw shape, their axis of rotation being parallel, or slightly inclined in a vertical plane, to the direction of the wind.
The velocity of the wind determines its pressure, and the pressure of the wind against the sails of the windmill determines the power developed by the mill. A mill of small diameter acted upon by a high pressure develops as much power as a large mill working under a lower pressure.
The mean average velocity of the wind for the entire United Statesis very nearly eight miles per hour. However, for large areas such as the great plains east of the Rocky Mountains, the mean average is about eleven miles per hour, and yet in certain small areas situated in the mountainous districts the mean average velocity is as low as five miles per hour. Therefore, in selecting and loading a mill, reference should be had to the wind velocity prevailing in that particular locality. In general, windmills loaded to operate in ten-mile winds can be depended upon to furnish a sufficient supply of water.
The variations in the velocity and pressure of the wind are considerable even within a brief time, and sometimes sudden and extreme. Winds of 100 miles per hour and upwards are on record. A very violent gale in Scotland registered by an excellent anemometer a pressure of 45 lbs. per square foot. During the severe storm at London, the anemometer at Lloyd’s registered a pressure of 35 lbs. to the square foot. The gauge at Girard College, Philadelphia, broke under a strain of 42 lbs. per square foot, a tornado passing at the moment within a quarter of a mile. At the Central Park Observatory, a wind was recorded of 28.5 lbs. pressure per square foot.
If the wind were to blow continuously a very small windmill would suffice to do a large quantity of work and no storage capacity would be required, but when it does blow it is “free” and experience dictates that a mill shall be erected sufficiently large to pump enough water, when the wind does blow, to last over, with the assistance of ample storage capacity.
Average hourly velocity of the wind at following stations of the U. S. Weather Bureau, given in miles per hour:
Albany, N. Y.7Alpena, Mich.9Atlanta, Ga.9Atlantic City, N. J.10.3Augusta, Ga.4.2Baltimore, Md.6Bismarck, N. D.9.4Boise City, Idaho4.2Boston, Mass.10.2Brownsville, Tex.7.4Buffalo, N. Y.10Cairo, Ill.7.6Cape Henry, Va.12.7Charleston, S. C.8Charlotte, N. C.5.6Chattanooga, Tenn.5.5Cheyenne, Wyo.10.5Chicago, Ill.10.5Cincinnati, Ohio6.3Cleveland, Ohio9.6Columbus, Ohio7.6Davenport, Iowa8.5Denver, Colo.6.7Des Moines, Iowa7Detroit, Mich.8.7Dodge City, Kan.11.8Duluth, Minn.7Eastport, Me.0.6El Paso, Tex.6.3Fort Grant, Ariz.7Fort Sill, I. T.10.7Galveston, Tex.10.3Grand Haven, Mich.10.7Hatteras, N. C.14Helena, Mont.6.7Huron, S. D.11Indianapolis, Ind.6Jacksonville, Fla.6.7Keokuk, Iowa8Key West, Fla.9.8La Crosse, Wis.7.3Leavenworth, Kan.7.1Little Rock, Ark.3.6Los Angeles, Cal.4.7Louisville, Ky.7.3Lynchburg, Va.4Madison, Wis.10.2Marquette, Mich.8.7Memphis, Tenn.5.8Mobile, Ala.6.7Montgomery, Ala.5.1New Haven, Conn.8New Orleans, La.7.6North Platte, Neb.10.3Olympia, Wash.3.8Omaha, Neb.8.5Oswego, N. Y.9.6Pensacola, Fla.8.2Philadelphia, Pa.10Pittsburg, Pa.6Portland, Me.8Portland, Ore.5.3Prescott, Ariz.6.5Red Bluff, Cal.7Roseburg, Ore.5.3Sacramento, Cal.6.7St. Louis, Mo.10.3St. Paul, Minn.7.6St. Vincent, Minn.9.4Salt Lake City, Utah5.3Sandy Hook, N. J.14.5San Diego, Cal.5.6San Francisco, Cal.9.4Savannah, Ga.7Shreveport, La.5.6Spokane Falls, Wash.4.7Springfield, Ill.8.7Vicksburg, Miss.5.8Washington, D. C.6.5Yuma, Ariz.6Yankton, S. D.9
Note.—Windmills are erected to be operated by the lightest winds. A wind which will carry off smoke will move a windmill; and the absence of a wind of this force means a perfect calm. Mr. Corcoran says: “My experience of thirty years teaches that a calm has seldom, if ever, held sway in this part of the world for a longer period than three days. Consequently, with a tank to hold a three days’ supply, it becomes possible to pass over any number of calms.”
Note.—Windmills are erected to be operated by the lightest winds. A wind which will carry off smoke will move a windmill; and the absence of a wind of this force means a perfect calm. Mr. Corcoran says: “My experience of thirty years teaches that a calm has seldom, if ever, held sway in this part of the world for a longer period than three days. Consequently, with a tank to hold a three days’ supply, it becomes possible to pass over any number of calms.”
Fig. 460.
Fig. 460.