GENERATION OF ELECTRICITY In Great Britain.—The development of the windmill for the generation of electricity is mainly—so far as Great Britain is concerned—a post-War movement. During the years 1924-25 the Institute of Agricultural Engineering, University of Oxford, carried out an investigation into the performance of eight sets, ranging in rated output from 0.300 kw. to io.o kw. The wind wheels were from 8 feet to 29.5 feet in diameter and the structures varied in height from 1 o feet to 6o feet.
The cost of the energy obtained is mainly due to capital charges and depreciation ; maintenance and repair are relatively small items. The plants tested by the Institute of Agricultural Engineer ing supplied energy at costs varying from 12.7 pence per kw.h. for a set giving an annual output of 316 kw.h. to 4.1 pence per kw.h. for a set giving an annual output of 7,640 kw.h.
It was established that continuity of supply is assured in average cases by installing a storage battery capable of supplying the heav iest winter demand for three successive days. The battery fulfils a further important function in acting as a "flywheel" between the dynamo and the load. The wind fluctuates in strength from sec ond to second, with corresponding fluctuations in the power sup plied by the dynamo : the battery enables the demand to be sup plied steadily without momentary reference to the state of the wind.
By the term "speed" in connection with a wind-wheel is meant the circumferential speed of the blade tips, and it is usual in wind wheel technique to compare this speed with the wind-speed at which it is produced. If the wind-speed is denoted by V and the circumferential speed of the blade tips at this wind-speed is de noted by U, the ratio expresses the speed characteristic of the wheel. Slow-speed wheels—multibladed with concave sheet-metal
blades—have a value for varying from 1.o to 1.5. Medium speed wheels vary from 1.5 to 3.o: the sweep-sail mills are in this class. High-speed wheels run at values for varying from 3.o to 6.o : this class contains wheels with the modern stream-lined blades, varying from two to five in number and covered with sheet metal or fabric.
The efficiency of a windwheel is the use which it makes of this power, and the figure by which efficiency is expressed shows what proportion of the energy of the wind acting on the windwheel is transformed into mechanical energy available at the hub of the wheel. It is not possible to use up all the energy in the wind, since some must be left to carry on the flow of air : this consideration results in the theoretical maximum possible efficiency being 0.59. The practical efficiencies are much below this.
Dr. Betz has a design for a high-speed wheel to run with reason able efficiency at =6.
The use of high-speed wheels for driving generators is increas ing in favour for four reasons : I. They are more efficient.
2. The higher speed of the wheel means that less gearing is required to drive the dynamo at the proper speed.
3. The surface area of the blades is small, thus reducing the risk of damage in gales.
4. They are more easily erected.
Observation of wind conditions over several years show that, when fixing the site for a windmill plant the greatest care should be taken to ensure freedom from interference from trees, etc., on the sides from which the winds mainly come. The wheel should be as high as possible to obtain the utmost use of the winds. At the coast and in exposed hilly districts there should be no hesita tion in adopting windpower to supply electricity. Even in ordinary inland parts there are very few districts where a suitable site can not be found. Thus, for isolated districts wind power may be adopted for generating purposes with every assurance of re liability and economy; for communal groupings and for medium power work there are larger sets providing the same service.