Wind Engine

an engine that uses the kinetic energy of the wind to generate mechanical energy. A rotor, a drum with blades, a windwheel, or other similar device that receives the energy (pressure) of the wind current and transforms it into the mechanical energy of the shaft’s rotation may be used as the working member of the wind engine.

A distinction is made among carrousel (or rotor), drum, and vane types of wind engines depending on the type of working member and the position of its axis in relation to the wind current. In carrousel wind engines, the axis of rotation of the working member is vertical. The wind presses on the vanes located along one side of the axle; the vanes along the other side are covered with a screen or, by means of a special device, turn edgewise to the wind. Since the vanes move in the direction of the current, their speed of rotation cannot exceed the speed of the wind. For this reason carrousel wind engines are relatively low-speed, more cumbersome, and less efficient than vane types. The largest coefficient of use of the wind’s energy ξ, which rates the energy efficiency of a wind engine and shows what fraction of the wind current is converted to mechanical energy, does not exceed 0.15 in carrousel wind engines. Of the first two types of wind engines, the rotor type, with two polycylindrical vanes has the highest ξ, equal to 0.18. The same disadvantages are inherent in the drum-type wind engine, in which the drum shaft is placed horizontally and perpendicular to the wind current. Vane-type wind engines, in which the axis of the windwheel is horizontal and parallel to the wind current, have become predominant. They have the highest ξ (up to 0.48) and are more reliable in operation. Since the vane and its tip for mounting to the hub is called a wing, the wind engine of this type is called winged (vaned).

Depending on the number of vanes, high-speed (less than four vanes), medium-speed (four to eight), and low-speed (more than eight windwheels) may be distinguished. The speed of a windwheel is rated by the size of moduli Z, equal to the ratio of the circumferential speed ω R of the tip of a vane of radius R rotating with an angular speed ω, to the velocity v of the oncoming current. With an identical Z, a windwheel of greater diameter has a smaller frequency of rotation. When all other conditions are identical, an increase in the number of vanes also decreases the frequency of rotation of the windwheel. A windwheel with a small number of vanes usually consists of a hub and vanes that are rigidly attached to it at a certain angle Φ to the plane of rotation (Figure 1) or are attached by means of bearing assemblies in which the vane rotates in order to change the mounting angle. The air current hits the vane with a relative speed w at a certain angle of attack a. The total aerodynamic force R_Σ, which arises at each vane, is resolved to the lifting force P_y, which creates the torque moment M; and to the ram pressure force P_X which acts on the axis of the windwheel.

Figure 1. Diagram of the vectors of aerodynamic forces and velocities in a cross section of a vane

In high-speed windwheels with swiveling vanes, the mechanisms for regulation of the frequency of rotation and limitation of power and the start-stop mechanism for the motor, which cause the deflection of the vanes in relation to the longitudinal axis of the wind engine, are often combined structurally. A multivane windwheel consists of a hub with a framework to which specially shaped vanes of sheet steel are rigidly mounted. In low-speed windwheels, the value of ξ may be as high as 0.38. Limitation of developed power is usually accomplished by the rotation of a low-speed wind-wheel relative to a surface perpendicular to the direction of action of the wind current. The power developed at the shaft of the windwheel depends on its diameter and the form and profile of the vanes and is practically independent of their number:

where P_w is the power at the shaft of the windwheel in kilowatts (kW), ρ is the density of the air (in kg/m³), v is the wind velocity (in m/sec), and D is the diameter of the wind-wheel (in m).

on the speed of the windwheel (Figure 2). In low-speed wind-wheels, the maximum value of M coincides with the initial momentum M₀; in high-speed windwheels, on the contrary, the nominal value M_n, corresponding to fmax, is several times greater than M₀.

Figure 2. Dependence of the values of the relative momenta M and the coefficient of use of the wind’s energy ξ on engine speed Z for varying numbers i of windwheel vanes

The theories of the airfoil and airscrew are applied in studying the physical phenomena of the passage of an air current over a windwheel. The theoretical foundations of the computations of the windwheel were laid down in 1914-22 by the Russian scientist N. E. Zhukovskii. In addition, he proved that the ξ of an ideal windwheel is equal to 0.593. His students V. P. Vetchinkin and G. Kh. Sabinin, as well as other Soviet scientists, developed the theory of the wind-wheel and worked out methods of calculating the aerodynamic characteristics and the regulatory systems of the wind engine.

One of two basic schemes for the vane-type wind engine is ordinarily used: either with a vertical transmission and a lower connecting mechanism or with all units located in the head of the motor. The head is mounted on a rotating tower support, and when there is a change of wind direction, it rotates relative to the vertical axis. The height of the tower is determined by the diameter of the windwheel and the height of obstacles hindering free passage of the air current to the wind engine. Multivane wind engines are usually used for operation with low-speed slave machines; and two- or three-vane engines are used for unitizing with generators, centrifugal pumps, and other high-speed machines. In addition to mechanical drive, electrical, pneumatic, hydraulic, and mixed drives are also used. The orientation of the wind-wheel according to wind direction is accomplished automatically by a tail assembly, small swiveling vanes, or the location of the wind engine behind the tower (self-orientation).

Since the power of a wind engine is proportional to the cube of the wind velocity, it is necessary under actual conditions of operation to limit the power when v > v_p and to regulate the frequency of rotation of the windwheel. The operation of various automatic regulation systems is based on the change in the aerodynamic characteristics of the blade or of the whole windwheel according to the effective velocity of the wind, the frequency of rotation of the wind-wheel, and the size of the load. Up to certain calculated values of the wind velocity v_p, the regulatory system is not activated and the wind engine operates with variable power. At velocities greater than v_p, power is maintained at a practically constant level by means of the regulatory system. In regions with average annual wind velocities ¯v_a of 4-5 m/sec, v_p is usually assumed to be 7-9 m/sec; at ¯v_a of 6-7 m/sec, 10-12 m/sec; and at ¯v_a greater than 7 m/sec, 13-14 m/sec. Table 1 shows the power that may be developed by a wind engine when ξ = 0.35 and v_p = 8 m/sec (for a motor with a windwheel diameter of 2-12 m) and v_p = 10 m/sec (for a motor with a windwheel diameter greater than 12 m).

In low-speed wind engines, the most common system of automatic regulation is by means of removal of the wind-wheel from the action of the wind by the pressure created by an air current on a supplementary surface (lateral planes) or by the pressure on a windwheel whose axis of rotation is shifted (located eccentrically) relative to the vertical axis of rotation of the head. The windwheel returns to its .initial position through the pull of a spring. Emergency stoppage of the wind engine is accomplished by means of a winch mounted on the tower through a system of cables whose tension removes the windwheel from the action of the wind. The system of regulation by means of a lateral plane has been used in the domestic TV-8, Buran, wind engine and in many foreign models; the regulatory system with eccentric location of the windwheel is used in the domestic TVM-3 and TV-5 wind engines and in a number of engines produced in the USA, Great Britain, Australia, and other countries.

In most high-speed wind engines, regulation is accomplished by deflection of the vane or of its end portion relative to the longitudinal axis. The high-speed wind engine developed by A. G. Ufimtsev and V. P. Vetchinkin regulates the frequency of rotation of its windwheel by turning its vanes edgewise to the wind current as a result of the combined action on it of the pressure of the air current and the momentum of its centrifugal force. In the USSR, such wind engines have a windwheel diameter of 10, 12, or 18 m and a capacity of 7.4-29.5 kW and are usually used as the primary motors of wind power plants. In wind engines of relatively low capacity (up to 5 kW), the vanes are either turned (during regulation) in the direction of an increase in the mounting angle Ø by the centrifugal force of the vanes and weights (V. S. Shamanin’s method), or regulation is effected by turning the vanes in the direction of a decrease in the angle Ø under the influence of the centrifugal force of the vanes and weights of the regulator. This method (the method of E. M. Fateev and G. A. Pechkovskii) has been used in wind engines of the types VBL-3, Ve-2M, and Berkut. In more powerful wind engines, a stabilizer method of regulation is used (the method of G. Kh. Sabinin and N. V. Krasovskii); regulation is usually accomplished by the end portion of the vane, which rotates relative to the axle under the action of forces arising at the stabilizer. The stabilizer is governed by a centrifugal regulator. Because of the great uniformity of rotation of such wind engines, they are used for work with electrical generators (the D-12, D-18, and D-30 motors). The Sokol wind engine, with an electric transmission, has a combined momentum-centrifugal regulation (the method of la. I. Shefter) based on the change in the lifting force of the vane when it turns relative to the longitudinal axis in the direction of a decrease or increase in the mounting angle under the action of the moving torque at the windwheel. To protect the wind engine from overspeeding when the value of the load moment is low, there is a centrifugal regulator, which also governs the rotation of the vanes. This type of wind engine can operate separately and parallel with other units or an electrical network. Regulators in the form of braking stub-wings, end valves, and other devices that decrease the aerodynamic moment are used in some wind engines. In the All-gaier wind engine (Federal Republic of Germany), the vanes are turned by a mechanical-hydraulic system; when the frequency of rotation is high, the engine stops automatically.

Table 2 shows the annual energy output at the shaft of a windwheel when ξ = 0.35 as a function of the average annual wind velocity ¯_a, diameter of windwheel D, and maximum possible hours of operation T_op of the wind engine in a year.