METHOD ON REGULATING THE AIR FLOW AROUND THE WINDMILL WING AND DEVICE FOR USE IN SUCH METHOD

Abstract: The invention concerns a method and a device for regulating the air flow around a wing on a wind mill. The device consists of a spoiler provided with a hollow. The spoiler is fastened to the leading part of the wing, and the spoiler may assume different shapes in order thereby to change the air flow around the wing. When the spoiler assumes a deactivated shape, no substantial change of the air flow occurs as the spoiler then extends continually and follows the contour of the wing. When the spoiler assumes an activated shape, the air flow is changed, however, by the spoiler no longer following the contour of the wing and creating a discontinuity or at least changing the wing section so that the flow conditions are changed. The change of shape occurs by supplying a fluid, such as pressurised air or hydraulic oil, to the hollow in the spoiler. The change in the air flow is an advantage in order thereby to regulate the rotational speed of the rotor onto which the wing is mounted.

Description: METHOD ON REGULATING THE AIR FLOW AROUND THE WINDMILL WING AND DEVICE FOR USE IN SUCH METHOD. The invention concerns a method for controlling a wind mill and an apparatus for use in the method. It is known that on every windmill it is necessary to limit the power in high wind oth- erwise the windmill may become overloaded. The normal methods for power limitation are stall control and pitch control. By usual stall control, the wings are fixed on the mill hub and cannot be turned about their longitudinal axis. The angular position of the wings on the hub is adjusted once for all at the erection and running in of the mill. The wings are designed so that the air flow over them by itself gives larger air resistance in high wind and thereby limits the power. Due to this passive utilisation of the aerodynamic properties of the wings, the control becomes simple and sturdy under all circumstances with only small peak loads. Common stall control has the drawback that the maximum power depends on the density of the air and the surface roughness of the wing. Hence there will be changes in maximum power from summer to winter and by dirty wings. To a large degree, the stall power also depends on the design of the leading edge of the wing. Small produc- tion tolerances in the shape of the leading edge may give significant differences in the power level at which a wind mill is stall controlled. By pitch control, the wings are mounted on bearings on the mill hub so that they may be turned about their longitudinal axis. In high wind, the angular position is adjusted away from stall all the time so that the lift is limited to give just the power desired. With the active regulation, there may be compensated for the density of the air. the surface roughness of the wing and the influence of production tolerances. Pitch control has the drawback that a relatively complicated, active regulation is re- quired which in high wind may be somewhat sensitive to turbulence. Therefore in practice, pitch control depends on a special generator with fully or partly variable rotational speed so that the mill may increase speed slightly by wind gusts. Otherwise. the active regulation cannot follow the variations in the wind, and too great peak loads are obtained. Compared with stall control with fixed wings, pitch control also has the drawback that the wings are to be mounted turning on the mill hub and therefore have to be provided with bearings and actuation systems. These components have to transmit large loads and imply an increased demand for service. A newer kind of control is active stall regulation. Here, the two normal methods of power limiting are combined. As by pitch control there are bearings between wings and mill hub so that the angular position may be adjusted but the power limiting itself in high wind occurs by stalling. Compared with usual stall control, active stalling has the advantage that the maximum power may be held at the desired valued with certainty irrespectively of air density, possible dirt on the wings and the influence from production tolerances. Compared with pitch control, active stall control has the advantage that the regulation itself still occurs by stalling, thus a passive utilisation of the aerodynamic properties of the wing so that sensitivity to turbulence is still small. Therefore, it is not necessary to use special generators, variable speed or the like for avoiding large peak loads. Active stall control has. however, also its drawbacks compared with passive stall control with fixed wings. As with pitch control there is the disadvantage that the wings may be mounted turning on the mill hub and therefore has to be provided with bearings and actuating systems. These components have to transmit great loads and imply an increased demand for service. By active stall control, the regulating is slower than by pitch control, and the demands to the actuating systems are therefore lesser but the complexity is, however, substantially greater than by passive stall regulation. Besides the regulating systems based on turning of whole wings, spoilers on fixed wings are also known where the regulating occurs by a spoiler effect, usually sup- ported by complete or part stall of the wing. The spoiler may typically be made as a rail disposed over the suction side of the wing, and which by extension gives increased air resistance and turbulence, and maybe also trips a real stall. Such spoilers were used on the windmills erected in Denmark by F.L.Schmidt during Second World War. Spoiler systems of this kind usually has the disadvantage that they imply mechanical parts far out on the wing. By experience, it is difficult to maintain a high availability on such systems as the operating conditions are very difficult and since the actuating mechanisms for spoilers usually are not very suited for resisting the hundreds of million actions to which the system is subjected during normal operation. To this is added that spoilers disposed over the wing surface, also when not actuated, normally have a certain continuous spoiler effect reducing the aerodynamic efficiency of the wing. The external mounting may also give a considerable noise contribution. Other spoiler systems exist, consisting of rails or bellows placed in a recess in the wing surface. Again the effect is depending on increased air resistance and turbulence but compared with external spoilers, the mechanism is somehow better protected. Such spoilers are seen to be used, among others, on early wind mills of the Wind- Matic-type in Denmark. By being placed in a recess in the wing surface, this type of spoilers has the advantage that it does not to any significant degree reduce the aerodynamic efficiency of the wing when it is not extended. Recessed spoilers usually have the disadvantage that they require special wing structures with recesses and hollows. Furthermore, normally they may be provided only with difficulty at the leading edge itself where the air forces are great but have to be disposed farther back at the suction side of the section. Here the operating conditions are better but in return the spoiler effect is more limited, and the spoiler therefore has to be of substantial size. Noise problems may exist at the partitioning faces between main wing and spoiler, and the effect may be uncertain under icing and by strong dirtying with dust etc. where extension and withdrawal may be impeded. The purpose with the present invention is to provide a method and an apparatus for power regulation of wind mills with fixed wings reducing the disadvantages connected with the known methods. This purpose is achieved by using a method based on a flexible spoiler disposed at the leading edge of the wing, and which is actuated by filling with a liquid or gaseous medium. This method has many advantages as compared with prior art systems. By being based on active regulation, the drawbacks usually connected with passive stall regulation with fixed wings are avoided. With the active regulation, compensation can be made for air density, wing roughness and influence from production toler- ances. By being based on a spoiler effect, the drawbacks normally connected with wing turning systems are avoided. The expense to the spoiler system is quite marginal compared with the considerable expenses to wing bearings and actuating systems, as well as the service demands are much reduced. By being based on a flexible type of spoiler mounted on the wing surface, the disadvantages usually connected with spoilers of prior art are avoided. There is no loss of aerodynamic efficiency when the spoiler is not extended, and by suitable design of the edges so that the height of the transition to the wing surface is small compared with the boundary layer, the risk of noise problems may be minimised. By being mounted on the leading edge of the wing, the spoiler may achieve maximum effect when extended. By being mounted on the wing surface, the spoiler may dispense with the recesses and hollows normally connected with recessed spoilers. By not having parti- tioning faces between spoiler and main wing, the function of the spoiler is also ensured under icing and by strong dirtying by dust etc. A special advantage by the method and the spoiler type comes from the mounting on the wing surface. This implies that the spoiler may be retrofitted so that the method of regulation may be applied to existing wind mills. Usually, this would not be possible by prior art type of spoilers, except maybe the original type which is mounted spaced above the wing surface. A further advantage by the method is that the flexible spoiler, as a side gain, may be used for removing ice at the leading edge of the wing by inflating when the leading edge is covered by ice. Reference is made to the patent claims for the means for achieving the effect. In the following, the invention is described more closely as reference is made to the Figures. Figure 1 shows a wind mill wing with an external spoiler according to older prior art. The wing section 1 is provided with a spoiler rail 2 carried by a row of fittings 3 with hinges 4. The spoiler is shown in normal, deactivated position where no spoiler effect is desired. Figure 2 shows the same spoiler in activated position where the spoiler is pivoted for maximum effect by mechanical means. Figure 3 shows a wind mill wing with a built-in spoiler according to newer prior art. The wing section 5 is provided with a spoiler rail 6 disposed in a hollow 7 in the wing surface and which may be pivoted about a hinge 8. The spoiler is shown in normal deactivated position where no spoiler effect is desired. Figure 4 shows the same spoiler in activated position where the spoiler is pivoted for maximum effect by mechanical means. Figure 5 shows a wind mill wing with a built-in flexible spoiler according to newer prior art. The wing section 9 is provided with a hollow 10 covered by a flexible membrane 11. The hollow 10 extends along a part of the length of the wing and is inwardly closed toward the wing root and outward toward the wing tip with valves not shown on the Figure. The spoiler is shown in normal, deactivated position where no spoiler effect is desired. Figure 6 shows the same spoiler in activated position where the spoiler membrane is inflated for maximum effect. The activation normally occurs by opening the innermost valve and closing the outermost. The inflation is then caused by the surpressure coming from the centrifugal force on the air column standing the hollow of the wing. De- activation occurs by closing the innermost valve and opening the outermost. Figure 7 shows the leading edge of a wind mill wing with a spoiler according to the invention. The wing section 12 is provided with a flexible spoiler 13 extending along a part of the wing length. The spoiler has a cross-section with a hollow 14. The hollow 14 is partly closed outward toward the wing tip and is connected inward toward the wing root to a pressurised air system which may apply a greater or lesser pressure to the hollow of the spoiler. The spoiler is shown in normal deactivated position where no spoiler effect is desired. Figure 8 shows the same spoiler in activated position where the spoiler is inflated for maximum effect. Activation usually takes place by applying pressure to the hollow of the spoiler. The spoiler may be inflated steplessly, depending on the applied surpressure. Deactivation occurs by reducing the filling pressure. The emptying is enhanced by the centrifugal effect on the air column standing in the hollow of the spoiler, pos- sibly supplemented by ejector effect from air flowing by at the outermost end of the spoiler. Figure 9 shows the same kind of spoiler in activated position but where the hollow is moved farther back toward the suction side of the wing. Here the stall effect is becom- ing more violent even by lesser angles of attack. Figure 10 shows examples of cross-sections of spoiler according to the invention before the spoilers are mounted on the wing. The spoilers may e.g. be extruded in EPDM Shore 45 whereby they may easily be mounted on leading edges with different curva- ture. A normally preferred embodiment 15 may have uniform cross-section of the hollow. Another preferred embodiment 16 may have a number of ribs 17 which maintains the shape of the hollow at possible underpressure from the suction effect under the centrifugal action but which gives good access for air for inflating anyway. A third preferred embodiment 18 may have non-uniform cross-section of the hollow where one part 19 has great material thickness in the side expanding by inflation, while another part 20 has less material thickness. Hereby the shape of the inflated spoiler may be adjusted for giving the most advantageous separation of the air flow. In a fourth, preferred embodiment, the hollow may be divided into two or more parallel ducts which possibly may be inflated to different pressures. Hereby the shape may be further differentiated as well as a larger part of the spoiler may be imparted a shape advantageous for separation.

Claims: CLAIMS 1. A method for regulating air flow around a wind mill wing, the method comprising changing a section of the wing in such a way that the air flow around the wing is also changed, characterised in that a first air flow around the wing is created around a stepless flexible device mounted at a front outer side of the wing by a deactivated shape of the wing, and that a second air flow around the wing is created by an activated shape of the device, and that the shape of the device is changed from the deactivated shape to the activated shape so that the rotational speed of the rotor onto which the wing is fixed is maintained within a given range for the rotational speed. 2. A device for use in the method according to claim 1, where the device is made of a flexible material which is provided with at least one hollow, and where the hollow has a first volume in a deactivated shape and has a second volume in an activated shape, and where the volume of the hollow may be changed from the deactivated shape to the activated shape by conducting a fluid to the hollow, characterised in that the at least one hollow is formed in a separate spoiler, and that the separate spoiler is mounted on a surface of the wing. 3. A device according to claim 1, characterised in that the spoiler constitutes a film with an outer side intended to face outward in relation to the wing surface, and an inner side intended to face inward in relation to the wing surface, that the inner side of the film is intended to be fastened to the wing surface and that the hollow is formed between the outer side and the inner side of the film. 4. A device according to claim 1 or 2, characterised in that the film has a central area where the hollow is formed and which has a thickness T, that the central part converges toward edge areas having a thickness t, and that the thickness t is less than the thickness T. 5. A device according to any of claims 2-4, characterised in that the central area of the spoiler has an outer side converging continuously toward the edge areas when the spoiler assumes the deactivated shape, and that the central area of the spoiler at least implies a changed section of the wing, preferably displays a discontinuity in relation to the wing surface, when the spoiler assumes the activated shape. 6. A device according to any of claims 2-5, characterised in that reinforcing ribs have been formed through the hollow between the outer side of the spoiler and the inner side of the spoiler, and which are capable of maintaining a given minimum distance between the outer side and the inner side of the spoiler when the device is in its deactivated shape. 7. A device according to any of claims 2-5, characterised in that passages are formed through the hollow, extending in narrower ducts in the hollow and between wider ducts in the hollow, that the passages are folded when the spoiler assumes its deactivated shape, and that the passages are distended when the spoiler assumes its activated shape. 8. A device according to any of claims 2-7, characterised in that the spoiler is brought from its deactivated shape to its activated shape by the hollow being filled with a give amount of fluid, preferably pressurised air or other pressurised gas, alter- natively hydraulic oil or other liquid. 9. A device according to claim 8, characterised in that the spoiler is brought from its activated shape back to its deactivated shape by the hollow becoming emptied from fluid by means of the air flow acting on the wing surface and at the outer side of the film, and/or by means of the centrifugal force acting on the fluid. 10. A device according to any of claims 2-9. characterised in that the spoiler is made of rubber, alternatively made of plastic, preferably made of an extruded section of EPDM with a hardness between Shore 45 and Shore 70 and with a width of 50 - 250 mm.

Inventors: STIESDAL HENRIK, STIESDAL, HENRIK
Applicants: BONUS ENERGY AS, STIESDAL HENRIK, BONUS ENERGY A/S, STIESDAL, HENRIK
Application year: 2000
Main classification: F03D7
IPC codes: F03D7