Fluid reaction surfaces (i.e. – impellers) – Specific blade structure – Laminated – embedded member or encased material
Statutory Invention Registration
2001-01-10
2003-01-07
Tudor, Harold J. (Department: 3641)
Fluid reaction surfaces (i.e., impellers)
Specific blade structure
Laminated, embedded member or encased material
C416S240000
Statutory Invention Registration
active
H0002057
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention (Technical Field)
The present invention relates to rotor blades, particularly wind turbine rotor blades, and specifically to an aeroelasticly tailored turbine blades.
2. Background Art
Whenever the blades on a wind turbine are twisted, the twist directly influences the blade's angle of attack, thereby changing loads and affecting output power. Classic pitch control used in not only wind turbines, but in rotors of all types, directly exploits these principles. When the pitch changes are sufficiently rapid, they can affect not only average rotor loads and turbine power, but vibratory loads as well, influencing fatigue life throughout the system. Even quite small angles of twist can have significant impact.
The general concept of rotor blades that passively adapt to the incident wind loading is not new. Mechanisms that adjusted blade angle of attack in response to the thrust loading were quite popular in the early days of the modem wind energy push of the late twentieth century. Approaches and objectives were quite varied. One effort regulated power with a centrifugally loaded mass on an elastic arm. Cheney, M. C. and Speirings, P. S. M. (1978) “Self Regulating Composite Bearingless Wind Turbine,” Solar Energy, Vol. 20. Another attempt employed a system for cyclically adjusting pitch for per rev load balancing. Bottrell, G. W. (1981) “Passive Cyclic Pitch Control for Horizontal Axis Wind Turbines,” Proceedings of Wind Turbine Dynamics, NASA Conf. Pub. 2185, DOE Pub. CONF-810226, Cleveland, Ohio. The North Wind 4KW had a system for passively adjusting pitch for both power and load control. Currin, H. (1981) “North Wind 4kW ‘Passive’ Control System Design,” Proc. Wind Turbine Dynamics, NASA Pub. 2185, DOE Pub. CONF-810226, Cleveland, Ohio. Others have studied alleviating yaw loads with cyclic pitch adjustments. Hohenemser, K. H. and Swift, A. H. P. (1981) “Dynamics of an Experimental Two Bladed Horizontal Axis Wind Turbine with Blade Cyclic Pitch Variation,” Wind Turbine Dynamics, NASA Pub. 2185, DOE Pub. CONF-810226, Cleveland, Ohio.
Also, a Garrad-Hassan report, for example, evaluated the use of all available blade loads to effect pitch changes that would regulate the power output of a turbine, aiming at a flat power curve in high winds. Corbet, D. C. and Morgan, C. A. (1992) “Report on the Passive Control of Horizontal Axis Wind Turbines,” ETSU WN 6043, Garrad Hassan and Partners, Bristol, UK, But only pitching to feather was evaluated to avoid the vagaries of predicting power output in the post-stall regime. The conclusion was that perfect regulation is very difficult to achieve, and that even less than perfect regulation is a challenge.
Regarding the construction of passively adaptive blades, Karaolis et al. introduced the concept of using biased lay-ups in blade skins on the surface of blades to achieve different types of twist coupling for wind turbine applications. By changing the blade skin surface from an orthotropic to a biased fiber lay-up, the blade can be aeroelastically tailored with minimal disturbance to the beam stiffness properties or manufacturing costs. Karaolis suggested that in addition to using the flapwise or centrifugal loading to twist a blade, it might be useful to internally pressurize a spar and use changes in the pressure to actively control the angle of blade twist. Karaolis, N. M., Mussgrove, P. J., and Jeronimidis, G. (1988) “Active and Passive Aeroelastic Power Control using Asymmetric Fibre Reinforced Laminates for Wind Turbine Blades,” Proc. 10
th
British Wind Energy Conf., D. J. Milbrow Ed., London, March 22-24, 1988; Karaolis, N. M., Jeronimidis, G., and Mussgrove, P. J. (1989) “Composite Wind Turbine Blades: Coupling Effects and Rotor Aerodynamic Performance,” Proc., EWEC'89, European Wind Energy Conf., Glasgow, Scotland, 1989. FIG. 1.1 from the prior art shows how different fiber orientations in a blade skin can be used to acheive bend-twist or stretch-twist coupling. In his 1988 report, Karaolis mapped out the combinations of two direction lay-ups to maintain strength and produce twist coupling in an airfoil shape.
Middleton et al. and Infield et al.designed, analysed, fabricated and tested a “stretch- twist coupled” blade developed to control the rotor in a runaway scenario. Their composite blade was fabricated using a helical layup with layers of glass and carbon fibers. Measured twist coupling agreed well with predictions and measured runaway speeds were actually less than predicted. Middleton, V., Fitches, P. Jeronimidis, G. and Feuchtwang, J. (1998) “Passive Blade Pitching for Overspeed Control of an HAWT,” Wind Energy 1998, Proceedings of the 20
th
British Wind Energy Association Conference, Cardiff University of Wales, Sep. 2-4, 1998; Infield, D. G., Feuchtwang, J. B. and Fitches, P. (1999) “Development and Testing of a Novel Self-Twisting Wind Turbine Rotor,” Proceedings of the 1999 European Wind Energy Conference, pp 329-332, Nice, France, Mar. 1-5, 1999.
Another report on aeroelastic tailoring concluded that the use of aeroelastic tailoring of the Fibre Reinforced Plastics to control limited torsional deformation is a promising way to improve rotor blade design. Kooijman evaluated building the elastic coupling into the blade skin. Some of his conclusions for blades designed for the “Smart Rotor” were that: (1) Bending-twist coupling gives the potential for a few percentages of energy yield improvement for constant-speed pitch-controlled turbines and improves starting torque by 10%; (2) Optimal constant-speed pitch-controlled rotor production is obtained with the inboard span twisting to feather and the outboard 60% of the span twisting toward stall as wind speed increases; (3) The coupling is best achieved with hybrid carbon/glass reinforcement in the cross ply direction; and (4) Bending-torsion flexibility is about 10% less than a standard construction. Kooijman, H. J. T. (1996) “Bending-Torsion Coupling of a Wind Turbine Rotor Blade,” ECN-I 96-060, Netherlands Energy Research Foundation ECN, Petten the Netherlands.
For constant speed rotors, enhanced stall control of wind turbines has been used in the past to improve the energy capture of rotors by allowing the rotor size to grow while maintaining a low maximum rating on other components in the system. Families of airfoils have been published that have since been used to stall regulate turbines at lower power levels with the associated reduced system cost of energy. Tangler, J. and Somers, D. (1995) “NREL Airfoil Families for HAWTs,” Proc. Windpower '95, American Wind Energy Association, Washington D.C.; Klimas, P. C. (1984) “Tailored Airfoils for Vertical Axis Wind Turbines,” SAND84-1062, Sandia National Laboratories, Albuquerque, N.Mex. An aeroelastically tailored blade that twists to stall in response to flap loads has a similar effect.
We have examined previously the generic coupling effects on annual energy production of a nominally 26 meter diameter stall regulated wind turbine. The blades were assumed to twist to stall, reducing maximum power. The rotor diameter was then increased to bring maximum power back up to its initial level. Twist distributions were specified by prescribing a maximum tip amplitude and a spanwise variation, varying with wind speed in either a linear or quadratic fashion. A twist proportional to power was also used. It was discovered that the details of spanwise variation or how the twist varied with wind speed (or power) had only minor impacts. The twist-coupled blades combined with larger rotors increase power in the important middle-range of wind speeds while power in high winds remains the same. Studies which investigated the increase in annual energy as a function of the annual average wind speed showed that for a maximum twist angle of one degree the energy capture is increased by about 5% and for two degrees, about 10%. The improvements are not overly sensitive to the wind resource. Lobitz, D. W., Veers, P. S., and Migliore, P. G. (1996) “Enhanced Performance of HAWTs Usi
Lobitz Donald W.
Veers Paul S.
Baker Rod D.
Elliott Russell D.
Sandia Corporation
Tudor Harold J.
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