Fluid reaction surfaces (i.e. – impellers) – With control means responsive to non-cyclic condition... – Responsive to relative working fluid velocity
Reexamination Certificate
2002-03-07
2004-06-15
Look, Edward K. (Department: 3745)
Fluid reaction surfaces (i.e., impellers)
With control means responsive to non-cyclic condition...
Responsive to relative working fluid velocity
C416S044000, C416S120000
Reexamination Certificate
active
06749399
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the field of wind turbine generators. Specifically, the invention relates to an array of wind turbine rotors on a single tower that are individually optimized to improve the economics of the entire system.
BACKGROUND OF THE INVENTION
Wind turbines have gained widespread use for electricity generation in recent years. The cumulative capacity of wind turbines installed worldwide has grown at a rate of approximately 32% per year over the past ten years. As of the end of 2001, the total installed capacity of wind turbines worldwide added up to over 20,000 MW. Future growth prospects for the industry are bright, although the economics of wind energy must continue to improve for the market to grow. There are signs that the potential for economic gains from current wind turbine technology is constrained.
As the market for wind turbines has grown in recent years, the size of the turbines has also grown.
FIG. 1
shows the typical rotor diameter and rated power for state of the art wind turbines that have been installed in Europe over the past 15 years. Most wind turbine manufacturers have recently introduced turbine designs in the range of 1.5 to 2.5 MW with rotor diameters of 66 to 80 m. Even larger turbines are on the drawing boards of most wind turbine manufacturers. The trend toward larger turbines has been driven partly by technological and economic improvements, but the trend has largely been driven by market demand. Bigger wind turbines have proven to be more productive than smaller designs, largely due to the taller tower heights of the larger machines. Also, there is an economy of scale that favors large turbines because there are certain fixed costs associated with road construction, project planning, SCADA equipment, and operations and maintenance that do not increase with larger turbines. However, large turbines are also considerably more expensive than smaller machines and the economy of scale does not completely explain the trend to multi-megawatt size wind turbines. Project developers have demanded larger wind turbines at least partly due to perception issues. In Europe, where population density is relatively high compared to the United States, it is easier to obtain permits for fewer large turbines compared to a larger number of small turbines. Also, as wind turbines are installed offshore, there is a growing market for very large turbines to be used in that market.
As the size of wind turbines grows, there are several technical issues that adversely affect the economics of wind energy and that can potentially lead to constraints in turbine size. Basic design principles indicate that the weight of the turbine increases approximately with the cube of the rotor diameter. System cost is generally proportional to the turbine's weight and so the cost of the turbine increases approximately with the rotor diameter cubed. The turbine weight and cost increase faster than the energy capture, which increases with the rotor diameter squared. For relatively small turbine sizes, there are other economies of scale that outweigh the increase in turbine weight and cost, however for turbines over approximately one megawatt in size the economies of scale are outweighed.
Another problem with large wind turbines is blade deflection. The wind turbine rotor is typically oriented upwind of the tower so that the blades bend downwind toward the tower. The turbine designer must take care so that the blade does not strike the tower, thereby causing catastrophic failure. The blade's stiffness, defined by the material modulus of elasticity multiplied by the cross-sectional moment of inertia, or EI, increases as the blade becomes longer. However, the loads that cause deflection also increase for longer blades. If all of the blade's dimensions are scaled proportionately to the blade length, then EI increases with the blade length to the fourth power whereas the bending moment increases with the blade length to the third power. This would lead to lower deflection for longer blades. However, practical considerations such as tooling, blade weight, and material cost constrain the design so that the blade's chord and thickness are smaller relative to the blade's length for large rotors. This causes a higher aspect ratio and lower solidity for large rotors. The lower solidity requires a higher tip speed for good aerodynamic performance and the higher tip speed can lead to increased centrifugal stiffening of the blade which reduces blade deflection. However, noise issues tend to constrain tip speed ratio so that centrifugal stiffening is less for very large rotors. Deflection thereofore becomes the design driver for very large rotors. Blade deflection can be mitigated by using large uptilt of the wind turbine nacelle. However, wind turbine designers are already using high (7 degrees) uptilt and negative coning to avoid tower strikes. Some blades are even being built curved to incorporate effective negative coning. All of this points toward blade deflection becoming a limiting design criteria for very large wind turbine rotors.
Another issue with very large rotors is that there is a large amount of composite material in each blade which can lead to material problems. Statistically, there is a higher probability of a defect existing in a large blade than in a small blade. If a defect is built into a blade, it can propogate to become a crack which will eventually lead to the blade's failure. As the thickness of the blade's laminate increases, it becomes more and more difficult to detect flaws in the material. Therefore, very large wind turbine blades may have a higher statistical probability of failure than a larger number of smaller blades.
Another issue for very large wind turbines is transportation and installation logistics. The long blade lengths being used on multi-megawatt wind turbines can exceed the capacity of public roads. Also, the tower heights necessary to support the large rotors can exceed the height capacity of cranes that are readily available.
Another problem experienced by the large wind turbines that are currently under development or being sold is that the rotors are so large that they experience a massive differential in wind speed from one side of the rotor to the other. Vertical wind shear exponents in the Midwest have been measured as high as 0.40 which can cause the wind speed across a 70 m rotor to vary by 62% from bottom to top if the turbine is mouted on a 65 m tower. The variation in wind loading is even more severe since loads are generally proportional to wind speed squared. In the example given, the bending load due to the wind at the top of the rotor would be 262% higher than the bending load due to the wind at the bottom of the rotor. Since each blade moves through this shear field, they are subjected to extreme fatigue loading conditions. From an energy standpoint, things are even worse. Energy in the wind is proportional to wind speed cubed which means that the energy content in the wind at the top of the rotor is 425% higher than the energy content at the bottom of the rotor.
Since all of the blades have the same rotational speed and pitch angle, it means that the entire rotor must be optimized for an average wind that is “seen” by the entire rotor. The rotor speed and blade pitch that work best for the wind speed at the rotor's center may not work well at all for the portions of the rotor at the top and bottom. Therefore, at least part of the rotor will be operating in a sub-optimal condition whenever a wind shear is present. This problem is made worse as the turbine's rotor diameter gets larger.
The issue of windspeed variation across the rotor also has negative implications for selecting an appropriate turbine for a given site. Wind turbine manufacturers generally offer their equipment with a range of rotor diameters for a given power rating or, conversely, with a range of power ratings for a given rotor diameter. For example, a 750 kW turbine may be sold with an option for a
Bosche John Vanden
Look Edward K.
McCoy Kimya N
Ocean Wind Energy Systems
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