Fluid reaction surfaces (i.e. – impellers) – Actuation directly responsive to magnetic or electrical effect
Reexamination Certificate
2002-10-08
2004-08-03
Look, Edward K. (Department: 3745)
Fluid reaction surfaces (i.e., impellers)
Actuation directly responsive to magnetic or electrical effect
C416S041000, C416S240000, C416S147000, C415S015000
Reexamination Certificate
active
06769873
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The invention relates to wind turbines and is directed more particularly to a turbine blade assembly in which the turbine blades are reconfigured for maximum performance automatically in the course of operation of the turbine.
(2) Description of the Prior Art
Wind turbines are alternative energy sources with low environmental impact. The basic physical principle of wind turbine operation is to extract energy from the wind environment to rotate a mechanism to convert mechanical energy to electrical energy. In
FIG. 1
, there is shown a typical horizontal axis wind turbine. The turbine generally includes two or three blades
20
attached to a hub
22
. Optimally, the blades
20
are lightweight but very stiff, to resist wind gusts. Many blades employ aerodynamic controls, such as ailerons or wind brakes, to control speed. The hub
22
is connected to a drive train (not shown) and is typically flexible to minimize structural loads. This mechanism is connected to an electrical generator
24
. Wind turbines usually employ constant rotational speed generators, though advances are underway to utilize variable speed generators with efficient transformers. Variable speed generators have an advantage in that expensive gearboxes can be reduced or eliminated. The entire mechanism is elevated by a tower structure
26
. The higher the tower, the stronger the wind, generally. A control room
28
usually is located near the turbine to monitor wind conditions and employ control strategies on the turbine.
Future applications envision wind turbines connected to a main power grid to provide energy to home and business users. At present, the cost of energy associated with wind turbines is significantly higher than the cost associated with non-renewable energy sources (coal or gas fired turbine generators, for example). The U.S. Department of Energy has a goal of substantially reducing the energy cost for sites where the average annual wind speed is about 15 mph. To do this, turbines must more efficiently generate power at lower wind speeds and must withstand excessive structural loading at high wind speeds. Wind turbines constructed based on current technology shut down at very low (below 6 mph) and very high (above 65 mph) wind speeds. This increases the cost of electricity.
The basis for electrical energy generation resides in the aerodynamics associated with a wind turbine. The turbine generates energy from lift produced on the blades in the presence of wind.
FIG. 2A
shows the effective lift and drag produced by a turbine blade
20
in operation. The two main sources of velocity the blade
20
“sees” are due to the rotation r&ohgr; of the rotor and the oncoming wind V
w
. The angle &bgr; is the physical angle of the blade either due to a pitch mechanism or due to the twist along the blade. The angle of attack &agr; the blade ‘sees’ is therefore:
&agr;=tan
−1
(
V
w
/r
&ohgr;)−&bgr; (1)
As wind speed increases, the angle of attack &agr; on the blades increases. The blade pitch and twist is typically designed to optimize the angle of attack near the average wind speed. Thus, at low wind speeds the angle of attack is lower than optimum and the turbine loses efficiency. At very low speeds, there is insufficient energy available to drive the turbine. At high wind speeds the angle of attack of the blade becomes excessively large and can drive the blade into a stall. As a result, the forces and moments on the turbine blades become too high and the turbine is shut down to prevent blade failure caused by excessive dynamic loading.
The above applies specifically to the case in which the wind across the turbine rotor is uniform and perpendicular to the flow. During normal operating conditions, neither assumption is typically valid. The flow across the rotor is usually very non-uniform with horizontal and vertical wind shear components. In addition, much of the time, the flow into the rotor (
FIG. 2B
, for example) is offset by a certain yaw angle &ggr;. Defining the position of the blade in the rotation cycle by &PSgr;, there is a normal V
n
and a crossflow V
c
component of the wind:
V
n
=V
w
cos &ggr;
V
c
=−V
w
sin &ggr; (2)
The wind velocity is also modified due to horizontal and vertical wind shear at a given position in the angular rotation cycle:
V
w
=V
mean
+(
r/R
)[
V
vshear
cos &PSgr;+
V
hshear
sin &PSgr;] (3)
The tangential velocity the blade experiences during the rotation cycle is then:
V
t
=r&ohgr;+V
c
cos &PSgr; (4)
The instantaneous angle of attach of the blade during the rotation cycle is then:
&agr;=tan
−1
(
V
n
/V
t
)−&bgr; (5)
During uncontrolled turbine operation, there is significant variation in the local blade angle of attack. For large angle of attack variations, this can result in a phenomena termed “dynamic stall”. Experimental field studies have demonstrated that significant dynamic loading can be experienced by the turbine blade resulting in fatigue and potential failure of the wind turbine. This problem is a major cause of increased operational and maintenance costs. An additional consequence is that for high wind speeds, the turbine is rarely operating under optimal conditions in terms of blade angle of attack. For both low and high wind speeds, it is desirable to control the local blade angle of attack to establish optimal operating conditions.
There are essentially two ways to control the blade angle of attack. The first is to vary the rotational velocity of the turbine. This is a major reason that research has been conducted to improve the efficiency of variable speed power transformers. During high wind speeds, it is desirable to increase the rotational velocity of the turbine, and decrease the rotational velocity during low wind speeds. Unfortunately, the efficiency of the transformers are such that it is still more cost effective to sacrifice operating the turbine during high wind states and maintain constant rotational velocity.
Accordingly, there is a need to provide an alternative wind turbine assembly which facilitates control of the angle of attack of the blades, as by actively or dynamically reconfiguring the blades to provide continuous adjustment of the angle of attack, as by local blade pitch angle adjustments and/or by pitching the entire blades.
SUMMARY OF THE INVENTION
An object of the invention is, therefore, to provide a wind turbine assembly adapted to twist the turbine blades dynamically to increase efficiency at low wind speeds, and reduce dynamic loads at high wind speeds.
A further object of the invention is to provide a wind turbine assembly having means to control the dynamics of the blade during instances of wind shear and non-zero yaw of the turbine with respect to the wind, such that optimal blade angles of attack can be maintained throughout the entire rotational cycle of the wind turbine.
A still further object of the invention is to provide a wind turbine assembly adapted to effect dynamic blade twist so that wind turbines will start at lower wind speeds, and continue to operate at higher wind speeds.
A still further object of the invention is to provide a wind turbine assembly adapted to adjust the twist of the wind turbine blades so as to increase the lift at low speeds and decrease the lift at high speeds, whereby to increase the range of wind speeds at which wind turbines can practically produce energy, and wherein at any specific wind speed the blades twist is optimized for that speed to improve the overall efficiency of the system.
With the above and other objects in view, a feature of the present invention is the provision of a dynamically reconfigurable wind turbine blade assembly comprising a plurality of reconfigurable twistable blades mounted on a hub, an actuator fixed to each of the blades and adapted to effect the reconfiguration thereof, and an actuator power regulator for regulating electrical power supplied to the a
Beauchamp Charles H.
Huyer Stephen A.
Plunkett Stephen J.
Kasischke James M.
Kershteyn Igor
Look Edward K.
Oglo Michael F.
Stanley Michael P.
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