The focus of CARE is on three areas that are currently being pursued by the U.S. Department of Energy (DOE), the national Renewable Energy Laboratory (NREL), and the wind power related industries. These three areas include:
1. Innovative designs of large turbines blades (Megawatt level)
2. Power-train (Gearbox) of the large turbines (Megawatt level)
3. Distributed wind power system for low-speed wind sites (Kilowatt level)
Below is a brief description of the current status of different components of typical wind turbines and the required research/design work to be done.
Current Status of Turbine Components
Typically, a modern turbine will cut-in and begin to produce power at a wind speed of about 5 m/s (Figure 1). It will reach its rated power at about 12 m/s to 14m/s, where the pitch control system begins to limit power output and prevent generator and drive-train overload. At around 22 m/s to 25 m/s, the control system pitches the blades to stop rotation, feathering the blades to prevent overloads and damage to the turbine’s components. The job of the rotor is to operate at the absolute highest efficiency possible between cut-in and rated wind speeds, to hold the power transmitted to the drive-train at the rated power when the winds go higher, and to stop the machine in extreme winds. Modern utility-scale wind turbines generally extract about 50% of the energy in this stream below the rated wind speed, compared to the maximum energy that a device can theoretically extract, which is 59% of the energy stream.
Figure 1 – Cut-in and cut-out speeds for a typical wind turbine
As wind turbines grow in size, so do their blades—from about 8 m long in 1980 to more than 40 m for many land-based commercial systems and more than 60 m for offshore applications today. Rigorous evaluation using the latest computer analysis tools has improved blade designs, enabling weight growth to be kept to a much lower rate than simple geometric scaling.
Designers are also starting to work with lighter and stronger carbon fiber in highly stressed locations to stiffen blades and improve fatigue resistance while reducing weight. (Carbon fiber, however, costs about 10 times as much as fiberglass.) Using lighter blades reduces the load-carrying requirements for the entire supporting structure and saves total costs far beyond the material savings of the blades alone.
The Drive-train (Gearbox, Generator, and Power Converter)
Generating electricity from the wind places an unusual set of requirements on electrical systems. Most applications for electrical drives are aimed at using electricity to produce torque, instead of using torque to produce electricity. The applications that generate electricity from torque usually operate at a constant rated power. Wind turbines, on the other hand, must generate at all power levels and spend a substantial amount of time at low power levels. Unlike most electrical machines, wind generators must operate at the highest possible aerodynamic and electrical efficiencies in the low-power/low-wind region to squeeze every kilowatt-hour out of the available energy. For wind systems, it is simply not critical for the generation system to be efficient in above-rated winds in which the rotor is letting energy flow through to keep the power down to the rated level. Therefore, wind systems can afford inefficiencies at high power, but they require maximum efficiency at low power—just the opposite of almost all other electrical applications in existence.
Torque has historically been converted to electrical power by using a speed-increasing gearbox and an induction generator. Many current megawatt-scale turbines use a three-stage gearbox consisting of varying arrangements of planetary gears and parallel shafts. Generators are either squirrel-cage induction or wound-rotor induction, with some newer machines using the doubly fed induction design for variable speed, in which the rotor’s variable frequency electrical output is fed into the collection system through a solid-state power converter. Full power conversion and synchronous machines are drawing interest because of their fault-ride-through and other grid support capacities.
As a result of fleet-wide gearbox maintenance issues and related failures with some designs in the past, it has become standard practice to perform extensive dynamometer testing of new gearbox configurations to prove durability and reliability before they are introduced into serial production. The long-term reliability of the current generation of megawatt-scale drive-trains has not yet been fully verified with long-term, real-world operating experience. There is a broad consensus that wind turbine drive-train technology will evolve significantly in the next several years to reduce weight and cost and improve reliability.
The tower configuration used almost exclusively in turbines today is a steel monopole on a concrete foundation that is custom designed for the local site conditions. The major tower variable is height. Depending on the wind characteristics at the site, the tower height is selected to optimize energy capture with respect to the cost of the tower. Generally, a turbine will be placed on a 60-m to 80-m tower, but 100-m towers are being used more frequently. Efforts to develop advanced tower configurations that are less costly and more easily transported and installed are ongoing.
Technology improvements on the horizon
The required technological improvements are relatively straightforward: taller towers, larger rotors, and continuing progress through the design and manufacturing learning curve. No single component or design innovation can fulfill the need for technology improvement. By combining a number of specific technological innovations, however, the industry can introduce new advanced architectures necessary for success. The 20% Wind Scenario does not require success in all areas; progress can be made even if only some of the technology innovations are achieved. Section IV
- Innovative blade design: Department of Mechanical Engineering