Wind Energy Enabling Research
Taking turbine designs to the "limit" of cost and performance will require advances in several research disciplines. While some of the near-term cost of energy reductions may be possible based on current levels of technology (e.g., tall towers) others will require investment in fundamental research in order to be successful.
Enabling research activities to support large wind technology and distributed wind technology program goals fall within four major topic areas:
- Advanced Rotor Development
- Site-Specific Design
- Generator, Drivetrain and Power Electronics Efficiency Improvements
- Systems and Controls
Advanced Rotor Development
The wind turbine rotor is the only component unique to wind turbines in the system. The rotor's blades control all the energy capture and almost all the loads, and are therefore a primary target of research efforts. The challenge for researchers is to create the knowledge and engineering tools to enable blade designers to squeeze the most performance out of the lowest possible cost, using new materials, improved manufacturing processes, and enhanced design tools. This work will assist the industry in meeting the large wind technology and distributed wind technology goals, by stretching rotors longer to produce electricity at wind sites that previously were not cost effective.
Advanced rotor development work can be segmented into three areas:
- Blade development
- Aerodynamic code development and validation
- Aeroacoustics research and testing
Blade Development
A significant step toward the wind turbine cost goals will be achieved with blades that are stiffer, stronger and span a greater area, while lighter and adaptive, to reduce system loads. Beyond that, design details need to be evaluated so that the entire industry is led in the direction of efficient material usage. Finally, substantial testing, both in the laboratory and in the field, is required to validate the tools, loads, and designs, and to make sure they can be linked to the site characteristics.
Aerodynamic Code Development and Validation
The current generation of aerodynamic performance codes don't accurately predict performance in turbulent winds. To gain a better understanding of wind turbine aerodynamics, researchers at the NWTC and Sandia are combining two-dimensional field and wind tunnel test data to predict aerodynamic loads on turbines under varied inflow conditions. To improve the predictive power of these analyses, researchers are adapting a more accurate approach used for helicopter and aircraft design. Their evaluation will be based on analyses of three-dimensional computations of fluid dynamics.
Aeroacoustic Research and Testing
Turbine noise can be caused by rotor speed, blade shape, tower shadow, and other factors. The program is sponsoring both wind tunnel and field tests to develop a noise prediction code that turbine manufacturers can use to ensure that new rotor designs and full systems aren't too noisy. This is especially true for high-growth U.S. markets for small wind turbines that will demand quieter rotors, especially when turbines are sited in residential neighborhoods. Small turbines operate at high rotational speeds and tend to spin even if they are furled (pointed out of the wind). Aeroacoustics research activities will be conducted to explore how to reduce noise produced by distributed wind turbines in a variety of wind regimes and to develop a noise standard with industry participants that can be used for the growing domestic distributed wind turbine market. This research will support the program's public-private partnerships, both directly in working with industry and indirectly in providing necessary underlying research.
In the longer term, program researchers will work to develop physics-based aeroacoustics codes for both design and problem solving applications. These will enable more slender blades and higher tip speeds, enhancing both cost and performance of future designs.
Site-Specific Design
For wind use to continue to grow, future wind energy installations will need to be in areas with more challenging winds. Wind farms will need to move into areas with less wind, using taller towers and longer blades to harvest the more rarified energy. Wind turbines designed to be strong enough to survive high wind areas will drive up the cost unnecessarily for less windy sites and could limit the area into which wind energy is able to provide cost effective energy. The benefits of designing significant installations (100 MW or more) for specific site conditions are substantial. The nature of the atmospheric loading at increasing heights must be assessed and documented. The types of blades, including aerodynamic geometry, controls, and structural details need to be tuned to the energy capture requirements and durability suitable for low energy and lightly loaded sites. Every structural strength requirement throughout the system is sized based on the expected maximum event and turbulence at the site.
This area of work is therefore two-fold. One is to create systematic methods of specifying specific site energy and load conditions. The other area will be to conduct the field measurements that validate the procedure, and to work in public-private partnerships that fill in the site-specific information at interesting regions of the country.
Inflow Characterization
To harvest electricity economically from low wind speed sites, turbines must reach higher up, where the winds are stronger. Wind speeds increase with altitude, but so do turbulent wind patterns. Turbulent winds can cause wind turbine damage that would reduce the life of the turbine.
Researchers need a much better understanding of the wind resource and the nature of inflow and its impact on turbine performance and reliability. A clear understanding of the nocturnal jets encountered at sites in the Great Plains is critical. (The nocturnal jet is a poorly understood phenomenon that occurs at night as cooling allows high-level, high-velocity winds to dip close to the earth's surface, creating violently turbulent wind regimes. Low wind speed turbines will be exposed to nocturnal jets in some areas.) New components and architectures, which reduce structural loads while increasing performance and energy output, must be explored. Design and performance codes must continue to improve if innovation is to be sustained.
Design Load Specification
The inherent uncertainties of site conditions, turbulent winds, extreme events, and component strength must also be accounted for in a manner that does not require overly conservative design margins. International design standards have traditionally been based on the worst-case situation over broadly defined site classes. As turbines become routinely designed for specific sites, where these standard load cases can be reduced and tuned to site-specific conditions, the ability to estimate and account for individual design uncertainties will become necessary. Site-specific design margins will be needed to avoid a catastrophic loss of a wind plant. Sophisticated financial institutions increasingly will require site-specific design for due-diligence before investing in large installations. Methods of estimating and designing to site-specific environments with uncertainty-based design margins will be established and integrated into standard design practices.
Generator, Drivetrain and Power Electronics Efficiency Improvements
Drivetrain components include generators, gearboxes, shafts, and bearings. These components convert the slow-rotating mechanical energy from the rotor into electrical energy. Large wind turbines and small, distributed turbines will need drive trains specially designed to operate in these challenging operating environments.
Systems and Controls
Both large and distributed wind turbines will be dynamically active and must be carefully designed to lessen unwanted structural dynamic loads and responses. New innovative hub control strategies are being developed to reduce unwanted aerodynamic loads at the rotor hub. Optimization of conventional control strategies such as blade pitching as well as developing new methods including twist-coupled blades and embedded micro-tabs are being evaluated. The control strategies have to be designed to meet two seemingly conflicting goals — to increase energy capture, yet reduce turbine structural loading. Studies indicate that large wind technology goals can be met if wind energy technology moves toward large slender turbines placed on tall towers. Designing these large structures to be long lasting and fatigue-resistant at minimal cost is a difficult task. While the rotor itself can be made more cost effective through innovative approaches to control, it is the entire wind turbine system that is the expected beneficiary as loads are reduced everywhere on the structure.
To address research needs specific to passively and actively controlled distributed wind turbines, improvements in current design model capabilities are needed. These models are implemented in computer codes used by industry to improve turbine designs as they go through the engineering design process. Many small turbines use a passive overspeed control such as furling. In furling, the force of the wind turns the rotor sideways, just as farm water-pumping windmills have done for 100 years. So far, no computer codes have been able to reliably predict the performance or assist in the design of furling mechanisms. This means such designs need to be performed empirically, raising development costs.