Sandia National Laboratories' wind energy researchers are re-evaluating vertical-axis wind turbines (VAWTs) to help solve some of the problems associated with generating electricity from offshore breezes.
The engineers are creating several concept designs, running those designs through modeling software and narrowing those options down to a single workable design for a VAWT blade, Sandia explains.
According to the lab, VAWTs offer three big advantages that could reduce the cost of wind energy: a lower turbine center of gravity, reduced machine complexity and better scalability to very large sizes.
A lower center of gravity means improved stability afloat and lower gravitational fatigue loads. Additionally, the drivetrain on a VAWT is at or near the surface, potentially making maintenance easier and less time-consuming, Sandia explains, adding that fewer parts, lower fatigue loads and simpler maintenance all lead to reduced maintenance costs.
The VAWT research is being conducted under a 2011 U.S. Department of Energy solicitation for advanced rotor technologies for U.S. offshore wind power generation. The five-year, $4.1 million project began in January.
Sandia's wind energy program aims to address the national energy challenge of increasing the use of low-carbon power generation.
"VAWTs are elegant in terms of their mechanical simplicity," says Josh Paquette, one of Sandia's two principal investigators on the project. "They have fewer parts because they don't need a control system to point them toward the blowing wind to generate power."
These characteristics fit the design constraints for offshore wind: the high cost of support structures; the need for simple, reliable designs; and economic scales that demand larger machines than current land-based designs.
Large offshore VAWT blades in excess of 300 meters will cost more to produce than blades for onshore wind turbines. But as the machines and their foundations get bigger – closer to the 10 MW to 20 MW scale – turbines and rotors become a much smaller percentage of the overall system cost for offshore turbines, so other benefits of the VAWT architecture could more than offset the increased rotor cost, according to Sandia Labs.
Challenges
However, challenges remain before VAWTs can be used for large-scale offshore power generation.
For instance, VAWT blades must overcome problems with cyclic loading on the drivetrain. Unlike horizontal-axis wind turbines (HAWTs), which maintain a steady torque if the wind remains steady, VAWTs have two "pulses" of torque and power for each blade, based on whether the blade is in the upwind or downwind position.
This "torque ripple" results in unsteady loading, which can lead to drivetrain fatigue. The project will evaluate new rotor designs that smooth out the amplitude of these torque oscillations without significantly increasing rotor cost.
Because first-generation VAWT development ended decades ago, updated designs must incorporate decades of research and development already built into current HAWT designs. Reinvigorating VAWT research means figuring out the models that will help speed up turbine design work.
"Underpinning this research effort will be a tool development effort that will synthesize and enhance existing aerodynamic and structural dynamic codes to create a publicly available aeroelastic design tool for VAWTs," explains Matt Barone, the project's other principal investigator.
In addition, new VAWT designs will need robust aerodynamic brakes that are reliable and cost-effective, with a secondary mechanical brake much like on modern-day HAWTs. Unlike HAWT brakes, new VAWT brakes will not have actively pitching blades, which have their own reliability and maintenance issues.
The first phase of Sandia's program will take place over two years and will involve creating several concept designs, running those designs through modern modeling software and narrowing those design options down to a single design. During this phase, Paquette, Barone and their colleagues will look at all types of aeroelastic rotor designs, including HAWTs and V-shaped VAWTs.
In phase two, researchers will build the chosen design over three years, eventually testing it against the extreme conditions that a turbine must endure in an offshore environment. In addition to rotor designs, the project will consider different foundation designs.
The University of Maine will develop floating VAWT platform dynamics code and subscale prototype wind/wave basin testing; Iowa State University will develop manufacturing techniques for offshore VAWT blades and subscale wind tunnel testing; TPI Composites will design a proof-of-concept subscale blade and develop a commercialization plan; TU-Delft will work on aeroelastic design and optimization tool development and modeling; and Texas A&M University will work on aeroelastic design tool development.
