Published on August 2nd, 2012 | by Glenn Meyers11
Offshore Use of Vertical-Axis Wind Turbines Gets Closer Look by Sandia Labs
Do the best offshore wind turbines operate with a horizontal or vertical-axis mechanism? That question is being reexamined by Sandia National Laboratories’ wind energy researchers who are now re-evaluating vertical-axis wind turbines (VAWTs) to help solve some of the problems of generating energy from offshore breezes using horizontal-axis wind turbines (HAWTs).
The economics of offshore wind power are different from land-based turbines, due to installation and operational challenges, Sandia Labs noted in a recent press announcement. Sandia contends that VAWTs can offer three significant advantages that can reduce the cost of wind energy:
- lower turbine center of gravity, meaning improved stability and lower gravitational fatigue loads;
- reduced machine complexity with fewer parts, which makes maintenance easier and less time-consuming;
- better scalability to very large sizes.
Sandia is conducting its research under a Department of Energy solicitation for advanced rotor technologies for U.S. offshore windpower generation. The five-year, $4.1-million project began in January of this year.
Wind Energy Technologies manager Dave Minster said Sandia’s wind energy program is aimed at addressing the national energy challenge of increasing the use of low-carbon power generation.
“VAWTs are elegant in terms of their mechanical simplicity,” said 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.”
But VAWT blades must overcome problems with cyclic loading on the drive train. Unlike horizontal-axis wind turbines, 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 drive train 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.
Another challenge involves braking. Older VAWT designs had no aerodynamic braking system and relied only on a mechanical braking system.
HAWTS use pitchable blades, which stop the turbine within one or two rotations without damage to the turbine and are based on multiple redundant, fail-safe designs. Barone said 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 won’t have actively pitching blades, which have their own reliability and maintenance issues.
In the 1970s and 1980s, VAWTs were actively developed as windpower generators until being overtaken by HAWTs.
“HAWTs emerged as the predominant technology for land-based wind over the past 15 years primarily due to advantages in rotor costs at the 1 to 5 megawatt scale,” Paquette said.
The first phase of the program will take place over two years and involve creating several concept designs, running those designs through modern modeling software and narrowing those design options down to a single, most-workable design. In the second phase, 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.
“Ultimately it’s all about the cost of energy. All these decisions need to lead to a design that’s efficient and economically viable,” said Paquette.
Source: Sandia National Laboratories