Offshore Energy: Vertical Wind Turbines

Wednesday, August 1, 2012 @ 05:08 PM gHale


There is a problem generating energy from offshore wind so one of the cures may involve a re-evaluation off vertical axis wind turbines (VAWTs).

While VAWTs have been around since the earliest days of wind energy research, a new architecture could transform offshore wind technology, said Sandia National Laboratories’ wind energy researchers.

RELATED STORIES
Siemens to Build Aussie Wind Farm
Boosting Wind Power from Memory
Ultracapacitor Delivers a Steady Jolt
Smart Grid Needs to get Smarter

The economics of offshore windpower are different from land-based turbines, due to installation and operational challenges. 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. Fewer parts, lower fatigue loads and simpler maintenance all lead to reduced maintenance costs.

Sandia is conducting the research under a 2011 Department of Energy (DoE) 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 can address 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.”

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 10–20 megawatt (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.

Challenges do remain before VAWTs can fire up for large-scale offshore power generation.

Curved VAWT blades are complex, making manufacturing them difficult. Producing very long VAWT blades demands innovative engineering solutions. Matt Barone, the project’s other principal investigator, said partners Iowa State University and TPI Composites will explore new techniques to enable manufacture of geometrically complex VAWT blade shapes at an unprecedented scale, but at acceptable cost.

VAWT blades must also 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,” Barone said.

Another challenge is brakes. Older VAWT designs didn’t have an aerodynamic braking system, and relied solely on a mechanical braking system that is more difficult to maintain and less reliable than the aerodynamic brakes used on HAWTs.

HAWTS use pitchable blades, which stop the turbine within one or two rotations without damage to the turbine and use 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.

“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.



Leave a Reply

You must be logged in to post a comment.