July 30, 2012 – Sandia National Laboratories’ wind energy researchers say they are re-evaluating vertical-axis wind turbines (VAWTs) to “help solve some of the problems of generating energy from offshore breezes”. Though VAWTs have been around since the earliest days of wind energy research at Sandia, VAWT architecture could transform offshore wind technology, say the researchers.

The economics of offshore windpower are different from land-based turbines due to installation and operational challenges, says Sandia. VAWTs offer three big advantages that could reduce the cost of wind energy: a lower turbine centre of gravity; reduced machine complexity; and better scalability to very large sizes.

A lower centre 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 U.S. Department of Energy (DoE) solicitation for advanced rotor technologies for American 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.”

Large offshore VAWT blades in excess of 300 metres will cost more to produce than blades for onshore wind turbines, says Sandia but, as the machines and their foundations get bigger (closer to the 10-20MW 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.

However, challenges remain before VAWTs can be used for large-scale offshore power generation, adds Sandia. Curved VAWT blades are complex, making manufacture difficult. Matt Barone, the project’s other principal investigator, said partners Iowa State University and TPI Composites will explore new techniques to enable the 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 when 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.

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 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.

“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.

Photo 1: Basing their work on decades of wind energy research and experience, Sandia engineers are creating several concept designs, running those designs through modern modelling software and narrowing those design options down to a single, most-workable design for a VAWT turbine-blade. Results aren’t in, but the early favourite for further testing is the Darrieus design. Illustration by Josh Paquette and Matt Barone, courtesy Sandia.

Photo 2: A Sandia team completes installation in the late 1980s of a vertical-axis wind turbine test platform in Bushland, Texas. Photo by Randy Montoya, courtesy Sandia.