Building a wind turbine that is both durable and more energy efficient requires thinking outside the props.

As the world races to find alternative energy sources away from depleting fossil fuels, attention focuses increasingly on the earth’s limitless supply of wind. The National Renewable Energy Laboratory (NREL) in Golden, Colorado, is driving international collaborative efforts to find innovations in gigantic wind turbines to better harness this invisible and powerful force.

Using wind power to generate electricity was developed first in the 1880s in Scotland and the United States. By the 1920s, the largest wind-driven electric generators were 79 feet high, had four 75-foot diameter rotor blades and generated 5 to 25 kilowatts. The blades, originally of sailcloth or wood, came to be made of cheap high-tensile steel placed atop open lattice towers. Today the typical utility-class–size tubular steel tower ranges between 180 to 350 feet tall and has three fiberglass blades 65 to 130 feet in length. The blades rotate at 10 to 22 revolutions per minute and normally can generate 1.5 to 3 megawatts of power. They can operate in winds up to 60 mph.

“The taller the tower the better it can access strong winds,” explained Jim Johnson, a 20-year NREL Senior Engineer. “But there are tradeoffs. The bigger and taller wind turbines become, the more logistical challenges, engineering problems and costs they incur.”

Problems of wind energy generation range from finding the right site in remote locations with constant high winds, a process called “micrositing,” to transport problems for the huge blade, to finding new composite materials for blades and towers and improving control systems to regulate turbine power.

NREL and the U.S. Department of Energy’s (DOE) Wind Energy Program are making huge strides in resolving these and other issues. One groundbreaking example is the Liberty Turbine, developed by Clipper Windpower in an R&D partnership with NREL and DOE. Those combined efforts have yielded one of the most advanced, and most efficient, wind turbines ever produced.

The 2.5 Megawatt Clipper Liberty wind turbine features a new lightweight, enlarged rotor that increases power production, a revolutionary generator design that improves reliability and reduced gearbox weight and size. Because the generator is lighter, it will also cost less to produce and be easier to maintain than similar machines. Researchers believe this advanced design will become a benchmark for future turbine design in the United States and Europe.

Not only are researchers pushing forward with revolutionary new turbine designs, they are grappling with some of the most vexing problems encountered by turbines with years of service in existing wind farms. Equipment fatigue, especially in turbine gearboxes, threatens to drive up wind power costs, just as the industry is poised to capture a greater share of U.S. generating capacity.

“The faster and harder the turbines run, the faster the ascendancy of fatigue damage,” said Johnson. “It became a particular problem of the wind turbine’s giant gearboxes. Instead of lasting the expected 20 years, these increasingly sophisticated and heavy boxes are found to be stressing out and stiffening after five to seven years.”

The DOE Wind Energy Program focused its efforts on this breakdown in gearbox reliability. It decided to help identify improved gearbox design opportunities.

NREL sponsored a drivetrain “Wind Turbine Reliability” workshop to jointly identify research needs. Some 90 participants from wind farms, service companies, consultants, manufacturers, universities and laboratories shared information on hardware reliability, and offered perspectives on reducing operating and maintenance costs.

A WindPACT drivetrain project was developed to examine the design of gearboxes, the way components interacted and moved and the way the control systems were designed to regulate turbine power and maintain stable, closed-loop behavior (especially in the presence of turbulent winds). It tested simple gearbox designs.

In 2007 NREL initiated a long-term Gearbox Reliability Collaborative (GRC) to improve gearbox performance and durability. The collaborative validated the design process, tested extensively instrumented gearboxes to identify weakness in current design approaches and pointed out ways to improve initial designs and retrofit packages.

“We helped the collaborative write a standard to improve the level of manipulation, concentrating on the design and standards of gearboxes,” said Johnson. Then came the surprise.

“NREL discovered that the fatigue failings of the gearbox were not due to its design, but were caused by problems with the bearings,” said Johnson with some satisfaction. “Awareness of this has been a kind of breakthrough in the problem of wind turbine component fatigue. Now we can focus on the real cause.”