Enormous blades for offshore energy

A new design for gigantic blades longer than two football fields could help bring offshore 50-megawatt (MW) wind turbines to the United States and the world.

Sandia’s research on the extreme-scale Segmented Ultralight Morphing Rotor (SUMR) is funded by DOE’s ARPA-E program. The challenge: Design a low-cost offshore 50-MW turbine requiring a rotor blade more than 650 feet (200 meters) long, two and a half times longer than any existing wind blade.

The team is led by the University of Virginia and includes, in addition to Sandia, researchers from the University of Illinois, the University of Colorado, the Colorado School of Mines, and the National Renewable Energy Laboratory. Corporate advisory partners include Dominion Resources, General Electric, Siemens, and Vestas.

“Exascale turbines take advantage of economies of scale,” says Todd Griffith, lead blade designer on the project and technical lead for Sandia’s Offshore Wind Energy Program.

Great offshore wind energy potential

Sandia’s previous work on 13-MW systems uses 100-meter blades on which the initial SUMR rotor designs are based. While a 50-MW horizontal wind turbine is well beyond the size of any current design, studies show that load alignment can dramatically reduce peak stresses and fatigue on the rotor blades. This reduces costs and allows construction of blades big enough for a 50-MW system.

Most current US wind turbines produce power in the 1-2 MW range, with blades about 165 feet (50 meters) long, while the largest commercially available turbine is rated at 8 MW, having blades 262 feet (80 meters) long.

“The US  has great offshore wind energy potential, but offshore installations are expensive, so larger turbines are needed to capture that energy at an affordable cost,” Todd says.

Barriers remain before designers can scale up to a 50-MW turbine — more than six times the power output of the largest current turbines.

“Conventional upwind blades are expensive to manufacture, deploy, and maintain beyond 10-15 MW. They must be stiff to avoid fatigue and eliminate the risk of tower strikes in strong gusts. Those stiff blades are heavy, and their mass, which is directly related to cost, becomes even more problematic at the extreme scale due to gravity loads and other changes,” Todd says.

He says the new blades could be more easily and cost effectively manufactured in segments, avoiding the unprecedented-scale equipment needed for transport and assembly of blades built as single units.

Huge blade design inspired by palm trees

The exascale turbines would be sited downwind, unlike conventional turbines that are configured with the rotor blades upwind of the tower.

SUMR’s load-alignment is bio-inspired by the way palm trees move in storms. The lightweight, segmented trunk approximates a series of cylindrical shells that bend in the wind while retaining segment stiffness. This alignment radically reduces the mass required for blade stiffening by reducing the forces on the blades using the palm-tree inspired load-alignment approach.

Segmented turbine blades have a significant advantage in parts of the world at risk for severe storms such as hurricanes, where offshore turbines must withstand tremendous wind speeds of more than 200 mph. The blades align themselves to reduce cantilever forces through a trunnion hinge near the hub that responds to changes in wind speed.

“At dangerous wind speeds, the blades are stowed and aligned with the wind direction, reducing the risk of damage. At lower wind speeds, the blades spread out more to maximize energy production.” Todd says.

DOE’s goal is to reduce the cost of wind power to support deployment that could provide 20 percent of the nation’s energy from wind by 2030. Exascale turbines made possible by giant blades could be an important way to meet that goal.