The average wind turbine rated power has increased 20x since 1985, with the largest commercially available wind turbines rated at 8MW with 262ft (80m) long blades. Economies of scale and high offshore wind speeds are driving the industry to increase output with larger systems. Based off a morphing downwind-aligned rotor concept – which protects blades by collapsing them during extreme wind conditions – engineers are gearing up to test a Segmented Ultralight Morphing Rotor (SUMR) on a 50MW system with blades longer than two football fields.

Though several barriers remain before designers can scale up to a 50MW turbine – more than 6x the power output of the largest current turbines – larger turbine blades have great offshore potential and are needed to capture energy at an affordable price.

Downwind-aligned rotor

Responding to a call for proposals from the Advanced Research Projects Agency-Energy (ARPA-E), the University of Virginia, Sandia National Laboratories, the University of Illinois, the University of Colorado, the Colorado School of Mines, and the DOE’s National Renewable Energy Laboratory (NREL) teamed up to develop a two-bladed morphing downwind-aligned Segmented Ultralight Morphing Rotor (SUMR). The SUMR design is based on the way palm tree leaves collapse and align during extreme wind conditions, such as a tropical storm. Under moderate wind conditions, the flexible living structure can bend downwind and alleviate destructive, cantilever aerodynamic loads. In high wind conditions, the flow-adaptive nature of the tree allows it to bend all the way to the ground, reducing aerodynamic loads. The tree’s trunk structure is one of the main reasons why palm trees are much more common near coast lines where hurricane-level winds are common, compared with oak trees or other heavy, stiff trees.

To mimic this, the SUMR uses blade elements with hub-joints that can be unlocked to allow for moment-free downwind alignment via a morphing hinge. The rotor, gravitational, centrifugal, and thrust forces are aligned along the blade path, reducing downwind cantilever loads, resulting in primarily tensile loading.

© Credit: RamonBerk

“We call that bioinspiration, using nature to design technology. And it’s something that is taking off now in the industry,” says Eric Loth, a lead researcher on the project and Mechanical and Aerospace Engineering chair at the University of Virginia. “We’re putting the tower first and the blades behind it so they don’t crash, much like a palm tree. Nature doesn’t try to fight the flow; it tries to bend with it. In 100mph wind conditions, a palm tree can bend all the way to the ground.”

For control simplicity, engineers designed the blade curvature to be fixed with a single morphing degree of freedom using a near-hub joint for 22° coning angle at rated conditions. Because of the reduced number of blades and coning, the SUMR concept has a 33% mass reduction.

“The dominant and consistent trend in wind energy over the years has been larger turbines, and in recent years that has stopped because it has become challenging to go bigger,” says Todd Griffith, principal member of the technical staff in the wind and water power technologies department at Sandia National Laboratories and blade structural designer on the project. “With this project we are proposing several new technologies that allow us to go even bigger.”

Biologically-inspired mechanical constructs have become an increasingly important research tool in experimental biology, offering the opportunity to focus research by creating model organisms that can be easily manipulated to fill a desired parameter space of structural and functional repertoires. Researchers have found ways to use bioinspired models to explore the biomechanics of organisms from all kingdoms. “Bioinspiration: Applying Mechanical Design to Experimental Biology” Brooke E. Flammang and Marianne E. Porter

Blade segmentation

By dramatically reducing peak stresses and fatigue on rotor blades the SUMR can support 200m segmented blades for the 50MW turbine.

Morphing angle schedule of the Segmented Ultralight Morphing Rotor (SUMR) blade as a function of wind speed (top figure) combined with images of rotor geometry (bottom row) in: (a) parked configuration, (b) aligned at rated speed, (c) aligned just below cut-out speed and (d) stowed for minimum aerodynamic load configuration.

“Segmented blades help on the manufacturing side because you don’t have to go to larger foundries,” says Lucy Pao, dynamics and controls professor at The University of Colorado, Boulder. She adds that since segmented, downwind blades can align with some of the forces acting on them, they have to withstand lower forces overall. Therefore, they do not need to be made as stiff and require less material.

Loth says segmentation makes the concept easier to fabricate, transport, assemble, and service.

“This concept allows much bigger turbines to be built,” Loth says. “Look at coal-fired plants that can generate huge amounts of energy. In order to compete economically, we need bigger turbines, and I think the best way to do that is offshore. What is driving this project is a way to reduce the cost of energy.”

The turbines are also lighter, and may need only two instead of three blades.

“Mass is money, if you can reduce mass, that is money,” Loth says.

(a) The non-azimuthal forces on a wind turbine blade, (b) the distribution of net force for a conventional upwind rotor blade and (c) the distribution of forces with alignment for a SUMR blade at rated conditions so downwind cantilever moments are eliminated.

Griffith explains that the morphing hinge and actuators allow the blades to fold back during storm conditions reducing the potential mass.

He says another unique aspect is in the way that the blades are designed to enable load alignment. Blades will now be designed to be very flexible and the idea is to better align the turbine’s aerodynamic load.

“We see this as an opportunity to take some of the weight and the cost off the blades to allow them to be more flexible,” Griffith continues.

Since the blades are downwind of the tower, as the wind comes along the blades are pushed away from the tower, reducing the likelihood of the blades striking it.

“This is one of the real challenges with the traditional upwind design,” Griffith says. “In an upwind design, the blades can be pushed toward the tower, sometimes resulting in a tower strike – a catastrophic turbine failure.”

Increase in wind turbine scale and power.

Griffith says the turbine’s control system is also very crucial for the system to operate.

“The controls development is a major aspect of this project to enable the load alignment and morphing capabilities to do it safely and in a way where energy capture is a low-cost energy solution,” Griffith explains.

Since this project is still in its infancy, the manufacturing details of the SUMR are still developing.

“We believe, with the exception of the hinge, that every component in this system can be fabricated by any group of companies doing blade fabrication, by processes that are very similar today. The hinge is an entirely new component,” Griffith explains. “Our project is just getting started, and we will be working on it over the course of the next three years to develop this.”

Todd Griffith shows a cross-section of a 50m blade to be used in the SUMR project.
Photo by Randy Montoya.

The SUMR team is very focused on their technology and market strategy. At the end of the three-year project, the team will produce a working design for an extreme-scale 50MW turbine and test it at the NREL lab in Boulder, Colorado.

“At the end of the project we expect to be making a case to move forward with commercialization,” Griffith says. “Though we’re at the beginning of this project, we have a plan.”

Offshore impact

One of the biggest aspects of this project is significantly driving down the cost of offshore wind energy.

“Wind energy has been effective inland and in shallow water for Europe and the U.S., however, when you go further, to floating and further offshore turbines, we will need new turbine technology,” Griffith says. “Even in shallow water, we may need new turbine technology.”

The Segmented Ultralight Morphing Rotor (SUMR) design is based on the way palm tree leaves collapse and align during high-wind activity for offshore wind deployment.

Loth and his team want this project to propel wind energy in all 50 states.

“Even with on-land turbines, going big can drive down costs,” Loth explains. “And more importantly is enabling offshore wind in the southern states where wind conditions are very consistent.”

Loth and Griffith believe this technology can help deploy offshore wind in Florida, South Carolina, Texas, and all the Gulf Coast states where hurricanes and high winds are prevalent.

“This is really important because there are several parts of the U.S. that we are looking at to deploy turbines where hurricanes and storms are holding this back,” Griffith says. “Our idea of a natural defense – bend but not break philosophy, is really important.” Loth adds, “ARP-E funds projects that are really blue sky technologies that take a large leap in advancement. This project is high-risk, high-reward. Now this is just a dream, but were trying to make that dream a reality in about 15 years.”

Advanced Research Projects Agency-Energy

arpa-e.energy.gov

University of Virginia

www.virginia.edu

Department of Energy Sandia National Laboratories

www.sandia.gov

Department of Energy National Renewable Energy Laboratory

www.nrel.gov

University of Illinois

www.uillinois.edu

University of Colorado

www.colorado.edu

Colorado School of Mines

www.mines.edu

Dominion Resources Co.

www.dom.com

General Electric Co.

www.ge.com

Siemens AG

www.siemens.com

Vestas Wind Systems

www.vestas.com

About the author: Arielle Campanalie is the associate editor of TES and can be reached at acampanalie@gie.net or 216.393.0240.