James F. Manwell, director of the UMass Wind Energy Center and a professor in the Mechanical and Industrial Engineering Department, was the primary author of a critical section of a wind-energy article published in the journal Science. The article is entitled "Grand challenges in the science of wind energy." The process that led to this article also generated the theme – the Grand Vision for Wind Energy – of a formative conference, which will take place at UMass Amherst from October 14 to 16.
The conference, called NAWEA/WindTech 2019, will be an opportunity for wind energy academics and researchers to discuss the Grand Vision for Wind Energy and its implications. Thought leaders from around the world will discuss the pivotal role wind energy will play in the transition from a system based primarily on fossil fuels to one primarily supported by renewable energy.
The Science article was published as part of the rollout for this crucial conference. Manwell’s function in the article was to serve as the primary author of the offshore portions of the section subtitled “Second grand challenge: aerodynamics, structural dynamics, and offshore wind hydrodynamics of enlarged wind turbines.”
As Manwell and his co-authors explained in the article, an operating wind turbine might appear to be very still except for the rotation of the blades, yet the entire system is constantly in motion because of forces and moments exerted in all directions.
According to the authors, numerical wind turbine simulation capabilities that incorporate up-to-date knowledge of wind turbine physics (including coupling aerodynamics, structural dynamics, controls engineering, and even hydrodynamics for offshore applications) have empowered the wind industry to design machines that produce efficient power for many years.
As one result, say the authors, wind turbines have grown to become the largest dynamic machines in the world. They are massive structures that must operate continuously for 20 years or more under constant complex loading. Blades approach 80 meters long, and towers are expanding well above 100 meters for maximum tip heights often going beyond 200 meters, equivalent to a building over 60 stories high.
To put these dimensions in context, the authors note, three of the largest passenger aircraft on the planet can easily fit within the swept area of one wind turbine rotor.
However, the authors state, the industry is seeking even larger turbines that access higher wind speeds aloft and provide economies of scale, thus reducing manufacturing, installation, and operating costs.
As turbines continue to mushroom in size, there are research questions around offshore wind turbine dynamics involving their interaction with the atmosphere, wakes, and other sources of complex forces upon the rotors. These questions, as the authors note, include the aeroelastic behavior of very large and flexible machines and additional dynamics associated with deployment offshore in extreme weather conditions.
When the blades flex into and out of the wind, as the authors explain, a rotor interacts with its own vorticity, calling into question the accuracy of the design assumptions. In addition, structural dynamics of blades incorporating composite materials, built-in curvature and sweep, and large nonlinear deflection further complicate models of the physics.
The authors say that offshore installations require the combined modeling of aerodynamics with the hydrodynamic forces from waves and currents. To explore configurations for offshore support structures specific to wind energy, the hydrodynamic models will need to include the combined nonlinearity and irregularity of sea states, breaking waves, viscous effects on bluff bodies at high Reynolds numbers, vortex-induced vibrations, dynamic soil-structure interactions, and more.
Particularly relevant for these offshore applications are extreme weather conditions, of course, such as hurricanes or tropical cyclones. The authors explain that the uncertainty of offshore turbines is amplified if the entire rotor is rocking into and out of its own wake, as could happen on a floating substructure.
As the authors say, new materials and manufacturing methods are a major part of supporting the development of offshore wind turbines. Understanding the dynamics will help establish the design requirements, but materials and manufacturing breakthroughs must be developed to produce low-cost and reliable machine designs. There is still a critical need, they write, to improve materials performance for particularly difficult environmental conditions and operational loads.
According to the authors, the unique challenge related to materials science and engineering for wind energy is the need, not only for materials to have tuned or customized properties for the specific application, but to also be commoditized. That is, turbines must be mass produced inexpensively and must be able to be recycled readily. (October 2019)