In the United States, wind power generation has more than tripled over the past decade, and wind turbines are getting steadily larger to meet an ever-increasing demand for renewable energy.
“Blades, towers, bearings, size of cranes—everything is being pushed to its absolute limits,” says Brian Ray, chief engineer for industrial bearings, The Timken Co.
While these trends are exciting for the future of wind energy, designing and using such turbines presents complex challenges for engineers to solve.
Bigger blades, more headaches
Design challenges start at the ground level. Taller, heavier towers require larger, stronger foundations, but scaling up existing designs is not enough. Eventually the size, cost, construction site logistics and concrete pouring become unmanageable. Foundation and anchoring bolt materials and configurations must handle increasingly larger vertical loads, shear forces and overturning moments.
There is a practical limit to tower heights as well. Under current market conditions, land-based towers taller than 120 meters cannot provide enough extra energy to offset installation costs. Transporting a tower from the factory to the construction site requires keeping tower section diameters under 4.3 meters to fit under highway overpasses, which makes it necessary to build towers with thicker walls to achieve the necessary structural strength.
One way around this challenge is to manufacture towers on-site. For example, offshore wind turbines are often manufactured near port facilities to minimize land transport. Outside the U.S., onshore towers are made from concrete, steel fabricated on-site or some combination of the two. Unfavorable economics have hampered similar efforts in the U.S., but domestic companies are developing metal lattice structures that can be transported on a flatbed truck and on-site thermal forming processes for making corrugated steel plates.
Transporting wind turbine blades poses another challenge. Blades longer than 53 meters require a turning radius greater than most roads and railways can currently accommodate. Constructing blades in segments is one way around this.
Standing up straight
Once a turbine is up and running, rotor systems must withstand loading extremes and oscillatory dynamic loads that can accelerate white etching cracks and fatigue failure. Doug Lucas, advanced engineering technologist for The Timken Co., says starting and stopping the turbine will cause torsional dynamics through the turbine’s entire drivetrain. “We are more concerned about wind gusts than lulls, where everything is moving but it is changing speed rapidly,” he says.
Turbine blades that deform under aerodynamic loads, (e.g., fabric-based blades) reduce lift to prevent excessive loading, and sensors can dynamically adapt the blades’ angle of attack and lift coefficient to changing wind conditions.
As land-based turbines clear the 5 megawatt (MW) hurdle and offshore turbines move into the 10-plus MW range, main shaft bearings can easily reach outside diameters of 2 to 4 meters. At these sizes, advanced materials and heat treatments are used to manage rising manufacturing costs. Finite element analysis plays an increasing role in examining the flexibility of the turbine system and how it affects the interaction between system components, according to Lucas.
The fundamentals of the gearbox between the rotor shaft and the electrical generator are evolving, according to Ray. Rotational speed at the hub decreases as blades get longer, but generator speeds typically remain the same, which may necessitate increased gearbox ratios. The traditional gearbox setup has one planetary gear with a couple of parallel stages. “Now you are seeing 2 to 3 stages of planetary gears with or without parallel stages,” he says.
Maintenance, tracking and modeling
Collecting more—and better—operational data helps turbine operators spot maintenance problems in the early stages, but it also improves computational modeling of new designs and materials. For the largest turbines, modeling components and systems under operating conditions is critical, because full-scale test rigs are expensive to operate and rare, and it is often not practical or economical to build full-scale prototypes.
“You cannot test these things at anywhere near the actual size—it is not physically feasible,” Ray says.
Today’s wind turbines can achieve about 75–80% of their theoretical efficiency limit because of improvements in aerodynamic design of the blades, reduced resistance from the internal components and optimized controls, according to Kyle Kingman, president of Offshore Power, LLC. European operations have approximately doubled performance over the past 5 years, but costs have not risen commensurately. Market and supply chain factors keep driving costs down, as well as improved manufacturing, more efficient transportation and installation methods, and an increased availability of skilled workers.