Improving efficiency & reducing the cost of advanced wind turbine technology
by Justin Martino
Wind power capacity has increased dramatically recently — and accompanying that, the turbines that produce it have become more powerful, more efficient and more affordable for power producers. "If you go back 10 years ago and look at wind power and then look at where is today, it's just dramatically different," says Keith Longtin, wind products general manager at General Electric. Those differences come in many different areas — rotors, controls, electronics and gearboxes — but the advancing technology used in wind power production has always aimed at the same goal: making wind a better choice for power generation.
While a previous focus of the industry was increasing the total name-plate capacity of wind turbines, the focus has shifted to the capacity factor of the turbine, which helps keeps energy cost low by providing the most possible power. "That is really the direction we're going," Longtin said. "If you go back 10 years ago, the turbine was at about 25% capacity factor. Today, it's over 50%. As we've improved the capacity factors and improved the cost of energy, that enables us to go into more and more locations where the wind is lower."
One of the deciding forces so far for increasing capacity factors has been an increase in the size of the rotors used on wind turbines. Longtin said GE's predominant turbine in the U.S., which has a 1.6 MW capacity, currently comes with a 100-meter rotor, compared to a 70-meter rotor in the past. Increasing the size of the turbine rotors creates new challenges for manufacturers, however. Longtin said rotors scale poorly with size, so the cost can go up faster than the revenue generated by the increased capacity factor.
Turbine rotors are affected by two different forces: torque, which turns the rotors and creates energy, and thrust, which pushes against the turbine. Dealing with thrust can be difficult when designing a rotor.
"The thrust is equivalent to five F-18 engines just trying to pull it over," Longtin said. "There are tremendous loads of forces up there, and so it needs great engineering technology to be able to create these very reliable turbines." Breakthroughs in the turbine controls led to being able to handle the additional thrust generated by wind, Longtin said. He compares the controls used on turbines to anti-lock braking systems on cars, saying the way the turbine is controlled and shut down, along with how it responds to wind gusts, allows for a bigger rotor on a turbine.
Alstom has made similar changes in the size of rotors for its platforms. The company took its eco100, a 3-MW turbine with a 100 meter rotor, and upgraded it to a 110-meter rotor in 2010. Last year, the company increased that to a 122-meter rotor. "We were able to increase the area of the rotor by nearly 40% in less than five years," said Albert Fisas-Camanes, Director of Innovation for Alstom Wind North America. "That lets us deliver a more efficient wind turbine to our customers. When you increase the area of the rotor, you are able to get more energy at lower wind speed."
While the focus on increasing the power produced from wind turbines may be on the capacity factor, another way is to make sure wind turbines are operational and available. Longtin says GE has made major strides in that area in the past 10 years. "The availability of wind turbines 10 years ago was about 80 - 85%" he said. "And the wind industry was OK with that because before that they were available 70%. We took a power-generation mindset to it and said 98% is what we do. That's what gas turbines run at, that's what nuclear plants run at, that's what steam turbines run at, so that's what we're going to do. We've made lots of investments to improve the overall availability of a wind turbine so that today we have 22,000 wind turbines with an average production-based availability around 98%."
To help achieve that sort of industry reliability - and to continue improving on it - Longtin said the company has worked to improve the individual components used in turbines, both electronics and gearboxes. For gearboxes, GE has combined the manufacturing processes and design processes so they're designing components that can be reliably manufactured. In addition, the company does highly accelerated lifetime testing on all its gearbox designs to validate the design on all new gearboxes. Longtin says the company is looking at the manufacturing of the parts used in the gearbox and at ways to harden gears, as well as different types of bearings and bearings configuration. He says: "Gearbox reliability is something we're incredibly focused on. There's a lot of work that goes into trying to maximize that reliability." In the future, the company will also focus on how to minimize the downtime when a gearbox does have a mechanical failure. "We're trying to make our gearboxes so if a problem occurs, we're able to service that up-tower as opposed to bring a crane in and replace the whole thing. That is probably one of the best things we can do."
Challenges in Offshore Production
While the onshore wind turbine industry is going strong, the U.S. is looking toward the possibility of adding offshore wind capacity in the future. Alstom recently signed a contract with Deepwater Wind to supply turbines for the 30-MW Block Island Wind Farm three miles off the coat of Rhode Island, and the company is also part of Dominion Virginia Power's Virginia Offshore Wind Technology Advancement Project, which was one of seven picked by the U.S. Department of Energy's advanced technology demonstration projects. The needs of offshore wind production require different solutions than onshore, Fisas-Camanes says: "From the Alstom perspective, we see two different worlds," he said. "Onshore has some contraints, and they are not the same constraints you are going to see in an offshore site. We are treating them as two different animals, and we are using different technologies for those two applications."
The use of different technologies for onshore and offshore wind power projects is another change that has occurred over the past 10 years. While companies used to take the same wind turbine used on land and installed it offshore, but Fisas-Camanes says Alstom took a different approach with its current generation of offshore wind turbines: "If you look at the new generation of offshore wind turbines, our wind turbine, the Haliade 150, has been designed from the very beginning to operate in offshore conditions," he said. "This has driven different aspects of the design of the wind turbine, with one of the main decisions being using direct drive technology." The Haliade 150 is a 6-MW turbine that uses a 150-meter rotor. Fisas-Camanes says the company plans to continue developing and investing in the improvement of the direct drive solution for its offshore wind turbines, including improving its efficiency and weight.
Companies are also looking at the use of floating wind turbines, which use floating structures instead of requiring wind towers be set into a foundation under water. "Ten years ago, it was hard to imagine floating wind turbines, and this is real now," Fisas-Camanes says. "We're in a demonstration phase right now, and I'm expecting to see the development of floating wind farms in the next 10 years."
Tools to Improve Efficiency
As companies look to make more sophisticated wind turbine technology, more sophisticated tools are required. One of those is 3D-modeling technology, which allows companies to use computer simulations to see how products will respond before manufacturing and testing the product in the field. Computer simulation can be useful to companies as they look to increase the capacity factor of turbines. The software allows companies to help design blades across a range of flow velocity without having to continuously make the rotors large.
Companies are able to use software to create a virtual lab and set up the blade in the lab. Designers can then vary the blade geometry, blade twist, yaw angle, angles of wind attack and wind velocities. The simulations allow designers to see the coefficient of lift and drag across the blade on both the top and bottom surface. "In wind turbine blades, blades could be very thin at the end, and there's also a little twist to it, so maintaining fluid dynamic contact on the blade across of range of wind velocities and different angles of attack and different yaw is a complex fluid mechanics problem," says Ahmad Haiari of ANSYS, "Once we have the rotor size as large as we can, the next thing is, 'How can we get the blade efficiencies up?' That means fluid dynamic efficiency across a range of wind speeds."
Modeling software can be used in more than blade design, however. Haidari said software can be used when siting wind power projects. Buildings, hills and even trees can change wind turbine behavior, so using software can help choose the correct installation for a given wind farm. Although siting may be less important in offshore wind power projects, the software can be used to help decide the best way to run power onshore as well as determine on the right strategy for installing the tower into the ocean floor.
The technology can be used for a variety of other simulations, including manufacturing components, as well as monitoring the potential performance of generators and components, performing structural analysis or looking at electronic controllers. "When you look at wind blades, you have to look at the whole system," Haidari says. "Looking at the blade is part of it, but one needs to look at the entire system design in order to have a full understanding of the blade's performance. Everything, including your washing machine, is becoming more sophisticated. Designing these around a single component doesn't help."
Future Focus on Innovation
The technological advances made with wind turbines have resulted in clear bottom line: Wind power is more efficient and affordable than it has ever been, which has helped drive its popularity along with power prices and incentives such as the U.S. production tax credit. Also, Fisas-Camanes says wind power has proven itself as a good option for companies looking for power that can come online quickly. "It's a fast track technology, it doesn't need water and it doesn't pollute the earth, so it's an easy and quick energy solution for increased generation," he said. "I think there are a number of reasons, technology being one of them, that have helped lower the cost for energy."
As quickly as wind technology has developed in the past 10 years, Fisas-Camanes says he expects that pace to at least continue and possibly accelerate. He expects many advances in the next 10 years, including a new generation of offshore and onshore wind turbines that will have more value for their owners and more developments in the components and controls of a turbine: "It's an open book. If you look at how this industry has been evolving, it's amazing. It's a very dynamic, technology-driven industry with a focus on cost, and I think we'll see a lot of interesting things in the next 10 years."
◊ Justin Martino is Associate Editor of Power Engineering.
Publ. here 7.4. 2014