Winds of change

Collaboration across disciplines works to overcome technical challenges in quest to develop economical, clean and reliable wind energy.

The answer to energy independence may be, as Bob Dylan penned, blowing in the wind. If the United States is to reach the U.S. Department of Energy's goal of 20 percent wind power by 2030, it will take innovative technology to improve the performance of new and existing wind power plants. And that's just what Purdue researchers from multiple disciplines are developing.

Doug Adams, the Kenninger Professor of Renewable Energy and Power Systems, and computer science professors Jan Vitek, Ananth Grama and Suresh Jagannathan received a $1.6 million three-year grant from the National Science Foundation's Division of Computer and Network Systems to advance sensor technology and computer simulation tools for tracking and improving the performance and reliability of "smart" wind turbines and wind farms.

The project builds on years of research by Adams, who is developing "smart" turbine blades that use sensors and computational software to improve energy capture by adjusting for changing wind conditions. Adams developed these sensing techniques working with Sandia National Laboratories.

One year into the project, a specialized facility has been built in Lafayette that allows researchers to study and manipulate the interaction of multiple turbines in a controlled environment — a nearly impossible task in the field.

Scott Dana (MSME '11) designed and built the Wind Energy Dynamics and Control Facility in Lafayette, where the testing is being conducted. He constructed an earlier test cell as his master's thesis. He is currently working as a researcher at the DOE's National Renewable Energy Laboratory just outside Denver.

Adams explains that wind turbines are self-contained units that pay no attention to what another turbine is doing. "As long as the turbines don't interact, there are no issues," he says. But the fact is in large wind farms with turbines 240 feet off the ground, the wind enters one rotor of one turbine extracting energy. What's left travels downstream creating a "wake" that reduces wind speed. Variables such as wind direction and blade pitch can compound the effects of this wake effect.

What is ideal, Adams says, is to optimize the system so the turbines are immune to the effects of wake. "There is existing research that shows the drop in productivity as a result of interaction can be as high as 25 percent in offshore wind farms," he says.

Attacking multiple challenges

Adams' co-principal investigators on the NSF project are writing computer code and interpreting the mathematical output for the code to make control decisions in real time.

"The wake moves downstream quickly so you don't have much time to react," Adams says. "They are developing computer code and validating that it will work properly all of the time. It's like the adage: ‘If you measure it, then you can control it.'"

One of the important parts of the project is exploring the long-term implications of turbines as a system. Ignoring this can result in the economically devastating loss of turbine longevity.

For example, most often the turbines that experience wakes — the dirty air —experience failures more frequently.

"By measuring the interaction better, and making the right control decisions, we can extend the life of the wind turbine," Adams explains — something that is critical to meeting the Department of Energy's 2030 goal.

From a turbine-life aspect, in 2030 the turbines are going to be at the end of their life, Adams says. "Turbines typically are designed to operate about 25 years. We have to look at what we're doing now to improve their life down the road."

The NSF project is just one part of a large, multidisciplinary approach to studying wind energy. Karen Marais, assistant professor of aeronautics and astronautics, is studying reliability of wind turbines. What kinds of measurements and preventive maintenance are needed to reduce the cost of wind energy?

Adams and Marais are co-principal investigators on a project at Sandia Laboratories studying how to anticipate repairs to avoid failure.

Finding solutions

To advance the use of wind energy, it's important to assess the environmental impact on people, flora and fauna.

Bryan Pijanowski, professor of forestry and natural resources, is researching frequency ranges of noise created by the turbines. Are the frequencies affecting the migration pattern of birds? Do bats communicate in a particular frequency range that overlaps with the blades? These are just a few of the questions being explored.

Adams says that with wind energy and other emerging technologies, people tend to focus on the good aspects, like renewability, but don't pay as much attention to the challenges associated with it.

One major misconception, he says, relates to a perceived negative impact on agriculture since many of the turbines are placed on agricultural farms. "People believe that wind turbines could have a negative impact but in the College of Agriculture, faculty researchers have instrumented a wind farm measuring the air coming into the farm and the temperature coming out," Adams says. "They found that the air warms up coming out, which could potentially extend the growing season in the Midwest."

Noise is another concern for nearby residents. Some landowners say the wind turbines are quiet and others complain, Adams says. "Who is right? Maybe both are because of the conditions in which the wind farm operates — the wind speed and topography of the land – these factors affect how sound is generated and propagated."

Improving power transmission

For a long time, wind turbines were placed in the center of the U.S. — a wind corridor that anyone driving through the area can attest to. The problem, Adams says, is there are few places to hook up the turbines to the power grid.

"You're transmitting power over long distances," he says, "and the longer the distance, the more power you lose." According to the Department of Energy, approximately 6 percent of electricity generated is lost in the transmission and distribution system, costing consumers roughly $25 billion annually.

The U.S. electric grid is a complex network of independently owned and operated power plants and transmission lines that powers our factories, homes and schools. Often taken for granted, the electrical power we depend on is most likely produced by a coal or gas-fired plant that can be ramped up or down to meet demand. The United States' current electric grid was designed more than a century ago, and its inefficiencies are expensive. Rising energy prices and increased power demands weigh heavily on the aging infrastructure, forcing experts to critically examine the status and health of the nation's electrical systems.

To answer these challenges, utilities will be turning to more efficient transmission and digital infrastructure and communication processes called the smart grid. Purdue, in partnership with Ivy Tech Community College, is training the workforce of tomorrow that can design, develop, install and maintain the electrical grid through the Crossroads Smart Grid Training program.

Maryam Saeedifard, assistant professor of electrical and computer engineering, is researching transmission issues related to offshore wind farms. Her focus is High Voltage Direct Current (HVDC) transmission technology as more wind parks are being planned farther offshore.

"When the undersea cables using threephase AC current exceed a specific length, power is lost along the way and practically no energy can be effectively transferred from the wind parks," she says. The solution is HVDC transmission in which the three-phase AC current produced by the wind turbines is collected and transformed to higher voltage levels at offshore substation platforms.

"The HVDC substation platforms are based on Voltage Sourced Converters (VSCs)," Saeedifard says. "The main technical challenges in the design of VSCs for HVDC transmission are to improve the efficiency of those VSCs and subsequently the HVDC transmission, and to increase the scalability and modularity of the system to meet any voltage or power level and to improve fault tolerance."

Understanding fatigue failure

Adams' colleague Sanford Fleeter, the McAllister Distinguished Professor of Mechanical Engineering, is an expert in gas turbine engines who became interested in wind power not just because he thought he could make a contribution to the rapidly advancing science, but also because he recognized its commercial potential.

"When I looked at wind power, I saw a growing business," Fleeter says. "With energy costs going up, wind power is a green alternative."

It also makes sense economically, since wind power is not subject to supply-anddemand fluctuations in the marketplace.

Fleeter constructed a model wind turbine facility at Purdue to study fatigue failure and other issues that arise from the dynamics of the turbine. He quantifies the interactions between the tower and the rotor in a wind tunnel. Because the rotors have three blades that are revolving past a columnar tower, they create a three-beat impulse that affects operational reliability. These vibrations, plus phenomena like acoustic noise from the blade tips, are factors that need to be accounted for. "We're trying to understand and monitor these vibrations, and hopefully be able to control them," he says.

Natalie Barrett, a doctoral mechanical engineering research assistant, is doing computational analysis of the costs associated with offshore wind farms. She is a former project engineer at Pratt & Whitney, a global company specializing in the design, manufacture and service of aircraft engines, industrial gas turbines and space propulsion systems.

"I am evaluating the cost benefit of a wind turbine blade condition monitoring system that detects damage early to prevent failure," she says — something that will contribute to reduced cost for both consumers and the wind industry. "Cost and reliability are critical elements. Consumers want a good price for their energy and wind power owners want to make a good profit."

Adams agrees. "Environmental impacts are here to stay so you have to make sure you design systems in such a way to give maximum benefit with minimum cost. You have to have credible analysis. No developer is going to make money if there are negative environmental consequences."