The wind energy industry is growing in the United States, and that means the country is going to need tens of thousands of new composite wind blades in the next decade. In 2019, it took more than 60,000 wind turbines to generate 109,919 megawatts (MWs) of electricity in the United States, Guam and Puerto Rico, according to the American Wind Energy Association (AWAE). Between projects already underway or currently in the final planning stages, AWAE projects another 44,000 MWs of wind power will be added over the next few years.

The hitch is that manufacturing a wind blade is currently a time-consuming and labor-intensive process. For example, GE Renewable Energy reports that it takes two days and 100 workers to produce a single 351-foot wind blade for LM Windpower’s factory in Spain. Although these are some of the longest blades manufactured today, the process is similar regardless of the blade’s length.

To improve and streamline the manufacturing process, a multidisciplinary team, led by GE Research, is currently investigating the feasibility of embedding sensors into GFRP wind blades during production. The sensors would measure temperature, resin flow and curing as the blade is being manufactured, with the goal of reducing cycle time, material usage and overall costs. The research is being funded by NextFlex, one of eight Manufacturing Innovation Institutes established by the U.S. Department of Defense as a public-private partnership. (IACMI – The Composites Institute is another.)

Through its consortium of 100 companies, academic institutions, non-profit organizations and government agencies, NextFlex works to foster the growth of flexible hybrid electronics (FHEs) in U.S. manufacturing. FHEs are formed by placing integrated circuits (ICs) and other electronic components onto flexible, stretchable and conformable substrates, such as film. This reduces the weight and size of these electronic circuits and opens up the possibility of incorporating them into products in many new forms. FHEs are currently being tested for medical wearables, asset and structural health monitoring systems, flexible array antennas and soft robotics.

“The advantage that you get with FHEs is that you can build electronics in form factors that you typically can’t [build] with conventional electronics,” says Scott Miller, director of technology at NextFlex. “You also have the potential to do some really important size, weight and power trade-offs that you couldn’t get without the lightweighting of the electronic systems.”

Building the Sensor

NextFlex consortium members selected the GE Research sensor project because of their widespread interest in using FHEs for asset monitoring. Nancy Stoffel, principle engineer of GE Research’s electronics and sensing group, is the team leader, and Shridhar Nath, GE’s platform leader for onshore wind, is the expert in wind blade technology. Binghamton University in New York is contributing knowledge and experience in electronics manufacturing, reliability and material systems, while Georgia Tech brings expertise in additive manufacturing of flexible radio frequency (RF) modules and passive wireless sensors. The project team will also create embedded sensors for carbon fiber composites, and helicopter manufacturer Sikorsky is participating to guide the specifications and needs for monitoring the structural health of aircraft components operating in the field.

The team’s early challenges included designing the sensors and finding a way to insert them into composite material without affecting its structural integrity. The sensors had to be wireless because running exterior wires could attract lightning strikes in the field, and since they were embedded, had to be passive and able to operate without battery power.

The team designed a passive, wireless sensor with a specific resonance frequency. When an electronic device called an interrogator sends out an energy wave, the sensor reflects it and the interrogator reads it. Changes in the resonance frequency or amplitude shifts in the reflected energy wave provide an indication of what’s going on inside the composite material.

“There are a couple of things that go into making the decision about what frequency bands to use,” says Stoffel. The team wanted the sensors to operate in ISM radio bands, which are reserved internationally for industrial, scientific and industrial purposes. To control costs, it decided to use the same interrogation devices used by automotive radar detectors that operate in the 25 GHz range. “This isn’t a large application where it makes sense for you to have a custom chip,” Stoffel explains.

“One of the challenges that we have is that we are burying [the sensor] inside the composite, which will modify it somewhat,” she continues. Embedding sensors into the composite also makes it more difficult for the interrogator to read. The team wanted the reading distance to be at least 6.5 feet. This would allow the interrogator to be suspended above the blade mold in the factory, so it could read data from the sensors without interfering with work on the production floor.

An additive printing process is used to manufacture the sensors. An aerosol jet printer sprays conductive metal inks onto a substrate such as a polyimide film. The printed circuits made through this process are fairly complex and include an array of small antennas (called the Van Atta array) to provide a good reading distance, according to Stoffel.

Minute changes in the spacing of the arrays will alter the response of the sensors, so they can be tuned for various properties. That made it possible to use the same sensors for both the GE production monitoring part of the project, including the extent of curing and the real-time temperature of the composite, as well as the longer-term, in-the-field strain monitoring that Sikorsky is testing.