Scientists from the National Renewable Energy Laboratory (NREL) have developed a lightning protection system for wind turbine blades using a new composite and an innovative thermal welding process.
Created in partnership with General Electric and LM Wind Power and with funding from the US Department of Energy’s Technology Commercialization Fund, the “lightning shield” of wind turbines can deflect approximately 80% of current at a stroke. lightning strikes from metallic heating elements, according to NREL.
Lightning protection systems exist for conventional wind turbine blades. But protection was needed for blades made from a new type of material – thermoplastic resin composites – and made using an innovative thermal (heat-based) welding process developed by scientists from around the world. NREL.
Thermoplastic materials, like plastic bottles, can be more easily recycled than the thermosetting materials commonly used to make wind turbine blades today. While thermosetting materials must be heated to harden, thermoplastics harden at room temperature, reducing both the time and cost of manufacturing the blades.
NREL’s patent pending thermal welding process for thermoplastic blades enables these benefits and even adds them by replacing the adhesives currently used to bond blade components. Using solder instead of adhesives eliminates the drawbacks of extra weight and the potential for cracking, according to NREL.
While thermal welding offers advantages, it also requires the addition of a metal heating element inside the blade, which can attract lightning. As a result, a team of NREL researchers led by Robynne Murray and supported by GE and LM Wind Power invented a new lightning protection system to protect new thermoplastic materials.
In 2018, Robynne Murray, an NREL engineer specializing in manufacturing methods and advanced materials for wind turbine blades, received a two-year NREL Lab-Led Research and Development Award for thermal welding research of thermoplastic wind turbine blades.
To make one of these new blades, a vacuum draws liquid thermoplastic resin into the fiberglass material which is placed in a mold for each blade half. To weld the blade halves together, scientists sandwich a conductive material, such as foil of expanded metal or carbon fiber, between the two blade components and attach a wire to a power source. This creates the heating element. When current passes through this element, thermoplastic materials melt. Once melted, the current is turned off and the bond cools under pressure.
Murray’s research has shown that thermal welding can effectively bond segments of thermoplastic wind turbine blades. She applied for a patent on the process in 2018.
“Thermal welding is an important step in advancing the commercialization of thermoplastic materials for wind turbine blades, but what happens when lightning strikes a heat-sealed blade? It was an unanswered question and a great concern ”, Murray noted.
“For thermal welding of thermoplastic sheets to become commercially viable, it is essential that conductive bond lines are protected against lightning strikes. “
In partnership with GE and LM Wind Power, Murray submitted a research proposal to the DOE’s Technology Commercialization Fund (TCF). The TCF awards are designed to advance technology developed in national laboratories towards commercialization while encouraging laboratory-industry partnerships.
“With our partnership with GE, a company that can take the thermal welding process to market, our TCF was a solid proposition. “ Murray noted.
“Together, we wanted to determine if we could protect these blades from lightning strikes and eliminate an important reason for preventing us from using the technology. “
In 2019, the team received $ 150,000 in TCF funding, and GE matched that amount.
Research partners have moved to NREL’s Composite Manufacturing Technology and Training (CoMET) facility to demonstrate that thermoplastic blades sealed by thermal welding can be protected against lightning strikes.
The team infused expanded aluminum foil into the skin of the blade to deflect lightning current from metallic heaters. They then performed experiments using a simulation which showed that a lightning strike would not cause the blades to fail with the lightning protection system in place.
Physical damage testing, which subjects the blades to high electrical currents, has shown that approximately 80% of the electrical current passes through the foil layer for lightning protection, not the skin of the blade. . The carbon fiber under the damaged area of the tip was also unharmed.
Research has confirmed that the design can protect wind turbine blades from failure caused by lightning, NREL said.
“LM Wind Power and GE Research were delighted to work with NREL on the development of this technology and appreciated the support from the DOE Technology Commercialization Fund. Thermal welding technology for thermoplastic and recyclable wind turbine blades offers a significant opportunity to impact the durability and carbon footprint of wind turbine blade structures ”, noted Jacques Martin, director of blade platform deployment for LM Wind Power.
“NREL’s focus on mitigating the risk of lightning damage associated with electrically conductive elements in the solder link is a key challenge, and their work has helped mature the technology into potential commercialization. “
The project has already delivered two lightning strikes. Murray’s work to develop a market-ready thermal welding system, however, is still ongoing, NREL said.
“We answered the question about lightning. But there are more questions to answer and more work to do ”, Murray noted.
“The next step for us is to validate the structure of the blade tie lines and heat-welded blade tip segments. Hope we can do it in the next year or so.