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Lightning protection of wind turbine blades with carbon fibre composite materials

Periodic Reporting for period 2 - SPARCARB (Lightning protection of wind turbine blades with carbon fibre composite materials)

Reporting period: 2017-01-01 to 2018-12-31

Wind turbines are often erected in remote areas of the world, off shore or in mountainous regions, to exploit better wind resources. In these areas, the cost of failures and repairs can be substantial, and increase the levelized cost of energy (LCOE) remarkably. Since blade failures due to lightning are known to cause the longest outage time and involve the largest repair cost, ensuring the lightning performance of the turbines and especially of the blades has become very important.
Modern wind turbine blades are to a large extent manufactured using Carbon Fiber Reinforced Polymers (CFRP) structural parts, due to the CFRP’s excellent mechanical tensile strength and stiffness, combined with a light weight. The use of CFRP decrease the weight of the overall rotor, and hence reduce the requirements for the remaining structure (drivetrain, nacelle structural parts, tower, foundation, etc.). By reducing all loads on the structure, the cost of the installed capacity can be minimized (CAPEX).
However, CFRP also exhibit highly an-isotropic electric and thermal conductivities, which require special attention in terms of lightning protection. The specific issues of concern include design of electrical bonding to CFRP, investigation of damage tolerance, resin chemistry to optimize thermal and electrical properties of the CFRP, guidelines on overall protection concepts, etc. The research project SPARCARB is dedicated to solve these challenges of enabling efficient lightning protection of wind turbine blades using these CFRP based materials.
Once the complete knowledge on the interaction between lightning and CFRP is achieved, and the knowledge is built into design guidelines and paradigms for blade manufacturing, blades can be designed to lower both CAPEX and OPEX of wind turbines. The reduced CAPEX will make future investment in wind plants more attractive, and the reduction in OPEX will ensure that the business case for investors remains attractive for the lifetime of the turbines. Both are important to ensure integration of more green energy into our system, and SPARCARB will via the generated knowledge assist in this process.
The work conducted in the 48 months of the project
Specific achievements are:
1 - A baseline CFRP material have been been defined to ensure the research and tests are conducted on realistic materials.
2 - The resin chemistry have been investigated, and efforts have been made to improve properties using different additives.
3 - Thermal, electrical, chemical and combined properties have been measured, to fully understand and model the complex processes involved in lightning interaction with CFRP.
4 - Several impulse current and arc entry tests have been completed by the ESRs on representative panels in both the SOTON laboratories as well as the GLPS lightning laboratory.
5 - Lightning induced structural damages have been investigated by means of CT scans and similar methods, and the damage mechanisms vs. the lightning impact have been described and modeled.
6 - Secondments scheduled in May/June 2017 and February/March 2018 have now been completed.

The list of key research outcomes in terms of methods/methodologies and groundbraking new knowledge generated though SPARCARB includes:
• Validated methodologies and analysis tools to define lightning attachment points, and further to predict current and voltage distributions within WT blades
• Validated Finite Element Analysis (FEA) codes to assess current distribution in both frequency and time domains, with special applications to assessment of electrical bonding techniques in general and so-called ‘equipotential’ bonding techniques in particular
• Validated multiphysics tools and codes (FEA based) to predict the electrical-thermal-mechanical response of composite WT blade components and structures, and further to predict lightning strike induced damage and its influence on the overall WT blade structure wrt. load carrying capability and failure behaviour
• New validated techniques for measurement of electrical and thermal conductivity specially adapted to CFRP WT blade materials
• New structural test methodology and test setup, referred to as ‘compression after lightning strike’ or CALS, to experimentally assess the post lightning strike mechanical performance and failure behaviour of WT blade CFRP panels and components
• Novel polymer resin systems enhanced with low cost Graphene-Oxide nanofillers with significantly improved electrical and thermal conductivities. The performance of the novel polymer material system has been validated in both bulk form and when used as matric material in infused CFRP panels
• Novel design concepts for ‘equipotential’ bonding of CFRP elements for WT blades
Considering the location of wind turbines, the risk of harming humans or livestock can be substantial. In several cases pieces/components, even entire blades, have been detached from wind turbines because of lightning induced damage. This governs both blades made of pure fiberglass (GFRP) but also blades where structural components are replaced by carbon fiber composites (CFRP). SPARCARB is tailored to provide the knowledge on CFRP and its interaction with lightning, and explicitly how to optimize the material and the lightning interaction to secure a reliable and safe operation.
In this sense, blades with improved lightning performance does not only save operational cost and downtime for the turbine owner, but is also considered far safer than today’s blade designs.
The specific results being achieved within SPARCARB are listed in the following:
1. Clear protection strategy for CFRP blades and the formulation of a process guideline from initial structural design to safe lightning design. This is an overall goal for SPARCARB involving all four ESRs.
2. The properties of the CFRP composite material is characterized by ESR1, leading to an important discussion on the role of thermal and electrical properties vs. lightning performance. The research challenges the perception of electrical conductivity being the dominant parameter for lightning immunity, and it seems as if ESR1 is approaching a breakthrough in the importance of thermal properties as well.
3. Investigation and formulation of the matrix resin, and how to enhance especially thermal and electrical properties. By including Graphene Oxide Nano fillers, we believe to change the view on the isotropy of CFRP remarkably. The work is driven by ESR3.
4. Well defined damage description - which failure modes exists for the CFRP material against the various stresses induced by lightning. By knowing the limits, it is easier to design cost efficient without excessive engineering margins in one end, or over testing in the other. The results are improved design paradigms, mostly driven by ESR4

With its dedicated research team and the excellent list of partner organizations hosting the ESRs during their secondments, SPARCARB will provide an extensive outlook into means of optimising the CFRP material and its integration with the lightning environment. These results will be of technical benefits of blade manufacturers, financial benefits of future operators of wind turbine blades, and general benefits for the society experiencing cost of energy from renewable sources.