Advancing Ceramic Matrix Composites and Environmental Barrier Coatings for Hydrogen Turbines: Challenges and Opportunities
Decarbonization goals call for advancing high-impact energy technologies.
The U.S. has pledged to achieve net-zero emissions by 2050, a goal that presents both challenges and opportunities in the field of advanced materials. This ambitious goal requires a wide range of technological advancements to enable the shift from fossil fuel-based systems to more sustainable and low-carbon solutions. The development and deployment of these solutions will be inherently complex and require coordination and collaboration between the public and private sectors.
Hydrogen as Pathway to Net-Zero Emissions and Necessity for New Material Development
Hydrogen, a clean energy carrier, is positioned to become a key platform to achieve national net-zero goals with the potential to decarbonize various heavy-emitting sectors and replace fossil fuels in power generation applications. However, harnessing the full potential of utilizing hydrogen fuel in land-based utility turbines is challenged by material limitations.
Hydrogen’s adiabatic temperature is approximately 500F higher than natural gas, making the fuel more prone to generating hot spots that would exceed the current temperature capacities of traditional superalloys. Historically, superalloys, advanced casting manufacturing methods, and thermal barrier coating technologies were used to enable the operation of components in high-temperature environments; however, these technologies are reaching their inherent temperature limitations. New materials to support the hydrogen economy are needed, yet the pace of materials innovation is notoriously slow, potentially impeding progress if ignored.
Materials Innovation: The Rise of CMCs and EBCs
Continued advancements in two ceramic-based materials are required to enable higher firing temperatures and, in turn, increased turbine efficiency: ceramic matrix composites (CMCs) and environmental barrier coatings (EBCs). CMCs are materials comprised of a ceramic matrix and embedded with reinforcing fibers—typically carbon, silicon carbide, or oxide—to enhance strength, durability, and temperature resistance. EBCs are specialized coatings applied to CMCs that permit their operation in extreme environments. They are often silicon-based, multi-layer systems developed from rare earth silicates to prevent surface recession, strength loss, and adverse chemical reactions during combustion. EBCs are critical for components designed for long-term use in extreme environments.
CMCs have been in development for decades, primarily driven by utilization in the aerospace and defense sectors, but they have only recently made a commercial debut in the General Electric Leading Edge Aviation Propulsion (LEAP) engine. While CMCs are still a relatively nascent market, they have great potential for use in land-based hydrogen power generation turbines if the materials can enable turbines to run at higher temperatures and efficiencies while maintaining durability. Although further work to improve the viability of CMCs and EBCs is needed, overcoming the existing challenges associated with these materials is critical to enable a hydrogen economy.
Challenges and R&D Opportunities in CMC/EBC Development for Hydrogen Power Generation
To understand the principal barriers to commercializing CMCs and EBCs in power generation applications, the National Energy Technology Laboratory (NETL) explored key challenges and opportunities by engaging with leading experts and technology innovators across the public and private sectors. Very few domestic organizations develop proprietary fibers and manufacture CMCs due to the extremely high barriers to entry. Despite their reliance on costly rare earth elements, EBCs present a more accessible entry point for organizations experienced in high-temperature coatings and advanced material development. As a result, many of the material challenges highlighted were focused on EBCs, a key enabler for CMCs in hydrogen environments. Overall, the challenges fit broadly across four dimensions:
- Material availability, supply chain limitations
- Material (EBCs). Coating durability and performance
- Design, Manufacturing, and Development. Optimizing material properties, minimizing material waste
- System Integration. Component and material reliability
An innovation strategy that spans the CMC/EBC continuum is needed to address these challenges (Figure 1).
Opportunities in CMC and EBC Design
New materials for novel applications call for innovative approaches to digital design and modeling of components. Digital design tools enable precise optimization and customization, especially as manufacturers navigate the intricacies of developing large and complex turbine components. Physics-based lifetime and performance models, an opportunity continuously highlighted by industry, can revolutionize material design and performance simulation but remain largely unrealized due to the complexity of capturing highly nuanced material behavior data.
To advance modeling capabilities beyond the empirical level, models must incorporate time-dependent data, component-specific data, physical and mechanical property changes over time, material degradation as a function of temperature, and more. Successful development of characterization databases that contain both CMC and EBC data can support modeling advancements that impact all industries utilizing the materials.
Processing Opportunities for Components and Coatings
Minimizing material waste and ensuring the durability and longevity of components are common challenges in advanced material development. Traditional processing techniques, such as thermal plasma spray, can lead to waste and represent an area in which further research and development (R&D) in highly efficient processing techniques could reduce costs. Additionally, EBC bond coats, which are typically silicon-based, are constrained by silicon’s melting point of 2,577F (1,414C).
As silicon-based bond coats approach their thermal limits, the heightened risks of delamination from increased water vapor content demand alternative material solutions with strong adhesion and thermal resistance properties. Furthermore, addressing the coefficient of thermal expansion (CTE) mismatch across EBC layers is a developmental imperative. Ensuring material compatibility and phase stability among layers to prevent cracking, delamination, and degradation is critical for managing mechanical stress.
Solving CTE mismatch will enhance coating performance, boosting the lifespan of the component. Industry insights suggest a need for either new material combinations that enable exceptional management of thermal flux among layers or simpler EBC designs that minimize CTE compatibility challenges.
Manufacturing Opportunities
Availability of materials and supply chain constraints are the primary obstacles to commercializing CMCs and EBCs. Most proprietary CMC fibers are made in Japan and require extensive infrastructure and know-how to develop. Developing a supply chain for these materials is both complex and capital-intensive, requiring the investment of up to a billion dollars to build from scratch.
Improving economies of scale is seen as the most realistic option for making the materials viable; however, no single solution to overcome this obstacle exists. Sustained innovation across the continuum can help reduce costs and facilitate market entry for new participants. Additionally, opportunities to improve the finishing, adhesion, and sealing of CMCs/EBCs can support manufacturing components that will maintain their durability in extreme environments.
Opportunities for Validating CMC and EBC Reliability
Advanced testing facilities can significantly accelerate the commercialization of CMCs and EBCs by helping manufacturers to efficiently generate high-quality data. Many testing facilities today are not equipped to evaluate materials in relevant operating environments, such as laser rigs for high-temperature gradients or natural gas/hydrogen rigs to simulate steam. For the facilities that do possess the required capabilities, availability is extremely limited and wait times are long. Accessible testing facilities that can accurately simulate real-world operating conditions across a variety of applications will enable better characterization of materials and are one of the highest impact opportunities that can support the advancement of CMC and EBC technologies.
Additionally, there is a noted gap in understanding the causes and effects of defects in CMCs. Industry is not able to determine what makes a part suitable for use due to a lack of information and standards. Collaboration and knowledge-sharing opportunities to develop databases that can support identifying and mitigating defects exist. Additionally, further innovation in non-destructive testing and evaluation methods can help original equipment manufacturers (OEMs) accurately assess and ensure the durability of components without damaging costly materials.
Opportunities Mitigating In-Service Challenges
The presence of water vapor dramatically accelerates the degradation of CMCs and EBCs. Opportunities for further R&D to develop EBCs that offer superior recession resistance are critical for enhancing the longevity of CMCs that operate in environments rich in water vapor and other reactive gases.
Current CMC/EBC technologies can withstand temperatures up to 2,400F (1,316C); however, the industry is already looking at next-generation materials that can withstand up to 2,700F (1,482C) to enable higher turbine efficiency and power outputs. Nevertheless, significant efforts to enhance existing EBC systems to prolong materials’ lifespans are needed.
Calcium-magnesium-alumino-silicates (CMAS) is a type of deposit that forms on the surface of CMCs when certain minerals and impurities in the environment react with the materials, causing erosion, wear, and loss of structural integrity, ultimately leading to premature failure. This effect is more easily managed in power generation applications where the operating environment can be controlled, but it remains a major challenge for aerospace applications. These challenges present an opportunity to explore novel material compositions and formulations that can improve the overall durability and performance of materials.
NETL’s Role in Addressing High Impact Challenges
Advancing state-of-the-art technology in CMCs and EBCs will require interdisciplinary collaboration across the public and private sectors. The Department of Energy (DOE) has a mandate to bolster America’s prosperity by addressing energy challenges through advanced R&D in clean energy technologies and facilitating collaboration across ecosystems to accelerate their commercialization. NETL, one of the DOE’s 17 national laboratories, possesses extensive experience in advanced material development, advanced combustion turbines, and computer science and modeling capabilities, playing a critical role in propelling the nation toward its decarbonization objectives.
NETL is engaged in several high-impact research areas spanning thermal and environmental barrier coating (T/EBC) development, physics-based modeling, and an imminent launch of a cutting-edge gas turbine rig designed for the environmental testing of T/EBCs, CMCs, and other materials. The gas turbine combustion rig closely simulates turbine environments through ultra-high surface temperatures, high gas velocities, high pressures, adjustable gas mixtures, thermal gradients, and prolonged exposure times. This rig provides a critical platform for developers to rigorously analyze, test, and gather high-quality data on the behavior and performance of advanced materials under realistic operational stresses, accelerating the pace of material innovation and application.
To realize the vast potential of CMCs and EBCs, and make land-based high-efficiency power generation utilizing hydrogen a reality, a concerted effort from government agencies as well as academic institutions, industry leaders, and technology innovators is imperative. Together, stakeholders must collaborate to navigate an extensive array of challenges and opportunities, synergizing strengths and resources to catalyze interdisciplinary advancements and accelerate the shift toward sustainable energy solutions.
—Haleigh Heil is a NETL support contractor and Erik Shuster is an expert at NETL focused on its National Energy Modeling System, Systems Analysis, and Modeling programs.
Disclaimer
This project was funded by the United States Department of Energy, National Energy Technology Laboratory, in part, through a site support contract. Neither the United States Government nor any agency thereof, nor any of its employees, nor the support contractor, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.