Boilers, Coal, Gas, Gas Turbines

Advanced Materials and Processes for the Next Generation of Gas Turbine Design

Issue 9 and Volume 120.

By Michael Aller, Timothy Franta, and Helge von Helldorff

Advanced materials and additive manufacturing processes provide the foundation for cleaner and more efficient power generation solutions. Photo courtesy: Energy Florida/CAPE

Gas turbines have become an increasingly important part of the U.S. power generation mix, especially as regulatory and market trends have made gas turbines much more competitive in terms of price and emissions signature, relative to alternatives such as coal power generation. Gas turbines are poised to become the most important thermal energy and propulsion device in the global economy within the next few years.

The U.S. Energy Information Administration (EIA) projects that almost 60 percent of new power generation capacity within the next 20 years will be provided by natural gas-fired, combined cycle power plants. Together with the rise in renewable power generation, especially wind power generation, this shift has profound implications for the trajectory of the global power generation sector in coming decades. The shift is already having a significant impact on power markets within the United States and Europe, where renewable and gas power generation are underpricing and therefore squeezing out coal-fired generation, as well as reducing the profitability of baseload power sources such as nuclear facilities. Gas-fired power generation surpassed coal as a source of electricity in the United States for the first time ever in the spring of 2015, and given current trends, natural gas does not seem likely to relinquish its leadership position as a source of power generation in North America at any point in the near future.

The large increase in gas production from the shale gas boom in North America has substantially reduced prices for natural gas since 2008, and significantly altered the economics and price structure of the gas power generation landscape. In many (though not all) international markets, the global glut in oil and gas supplies has similarly reduced natural gas spot prices and improved the economics of natural gas-fired facilities. Contributing to their attractiveness, gas-fired power generation facilities often have a smaller footprint and many balance-of-plant components are available “off the shelf”, allowing for rapid permitting and set up of new facilities, and reducing the cost of construction. In a world where emission signatures are becoming increasingly important and regulated, gas turbine power generation facilities emit relatively low levels of pollution: no sulfur dioxide, low levels of nitrogen oxides and particulates, and less than half the carbon dioxide of comparable coal-fired facilities.

All of these factors contribute to a significant rise in the attractiveness of gas turbines for power generation systems. Furthermore, gas turbines provide significant short-term ramping and responsive power production capability, allowing them to provide backup capacity for variable renewable power resources such as wind and solar installations. As more renewable generation capacity comes online, this ability to provide responsive, cost-effective power is driving significant growth in new gas-fired power generation installations.

As gas turbines become ever more prevalent in the power generation community and adapted for a range of applications including industrial energy efficiency through combined heat and power installations, their operational parameters and efficiency ratings are in need of consistent improvement. Current gas-fired power generation facilities are approximately 42 to 44 percent efficient in simple cycle (one gas-fired turbine, hooked to a generator) and approximately 60 to 61 percent efficient in combined-cycle operation (including an attached heat recovery steam generation loop, steam turbine and generator(s) to harness waste heat for additional power generation).

There are a broad range of new technologies in advanced manufacturing that are being applied within the gas turbine design, engineering, manufacturing, and repair communities.

These new technologies allow for enhanced performance and shorter time to market for new improvements and upgrades.

In order to improve the efficiency and performance of today’s turbine designs, manufacturers are looking to improve aerodynamics, produce higher pressure ratios, and enable higher turbine inlet temperatures in the next generation of machines. A number of different advanced manufacturing techniques can help achieve these goals, including new materials, more precise or complex geometries, enhanced cooling architectures, and new types of high-performance coatings.

A number of companies and research teams are demonstrating printing entire engine components, such as this fuel injector, as one integrated piece. Photo Courtesy: NASA

A key enabler for increased efficiency and reduced emissions in industrial gas turbines is the development of high temperature materials applicable to the harsh and high vibration environment in these machines, and associated manufacturing techniques suitable for prototyping, production, and repair.

Although these have each been a primary focus of technology development within the industry for decades, new techniques and applications are emerging to address the challenges of high temperature materials and related designs.

Additive Manufacturing

One of the most exciting areas of new technology in gas turbine engineering is in additive manufacturing or 3D printing. This technology has been in commercial use for nearly three decades, although it is only recently being applied to metals at commercial scale. Additive manufacturing allows a machine operator to take digitized engineering designs and transform them into fully functional objects. The material is added in layers and bonded by heat deposition or chemical processes; then more layers are added to produce the desired three-dimensional shape.

Additive manufacturing technology opens up an entirely new design philosophy for turbine parts and components. This process can reduce component complexity by reducing the number of steps necessary to produce a part or component, or by consolidating several components into a single integrated piece. Additive manufacturing of metallic parts for the turbine industry is an active area of research for many manufacturers and university research institutes around the world.

The process of adapting additive manufacturing techniques to the turbine environment is still a work in progress. As just one example, the underlying characteristics and properties of many metal powders (especially high-temperature super-alloys) are not yet well-understood and tested to the industry’s exacting standards. In addition, the replicability of the build characteristics produced by different additive manufacturing machines, even of the same model and manufacturer, remains a challenge. Many industry participants have found that in practice, the output of each individual additive manufacturing machine must be assessed and certified separately, and all production information and parameters must be tracked meticulously as part of the quality assurance process.

Given the challenges in adapting this technology to scale up for manufacturing, one area of particular interest in the short term is additive manufacturing for castings. Many industry participants are looking at new ways to utilize this technology to accelerate the design and iteration process by providing rapid tooling and other capabilities (such as replacing the lost wax process in complex castings with 3D printed cores). This can save months or years and millions of dollars of engineering time within a development program, whereby new iterations of parts can be produced and tested within days rather than months.

Several companies are already implementing additive manufacturing in the production of small components and the repair and refurbishment of legacy parts. The 3D printing environment allows for companies to produce parts for repair or refurbishment on demand rather than having to stock large numbers of legacy parts for their customers. This reduces costs and improves response time for customers, and provides a great opportunity for improved efficiency within the repair and refurbishment business. For example, a pump manufacturer reduced impeller replacement costs by up to 90 percent and cut lead times for new part(s) by over a month using 3D printing. According to 3D print supplier ExOne, the finished part took two weeks and cost less than $1,500 using 3D printing methods, while a traditional pattern-based replacement would have taken 6 to 12 weeks and cost between $5,000 and $15,000 by comparison.

Several companies are looking at ways to repair blade tips, burner tips, and other small repairs which might have traditionally necessitated the disposal of a part that now can be safely and effectively refurbished and reused. For example, Siemens Power & Gas is actively utilizing 3D printing technology to repair burner tips, allowing it to recycle and reuse components that normally would have been disposed of, and saving its customers time and money.

A number of companies and research teams are pursuing 3D printing of full blade assemblies for testing as well as airfoils, fuel injectors, and a variety of other components. Safran Microturbo, an engineering firm based in Toulouse, France, recently demonstrated the ability to 3D print an entire gas turbine assembly (albeit on a small scale). Many of these applications are being implemented first in the aviation engine industry, as the weight savings and value-add in that sector is more significant than in the industrial gas turbine market, but many of these innovations will soon also be integrated into the industrial gas turbine fleets of major manufacturers.

Ceramic Matrix Composites

Another advanced material with great promise for gas turbine performance improvements is Ceramic Matrix Composites (CMC), which promise highly superior thermal resistance at approximately one-third the weight of current nickel-based super-alloys. CMC are a composite of intertwined ceramic silicon carbide fibers embedded in and reinforcing a continuous silicon carbide-carbon ceramic matrix. This material has been in development for nearly 30 years, and is being implemented in aviation engines in the shroud for first stage turbine inlets in GE’s LEAP engines. CMC are also being assessed by a number of other manufacturers for applications within the industrial gas turbine environment. For small turbines, the use of CMC could result in turbine blade designs without the requirements for advanced cooling and thus improved efficiency through reduction of bleed-air. However, for large turbines, concerns regarding elevated mechanical stresses and fracture properties of the CMC are still prohibiting their practical implementation in turbine blades. Non-rotating components such as combustor liners, stators, and shrouds will see increasing implementation of CMC, and as their mechanical properties are better understood and design parameters improved, CMC may be targeted for implementation in other parts of the engine as well.

A considerable amount of work is also being done in new coatings and coating application technologies. Significant new advances in coatings have not occurred for nearly 40 years. Most major coating compounds were originally identified in the 1950s and 1960s, and a great deal of fundamental research is being conducted in order to identify and improve upon the existing coating formulations and deposition techniques, allowing for higher firing temperatures and greater operational efficiency of next-generation gas turbines. Substantial effort is also being made on methods to embed instrumentation and sensors directly into turbine components utilizing additive manufacturing or other techniques. Enabling reliable sensors within machines will allow for much more direct measurement of operating parameters and anomalies, resulting in much greater precision in maintenance intervals, reduced downtime, and substantial improvements in operational performance.

The “Rising Tide” of Technology

Although many of these improvements are driven by the large gas turbine original equipment manufacturers (OEM), it is clear that the application of these new advances in production technology should not be restricted only to the largest and/or most sophisticated classes of engines. There are substantial opportunities for improvements in maintenance, repair, and overhaul of existing generation fleets, including legacy gas-fired, aero-derivative, and peaking power facilities. These new technologies will allow more rapid, efficient, and customizable retrofits and upgrades for existing power generation facilities, as well as improvements for new generations of power generation equipment.

A group of turbine OEMs, design firms, major research institutions and other public and private stakeholders are currently involved in an effort supported through the National Institute of Science and Technology’s Advanced Manufacturing (AMTech) program to support the assessment of advanced materials and manufacturing processes on behalf of the next generation of gas turbines and rotating machinery equipment. Coordinated by Energy Florida and the Gas Turbine Association, the stakeholders in the Consortium for Advanced Production and Engineering of Gas Turbines and Rotating Machinery (CAPE) have been evaluating a variety of techniques and technologies necessary to underpin the next generation of highly efficient natural gas-fired, combined-cycle turbines and generator sets. The partners have been reviewing the state of knowledge in a variety of related areas, and have developed a set of recommendations associated with further development of related technologies. These priorities are being coordinated with national funders, research institutions, and key stakeholders across the industry. An update on the AMTech CAPE effort will be presented during the PowerGen International conference and trade show in December 2016.

Diversity of Applications and Solutions

Although the centralized power production model remains strong in certain regulated markets and in many emerging markets that are still working to deliver fundamental capacity improvements to their grid, the power production landscape is more diverse. The power sector takes decades to fully integrate new technology due to the high complexity, long production lead times, high capital investment, and operational lifetimes of plants and equipment. Due to this fact, the gas turbine design and engineering community must actively cultivate an understanding of the needs of its customer base, both today and in years to come. The industry must adapt its design and engineering solutions to be flexible enough to meet its customers’ future requirements, considering the trends and development of the quickly changing renewable energy markets. The decades-long development cycle for gas turbine technology requires some educated guesswork regarding the fundamental technologies that will underpin future technology cycles. Customers will always need power, but as we have seen, the ways they receive and use that power can change.

The gas turbine industry should consider ways to “hedge its bets” by investing in a range of materials and technical solutions that support improvements in smaller, less expensive machines, in addition to supporting large-scale material science efforts to improve performance and longevity of increasingly sizable and complex combined-cycle power systems. The power generation community should work to ensure that all voices are being heard, and that we are actively sharing information and driving improvements across the range and breadth of power generation equipment and applications. Today’s world demands nothing less.

Author

Michael Aller is executive director of Energy Florida and the Consortium for Advanced Production and Engineering of Gas Turbines and Rotating Machinery (CAPE), based in Cape Canaveral, Florida. Timothy Franta is project manager of the CAPE. Helge von Helldorff is a project associate with the CAPE, and a doctoral candidate in aerospace engineering at the Florida Institute of Technology in Melbourne, Florida.