In England in 1930, Frank Whittle submitted the first turbojet patent. Seven years later, Whittle succeeded in conducting the first flight test run of a complete turbojet experimental engine. The engine, produced by Power Jets, was liquid fueled, with a single-stage centrifugal compressor. It was designed to power a small mail plane to a speed of 500 mph (800 km/hr).1 Since Frank Whittle and Power Jets’ experimental engine, gas turbines have seen tremendous progress. They can now operate above the melting point of engine structural materials thanks to advanced cooling and coating technologies; they offer some of the highest power density levels for propulsion and power generation; they offer important flexibility in terms of load levels and fuel types; and they have shown reliable operation, providing electricity and transporting billions of people worldwide. Because of these characteristics, gas turbines are now used across major global industrial applications. Altogether, the yearly global value of production of gas turbines is projected to grow from about $90 billion today to $110 billion by 2032, with aviation gas turbines consistently accounting for about 85 percent of the total market.2Box 1.1 provides a basic understanding of gas turbines.
This report identifies key research goals and topics that the United States could pursue to continue its technological leadership in gas turbines. The report is organized around three applications of particular interest:
- Power generation (i.e., large ground-based gas turbines that generate electricity to power the electrical grid);
- Aviation (i.e., gas turbines for commercial and military aircraft propulsion); and
- Oil and gas industry.3
2 L. Langston, 2018, Anticipated but unwelcome, Mechanical Engineering 140:36-41, doi: 10.1115/1.2018-JUN-2, https://asmedigitalcollection.asme.org/memagazineselect/article/140/06/37/447709/Anticipated-but-UnwelcomeEven-as-Gas-Turbines-Get.
3 The committee determined that within the oil and gas industry, the highest priority research should focus on gas turbines that power natural gas compressors in pipeline applications.
Gas Turbine Research and Development
Because of their technical characteristics, gas turbines still have important technological and economic growth potential. For this reason, regions around the globe are heavily investing in research and development (R&D) for gas turbines. The European Union provides substantial support for power generation and aviation research and development programs, such as the Clean Sky 1 and 2 programs, which were initiated in 2008 and 2014, respectively. Similarly, in 2016 China launched its state-owned Aero-Engine Group of China, which has about 50 billion yuan ($7.5 billion) in registered capital to develop military and commercial engines.4 In addition, in 2018 the Natural Science Foundation of China launched a major national research initiative on advanced aircraft engines to address advanced manufacturing, smart failure diagnostics, active safety controls, high-temperature materials, and turbulent combustion.5 China has also launched a domestic heavy-duty gas turbine development program to support electrification and decarbonization.
The U.S. federal R&D investment in gas turbines seems modest in comparison. For power generation, advanced gas turbine technologies are funded under the Department of Energy (DOE) Office of Fossil Energy, with a budget of approximately $15 to $20 million annually since 2015 (see Figure 1.1).
For aviation propulsion, advanced gas turbines are funded through the Department of Defense (DoD) for military aviation engine technologies and through NASA for commercial aviation propulsion technologies. The amount of funding for military aviation engine technology development is classified, but the committee believes that it is substantially less than what Asian nations have been investing in advanced gas turbine technology in recent years. Funding for commercial aviation propulsion technologies from NASA has meanwhile been relatively level and modest in size (between $600 million and $650 million per year) over the past decade, with no change projected in future budgets through 2023.6
6 National Aeronautics and Space Administration, 2019, FY2019 Budget Estimates, https://www.nasa.gov/sites/default/files/atoms/files/fy19_nasa_budget_estimates.pdf.
Globally, electricity generation has more than doubled since 1990,7 and it could grow by more than 50 percent over the next two decades, as depicted in Figure 1.2. By capacity type, power generation for natural gas could also grow by more than 50 percent by 2040.8,9
In aviation, essentially all large military and commercial airplanes are powered by gas turbine propulsion systems. During the 20 years preceding 2017, the number of commercial airline passengers globally almost tripled,10 and over the next 20 years it is projected that more than 40,000 new aircraft will be produced, with the total number of operational aircraft doubling (see Figure 1.3).
The global demand for oil and gas could increase by 20 percent over the next 20 years, with China accounting for one-third of the growth.11 However, this growth is expected to plateau or contract after 2035, depending on energy policies regarding emissions and the use of renewable energy that are implemented internationally because of concerns about climate change, air quality, and overall sustainability.12 Research and development to substantially reduce carbon dioxide (CO2) emissions may mitigate concerns with respect to climate change. Even so, demand for natural gas could decrease if utility-scale energy storage technologies improve the ability of solar and wind energy systems to provide power when real-time demand for electricity exceeds the real-time generating capacity of these systems. In one potential scenario, excess solar or wind power could be used to create hydrogen as an important energy storage mechanism, which in turn could be burned in gas turbines to create electricity.
9 The accuracy of long-term projections is of course uncertain. This projected growth is per the new policies scenario in World Energy Outlook 2018 (International Energy Agency, 2018, https://www.iea.org/weo2018/).
In the United States, gas turbine facilities powered by natural gas generate about one-third of the electrical power; and gas turbines are the predominant means of using energy from the combustion of natural gas to produce electricity.13
Within the United States, the power produced by gas turbines and by renewable energy sources are each projected to grow in absolute terms and as a percentage of the national market through 2050 (see Figure 1.4).14 These two power sources will satisfy all of the projected growth in national demand for electrical power, as the contribution by nuclear power and coal diminishes (see Figure 1.5).
14 Renewable energy sources include solar power, wind power, hydroelectric power, energy storage systems that store renewable energy when generating capacity exceeds demand, and gas turbines that are powered by renewable fuels. Renewable fuels include hydrogen, ammonia, biofuels, and synthetic gaseous or liquid hydrocarbons that are generated by electricity from renewable sources of energy.
The growth in natural gas demand is projected to drive a corresponding growth in the need for natural gas pipelines. In the United States, for example, the production of natural gas will grow in the East and Upper Midwest as well as in West Texas (the Permian Basin) much more rapidly than in any other region of the United States. Meanwhile, the South Central region is projected to experience a greater increase in the demand for natural gas than any other region. This increase in demand will be driven by industry, electrical power generation, natural gas exports to Mexico, and liquefied natural gas (LNG) exports globally. As a result, there will be a tremendous growth in the demand for natural gas pipelines from the East and Midwest to the South Central region (see Figure 1.6). The production of new pipelines to meet this demand—and the need for increased natural gas pipelines elsewhere in the United States—will result in a concomitant increase in demand for pipeline compressor stations, the vast majority of which are driven by gas turbines.
Gas turbines are a proven and reliable technology. Their power density, reliability, and safety are well established. For all three applications of interest to this report, gas turbines have and will continue to play a key role domestically and globally. Continued research and technology investments will enable gas turbines to maintain their place in the market.
The power generation market is undergoing important transformations with renewable energy sources and decarbonization. However, because of their power density, relatively small carbon footprint, and potential to burn environmentally friendly fuels—and because appropriately designed gas turbines can respond rapidly to changes in power demand—gas turbines will continue to be an important element in this energy mix.
The aviation market, which is much larger than the market for all other applications combined (see Figure 1.7), shows a strong preference for gas turbines given their proven reliability, safety, and performance. They will therefore likely continue to dominate the growing aviation propulsion market for the foreseeable future.
The oil and gas market has a large pipeline network that can serve as an important energy storage mechanism. Gas turbines are powering this oil and gas industry all along the value chain. Their reliability, power density, and fuel availability also make gas turbines the technology of choice for the future.
Continued investments in gas turbine research and development have important implications for the economic and technological leadership in this industry. This technological leadership is important, as the United States is the principal exporter of gas turbines, with an export value of $32 billion in 2017.15 Overall, gas turbines represent a $90 billion global market (see Figure 1.7), and they are an important factor to U.S economic prosperity and to sustaining the highly skilled manufacturing and engineering workforce that is essential to the gas turbine industry. This workforce can also cross-pollinate other important U.S. industrial sectors (e.g., transportation, electronics, petrochemical, and general manufacturing).
The future of the power generation, aviation, and oil and gas markets for gas turbines will face many challenges as changes occur to the overall energy landscape. The market trends described in this section will affect the design and manufacture of gas turbines in the future and will be important to consider when framing a research portfolio. The goal of this section is to answer the following key questions:
- Who will be the future customers for gas turbines?
- What will they need for power generation, aviation, and oil and gas applications?
- What are their buying preferences going to be?
The following four market trends are particularly relevant:
- Energy security and resilience;
- Decarbonization; and
- Competition and customer profiles.
As discussed in more detail below, global market projections show important growth opportunities for power generation, aviation, and oil and gas applications. The market trends also show that gas turbines will need to adapt to accommodate future customer needs. Investments in advanced technologies in the United States will enhance the ability of the gas turbine industry to adapt to future needs and meet customer demands.
Changes in Demographics
The global population is growing, from 7.4 billion in 2015 to a projected 9.2 billion in 2040, with increasing urbanization. More than 90 percent of the projected population growth will occur in Africa (48 percent) and Asia (43 percent).16 As such, countries in these regions will have an increasing demand for reliable power, energy supplies, and air transportation. Unlike previous markets penetrated by gas turbines, these countries often lack capital, local sources of natural gas, and a robust grid infrastructure for power generation and gas pipelines.
Strong aviation growth is also forecast. During the next 20 years, air traffic is projected to increase by 7 percent annually in India and 6 percent annually in China and Africa. This is about double the projected growth in North America and Europe.17
Meeting the power demands of developing nations will be difficult given the projected population growth, the drive of individuals and nations to improve their standards of living, and the lack of traditional power and energy infrastructure that exists in developed nations. Interconnected transmission and distribution networks will be less widespread than in developed countries.18 New gas turbines will be much more competitive if they fit the needs of these evolving markets.
Energy Security and Resilience
In addition to changes in demographics previously described, approaches to energy security will also drive the diversification of power generation assets and the construction of power generation assets that are generally closer to consumers than is typical today. Natural gas pipelines will act as energy carriers as well as energy stor-
17 Boeing, 2019, Commercial Market Outlook 2019-2038, https://www.boeing.com/commercial/market/commercial-market-outlook, accessed November 5, 2019.
18 T. Levin and V.M. Thomas, 2019, Can developing countries leapfrog the centralized electrification paradigm? Energy for Sustainable Development 31(2016):97-107, https://www.osti.gov/pages/servlets/purl/1391710.
age facilities. With the availability and low pricing of natural gas, specifically in the United States, a shift will continue from the use of coal and diesel fuels toward natural gas (which will also help decarbonization); many cities and countries are moving in this direction already.
Liquefied natural gas (LNG) has been a means of transporting natural gas in liquid form in places where pipeline infrastructure is insufficient or nonexistent. Ongoing discoveries of natural gas fields are sufficient to meet demand for many decades, but they are located far away from consumers. New methodologies are making LNG plants more attractive to build and operate (mainly in Australia and the United States). Consequently, LNG is developing into a commodity, much like gasoline, with the global trade in LNG projected to double by 2030.19 This change has the potential to improve energy security in Africa, China, Europe, India, and elsewhere.
The need for secure and reliable power generation will be a continued trend for countries that are facing geopolitical conflicts, and more broadly due to changes in the evolving set of energy suppliers and buyers. In this context, gas turbines will be impacted, as the regions where they will operate will vary in terms of fuel supply, electrical grid characteristics, and power generation mixes. Severe weather and the lack of capital for large infrastructures in certain regions will also tend to drive the need for smaller electrical grids and for smaller and more reliable gas turbines to meet increased needs in grid reliability and counterbalance the unpredictable availability of many renewable energy sources.
Policies and standards established by governments (in the United States, Europe, and elsewhere) and by intergovernmental bodies such as the International Civil Aviation Organization, as well as end-customer preferences, are set to significantly reduce greenhouse gas emissions to address global warming. This trend toward decarbonization and sustainability will incentivize the replacement of conventional gas turbine power generation systems with renewable systems. It will also incentivize the use of gas turbines as a replacement for power generation facilities fueled by coal or petroleum products because those facilities emit more CO2 per kilowatt hour than gas turbine facilities. In addition, gas turbines can become a renewable, carbon-free source of power for ground and aviation applications to the extent that they are fueled by synthetic hydrocarbons, hydrogen, methane, and other fuels that can be generated and transported from renewable energy sources. The supplies of such fuels may substantially increase over the next several decades, which may impact the design of gas turbines, primarily in the combustor module.
Continued increases in the prevalence of renewable energy sources and energy storage systems will also impact gas turbine performance requirements, demanding more flexible and fast ramping generators (as well as practical, large-scale energy storage approaches) to accommodate the integration of renewable energy sources into the power grid. In the case of new energy storage approaches, gas turbines will be impacted by having to burn different types of fuels or by operating in different power plant setups than is customary.
The growing demand for electric vehicles will add to the overall demand for electricity. This will also increase the variability of demand throughout the day to the extent that owners of electric vehicles predominantly recharge their vehicles at the same time each day (e.g., when arriving home after the end of the workday). The power network will need to grow and adapt to accommodate these changes.
Competition and Customer Profiles
Production and engineering overcapacity in the power generation gas turbine market has hit all major original equipment manufacturers (OEMs).20 Despite the increase for global demand in electricity, this reality will continue to impact the gas turbine industry by limiting investment by OEMs in research and development. The rise of new international competitors in power generation gas turbines (e.g., in China and Korea) will put further pressure on existing OEMs’ market shares, profits, and investment in research and development. These international
competitors are seeking to grow their gas turbine industries across all sectors of the product life cycle, including research and development, supply chain, and servicing. Government intervention and subsidies to support these new competitors could significantly outspend U.S.-based OEMs, which will increase the competitiveness of foreign OEMs.
The increasing need for smaller, more decentralized power sources and the declining needs for large, centralized facilities in the developed world, coupled with competition with renewable energy sources, will tend to threaten the market share of gas turbines in the power generation market, especially with respect to the very large gas turbines that are used to provide base load power in existing, centralized facilities. Gas turbines will be required to work in concert with renewable energy sources within a more-distributed power generation system.
Gas turbines are complex products that exist within a deep and diverse technology development ecosystem. Many advanced technologies have broad applications, and substantial resources are often being invested in these technologies by many different organizations in government and industry. Global technology trends, therefore, are important to understand so that gas turbine technology development efforts can leverage the progress being made by others. For example, gas turbine research and technology development programs can leverage and adapt research performed in cybersecurity (outside the gas turbine community) to develop secure communications protocols and firewalls.
The global technology trends are different from the research topics recommended in this report, as the former represent global mega-technological trends supported across multiple industries and markets. Key global technology trends include the following:
- Inexpensive, large-scale computational capabilities;
- Highly autonomous systems;
- Additive manufacturing;
- Artificial intelligence (AI); and
Inexpensive, Large-Scale Computational Capabilities
Inexpensive, large-scale computational capabilities will help design, manufacture, and operate gas turbines faster and with higher accuracy. On the design side, the development of these technologies will provide the ability to overcome system integration challenges such as dynamics coupling, turbulence distortion, unanticipated heating, or aerodynamic loading from complex mixing phenomena.21 On the manufacturing side, the simulation of new manufacturing technologies like additive manufacturing at different levels of resolution will help better exploit these technologies. The generation of virtual sensors or the physics-based interpretation of real-time sensor data will also help the operation and maintenance of gas turbines.
Large-scale simulations, described above, will likely leverage emerging hardware for accelerated computing to reduce simulation costs (in both time and real-dollar costs). Whether accessed via the Internet in the cloud or on premises, reformulated computational methods and models can be leveraged to efficiently operate on these hardware architectures. Figure 1.8 depicts an estimated cost trend for large-scale simulations based on available commodity (×86) and accelerated (graphics processing unit) architectures for a high-fidelity flow solver.
Highly Autonomous Systems
Highly autonomous systems will help reduce the operation and maintenance costs of gas turbines. Through better control, improved inspection through robotics, improved manufacturing and repair processes, and by
21 Briefing by Frank Ham, Cascade Technologies, to the Committee on Advanced Technologies for Gas Turbines on December 18, 2018.
leveraging advances in AI (see below) this trend will significantly impact the entire gas turbine product life cycle. Autonomous systems will increase the level of flexibility and intelligence of the ecosystem of activities supporting a gas turbine by improving the repair, maintenance, and inspection processes, which will improve the reliability of these assets as well as reduce product life-cycle cost. In particular, the following emerging technologies will enable substantial changes for the inspection and repair paradigm for gas turbines:
- Miniaturized robotic mechanisms;
- Autonomous navigation;
- Automated repair and inspection;
- Teleoperation and telepresence; and
- Virtual and augmented reality.
Additive manufacturing represents an opportunity to institute digitally based product development capabilities that can marry conception and validation of new and complex designs with print-to-part manufacturing capabilities of radical, previously unavailable three-dimensional (3D) shapes. A wide range of industries are benefiting from the rapid maturation and deployment of additive manufacturing processes for tooling and prototyping and production of components. The possibilities for higher efficiencies in delivering new, higher performing hardware in shorter periods of time are potentially enormous.
Many robust activities are under way in academia, government laboratories, and industry to advance the state of the art in additive manufacturing. The U.S. government’s investment alone exceeds $400 million per year. Enabling the next expansion of additive manufacturing capabilities means creating a new generation of 3D
additively manufactured components that will require larger machines, with multiple heating sources and larger processing volumes to meet ever growing demands. Ongoing efforts will also improve the ability to evaluate the capabilities and durability of additively manufactured components.
The additive process modalities and design methodologies will enable a systems approach to component design that will reduce the number of individual parts, enhance vendor involvement, reduce system weight, improve durability, and reduce life-cycle costs.
AI will help improve the operational efficiency and quality of gas turbines throughout the design, manufacturing, operation, and service phases of the product life cycle. Advances in AI (including machine learning)22 and data warehousing will also (1) enable human operators to make better and faster decisions, (2) improve maintenance processes, (3) improve reliability and resilience, and (4) decrease operational costs. Expectations regarding the potential return on investment from AI research is indicated by the level of investments being made by U.S. venture capital firms (see Figure 1.9).
As gas turbines become even more connected, and as the amount of highly valuable company data stored in design, manufacturing, operational, and maintenance systems increases, the importance of cybersecurity and the required levels of security will also increase. In an ideal world, distributed assets, such as electrical distribution systems, would be connected to a larger network, enabling experts to look for potential problematic behaviors and identify opportunities to increase efficiency or reduce costs. Such data would then be aggregated across a large number of locations to build the databases needed to sleuth out more subtle or lower probability issues. However, such access creates vulnerabilities. While many of the ongoing cyberattacks on critical infrastructure are classified, a number of high-profile incidents have made it into the open press. One of the most high-profile attacks was the 2015 hacking of the Ukrainian power grid, which led to widespread power outages and has been attributed to state-sponsored actors. Similarly, the Department of Homeland Security (DHS) and the Federal Bureau of Investigation (FBI) issued a public alert in 2018 regarding state-sponsored cyber actors who gained remote access into U.S. energy networks. Indeed, it seems increasingly clear that it may not be possible to completely prevent suitably resourced hostile actors from accessing critical industrial infrastructure, leading to discussions of how to
22 Machine learning is a branch of artificial intelligence relating to the creation of models that can learn from large volumes of data and prior conditions.
limit or manage risk. Such issues will inherently limit the ability to control gas turbines and manage data from industrial facilities. Cybersecurity issues are also important to other aspects of the oil and gas industry, including production and distribution.23
The committee fashioned a multistep process to identify high-priority research. First it agreed upon prioritization criteria, and then it used those criteria to identify aggressive goals for gas turbines in each of the three applications of interest: power generation, aviation, and oil and gas. The committee subsequently identified 10 high-priority research areas that address these goals. Within each area, the committee identified up to three research topics to provide more specific guidance on the recommended research.
The prioritization process is based on the assumption that the global market and technology trends described above will continue. Substantial changes in these trends may well justify reassessing the priorities recommended in this report.
An unstated element of the prioritization process is that acceptable approaches for achieving the goals must not degrade the ability to meet current standards and expectations for key parameters such as safety, emission of oxides of nitrogen (NOx) and other regulated compounds, life-cycle cost, commercial viability, reliability, maintainability, and availability.
The committee believes that this report would provide little value added if it simply endorsed a continuation of ongoing research. Rather, the value added by this report lies in identifying (1) new research directions and (2) known research directions with substantial potential that for whatever reason are receiving no or minimal funding.
As noted above, this report focuses on three applications of gas turbines: powering the electrical grid, propulsion for both military and commercial aircraft, and transporting natural gas from gas fields to users. In all three applications of interest, gas turbines are but one element of a larger system such as the nation’s electrical power grid, an aircraft, or a natural gas pipeline. In addition, gas turbines are also closely linked to ancillary systems. Depending on the type of turbine and the applications, this may include external cooling systems, fuel storage systems, electrical generators, structures for integrating gas turbine engines into the airframes of aircraft, cogeneration systems (which typically use steam turbines to generate electricity from the waste heat of gas turbines), and alternative fuels. As specified by the statement of task (see Appendix A), however, the scope of the report’s recommended research is limited to goals and research topics that are directly related to the design and manufacture of gas turbines. Thus, even though some alternative fuels could help improve the performance of gas turbines, developing alternative fuels is not a gas turbine research and development goal. On the other hand, a goal related to the development of gas turbine technology that would enable gas turbines to accommodate a variety of alternative fuels is within the scope of this study.
Selection Criteria for the Goals
The committee used three criteria to select aggressive goals for improving and creating advanced technologies that can be introduced into gas turbines considering the current and future state of the art of gas turbine design and manufacturing for each of the three applications. The three criteria are:
Performance improvement. This selection criterion was used to assess the extent to which accomplishment of a goal could have the potential to improve the performance of gas turbines in their respective applications. Performance parameters of particular interest are as follows:
- Full rated load
- Partial load (across the operating envelope)
23 T. Lieuwen and B. Noble, 2019, The new industrial data economy, Mechanical Engineering 141(5):38-41.
- Design and development time and cost
- Manufacturing time and cost
- Reliability, availability, and maintainability (RAM)
- Fuel flexibility
- CO2 emissions
- Compatibility with renewable energy sources and the future electrical grid (for power generation and oil and gas applications only)
- Life-cycle cost
- Technical risk. This selection criterion was used to assess the extent to which a goal faces an appropriate level of technical risk. Goals are expected to be aggressive in that efforts to achieve the goal will face medium-to-high risk and may fall short. The high-priority recommendations do not include low-risk activities given the expectation that low-risk research that could substantially accelerate improvements to the performance of gas turbines would have already been addressed by government, industry, or academic members of the gas turbine community. However, the goals should not be overly aggressive to the extent that there is little or no prospect that substantial progress in achieving the goal will be accomplished between now and 2030.
- Breadth of application. This selection criterion was used to assess the extent to which accomplishment of a goal could support the accomplishment of other goals, and the extent to which a goal is related to multiple research areas and topics.
Background Information for the Performance Improvement Criteria
As detailed above, performance improvement was assessed using five parameters. This section describes the relevant aspects of each parameter as it applies to the process of assessing potential gas turbine goals.
Increasing system efficiencies is a common goal across all gas turbine applications for both cost and environmental reasons. Given that fuel costs are a significant component of overall operational costs for the majority of gas turbine applications, increases in efficiency directly translate to fuel savings. In addition, for systems that use hydrocarbon fuels, increases in efficiency directly translate into reduced CO2 emissions. As will be explained in Chapter 2, however, specific efficiency goals differ for each of the three applications of interest. In addition, depending on the intended operational use of a particular gas turbine, its efficiency when operating under its full rated load may be of more, equal, or less importance than when operating under partial load.
The thermal efficiency of a gas turbine is a function of site characteristics, prevailing ambient conditions (temperature and humidity), altitude and thrust (for aircraft propulsion), and load (for power generation and oil and gas applications). The performance of a gas turbine at partial load is determined by the hardware design and the design philosophy of the manufacturer. In general, gas turbine efficiency decreases with decreasing load, increasing altitude (i.e., decreasing inlet pressure), and increasing ambient temperature. Thermal efficiency also varies with size. In many cases, design features that improve the efficiency of very large turbines are less effective when scaled down for application to smaller turbines. This consideration arises in power generation applications (because the growing prominence of smaller distributed electrical generators are changing the market dynamic), in aviation applications (for relatively small commercial transports), and for oil and gas applications (which tend to use small gas turbines compared to power generation gas turbines).
Gas turbine thermal efficiency can be improved by higher cycle pressure ratio and higher turbine inlet temperature. Power output is controlled mainly by air flow, which dictates the physical size of the machine. While cycle pressure ratio has the strongest effect on efficiency, it is detrimental to net power output via increased compressor power consumption and reduced output from the bottoming steam cycle (in combined cycle electric
power applications). Therefore, turbine inlet temperature is the most important design parameter for increasing the efficiency for both simple and combined cycle gas turbines.
Historically, there have been incremental improvements to gas turbine efficiency over time; improvements in gas turbine efficiency for power generation gas turbines over the past four decades and the class hierarchy of heavy-duty industrial gas turbines is summarized in Figure 1.10.24 Values of turbine inlet temperature (in °C) in the embedded table reflect the state of the art at the time of the introduction of the particular gas turbine class. Over time, advances achieved in materials, casting and manufacturing techniques, thermal barrier coatings, and film cooling technologies flow down into the earlier products and push their efficiencies upward as well, largely because they enable higher turbine inlet temperatures. Cycle pressure ratio increases in lockstep with turbine inlet temperature in order to keep the gas turbine exhaust at a temperature commensurate with the temperature limits on the materials that are typically used in the gas turbine last stage, the exhaust diffuser, and the downstream heat recovery steam generator (in combined cycle applications).25
25 This practice minimizes the number of gas turbine parts that must be designed with expensive high-temperature materials and complex cooling systems.
The committee considered three key aspects of life-cycle cost:
- Design and development time and cost;
- Manufacturing time and cost; and
Given the low cost of shale gas in the United States, gas turbines will be the least expensive option for generating electricity using fossil fuel for the foreseeable future. The situation is similar in the rest of the world except for India and China, which have large coal reserves and a gigantic and growing demand for electrical power.
Technological advances related to subjects such as additive manufacturing and rapid prototyping can significantly reduce the first two items in the list above. Additive manufacturing represents an opportunity to institute digitally based new product introduction capabilities that can marry conception and validation of new and complex designs with print-to-part manufacturing capabilities of radical, previously unavailable shapes. A wide range of industries are benefiting from the rapid maturation and deployment of additive manufacturing processes for tooling and for prototyping and production of components. The possibilities for higher efficiencies in delivering new, higher performing hardware in shorter periods of time are potentially enormous. The additive process modalities and design methodology enables a systems approach to component design resulting in a fewer number of parts, reduced weight, improved durability, reduced design cycle times, and reduced life-cycle costs.
RAM, which appear as the last item in the above list, are vital to the profitability of a power plant, airline, or pipeline. High efficiency means very little if a power plant cannot start on command, if an aircraft is grounded, or if a pipeline is shut down because compressor stations are off-line. Improvements in RAM have a major payoff in terms of overall gas turbine performance, and the payoff of improvements in other parameters may be greatly or completely offset if they reduce RAM.
Gas turbines can be designed to use a broad range of liquids and gases as fuel. This includes conventional fuels such as natural gas and jet fuel, as well as alternative fuels, including those that are derived from waste gases (e.g., methane from landfills) as well as renewable fuels (e.g., hydrogen, ammonia, biofuels, and synthetic gaseous or liquid hydrocarbons) that can be generated by electricity when the supply of electricity from renewable sources exceeds demand. The ability of gas turbines to use alternative fuels is becoming increasingly important to the extent that they (1) allow gas turbines to operate with no net CO2 emissions; (2) facilitate compliance with increasingly stringent regulatory limits on emissions; and (3) enable the use of renewable sources of electrical power (e.g., wind and solar power) even when the power from those sources exceeds demand on the electrical grid.
Natural gas power plants have inherently low levels of harmful emissions such as mercury, particulates, and sulfur oxides. The harmful emissions of particular concern with gas turbines fueled by natural gas are NOx and CO2.
Gas turbines that must comply with very stringent regulatory standards on NOx emissions currently come with tight fuel specifications in terms of, for example, the levels of hydrogen or higher hydrocarbons because NOx emissions are very sensitive to fuel composition. This is a particular challenge when it comes to using alternative fuels such as biofuels for aviation, in part because it is technically challenging and expensive to certify alternative aviation fuels.
Hydrogen fuels have been researched to some extent in large-scale industrial turbines, and gas turbine manufacturers are continuing to invest in the development of turbines that can use fuels with a mix of natural gas and hydrogen, with the goal of increasing the amount of hydrogen in the fuel to the point that the turbines can operate on hydrogen alone.26 The introduction of significant levels of hydrogen into the existing natural gas pipeline network would raise significant issues associated with the pipelines themselves and with the gas turbines that would be fueled by the mix of natural gas and hydrogen. For gas turbines, the primary issue would be the inher-
ent safety risks to the gas turbine combustion module and the fuel distribution system that are associated with natural gas–hydrogen fuel mixtures. In general, the difficulty of overcoming these challenges increases as the percentage of hydrogen increases. Challenges associated with alternative fuels (including hydrogen) that impact gas turbine design and operations fall within the scope of this report and the recommended research goals and areas in Chapters 2 and 3. Because challenges associated with hydrogen would perhaps be the most difficult to overcome, they are detailed specifically below:
- Hydrogen has a higher probability of system leakages than natural gas because of the very low molecular weight of hydrogen.
- Hydrogen has substantially broader flammability and detonation limits than natural gas, which increases safety and explosion risks.
- New sensors are needed to monitor gas turbine operations.
- New methods and associated technologies are needed to safely handle hydrogen–natural gas fuels.
- New control systems are needed to respond quickly in case of flameout caused by the interruption of gas supplies.
- Hydrogen has a much higher turbulent flame speed and extinction resistance than natural gas, which increases the risk of flashback. As a result, premixing nozzles in the combustor that can operate safely while producing low levels of NOx emissions are significantly different from existing designs for natural gas.
Other challenges that impact the natural gas pipeline system as a whole, such as those listed below, fall outside the scope of this report and the recommended research goals and areas in Chapters 2 and 3:
- The hydrogen concentration in a particular pipeline could vary substantially along the length of the pipeline because hydrogen would be produced on an intermittent basis and at certain locations only.
- To transport the same amount of energy as natural gas, the flow volume of hydrogen must be three times the flow volume of natural gas.
- Pipeline materials will need to be validated and perhaps replaced to avoid hydrogen embrittlement that could lead to pipeline ruptures.
Gas turbines fueled by natural gas are the cleanest source of electrical power generated by fossil fuels. For example, they produce less than half of the CO2 that is produced by a coal-fired power plant with the same power-generating capacity.
There are two options for reducing or eliminating net CO2 emissions from gas turbines. Improving gas turbine efficiency will directly reduce the amount of CO2 produced by gas turbines powered by hydrocarbon fuels. Replacing hydrocarbon fuels with hydrogen or other renewable fuels, in part or in whole, will correspondingly reduce CO2 emissions. Both of these options have already been addressed above in the discussions of efficiency and fuel flexibility. In any case, the ultimate goal is to reduce total systemic emissions of CO2. Thus, while replacing hydrocarbon fuels with renewable fuels would reduce CO2 emissions from gas turbine exhaust, the net benefit would vary depending on how much CO2 is released in the process of generating the renewable fuel.
Compatibility with Renewable Energy Sources and the Future Electrical Grid
Renewable energy sources are playing an ever-increasing role in current and future energy networks. The amount of power generated from renewable sources is increasing rapidly, and gas turbines will play a vital role in complementing renewable energy sources.27 This will include both generating electricity when the power from renewable energy sources is not available and using renewable fuels. Although power generation gas turbines compete with renewable energy sources, all sources of ground power will increasingly be integrated within the
27 This parameter does not apply to aviation.
energy infrastructure and complement each other. Gas turbines used for power generation will need to ramp up faster and overfire28 more frequently. In addition, modes of operation that have not been considered in the past will be increasingly important. For example, some gas turbines will likely be required to operate in hybrid power plants, where they will need to work in a symbiotic manner with solar power generation and fuel cells. The growing presence of solar and wind power also means that the electrical grid is becoming more distributed, with a larger number of generating stations with a larger range of generating capacities. Gas turbines that are not integrated as part of the growing infrastructure of renewable power will face the risk of becoming obsolete.
As noted earlier, the demand for electricity is increasing and the nature of that demand is changing. When intermittent renewable energy sources are producing more energy than needed, alternatives are needed to store such excess energy. One form of storage is the production of hydrogen through electrolysis or other methods. The produced hydrogen can then be introduced into the existing natural gas pipeline network for further storage, distribution, and use. All existing and future pipelines, including their compressor stations, would be affected by introducing hydrogen into the transport media.
Gas turbines for power generation are able to start up much more rapidly than alternatives such as steam turbines, which have much thicker metal casings and rotors. Even a large, heavy-duty industrial gas turbine rated at several hundred megawatts can reach full load in about 20 minutes from standstill. Aeroderivatives (i.e., power generation gas turbines that are derivatives of aviation gas turbines) such as the General Electric (GE) LMS100 can achieve full power in about 10 minutes. Another possibility is pairing a small aeroderivative such as the GE LM6000 with a lithium-ion battery. In this arrangement, instant power is available on demand from standstill because while the gas turbine rolls to full speed, synchronization, and full load, the power demand is instantaneously met by the battery.
Aggressive Goals for Improving Gas Turbine Performance
The committee generated a list of candidate goals for improving gas turbines for each application. These goals were qualitatively evaluated using the selection criteria above, and up to five high-priority goals were selected for each application. Achieving the selected goals would lead to substantial improvements in gas turbine performance (selection criterion 1), and these goals would be aggressive (selection criterion 2). In some cases, they would also support the achievement of other goals (selection criterion 3). The selected goals are discussed in Chapter 2.
Selection Criteria for the Research Areas and Topics
This report is not intended to provide comprehensive research plans for accomplishing the selected goals. Rather, the report is intended to highlight the most important research areas that should be funded to lead the way in pursuit of these goals. Referring again to relevant aspects of the statement of task and relying again on the expertise and experience of the committee members, the committee selected the criteria below to use in selecting research areas and the research topics they encompass.29
The benefit selection criterion was used to assess the following:
- The extent to which a proposed research topic could accelerate current efforts or establish new efforts to address issues or uncertainties that are essential to achieving relevant goals and for which there is a particularly large gap between the currently projected state of the art and the state of the art that is needed to achieve relevant goals.
28 Overfiring in this context means operating the gas turbine at a temperature higher than the designed combustion firing temperature.
29 The criterion that appears first in this list (i.e., Benefit) is named differently from the criterion that appears first in the list for prioritizing the goals (i.e., Performance) because the two criteria are assessed using different factors. The second and third criteria in each list (i.e., Technical Risk and Breadth of Application) are more closely linked, and so they are identically named.
- The extent to which a proposed research topic could advance the state of the art beyond that which is currently projected based on a continuation of ongoing research.
- The state of development that a reasonably funded research program could achieve by 2030.
The technical risk selection criterion was used to assess the extent to which a proposed research topic would achieve a balance between (1) offering a high payoff (and high risk) and (2) the potential to make substantial progress between now and 2030.
Breadth of Application
The breadth of application selection criterion was used to assess the extent to which a proposed research topic would support the accomplishment of multiple research goals.
High-Priority Research Areas and Topics
The committee generated a list of candidate research areas that would contribute to achieving the gas turbine goals. However, the committee recognized that not all research topics associated with a particular research area would have equal levels of benefit, technical risk, and breadth of application. To ensure that the specific research topics recommended by this report would each be of high priority, the committee assessed the priority of candidate research topics individually using the above criteria. Ultimately, 10 high-priority research areas were identified, and up to three high-priority research topics were identified for each area. The selected research areas and topics are discussed in Chapter 3.