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Introduction

The primary human activities that release carbon dioxide (CO₂) into the atmosphere are the combustion of fossil fuels (coal, natural gas, and oil) to generate electricity, the provision of energy for transportation, and as a consequence of some industrial processes. This report focuses on reducing life-cycle CO₂ emissions from commercial aviation.

Human activities also produce or release additional greenhouse gases other than CO₂, most importantly methane, nitrous oxide, and fluorinated gases such as hydrofluorocarbons and perfluorocarbons. The physical mechanisms by which greenhouse gases cause climate change are well understood.¹ Since 1750, CO₂ has contributed more to global warming than any other greenhouse gas, and there is growing recognition of the need for aviation to reduce its CO₂ emissions.² One manifestation of this is the recent agreement at the International Civil Aviation Organization, a United Nations standards organization, on a fuel economy standard for new aircraft. It will apply to new aircraft designs starting in 2020 and to in-production types in 2023. As has been the case for standards on aircraft emissions and noise, it is expected that national governments will incorporate the new fuel economy standard into their national rules. If the CO₂ rules follow the precedent of other aviation emissions regulations, they will be periodically reviewed and tightened, necessitating an ongoing investment to reduce the net CO₂ emitted by aircraft.

CARBON DIOXIDE EMISSIONS FROM COMMERCIAL AVIATION

This report is focused on propulsion and energy technologies that have the potential to start reducing global emissions from commercial aviation within the next 20-30 years.³ Aviation CO₂ emissions presently make up

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¹ Intergovernmental Panel on Climate Change, 2007, Climate Change 2007: The Physical Science Basis, Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (2007). https://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4_wg1_full_report.pdf (accessed June 29, 2016).

² Forster, P., V. Ramaswamy, P. Artaxo, T. Berntsen, R. Betts, D.W. Fahey, J. Haywood, J. Lean, D.C. Lowe, G. Myhre, J. Nganga, R. Prinn, G. Raga, M. Schulz, and R. Van Dorland, 2007: Changes in Atmospheric Constituents and in Radiative Forcing. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M.Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom, and New York, N.Y., p. 135.

³ Both propulsion and energy technologies are of interest to cover the entire process from energy storage (e.g., using jet fuel, alternative fuels, or batteries) to the generation of propulsive power (e.g., using gas turbines, generators, and/or motors).

approximately 2.0 to 2.5 percent of total global annual CO₂ emissions.⁴ In the United States, the aviation sector contributes about 11 percent of transportation greenhouse gases, with commercial aviation contributing 9 percent of total transportation emissions.⁵

As commercial aviation continues to grow in terms of revenue-passenger miles and cargo ton miles, CO₂ emissions are expected to increase. To reduce the contribution of aviation to climate change, it is essential to improve the effectiveness of ongoing efforts to reduce emissions and initiate research into new approaches. Although aviation CO₂ emissions are a small part of total CO₂ emissions, action to reduce them is urgent for the reasons stated above and because it takes new technology a long time to propagate into and through the aviation fleet.

This report classifies commercial aircraft as indicated below based on typical passenger capacity (all figures approximate):

• General aviation: fewer than 6 passengers
• Commuter: fewer than 20 passengers
• Regional: 30-100 passengers
• Single-aisle: 100-200 passengers
• Twin-aisle: more than 200 passengers

In general, the range of each class of aircraft is greater than that of the class that precedes it, but there are

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⁴ D.S. Lee, D.W. Fahey, P.M. Forster, PJ. Newton, R.C. Wit, L.L. Lim, B. Owen, R. Sausen, 2009, Aviation and global climate change in the 21st century, Atmospheric Environment 43:3520-3537.

⁵ EPA, 2015, EPA Inventory of US Greenhouse Gas Emissions, EPA-430-R-15-004, Washington, D.C.

some exceptions given the wide array of aircraft designs within each class. This report focuses on large commercial aircraft—that is, single-aisle and twin-aisle subsonic transport aircraft—because they are the source of the bulk of aviation emissions (see Figure 1.1). Smaller aircraft also emit CO₂; however, they make only a minor contribution to global emissions, and in any case many technologies that reduce CO₂ emissions for larger aircraft are also applicable to smaller jet aircraft.

SYSTEMIC CHALLENGES TO LOWERING GLOBAL EMISSIONS FROM COMMERCIAL AVIATION

Each year, about 20,000 commercial aircraft carry 3 billion passengers over 3 trillion passenger-miles, connecting 35,000 city pairs with 30 million aircraft movements. These aircraft also carry more than $6 trillion in air cargo, about 35 percent of world trade by value. Commercial aircraft now consume more than 70 billion gallons of jet fuel per year. Thus, any discussion of reducing the carbon emission from commercial aircraft will need to be applicable to and effective at this scale.

CO₂ emissions from a commercial aircraft can be reduced in the following ways:

• Reduce the energy required to fly the aircraft by reducing its weight and/or drag.
• Improve the efficiency with which the energy is converted from fuel into thrust—in other words, improve the propulsion system efficiency.
• Reduce the carbon intensity of the energy required—in other words, reduce the net amount of carbon that is emitted into the atmosphere for each joule of energy that is generated. This includes total life-cycle carbon emissions during production of the fuel. For electric aircraft, this would also include carbon emissions produced by the source of electricity, either on the ground (for battery-powered aircraft) or on the aircraft (for generator-equipped electric aircraft).

Making additional progress in any of these areas is challenging, but there are also many promising approaches, as discussed in more detail in the next section and in subsequent chapters. In addition to the technical, economic, and policy challenges identified in Chapters 2-5 for specific approaches, there are also two systemic challenges that all approaches need to overcome.

Economic Competitiveness

Commercial aviation is a highly competitive industry for which reduction in fuel burn (and, thus, CO₂) is a major technology driver. Cost considerations can be a challenge and have to be taken into account as new systems are proposed for commercial development.

Individual aircraft can be in service for more than 30 years. Airlines and aircraft manufacturers are constantly seeking ways to increase efficiency, and in particular fuel burn efficiency. Decisions by airlines to invest in new aircraft—and in which kind—are driven by complex assessments involving many considerations, including near- and long-term projections of economic conditions; fuel costs; societal expectations regarding the environmental impact of aviation in terms of noise and emissions; the cost of retraining operational or maintenance personnel and of acquiring new facilities for maintenance and fuel distribution and other potential operational and capital costs; and national and international policies and regulations that impact aviation, and so on. These considerations are also important to aircraft and engine manufacturers as they try to anticipate the factors that will drive future purchase decisions by airlines.

Aircraft Systems Complexity and Integration

Commercial aircraft are composed of many distinct systems that are carefully integrated and regulated to maximize performance and safety. Disciplined system integration is required to introduce new technologies so that the improvement of one system does not adversely impact the performance of other systems or the performance of the aircraft as a whole.

Transitioning new aircraft propulsion technologies into an operational aircraft is a complex systems engineering task. For example, the motors, generators, and other electrical components of an electric aircraft propulsion system will generate heat that must be dissipated by a robust thermal management system, which may affect aircraft flight performance. Even after engineering solutions to such problems are developed, it usually takes a long time to build the confidence needed to introduce new technologies into commercial aviation. Because safety is essential to the commercial aviation industry’s success, new technologies and designs need to also go through extensive certification processes that can take up to a decade. In addition, airlines expect new aircraft to have at least the same level of operational reliability as the aircraft they are replacing.

REPORT ORGANIZATION AND PRIORITIZATION PROCESS

After considering various potential prioritization methodologies, the committee came up with the following two-step process for developing a national research agenda on propulsion and energy systems for reducing CO₂ emissions from commercial aircraft:⁶

High-Priority Approaches

In the first step, the committee examined several potential high-priority approaches to CO₂ reduction based on three considerations:

1. Improvement potential. This criterion considers the level of CO₂ reduction per passenger mile or stored power energy in the fuel that a given approach may help achieve.
2. Timeline. This criterion considers the nominal time for relevant technologies to substantially reduce global aviation emissions. To meet the 30-year time frame considered by this study, the new technology needs to have reached at least technology readiness level 6 (TRL 6)⁷ within 20 to 25 years from now.
3. Risk. This criterion considers the levels of technical and economic and policy risk associated with achieving projected improvements within the time frame of interest.

Next, the committee identified the four top-level approaches that are most likely to meet the goal of developing low-carbon propulsion and energy system technologies that could be introduced into service during the next 10 to 30 years. These approaches—and the rationales for investing in them—are as follows:

• Aircraft–propulsion integration research. Advances in the integration of aircraft and propulsion are needed to enable many aspects of low carbon aviation that are not achievable with the incorporation of discrete improvements into individual component technologies. Areas of interest include evolutionary configurations such as lower fan pressure ratio engines in nacelles on standard tube-and-wing aircraft as well as significant

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⁶ As described in the study statement of task (see Appendix A), the committee’s deliberations focused on research related to propulsion and energy systems. This report does not include recommendations concerning other avenues for reducing carbon emission, such as operational improvements, changes to airport ground equipment (e.g., electric airport shuttles), airframe improvements not related to advanced propulsion concepts, and nontechnology policy approaches such as the imposition of carbon taxes, the use of carbon offsets, or legislative limits on carbon emissions.

⁷ NASA uses technology readiness levels (TRLs) to track the maturity of a new technology under development. TRL 6 is achieved when a system or subsystem model or prototype has been verified in a relevant environment.

• departures from standard configurations including modified aircraft platforms, distributed propulsion concepts, and boundary layer ingestion configurations.

• Gas turbine engine research. Gas turbine engines have considerable room for improvement, with a potential to reach overall efficiencies perhaps 30 percent higher than the best engines in service today, with a concomitant reduction in CO₂ emissions. This magnitude of gain requires investment in a host of technologies to improve thermodynamic and propulsive efficiency of engines, with each discrete technology contributing only a few percent or less.
• Turboelectric propulsion research.⁸ Turboelectric propulsion systems are likely the only approach for developing electric propulsion systems for a single-aisle passenger aircraft that is feasible in the time frame considered by this study. System studies indicate that turboelectric propulsion systems, in concert with distributed propulsion and boundary layer ingestion, have the potential to ultimately reduce fuel burn up to 20 percent or more compared to the current state of the art for large commercial aircraft.
• Sustainable alternative jet fuels research.⁹Sustainable alternative jet fuels (SAJF) will be able to reduce life-cycle CO₂, emissions, and in some cases the reductions may be substantial. SAJF have the potential for immediate impact on lowering net global CO₂ emissions from commercial aviation because, as drop-in fuels, they are compatible with existing aircraft and infrastructure. Thus, their widespread use will not be limited by the rate at which new aircraft replace existing aircraft. The combustion of SAJF will likely also produce lesser amounts of other harmful emissions, such as oxides of sulfur and particulate matter, than the combustion of equivalent amounts of conventional jet fuel. SAJF are also compatible with and complementary to the three other high-priority approaches recommended in this report for reducing carbon emissions.

More information on each of the four approaches appears in Chapters 2, 3, 4, and 5, respectively.¹⁰

High-Priority Research Projects

As a second step, after identifying the high-priority approaches above, the committee examined potential research projects for each approach based on four considerations:

1. Breadth of applicability. This criterion considers the range of aircraft to which a particular improvement could be applied. For example, drop-in fuels can be used in all aircraft, by definition. In contrast, electrical propulsion systems are currently limited to small aircraft.
2. Ease of integration. This criterion considers how easily a particular improvement could be incorporated into an aircraft or the air transportation system. For example, fuels that are not drop-in would require changes to aircraft engines, aircraft architecture, and to the fuel manufacturing and distribution systems.
3. Technical and economic risk. This criterion considers the extent to which a research project could mitigate

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⁸ Turboelectric propulsion systems use gas turbines to drive electrical generators that power electric motors that drive propulsors (fans or propellers). A partial-turboelectric system is a promising variant of the full turboelectric system that uses electric propulsion to provide part of the propulsive power; the rest is provided by a turbofan driven by a gas turbine. In contrast, hybrid electric systems use high-capacity batteries to provide some or all of the propulsive power during one or more phases of flight, and all-electric systems rely solely on batteries for propulsive power. The term “electric propulsion” encompasses all of these concepts.

⁹ This report uses the term sustainable alternative jet fuels (SAJF) to characterize a family of drop-in fuels that are intended to lower the net life-cycle carbon emissions of commercial aviation. First and foremost, SAJF must meet current specifications for jet fuel, either on their own or when blended with conventional jet fuel. As such, SAJF (blended as necessary) are drop-in replacements for conventional jet fuel. SAJF are alternative in that they are produced primarily from nonpetroleum sources of hydrocarbons using a potentially broad range of biochemical and thermochemical conversion processes. To date, four pathways have been approved for producing alternative fuels that meet the specifications necessary to be considered jet fuel while others are pending. To be successful over the long term, alternate fuels must be sustainable both in terms of their ability to reduce net life-cycle carbon emissions relative to conventional jet fuel and in terms of environmental, societal, and economic factors. Not all alternative fuels will result in a net reduction in life cycle carbon depending, for example, on their source materials.

¹⁰ The approaches on aircraft and propulsion integration, gas turbine engines, and turboelectric propulsion focus on propulsion research. The SAJF approach focuses on energy.

4. the technical and economic risk and/or shorten the timeline for relevant CO₂ technology to become operational within the time frame of interest.

5. Improvement potential. This criterion considers the level of CO₂ reduction per passenger mile or stored power energy in the fuel that a technology may help achieve.

The high-priority research projects identified for each approach are detailed in Chapters 2-5 and summarized in Chapter 6, which also includes a list of all findings, the challenges, high-priority research projects, and recommendations.

Other Potential Approaches and Research Projects

Many potential approaches and technologies for reducing CO₂ emissions by means of advanced propulsion and energy systems are not included with the four high-priority approaches recommended here or the associated high-priority research projects. This does not mean that the committee is recommending that all research to support other approaches should be discontinued, nor does it imply that the committee believes it can predict with certainty how far the state of the art may advance in any of these areas over the next 30 years. Over the long term, only time will tell which breakthroughs in which technologies will revolutionize approaches for reducing CO₂ emissions for commercial aviation, and a broad-based program of basic research is more likely to make breakthroughs than one that is narrowly focused. On the other hand, the approaches and research projects detailed in this report are necessarily focused on a very particular goal: identifying the most promising propulsion and energy system technologies for reducing CO₂ emissions that could be introduced into service during the next 10 to 30 years.

Many approaches other than the four recommended as a high priority show some promise for reducing CO₂ emissions, but 30 years is not a particularly long time when it comes to developing and introducing new technologies into commercial aviation, and 10 years is almost no time at all. Even after new technologies are well understood, and even after they have been demonstrated in full-scale prototypes, it can take a decade or more to accomplish both of the following:

• Ensure that the components and systems that incorporate these technologies meet expectations with respect to reliability, safety, performance, economic return, and so on.
• Develop and validate certification standards to enable their use on commercial aircraft.

The timeline for inserting new technology into commercial aviation generally increases as the approaches and technologies differ more from those currently in use. For example, the design of any electric propulsion system will depart substantially from the design of current systems. One option would be to pursue a broad research program that supports a wide array of electric propulsion concepts. The committee concluded, however, that batteries with the power capacity and specific power¹¹ required by all-electric or hybrid-electric systems for aircraft at least as large as a regional jet are unlikely to be matured to the point that products satisfying FAA certification requirements can be developed within the 30-year time frame addressed by this report. The design of turboelectric electric propulsion systems, however, does not include high-power batteries, thereby eliminating a major technical risk. Accordingly, turboelectric systems are recommended as the highest-priority approach to achieving electric propulsion for large commercial aircraft. Of course, turboelectric systems still face substantial technology challenges associated with generators, inverters, power distribution, and so on. However, all of these components are also included in various other electric propulsion concepts, so advances in turboelectric technologies will also enhance the feasibility of other concepts.

It may seem counterintuitive to pass over all-electric and hybrid-electric propulsion concepts as high-priority approaches given that small general-aviation aircraft using these concepts are already flying. Indeed, small aircraft can be useful to conduct flight tests of new technology, and in some cases technologies developed for use in large

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¹¹ In this report, specific power and specific energy refer to power and energy per unit mass, respectively, and power density and energy density refer to power and energy per unit volume.

aircraft have later migrated to small aircraft. Even so, not all technologies that are suitable for general aviation aircraft are scalable to the large sizes, long flight distances, and high operational tempos that are key characteristics of large commercial aircraft. This is the case for all-electric and for hybrid-electric systems given the time frame of interest for this study. In addition, the committee is not aware of any system studies showing that hybrid systems, including variants such as series/parallel partial hybrid systems, would reduce CO₂ emissions more than turboelectric systems (including a partial turboelectric system), which are recommended as a high-priority approach.

Advances in superconducting materials could eventually result in high-power superconducting motors and generators suitable for use on aircraft. Superconducting components could greatly reduce the weight of a wide array of electric propulsion concepts. However, the committee does not anticipate that the technical and certification challenges to developing such devices will be overcome during the next 20 to 30 years.

Another approach for electric propulsion would be to use fuel cells to convert hydrocarbon or hydrogen to electricity. If fueled by hydrocarbons, the amount of CO₂ reduction becomes problematic. In any case, fuel cells are unlikely to be able to provide the power needed for propulsion of a regional jet or larger commercial aircraft in the foreseeable future. Advanced fuel cells will likely be able to replace the gas turbine auxiliary power units that aircraft use to produce electrical power when the main engines are shut down. If fueled by hydrogen, this application of fuel cells would reduce total CO₂ emissions from commercial aviation, but only by a relatively small amount. Therefore, even though fuel cell auxiliary power units should be considered as part of the process for reducing CO₂ emissions, the committee did not classify them as a high priority.

The national research agenda recommended by this report could lower the priority assigned to research programs that are developing high-power batteries, fuel cells, and superconducting motors for the purpose of incorporating them in the propulsion systems of large commercial aircraft. Even if that happens, however, a substantial national research investment in each of these technologies is certain to continue because of their potential to benefit a wide array of other applications. Aircraft-specific research in these areas may then take on a higher priority as the general state of the art advances.

Many different types of internal combustion engines have been used to power commercial aviation, and even more types of engines have been studied. This includes piston engines, hybrid compound engines (which combine a gas turbine with another internal combustion engine such as a diesel), and engines that use thermodynamic cycles other than the simple Brayton cycle used by conventional gas turbine engines. Gas turbines dominate the current fleet of commercial aviation aircraft because their inherent characteristics, combined with decades of research, development, and operational experience, have resulted in very high levels of safety, reliability, performance, and efficiency. In addition, ongoing investments in gas turbine research have produced a steady trend of increasing efficiency, lower fuel consumption, and reduced emissions per passenger mile. Accordingly, the committee concluded that continued research in gas turbines should be the highest priority approach for developing advanced internal combustion engines for commercial aviation. Some specific areas of research, such as acoustics and open rotors, could contribute to reducing CO₂ emissions from gas turbine engines, but the committee concluded that they are unlikely to reduce CO₂ emissions by commercial aviation as much as research in the areas that are identified as a high priority.

There are many options other than conventional jet fuel to use as sources of energy to drive aircraft propulsion systems. Potential alternatives include hydrogen, blending of ethanol and biodiesel with conventional jet fuel, nuclear power, and compressed or liquefied natural gas. All of these have been studied, and some are more feasible than others. Unlike all of these options, however, SAJF is well suited as a drop-in aviation fuel that can reduce CO₂ emissions throughout the existing fleet of commercial aircraft, and that is why SAJF is the committee’s highest-priority approach for energy systems.