Despite being small relative to other transportation methods, the aviation sector is a significant contributor in the realm of the global energy crisis. The number of airline passengers is steadily increasing each year, and relative to the industry’s growth, the amount of fossil fuels consumed, greenhouse gases emitted, and jet fuel prices are going up. Despite improvements in less polluting engines and other modernizations, air travel’s growth has still caused a net increase in pollution from aviation. Looking to the future, the industry is predicted to continue expanding as commercial flight becomes increasingly common, and alternative fuels certainly must and will be implemented to decrease harmful emissions and fossil fuel use from aviation. However, with the United States being a major player in the aviation world, and with the new presidential administration that clearly does not prioritize the battle against climate change, the future of aircraft and fuel development is somewhat uncertain.
Introduction and Background
The biggest energy-related problem in commercial flight is inefficient, greenhouse gas emitting fuel. Previously, the world was mostly concerned with ground vehicles and developing innovative technology like energy efficient electric cars and hybrid vehicles. Now, in our globalizing world, we must turn our attention to the rising star of long-distance travel: flight. International travel has become increasingly common—and the aviation sector dominates the market for long-range transport. With annual passenger counts steadily climbing since 2009, last year alone nearly 4 billion passengers flew commercially, a number that 2018’s passenger count has already surpassed (Future Aircraft Concepts). The industry is booming and is only projected to continue growing. The issue then, is whether or not fuel efficiency and aircraft designs are evolving at the same rate.
According to the International Energy Association (IEA), global energy consumption is projected to increase by 36 percent from 2008 to 2035. Fossil fuels are predicted to remain the mainstay of fuel consumption (Air Transport and Energy Efficiency). Improving aircraft’s energy efficiency relative to this increase will take a hefty effort as aviation fuel already accounts for 12 percent of world fossil fuel use. In an effort to tackle climate change and minimize emissions and fossil fuel use, alternative aircraft designs and fuels are being researched.
With international flight becoming more common and climate change encroaching from GHG emissions, the worlds’ agencies have sprung to action. The Advisory Council for Aeronautical Research in Europe (ACARE) has set a vision to reduce CO2 emissions from aircraft by 75 percent, NOx by 90 percent, and perceived noise pollution by 65 percent (ACARE). This vision has even been further amended to include efficiency measures surrounding aircraft taxing and most importantly, efficient fuels.
As for actual legislative work, no real regulation for aircraft CO2 emissions existed until recently. In 2016, the U.S. Environmental Protection Agency updated parts of the Clean Air Act, a federal law that has been in place to maintain air quality since 1970 (EPA). This update adopted old emission standards for transportation by an agency of the United Nations called the International Civil Aviation Organization (ICAO), but even ICAO’s regulations did not contain standards specifically for aircraft CO2 emissions.
Thankfully, in March of 2017, ICAO’s council adopted new standards to reduce GHG emission impacts of subsonic aircraft (which includes most commercial planes), producing the world’s first global standard governing CO2 for any transportation sector (ICAO.int). These changes will apply to aircraft designs starting in 2020, and all existing designs must follow regulations by 2028. Regulations like this are the excellent first steps in the right direction to reducing GHG emissions from aviation.
Jet-A & Alternatives
Jet-A is part of the aviation kerosene fuel family and is the most widely-used jet fuel in the world. At only 0.46 USD per kilogram, kerosene does have the advantage of being a dirt-cheap fuel source (NYSERDA). However, it is derived from crude oil, and, like most fossil fuels, has a negative impact on the environment.
As it is burned, kerosene emits three major greenhouse gases: water vapor, nitrogen oxide, and carbon dioxide. Its carbon emissions are particularly concerning. As of 2016, aircraft produce 2 percent of global carbon dioxide emissions. According to one study, it is forecasted that transportation-related CO2 emissions will represent 23 percent of world emissions by 2050 if no improvements are made (Future Aircraft Concepts).
In this race against time, many alternative fuel sources for aviation have been investigated. Three main sources of energy are under developmental research: solar, fuel-cell, and biofuel. Solar powered aircraft is an important option, is certainly one of the most efficient models, and has seen success in small aircraft. However, solar technology has a long way to advance before solar is viable for long distance flights with commercial aircraft. Fuel cell technology, which converts chemical energy into electricity, has likewise only been applied to short-distance aircraft. Lastly, biofuels are another alternative fuel type that has been considered. While it is much “cleaner” than regular Jet-A type fuels, it is comparatively much pricier. Biofuels must also be mixed with kerosene, so it is technically not a “new” fuel type. All three of these energy sources may be viable in the future, but until technology advances to meet the complexity of using these fuel types, more feasible alternatives must be considered instead.
One attainable method that has risen to the forefront of efficient aviation energy in recent years is the More Electric Aircraft (MEA) system. While it is technically not a fuel type, it attacks the root of the problem—that is, how efficiently aircraft use kerosene fuel-- rather than the symptoms. MEA, which has been under development since World War II, aims to replace hydraulics and mechanical systems with electric ones (“Electric Aircraft the Future of Aviation?”). These MEA systems also have already been employed in commercial flight—most notably by All Nippon Airways starting in 2011 with the Boeing 787-8. For comparison, for a medium-long range flight, the similarly-sized but non-MEA Boeing 767 consumed 2.74 L/100 km per seat, while the 787-8 only consumed 2.26 L/100 km per seat. The Boeing 787-8 also uses 20% less fuel and is reduces sound pollution slightly as it is ~3 decibels quieter than the 767 during departure and landing. Other benefits of MEA systems include better system reliability and easier maintenance, reduced installation costs, and reduction in aircraft weight. Overall, MEAs are more efficient than traditional aircraft, and will hopefully phase out traditional aircraft in the coming years.
In regard to actual alternative fuels, the most reasonable resource being researched currently is liquid hydrogen (LH2). Hydrogen is the most abundant resource in the universe and is easily accessible through gasification, electrolysis, or most commonly, natural gas reforming. LH2 specifically has been researched since the 18th century when it was typically used to inflate balloons, and eventually even used to fly a Zepplin in the 19th century (“Hydrogen Aircraft: the future of air transport”). It is considered to be the best alternate fuel because it contains over three times higher energy per weight than kerosene fuel and is projected to be 22 percent more efficient for long range flights (“Hydrogen powered aircraft”). Hydrogen fuel is carbonless, and emits relatively low levels of NOx, making it a prime candidate for alternate fuel.
However, LH2 has its tradeoffs-- the biggest challenge being proper storage. LH2 must be stored in insulated storage containers that regulate temperature to avoid boil-offs. The tanks used for liquid hydrogen fuels are typically an awkward cylindrical shape and are larger than those of a traditional aircraft, requiring them to be configured differently. The size of these tanks means hydrogen fuel is not a possible source for short-to-medium distance flights but is not much of a problem for long distance travel. This is because the larger the aircraft, the larger the ratio of plane-to-storage tank becomes. Further, one study on flights using hydrogen fuel reported a lighter gross weight upon take-off for long range transportation and improved life cycle of the aircraft’s engine, so the pros balance out the cons (“Long range transport aircraft using hydrogen fuel”). As is a trend in most energy efficient alternatives, another downfall LH2 suffers from is its price. At 1.2 USD/kg, LH2 is three times the cost of kerosene jet fuel. This does not negate the fact that liquid hydrogen fuel contributes far less the GHG crisis, and for that reason is still the best alternate fuel all things considered.
One final idea to consider is that MEA and LH2 fuels can be combined to amplify their respective benefits. A hybrid aircraft would negate hydrogen fuel’s issue of being unusable in short-to-medium range flights. Also, maintaining MEA’s are much easier without having to locate problems within a hydraulic system. Liquid hydrogen fuel fires are also far more manageable than kerosene fuel fires. The MEA’s electrical system coupled with the increased safety of LH2 would make a hybrid aircraft a safer and more reliable option.
The only issue, which is a reoccurring theme in improving energy efficiency, is waiting for the technology to catch up. Implementing a liquid hydrogen fuel system in a MEA is a complex process that will hopefully be possible in the coming years. In the next decade or so, hopefully the world will see the rise of carbonless, reliable, energy efficient aircraft.
The aviation industry is still becoming more and more accessible—no other long-distance means of transportation comes close to the speed and affordability of flying internationally. Technology development will also continue making flight more accessible for everyone. Moreover, the industry provides 15.5 million direct jobs, 46.4 million indirect jobs, and $1.5 trillion of gross domestic product. Also considering the impact of global tourism, air transport contributes $5.7 trillion in GDP (Aviation Benefits). It is logical then, to predict that aviation will continue being profitable ten years into the future. However, with the current political climate which is fraught with controversy surrounding energy efficiency, it is more difficult to look beyond the next decade or so .
In the United States, Trump’s presidential administration attempted to freeze the Obama-era fuel efficiency standards for cars and trucks until 2026 (USA Today). This regulatory rollback attack even proposed to revoke certain state’s rights to set their own regulatory standards. President Trump has made his stance on climate change and energy efficiency clear on numerous other occasions which include but are not limited to: withdrawing from the Paris Agreement on climate change, rolling back GHG restrictions for coal plants, opening the Arctic to offshore drilling, and said during an interview, “As to whether or not it’s man-made and whether or not the effects that you’re talking about are there, I don’t see it.” (National Geographic).
These views from the president can reasonably be extended to energy efficiency in the commercial aviation sector. His administration clearly does not intend to prioritize efficiency research, and the most he has done in terms of aviation legislation is regulate airline seating sizes (USA Today). President Trump and the U.S. are not the only players in aviation and climate change, however.
Looking at the industry’s future from a global perspective, other organizations are still pushing for improvement. The European Union has set standards for its preexisting cap-and-trade system that intends to internalize carbon cost and recirculate the funds to other sectors. ACARE is still pursuing its vision for 2020, and ICAO’s recent regulations on CO2 emissions were a big step towards reducing GHG emissions.
Regardless of how clean the energy is, technology will continue improving relative to the demand for it. The creation of the Boeing 767 aircraft and Boeing 787-8 were only two decades apart, and the Boeing 767’s components, from the engine to its aerodynamic design improved massively. And while the current political state with regards to energy efficiency may be somewhat discouraging, many global organizations are working to fight climate change, and either way aviation efficiency will be forced to adapt and improve with the passage of time. Current forecasts predict that the industry will continue growing at breakneck pace, and the rate of technology development will follow, which will open opportunities for cleaner, more efficient hybrid aircraft.