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by Tony Hays
In the United States, about 2.5% of greenhouse gases are produced by air transportation. Although business travelers account for only about 12% of airline passengers, they contribute a disproportionate amount to greenhouse gases, because they typically sit in business class, which has much heavier seats and more space. For example, a round-trip, economy-class trip from LAX to JFK generates about 0.57 metric tons of CO2, whereas the same trip in business class generates 1.1 tons. Making the trip by car would be even worse. Driving a car that gets 25 miles per gallon would generate about 2 tons. By comparison, if your monthly household electricity bill is about $100/month, then solar panels on your roof would save about 2.7 tons/year. If you often fly on business and want to reduce your carbon footprint, the answer is to fly less. In Europe, “flight shaming” has had success in reducing air travel. This works best if there is a viable alternative such as high-speed rail, but that is only available in Europe, China, and Japan.
Looking to the future, batteries are a good solution for cars and trucks, but not for nearly all commercial aircraft operations. For aircraft, light weight is important, and for aircraft, specific energy (stored energy per unit weight) is critically important. For a long-range jet airliner, about half the weight of the airplane at takeoff is fuel. Compared with lithium-ion batteries, jet fuel has about 40 times the specific energy, and hydrogen has over 130 times the specific energy. Hoped-for improvements in both battery-specific energy and recharging rates have not materialized, at least not yet, so hydrogen is currently the preferred energy source for the future.
Boeing believes that commercial aircraft could be operational with zero net carbon emissions by 2030 using fuel produced from plant and animal sources plus other synthetic hydrocarbons, and that hydrogen-powered aircraft will take too long to develop. Airbus is confident that hydrogen-powered aircraft can be in operation by 2035, but there are numerous challenges. In an aircraft, hydrogen must be stored as a liquid at about -253o C in an insulated tank. The tank weighs about twice as much as the hydrogen (which negates some of the specific energy benefits), and liquid hydrogen occupies a much larger volume than jet fuel. We don’t have a hydrogen-based economy, so it’s expensive to distribute and store it. The decreasing cost of electricity from solar and wind sources makes hydrogen generated locally by electrolysis more economical, so this problem may eventually solve itself. Both these approaches must be used to achieve the industry goal of reducing carbon emissions to half the 2005 value by 2050.
Tony Hays has worked in aircraft design and operations in the UK, Canada, and the U.S. since 1962, and he currently teaches classes in aircraft design, mostly in Asia. He has a special interest in the design of aircraft and their propulsion systems to reduce carbon emissions. He is an active member of the South Orange County chapter of the Citizens’ Climate Education/Lobby.