Flying greener: sustainable aviation

As I type this I’m listening to my colleague Ilan Kroo, professor in Aerodynamics at Stanford University and a world renowned airplane designer. The topic of his lecture is Sustainable Aviation: Future Air Transportation and the Environment.
A few blogs ago I gave some current numbers on the efficiency of air travel vs car travel. A Boeing 737 with 75% occupancy gives about 80 miles/gallon for each passenger. This compares favorably to a car, but then, we would not drive across the Atlantic.
What is encouraging is that fuel efficiency has dramatically improved over the years and is expected to improve further. Current jets, such as the 767, use around 70% less fuel compared to the early jets, such as the 707-320. But, perhaps that is because these early jets were so bad! The next generation (787) will give about 20% improvement over the current (second) generation planes.
Aviation accounts for about a seventh of transport in terms of carbon emissions. With transport making up around a seventh of total green house gas emissions, this means that total carbon emissions from airtravel is about 3% of total. Sounds OK, but the effects of emissions at altitude are a fair bit higher. Looking at carbon calculators, one round trip San Francisco to Paris is about 15% of the emissions of an average US household per year (aye, this hurts when I think about my own air travel….).
What is troubling is that air travel is growing very fast. Growth is about 5-6% per year worldwide in passenger and cargo air travel, which could mean a three-fold increase in the next decades. So, it’s much worth while looking at ways to reduce emissions.

At the government level, the US, Europe (Clean Skies Initiative) and Canada are starting new initiatives. In Stanford’s AA department a new sustainable aviation program has also been started. It includes work on new concepts and technologies, as well as safely increasing air transportation system capacity, and monitoring of its environmental impact.

One example of an environmental impact, apart from green house gas emissions, is contrail formation in super-saturated and cold air. This can often be avoided, quite simply, by changing the cruise altitude by a few thousand feet. Contrails are of interest because, like the carbon dioxide emitted, they lead to a positive radiative forcing. This in itself is not a great measure of climate impact. In contrast to carbon dioxide or other particles, contrails do not stick around for hundreds of years. But, aircraft generally cruise in a narrow altitude band where control persistence conditions are most likely. So, although accurate estimation of the impact on global climate of contrails is rather challening and uncertain, it is probable that it has an impact.

Taking into account environmental impacts, the AA department has run optimization models. It is possible to adapt designs to minimize climate impacts, but then generally one pays penalties in terms of cost or travel time or both, for example. What is very encouraging is that the preliminary studies indicate that a relatively small reduction in speed with a 1-2% increase in costs can reduce climate impact by 50-70% (that is, if contrails really are important contributors to global warming). Uncertainties are however still high, but it’s really worth looking at this in more detail.

In terms of new technologies, Ilan offered various ideas. One is to look at alternative fuels. But hydrogen, for example, is not great as the energy density per volume is much lower than kerosene. Ethanol is also not ideal in terms of energy density. Gas-to-liquid synthetic jet fuels may be an option.
Another idea is to use aerial refueling so that airplanes do not have to carry all the fuel for the full flight. In other words, design airplanes to design, say, 3000 miles rather than 9000 miles and refuel along the way. The military has great expertise in aerial refueling and this could be utilized.
A really interesting idea is to fly airplanes in formation (like geese) which can lead to potentially large drag reductions.
The new 787 has more efficient engines which leads to a 20% improvement in fuel efficiency. New composite materials can lead to weight reductions, which in turn will lead to fuel efficiencies. Larger span wings supported by struts, gust load control and airplanes with tail in front, or blended wing bodies, are all ideas that can be and are being considered.

So, in summary there are large potential savings possible with optimized designs for low noise and low emissions with limited cost and/or utility changes. New technologies could improve things (much) more. All very positive.

This talk will be available on Stanford I-tunes shortly and I will include the link when it is available.

 

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