Last February, I attended The Energy Conference 2013: Energy At The Crossroads, which was held by NERI. Over the next month or two, I’m going to talk about some of the work presented at the conference – starting today with a presentation by Robert Vale from Vic University. You can find his Powerpoint slides here, along with the slides used by most of the other speakers.
The aircraft vs. the dove pigeon
Robert’s presentation, poetically titled “Oh for the wings of a dove”: the future of flight in a resource constrained world, looked at the history of passenger aviation over the last century or so, and the likely future. He started off with an interesting analogy: how does the efficiency of modern planes compare with the efficiency of that well-known flying rat, the domestic pigeon?
It turns out that pigeons have an energy efficiency of about 7.5 kJ/kg/km – that is, it takes 7.5 kilojoules to move one kilogram of bird over a distance of one kilometre. Robert assumes that the average weight of a passenger plus baggage is 90 kg. As such, using the pigeon’s energy efficiency, 0.7 MJ (megajoules) is needed to move a single passenger plus baggage for 1 km.
So, how does that compare with modern aircraft? Modern jets average around 1.5 MJ per “available seat kilometre” – ASKs being a common measure in the aviation industry, and fairly self-explanatory. This is more than twice the energy used by the pigeon! Note that planes aren’t usually full, so actual energy use per passenger would be slightly higher again.
The latest generation aircraft, like the Airbus A380 and Boeing 787 “Dreamliner”, represent another step forward in efficiency – by my rough calculations, about 1.2 MJ per available seat kilometre. But it’ll be some years before these kinds of aircraft are dominant globally.
Aircraft fuel efficiency over time
Robert makes the point that “airliners now are no more efficient, in terms of fuel used per passenger-km travelled, than airliners in the 1950s”. This essentially comes down to the shift from propeller-driven aircraft to jet aircraft – and the higher speeds that have gone along with this shift. The first jet airliners used horrendous amounts of fuel, but they were much faster than propeller aircraft.
Of course, passengers have benefited massively from the switch from propellers to jets. The higher speeds have made air travel much more attractive, and made long-haul international flights much more viable. So no one’s saying that today’s planes aren’t better than those of the ’50s. But it’s interesting that it’s taken jets 50 years to to match propeller aircraft for fuel efficiency. (and, beginning with the A380 and Boeing 787, to beat propeller aircraft for fuel efficiency).
Robert also compares the energy efficiency of the ill-fated Hindenburg airship: at 3.44 MJ/ passenger kilometre, it’s much worse than the other aircraft discussed here – its figure includes hydrogen gas which had to be vented during flight. He sums up with the following slide:
Note: despite the figure given in the slide above, the ‘average’ fuel consumption for the Boeing 747-400 is 1.4 MJ/ passenger kilometre, as given later in Robert’s presentation
This slide raises another relevant issue, which is that much of the weight carried by aircraft (or road vehicles, for that matter) is essentially dead weight – including the aircraft itself. The ultimate goal of the flight is to move passengers and baggage, and nothing else really matters. This is why airlines keep a close eye on their seat kilometres, and passenger kilometres, and of course their loadings. Vehicles can be made more efficient by upping the percentage of ‘useful’ weight, and this is an important consideration for aircraft and car engineers alike.