# 5   Planes

Imagine that you make one intercontinental trip per year by plane. How
much energy does that cost?

A Boeing 747-400 with 240 000 litres of fuel carries 416 passengers about
8 800 miles (14 200 km). And fuel’s calorific value is 10 kWh per litre. (We
learned that in Chapter 3.) So the energy cost of one full-distance roundtrip
on such a plane, if divided equally among the passengers, is

 2 × 240 000 litre × 10 kWh/litre ≈ 12 000 kWh per passenger 416 passengers

If you make one such trip per year, then your average energy consumption
per day is

 12 000 kWh ≈ 33 kWh/day 365 days

14 200 km is a little further than London to Cape Town (10 000 km) and
London to Los Angeles (9000km), so I think we’ve slightly overestimated
the distance of a typical long-range intercontinental trip; but we’ve also
overestimated the fullness of the plane, and the energy cost per person is
more if the plane’s not full. Scaling down by 10 000 km/14 200 km to get an
estimate for Cape Town, then up again by 100/80 to allow for the plane’s
being 80% full, we arrive at 29 kWh per day. For ease of memorization, I’ll
round this up to 30 kWh per day.

Let’s make clear what this means. Flying once per year has an energy
cost slightly bigger than leaving a 1 kW electric fire on, non-stop, 24 hours
a day, all year.

Just as Chapter 3, in which we estimated consumption by cars, was
accompanied by Chapter A, offering a model of where the energy goes in
cars, this chapter’s technical partner (Chapter C, p269), discusses where
the energy goes in planes. Chapter C allows us to answer questions such
as “would air travel consume significantly less energy if we travelled in
slower planes?” The answer is no: in contrast to wheeled vehicles, which
can get more efficient the slower they go, planes are already almost as
energy-efficient as they could possibly be. Planes unavoidably have to use
energy for two reasons: they have to throw air down in order to stay up,
and they need energy to overcome air resistance. No redesign of a plane
is going to radically improve its efficiency. A 10% improvement? Yes,
possible. A doubling of efficiency? I’d eat my complimentary socks.

## Queries

#### Aren’t turboprop aircraft far more energy-efficient?

No. The “comfortably greener” Bombardier Q400 NextGen, “the most
technologically advanced turboprop in the world,” according to its manu-

Figure 5.1. Taking one intercontinental trip per year uses about 30 kWh per day.
Figure 5.2. Bombardier Q400 NextGen www.q400.com
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