This moment of celebration feels like a good time to unveil this chapter’s
big summary diagram, figure 20.23, which shows the energy requirements
of all the forms of passenger-transport we have discussed and a
couple that are still to come.

OK, the race is over, and I’ve announced two winners – public transport,
and electric vehicles. But are there any other options crossing the
finishing line? We have yet to hear about the compressed-air-powered car
and the hydrogen car. If either of these turns out to be better than electric
car, it won’t affect the long-term picture very much: whichever of these
three technologies we went for, the vehicles would be charged up using
energy generated from a “green” source.

Compressed-air cars

Air-powered vehicles are not a new idea. Hundreds of trams powered by
compressed air and hot water plied the streets of Nantes and Paris from
1879 to 1911. Figure 20.24 shows a German pneumatic locomotive from
1958. I think that in terms of energy efficiency the compressed-air tech-
nique for storing energy isn’t as good as electric batteries. The problem is
that compressing the air generates heat that’s unlikely to be used efficiently;
and expanding the air generates cold, another by-product that is unlikely
to be used efficiently. But compressed air may be a superior technology to
electric batteries in other ways. For example, air can be compressed thous-
ands of times and doesn’t wear out! It’s interesting to note, however, that
the first product sold by the Aircar company is actually an electric scooter.
[www.theaircar.com/acf]

There’s talk of Tata Motors in India manufacturing air-cars, but it’s
hard to be sure whether the compressed-air vehicle is going to see a revival,
because no-one has published the specifications of any modern prototypes.
Here’s the fundamental limitation: the energy-density of compressed-air
energy-stores is only about 11–28 Wh per kg, which is similar to lead-acid
batteries, and roughly five times smaller than lithium-ion batteries. (See
figure 26.13, p199, for details of other storage technologies.) So the range of
a compressed-air car will only ever be as good as the range of the earliest
electric cars. Compressed-air storage systems do have three advantages
over batteries: longer life, cheaper construction, and fewer nasty chemicals.

Hydrogen cars – blimp your ride

I think hydrogen is a hyped-up bandwagon. I’ll be delighted to be proved
wrong, but I don’t see how hydrogen is going to help us with our energy
problems. Hydrogen is not a miraculous source of energy; it’s just an en-
ergy carrier, like a rechargeable battery. And it is a rather inefficient energy
carrier, with a whole bunch of practical defects.

The “hydrogen economy” received support from Nature magazine in

Figure 20.24. Top: A compressed-air tram taking on air and steam in Nantes. Powering the trams of Nantes used 4.4 kg of coal (36 kWh) per vehicle-km, or 115 kWh per 100 p-km, if the trams were full. [5qhvcb]
Bottom: A compressed-air locomotive; weight 9.2 t, pressure 175 bar, power 26 kW; photo courtesy of Rüdiger Fach, Rolf-Dieter Reichert, and Frankfurter Feldbahnmuseum.
Figure 20.25. The Hummer H2H: embracing the green revolution, the American way. Photo courtesy of General Motors.