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.

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.