Here are some other problems with hydrogen. Hydrogen is a less convenient
energy storage medium than most liquid fuels, because of its bulk,
whether stored as a high pressure gas or as a liquid (which requires a
temperature of -253 °C). Even at a pressure of 700 bar (which requires a
hefty pressure vessel) its energy density (energy per unit volume) is 22%
of gasoline’s. The cryogenic tank of the BMW Hydrogen 7 weighs 120 kg
and stores 8 kg of hydrogen. Furthermore, hydrogen gradually leaks out
of any practical container.E If you park your hydrogen car at the railway
station with a full tank and come back a week later, you should expect to
find most of the hydrogen has gone.E

### Some questions about electric vehicles

#### You’ve shown that electric cars are more energy-efficient than fossil cars. But are they better if our objective is to reduce CO2 emissions, and the electricity is still generated by fossil power- stations?

This is quite an easy calculation to do. Assume the electric vehicle’s
energy cost is 20 kWh(e) per 100 km. (I think 15 kWh(e) per 100 km is perfectly
possible, but let’s play sceptical in this calculation.) If grid electricity
has a carbon footprint of 500 g per kWh(e) then the effective emissions of
this vehicle are 100 g CO2 per km, which is as good as the best fossil cars
(figure 20.9). So I conclude that switching to electric cars is already a good
idea, even before we green our electricity supply.

#### Electric cars, like fossil cars, have costs of both manufacture and use. Electric cars may cost less to use, but if the batteries don’t last very long, shouldn’t you pay more attention to the manufacturing cost?

Yes, that’s a good point. My transport diagram shows only the use cost.
If electric cars require new batteries every few years, my numbers may be
underestimates. The batteries in a Prius are expected to last just 10 years,
and a new set would cost £3500. Will anyone want to own a 10-year old
Prius and pay that cost? It could be predicted that most Priuses will be
junked at age 10 years. This is certainly a concern for all electric vehicles
that have batteries. I guess I’m optimistic that, as we switch to electric
vehicles, battery technology is going to improve.

#### I live in a hot place. How could I drive an electric car? I demand power-hungry air-conditioning!

There’s an elegant fix for this demand: fit 4 m2 of photovoltaic panels
in the upward-facing surfaces of the electric car. If the air-conditioning is
needed, the sun must surely be shining. 20%-efficient panels will generate
up to 800 W, which is enough to power a car’s air-conditioning. The
panels might even make a useful contribution to charging the car when
it’s parked, too. Solar-powered vehicle cooling was included in a Mazda
in 1993; the solar cells were embedded in the glass sunroof.