3.2. Better transport http://www.ck12.org
this hydrogen car.(The only transport methods I know that are worse are jet-skis – using about 500 kWh per 100
km – and theEarthracebiodiesel-powered speed-boat, absurdly called an eco-boat, which uses 800 kWh per 100
p-km.)
Figure 20.27:The Earthrace “eco-boat.” Photo by David Castor.
Hydrogen advocates may say “the BMW Hydrogen 7 is just an early prototype, and it’s a luxury car with lots of
muscle – the technology is going to get more efficient.” Well, I hope so, because it has a lot of catching up to do.
The Tesla Roadster (figure 20.22) is an early prototype too, and it’s also a luxury car with lots of muscle. And it’s
more than ten times more energy-efficient than the Hydrogen 7! Feel free to put your money on the hydrogen horse
if you want, and if it wins in the end, fine. But it seems daft to back the horse that’s so far behind in the race. Just
look at figure 20.23 – if I hadn’t squished the top of the vertical axis, the hydrogen car would not have fitted on the
page!
Figure 20.28:The Honda FCX Clarity hydrogen-powered fuel-cell sedan, with a Jamie Lee Curtis for scale. Photo
courtesy of automobiles.honda.com.
Yes, the Honda fuel-cell car, the FCX Clarity, does better – it rolls in at 69 kWh per 100 km – but my prediction is
that after all the “zero-emissions” trumpeting is over, we’ll find that hydrogen cars use just as much energy as the
average fossil car of today.
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. If you park your hydrogen car at