http://www.ck12.org Chapter 2. Numbers, Not Adjectives
Figure 15.8:Energy requirements of different forms of freight-transport. The vertical coordinate shows the energy
consumed in kWh per net ton-km, (that is, the energy per t-km of freight moved, not including the weight of the
vehicle). See also figure 20.23 (energy requirements of passenger transport).
Water transport requires energy because boats make waves. Nevertheless, transporting freight by ship is surprisingly
energy efficient.
Desalination
At the moment the UK doesn’t spend energy on water desalination. But there’s talk of creating desalination plants
in London. What’s the energy cost of turning salt water into drinking water? The least energy-intensive method is
reverse osmosis. Take a membrane that lets through only water, put salt water on one side of it, and pressurize the
salt water. Water reluctantly oozes through the membrane, producing purer water – reluctantly, because pure water
separated from salt has low entropy, and nature prefers high entropy states where everything is mixed up. We must
pay high-grade energy to achieve unmixing.