Transport makes up around 20 per cent of our energy use around the world and that figure is set to grow, according to the International Energy Agency. With sustainable solutions in mind, a new study published by eminent physicist Jo Hermans, looks at the energy efficiency of current modes of transport from bicycles to buses, from air transport to cruise ships and concludes that liquid hydrogen seems to be a realistic option for what is probably the most problematic of transportation modes in terms of sustainability, future air travel.

The physicist acknowledges that achieving sustainable transport in the post-fossil fuel era will be a huge challenge but finds that liquid hydrogen could offer a potential solution for future air travel. "Given the severe weight limitations for fuel in aircraft, liquid hydrogen may be a viable alternative in the long run," he argues with following reasons: First, handling of liquid hydrogen would be carried out by professionals, which reduces the safety issues involved with liquid hydrogen to the same level of risk involved in handling kerosene; Second, liquid hydrogen itself is very light (in fact, it is in a gaseous state at ordinary temperatures), which is an important advantage for air travel; Third, the disadvantages of "boil off" (created by the low boiling point of liquid hydrogen) would be reduced in air travel because of the low outside temperature at cruising altitudes.

Hermans discounts the use of solar power for air travel without revolutionary changes in the airplane concept, but concludes that it seems wise to extend the availability of oil products as long as possible. However, he argues that the low cost of kerosene is a huge disincentive in this respect. "It is a defect that kerosene is so irrationally cheap, which triggers much unnecessary air travel," he writes. "A worldwide tax on kerosene if at all politically possible should be something to pursue."

Alternatively, Hermans writes, the most efficient way for us to reduce energy use in future is to reduce our mobility, for example, by having shorter distances between the workplace and home. "In other words, urban planning provides an important key," he concludes.