tisdag 27 mars 2018

Phasing out diesel

British politicians are now saying that the future for the railways is hydrogen or battery power and that diesel traction should be phased out by 2040.

Batteries have made vast improvements over the past couple of decades. Lithium supply is a problem but several of the elements on the top left hand side of the Periodic Table are candidates and we can expect substitutes to be adopted. However, the underlying problem of energy density is unlikely to be solved since there is no Moore’s Law in operation. The likely use of battery power will be for use on routes which are electrified for most of their length; one could envisage a train running from Paddington to Maidenhead on electric power and continuing to Bourne End and Marlow under battery power, where it could receive a top-up before returning; similar trains could also provide the all-day shuttle service on the branch. Apart from the provision of batteries, they would be similar in almost all respects to the regular fleet of electric trains running only on electrified routes.

Hydrogen power dead end?
Hydrogen powered trains, on the other hand, look like a specialised niche. The hydrogen has to be made somehow, probably by electrolysis of water. This energy is recovered in a fuel cell where it is converted into electricity. Both processes result in losses, on top of the usual losses associated with the drive train and control systems. That is not the end of the energy losses. There are also losses associated with the transport of the hydrogen, which is not a portable fuel. Hydrogen will liquify only at extremely low temperatures, below 33°K. That is cold. At ambient temperatures is has to be compressed and put in tanks capable of withstanding extreme high pressures, which means they are heavy, and both compression and liquefaction consume large amounts of energy. A German experiment aims to use otherwise unusable electricity from wind generation to produce the hydrogen but this seems an inefficient and expensive way of making use of it.

What is the overall thermal efficiency when all of this is taken into account? There is a discussion of the subject here, in relation to automotive applications of hydrogen fuel cells. Then there is platinum to consider. Fuel cells require platinum catalysts. Alternatives are not even on the horizon. It is one of the rarest of elements. Platinum mines are not environmentally friendly. Taking one thing with another, this technology is nothing like as clean as it seems, and not particularly cost effective.

Battery power might have specialised applications such as the branch line off an electrified main line, referred to above. Hydrogen power looks like a dead end. Neither is a candidate for the hoped-for replacement of diesel power. Politicians should get to grips with basic chemistry and physics before going public about their aspirations.

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