Thermodynamics spoil the party

If LN2 is a waste product, one can see there may be attractions, but such vehicles could only occupy a niche market

I read with interest the article "Solid future for liquid air" in PE July 2013. As is often the case, thermodynamics spoil the party. 

The current energy cost of manufacture of liquid N2 (LN2) is approximately 1.8MJ/kg. A perfect engine using LN2 would produce an output of ~0.77 MJ/kg. I am uncertain about the definition of engine efficiency used in the article, but presumably the 60% figure refers to the output relative to the ideal, so the output would have been ~0.46 MJ/kg, i.e. about  26% of the energy required to manufacture the original LN2. (Intriguingly, this is roughly the same round-trip efficiency of the electricity - electrolysis - hydrogen - fuel cell - electricity route which is sometimes touted as an attractive proposition for vehicular transport, and thus an alternative method of throwing away ~3/4 of the energy one started with.) For further details on LN2 prime movers, readers might wish to consult Knowlen, C., Mattick, A., Bruckner, A., and Hertzberg, A., "High Efficiency Energy Conversion Systems for Liquid Nitrogen Automobiles," SAE Technical Paper 981898, 1998, doi: 10.4271/981898.

If LN2 is a waste product, one can see there may be attractions, but such vehicles could only occupy a niche market, given the fact that the 6.5 million car-km LN2 resource quoted is roughly 0.2% of the current UK total. - One wonders whether converting the waste LN2 on-site (to electricity?) might not more more sense - the fact that this seems not to be considered economically viable perhaps gives further pause for thought. 

Finally the importance of the "clean" nature of such engines needs to be treated with caution - the main problems with air quality, in so far as they are affected by road transport, are currently mainly due to "legacy" vehicles - new vehicles have extremely low noxious emission levels, by law.

Nick Collings, Professor of Applied Thermodynamics, Cambridge

Next letter: True cost of wind