There was an excellent workshop last week, entitled ‘How is Technology Shaping the Energy Industry’, organised by Cambridge University Energy Network (CUEN). There was an interesting discussion about use of Hydrogen as an energy vector for powering heavy goods vehicles. The discussion prompted me to write a summary of the issues in this blog.
There is nothing new here. Much of my analysis comes from an excellent article written by Ulf Bossel in 2006, entitled ‘Does a Hydrogen Economy Make Sense?’ (Proc IEEE, Vol 94, No 10, pp1826–1837, 2006)
Although generating and storing hydrogen seems like an attractive way to manage excess electricity production, the fundamental thermodynamics of the various processes makes it a very wasteful solution. Use of Hydrogen as a fuel only makes sense if sustainable energy is so abundant that we are willing to throw away most of it. This was the “inexhaustible energy” promise of Nuclear Fusion advocates in the 1970s. Unfortunately Nuclear Fusion is still “only 40 years away”.
A more realistic view of the future is that it will be a significant challenge to generate enough sustainable electricity to replace fossil fuels for electricity generation and heat supply, worldwide. (See the late David MacKay’s excellent book on the subject ‘Sustainable Energy – Without the Hot Air’ for a numerate discourse on this subject.)
A key figure from Bossel’s paper (reproduced in Figure 1, below) summarises the main problem with hydrogen as a vehicle fuel. Generation, storage and transportation of Hydrogen is very energy-intensive and therefore very costly. If you take 100kWh of electricity (which is assumed to come from low carbon sources in future), transmit it via the grid, put it into a battery, then run an efficient electric vehicle with it, you will end up with about 69kWh of energy at the wheels (right-hand path on Fig. 1).
If you take the same 100kWh of electricity, use it to generate hydrogen (by electrolysis of water), compress, store and transport the hydrogen in a vehicle, then run it through a fuel cell to create electricity and use that to drive an efficient electric vehicle, you will end up with about 23kWh at the wheels (left-hand path on Fig. 1).
The route via hydrogen is extremely wasteful, because converting electricity into hydrogen (by electrolysis) is only about 75% efficient and converting hydrogen into electricity (in a fuel cell) is only about 50% efficient, at best.
There are other ways to make hydrogen, particularly by steam reforming of methane (see figure 2). However unless there is a viable Carbon Capture and Storage (CCS) scheme to sequester the CO2, the total greenhouse gas emissions are just as bad as burning the methane directly in a gas engine (which is commercially available now). The latter is about 43% efficient, compared to 29% for methane-via hydrogen-to electricity. So the only way that hydrogen can possibly make sense from an energy viewpoint requires CCS as a pre-requisite. This looks increasingly unlikely, particularly since the UK Government cancelled CCS research in 2015.
The best strategy to reduce GHG emissions and energy cost is simply to use electricity to charge the batteries of electric vehicles. Forget the hydrogen! Even better is to power the vehicles directly without going through batteries at all: for example, using overhead powerlines with conductive power transfer. This is about 80% efficient (windmill-to-wheel).
Look at this another way… A 40t lorry driving along a highway at steady speed uses approximately 200kW of power. If there were 10 such lorries, the electricity needed to power them by battery electrification would be 10x200kW/0.69 = 2900 kW, ie 3 MW. This is approximately the power supplied by one of the largest available wind turbines. If you used Hydrogen fuel cells instead, the equivalent number of wind turbines needed to generate the electricity to make the hydrogen to power the 10 lorries would be 10×200/0.23 = 8.7 MW. Consequently you would need 3 wind turbines to power the same 10 lorries that would require one wind turbine for battery electrification. Can we afford a system that requires generation of three times more renewable electricity?
So let me rephrase the question in the title of this article: ‘Does it make sense to use 23% of sustainable electricity (and waste 77%) with a hydrogen solution for road freight, or is it more sensible to use 69% by battery electrification or 80% via overhead powerlines?’
The answer is clearly a no-brainer. Hydrogen is far too inefficient and far too costly on a well-to-wheel basis. Direct electrification is the way to go…