In all truth, hydrogen fuel cell technology is around a decade behind battery electric propulsion systems. It may yet be an answer for heavy duty/long distance transport but not just yet.
Here we will consider three energy storage media:
- ‘Green hydrogen’ is where renewable energy is used to electrolyse water to extract the hydrogen
- ‘Green ammonia’ is where renewable energy is used to create ammonia as an energy store
- Nickel manganese cobalt (NMC) lithium batteries, the gold standard of EV propulsion batteries today
‘Grey hydrogen’ and ‘brown ammonia’ are produced by the megaton in existing industrial processes from fossil fuels all over the world today. Given that these are often more polluting to produce than many fossil fuels, here we will look at the minimal carbon emitting examples above.
Efficiency – Batteries, hydrogen and ammonia
The best available research on energy storage efficiency is:
- A typical Nickel Manganese Cobalt (NMC) lithium car battery will store 85% of the energy used to charge it
- Green hydrogen pressurised at 700 bar, 50-60%
- Ammonia – 33%
In terms of energy stored, batteries are the clear winner.
Energy density
Where it comes to usable energy density, hydrogen blows batteries out of the water:
- Hydrogen stores 33 kilowatt-hours (kWh) per kg
- Ammonia, 5.2kWh/kg
- NMC lithium batteries, 280Wh/kg
Looking at the above figures, one can immediately see the conundrum of hydrogen – it is very inefficient as an energy store but stores a lot more energy per kg.
Hydrogen – large?
Where it comes to cars and small to medium vans, batteries have already won the race. Battery packs in EVs can do 450km per charge. Hydrogen is never going to compete in cars, no matter the hype. With current technologies fuel cell hydrogen stored at 700 bar is 4-5 times the volume of petrol. To add to this, a carbon fibre tank to hold 4.5kg of hydrogen at that pressure will weigh 10 times that of a petrol tank.
Hydrogen still has its place though. Scaled up to HGV level, a hydrogen storage system capable of a 1000km run will take up a lot more space than the diesel tank (some even speak of replacing the sleeper cab). Space is available at that scale however, unlike cars and vans, and it can be refuelled in just minutes. An 850kWh battery in a Tesla Semi HGV tractor would take considerably longer, even when the megawatt charging protocol is rolled out for HGVs.
Sitting between batteries and hydrogen is ammonia. This can be stored at minus 35 degrees C, similar to that of LNG used in some HGVs today. It is 20 times as energy dense as NMC batteries. However it is extremely toxic, flammable and explosive! There are over 130 orders for ammonia powered oceangoing transport ships for example even while no one will currently license them as a fuel due to the environmental holocaust that would occur when one sinks.
Cost per mile
With sodium-ion batteries on the horizon, battery electric vehicle prices are set to plummet in the medium term. For van and car owners this is great news, but it won’t cover the need of HGVs to cover 1000km per day because sodium is so much less energy dense than lithium. Solid state batteries, potentially on the market by 2028, will likely be 20% more dense than NMC today. You can bet these won’t be cheap though!
HGV fleet owners are in for a rocky ride in terms of cost per mile, no matter where they look. Hydrogen fuel is ridiculously expensive – the RAC estimated it at £11-£16/kg in 2020, and this is grey/blue hydrogen as opposed to the green that we wish for here. The Royal Society reported the same year that green hydrogen will be competitive with grey hydrogen if the energy used costs GBP£0.01-£0.04 per kWh, not at all possible given energy costs today.
No easy answers!
Decarbonising the supply chain cannot be pain free. If HGVs and ships are to run on fuel costing an order of magnitude more than fossil fuels, the consumer will pay for it in eye-watering inflation. The competing issues of energy density and storage efficiency (hydrogen both the clear winner and clear loser) show that there is no straight up answer today.
As we stated at the beginning of this article, hydrogen fuel cell technology is a decade behind that of battery tech. Just because batteries are winning the race today doesn’t mean that by 2033 they will still be ahead.
Source: Ti Insight
