Why do people think hydrogen-powered cars are sustainable?
A decade or so ago, cars of the future were to be electric, powered either by batteries or by fuel cells, using hydrogen. Major car manufacturers produced hydrogen-fuelled demonstration models, some hydrogen-powered buses were put into service and a handful of hydrogen filling stations were built. More recently, however, it seemed that batteries had won the race to provide motive power for the next generation of vehicles, as technology evolved and range became more respectable.
But that doesn’t mean that the alternative technology doesn’t still have its supporters. One is Toyota, the world’s largest car manufacturer. Another is a startup company in Wales, Riversimple. With the recent growth in sales of electric cars (now seen on a daily basis in many parts of the country) we might be forgiven for thinking that batteries had won the technology race. Up to a point, maybe, but there are still significant issues that will prove very difficult to solve to the extent that electric cars can completely replace conventional ones.
The most evident problem is the need to charge batteries. For commuting or relatively short journeys this is currently not much of an issue. The comparatively few cars in regular use can be left charging at one of the quite numerous (in some locations) charging points or left to charge at home overnight (at least for those people who do not have to rely on on-street parking).
The real problem comes if electric cars become a large sector of the market, as governments envision. At this stage, the charging network would have to be much more extensive to avoid drivers being effectively stranded while waiting in line for a half-hour charging slot before they can continue their journey. Doubtless some organisations have modelled this situation, in which case they will also have come to the conclusion that this could be a growth-limiting issue.
Which brings us back to hydrogen-fuelled cars. These have been the Cinderella of the motoring world; as batteries have become smaller and lighter, fuel cell technology has progressed relatively slowly. But a realisation of the downside of battery power (and that’s before we get into the future supply of lithium, or the potential fire hazards) has led to a tentative renaissance in the hydrogen-fuelled car. Toyota and other major manufacturers are still hedging their bets.
The company virtually created the market for petrol-electric hybrids with the launch of the Prius a couple of decades ago, and this has led to the development of plug-in hybrids, with more batteries and an electric-only range of perhaps 30 miles. Plug-in hybrids are the most likely sector to enjoy continued growth, since they offer environmental advantages for short, urban journeys while retaining the range advantage of the petrol engine.
But all-electric cars remain a niche for early adopters, with little immediate prospect of breaking into the mass market. Toyota produce only two all-electric vehicles, the small but conventional-looking iQ and the motorbike/car hybrid i-Road. Now, they have also launched a Prius-sized hydrogen fuel cell car, the Mirai. This also has a battery pack to store the energy regenerated during deceleration and to add additional power for acceleration. The hydrogen tanks are made of a carbon-fibre composite to contain the gas under high pressure and minimise leakage, a big problem when trying to store the lightest element. Importantly, a range of over 300 miles is claimed.
Toyota have re-engineered a conventional-looking car to be powered by fuel cells. At the other end of the spectrum, Riversimple has developed a more radical car from first principles. It is small and very light, a two-seater weighing only 580 kg, or not much more than a quarter of a Mirai. The car has a much smaller fuel cell pack but, because of its lightness, a reasonable power-weight ratio and a claimed range close to that of the Toyota.
Other manufacturers, including Honda, Mercedes and Hyundai, also now offer hydrogen cars. Toyota has delivered a handful of Mirais for use as London taxis, and over 1,000 are on the roads in California, the only US state in which they are sold. They are real cars that are driveable on normal road, and the fact that a number of mainstream manufacturers are still involved in their manufacture or development suggests that the future of battery-powered cars is not assured.
However, despite the logistical problems associated with keeping large numbers of battery vehicles on the road, the barriers to domination of the market by hydrogen-powered fuel cells look even higher at present.
The obvious problem is that cars can only be driven a certain distance from the handful of filling stations they can currently use. And hydrogen is not easy to distribute. It seems that current facilities receive cylinders of compressed gas, as used industrially, and cool and compress it ready to deliver it to in-car tanks. According to Toyota, refuelling takes five minutes, much faster than recharging batteries, but rather slower than filling up with petrol or diesel.
Providing a network of such filling stations would be a nightmare, suggesting that the way forward for hydrogen – if it has a future as a transport fuel – would be to generate it at each filling point. But an even bigger issue than this is that the rationale of using hydrogen is to reduce carbon dioxide emissions from transport. Currently emissions from hydrogen cars are simply shifted from exhaust pipes to power stations.
In fact, hydrogen generation, compression and use in fuel cells requires more energy overall (because of inevitable inefficiencies at each stage), while making little or no difference to emissions without a revamp of the electricity generating. At present, most hydrogen is actually not produced by electrolysis of water, but by steam reforming of methane. Sufficient low-carbon generating plant, such as nuclear, would need to be built to cope with the additional demand from transport.
In energy efficiency terms, it would be better to use this electricity to charge batteries and drive motors directly rather than go through the intermediate stage of generating hydrogen as an energy carrier. With current technology, sustainable (in the poorly-defined but normally accepted sense) is not a word that can reasonably be applied to hydrogen-powered road transport. Nevertheless, manufacturers do not have sufficient confidence in all-electric battery-powered cars to stop work on this alternative technology.