Finding new ways to power the world’s vehicles has long been an essential part of tackling the climate crisis. When it comes to small cars, there is no question that the future lies more in battery-electric cars than in those powered by hydrogen fuel cells – the other viable alternative. However, as the size of the vehicle increases, hydrogen can become an increasingly attractive option. In the case of buses, some argue that hydrogen propulsion offers several key advantages over their battery-electric counterparts. Which of these will ultimately become the main technology in buses could also influence other modes of transport.
Battery electric and hydrogen fuel cell vehicles have similar propulsion systems. Both store energy to drive an electric motor. In the latter, however, the energy stored as hydrogen is converted into electricity by the fuel cell and not stored in a rechargeable battery.
Sales of electric cars reached 3 million in 2020, an increase of 40 percent compared to 2019, with around 10 million electric cars currently on the world’s roads. Hydrogen car registrations are still three orders of magnitude lower, and there are only 26,000 on the world’s roads, concentrated in three countries: Korea, the United States (mostly California) and Japan. While several hydrogen fuel cell cars made by Toyota and Hyundai are still available in the market, they are typically more expensive than battery electric cars and can be difficult to refuel right now: hydrogen is expensive to buy, and there is far fewer petrol stations than charging points in most places.
But with larger vehicles the picture is not quite as clear. The larger the vehicles get, the more difficult it becomes to electrify them as larger and larger batteries are required. For energy-intensive applications such as long-haul trucks, some experts consider hydrogen to be the best option.
Buses are somewhere between cars and trucks on this spectrum. “The big problem is the size of the buses,” says James Dixon, researcher in the field of modeling of energy and transport systems at Oxford University. “Batteries have a comparatively low energy density: The energy density is around 1/40 of the energy density of a liquid hydrocarbon fuel such as gasoline or diesel.” Hydrogen also has a relatively low energy density (the amount of energy that can be stored per unit volume of mass or area) – about four to five times lower than petroleum fuels, but far higher than electric batteries, he adds.
