As the world acts to decarbonise transport, sales of electric cars are booming.
More than 10 million EVs were driven off dealers’ forecourts in 2022, according to the International Energy Agency, and this year that number is expected to jump to 14 million.
The IEA forecasts that by 2030 electric cars will reduce the use of oil by five million barrels a day.
In a global economy moving away from oil and gas, it is not just battery-powered electric vehicles that are coming to prominence.
Other alternative fuels, among them biodiesel, ethanol, hydrotreated vegetable oil (HVO), hydrogen and green ammonia may also play a role in the post-fossil-fuel economy.
As researchers reduce the lifecycle effects of alternative fuels – the environmental impact they have when taking into account all stages of their production as well as use – some could play an important role in road, sea and air transport.
Renewable diesel
Paul Hellier, an associate professor in engines at University College London who researches low and net zero carbon fuels, says HVO, or renewable diesel is "increasingly popular" and can be used on its own without being blended with a fossil diesel.
It already powers vehicles such as refuse lorries, for example.
"HVO is actually a very high quality fuel relative to a fossil diesel or a standard biodiesel," Dr Hellier says.
Its composition means that it could work well for aviation, an industry where electrification is less advanced than it is for road transport.
HVO reduces greenhouse gas emissions by up to 90 per cent compared with standard diesel, although it is typically slightly more expensive than standard diesel.
Hydrogen
Another key alternative fuel is hydrogen, which can be generated sustainably using renewable energy, although there are challenges in hydrogen storage and delivery.
With hydrogen fuel cell electric vehicles, a chemical reaction between hydrogen and oxygen powers an electric motor, and the exhaust pipe gives off water vapour.
Toyota and Hyundai have production hydrogen-powered vehicles, while other manufacturers, including BMW, have models under development or in limited production.
BMW chairman Oliver Zipse said in August that hydrogen was expected to "gain substantially in importance as far as personal mobility is concerned".
"We think hydrogen-powered vehicles are ideally placed technologically to fit alongside battery-electric vehicles and complete the electric mobility picture," he added.
The UK government recently awarded a British company – Compact Syngas Solutions (CSS) – nearly £4 million ($5.05 million) to develop its method of generating hydrogen from waste. This waste may include waste wood that would have gone to landfill and released greenhouse gases as it decomposed.
The funding will help CSS develop a full-scale rig to demonstrate that it can separate and store CO2 generated during the hydrogen production process.
It plans to build 50 hydrogen modules at 15 UK sites, with each module said to be able to produce 750kg of hydrogen a day, enough to fuel a fleet of lorries.
CSS has had discussions with a UK company that has developed dual fuel technology for lorries.
"They would retrofit a hydrogen conversion to an existing diesel vehicle, so it de-risks it for everybody," says Paul Willacy, the managing director.
"It could still run on 100 per cent diesel or it could run up to 50 per cent hydrogen. It’s a nice stepping-stone technology while we get ourselves ready and so does the market."
Mr Willacy, who used to work in the oil and gas industry in locations including Kuwait, says hydrogen is likely to be a better solution than batteries for some vehicles, such as large excavators that would require batteries that would be prohibitively heavy and expensive.
Green ammonia
Another fuel attracting growing interest is ammonia produced using renewable power – or green ammonia – where vehicles use ammonia fuel cells to generate electricity to power the motor.
On Tuesday, Chinese carmaker GAC unveiled in Beijing what was described as the world’s first ammonia-powered car engine.
It claims a 90 per cent cut in carbon emissions compared with traditional internal combustion engines.
Shanwen Tao, a professor of chemical engineering and sustainable processes at the University of Warwick in the UK, is among those researching ammonia as a fuel for vehicles.
"[For] long-distance vehicles, the fuel-cell technology, particularly ammonia, has huge advantages over batteries," Prof Tao says.
As with hydrogen, refuelling would be quick, without the charge-time issues that affect electric vehicles. Prof Tao also says that ammonia fuel cells are cheaper to produce than those of hydrogen.
"Big progress has been made in direct ammonia fuel cells, so green ammonia is the rising star, in my opinion," he says.
There are toxicity concerns with ammonia, but the technology is attracting significant interest and the infrastructure and storage it requires could be simpler than with hydrogen.
Niche fuels
Demonstrating that other new technologies may emerge, Prof Tao is also interested in urea as a potential fuel for vehicles, again involving fuel cells that power an electric motor.
"I think direct ammonia fuel cells and urea fuel cells have a good chance of being used to power electric vehicles such as cars, vessels, ships, planes, trains, lorries and buses, etc, in the near future," he says.
Other more niche fuels are also being developed, some with regional links.
Dr Hellier is collaborating with researchers in Oman who have developed a biodiesel made from date pits.
"I think we’ll see a bit more of that. In different places around the world there are different local feedstocks to make these fuels," he says.
Ultimately he expects there to be "several alternative fuels" used at significant levels, depending on the application.
"I think we’ll go away from what we’ve had for decades and decades, which has been really two or three fuels fractionated from fossil oil that have served a huge variety of purposes," Dr Hellier says.
Perhaps working against this is the historical tendency, cited by Tim Lenton, professor of climate change and earth system science at the University of Exeter in the UK, for people to "go all in on one technology".
Diverse options
So some alternative fuels being touted as the next big thing may fall by the wayside.
Prof Lenton, who is writing a book on tipping points that may lead to the adoption of particular technologies, says that he would like to see a diverse range of options used, with the most appropriate technology employed for any given purpose.
He says some manufacturers are "hedging their bets" by building lorry platforms suitable for hydrogen and battery technology. The solution adopted may depend on context, he says.
In many countries the time taken to recharge a battery may be less of an issue because drivers must have lengthy rest periods for safety reasons. But in Brazil, where drivers often work in pairs, recharging time "would make it really difficult to switch to battery technology".
"For HGVs you can build a good case for hydrogen fuel cells," Prof Lenton says, with renewable electricity producing green hydrogen at times when domestic and industrial electricity demand is lower.
"I can see that happening in future. That could support a niche for green hydrogen for which the primary use would be goods transport."
