To stop global warming and meet the goals set out in the Paris Climate Accord, power delivery systems and energy systems will have to become carbon-neutral. Fuels are one of the key levers we have. With e-fuels produced from green electricity, electricity generation and drive-power technology stands a good chance of going net-zero.
Europe aims to be climate-neutral by 2050. Incoming US President Joe Biden has also announced this is a goal he will be pursuing. This demands a technology rethink not just on power delivery but also on new fuels. No matter how clean fossil fuel combustion engines may be, they will always emit CO2. E-fuels such as e-hydrogen, e-methane, e-methanol or e-diesel – produced from green electricity and carbon from the air with some subsequent processing – are now opening up a new path into the future. The demand for e-fuels is huge, with 20,000 terawatt-hours of fuel-based energy needed in 2050 according to an analysis undertaken by Rolls-Royce Power Systems. In diesel terms, this equates to two trillion liters.
Power-to-X: electricity turned into fuel
These new fuels are produced in a process known as Power-to-X. It works like this: Electricity produced from renewables such as wind or solar power is used to break water down into its components (hydrogen and oxygen) by a process of electrolysis. The hydrogen can be then used without further ado in a hydrogen engine, aircraft turbine or fuel cell. However, its relatively low energy density means it needs a lot of tank space and is anything but easy to store and transport. Other fuels that can be produced from hydrogen have higher energy density and are easier to store. By applying more energy and adding carbon (either air-borne carbon or from biomass), it is possible to produce other synthetic fuels such as e-hydrogen, e-methane or e-diesel. This is how electricity is turned into fuel. It can be burned carbon-neutrally, as no extra CO2 is produced.
Other synthetic options are methanol and ammonia, the latter requiring no CO2 during synthesis. However, combustion engines have to be modified to run on synthetic fuels.
What are set to be the key fuels of tomorrow?
The big question now facing experts at Rolls-Royce is which fuel is set to be the most economical and most energy-efficient. “When producing methane, methanol or kerosene from hydrogen, you have to apply additional energy and add CO2,” explained Daniel Chatterjee, Director of Technology Management & Regulatory Affairs at Rolls-Royce Power Systems. “Methane or diesel could be used with existing technology and infrastructure. Even so, ammonia or methanol might also come to the fore, especially in shipping, although there are still safety and infrastructure issues to be overcome,” he added.
The whole scenario is different for stationary engines used in power generation. For example, existing natural gas grids can be used to transport e-methane, and it is possible to produce e-hydrogen locally.
“I'm assuming we're going to see various different fuels in future, without any one of them becoming a panacea,” summed up Chatterjee, adding, “The question of which fuels we consider so future-proof that we end up developing new mtu engines or other energy converters for our customers is going to be occupying our minds a lot in the coming months and years.”