Listen to this story
(Image courtesy of Porsche.)
Porsche and Siemens Energy have recently announced their entry into the eFuels industry with a plan to initially produce 130,000 liters a year starting in 2022. This is just the start, and the two giants have goals to scale up production to 55 million liters by 2024 and 550 million liters by 2026.
While most of the attention on automotive decarbonization is currently focused on battery electric vehicles (BEVs) and hydrogen fuel cell electric vehicles (FCEVs), there are several alternative approaches. I’ve recently covered electric road systems, which can provide electricity directly to vehicles in motion, greatly reducing or even eliminating the need for on-board energy storage.
Although there was once great hope for biofuels, much of this enthusiasm has now dissipated. Firstly, it became apparent that there is insufficient agricultural land to produce both food and first- or second-generation biofuels. Third-generation biofuels address this by growing algae in open lakes or the sea. In theory, this could be a highly efficient way of producing biofuels without displacing agriculture, but in practice it has proved uneconomic.
Attention is now moving to fourth-generation biofuels, which use renewable power to produce hydrocarbon fuels without using any plants or other biological processes. These processes use water and carbon dioxide to produce hydrocarbons, releasing oxygen just as plants do. Examples include using solar reactors in which the heat of the sun is used to produce fuel, and electrofuels, or eFuels, which use electricity from renewable sources.
The advantages of storing energy as a hydrocarbon are significant. For vehicles, hydrocarbons provide a lightweight and compact way to store large quantities of energy. Compared to the best lithium-ion batteries, gasoline can store almost 50 times the energy for the same weight, and 14 times for the same volume. Even when compared to hydrogen, hydrocarbons are far lighter and more compact. Although when unpressurized, hydrogen has three times as much energy per kilogram as gasoline, it cannot be transported in this form. Uncompressed hydrogen has an extremely low density and would require 3,400 times as much volume as a gasoline tank. The very high pressures required to significantly compress hydrogen means that the tanks are extremely heavy. Using a state-of-the-art composite tank to store hydrogen at 700 bar results in an energy store over seven times as heavy as a tank of gasoline—and still requires 130 times as much volume.
The utility of hydrocarbon fuels is not limited to a dense store for vehicles. The cost of tanks to store large quantities of liquid fuel are very low, so fuels can be stored for long periods of time. This means that once the fuel is produced, there are no practical limits to how much energy you can store. With significant seasonal variations in energy demand and renewable production, this is a major advantage. It is also very easy to ship these fuels around the world, just as we do now with fossil fuel petroleum-derived fuels. eFuels could, therefore, solve the significant issues with moving large quantities of renewable power in time and space.
Synthetic eFuels can directly replace fuels used in existing vehicles, such as jet aircraft, heavy trucks, private cars and ships. The existing refueling infrastructure can be used, including distribution, storage and delivery to vehicles. eFuels can also be designed to burn more cleanly than fossil fuels, producing much lower levels of particulates and nitrogen oxides.
At this point it may seem as though eFuels are the perfect energy carrier, set to replace batteries. There is, however, a downside. Lithium-ion batteries can store and release electrical energy with very high efficiency, typically 95 percent in real use. Producing chemical fuels from electricity is far less efficient, and if these fuels are burned in a heat engine, then the efficiency falls even lower since the engine is only able to convert about 30 percent of the energy into mechanical work.
The most efficient process for storing electricity as a fuel is using hydrogen. With the best current technology an electrolyzer requires 3.8 to 4.4 kWh to produce a normal cubic meter of hydrogen. This equates to an energy efficiency of between 68 percent and 79 percent. However, the most efficient units are also very expensive and in commercial operation an efficiency of 50 percent is more typical. Creating liquid fuels from hydrogen requires significant further energy inputs that can result in an efficiency from electricity to hydrocarbon of just 20 percent. If the fuel is burned in an engine with an efficiency of 30 percent, then the combined efficiency would be just 6 percent. Therefore, eFuel production would require almost 16 times the wind and solar energy to power vehicles as direct electrification using battery electric vehicles. Despite the excellent energy density of eFuels, they are not a replacement for battery electric vehicles.
How Are eFuels Produced?
eFuels are hydrocarbon fuels that require a feed of both hydrogen and carbon dioxide. Hydrogen is produced from water using an electrolyzer, and carbon dioxide may be captured from the air or from a more concentrated waste stream, such as cement production. The next stage is to combine these feeds to produce a hydrocarbon fuel, and this may involve multiple steps such as first producing methane, then methanol, and finally diesel or gasoline.
Some common processes used to produce hydrocarbon eFuels include:
- Methanol synthesis, which mixes together two feed streams of hydrogen and carbon monoxide and reacts the mixture at high temperature with a copper catalyst to produce methanol. This process has been in use for over 60 years and is currently operating at commercial scale to produce over 70 million tons of fuel a year.
- Fischer-Tropsch (FT) synthesis, which can be used to produce a range of different hydrocarbons, also reacts to carbon monoxide and hydrogen at high temperatures (150–300 °C) and in the presence of a metal catalyst.
All eFuel production processes include the following steps:
- Hydrogen production, which for a true renewable eFuel involves electrolysis from water using renewable electricity. Currently, synthetic fuels are created using hydrogen produced by steam reformation of natural gas. In the future, it may also be produced directly from solar energy, without the intermediate step of producing electricity, in a solar reactor.
- Carbon dioxide capture, which is most easily performed using a high concentration output from an industrial process such as cement or steel production, or fossil fuel power generation. Direct air capture (DAC) is also possible, for example, by using amine absorption or the lime-soda process.
- Synthesis, which involves reacting carbon dioxide with hydrogen to create a hydrocarbon fuel. Current processes require a carbon monoxide stream, although it is expected that research will enable carbon dioxide to be used directly.
Who Currently Produces eFuels?
Although synthetic eFuel production is at an early stage, there are several commercial plants already in operation. For example, Carbon Recycling International (CRI) produces 4,000 tons a year of renewable methanol at its Vulcanol plant, recycling 5,500 tons of carbon dioxide from a power plant’s flue gas and combining this with green hydrogen from electrolysis.
Norsk e-fuel is one of the first companies to start large-scale commercial eFuel operations. It is using Sunfire’s electrolyzers to produce hydrogen and Climeworks’ Direct Air Capture to produce carbon dioxide. The first plant aims to be producing 10 million liters a year by 2023, and it is then planning to ramp up production to 100 million liters by 2026.
eFuels are not a panacea for the widespread decarbonization of transport. Although they offer “drop-in” compatibility with existing vehicles and infrastructure, the very low energy efficiency means that widespread use would not be practical. However, they may have a significant niche role in decarbonizing aviation, some heavy vehicles that need to access remote locations, and perhaps also some luxury vehicles.