With solar fuel to the climate target

Mobility analyses show: Only a small proportion of all vehicles are responsible for the majority of kilometers driven. We are talking primarily about heavy goods traffic, frequent and long-distance drivers who transport their goods right across Europe. If these many kilometers continue to be driven with fossil fuels, it will hardly be possible to reduce CO2 emissions in road traffic sufficiently. Synthetic fuels make a very significant contribution to such applications.

With electromobility, hydrogen mobility and synthetic fuels, Empa's mobility demonstrator "move" is investigating three technological paths for CO2 reduction in road transport against the backdrop of a rapidly changing energy system. "All of these concepts have energy, operational and economic advantages and disadvantages. To use them properly, you need an in-depth understanding of the overall system," says Christian Bach, head of Empa's Vehicle Propulsion Systems department. "Together with the 'move' partners, we are developing actionable knowledge on this."

The latest project revolves around the production of synthetic methane from hydrogen and CO2 - so-called methanation. Such fuels artificially produced with renewable energy - called synfuel or syngas - can be transported via conventional routes and made available through the existing infrastructure, which is interesting for Switzerland as well as globally because it opens up an enormous potential for renewable energy.

Methanation process developed at Empa

The basic chemical process of methanation has been known for over 100 years as the Sabatier reaction. In "move", a process developed further at Empa is to be used: so-called sorption-enhanced methanation. Empa researchers hope that this novel process engineering concept will simplify process control, increase efficiency and improve suitability for dynamic operation.

Methanation works as follows: Methane (CH4) and water (H2O) are produced from carbon dioxide (CO2) and hydrogen (H2) by means of catalytic conversion. The latter is a problem in conventional processes: In order to separate the water, several methanation stages are typically required in succession - with condensation areas in between. Due to the high temperatures involved, some of the water produced is also converted back into hydrogen by the water-gas shift reaction. The gaseous product of the methanation reaction therefore contains a few percent hydrogen, which prevents it from being fed directly into the gas grid; the hydrogen must first be separated again.

In contrast, sorption-enhanced methanation in the "move" is a single-stage process that does not require hydrogen separation in the product gas. The idea behind it: The reaction water is adsorbed on a porous catalyst support during the methanation process. This continuous removal of water shifts the reaction equilibrium toward a nearly 100 percent methane yield. "The gaseous product can therefore be fed directly into the gas network without additional purification and used, for example, to fuel gas-powered vehicles," explains Bach.

Carbon dioxide and water from ambient air

The CO2 for methanation as well as the water for the production of hydrogen is extracted from the atmosphere directly on site using a CO2 collector from the ETH spin-off Climeworks. The system draws in ambient air, leaving the CO2 molecules attached to the filter material. Using heat - around 100°C - the CO2 molecules are then detached from the filter again. The Empa researchers see further potential for optimization in the heat required for this CO2 desorption. "Hydrogen production as well as methanation continuously generate waste heat," says Bach. "Through clever heat management, we want to cover the heat demand of the CO2 collector to the greatest possible extent with this waste heat." In addition to CO2, the Climeworks plant also extracts water from the air, which is used via a condensate line for hydrogen production in the electrolysis plant. This means that such plants are also conceivable in regions without a water supply, for example in deserts (see box).

In addition to new insights into the technical and energy aspects, statements on the economic viability of synthetic methane are also an objective of the project. "To ensure this holistic perspective, the project consortium consists of partners covering the entire value chain - from Empa researchers to energy suppliers, filling station and fleet operators, and industrial partners in the technology and plant sector," says Brigitte Buchmann, member of the Empa Directorate and strategic head of "move". The project is supported by the Canton of Zurich, the ETH Board, Avenergy Suisse, Migros, Lidl Switzerland, Glattwerk, Armasuisse and Swisspower.

Currently, Christian Bach's team is focusing on the studies of water adsorption on porous materials and the process control of the catalytic reaction. Construction of the plant is scheduled for the middle of next year. "About a year later, we want to fuel the first vehicle," Buchmann says. "With methane from solar energy."

Synthetic fuels from the desert?

There is a major challenge in converting our energy system to renewable energy: Renewable sources such as the sun or wind are not always available everywhere. In winter we have too little renewable electricity and in summer too much - in the northern hemisphere. In the southern hemisphere, it's the other way around. But there are also areas with almost continuous sunshine - the so-called Sun Belt, where the Earth's great deserts are located. "From a global perspective, we do not have too little renewable energy worldwide, but "merely" an energy transport problem," says Christian Bach. Synthetic energy sources could solve this.

Smaller plants in Switzerland can make a valuable contribution to the national energy system by harnessing surplus summer electricity and linking different energy sectors. Large plants, however, could exploit their full potential primarily in the earth's sunbelt. This is illustrated by a calculation example: To cover Switzerland's energy demand in the winter half-year, which is not covered by hydropower, as well as all domestic long-distance traffic exclusively with (imported) synthetic energy sources, a solar plant would be required in a desert with an area of approximately 700 km2 ; that is 27 x 27 km or, expressed differently, 0.008% of the area of the Sahara. The water and CO2 needed for production could be extracted locally from the atmosphere. "The trading mechanisms, transport infrastructures, standards and expertise already in place could be reused in the process," Bach says. So could the plant in the "move" soon be the model for a gigawatt plant in the desert?

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