Moving towards low-carbon road transport

Researchers at the Paul Scherrer Institute PSI have presented a concept for decarbonising transport while keeping it economically competitive. Increasing the utilisation of carbon dioxide from biogas plants plays an important role.

 

 

In 2022, Switzerland’s transport-related CO₂ emissions amounted to 13.6 million tonnes, not including international aviation. This corresponds to 41 percent of Switzerland’s overall emissions. Transport can only become more climate-friendly by switching to low-carbon technologies. Electric drives are suitable for passenger cars and are already well established in this segment. However from a technical point of view, hydrogen propulsion is more suitable for heavy commercial vehicles, because a hydrogen-powered lorry outperforms its battery-powered counterpart in terms of weight, load, recharging time and range. In both cases, however, the electricity required must come from renewable sources. Otherwise decarbonisation will not work.

 

The problem is that there is less sunshine in winter, for example, so less solar power is produced. Is it possible to supply enough renewable electricity and green hydrogen for road transport throughout the year while still remaining economically competitive? PSI researchers Emanuele Moioli, Tilman Schildhauer and Hossein Madi have now shown how this might be achieved. Their concept is based on a clever combination of electricity generation and biogas production.

 

Two-step conversion

 

The concept proposes storing surpluses when more renewable electricity is available than required – in the summer and sometimes also in spring and autumn – and making these available to transport systems in the winter when electricity from renewable sources is scarce. This is to be achieved via a number of conversion and re-conversion steps.

 

First, the surplus electricity is used to electrolyse water, splitting it into its molecular components, hydrogen and oxygen. The surplus electrical energy is stored in a converted form in the hydrogen. In a further step, two synthetic fuels are then produced, methane gas (CH₄) and liquid methanol (CH₃OH). “These are the molecules we use to store the energy, and they play a central part in the concept,” says Emanuele Moioli from the Laboratory for Sustainable Energy Carriers and Processes at the PSI Center for Energy and Environmental Sciences.

 

The reason why the hydrogen itself is not stored is that hydrogen cannot be liquefied at ambient temperatures, and even in compressed form it still has a large specific volume. This makes it difficult to store and transport. Also, the necessary infrastructure for this is lacking. “It is much better to convert the hydrogen further into methane or methanol, because they have a much higher energy density than hydrogen. This significantly reduces the amount of space needed, making both storage and transport much simpler.”

 

Biogas plants as a third sector

 

However, producing the molecules that store this energy requires another reagent: carbon dioxide. In the decarbonisation concept presented, biogas plants are used as a source of CO₂. CO₂ is produced here as a waste product, when biogas is processed, making it very cheap. It can be used directly to produce methane and methanol. According to Moioli, this will in fact be done on site. In addition to the bioreactors, in which biomass is fermented into biogas, additional tanks will be installed in which the captured carbon dioxide and the previously produced hydrogen will be mixed and converted into either methane or methanol. So much for the storage side.

 

In winter, when renewable electricity is in short supply, the second part of the concept comes into effect. The two synthetic fuels are transported to a centrally located electricity/H₂ fuelling station. The liquid methanol will be transported there by lorry, the gaseous methane ideally via existing natural gas pipelines. “This is often possible because biogas plants already produce methane and feed it into the grid,” says Moioli. Alternatively, the methane could be compressed and transported by rail in gas cylinders.

 

At the electricity/H₂ fuelling station, the stored energy is converted back into hydrogen by steam reforming, i.e. recovering the hydrogen from the methane. During the chemical process, the hydrogen is separated from the methane or methanol and is immediately available for “refuelling” hydrogen-powered vehicles. The further conversion back into electricity for the electric vehicles also takes place on site through the combustion of the methane in a gas turbine and the methanol in a fuel cell. In view of their individual characteristics, the two molecules would be used differently. Methanol is more suitable for producing H₂, because steam reforming is more efficient with methanol than with methane. Methane, on the other hand, is more suitable for electricity generation because of its higher efficiency during combustion.

 

Narrowing the cost gap

 

But does decarbonising transport along these lines make economic sense? To answer this question, the authors of the study analysed publicly available data from the Werdhölzi biogas plant in Zurich and calculated the size of the proposed fuel synthesis station that would be required, as well as the capital costs associated with building such a plant. Moioli summarises the results of their calculations: providing electricity and hydrogen from stored methanol or methane, as proposed in the study, is more expensive than today’s petrol. But by carefully combining electricity, gas and infrastructure, the costs of decarbonising transport can be reduced to a level at which even “moderate incentives”, such as CO₂ certificates, can make it cost-competitive with fossil fuels. And the cost calculation did not even take into account the reduction in greenhouse gas emissions. Moioli points out that carbon capture and the long-term storage of carbon dioxide can actually result in negative carbon emissions.

 

The proposed approach is therefore viable. However, the complete decarbonisation of a sector such as mobility will require very large amounts of renewable electricity. The quantities produced today are nowhere near sufficient. According to Moioli, there are several obstacles to increasing the supply: “The imbalance between demand and supply limits the direct use of renewable electricity and hydrogen, especially for transport. This can be seen in the field of photovoltaics. Installing additional PV systems means that the new systems will have fewer customers. Because in summer, when solar power is available in large quantities, the existing systems already cover the demand.” This is slowing down the expansion of PV which, according to Moioli, means that many more energy storage systems need to be installed. A significant increase in the storage capacity would speed up the expansion of photovoltaic systems – thereby creating the basis for the climate-friendly transport of the future.

 

Text: Andreas Lorenz-Meyer

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