A technical, economic, and environmental assessment of Power-to-X applications for heavy-duty transport

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Date
2023
Authors
Gray, Nathan
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University College Cork
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Abstract
Deep decarbonisation of the global energy sector is essential in order to avoid the worst impacts of anthropogenic climate change. Finding more sustainable ways to transport both people and goods is key to meeting ambitious emissions targets. The low energy density of current battery technology presents significant challenges for use in long-distance, heavy-duty transport. The Power-to-X concept converts electricity to hydrogen via electrolysis. The hydrogen can either be used directly or used as a feedstock, in conjunction with a source of CO2, for the production of synthetic hydrocarbons. Power-to-X systems have been suggested as a novel pathway to indirectly electrify modes of transport where direct electrification is not technically feasible but there is still uncertainty surrounding it’s economic and environmental sustainability. This thesis examines the use of Power-to-X fuels in road haulage, shipping, and aviation from a technical, economic, and environmental perspective. Integrating Power-to-X fuel production with biogas facilities can increase the gross energy of the system by between 32% and 63%, depending on the synthetic fuel produced. However, the large energy inputs required for electrolysis means that the net energy returns of the integrated system are lower than the biogas facility in isolation. Optimising the anaerobic digestion process to maximise biogas methane content and consuming electricity that would otherwise have been curtailed are key to improving the energy return of the integrated system. For the road haulage sector, battery electric vehicles are technically feasible, and offer lower total lifecycle costs than the diesel incumbent despite higher vehicle capital costs. However, the low-energy density of battery technology means that they are limited in range to 450 km. Hydrogen fuel cell vehicles offer zero tailpipe emissions and a greater range than battery electric vehicles but come with increased costs. High fuel costs combined with low powertrain efficiencies mean that synthetic methane and synthetic diesel are not economically competitive for land transport, increasing total costs by up to 280%. The use of batteries in the shipping sector is highly challenging, with the use of energy dense liquid fuels are most applicable. There is a trade-off between vessel range and its cargo capacity due to the lower energy density of Power-to-X fuels such as methane, methanol, ammonia, and hydrogen. To maintain the range of a vessel powered by traditional marine fuels, the cargo capacity of an alternatively fuelled vessel would decrease by between 3% and 16%. Furthermore, the use of Power-to-X fuels increases total costs by between 124% and 731%, depending on the fuel. Fuels which can be stored as a liquid, such as methanol, result in lower payload losses and lower costs than fuels which require complex storage systems, such as methane or hydrogen. As aviation is often classed as the most difficult mode of transport to decarbonise, offsetting continued use of fossil jet fuel using direct air capture technology was considered. It was found that using direct air capture technology to offset emissions has a lower cost of abatement than using a synthetic jet fuel. Additionally, the use of direct air capture technology to offset emissions consumes half as much energy and 10 times less electricity than producing Power-to-X jet fuel. However, offsetting the continued use of fossil fuels has broader sustainability issues. The majority of lifecycle emissions of Power-to-X systems is associated with the electricity consumed in the process. Due to the inefficiencies of electrolysis and fuel synthesis, the carbon footprint of hydrogen or synthetic fuels will always be higher than the electricity used to produce them. For this reason, utilising electricity directly wherever possible, such as in battery electric heavy goods vehicles, will result in the lowest well-to-wheel greenhouse gas emissions. For the sustainable production of hydrogen or synthetic fuels, electricity with a carbon footprint of below 40 gCO2e/kWh is required. This implies that direct connection to a source of renewable electricity is required. The results presented in this thesis provide a comprehensive overview of the use of Power-to-X fuels in heavy-duty transport. The results contained within provide stakeholders with the requisite knowledge to help accelerate the uptake of these fuels.
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Keywords
Electrofuels , Power-to-X , Hydrogen , Heavy transport , Heavy goods vehicles , Shipping , Aviation , Lifecycle assessment , Total cost of ownership , Renewable fuels
Citation
Gray, N. 2023. A technical, economic, and environmental assessment of Power-to-X applications for heavy-duty transport. PhD Thesis, University College Cork.
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