The results in this paper suggest that electricity is the most
efficient method of supplying transport fuel in the future. Where
energy dense fuel is required for other applications such as longdistance driving or for heavy-duty transport such as trucks, then
hydrogen is the most efficient way to supply these vehicles. However, in the short term, based on the production costs only,
hydrogen is an expensive way to supply this energy dense fuel.
These costs are likely to be even more significant when additional
costs relating to hydrogen are taken into account, such as hydrogen
vehicles and its infrastructure are correct. Therefore, it is likely that
some form of gaseous or liquid based fuel will be necessary to
supplement electricity in a future 100% renewable energy system.
According the results in this study, the most attractive option at
present is liquid fuel in the form of methanol/DME. Producing
methanol/DME is more efficient than methane and it is anticipated
that the cost of adjusting existing infrastructure to methanol/DME
is relatively low. Therefore, methanol/DME is recommended when
completing an energy systems analysis of a 100% renewable energy
system. In either case, this distinction is not as critical as it may
seem: both the methanol/DME and methane pathways share a lot
of technologies in their pathways so the key message for the short
term is that these technologies should be developed further before
a final fuel is pursued. Most significantly, these technologies are
biomass gasification and electrolysers.
Two distinct methods of producing this liquid/gaseous fuel have
been presented here: one where existing bioenergy resources are
boosted (i.e. bioenergy hydrogenation and fermentation) and one
where CO2 is combined with hydrogen to produce a synthetic
methanol/DME fuel (i.e. CO2Hydro and co-electrolysis). The cost
estimates in this paper suggest that the final mix between all of
these fuels will fundamentally depend on the amount of ‘affordable’
bioenergy available (i.e. the price of the bioenergy resource
compared to the production of electricity). If bioenergy resources
are limited like they are expected to be in future 100% renewable
energy systems, then in the early stages the bioenergy resource can
be boosted using biomass hydrogenation. Once the bioenergy
resource has been utilised, or in other words once the price of
electricity production becomes cheaper than bioenergy, then the
final demand could be supplied using CO2Hydro and/or coelectrolysis, which do not require any bioenergy input. In this way,
the fuel required for transport can be met using a mix of electrification, bioenergy hydrogenation, and CO2Hydro/co-electrolysis,
without consuming an unsustainable level of bioenergy resources.
The cost estimates developed in this study suggest that by 2050,
electrification and bioenergy hydrogenation fuels will have cheaper
production costs than oil, while CO2Hydro/co-electrolysis will be
20% more expensive. Considering the risks and uncertainties related
to oil, this seems relatively low. Based on these results, the following
conclusions have been made from this paper:
Direct electrification should be promoted as much as possible.
For example, electric vehicles, urban electric rail, and intercity
high-speed electric rail should be facilitated to move both passengers and freight from road to rail transport.
Biomass gasification is a key technological bottleneck in the
bioenergy hydrogenation pathway (as well as for other sectors
in the energy system). The development of this technology
should be prioritised in the short term.
Electrolysers are key technologies which need further development to produce energy dense liquid or gaseous fuels,
particularly solid oxide electrolysers.
In the longer term, carbon capture will become important,
especially once the bioenergy resource begins to reach its limit.
Therefore, the development of biomass gasification and electrolysers should be prioritised for now, but carbon capture is
also a key technology in the future.
The results in this paper are significant since they provide a first
comparison between a variety of different transport pathways,
which illustrates the type of key technologies that will be necessary
in the future. The detailed energy flows constructed for each
pathway enable these fuels to be considered in energy systems
analysis, which is crucial due to the lifetime of energy technologies
and the key role that fuel storage could play when integrating
intermittent renewable energy [88]. Finally, by providing a detailed
breakdown of the energy and cost assumptions in each key step of
the pathways created here, the results provide a starting point for
the development of more detailed and refined pathways in the
future, as well as a useful tool for modelling the transport sector in
future renewable energy systems.