Solutions for passenger ‘green’ cars with market maturity and infrastructure availability. Click to enlarge.

The most important source of decarbonized energy in 2050 will come from electricity produced from renewable sources like wind, solar and hydroelectric and used in plug-in vehicles, the report notes. Surplus electricity generated from renewables could be stored in batteries or could be converted via power-to-gas technology into synthetic methane (SNG), liquid fuels or hydrogen.

Liquid Air
Among the carriers explored in the report was liquid air. (Earlier post.) Liquid air, the report notes, is an adaptable energy vector which can be created and consumed using traditional mechanical engineering technologies, stored safely in un-pressurized containers, and made from a free abundant raw material. It can be used in many applications to improve or replace existing transport solutions and deployed at electricity grid scale.
Air—and its primary constituent nitrogen—is 700 times denser than ambient air when in liquid form and of comparable density to diesel.
Liquid Air technically does not store any energy, as energy has been removed to create it. As such it is singular in the family of potential energy vectors as it provides an energy sink rather than energy source, the report notes. Returning liquid air to ambient temperatures and pressure will absorb 0.77 MJ or 213 Wh/kg, competitive with high energy density Li-ion batteries. The supply temperature of -169°C is a key enabler for harvesting low grade waste heat sources (around 100°C) such as combustion engine cooling systems.
Liquid Air in the energy and transport systems - The Centre for Low Carbon Futures May 2013. Click to enlarge.
Use of liquid air in propulsion systems has progressed from simple external heating to direct injection of the cryogenic liquid into a piston engine such as that developed by the Dearman Engine Company. Research work on split cycle concepts mixing both internal combustion and liquid air injection has also shown the potential for thermal efficiencies of around 60%.
Zero emission applications as a primary source is certainly of interest in urban scenarios for light duty, short range applications, the report notes. For heavy duty applications, the energy density of liquid air is inappropriate for use as a primary fuel but it may provide opportunities for more efficient and cost effective waste heat recovery from internal combustion engines.

Specific major high-level findings from the assessment are:

• Biofuels based on biomass have the potential to substitute between 15 and 30% of fossil fuels, due to the sustainable availability on biomass; i.e., overall potential is limited by the availability of sustainable biomass.

• Replacing more than 20% of fossil energy with new biogenic fuels will require direct CO₂ recycling, without the production of biomass on agricultural land. Technologies based on ‘CO₂ + Sunlight’ to fuel are under research and offer a huge potential which should be exploited.

• The most economical biofuels today and in future seem to be ethanol. Similar pathways to butane are more compatible to vehicle and infrastructure.

• For gasoline use, blend rates of alcohols will increase. For diesel use, drop-in components (e.g. ‘Hydrotreated vegetable oils’ (HVO), BtL diesel and sugar-to-diesel technologies) will be important.

• Due to technological barriers and backward compatibility in the vehicles and the infrastructure 1st generation Fatty acid methyl ester (FAME / biodiesel) are limited to 7% (vol) biodiesel contend in diesel fuel and ethanol up to 10% (vol). Additionally the sustainable European availability of oil plants is exploited at this level.

• In the overall economic assessment costs for fuel production and costs for possible new infrastructure and new vehicles have to be taken into account. In this respect drop-in renewable fuels offer substantial benefit.

• Extending biogenous diesel components pathways to drop-in fuels is a focus (e.g. HVO, sugar to diesel, BtL). The feedstock will be based on residuals.

• Captured fleets offer the potential to bring higher blends of biofuels (like e.g. E20 and B7) into the market.

• Methanol and DME have to potential to be a cost efficient way to become fuels in dedicated fleets of HD transports, busses and ships as well as, for methanol, as a blend component in gasoline in captured fleets.

• Natural gas offers the possibility to overcome the blend wall discussion. No matter where the molecule methane (CH₄) comes from (biomethane or power-to-gas methane), it can be injected into the network or liquefied and used in all CNG and LNG vehicles in any volume without limitation.

• In today’s powertrains, up to 25% CO₂ emissions can be saved by the use of natural gas compared to gasoline. Until 2030, the market share of new natural gas vehicles may increase towards 10%; a European-wide refueling infrastructure is essential in order to achieve this level. For long haul heavy-duty truck and corridor related applications, methane is expected to be an option as liquefied natural gas (LNG) on the TEN-T network.

• LPG on the other hand is a dead-end fuel in terms of research needs.

• For gaseous fuels, there is no blending restriction on the use of biomethane; a second source for decarbonized methane is from power-to-gas technology to synthetic methane, also fully interoperable with existing natural gas infrastructures, refueling and vehicle technologies.

• Green electricity is the very promising feedstock for energy carries for mobility in the future. The electricity might be used in batteries or converted to chemical energy carriers (e.g. power-to-gas methane, hydrogen or liquids).

Resources

• Energy Carriers for Powertrains