Transport of people and goods is generally considered as a distinct category within the wider energy debate. Not only is it a significant, identifiable economic bloc responsible for over a third of all energy consumed in the UK, it also places unique demands on the energy vectors deployed. Imposing the additional requirement to reduce greenhouse gas emissions increases the difficulty of satisfying all those demands within a single vector, and this is reflected in the fact that progress on biofuels, electric vehicles (EVs) and hydrogen fuel cell vehicles (FCVs) is proving far slower than expected or required by policymakers.
The self-evident but fundamentally defining fact about vehicles is that they are mobile, and usually need to be able to operate while disconnected from their source of energy. This in turn means they must carry ‘batches’ of energy on board and stop periodically to refuel. Most vehicles must also be able to cope with ‘mission variation’, since each trip may vary by destination, route, duration, speed and payload. This defines a series of factors against which transport energy vectors and technologies must be judged: energy density; power density; refuelling rate; and refuelling infrastructure. The relative importance of these factors varies between transport modes, and our detailed analysis reveals that liquid air could provide an attractive low carbon energy vector in several vehicle types and functions:
• Prime mover: a cryogenic engine such as the Dearman Engine produces zero emissions at the point of use; has low greenhouse gas emissions provided the liquid air or nitrogen is produced from low carbon electricity; has energy and power density on a level with battery electric technology; and has the potential for rapid refuelling. This makes it potentially attractive as a ‘prime mover’ (main engine) for use in small cars and vans for short range urban use, scooters, short range marine craft, forklift trucks and mining equipment.
• Heat hybrid: a cryogenic engine such as the Dearman Engine could also be used as a ‘heat hybrid’ in combination with an internal combustion engine or hydrogen fuel cell (see next section), to convert waste heat into additional shaft power at high levels of efficiency, reducing both fuel consumption and emissions. This approach would be viable in passenger ferries, commuter trains, heavy duty trucks and urban buses, and could also deliver ‘free’ cooling for passengers or goods.
• High efficiency internal combustion engine: heat recovery could also be achieved using the Ricardo split cycle engine, a novel internal combustion engine design that incorporates liquid nitrogen to capture exhaust heat and increase fuel efficiency. Detailed modelling of this approach undertaken through the Technology Strategy Board-funded ‘CoolR’ project has suggested that efficiencies of more than 60% are possible, compared to around 40% for modern diesel engines. Nitrogen could be supplied from a modest-sized onboard tank or an onboard liquefier driven by the engine and boosted by regenerative braking. This approach would be suitable for heavy duty trucks and container ships, and potentially rail locomotives, other commercial vehicles and even larger passenger cars.
• Refrigerated food transport: some food delivery vehicles already use liquid nitrogen as a heat sink to provide refrigeration, which cuts noise, complexity and carbon dioxide emissions substantially compared to conventional diesel powered refrigeration. However, current systems fail to capture any additional shaft power from the nitrogen evaporation process. We calculate that a vehicle food refrigeration system using liquid nitrogen or liquid air to provide both additional shaft power and cooling would cut emissions from 47 tonnes per lorry per year (diesel refrigeration) to 10 tonnes, a reduction of almost 80% on the basis of current grid average electricity (chapter 10 of the Full Report). The same approach could also provide refrigeration or air conditioning for passenger ferries, cruise ships, freight trains and buses, with greatest benefits in hot climates.