Chapter 4: Transport
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 (Figure 4.1), it also places unique demands on the energy vectors deployed. In this chapter we assess the potential for liquid air in transport by first defining the essential characteristics of transport energy vectors; analysing the requirements of different market segments or modes of transport; describing various ways liquid air could be used; and finally mapping liquid air technologies against the transport energy landscape. We have restricted the discussion to surface transport since the energy density of liquid air is too low to be relevant to aviation.
Figure 4.1: UK energy consumption by end use, 2011. Source: DECC1
1. Why transport is different
The self evident but fundamentally defining fact about vehicles is that they are mobile. In most circumstances – with the exception of electrified trains and trams – this means they must be able to operate while disconnected from their source of energy. This in turn means that unlike static equipment – a fridge or a production line, for example – vehicles must carry ‘batches’ of energy on board, and must periodically stop 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.
The need for vehicles to operate untethered from their source of energy, and to be able to cope with mission variation, means the onboard batches of energy must be sufficiently energy dense to give the vehicle a range adequate to its role. It also means the refuelling network must be commensurate to the likely mission variation of the vehicle.
The capability of a vehicle deploying such a batch energy system is limited by the practical considerations of loading another batch of energy. These include safety and ergonomic factors, but the time taken to recharge the onboard energy store is critical to deciding the usefulness of an energy vector to any given transport application. We list the critical attributes of an onboard energy vector associated with mission variation overleaf.
Critical attributes of an onboard energy vector
The onboard energy store must be able to hold sufficient energy without compromising the vehicle’s ability to do its job. That means the weight of the energy vector required to achieve the necessary range must not unduly limit the vehicle’s capability to carry its load. Any increase in the weight of a vehicle will, all other things being equal, increase its energy and power requirements, so a vector with low energy density can have an adverse effect on range, payload and general flexibility. Similarly, the volume of fuel must not unduly limit the vehicle’s ability to accommodate its load. The key units to express energy density are MJ/kg and MJ/m3.
Power density and the efficiency of energy conversion
A vehicle must also be able to convert the stored fuel into propulsion - or other activities such as hoisting or digging - at a sufficient rate to do its job, and at an efficiency that makes commercial and legislative sense. This is partly a characteristic of the engine – a wide range of power densities can be achieved in diesel engines, for example, through variations in design and capacity – but the characteristics of the energy vector should allow sufficient energy conversion rates to give a suitable range of power output.
Energy is stored as chemical energy. There are few practical limits on how rapidly the fuel can be pumped from the tank, so the only factors that constrain the rate at which that energy is converted into torque are the characteristics of the engine itself. By contrast, for battery electric vehicles, the power requirements of the vehicle are a fundamental influence not only on the capacity but also the design of the battery. In general, there is a need to fit the powertrain into as small a space as possible to maximise the room for passengers or goods. Any increase in the weight of the engine will either reduce the payload the vehicle can transport or increase the weight of the vehicle. If the vehicle’s weight is increased, its power and energy requirements will rise, as will its embedded CO2 and cost. The key unit to express power density are kW/kg. Efficiency is usually expressed as a percentage.
Current expectations for the passenger car market are that refuelling can be carried out in a few minutes. For other applications - such as electric buses, for example - where there is a predictable duty cycle with significant idle periods, longer charge times may be acceptable in exchange for significant perceived benefits. In any event, for an energy vector to succeed, its recharging times must match the expectations of the market.
Mission variation implies that the timing and location of recharging is generally unpredictable. The extent to which this is true depends on the vehicle sector, but in general a significant network of recharging stations is required to enable the full range of missions.
2. Variation in the transport sector
While the general requirements for energy vectors used in vehicles are distinct from those of other energy consuming equipment, and apply broadly across the sector, there is considerable variation in the needs of different modes of transport. Clearly the energy needs of a passenger ferry are quite different from those of a motor scooter, but it is important to understand precisely why. The reason for such contrasts – and for many more subtle distinctions – lies in the fact that each of the criteria described above is more or less important to different types of vehicle depending on their function. We discuss the reasons for these differences overleaf, and then provide a short characterisation of the energy requirements of the most important categories of vehicle.
Why the demands on transport energy vectors differ between vehicle types
Energy and power
The most fundamental requirement for a vehicle’s energy storage system is the total quantity of energy that must be stored on board; this is determined largely by another fundamental factor, the rate at which energy must be supplied to the engine – in other words, its power rating. Clearly, larger and heavier vehicles such as ships, trains and trucks tend to require more power in operation than smaller ones such as cars and scooters, but other factors such as speed are also important. For example the locomotive for a heavy freight train typically requires 2.5MW of power, the equivalent of around 20 family cars, yet a modern intercity express train requires around 5MW, despite being shorter and lighter; the high power requirement stems from the need to accelerate rapidly up to a high speed and the large air resistance that must be overcome at this speed.
Distance between refuelling stops
A second fundamental factor that determines the quantity of energy to be stored on board is the distance over which it must be supplied between refuelling stops. In general, long journeys require more energy than short ones, so vehicles that operate over long ranges tend to need to carry larger quantities of energy. As range and energy storage requirements increase, storing energy becomes increasingly critical in terms of its impact on the vehicle’s overall mass and its available space to carry goods or passengers. For example, a heavy goods vehicle must travel long distances to deliver its payload across continents, but is limited by law in its total weight and in its size. To minimise unproductive time en route the truck must refuel as infrequently as possible, but the amount of fuel carried must be traded off against the payload. Therefore it is absolutely critical for productivity to maximise the ‘energy density’ of its on board energy storage, and an energy vector with a low energy density will be significantly less attractive in terms of operating costs than the incumbent - diesel fuel. In contrast, for a similar truck covering much smaller distances between deliveries, more time is spent loading and unloading and the range between refuelling stops is far less critical; here, alternative, less energy-dense storage media such as compressed natural gas may pose no significant disadvantage compared to diesel.
Refuelling infrastructure cost
Another critical factor affecting the relative attractiveness of different energy storage media is the complexity and cost of refuelling infrastructure, which is mainly determined by the number of different locations at which a vehicle may need to refuel. The most difficult operating patterns to support are those where transport routes extend over long distances (eg long-range heavy goods haulage), where they are highly variable (eg general-use private passenger car), and where vehicle range is limited (eg in the case of low-density energy storage). Infrastructure requirements are less demanding where vehicles operate within a relatively restricted zone. Examples include small cars in an urban area; specialist vehicles on a single site such as an airport or a warehouse; or where vehicles such as buses or delivery vans return to a limited number of depots to refuel. Clearly where an existing infrastructure exists - as with petrol and diesel for use on the roads, or existing electrified rail routes – it may confer an advantage to incumbent technologies against those for which a new infrastructure must be established.
This is another factor that differentiates potential energy storage media, with different transport applications having different degrees of sensitivity. Incumbent fossil fuels have the advantage of very rapid rates of energy transfer of several MW during refuelling at a filling station pump. Refuelling rate - and therefore time - is least critical for vehicles that spend significant downtime at dedicated facilities for loading and unloading such as commercial shipping, which may refuel simultaneously while goods and passengers are transferred. Depot-based commercial vehicles may also spend significant time standing overnight, during which refuelling can potentially be carried out over several hours without impacting their operating schedules. Private vehicles also tend to spend significant amounts of time during the day at rest and if the appropriate infrastructure is available then these too may be tolerant of low recharging rates. In all cases where vehicles may need to break a journey to refuel, however, a slow refuelling rate is likely to be prohibitively unattractive.
National and international regulations that limit pollutant emissions are now driving up the cost and complexity of ICE vehicles across virtually all transport sectors. These may be supplemented by more stringent local regulations designed to improve air quality in target zones such as dense urban or conservation areas, which in some cases explicitly encourage or even restrict vehicle use to ‘zero emissions’ technologies. Beyond regulatory control of vehicle emissions, some more specialist applications may, by their nature, favour low or zero emission technologies, such as those operating in confined spaces including warehouses or mines.
The typical energy characteristics of key vehicle types
A large container ship typically travels long distances over long periods of time at a continuous, high power level. Efficient power delivery is therefore essential to minimise the inevitably significant energy costs involved and for the vehicle to remain competitive. In order to maximise productivity from each ship, cargo volume must be maximised, and because the quantity of energy that must be stored on board is large relative to the size of the vessel - fuel may be greater than 2% of cargo volume - volumetric energy density is an important factor. Refuelling is carried out at the vessel’s dedicated berth while cargo is loaded and unloaded, so the rate at which this happens is relatively uncritical. Regulated exhaust emissions limits are comparatively lax in mid-ocean, but Emissions Control Areas on many coasts are much more stringent. There may also be additional pressure for still lower emissions within busy and densely packed port environments.
A small passenger ferry has a very different operating profile to a large container ship. Voyage lengths are typically much shorter, although several trips may be undertaken between refuellings, with more variable speed and power levels. The fuel volume to be carried is relatively small, so energy density may be less important than in other transport applications, although this is offset by the need to maximise the space available for passenger accommodation and facilities. Refuelling typically takes place outside normal operating hours, so the refuelling rate is only moderately important. Since ferries often operate close to population centres there is more pressure to minimise exhaust emissions than for long-range cargo ships.
The widespread existence of electrified tracks in regions such as western Europe means many trains have no need for onboard energy storage, at least for primary propulsion. However, there remain many routes worldwide that are not electrified and for which trains must carry their own energy supply. Where new routes are being considered, the high cost of electrical supply infrastructure makes on-board energy storage potentially attractive. Refuelling can be carried out at a relatively limited number of depots, so the infrastructure is not as extensive as for most road transport applications. Energy storage density is not a particularly strong factor for locomotives, since there is some flexibility in the size of the vehicle. However, where propulsion units are integrated within passenger cars, as with Diesel Multiple Units (DMUs), then the space available on board is more limited, especially in the case of retrofitted technology.
Apart from these shared factors, operating patterns and requirements for different classes of rail vehicle vary widely. High speed passenger trains operate over long distances at the continuous and high power levels needed to overcome resistance forces. Refuelling rate becomes critical if trains cannot carry enough energy to operate continuously throughout a working day, as long refuelling breaks constrain scheduling and reduce asset utilisation. Freight locomotives must deliver high power levels for limited periods to accelerate away from halts and to overcome adverse gradients, but once the maximum speed has been reached and on downhill sections, as well as for long periods during loading and unloading, the power demand is very low. Local commuter trains feature highly transient operation, with frequent stops and associated acceleration and deceleration. Trains are not heavily loaded and maximum speeds are relatively low, however, so maximum power levels, required range and energy storage capacity are moderate. Because they operate in densely populated areas - and on some routes also underground - there may be additional local pressure to reduce exhaust emissions below otherwise regulated levels.
Long haul trucks
The energy storage needs of long distance, heavy duty haulage trucks are some of the most severe of any transport application. These vehicles typically operate point-to-point between distribution centres outside urban areas, with a single national or international return trip often taking several days. They spend a large proportion of time at close to their maximum speed and at high power levels, covering long distances over highly flexible routes with heavy or bulky loads, with tight constraints on their maximum size and weight. Energy conversion efficiency and energy storage density are highly critical to productivity. Refuelling must be performed relatively frequently, in many cases several times per trip, so refuelling rate also has a significant impact on productivity. Refuelling may sometimes be performed at base depots, but in general it is necessary for the refuelling infrastructure to be comprehensive across the entire route network. Exhaust emissions are tightly regulated, and technology solutions to achieve the regulated limits represent a significant up-front cost, as well as impacting on operating costs and range.
Medium duty trucks
Medium duty delivery trucks typically operate on inter-urban distribution routes, with a mix of urban and highway operation. They are likely to operate from depots on fixed routes. The requirements for trucks of this type share some similarities with those of heavy duty haulage vehicles, although the reduced range and power levels and the larger number of stops mean the demands on power, energy storage density and refuelling rate are less severe. Urban operation places higher priorities on compact vehicle dimensions, low noise and exhaust emissions, however.
Urban buses tend to operate under very different conditions to most heavy or medium duty trucks. They run almost exclusively within urban centres at low speeds, with very frequent stops and associated acceleration and decelerations. As a result, the distances covered in a single day are small and operation from a single depot means that no distributed refuelling infrastructure is required. Although acceleration and hill climbing often require full engine power, maximum loads are relatively low compared with heavy goods vehicles and, since passenger numbers often vary widely during a working day, average loads are lower still. This combination of factors means that there are not particularly strong requirements for high energy storage density or rapid refuelling. Buses are high profile contributors to local air quality and noise issues, so quiet operation and low exhaust emissions are high priorities.
Light duty commercial vehicles
Vans perform a variety of different functions, but many operate mainly in urban areas, for example delivering goods street-to-street, with limited extra-urban collection trips. As with buses, average loads and required daily ranges for these vehicles are not high, so the required levels of energy storage capacity and energy density are moderate. Again, like buses, there may be an imperative for low exhaust emissions within cities, particularly for large fleets operated by local authorities or high profile companies. Where vehicles operate from depots, refuelling can be performed at a single location outside core operating hours.
The uses to which cars are put varies widely, and it is useful to distinguish between two broad market segments:
Larger cars in segments D or E tend to be used for longer trips, often on motorways at relatively high and constant speeds. Driving range between refuelling stops is important, although ranges of well over 1,000km that can now be achieved by modern diesels may actually be greater than the market requires.
The unpredictable and potentially unlimited length of journeys undertaken means that refuelling infrastructure must be widely available with a high density of refuelling points to satisfy market demand. Refuelling must be performed flexibly and usually mid-journey, so a short refuelling time is important. Exhaust emissions for passenger cars - as for all light duty vehicles - are some of the most tightly regulated in transport, although there is unlikely to be pressure for zero emissions on long distance, cross-country routes for the foreseeable future.
Small cars in segments A or B are most frequently used for short journeys within urban areas, and long range capability is not a dominating requirement. The short distance of most journeys travelled means that energy vectors without an extensive refuelling infrastructure could still be viable especially in urban areas. Private cars are typically left parked for long periods, so charging overnight or at public parking locations may be feasible. Small cars’ emissions are as stringently regulated as those of larger cars, and in urban areas zero emissions capability may be attractive.
The definition of personal mobility vehicles includes small two-wheelers such as scooters and potential future ultra-light and highly compact vehicles for one or two people. This segment is expected to grow in future, particularly in developing markets and ‘mega-cities’. Such vehicles are intended to be used in congested urban centres only, so the top speed and driving range can be low. However, they must be highly compact to make them manoeuvrable, affordable and energy efficient. Thus energy storage requirements are very low but tight packaging space and the low vehicle weight mean good energy storage density remains important. Refuelling time may be important, but the vehicle’s low energy storage capacity means the refuelling rate is relatively uncritical. Exhaust emissions are currently mostly governed by two-wheeler regulations, but future growth in the market is likely to favour much lower or zero emissions capability to reduce air pollution in cities.
Other vehicle types
Other specialist vehicle types feature a combination of the requirements discussed above, as well as having some particular needs of their own. Some key requirements for selected specialist transport applications of interest to liquid air technology – such as forklift trucks - are covered in chapter 7, with a particular focus on refuelling infrastructure.
3. Uses of liquid air in transport
In the first decade of the 20th century there were a number of engine technologies competing to power the horseless carriage. Among these were steam, the internal combustion engine (ICE), battery electric, compressed air and liquid air. Figure 4.2 shows a liquid air vehicle in 1903.
Figure 4.2: Liquid air vehicle 1903
Since then the ICE has become the dominant technology for reasons which are many and complex, but which reduce to convenience and range. The ICE was quicker to start and cleaner to refuel than the steam engine, quicker to refuel than battery electric, and had superior range to electric, compressed air and liquid air.
Circumstances have changed radically over the intervening century, however, and are now very different from those which fostered the dominance of the ICE. Populations and cities have boomed, societies are more affluent and vehicle ownership has soared, leading to increasing pressure on transport infrastructure and the environment. Transport CO2 emissions represent around a fifth of the total, and in cities, local air pollution from transport is also a major issue, particularly in the developing world.
The urgent need to reduce both GHG emissions and local air pollution has increased the importance of developing low carbon and zero emissions energy vectors and some of the discarded technologies of the early 20th century may now be worth reviving. In particular, they may be suitable for light, urban vehicles with a range of about 100km. However, to be attractive in this market they would need to offer quick and easy refuelling and be competitively priced.
Characteristics and roles of liquid air in transport
Most common energy storage media such as batteries, capacitors or flywheels store ’positive’ energy; energy levels within the store rise as work done to them. For example, supplying electrical power to a lithium ion cell will cause the energy levels in the cell to increase. Liquid air is uncommon in that work supplied to the plant will see the internal energy of the stored element fall. Liquid air can therefore be described as a ‘negative’ energy store; energy is released when the liquid air is exposed to ambient temperatures (or higher), causing it to expand back to its natural gaseous state. In the context of thermodynamic cycles in transport, this enables liquid air to act both as a heat sink and working fluid. An additional attraction is that low grade waste heat of up to 100C, traditionally considered too low in temperature to be useful in power hungry transport applications, could be exploited to generate meaningful amounts of additional energy.
The potential roles for liquid air considered below are those where the air is used as a heat sink and then as a working fluid within a heat engine. The heat engine can be deployed as either the prime mover – the only or principal source of power in a vehicle – or in a supporting role to recover waste heat from a conventional engine or fuel cell (chapter 5). In this secondary role the liquid air device can either be used to produce shaft power to reduce the load on the primary engine, or to power auxiliary functions such as refrigeration.
Liquid air as a prime mover
One way to exploit liquid air in transport would be to increase the energy storage density of compressed air vehicles.
Expanding compressed air from an ambient temperature store creates drive. At 300bar air has a specific energy of 140Whr/kg or 0.5MJ/kg. The vehicle manufacturer MDI deploys such an approach to power lightweight urban vehicles of the type shown in Figure 4.3. This 320kg vehicle has a 300 litre tank and claims an urban range of 100km, which falls to 50km at higher speed.2
Figure 4.3: MDI compressed air car. Source: MDI3
If a vehicle of this type were to store the energy as liquid air, which has a specific energy of 214Whr/kg or 0.77MJ/kg, rather than compressed air, it would increase the specific energy density of the store by a factor of 1.52, increasing the urban range of the MDI vehicle to 152km. However, the density of liquid air is greater than compressed air, so for the same size of tank the energy content could increase by a factor of three, increasing the urban range to 300km.
The difficulty with increasing the range in this way would be to harvest sufficient heat from the environment to facilitate the pressurisation. Designing a heat exchanger capable of doing so would be challenging, but the large temperature difference between the liquid store and ambient – around 210C - would aid the process.
Liquid air directly into the engine
Using liquid air as the energy store for a compressed air engine as described in the previous section raises the energy density of the onboard store, which increases the range. However, if the compressed air store is at room temperature then the exhausted gas from the engine will be significantly colder, which reduces the work from the engine and creates practical issues with icing. These effects could be mitigated by designing an engine with multiple expansion stages interspersed with heat exchange, but this would add weight and reduce efficiency.
Another solution to the problem of temperature drop during expansion is to introduce further heat inside the expansion chamber. This would tend to make the expansion closer to an isothermal event (where temperature stays constant), so increasing the work output of the engine. Figure 4.4 shows the theoretical benefit of moving from the adiabatic case, where no further heat is added during the expansion, to the isothermal gas expansion case assuming 300K or 27C. The results are shown both with the impact of pumping work (dashed lines) and without (solid lines). At realistic working pressures this shows that an engine using an isothermal cycle could deliver as much as three times the work per kg of air as one using an adiabatic cycle. This means that compared to a compressed air engine with an air store at 300bar, a vehicle using this approach could produce around 7.5 times the work from the same size tank - depending on the maximum cylinder pressure and how close the cycle gets to isothermal expansion. This is because liquid air has around three times the volumetric energy density of compressed air, and the engine would use it 2.5 times more efficiently.
Figure 4.4: Work produced by adiabatic and isothermal expansion of 1kg of liquid air. Source: Ricardo
The engine being developed by the Dearman Engine Company and Ricardo, which uses this kind of heat exchange approach, should achieve a result close to isothermal expansion. The heat exchange is undertaken by passing a second Heat Exchange Fluid (HEF) into the expansion chamber, which mixes with the liquid air causing it to expand. The rapidity of the expansion defines the power density of the engine and the ultimate efficiency with which it extracts work from each kilogramme of liquid air. After each expansion cycle the heat exchange fluid is recovered from the exhaust and reheated to ambient temperature via a heat exchanger similar to a conventional radiator.
Figure 4.5: Thermodynamic cycle of the Dearman Engine. Source: Ricardo
The use of liquid air as a main energy source for a prime mover would require its storage in significant quantities on board a vehicle. Synergies with existing liquefied natural gas (LNG) technology for vehicles are likely to enable crossover solutions for onboard liquid air storage systems, and the growing popularity of LNG as a transport fuel will help to minimise costs.
Liquid air for heat recovery
The energy density of liquid air is relatively low when compared to diesel. As a result, it becomes increasingly inappropriate as the sole energy source as the power, range and mass of the vehicle increases, and the space and weight required for energy storage become more and more significant. The energy stored per kilogram of liquid air (0.77 MJ/kg) is around 56 times poorer than that of diesel fuel (43MJ/kg). Both fluids have a similar density, so this means that a liquid air storage tank must be 56 times larger than a diesel tank with the same energy content. In practical terms, however, the comparison is more favourable, because of the different way in which the energy is used in diesel and liquid air engines. In order to deliver the same amount of useful work, the quantity of liquid air required for an isothermal expansion engine, such as the Dearman engine, is reduced to around 20 times the equivalent quantity of fuel required for a diesel engine. As such, a truck with a fuel tank containing 350 litres of diesel fuel would still need a liquid air tank of more than 7,000 litres to deliver the same amount of useful work. Any operator opting for such a liquid air truck would therefore be faced with a far shorter range, much more frequent refuelling stops or a significant reduction in payload mass and volume.
These considerations make diesel the energy vector of choice for heavier duty vehicles. However, the heavy duty engines consume an enormous value of fuel per accounting period, and this makes operators highly focussed on reducing fuel bills. This translates into constant pressure on vehicle manufacturers to increase engine efficiency by measures such as improved combustion management, turbo-charging and exhaust heat recovery. Each technology will produce a small incremental improvement, but together they will aggregate to a substantial improvement with time.
However, it is possible that disruptive technologies could be introduced that would produce a step change in efficiency, as illustrated in Figure 4.6. These technologies could come about either from fundamental breakthroughs in our basic understanding, or by cross-fertilisation from other market sectors.
Figure 4.6: Incremental and disruptive technology innovation. Source: Ricardo
Heat recovery - intra-cycle: the liquid nitrogen split cycle engine
One example of a crossover technology is the liquid nitrogen split cycle engine proposed by Ricardo (US patent 20120103314), which borrows elements from a technology developed for static power generation to raise the efficiency of the internal combustion engine (ICE).
The efficiency of a simple gas turbine is generally less than a reciprocating engine but it can be increased by the use of an exhaust gas recuperator. The relatively low compression ratio of a simple turbine gives rise to a relatively low gas temperature at the end of compression as well as a high exhaust gas temperature. This situation can be used to drive heat transfer from the exhaust, in order to raise the temperature of the compressed air through a heat exchanger or ‘recuperator’ before combustion. This reduces the amount of fuel required to achieve the same output.
Figure 4.7: Recuperated Gas Turbine. Source: Ricardo
By contrast, the ICE derives its efficiency from having a high compression ratio, and this means temperature of the compressed air is too close to that of the exhaust to allow effective heat transfer; waste heat from the exhaust cannot be recovered in this way. In addition, piston-ICE designs almost always compress and expand the air in the same cylinder, making it impractical to introduce a recuperator into the system.
This problem can be overcome using a split cycle engine design similar to the Isoengine concept first developed by Ricardo in the 1990s and the Scuderi engine today. In such designs, compression takes place in one cylinder and expansion in another, which is similar in concept to a gas turbine. However, to make the split cycle thermodynamically efficient requires isothermal compression, in which the air remains at a relatively constant temperature despite being compressed. The temperature-entropy diagram in Figure 4.8 shows how isothermal compression allows the temperature difference between the compressed intake air and the exhaust gas to be maximised, so creating an opportunity for waste heat recovery.
Figure 4.8: Recuperated diesel cycle with high compression ratio and isothermal compression. Source: Ricardo
Isothermal compression can be achieved by spraying a fluid into the compression chamber to absorb heat from the gas being compressed, an approach that was tested in the 3MW Isoengine power generation demonstrator using water.4 However, although this produced a large demonstrable gain, raising gas to electricity conversion efficiency to 59%, it also required large quantities of water, because of the small temperature difference between it and the air being compressed, along with complex and expensive water management equipment.
The Ricardo split cycle invention replaces water with liquid nitrogen which is far colder at about -200C, meaning that far smaller volumes are required. In addition, once vaporised during compression the nitrogen can then pass straight through the combustor and be exhausted to the atmosphere. As a result, the system can be made far more compact and suitable for vehicle engines.
Detailed modelling of this approach undertaken through the Technology Strategy Board-funded ‘CoolR’ project has suggested that a thermal efficiency of more than 60% is possible. As a result a modest onboard tank of liquid nitrogen would extend the range of the vehicle by increasing the efficiency of the primary engine. Liquid nitrogen could also be produced by an onboard liquefier driven by the engine and boosted by regenerative braking.
Heat recovery – auxiliary plant
The liquid air split cycle engine uses liquid air or nitrogen to recover waste heat intra-cycle – within the design of a single engine – but it is also possible to recover waste heat using a secondary unit to absorb heat from the cooling loop or exhaust of the prime mover. The ICE remains a flexible, highly evolved and low cost technology, but as fuel prices rise and emissions legislation becomes ever tighter, such secondary heat recovery approaches look increasingly interesting.
Technologies currently being developed to absorb ICE exhaust heat include organic Rankine cycle (ORC), turbo compounding, thermo-electric generation (TEG) and fuel reformation. However, a secondary cryogenic engine such as the Dearman Engine could also perform this role, and could offer major advantages because of the ultra-low starting temperature of the working fluid.
This secondary heat-engine would be implemented alongside the main internal-combustion engine, either as a simple tandem device that delivers power in proportion to the main engine’s heat output, or as a hybrid device that uses the thermal inertia of a cooling circuit to offer a power peak-lopping capability. The second approach allows the main ICE to operate more steadily and closer to its ideal efficiency - like an electric hybrid - and thus reduces air-quality emissions ‘spikes’ that can arise in transient operation, or the efficiency compromises that result from avoiding them.
Figure 4.9 shows the theoretical Carnot efficiencies of an ORC device and liquid air for converting heat into power at a range of source temperatures. At a typical exhaust temperature of 400C, the maximum theoretical ORC efficiency is 56% whereas the liquid air efficiency is 89%. In addition, as the exhaust temperature is not fixed but dependent on load, the efficiency variation due to exhaust temperature excursion is lower for the liquid air device, making the heat recovery process less sensitive to vehicle transients.
Figure 4.9: Carnot efficiency of liquid air and ORC. Source: Ricardo
The higher Carnot efficiencies of a liquid air cycle mean that unlike the other heat recovery technologies mentioned above, it could also recover meaningful amounts of heat from ICE cooling circuits as well as exhaust streams. At the typical coolant temperature of around 90C, systems that use ambient air or water as the heat sink would have a maximum theoretical efficiency of 20%, whereas for the liquid air approach this would be 79%, which is more than the other technologies achieve with a source temperature of 400C. The operating temperatures of automotive hydrogen fuel cells range from 60-80C, which could also be exploited using liquid air, as we explore in more detail in chapter 5.
It should be stressed that the efficiencies quoted here are Carnot efficiencies and represent the maximum theoretical efficiencies that could be achieved at the given temperatures. Only a fraction of these translate into vehicle fuel savings because of the transient nature of engine performance and the complexities of integration; analysis by the Dearman Engine Company suggests that a practical Dearman engine could achieve a real-world thermal efficiency of around 50% using coolant to warm the HEF.
Evidence suggests some of the conventional heat recovery methods cited earlier could deliver hydrocarbon fuel efficiency savings from 5% to 15%. Liquid air, because of its higher temperature differential and the lower impact of transients, could deliver bigger reductions of up to 25% in the case of a city bus. Against these benefits, an additional tank would need to be installed on the vehicle to hold the working fluid, which is consumed and not recycled. Any comparative assessment would also need to consider the purchase and operating costs of each technology.
Liquid air may also be interesting for vehicles such as buses and refrigerated lorries (see next section) that need to maintain cooling while stationary. In buses for example, it is possible to increase the volume of coolant to create a store of waste heat from the main engine, which could then power a small liquid air engine to keep the air conditioning going during stops. This would allow the prime mover to be fitted with start-stop technology to give significant fuel savings on urban routes.
Conventional refrigeration equipment in trucks, trains and ships generally consists of a small diesel engine to drive the compressor of a closed cooling circuit, which is used to cool air for circulation in the cargo or passenger space. Such systems are typically inefficient, noisy and require regular maintenance. This inefficiency, combined with growing global demand, means transport cooling and refrigeration is a significant and expanding source of carbon emissions and there is increasing pressure on operators to develop alternative approaches.
Figure 4.10: Direct refrigeration by liquid nitrogen. Source: natureFridge5
A number of industrial gas companies such as Linde, Air Liquide and other market participants such as EcoFridge, have already developed systems that use liquid nitrogen as a heat sink to provide refrigeration in food transport.6 These systems either pass liquid nitrogen through a heat exchanger where it vaporises to absorb heat indirectly, or spray liquid nitrogen directly into the goods compartment. The second method has the advantage of being about a third more efficient, but means oxygen monitors and other safety equipment must be installed to prevent the operator entering the compartment until the atmosphere is breathable. Neither approach recovers any shaft power from the evaporation process, but it is no great leap to imagine that if substantial quantities of a cryogenic fluid such as liquid air were carried on board a vehicle as fuel for a cryogenic engine, the cold exhaust could also be used to keep goods or passengers cool.
Refrigeration by direct spraying
The simplest approach to cooling with liquid nitrogen or air is to spray it directly into the goods or passenger space (obviously nitrogen cannot be used in passenger transport). A heat exchanger vaporises the liquid air or nitrogen using ambient heat, driving it under pressure out of nozzles at the top of the space to be cooled. The cold gas sinks naturally, circulating air within the space and displacing warmer air to the environment. Such a system requires very few moving parts, is lightweight and operates silently, as well as being inexpensive. This approach displaces existing warm air and dissipates very effectively, and so can cool a volume of air rapidly.
In food transport, if liquid nitrogen is used rather than liquid air, there is the additional benefit of reduced spoilage since the produce is surrounded by an inert gas. However, this requires additional safety equipment to ensure the atmosphere in the goods compartment is breathable before the operator opens the doors to load or unload.
Systems of this type are already on the market using liquid nitrogen stored in tanks on board. For a large haulage truck with a fully refrigerated trailer, liquid nitrogen consumption is claimed to be 20-40 litres per hour, depending on ambient conditions.7
Refrigeration in combination with liquid air engine
A more complex but more efficient approach to cooling using onboard liquid air is in combination with a liquid air engine, which could be either the vehicle’s prime mover or an auxiliary unit. In this approach, waste air from the engine is pumped into the space to be cooled. The waste air is considerably colder than the ambient air, so a similar cooling effect is achieved as with the direct spray approach. Since the liquid air has already been expanded in the engine to generate mechanical power, the additional consumption of cryogenic liquid would be small and the cooling effect achieved very efficiently. Such a system has been proposed by the Dearman Engine Company, and the potential fuel and carbon emissions savings of this approach are explored in chapter 10. If the cryogenic fuel were liquid nitrogen, in passenger transport the cooling system would have to be indirect.
Depending on the system design, further efficiency gains may also be available if the engine takes its intake air from the cooled compartment as well. This would allow further heat to be extracted from it, reducing the cooling energy required and thus minimising the consumption of liquid air.
Refrigerated food transport is a significant and growing source of carbon emissions. In the EU there are about 650,000 refrigerated road vehicles in use primarily for food distribution, of which about 8% or 52,000 are in the UK. In the UK, food transport – including motive power and refrigeration – accounts for 1.8% of total emissions.8 The potential emissions reductions that could be achieved by using liquid air or nitrogen in refrigerated food transport are explored in chapter 10.
4. Mapping liquid air to vehicle types
Having discussed the energy requirements of a wide range of vehicle types, and explored the characteristics of liquid air or nitrogen, we are now able to compare the two. In this section we assess the relative attraction for the main vehicle types of the main potential applications of liquid air - as prime mover, heat recovery device and refrigeration unit.
Liquid air as a prime mover
In order to evaluate the suitability of liquid air for use as the main energy source for propulsion in different vehicle types, the energy storage requirements of those vehicles are summarised by key criteria:
- Energy and power density sensitivity: how important is it for each application that energy storage and conversion technology is compact and/or lightweight?
- Infrastructure: how extensive must refuelling infrastructure be for each vehicle type to enable them to operate with the reach and flexibility required?
- Refuelling rate sensitivity: how important is it that refuelling can be carried out quickly?
- Emissions sensitivity: how highly is low- or zero-emissions capability valued in each application?
The relative importance of each of these criteria is shown qualitatively in Table 4.1, and the ability of liquid air to serve those requirements is shown in Table 4.2.
Table 4.1: Relative importance of energy storage criteria to various vehicle types
For prime mover engines, the key characteristics of liquid air or nitrogen that influence its attractiveness as a main energy source in different vehicle types can be summarised as:
- A relatively high consumption of liquid air - around twenty times the mass of ‘fuel’ per unit of useful work delivered compared to an internal combustion engine, which limits its attraction in vehicles that are used intensively.
- Zero emissions at the point of use except for air or nitrogen, which is attractive not only in environmentally sensitive outdoor environments, but also useful for indoor operation.
- A relatively low capital cost due to a lack of exotic materials in the heat engine or cryogenic tank, which means that the system is more attractive where the application is used less intensively or at low power levels.
- Currently no accessible refuelling infrastructure network, especially for long range road transport applications, although (as discussed in chapter 7) the establishment of a refuelling supply chain for applications such as on-site and return-to-base applications may be relatively straightforward.
- Potential for similarly rapid refuelling rates to hydrocarbon fuels, in contrast to some competing technologies – see discussion below.
These characteristics are captured in Table 4.2.
Table 4.2: The ability of liquid air to satisfy key energy storage criteria
The scores in Tables 4.1 and 4.2 can be combined to assess how well the strengths and weaknesses of liquid air match the energy storage requirements of various vehicle types, and the results are presented in Table 4.3. The costs associated with liquid air energy conversion technologies and refuelling infrastructure are considered in other chapters of this paper. A full consideration of application-specific system costs is beyond the scope of this study and is in any case difficult to quantify robustly at the current state of technology development. However, we assume the costs of liquid air technologies could become competitive against competing technologies with future development.
Table 4.3: How well does liquid air match the energy storage requirements of various vehicle types?
In general, liquid air is most attractive as a main energy vector for prime movers in applications that do not require high levels of energy density; do not require an extensive distribution network for refuelling; value high refuelling rates; and value zero-emissions at the point of use. For applications where liquid air is competitive with other energy storage options, its attractiveness may be increased if secondary benefits such as its use for refrigeration are considered important. On this basis, liquid air stands out as relatively attractive in several urban and/or limited range applications. In particular, buses match the profile of strengths of liquid air well on most counts. Vans and to a lesser extent medium duty commercial vehicles also rank relatively well, as do small cars and personal mobility vehicles - assuming that they are used almost exclusively within a city and that zero emissions capability is valued there. For shipping, rail and long-range passenger car applications, the lack of a strong requirement for zero emissions at the point of use means that there is little to compensate for liquid air’s relatively poor energy storage density and distribution infrastructure challenges. For heavy duty haulage prime movers, liquid air is uncompetitive with incumbent energy vectors in almost every respect.
This analysis has examined the attractiveness of liquid air as a prime mover fuel against the primary requirements for transport energy in mainstream applications. However, there are other, more specialist vehicle types that have their own additional, application-specific requirements, for which liquid air propulsion may be particularly attractive. On this basis both fork-lift trucks used indoors and underground mining applications appear to be attractive for liquid air power. Both value the zero emission output of a liquid-air prime mover and the safety benefit of a non-flammable fuel, whilst having relatively undemanding infrastructure requirements through their operation on a single site.
The simple evaluation performed here gives an indication of the types of applications that may be more suited to the use of liquid air as an energy vector judged against a small number of fundamental criteria. A number of other factors are not taken into account that may affect feasibility but which are difficult to assess for generic concepts. These include: system durability and reliability (real or perceived), relative fuel prices, purchase or usage incentives, operability under different ambient conditions, technology cost, maintenance requirements and total cost of ownership. Likewise, safety and reliability are not considered for mainstream applications, since it is a given that vehicles must meet the requirements of the markets in which they operate. Another factor not examined here is the ready availability of ambient heat to facilitate continued vaporisation of liquid air within a prime mover. This potentially poses spatial challenges in vehicle design, but at this stage it is not possible to assess how critical these issues might be for each application. However, this issue might make the use of liquid air as a prime mover relatively more attractive in marine applications, since heat exchange with the sea is likely to be more effective than with air.
Liquid air vs competing low carbon prime mover energy sources
The success of liquid air as a main prime mover energy source would of course depend on the strength of its merits against other competing energy vectors. The key competitors, on the basis of good performance and a similar potential for low CO2 emissions and zero emissions at point of use, are:
- hydrogen - stored as compressed gas, as a cryogenic liquid or by adsorption.
- batteries - using any chemistry with high energy density, such as Lithium-Ion.
- compressed air.
We have discounted ultracapacitors (electrostatic storage devices), flywheels and other mechanical energy storage devices, all of which can offer excellent power densities (i.e. high rates of charge/discharge) but have energy densities too low to be competitive for primary energy storage for vehicle propulsion, as shown in Figure 4.11.
Figure 4.11: Power and energy densities of selected energy storage technologies Source: Ricardo
Compressed hydrogen at 700 bar using today’s best available technologies is currently relatively costly but has an energy density around twice that of liquid air. For this reason it would represent liquid air’s strongest competitor in the transport applications with more severe range, weight and space requirements – for example in the marine and rail applications covered here, as well as many of the road vehicle applications. Which technology might have more success would depend on several factors that are currently unknown, such as relative technology costs, raw material prices - to which a hydrogen fuel cell prime mover would be far more sensitive than a liquid air engine - and the prevailing refuelling infrastructure. In chapter 7 we discuss the extensive industrial and commercial distribution network that exists for liquid nitrogen, which does not exist for hydrogen. The low score for liquid air infrastructure in Table 4.2 reflects the fact no publicly accessible network of refuelling stations for liquid air or nitrogen currently exists, which would be a critical factor for usage by non-depot-based vehicles.
The nature of cost pressures on specific applications would also play an important role in any competition between hydrogen and liquid air. For example, for a heavy freight locomotive a hydrogen prime mover would be more expensive than a liquid air engine, but liquid air would require the vehicle to be made larger to cope with its relatively low energy density, so it is not yet clear which option would involve the higher incremental cost.
Batteries represent the most mature energy storage technology for zero emissions today, with many vehicles in production on the open market. Improvements in cost, energy density and longevity are set to continue and will improve their competitiveness during the time it would take liquid air technologies to come to market. Innovative models for battery ownership and reuse may also help to reduce costs. However, since battery energy density is currently not significantly better than for liquid air, it appears that the two technologies could well compete head to head.
Battery electric power is likely to be most attractive in situations where vehicles can be readily charged overnight or between shifts (eg private ownership or limited daytime business use), and in locations where there is easy access to charging infrastructure. The controllability and flexibility of electric power on board vehicles is likely to make BEVs a more attractive choice for applications that benefit from significant kinetic energy recovery (eg through frequent acceleration and braking) or that make significant use of onboard auxiliary equipment, such as utility trucks. For buses, inductive charging whilst stationary at stops also may prove to be a way to minimise battery size and cost that makes battery energy storage much more attractive in this application.
Nevertheless, a number of factors give liquid air an advantage over battery technology. First, batteries remain expensive, while liquid air appears to have the potential to deliver much lower costs per unit of energy stored. Second, the cost uncertainty caused by electric powertrains’ dependence on valuable raw materials makes alternative vectors such as liquid air that avoid this vulnerability appear particularly attractive. A further related issue is the relatively high levels of ‘embedded’ CO2 associated with current battery and electric drive technology; while little analysis has been performed to date of the likely embedded emissions impact of liquid air technology, the lack of exotic raw materials or complex processing needed for their manufacture suggests that liquid air-based powertrains of the future may have the edge over electric powertrains in this respect (see chapter 10).
Compressed air, as explained earlier in this chapter, may be seen as a simpler and cheaper alternative to liquid air, but its performance in terms of energy storage is inferior. On this basis, we conclude it is best suited to vehicles for which the required driving range is very low and for which there is a strong emphasis on low cost. This might include personal mobility vehicles and small cars for very restricted urban use, as well as some specialist industrial vehicles for indoor use, such as materials handling.
For the applications with the greatest need for high energy density, such as long-distance road haulage, it is highly unlikely that any of the competing zero-emission technologies would be feasible versus the incumbent hydrocarbon fuels such as diesel or natural gas. In these applications, however, liquid air is at its most attractive as an enabler for waste heat recovery – as discussed below.
Liquid air in heat recovery
Apart from their potential use as prime movers in transport, liquid air engines can also be employed to reduce vehicles’ fossil fuel consumption through waste heat recovery. The characteristic that makes liquid air technology attractive here is its ability to use ambient or low grade waste heat, which is particularly useful in harvesting heat from internal combustion engine or fuel cell cooling systems, whose heat quality is typically too low (~100C) for more conventional heat recovery devices.
There are three main ways in which this can be achieved:
- Through the use of a split cycle internal combustion engine to reduce the fuel consumption required for propulsion.
- Through the use of a secondary liquid air engine to supplement the motive power of an internal combustion engine, and possibly allow it to operate more steadily and at higher efficiency.
- Through the use of a secondary liquid air engine to provide power for auxiliary loads, driven by waste heat from the main engine.
The first option is most attractive in applications for which fuel costs make up a significant proportion of total operating costs, and where the engine operates at high power levels for much of the time, such as heavy duty road haulage trucks and container ships. Here the extreme pressure on vehicle productivity and therefore energy storage density make it unlikely that liquid air or other zero emission energy vectors could compete with the incumbent hydrocarbon fuels. However, if liquid air is generated on board then minimal storage capacity is required, and this can be used to recapture energy from the exhaust stream and reduce the quantity of primary fuel needed to drive the engine. Depending on the cost and space within the vehicle, this system could also be applicable more widely, perhaps including rail locomotives, other commercial vehicles or even larger passenger cars.
The first option might employ onboard liquefaction, but the second does not, and therefore becomes attractive in return-to-base vehicles such as buses and urban delivery trucks. Here the daily duty cycle may mean that use of liquid air or nitrogen as a sole prime mover fuel is impractical, but it could be attractive as a supplementary heat-harvesting fuel.
Under the third option, the attractiveness of liquid air for providing auxiliary power depends on two factors:
- a requirement for onboard auxiliary power in significant quantities relative to the power required for propulsion.
- a need for onboard auxiliary power for significant proportion of the time when the vehicle is at rest.
The greater the importance of these factors to a particular vehicle, the more valuable will be a dedicated, high-efficiency, quiet and clean-running auxiliary power unit, such as a liquid air system. Vehicles that satisfy these criteria include passenger ferries, passenger trains and buses, as shown in Table 4.4.
Table 4.4: Vehicles’ suitability for liquid air/liquid nitrogen auxiliary power
Liquid nitrogen is already used in commercial vehicles to provide continuous, efficient and silent refrigeration, particularly for those operating at night. However, at present the nitrogen is used only to provide cooling and no power is extracted from its evaporation. In any vehicle applications that would benefit from the use of liquid air as a prime mover fuel or for heat recovery, therefore, its additional use for refrigeration would represent a significant synergy with additional savings in complexity, weight and cost. Transport applications to which this could apply include all those carrying perishable goods or requiring extensive air conditioning for passenger accommodation, particularly in hot ambient conditions. So examples could include passenger ferries and cruise ships, freight trains, buses and all types of refrigerated haulage or delivery truck, with all likely to enjoy the greatest benefits in hot climates. However, the practicalities of supplying liquid air directly to each compartment that requires cooling are likely to make this most attractive for relatively compact applications such as road transport.
Liquid air is potentially attractive as an energy vector for future low CO2 transport through the benefits it offers in terms of its zero emissions at point of use, energy and power density on a level with battery electric technology and its potential for high refuelling rates.
- A number of transport applications exist for which these benefits are valued, and for which the use of liquid air as a prime mover energy vector could be feasible.
- Key mainstream applications of interest include urban buses, small city cars and personal mobility vehicles, urban delivery vehicles and short-range marine craft.
- Further specialist applications for operation in enclosed spaces may also be a good match with liquid air’s characteristics, such as indoor forklift trucks and mining vehicles.
- Beyond the liquid air’s use as a primary fuel for propulsion, it has the potential to improve the energy consumption of internal combustion engines by capturing waste heat, either through ‘heat hybrid’ designs or split cycle engines with high thermal efficiencies.
- The use of liquid air on board vehicles in any of the above forms also gives rise to further potential synergies, such as through the implementation of simple, efficient and quiet refrigeration or air conditioning systems.
- Compared to the key alternative energy vectors for low CO2 transport – hydrogen, batteries and compressed air – liquid air appears to offer a balance of characteristics that could make it competitive and worthy of further development.
Digest of United Kingdom energy statistics (DUKES) 2012,
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