Chapter 7: Infrastructure
The emergence of a liquid air or ‘nitrogen economy’, in which cryogenic liquids are widely used as an energy vector in transport and small-scale electricity generation, would require an extensive distribution network. It is one of the strengths of liquid air compared to some other potential energy vectors that this requirement is already broadly satisfied; thousands of tonnes of liquid oxygen and nitrogen are already distributed across the country every day. In this chapter we describe the existing cryogenic liquids distribution network and assess its ability to support an emerging liquid air economy. We find that the existing distribution system has ample capacity in the short to medium term.
1. Existing distribution infrastructure
The industrial gases industry supplies customers across the UK in a number of ways, the choice being determined by technical factors such as volume, pressure, flow rate and purity:
- Cylinders are normally used when customers need small volumes of gas at high pressure. Cylinders can be delivered by the supplier or picked up by the customer from some 600 outlets around the country.
- Liquid deliveries (Bulk Gas Supply) are normally used when customers require between 0.25 and 5 tonnes per day. A vacuum insulated storage tank is installed at the customer’s site, which is filled by road tanker as required. The customer can also opt for a network-connected level measure on the tank which automatically re-orders product as necessary.
- When the customer needs to use the product as a gas, the liquid is pumped through a vaporiser where it is brought up to ambient temperature and delivered at the required flow rate. At present the energy released by this expansion is not exploited, but in principle it could be used to drive a small cryogenic motor such as the Dearman Engine to generate ‘free’ electricity. We estimate there are around 1,100 larger industrial gas users with tanks of 15 tonnes or bigger that could benefit from this idea.
- Liquid supplies (Mini-Bulk Supply) are suitable for customers who require small amounts of cryogenic liquid such has hospitals and laboratories. A small tank of 200 – 1,000 litres is installed on site, and refilled from small delivery vehicles with a maximum capacity of 5 tonnes.
- Onsite or pipeline supply is used by customers who need oxygen and/or nitrogen in volumes ranging from 5 – 2,000tpd. This can be supplied either by pipeline from the gas company’s site to the customer, or produced by a plant installed at the customer’s site; all the gas suppliers have extensive portfolios of production plants to meet any combination of flows and pressures. Ideally the gas companies would prefer to build a production facility on their own site and send gas by pipeline to the customer, because this allows them to co-produce liquid products to distribute to other customers by road tanker (the ‘merchant’ market).
All the industrial gas production sites listed in chapter 6 and Figure 7.1 on page 84 will keep 2-4 days liquid storage. The actual volume will be set based on the liquid back-up required by the pipeline (gas) customers, and the number of liquid customers in the catchment area. Some sites may have additional storage left over from a previous pipeline contract. The risk to customers in the event of a plant failure is minimised by the fact that:
- The distance between the various production sites is such that in an emergency customers can be supplied from a site outside their optimum location.
- Gas companies can pick up product from a competitor’s site.
- In a medical or safety related emergency a gas company will supply a competitor customer on a short term basis.
In some cases gas companies will rationalise their production facilities and replace any shortfall by sourcing from a competitor’s site as a permanent arrangement. This practice is well established in the petroleum and diesel industry where, for example, all fuel supplied into the south and south west of England is likely to have been supplied from the Esso refinery at Fawley.
The level of storage on production sites is highly unlikely to be sufficient to provide liquid nitrogen for regular use as an energy vector. The supply chain is very short for liquids, and in almost all cases the product is essential for the customer’s business. In chemical plants it is also vital for safety purposes. If the decision were taken to install liquid air or liquid nitrogen power generation equipment at an industrial gases production site additional storage capacity would be required.
There are 5,500-6,000 storage tanks on customer sites, with capacities ranging 3 to 60 tonnes. The size of a customer storage tank is determined by a number of factors such as daily usage, distance from production sites, vehicle access, safety and the level of product security required.
These tanks are serviced by a fleet of about 400 tankers with carrying capacities of 5-22 tonnes, which is the maximum payload that can be delivered under current UK road regulations. The maximum delivery distance from a production site will be in the region of 100 miles with the average being in the range 20-50 miles. There are a few exceptions to this, one example being a delivery from Fawley to the east side of London. Typically a driver will make two to five deliveries in a shift. Most of the deliveries are done on a two shift system, night deliveries being restricted by customer access hours and in some cases noise. The logistics, driver and transport engineering standards used by the gas companies are almost identical to those used in the petroleum industry. The transport and logistics part of the industrial gases business has, in common with the production facilities, an excellent safety record (chapter 9).
The population of storage tanks is serviced by a network of regionally based project and service engineers whose main roles include site surveys, installation, maintenance and responding to customer emergencies. The installation work undertaken by these teams includes applications and end use equipment, electrics and control systems as well as the storage tanks. All the gas companies have centrally based ‘tank farms’ (where customer storage tanks are refurbished) and a stock of tanks to enable them to respond quickly to customer requirements.
The current network of production locations, distribution centres, vehicle fleets and customer service engineers provides good geographical coverage and could certainly be used and adapted to support the introduction of liquid nitrogen or liquid air as an energy vector.
Figure 7.1 shows the location of the major UK industrial gas production sites and LNG terminals where liquid air could be produced extremely cheaply by exploiting the cold from LNG regasification (see chapter 3 section 3). Each site is marked with a 50 and 100 mile radius to indicate its potential delivery catchment area. The map also shows the location of major logistics warehouses, haulage depots and supermarket distribution centres, which could be among the earliest bulk users of liquid air in transport, particularly for refrigerated food distribution. It is clearly shown that almost all such potential users fall within the catchment areas of one or more existing or potential liquid air production sites.
The industrial gas companies have an estimated surplus of 8,500 tonnes per day of gaseous nitrogen available for liquefaction, which, for illustration, could fuel the equivalent of 6.5 million car kilometers daily.1 If all the cold available from British LNG import terminals were used to assist air liquefaction, it could produce enough cryogen to fuel the equivalent of a further 45million daily car kilometres, equivalent to 4.2% of the daily distance driven by cars in Great Britain in 2011.2
Figure 7.1: Potential liquid air suppliers and users
2. Future transport fuel infrastructure requirements
Liquid air or liquid nitrogen could be used as a “fuel” in a number of transport or mobile applications, as explored in chapter 4. The characteristics of liquid air and the heat engines that run on it determine the most suitable transport applications for the technology, and these in turn determine the refuelling/infrastructure requirements. To date there has been very little deep analysis of the economics of distribution of liquid air or nitrogen as a fuel, although its distribution for process industries is well known and described in section 1 above. For this reason we have developed a qualitative narrative based on lessons from the distribution of process-industry liquid nitrogen, and from other new fuels such as hydrogen and liquefied natural gas (LNG).
Liquid air applications and refuelling models
The most attractive applications for liquid air in transport are defined by the characteristics of liquid air heat-engines and their fuel, as explored in chapter 4. These factors govern the type of vehicles that might use liquid air or nitrogen as either a primary fuel or secondary fuel, and hence their refuelling needs:
- On-site applications: as we found in chapter 4, potential on-site applications could include prime-movers (the main engine) for fork-lift trucks and other industrial equipment; mining equipment where safety is an important consideration; and short-range marine craft.
- Return-to-base applications: chapter 4 identified a number of these, either as prime movers, or as waste heat recovery devices that increase the efficiency of conventionally powered vehicles, especially where a cooling load can be integrated into the cycle. Analysis by the Dearman Engine Company and E4tech has shown that buses with air-conditioning and refrigerated delivery lorries are the most promising initial applications.
- Long-haul applications: In this category, the relatively high fuel consumption of the liquid air heat engine limits its use. It is more likely that the cryogen would be used in an advanced internal-combustion concept such as the Ricardo split-cycle engine, applied to long-haul trucks, locomotives or shipping (chapter 4).
Each type of application has its own fuelling needs, which are explored below.
On-site industrial applications
Zero-emission prime movers such as fork-lift trucks would require a supply of cryogen to a refuelling point, either by regular delivery or on-site production. To place the fuelling requirement in context, initial estimates by Dearman and E4tech suggest that an indoor fork-lift of the type commonly powered by battery today, of 20kW peak power and 5kW mean load factor, would use around 120 tonnes of liquid nitrogen per year if operated for 12 hours per day and five days per week. However, the same fork-lift would use just 10 tonnes per year (200kg/week) if used for around an hour a day. The options for meeting this range of needs for a small fleet of equipment are:
- Regular delivery: Industrial equipment is likely to be found on industrial estates where there is already a regular delivery of liquid nitrogen for purposes such as engineering and food processing. It would appear logical that these cryogen-fuelled prime movers would be fuelled by extension of the local liquid nitrogen supply, as this yields economy of scale and requires a minimum of new equipment or training. However, the existing high-value industrial processes may use relatively small quantities of liquid nitrogen, in which case the market tolerates a much higher price-point, typically 10p/kg or more. Robust viability as a ‘fuel’ in high utilisation applications requires a lower price, of 5p/kg or less, which is only realised in the highest volumes of supply. To put this in context, a single, low utilisation vehicle requiring a 200kg weekly delivery offers a fuel value of just £10-20 per delivery with this price range. The challenge for the cryogen supplier is to secure a profitable balance between increased market volume and potentially lower price per unit of liquid nitrogen. However, at a site with five or more forklift trucks, a small liquid air or nitrogen tank would be economic, and would fit exactly with the operating model of industrial gas suppliers. If a cryogenic engine is required for safety reasons – such as indoor operation – then it would be less sensitive to the price of liquid air.
- On-site production: Hydrogen fuel is often produced on-site, so it is worth considering the case for doing the same with liquid air or nitrogen. However, liquid nitrogen can be transported as a liquid at higher temperatures than hydrogen; is a larger molecule and therefore less prone to leak and presents no fire or explosion hazard. It is, therefore, easier to transport - apart from the fact that more energy is required to deliver each unit of energy - and benefits from a mature distribution network covering the industrialised world. Taken together these factors start to erode the case for small-scale, on-site production of liquid air or nitrogen.
Air and nitrogen are also harder to liquefy at small scale because of the characteristics of the Joule-Thompson Cycle. This process has an ideal coefficient of performance - heat extracted per unit of work input – of just 34%, and in practice this number is challenging to approach. This efficiency tends to fall even further at lower production volumes due to the inefficiency of small turbo-compressors. By contrast, the ideal efficiency is easier to approach at large scale, and in this type of centralised plant synergies with hot and cold fluid streams in other industrial processes can be realised to further improve efficiency, as can energy storage or buffering principles (chapter 2). Although liquefaction equipment is available in almost any size, existing markets have tended to focus on liquefiers that produce either:
• Very small amounts of cryogen - a few kilogrammes per day for a laboratory, for instance - and in these quantities the efficiency of the process is irrelevant and has not been the subject of much development; or,
• Larger quantities of cryogen – 100 to 1,000 tonnes per day, for example, for industrial applications - where the plant is most efficient at the upper end of this range, and may require around 30% more energy per unit of cryogen at the lower end.
There is currently a lack of mature technology for highly efficient localised production in volumes suitable for small fleets of vehicles, of the order of 1 tonne/day. A contemporary unit of this size has an electric energy consumption of around 1.6kWh per kg of liquid nitrogen produced3, whereas the largest plants (1,000 tonnes/day) consume just 0.4kWh/kg. For this reason, it is likely that delivery from a central production site will remain the preferred option for liquid air and nitrogen – in contrast to hydrogen. For orders of 1 tonne/day the industrial gas companies will always want to ensure that a liquid delivery is the most economic way to supply and will price accordingly.
On-site mining applications
Mining applications would differ significantly from the light industrial requirement described on previous pages because:
- The power and utilisation levels would both be higher, indicating a higher fuel demand;
- The asphyxiation risk presented by using large quantities of liquid nitrogen underground means mining equipment powered by a cryogenic engine would almost certainly run on liquid air;
- For mining in remote locations, tanker supply of liquid cryogen may be challenging and expensive. Some large mines in Africa and South America already have their own air separation units, but no mines in the UK do.
These factors may suggest a preference for local manufacture of liquid air, which could also serve as a two-way buffer for the mine’s main supply of electricity, thus relieving the peak loads on either grid supply or local generation. The economics of such a proposition would be very much application-dependent and have not been the subject of published study.
Return-to-base vehicles such as city buses and goods delivery vehicles are seen as an attractive target for new fuels or energy vectors. This is mainly because they are able to use a single refuelling point at or near their operational base. However, another important factor is that these vehicles tend to have high public visibility and are often operated or licensed either by city authorities with an environmental agenda, or by companies with a desire to meet social responsibility targets and promote a ‘green’ image. These factors combine to improve the business case and have been the subject of much study for hydrogen and electricity as transport fuels.
One promising application described in chapter 4 is the combination of an ICE and liquid air engine for heat recovery. Some initial analysis performed by Dearman and E4tech based on a city bus has indicated a 12-hour shift would consume around 200-300kg of liquid nitrogen, around 4-6 times the diesel requirement by weight. This indicates a daily requirement of 2-5 tonnes of liquid nitrogen for a small fleet of ten vehicles, depending on whether day-only or 24-hour operation is assumed and 20-50 tonnes for a 100-vehicle fleet. The upper extreme of these volumes is approaching the range in which mature industrial liquefaction technologies are available. Dearman estimates that a 100 tonne/day plant would have a capital cost of around £5m, compared for illustration to around £10m for 100 standard buses and £5m annual diesel costs (100 hybrid buses would cost around £25m). At this level of demand local production could be justified, especially if it could be matched to local grid management needs.
However, this level of demand could equally well be supplied by a cryogenic tanker making one or more daily deliveries. Centralised liquid air or nitrogen production will always be more efficient than local production and the industrial gases companies would prefer to install storage tanks at bus depots and supply them by tanker. Operationally this would not be difficult, since they have fleets of vehicles of different sizes which can operate in inner cities, and frequently deliver at night to avoid congestion.
Heavy duty, long-haul applications such as trucks, diesel locomotives and ships tend to make the best business case for exhaust heat recovery, due to the high and relatively constant engine load factors imposed by their duty cycles. In these applications a supplementary heat-engine such as the Dearman Engine can be effective at saving fuel, but the weight and cost of cryogen may weaken the case unless there is a specific need for refrigeration.
An alternative is the ‘split cycle’ engine proposed by Ricardo (chapters 2 and 4). This concept uses the cryogen to reduce the air compression losses of an internal combustion engine as a form of intercooling and allows internal recuperation of exhaust heat. The consumption of cryogen in weight terms is similar to that of diesel, so carrying cryogen on board and refuelling would be logistically possible. However, the concept can also generate its own cryogen on board via a small engine-driven liquefier. This device gives a theoretical efficiency gain in the steady-state, but is over-driven during deceleration as a form of regenerative braking for greater system efficiency gain. The technology is too immature to assess the relative benefits of onboard liquefaction and external refuelling, but the quantities involved would require cryogen delivery logistics on a similar scale to those of truck diesel fuel. The benefit of this doubling of fuel delivery logistics would be a substantial reduction in fossil fuel use – Ricardo claims that up to 30% is possible. Again, there has not yet been any detailed study of the economics of cryogen supply for this application.
A final factor that could prove relevant in long-haul applications is the resurgent interest in liquefied natural gas (LNG) as a fuel. A number of factors are driving this:
- From the supply side, rising crude oil prices have coincided with the prospect of relatively cheap shale gas - already a reality in the US.
- On the demand side, the rising popularity of diesel cars in Europe and the requirement to remove sulphur from marine fuels have led to pressure on middle distillates, while growing awareness and potential regulation of environmental issues such as land-use change limits the likelihood of a substantial increase in bio-diesel use.
- Adaptation of a heavy duty engine to use LNG is relatively low in risk and cost compared to other alternatives and yields an immediate reduction in CO2 emissions.
LNG is relevant because, like liquid air or nitrogen, it is a low temperature liquid. This means that:
- Some synergy in distribution and fuel stations may be possible, at least in terms of a shared need for insulation and maintenance of low temperatures; boil-off of liquid air or nitrogen - which is harmless if vented to an open atmosphere - can be used to maintain the more damaging LNG at low temperature.
- Similar synergies (insulation, boil-off) can be exploited in the vehicle’s fuel tanks, while the evaporation of LNG supplied to the engine can be used either to keep liquid nitrogen cool or assist onboard liquefaction.
LNG is currently a niche fuel, with most vehicles having converted ‘dual fuel’ diesel engines which retain the use of diesel injection both as an ignition source and as a backup fuel supply should LNG be unavailable. However, engine manufacturers are known to be working on dedicated LNG engines in the truck sector, and products are already available for marine propulsion. So the synergies described above could become relevant, especially in the 2020-30 timeframe as demand for LNG transport fuel rises.
Prospects for liquid air fuelling infrastructure
The discussion above has explored the refuelling options for liquid air and nitrogen in a number of potential scenarios. It is clear that the future of transport fuels itself will inevitably become more complex, with the traditional choices of gasoline, diesel and kerosene being supplemented or displaced by biofuels, natural gas, hydrogen and electricity. In this context, it is unlikely that liquid air will dominate or displace these, but there is a substantial case to be made for it finding a place alongside them in the right applications.
Building on the analysis presented in chapter 4 Table 7.1 provides an assessment of the potential for liquid air under four main scenarios, based on the characteristics of each transport application and the potential refuelling infrastructure. The colour of the cells indicates the relative risk presented by each factor, with green and indicating the lowest risk and orange the highest.
Table 7.1: Comparison of several liquid air transport applications by technology and refuelling options
This exercise exposes some interesting points. First the immaturity of liquid air engine technologies generally presents a greater risk to success than the availability of fuel, because the favoured applications in the table do not require a widespread new fuelling infrastructure (those applications were selected because they were suited to liquid-air engines, not because they required no new infrastructure). Second, the existing industrial cryogenic gas distribution network is a promising starting-point for at least two types of application and three specific vehicle types - fork-lifts, refrigerated trucks (see also chapter 10), and buses - and between them these might represent a sufficient market to stimulate further technology development. And third, every application sees other benefits from a move to cryogenic fuels: fast, safe refuelling of fork-lifts; fire-safe fuels; the ability of the cryogen to deliver rapid temperature reduction in refrigerated vehicles; the possibility of using injected nitrogen as a NOx suppressant in diesel engines; and the potential to use over-driven onboard liquefaction as a form of regenerative braking.
On the basis of the discussion presented above, we conclude:
- There exists already a well-established distribution network for cryogenic fluids in the UK and across the industrialised world.
- Surplus production capacity in liquid nitrogen (chapter 6) and the existing distribution network are more than adequate to supply the short to medium term fuel needs of an emerging ‘nitrogen economy’.
- Specifically, the existing distribution infrastructure is more than adequate to supply the early development of on-site, return to base and some long-haul transport applications.
- In the longer term, a mix of local production of liquid air and nitrogen, and centralised production combined with distribution by cryogenic tanker, is likely be able to satisfy any foreseeable demand.
- There is a need to develop higher efficiency mid-sized liquefiers in the low single-digit tonnes per day range.
We assume a small car has an energy requirement of 0.13kWh/km, on the basis that the Nissan Leaf has a 24kWh battery and range of 175km (24/175 = 0.13). At a practical energy density of 0.1kWh/kg this translates to a requirement of 1.3kg of liquid air per km for a liquid air prime mover, and 1.04kg/km for one operating with the benefit of waste heat from an ICE engine. The UK could produce 8,500T (8.5 million kg) per day of additional liquid nitrogen. At 1.3kg/km this would equate to 6.5m vehicle miles, increasing to more than 8m vehicle miles with waste heat. Cf http://www.nissan.co.uk/?cid=ps-63_296991&gclid=CIX476ulyrUCFcbKtAodfw0AbA#vehicles/electric-vehicles/electric-leaf/leaf/pricing-and-specifications/brochure.
See chapter 3 section 3. UK LNG cold could be used to produce 14.2mt of liquid air per year. 1 tonne of liquid air contains 1,150 litres, and Dearman Engine Company expects a car running on liquid air to achieve 1km/litre. This multiplies to 16.3 billion car kilometers per year or 44.7 million per day. Transport Statistics Great Britain 2012.