Why do we need another energy vector?
Energy policy in Britain and Europe rests on three pillars: decarbonisation, energy security and affordability. In order to reduce emissions and keep the lights on at an acceptable cost, much of the policy debate has centred on how to generate sufficient low carbon energy. However, the fundamental problem is not the adequacy of low carbon energy resources – wind, solar, nuclear etc are in principle sufficient to meet our needs many times over – but how to package that energy into useful forms. The imperative to decarbonise is forcing us to rethink the way energy has been transformed, transported and consumed for decades, and many of the trickiest problems relate to the mismatch between the forms in which low carbon energy is produced and the forms in which we need to consume it. Arguably one of our biggest challenges is to develop new energy vectors.
A vector is not a source of energy but a means of transporting it from one time and place to another. Unlike primary fuels – coal, gas and oil – vectors are man-made, resulting from the transformation of one source of energy into another more useful form – such as steam, electricity, hydrogen or biofuels. An ideal vector should be able to transport energy in both time and space, so that consumption can be decoupled from production, and the vector can serve as a transport fuel. However, existing vectors all suffer significant drawbacks and are not progressing as quickly as promised or required.
The need for new vectors is becoming more acute because of rapid changes in the energy system brought on by decarbonisation. Until recently our energy was almost exclusively derived from primary fuels such as coal, oil and gas that are easy to store and transport, and which can deliver power or heat whenever necessary. Today we are shifting rapidly to renewable forms of generation such as wind and solar, which, because they are intermittent, produce energy rather than despatchable power that is available on demand. This energy comes in the form of electricity, which is easy to move but harder and more expensive to store, making it particularly unwieldy as a transport fuel. It is the widening disconnect between energy and despatchable power that creates the need for new vectors.
It is widely accepted that cutting carbon dioxide emissions will mean a much larger role for electricity. Analysis by the Committee on Climate Change, the Government’s independent advisor, has shown that decarbonising the electricity supply is vital to achieving the country’s overall climate targets. This is because power sector emissions account for almost 30% of total emissions; cutting emissions is generally cheaper in electricity generation than in other sectors; and low carbon electricity can then be used to help decarbonise heat and transport.
However, because electricity is difficult and expensive to store, a strategy of decarbonisation that relies on electrification presents two major challenges:
- balancing supply and demand on a grid increasingly dominated by intermittent renewable generation, and
- transforming low carbon electricity into a form suitable for use in transport.
Both challenges might be amenable to a new low carbon energy vector such as liquid air.
What is liquid air?
Air can be turned into a liquid by cooling it to around -196C using standard industrial equipment. 700 litres of ambient air becomes about 1 litre of liquid air, which can then be stored in an unpressurised insulated vessel. When heat is reintroduced to liquid air it boils and turns back into a gas, expanding 700 times in volume. This expansion can be used to drive a piston engine or turbine to do useful work. The main potential applications are in electricity storage, transport and the recovery of waste heat.
Since the boiling point of liquid air (-196C) is far below ambient temperatures, the environment can provide all the heat needed to make liquid air boil. However, the low boiling point also means the expansion process can be boosted by the addition of low grade waste heat (up to +100C), which other technologies would find difficult to exploit and which significantly improves the energy return. There are myriad sources of low grade waste heat throughout the economy from power stations to factories to vehicle engines.
The industrial gases industry has been producing liquid nitrogen and liquid oxygen – the main components of liquid air – for over a century. Cryogenic gases have a wide range of applications including steel-making, food processing, medicine and superconducting technologies. The thermo-physical properties of liquid nitrogen and liquid air are similar, so a cryogenic energy vector could be provided by either.
The industry has a glut of gaseous nitrogen that could be made available for liquefaction, because there is four times as much nitrogen as oxygen in the atmosphere but much less demand for it commercially. Currently an estimated 8,500 tonnes per day of waste gaseous nitrogen is vented back to the atmosphere, which, if liquefied and used as transport fuel, would be enough to power the equivalent of 6.5 million car kilometres daily.
There have been several attempts to exploit liquid air or liquid nitrogen as an energy vector over the past century without commercial success. However, technological advances and market evolution in the early years of this century appear to have made it a practical and economic possibility worth considering again. Emerging liquid air technologies include:
• a novel piston engine that runs on liquid air or Nitrogen (the Dearman Engine) which could be used either as a prime mover (main engine) or as a secondary unit to recover waste heat from an internal combustion engine (ICE) or hydrogen fuel cell and so raise efficiency;
• a novel split cycle ICE engine design developed by Ricardo that incorporates liquid nitrogen to increase efficiency by capturing its own exhaust heat; and
• the Liquid Air Energy Storage (LAES) system developed by Highview Power Storage, a plant which generates liquid air using cheaper, off-peak electricity, stores it for some hours or days, and then expands it through a turbine to deliver power back to the grid at times of peak demand.
Liquid air technologies can also be used to recover waste heat from industrial sources and in hybrid combinations with internal combustion engines and even hydrogen fuel cells.
Liquid air is inherently capable of converting waste heat into power because of its low starting temperature. The liquid air cycle works between -196C and ambient temperatures, meaning the addition of even low grade waste of less than 100C, which is otherwise difficult to exploit, can increase the work output significantly. Sources of waste heat that could be exploited by liquid air technologies include conventional and novel internal combustion engines, power generation, industrial processes, and in future potentially hydrogen fuel cells.
In the UK, industrial processes provide myriad sources of waste heat, which total as much as 40TWh per year – enough to heat 2.4 million British homes.Industrial demand for heat, at around 180TWh, is easily large enough to absorb this waste, but this takes no account of the obvious fact that sources of waste heat are rarely co-located and coincident with demand. Even if all opportunities to make use of waste heat as heat were exploited, there would still be a very substantial waste heat resource available from manufacturing and process industries, and the best way to access this is to generate electricity. Our analysis suggests that if the ratio of peak to off-peak electricity prices is 2.5 or greater, liquid air could represent an economically attractive proposition to process plant operators with a waste heat source to exploit.
In countries with inadequate primary generating capacity, such as South Africa and Thailand, peak electricity prices can be as much as 8 times higher than off-peak prices, even today. In countries or regions with rising renewable generating capacity such as Germany, Texas and Great Britain, power prices can already turn negative in periods of high wind and low demand, and the effects of weather and renewable intermittency are expected to increase price volatility in the coming decades. By some forecasts the peak to off-peak ratio in such countries could be substantially higher than 2.5 within the next two decades.
In transport, PEM (Proton Exchange Membrane) hydrogen fuel cells operate at around 80C, not dissimilar to the coolant temperatures of internal combustion engines, meaning they too could be combined into heat hybrids with a Dearman engine or similar. This could improve the economics of hydrogen vehicles by allowing the PEMFC to be downsized.
Other advantages of such an arrangement include:
• Fuel cells are less efficient when running under dynamic conditions than at steady state, and a hybrid FC-liquid air engine may allow for greater efficiencies and component lifetime by load levelling.
• Manufacturers are constantly trying to reduce the amount of platinum used in fuel cells to cut costs, but this may increase heat generation, meaning thermal management could be increasingly important.
The markets where a PEMFC-liquid air hybrid would offer most immediate benefit and greatest chance of success have been identified as buses, taxis and forklift trucks.
Liquid air production and cost
Liquid air is not produced commercially today since demand is for the individual components of air: oxygen, nitrogen and argon. The industrial gases industry in the UK sells 9,000 tonnes per day (tpd) of oxygen (gas and liquid) and 8,000tpd of nitrogen.
However, Air Separation Units (ASUs) inevitably produce excess gaseous nitrogen, because there is four times as much nitrogen as oxygen in the atmosphere but much less demand for it commercially. Spiritus Consulting estimates conservatively that excess gaseous nitrogen production capacity in the UK amounts to at least 8,500tpd, and the glut would be even larger but for the fact that producers adopt various measures to optimise the oxygen output of their ASUs. This surplus gas is currently vented harmlessly to the atmosphere.
The thermo-physical properties of air and nitrogen are similar, and either could serve as a cryogenic energy vector. In the early stages of a liquid air economy, therefore, waste nitrogen gas could be liquefied to use in place of liquid air. If the entire estimated daily nitrogen surplus were used for this purpose, it could potentially fuel the equivalent of 6.5 million car kilometres daily.
Producing liquid air directly would be simpler and cheaper than producing liquid oxygen and nitrogen, since the gases need not be separated. Air liquefaction can be achieved with less equipment than required to separate oxygen and nitrogen, and consumes about a fifth less energy.
We calculate that the production costs of liquid air are between 3 and 4.5 pence per kilogramme, or 2.5 to 3.6 pence per litre on the basis of current electricity prices. There is potential to reduce these costs by almost half through measures such as co-locating production with LNG terminals to exploit waste cold. The delivered cost, after distribution by road tanker, would be roughly double, but local production at refueling stations could eliminate this cost.
These costs translate to competitive per-kilometre fuel costs compared to incumbent technologies. The graph shows that in all but one scenario the per-kilometre fuel costs are lower for a Dearman Engine car than for a petrol car of average UK fuel economy (including duty and tax). The running costs of an EV are lower still, but these should be seen in the context of much higher capital costs; the Nissan Leaf costs £26,000 even after a government grant of £5,000, around twice the price of an equivalent sized ICE.
A Dearman car would have similar capital costs to an ICE in the early stages of production. On balance we conclude the likely costs of liquid air mean it is likely be competitive – and perhaps highly competitive – with fossil fuels in a range of transport and other applications.
Cryogenic liquids present significant hazards because of their intense cold and substantial gas production when warmed. However, these hazards are well understood and amenable to established safety management protocols. Some hazards are common to both liquid air and liquid nitrogen, while others are more specific to one or other cryogen, but the issues are fundamentally identical in grid and transport applications. The most likely problems relating to the use of liquid air or nitrogen as an energy vector are:
• Cold burn or frostbite (both liquid air and liquid nitrogen)
• Materials structure and integrity (both)
• Pressure build-up (both)
• Oxygen deficiency (mainly liquid nitrogen)
• Oxygen enrichment (mainly liquid air)
For the purposes of this brief summary (please see chapter 9 for full review of safety issues), cold hazards are easily solved by the use of materials suited to low temperature service, and appropriately insulated systems. Pressure build-up in storage or fuel tanks is also easily solved with pressure relief valves and burst discs. However, oxygen deficiency and enrichment deserve further discussion.
Cryogenic liquids can be stored for substantial periods in insulated vessels, at atmospheric or slightly above atmospheric pressure. However, all cryogenic liquids will boil off in time, as ambient heat gradually penetrates the insulation. Pressure will rise in the vessel, and gas will then be released through a relief valve. Boil off generally occurs at a rate of around 1% per day in small tanks, and at 0.2% or lower for larger tanks, where the greater ratio of volume to surface area favours cold retention.
If the cryogen is liquid nitrogen, and if the tank or vehicle is housed in an enclosed space with inadequate ventilation, and if it is left unmonitored for an extended period, then there is a risk that the vented nitrogen will displace the original air and render the atmosphere unbreathable. Anyone entering such a space would be at risk of asphyxiation. However, this hazard could be eliminated by mandating appropriate passive ventilation standards for any building containing such equipment, oxygen monitoring or both. Health and safety procedures for the amount of ventilation required for an environment with cryogenic inert gases are well established, and oxygen monitoring equipment is routinely used at industrial gas production sites.
If the cryogen is liquid air, a different hazard predominates. The asphyxiation risk is lower since both nitrogen and oxygen boil off. However, nitrogen boils off preferentially to oxygen, since it has a higher partial vapour pressure at the same temperature and this factor drives evaporation. As a result, a tank of liquid air left unmonitored for extended periods may see the proportion of oxygen in the mix rise above 21%, the proportion that occurs naturally in the atmosphere. Any concentration above 23.5% is considered dangerous, since oxygen is highly reactive if it comes into contact with hydrocarbons or other organic material.
The risk of oxygen enrichment is clearly linked to the size of the storage vessel and the length of time liquid air is held in storage. Larger tanks retain cold better, and grid-scale applications such as Liquid Air Energy Storage that cycle frequently would store liquid air too briefly for enrichment to take place. Smaller tanks holding liquid air for longer periods would be at greater risk of oxygen enrichment. We suggest for safety reasons that in circumstances where liquid air may be stored for long periods it should be handled according to liquid oxygen handling procedures. More generally, there is good reason to believe the hazards associated with the use of liquid air and liquid nitrogen as transport fuel can be managed to acceptable levels, because:
• the industrial gas industry transports thousands of tonnes of cryogenic liquids by road tanker daily (chapters 6 and 7 of the Full Report);
• LNG and LPG are increasingly used as lorry fuel, and the hazards of transporting liquid air are expected to be much lower than for these or for liquid oxygen, which is also commonly transported by road;
• early applications of liquid air in transport are likely to involve commercial vehicles with fully trained operators;
• hazardous fuels such as petrol and diesel are routinely used by public, and the hazards have been managed to acceptable levels; and
• any use of liquid air or liquid nitrogen by members of the public would require it to be as safe or safer than using petrol or diesel, and all relevant technologies would need to be designed and engineered to ensure this.
There is no insuperable safety reason why liquid air and/or liquid nitrogen should not be widely deployed as an energy vector in both grid and transport applications.