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9 May 2013




LAEN, which was an outcome of the report, will serve as the global hub where new ideas are demonstrated and shared, and promote liquid air as a potential energy solution among researchers, technology developers, manufacturers, energy producers and consumers, and government.

Air can be turned into a liquid by cooling it to around -196 °C using standard industrial equipment; 700 liters of ambient air becomes about 1 liter of liquid air, which can then be stored in an unpressurised insulated vessel.

The expansion of the liquid air when heat is reintroduced 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 is far below ambient temperatures, the environment can provide all the heat needed to make liquid air boil.

The low boiling point also means the expansion process can be boosted by the addition of low-grade waste heat (up to +100° C), which other technologies would find difficult to exploit and which significantly improves the energy return. Sources of low-grade waste heat include power stations to factories to vehicle engines.

Despite their considerable industrial heritage, cryogenic liquids are a relative newcomer to the debate around energy vectors, as we explore in the papers that make up the Full Report. But it is this application of established technology to new challenges that makes liquid air such an attractive proposition, especially to a mature industrial nation such as the UK.

...More immediately, liquid air can prove its worth in specific sectors, including providing the cooling in refrigerated lorries, serving as the primary fuel for vehicles where zero-emissions are critical such as warehouses or mines, and exploiting sources of waste heat or coolth found in industrial processes or LNG terminals.

—Dr. Jonathan Radcliffe and Prof. Richard A. Williams, University of Birmingham

Liquid Air in Transportation Applications

Ricardo, which has been researching a split-cycle engine with cryogenic injection (earlier post), wrote the report chapter focusing on the potential of liquid air for transport applications.

Liquid air offers significant potential benefits as a future energy vector, both for use in light duty propulsion and as an enabler for other promising low carbon powertrain innovations, particularly waste heat harvesting. It is clearly worthy of further research and development effort to create a better understanding of its potential alongside more widely recognized potential future low carbon technologies such as advanced battery systems and hydrogen.

—Professor Neville Jackson, Ricardo chief technology and innovation officer

In its chapter, Ricardo considered a number of potential roles for liquid air considered in which 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 or in a supporting role to recover waste heat from a conventional engine or fuel cell. 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.

Among the transportation uses of liquid air outlined in the report are:

  • Prime mover. A cryogenic engine such as the Dearman piston engine produces zero emissions at the point of use; has low greenhouse gas emissions provided the liquid air or nitrogen is produced from low carbon electricity; has energy and power density on a level with battery electric technology; and has the potential for rapid refueling. It potentially is attractive for use in small cars and vans for short range urban use, scooters, short range marine craft, forklift trucks and mining equipment.

  • Heat hybrid. A cryogenic engine such as the Dearman engine could also be used as a heat hybrid in combination with an internal combustion engine or hydrogen fuel cell to convert waste heat into additional shaft power at high levels of efficiency, reducing both fuel consumption and emissions. This approach would be viable in passenger ferries, commuter trains, heavy duty trucks and urban buses, and could also deliver ‘free’ cooling for passengers or goods.

  • High efficiency internal combustion engine. Heat recovery could also be achieved using the Ricardo split cycle engine, a novel internal combustion engine design that incorporates liquid nitrogen to capture exhaust heat and increase fuel efficiency. Detailed modeling of this approach undertaken through the Technology Strategy Board-funded “CoolR” project has suggested that efficiencies of more than 60% are possible, compared to around 40% for current diesel engines.

    Nitrogen could be supplied from a modest-sized onboard tank or an onboard liquefier driven by the engine and boosted by regenerative braking. This approach would be suitable for heavy duty trucks and container ships, and potentially rail locomotives, other commercial vehicles and even larger passenger cars.

  • Refrigerated food transport. Some food delivery vehicles already use liquid nitrogen as a heat sink to provide refrigeration, which cuts noise, complexity and carbon dioxide emissions substantially compared to conventional diesel- powered refrigeration. However, current systems fail to capture any additional shaft power from the nitrogen evaporation process.

    The report calculates that a vehicle food refrigeration system using liquid nitrogen or liquid air to provide both additional shaft power and cooling would cut emissions from 47 tonnes per truck per year (diesel refrigeration) to 10 tonnes, a reduction of almost 80% on the basis of current grid average electricity. The same approach could also provide refrigeration or air conditioning for passenger ferries, cruise ships, freight trains and buses, with greatest benefits in hot climates.

The Dearman engine. The Dearman Engine Company Limited (DEC) is developing a novel, zero-emission piston engine that runs on liquid air (or liquid nitrogen); the exhaust is cold air. With Ricardo as a partner, DEC is tracking to deliver a test engine by end 2013 for testing and demonstration in Q1 2014.

Compressed air vs. liquid air
Compressed air engines (e.g., MDI, earlier post) expand compressed air from an ambient temperature store to create drive.
At 300 bar, air has a specific energy of 140 Wh/kg (0.5MJ/ kg). Using such an approach, the MDI vehicle has a 300 liter tank and claims an urban range of 100 km (62 miles), which falls to 50 km (31 miles) at higher speed.
In the report, Ricardo notes that if an MDI-type vehicle were to store the energy as liquid air (specific energy of 214 Wh/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 152 km (94 miles).
Since the density of liquid air is greater than compressed air, with the same size of tank the energy content could increase by a factor of three, increasing the urban range to 300 km (186 miles), Ricardo says.

The company says that it also intends to deliver a proof-of-concept vehicle by Summer 2014 to prepare for full application specific field trials and is working with MIRA ((Motor Industry Research Association) on this program.


Prior cryogenic expansion engine have worked on an open Rankine cycle akin to a traditional steam engine but operating across a different temperature range. The cryogenic fluid is pumped to operating pressure and vaporized through a heat exchanger, before expansion in the engine cylinder.


Specific work available from an expansion over a variety of pressures for isothermal and adiabatic cases. Dashed lines indicate the specific work rom the expansion net of pumping work. Source: DEC. Click to enlarge.

The Dearman Engine instead uses a heat exchange fluid (HEF) to facilitate extremely rapid rates of heat transfer within the engine. This allows injection of the liquid cryogen directly into the engine cylinder whereupon heat transfer occurs via direct contact mixing with the HEF. The heat transfer on injection generates very rapid pressurization in the engine cylinder.



Policy recommendations. The report notes that policy support for early stage transport technologies such as liquid air is “somewhat insensitive to the potential of real disruptors and the needs of the small companies that typically develop them.” To counter this, the report recommends considering the following changes to transport technology policy:

  1. Grant funding calls should offer appropriate opportunities for disruptive technologies, and make allowance in their structure for a less widespread level of understanding of those technologies; objectives should be set but the means should be technology-agnostic where possible.

  2. New technologies should be supported by a process of pre-clearance, to establish their basic scientific feasibility. This pre-clearance should then be publicly available, so that fund assessors can quickly verify the unfamiliar technology’s credibility. The costs of pre-clearance should be grant funded.

  3. A rigorous review should be undertaken periodically of existing visions for longer term CO2 abatement, to quantify progress against targets and identify emerging roles for disruptors. In the context of liquid air or nitrogen, this would need to embrace not only its role as a main or supplementary fuel in some applications, but also its energy-chain interaction with electricity grid buffering and with bulk LNG evaporation.

  4. Support mechanisms such as research and infrastructure grants should evolve to embrace the increasingly complex interaction of energy systems—for example, some of the liquid air vehicle-fuel systems described could involve vehicles, refrigeration, grid buffering, the industrial gas industry, and bulk LNG supply within a single concept.

  5. A specific program should be developed to support the field trial and deployment of technologies that replace or reduce diesel use in refrigerated food transport, which would be equally open to batteries and hydrogen fuel cells.


May 9, 2013 in Compressed Air Engines, Concept Engines, Engines | Permalink | Comments (25) | TrackBack (0)


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What's the round trip energy efficiency of "Air can be turned into a liquid by cooling it to around -196 °C using standard industrial equipment.."?

Not so sure about -196 °C on scooters, cars, ..

The use of a heat-transfer liquid is a clever idea (it can make the expansion nearly isothermal), but the pumping work for liquid air is very small given the 700:1 expansion; pre-warming the air would probably increase efficiency and also reduce entropy losses (heat engines could be run off the ΔT between ambient and the air boiler).  Recovery of the HTF will have to be very high, and it must also be non-toxic or very efficiently filtered because catalytic converters are not an option for eliminating fugitive emissions.

Good points, E-P.

I'm concerned about the energy density. Theoretical energy density for an engine with 300-bar max pressure limit is only 150 Wh/kg. Adding storage container with insulation and it would be lower, no more than Lithium battery. Battery retains charge much longer and won't evaporate like liquid air, and has much higher round-trip efficiency.

It takes a lot of energy to produce liquid air, due to the low temperature required, and the Carnot efficiency is quite low. For a heat engine operating at 300 K down to 77 k, the Carnot efficiency is 74%. The Carnot efficiency of a refrigerator, which is a reverse of a Carnot heat engine, between those two temperatures will be only 26%. Actual efficiency will be even lower. One can see that round trip efficiency will be very low, even for Carnot process. By contrast, round trip efficiency of battery is 80-90%.

Theoretical calculations must be done first before any further consideration. I hope my numbers are correct, so feel free to correct me.

H2 has much better round-trip efficiency, and can be stored in compressed form for long times, and has energy density of ~2000Wh/kg in compressed form using carbon fiber tank.

Correction to above regarding Carnot efficiency of refrigeration from 300 degree K to 77 K: The Carnot efficiency is 35%, not 26% as I mentioned. The round trip Carnot efficiencies will be .74 x .35 = 26%.

By contrast, the round trip efficiency for H2 -FCV will be ~50% or higher when waste heat is used for cabin heating, and when waste heat is used for home water heating.

“harness waste and surplus energy within power and transport ”

No such things in practical thermodynamics terms. It is a case of misusing words to deceive.

All heat engines reject heat. Thermal efficiencies (not to be confused with overall efficiency) can be improved but it comes at a cost in larger and more expensive equipment. It also add a new hazard to the risk analysis.

Something is only a waste if there is a use for it.

“home water heating. ”

You hear this a lot from people who do not understand how much energy a home uses. I have an all electric house. A more efficient heat pump makes a lot of sense in hot humid climates but hot water heating is not a big enough use to justify anything but the standard electric how water heater.

If you live in a cold climate, direct heating with natural gas or propane is done with very efficient equipment. While you can spend a huge amount of money improving the efficiencies of making electricity a small amount, it is unlikely that you will improve overall efficiency.

There is a shortage of engineers and equipment so there will never be 'surplus energy' because we only produce what we need.

It would be like storing CPR because no one is having a heart attack at the moment.

I thought the 2/4S engine is foolish enough and here's another one from Ricardo.

Liquifying air takes huge amount of energy and we will get lots of other contaminants before oxygen and nitrogen finally turn to liquid. What's missing is on how the fuel will be ignited when the cryogenic air is injected. Even when the fuel can be ignited, the super low temperature charge will give flame tough time to propagate.

With the port built onto the cylinder bore, I wonder how the HC from the engine oil can be controlled?

Gee, the professors are giving bad names to the academicians.


Yet the increase in entropy in even condensing furnaces is enormous.  If you are burning NG or propane, an engine-driven heat pump with heat recovery can push the CoP to well over 2, compared to less than 1 for any kind of furnace.


Overall efficiency can be improved by a number of means; even the engine-driven generator can be far more efficient in net fuel consumption if the heat in the coolant and exhaust is used for DHW and space heat compared to even a CCGT.  It is a question of balance.


It is not mentioned on how the consortium is going to recover and reuse the coolant and exhaust heat, any idea how?

“It is a question of balance. ”

What does that mean? Is that control system engineer speak for it is not a very economical way to save energy and therefore hardly anyone will do it?

Like a PHEV.

“any idea how? ”

Well they are not going to actually do it just issue press releases. My company is doing something very interesting with government money. R&D is really fun but I know what the answer will be, it is not practical, same as last time 30 years ago.

Thirty years ago the problems was fuel shortage. Of course the real problem was not a shortage of fuel but an inadequate fuel transportation system. Now ghg is the issue.

Let say we make the same 5% improvement in the steam plant at a coal or nuke plant. At the coal plant we get a improvement of 50 MWe (more income) and a reduction of 50 kg/MWh in ghg. At the nuke plant we get a improvement of 50 MWe (more income) and a reduction of 5 kg/MWh in ghg.

Since the 50 MWe at the nuke offsets the coal generation, the grid reduction is 950 kg/MWh.

At the risk of being too subtle, good old sound but boring engineering is how we solve problems most of the time.

Research into the old, boring topic of secondary batteries has already solved lots of problems (your cell phone doesn't need a shoulder strap any more, because we've moved beyond lead-acid gel-cells), and will solve many more over time.  Existing cells used in traction batteries can cut liquid-fuel demand by half with relative ease, and relocate noise and emissions to remote generating plants.

Maybe liquid air is a cheap, low-tech alternative for urban transport in warmer climates.  One thing is for certain, a vehicle running on it wouldn't lack for air conditioning.

This idea is old news to me. I first heard about it a decade ago when I found this website;

And here's another guy who has been working on the idea for 40 years;

And digging into wikipedia tells me one such vehicle, Liquid Air, was demonstrated in 1902!


What you're missing is that the cryogenic air is only used as a working fluid, not an oxidizer. It would be heated externally like water is in a steam engine.


Yes but liquifying the air is done in stages so when the contaminants condense out they can be removed while the rest is still a gas. And because the "contaminants" condense out at different temperatures they can be removed separately as pure and sold as commodities: Condensed H2O is of course fresh water and there is already a market for the other gases;

BTW this market for liquified gases could make this idea profitable even before you start recovering waste heat.

Liquified gases don't appear more efficient.

Efficiency isn't everything.  If a system is reliable and safe, inefficiency may be tolerable.


Agree, sometimes you have to take a hit on one factor and think holistically. For one thing it would be lighter.

Also there is price to consider, not only would the propulsion system in a LN car be cheaper than a BEV but the fuel is currently a throw-away of the liquid oxygen industry.

Ai Vin,

Thanks a lot for your explanation.

How much hot gas can brought into the the cylinder considering the filling time is very limited as the piston is approaching the TDC? Inject it too soon and the pressure build up will resist the piston upward movemement and injecting it too late, it will be very little thermal energy being brought into the system.

The beauty of an internal and external combustion engine is that quite a high amount of thermal energy can be made available in the chamber to expand the working gas. If the hot gases is all that this engine has to bring in the thermal energy, I dont see how the power density will ever be good.

My first thought was to refer you again to how a steam engine works but this Dearman engine seems to do it a little differently. They are not, as you suggest, bringing hot "gas" into the cylinder. If I'm understanding it right, the Dearman Engine process involves the heat being introduced to the cryogenic fluid through direct contact heat exchange with a heat exchange fluid inside the engine: The nitrogen goes into the cylinder as a dense "cryogenic fluid" and then another "heat exchange fluid" is injected into the cylinder. This would mean, if I'm understanding it right, thermal energy gets transfered from the second fluid to the first which, being nitrogen, expands into a gas at a much lower temperature than the "heat exchange fluid" could.

Once again I'm speculating here because while liquid air cars are old news to me this is the first time I've heard of this Dearman Engine so - YMMV.

The Dearman engine injects two liquids into the cylinder.  The heat-transfer fluid boils the cryogenic liquid, generating high-pressure cold gas.  If there's sufficiently intimate mixture of liquid and gas, the temperature of the gas will not drop very much during the expansion and the efficiency will be improved.

I suspect that this is not the best arrangement because the expansion starts well after TDC, wasting some expansion energy.  There's also the rub that the heat-transfer fluid must be compatible with cryogenic liquids and not freeze.  These problems would be eliminated by using an "air boiler" to produce high-pressure cold gas outside the motor, and then use a dense mist of the heat-transfer fluid to maintain its temperature during expansion.  In warm environments this arrangement might be compatible with water as the fluid, eliminating considerations of cost and toxicity.

I'm wondering if anybody's working on using this idea in a liquid piston hydraulic pump. Inject the cryogenic liquid into the bottom of one side of a "U" tube filled with hot oil and let the gas bubble up to the top to push the oil down.

I wonder why Ricardo is not thinking about injecting water? I dont think the liquid nitrogen will ever have high specific heat capacity and expansion potential.

Water wouldn't work well if the outside temperature was low.  But in tropical climates, it would be terrific.

considering that cryogenic air needs some time to be processed, frozen water can also be heated up quickly. Once the engine is in operation, the rest of the frozen water can be melted for close loop operation.

Err no, this engine is designed to work at ambient air temperatures. Extra heat is only added to the system as a boost if it's available as waste from another source.

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