Liquid Air Storage for Port Vehicle Propulsion

liquid air
The Dearman engine (image courtesy Dearman)

Published Oct 14, 2020 9:57 PM by Harry Valentine

During the early to mid-20th century, onboard compressed air storage provided propulsive energy for mining locomotives that pulled rail cars loaded with mined ore from deep within the earth. Advances in liquid air storage technology now provide the basis for an alternative form of propulsive energy, including for port vehicles.


Initiatives aimed at reducing carbon emissions at ports have focused on electric battery storage technology. Most electrochemical battery technologies intended for mobile usage incur the problem of comparatively short useable life expectancy when repeatedly operated on deep-drain discharge cycles. Some mega-scale electrochemical battery technologies intended for stationery power-grid operation offer greatly extended deep-drain discharge cycles, with mobile application restricted to mega-scale vehicles such as ships. There would likely be market application for a mobile energy storage technology that combines competitive energy storage density to electrochemical batteries with repeated deep-cycle discharge life expectancy of mega-scale batteries.

The liquid air battery stores liquid air at low pressure at temperatures down to minus 196o C in either massive scale stationery insulated storage tanks or in heavily insulated smaller-scale mobile storage tanks. Developers of small-scale liquid air energy storage and propulsion are aiming at a useable life expectancy of 25-years with 5-minute recharge capability, with the ability to transfer waste heat into mobile storage to enhance performance of liquid air propulsion. While the scale of a ship makes liquid air propulsive storage suitable for short-sea sailing application, the technology might also find short-distance application in port areas.

Compressed Air-over-water

Compressed air-over-water propulsion is well proven in small-scale rocket propulsion and in small-scale boat propulsion. While air at 250-psia at 220-deg F has a density of 1-lb/cubic-foot, water has over 60-times the density. Air density increases by 20% at 100-deg F, with water still holding 50-times the density and engine power is based on mass flow rate through an engine like a hydraulic turbine. Water will still hold 3.8-times the density as compressed at 4,000-psia at 200-deg F. The premium piston water pumps can move water up to 5,000-psia when recharging a compressed air-over-water propulsion system.

In short-haul ferry boat service, compressed air-over-water propulsion requires a larger vessel to carry the weight of water. The cost of electric power compared to the cost of fossil fuel would determine its cost competitiveness. There is scope for modern low-speed hydrofoils to raise the vessel hull above water at around 10-knots, reducing water drag in cross-channel service and likely extending operating range. Onboard thermal energy storage would capture heat of compression and reheat compressed air as pressure decreases when producing propulsion. Compressed air propulsion can be competitive in certain transportation vehicle applications.

Liquid Air

Liquid air at -196-deg C offers 87% of the density of water at room temperature, making liquid air storage competitive for propulsive applications. Producing liquid air requires removal of thermal energy that may be transferred into a companion thermal energy storage system to enhance energy recovery performance. There would be additional potential to install a high-temperature thermal energy storage system to superheat the air that flows from liquid storage to an engine. Grid-scale precedent indicates potential to install liquid air propulsive energy storage into mega-size transportation vehicles such as ships assigned to short-sea shipping up to 1,000-miles.

The density of liquid air suggests potential application in small-scale, short-distance mobile propulsive application, provided that such installations include thermal storage technology to reheat and even superheat the air prior to it entering an engine and being expanded. Dearman Engine Company and EpiQair/Epicam in the UK along with Cryomatiks in the USA are developing small-scale mobile versions of liquid air technology for road vehicle vehicular propulsion, with energy storage density approaching that of lithium-ion battery technology. Dearman installed a liquid air powered engine into a car and drove the vehicle, their proof of concept.

Useable deep-discharge cycles

A small number of large-scale battery technologies can offer excess of 20,000-deep-discharge cycles at over 80 percent discharge, with AMBRI technologies of Boston having tested a small-scale version of their high-temperature liquid metal battery to 100,000-cycles. Many small-scale mobile electrochemical battery technologies incur reduced useable life expectancy when drained from maximum to less than 50% of storage capacity. Some mobile purpose electrochemical battery technologies can offer up to 7,500-discharge cycles when discharged to 50% of storage capacity. By comparison, developers of liquid air propulsive storage are claiming in excess of the equivalent 20,000-deep-discharge cycles in wheeled vehicle applications.

Port area wheeled vehicles would include fork lift trucks, picker trucks that move containers, port area railway locomotives and port area shunting trucks that pull trailers carrying containers. Liquid air energy storage technology is claimed to remain fully operational in frigid sub-zero winter temperatures that impair operation of several electrochemical battery technologies. Both flow redox battery technology and liquid air battery technology can be recharged while simultaneously producing power. The five-minute to full recharge capability of small-scale liquid air technology enhances prospects to apply the technology to port area transportation vehicles.

Energy System

Highview Power of the UK has developed stationery grid-scale liquid air energy storage to operate along with any of wind power conversion, ocean dynamic energy conversion and even nuclear power generation. The essential need to provide sufficient cooling capacity for thermal power stations enhances the attractiveness of coastal locations, including within close proximity to a major transportation terminal such as a maritime port or even airport.

In this era, port-supplied shore power sustains operation of various onboard systems installed into large ships. The power requirement of major transportation terminals that includes port power for transportation vehicles enhances the attractiveness of installing grid-scale energy storage at or near such terminals. Liquid air energy storage installed at or near a port could serve the energy requirements of the port while providing additional liquid air to be transferred into super-cooled insulated storage tanks installed aboard short-sea service coastal ships. The energy storage system could further be expanded to include sustaining the operation of a fleet of liquid air powered port vehicles that move containers around the port.

Performance and Applications

While Cryomatiks is presently testing their proof-of-concept technology in a golf cart, their next developmental step would be a fork-lift truck or the port equivalent of a container picker truck. At several locations internationally, seaports and coastal airports are located within close proximity as is the case at Hong Kong, Macau, Osaka, Tokyo, Marseilles, Nice, Barcelona, Genoa, Rome, Beirut, Sydney, Newark, Vancouver, Christchurch, Auckland, Rio de Janeiro and a few other locations. At such locations, liquid air technology could provide energy for airport buildings, maritime port buildings along with a variety of seaport terminal vehicle and airport vehicles.


Port areas may be suitable locations for grid-scale liquid air energy storage stations as ports could utilize a significant proportion of that stored energy for port energy operations as well as to resupply the energy requirements of short-sea sailing ships, port tug boats along with port area wheeled and railway vehicles. While a large-scale stationery installation is operational in the UK and provides the basis to develop liquid air storage for mega-scale mobile applications such as ships, developing small-scale mobile versions of the technology for short-distance road vehicle and railway applications offers future promise.

Large-scale liquid air energy storage installations offer the promise of being competitive in port vehicle application due to the combination of deep-drain energy availability with deep-cycle recharge life expectancy that exceeds that of lithium technology. While liquid metal energy storage and flow redox batteries have potential application in large-scale vehicles such as waterway vessels, such technology would be less suitable for small-scale mobile applications. The long-life, multiple deep-cycle discharge and recharge capability of liquid air technology makes it a contender for mobile small-scale application in port area road and railway vehicles.

Harry Valentine is a regular contributor to The Maritime Executive.

The opinions expressed herein are the author's and not necessarily those of The Maritime Executive.