Liquefied Air for Short-distance Maritime Propulsion

The history of using compressed air for vehicle propulsion dates back into the latter 19^th century when inventors experimented with buses, cars and even self-powered railway carriages. For several decades, the mining industry made extensive use of compressed air powered locomotives that pulled ore trains through tunnels. During that period, compressed air powered mining locomotives shared components with steam-powered locomotives. Even at the present day, compressed air powered locomotives are still at work in some mines. This is the result of easy and ready availability of compressed air motors, high-pressure air tanks, gauges and control valves.

While some active mining locomotives store air at some 3,000-psi or 200-atmospheres, liquefied air and liquefied nitrogen can be super-cooled to near –200ºC courtesy of cascade refrigeration. Many industries buy liquefied air and liquefied nitrogen that can be transported by road, inside heavily insulated tanks that are kept at comparatively low-pressure. The British company Highview Power has chosen to develop liquefied air technology as an energy storage technology. While they have initially aimed for 15 to 50 MW-hr of energy storage capability, their long-term aim is 200 MW-hr of energy storage capacity at some 70% efficiency. 

An energy storage capacity of 50 MW-hr translates to 10-hours at 5,000kW or 6,700-Hp. Highview plans to purchase off-peak electric power at bargain basement rates to produce liquefied air that will be stored in heavily insulated vertical tanks. It is also possible to develop horizontal versions of these tanks that may be incorporated into the design and construction of an extended-length tug that will push and navigate large barges. Their plans call for building their energy storage systems at or near thermal power stations, some of which may be located on a river or next to the ocean.

Water has some 3,300-times the heat capacity as the equivalent volume of air at atmospheric pressure. The cascade refrigeration technology that would cool the air to the liquid state, would operate much consume far less energy using water as the heat sink, rather than air. It would cost less per ton more maritime vessels to deliver fuel to a dock located next to a clean coal fired power station that is located next to the river or ocean. As well, maritime is far more adept than road or rail at moving components at lower cost, to/from nuclear power stations.

Tugs on Liquefied Air:

Maritime vessels would have ready access to several coastal or riverside thermal power stations that may also be the future sites for liquefied air energy storage installations. While the liquid air technology would produce super-cooled liquid air during power station off-peak periods, there may be scope to adapt the technology to transfer super-cooled liquefied air (or nitrogen) from stationary storage to heavily insulated storage tanks on board a large tug vessel. The same tug vessel would also need to carry a heavily insulated tank of heat-of-fusion thermal storage material, to preheat the liquid air prior to expansion in an engine.

A large tug of 600-ft length could carry tanks of liquefied air along with heater tanks. It could push and navigate a barge of up to 1000-ft length on short/overnight voyages such as:

Helsinki – Tallinn (Gulf of Finland)

Genoa – Corsica (Ligurian Sea)

Barcelona – Majorca (Mediterranean Sea)

North Island – South Island (Cook Strait, New Zealand)

Buenos Aires – Montevideo (Rio del Plata)

Nova Scotia - Newfoundland (Cabot Strait, Gulf of St Lawrence)

Engines:

While the stationary application involves the use of turbines driving electrical generators, a mobile maritime application has the option of using a multi-stage expansion, positive-displacement engine directly driving the propeller(s). Rotary engines with bi-directional rotational capability would occupy less space than piston engines, allowing a modified sliding vane pump to operate as an engine. Quasiturbine of Montreal offers a rotary engine capable of operating on heated or superheated high-pressure air and a bi-directional rotation multi-stage expansion version of that engine may be sufficiently powerful to drive a ship propeller. 

Advantage of Scale:

In recent years, many inventors and researchers have shown plans to develop mobile pneumatic energy storage to propel powered small road vehicles, except that nothing has materialized. The only proven mobile pneumatic energy storage systems have worked in railway locomotives, the result of the advantage of scale. Pneumatic energy storage is a large-scale technology that is proven in stationary, grid-scale energy storage application with mining railway locomotives perhaps being the smallest allowable scale. Maritime transportation allows for a much larger scale of mobile pneumatic energy storage that railways, a scale that may have viable commercial application in niche markets.

Competing Technologies:

The scale of maritime vehicles allows application of large-volume energy storage technologies that usually have stationary, grid-scale applications. Maritime scale allows developers to adapt technologies such as steam-based heat-of-fusion thermal energy storage and liquefied air combined with heat-of-fusion energy storage to marine propulsion. These technologies would compete with a variety of large-scale and high-temperature chemical battery technologies that serve stationary, grid-scale energy storage applications. The list would include flow batteries, molten sulfur-sodium batteries and high-temperature metallic battery technologies. Liquefied air and heat-of-fusion technologies offer competing efficiencies, greatly extended useful service lives that reduce long-term costs.

Conclusions:

Liquefied air is a long-proven technology that is now being developed for grid-scale energy storage applications. The scale of maritime technology allows for such energy storage to operate in short-distance maritime transportation service at locations that impose stringent exhaust emission standards.

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Harry Valentine is a frequent contributor to the MarEx newsletter. He can be reached at [email protected].