In the area of renewable energy vessel propulsion, the scale of maritime vessel technology allows for application of grid-scale energy storage technologies that would otherwise be impractical for road or rail vehicle propulsion. Initiatives are underway in the UK to develop grid-scale liquid air energy storage, with potential for a mega-scale plant at a coastal location being able to provide maritime vessels with liquid air.

Introduction

The cost-competitiveness of many renewable energy technologies requires access to energy storage technology. As a result, there has been much investment into the development of grid-scale energy storage technologies such as compressed air storage for wind power, ocean wave and ocean tidal current conversion. The solar thermal sector has developed heat-of-fusion thermal energy storage to store energy for use after sunset. The technology can also be applied to nuclear power stations to store off-peak thermal energy that can be redeployed during peak demand periods. Much progress has also occurred in grid-scale electrical battery technology.

Stationary grid-scale technology such as the vanadium-oxide flow battery and its competitor, the high-temperature liquid metal battery can both deliver over 20,000-deep discharge cycles. There is scope to apply both technologies to inland waterway maritime propulsion courtesy of the nature of the scale of the transportation technology and its operating characteristics. High-pressure spherical storage tanks built to 48-inches internal diameter can hold compressed air to sustain the operation of air-over-water propulsion in ferry vessels, with water under pressure driving a motor and in turn a propeller. Development of grid-scale liquid air energy storage offers possible maritime propulsive applications.

Compressed Air Propulsion

During the early to mid-20th century, mining locomotives were powered by compressed air and carried tanks of hot water to preheat the air prior to expansion in the cylinders. Air temperature rises under compression. Heat pumps can transfer the heat into the thermal storage material, so as to increase the density of stored compressed air. Ultra-high pressure spherical tanks built to 48-inches inside diameter can hold 10.7-cu.ft of compressed air at 5000-psi (70-bar) that if cooled to seawater temperature, incur a density of 20 to 25-lb per sq. ft. or less than half that of water.

For temperatures above 85-deg C, water functions well as a refrigerant for use in high-temperature heat pumps. A small ship carrying several spherical tanks of compressed air, would release bursts of air into a low pressure tank that would supply an engine with air at variable low pressure. Heat of fusion thermal storage would preheat the compressed air prior to expansion in a turbine engine. Alternatively, combustion of LNG could also preheat the air, allowing such compressed air energy storage to be applied to local short-distance tug boat operation.

Liquid Air Propulsion

Grid-scale installations super-cool air to -196-deg C allows for storage of liquid air at low pressure inside massive insulated cylindrical tanks that hold many times the volume of an array of spherical tanks. There appears scope for ships to carry such large tanks to provide for propulsion. Ambient heat from seawater could convert liquid air pumped from the super-cooled storage tanks to the gaseous state, increasing volume by a factor of 700 times. The air would be further heated to high temperature from heat-of-fusion thermal storage before being expanded in a turbine engine driving an electrical generator.

Varying the air pressure upstream of the turbine would allow energy efficient turbine operation over a range of power output, with the turbine spinning at constant maximum RPM with constant maximum air inlet temperature. The sheer size and internal storage volume of grid-scale super-cooled tanks would extend the operating range of a liquid air powered ship to operate domestic coastal service or ferry service across comparatively wide water channels. Liquid air energy storage is a recyclable technology that offers potential for long-life service involving several thousand deep-cycle discharges that would rival the best electrochemical storage batteries.

System Hardware

For stationary mega-size grid-scale applications, industry already provides proven technology and components required to repeatedly pump and cool atmospheric air to the liquid state. Industry also offers proven insulated mega-size storage tanks capable of storing liquid air at -196-deg C, at low pressure. The British based Highview Company has developed proprietary thermal storage compounds to maintain super-cold temperature as well as to preheat liquid air prior to combustion-free expansion in a turbine engine. There may be scope to negotiate with Highview to adapt their thermal storage technology for installation and application onboard a ship.

The development of offshore and coastal wind power installations along with ocean wave and ocean current energy conversion technology provides scope to develop grid-scale liquid air production and storage installations at coastal locations, including at or near to ports. Close proximity to ports provides the basis by which to transfer liquid air into cooled and insulated storage tanks installed aboard coastal ships. Should the technology gain acceptance, a pair of coastal liquid air stations located within the sailing distance of a liquid air ship would be able to provide for the operation of such renewable energy ships.

Competing Technologies

Liquid air energy storage technology promises to be a long-life technology capable of delivering tens of thousands of repeated deep discharge cycles over a period of many years. Grid-scale vanadium-oxide flow batteries and high-temperature liquid metal batteries offer useable life expectancies of over 20,000-deep cycle discharge cycles and could be installed aboard ships that sail along inland waterways, to be partially recharged as vessels transit across navigation locks. Heat-of-fusion thermal storage technology capable of raising steam can offer useable life expectancies in excess of 100,000 deep discharge cycles, with potential for energy recharging at coastal thermal power stations.

Solar thermal power stations have for several years transferred thermal energy into heat-of-fusion thermal storage to provide after sunset power generation. The nuclear power industry is considering transferring off-peak thermal energy into heat-of-fusion thermal storage. At coastal power stations, the stored thermal energy could provide for both peak electrical power generation as well as transfer thermal energy into thermal batteries installed aboard ships. Such power stations located within the sailing distance of a thermal rechargeable ship would sustain the operation of such ships. Thermal rechargeable and liquid air energy storage technologies appear cost-competitive against electrochemical battery technologies.

Proven option

The sheer scale of some maritime transportation technologies allows for installations of energy storage technologies for propulsion that would be beyond road and railway propulsion. While the road industry seeks to develop a battery capable of delivering one million miles of service, proven technology is already available to the large vessels. A battery that offers 20,000-deep cycle discharges and propels a vessel for over 100 miles along a waterway could deliver two million miles of service. There is scope to adapt liquid air energy storage technology for short-distance coastal or waterway ship propulsion.

Liquid air energy storage promises to be a cost-competitive technology that is free from toxic compounds and that can offer extremely long service lives in both stationary and possibly mobile application. Proven technology to produce and store liquid air is available from industry, with potential to install some of the technology such as storage tanks, thermal storage and engine aboard ship. There may also be scope to introduce liquid air energy storage technology at major transportation terminals, to provide peak hour electric power and to recharge vessels powered by liquid air energy technology.