Competition Increases in Maritime Battery Technology

There are geographic locations where fuel oil prices are high and electric power prices comparatively low. Such locations enhance the business case to operate electric propulsion along inland waterways and coastal services. New battery technologies include carbon-foam sulfuric acid technology, sodium sulfate electrolyte technology and silicon dioxide batteries.

Introduction

In maritime history, submarines sailing near their depth limits utilized rechargeable electric battery propulsion. Lead-acid battery technology of an earlier era is now becoming obsolete for transportation based applications. In the area of battery-powered vehicles, maritime transportation offers several advantages over battery-powered road or railway transportation. In road transportation, the space requirements and weight of batteries becomes problematic while the vibration, jolts and shock loads involved in railway transportation puts many electric battery technologies at a competitive disadvantage. Maritime transportation can provide the weight carrying capacity, the volumetric space and the jolt-free smoothness to utilize electric battery energy storage.

Maritime transportation is compatible with electric battery energy storage technologies that would otherwise be impractical in road transportation and even in railway transportation. Maritime transportation can utilize grid-scale storage battery technologies that involve the physical movement of electrolyte or that involve lower levels of energy storage density that would in turn require additional volumetric and weight carrying capacity. A battery-electric tug boat built to double the width and triple the length of existing tugs and able to carry high weight levels could propel and navigate a train of coupled barges along an inland waterway.

Present Technologies

At present, battery-powered maritime vessels use one of three grid-scale storing technologies that include lithium-ion battery technology, flow-batteries that involve movement of the electrolyte and molten sodium-sulfur battery technology that involves high temperature. While maritime vessels can carry the weight of sodium-sulfur batteries, the technology also requires considerable insulation as these batteries typically operate in excess of 300^oC with up to six hours of power delivery, with delivery durations of up to 18 hours being possible. More recent research aims to reduce the operating temperature of the battery to 100^oC, making the technology more attractive to maritime application.

There are several variations of flow battery technology that include using such materials as vanadium and even uranium in the electrolyte. When electrolyte flows through the battery, it can simultaneously deliver electric power while also being recharged. Such an event could occur when a vessel sails at low speed through a narrow canal or transits through a series of navigation locks where overhead trolley power collection is available. Lithium-ion storage technology avoids the problem of having to operate at sustained high temperature and has no need to flow the electrolyte through the battery in order to operate.

New Battery Contenders

While molten sodium-sulfur batteries need to operate at elevated temperature, batteries that use an electrolyte based on sodium sulfate (NaSO4) can operate at much lower temperature. These “salt-water” batteries are built using low-cost, non-toxic materials and combine a manganese oxide cathode with a carbon composite anode with the potential to offer deep-cycle discharge/recharge life expectancy comparable to that of lithium-ion technology. While offering lower energy storage density that lithium-ion, the ability of maritime application to offer increased weight carrying capacity and increased storage volume enhances the competitive nature of batteries for maritime propulsion.  

While carbon foam batteries utilize sulfuric acid electrolyte, the technology avoids the problem of sulfur build-up of traditional lead-acid batteries. It can apparently offer comparable deep-cycle discharge/recharge performance as lithium ion batteries at much lower cost. Recent developments in silicon dioxide battery technology have resulted in comparable deep-cycle discharge/recycle performance as lithium-ion batteries at lower cost. While lithium-ion technology may offer higher energy storage density that is suitable for road vehicles, stationary applications and mobile maritime applications that offer high weight carrying capacity and high storage volume make the lower costing battery technologies especially attractive.

Short Distance Rapid Recharge

Some designs of rapid-recharge energy storage devices are well suited to very short distance sailing such as ferry services, where rapid recharge can occurring during layovers at terminals. A pair of large counter-rotating flywheels enclosed within a vacuum chamber could undergo many thousands of recharges. Compressed air-over-water would involve high-pressure water tanks connected to pre-pressurized, high-pressure air tanks. During a layover, electrically driven water pumps would rapidly recharge the energy storage system. During operation over short distances, water-under-high-pressure would activate a hydraulic motor that would in turn drive a propeller.

Compressed-air-over water allows a high mass flow rate of working fluid to pass through the hydraulic motor, compared to the lower mass flow rate of air passing through a motor. On a small scale, compressed-air-over-oil technology has been applied to city transport buses to reduce energy consumption during acceleration. The larger scale of maritime technology allows compressed-air-over-water to be applied for short-distance sailing such as ferry services. By comparison, chemical batteries would provide many hours of energy for longer-distance sailing.

Conclusions

While six different grid-scale battery technologies show promise for and could be adapted for short-distance sailing, only four technologies offer low operational complexity and competitive deep-cycle discharge/recharge performance. The heat involved in molten sodium-sulfur battery technology and complexity of flow-batteries would likely relegate such technologies to stationary applications. The high storage density of lithium-ion battery technology makes it the popular choice for on-road applications. 

However, the maritime sector would likely select lower costing batteries that offer equivalent durations of repeated deep-cycle recharge/discharge operation as lithium-ion battery technology. For sailing distances in excess of 20 miles, battery contenders would include any of carbon foam, silicon dioxide or sodium sulfate electrolyte batteries. For extremely short distance sailing, there may be scope to utilize flywheel technology, compressed-air-over-water technology or even mega-scale wind-up clockwork technology that would provide rapid recharge capability.