Prospects for Battery Electric Ferryboats

The history of using electrical storage batteries for marine propulsion dates back for almost a century. Batteries allowed the early submarines to travel submerged until the development of snorkels that could carry air for diesel engines from above the water surface. Lead-acid batteries also stored propulsive power for low-speed trolling fishing vessels. Modern electrical storage battery technology has evolved beyond the limitations of the early lead-acid battery and can find application in large-scale marine transportation.

The development of renewable energy technologies such wind energy, ocean wave and ocean tidal current conversion has created a demand for grid-scale energy storage. Several companies are developing a variety of electro-chemical storage batteries that are capable of grid-scale energy storage capacity. The volume (size) of these technologies and their storage capabilities allow for possible application in a variety of marine transportation services, especially ferry services and short-distance sightseeing cruise operations.

Redox-Flow Battery:

The redox-flow battery stores electrical energy in a metallic oxide electrolyte that can flow between a storage tank and the battery, where an electro-chemical process transfers energy into electric power lines. Several makers of redox-flow batteries use vanadium oxide as the electrolyte and experiments are underway that involve other compounds such as uranium oxide. It is possible to recharge the liquid electrolyte of a redox-flow battery while the battery is delivering power.

Suppliers such as Prudent Energy offer banks of redox-flow batteries of 22Mw output over 7.5-hours. The volume of the battery energy storage system can fit into the hold of a ship used in ferry service. Variations in the volume and capacity of the energy storage system can offer higher output over an extended service cycle. During layovers at port or at ferry terminals, it may be possible to exchange the electrolyte in a similar operation to dialysis. The operation would involve pumping out the spent electrolyte and replacing it with freshly recharged electrolyte.

Molten Sodium-Sulfur (Na-S) Battery:

The Japanese company NGK Insulators has developed a grid-scale storage battery that stores electric power in molten sodium-sulfur system held at a temperature of 900?F or 483?C. A bank of Na-S batteries can store 34MW that may be available over a period of 7.5-hours, with volume that may be accommodated in the insulated hold of a ship. Variations of the technology may allow ships to operate voyages that last between 15 to 20-hours.

NGK’s Na-S battery technology can be recharged each day and has a rated life expectancy of 4,500-cycles. The Na-S batteries would provide over 12-years of service providing propulsive power for ship being recharged during daylight and undertaking overnight ferry voyages. As with grid application, the batteries may be recharged at night during power station off-peak hours and undertake daytime voyages.

Lithium-Polymer Battery:

A company called Electrovaya has developed a proprietary design of lithium-ion battery. They advise that their technology may be adapted for grid-scale energy storage application and have so far proposed a prototype/demonstration concept of 100Mw-hrs capacity. Electrically assisted bicycles that are sold across North America store energy using rechargeable lithium-ion batteries and several makers of electrically powered cars are testing large versions of the same technology.

Electrovaya is so far one of the few makers of lithium-polymer batteries that envision using the technology for grid-scale energy storage. There is scope to adapt the technology to marine ferry operations. The life expectance of lithium-ion batteries is estimated at 5,000-cycles with possibilities extending to 10,000-cycles. The extended-life version of the batteries lends themselves to multiple daily recharges. Quite recently, an experimental electrically powered city transport bus entered service in Hong Kong, storing energy in lithium-ion batteries that are rapidly recharged at several time-stop points en route during each trip.
    
The precedent in Hong Kong suggests that electrically powered ferryboats may recharge the lithium-based batteries during the stopover at the turn-around point at the end of each crossing. Ferryboats powered by lithium battery technology may operate services across channels of 10-miles to 50-miles width. Premium routes for such vessels would include such links as Helsinki-Tallinn across the Gulf of Finland and Liverpool-Douglas in the Irish Sea.

Super-Capacitor Storage:

A capacitor is an electrical device that stores an electrostatic charge on the surface of conductive and/or semi-conductive materials. Certain types of bi-metallic oxide molecules such as barium titanate (BaTiO3) can actually store the electrostatic charge within their chemical structure. Coating the conductive plates of capacitors with barium titanate increases the electrostatic energy storage capacity of capacitors. They then become known as super capacitors or ultra capacitors. 

There is potential for ultra-large volume, low-tech super capacitors to store enough energy to propel ferryboats over a short-distance for up to 10 or 15-minutes duration, the time required to cross a short and deep channel. The technical challenge is to increase available surface area within the available volume being occupied by each super capacitor. The metal making industry is able to mass-produce ultra-thin hollow tubes for the medical sector for use as syringes.

Both inside and outside surfaces of the thin tubes can hold an electrostatic charge, allowing stacks of such tubes to store energy in a super-capacitor. A metal factory may be able to coat the outside of each tube with a layer of semi-conductive, bi-metallic oxide material to increase electrostatic storage capacity. Several such materials occur naturally in the earth as ores and include ilmenite a.k.a. iron titanate (FeTiO3) and iron chromate [Fe2(CrO4)3].

Arrays of hollow thin metallic tubes coated with semi-conductive material may be fused perpendicular to the conductive plates in large-size, low-tech super capacitors capable of storing propulsive energy for short-distance ferryboats. Such storage devices may be capable of 100,000 to over 1,000,000-deep-cycle recharges and discharges will require recharge during the brief layover at the end of each ferry crossing. While the technology may be several years into the future, it may fulfill the propulsive energy requirements of several short-distance ferry operations.

Recharging the Marine Batteries: 

If the grid-scale battery powered ferryboats operate daytime voyages, they may recharge the batteries during the power station overnight off-peak hours. Vessels that operate overnight voyages would recharge at higher cost on daytime electric power, between and after the peak demand periods for electric power. Ferry operators may invest in their own electrical energy storage technology that may recharge at lower cost during the overnight off-peak periods. They may then use the stored energy to recharge their electrically powered ferryboats during daylight hours.

Conclusions:

The economics of energy will determine the viability and practicality of operating battery-powered ferryboats, using grid-scale energy storage technology adapted for mobile application. Such technology may replace diesel-powered ferryboats at locations where the cost of the fuel, engines and engine maintenance exceeds the combined cost of electric power, electrical storage batteries and related maintenance costs. The prospect of higher future fuel prices enhances the long-term future prospects for rechargeable battery powered ferryboats.

-Harry Valentine