Options for Ship Propulsion Using Grid Scale Batteries

Image courtesy Form Energy

Published Aug 16, 2021 2:40 PM by Harry Valentine

The search for carbon free ship propulsion includes adapting grid-scale batteries for maritime applications. Tech billionaires Bill Gates and Jeff Bezos are providing funding to the Boston-based startup Form Energy, which is developing grid-scale iron-air battery technology projected to cost about one quarter the price of lithium batteries, and would also compete with liquid metal battery technology.


The original battery powered boats used lead-acid batteries to sustain operation of electrically driven propellers on early submarines and on low-speed, short-distance trolling boats, which were used for fishing along rivers. While new-generation grid-scale batteries and energy storage technologies are intended for stationary operation, there is actually scope to adapt some grid-scale battery and energy storage technologies to short-distance coastal ship propulsion. The competing long-life, deep-cycle technologies include lower-cost technologies such liquid metal batteries, vanadium flow-batteries and liquid-air technology. A new competitor called Form Energy is developing a deep-cycle, grid-scale iron-air battery.

Iron-Air Battery

Form Energy is developing a battery that is based on the oxidation of iron, using reversible rusting based on iron, water, air and electric power. During the discharge cycle, it takes in oxygen, which causes iron to rust. The reverse occurs during the recharge cycle when a supply of electrical power converts the rust to iron, which releases or expels the oxygen. An acre-sized battery (which is equivalent to the space covered by 140 forty-foot containers) is estimated to provide over three megawatts for 100 hours.

A large container ship would be able to carry two acres of iron-air batteries (20 widths by 14 lengths of forty-foot containers) to provide over six megawatts (8,000 horsepower) for 100 hours, which should sustain the propulsive requirements of a large ship sailing at 10 to 12-knots. Sailing a large ship with a 183-foot beam and 50-foot draft at 24 knots could require 40,000 horsepower, with 6,000 horsepower being able to maintain a sailing speed of 10 to 12 knots with reduced propeller propulsive efficiency. A more efficient propulsive system designed for 12 knots cruising speed could provide the ship with a range of around 1,000 nautical miles.

Liquid Metal Battery

The Ambri Group of Boston is offering the liquid metal battery developed by M.I.T. Professor Dr. Donald Sadoway. A 40-ft container with this battery is estimated to be able to hold over four megawatt-hours of power. Shipping containers arranged 20 wide by 14 long would occupy two acres of area in the bottom hold of a container ship and offer 1,120 megawatt-hours of power, or 1.5 million horsepower-hours. A ship requiring 6,000 horsepower to sail at 10 knots would be capable of over 200 hours of sailing and covering a possible 2,000 nautical miles.

Small versions of the liquid metal battery have been deep-cycle tested to 100,000 cycles with optimal performance occurring over the first 20,000 hours and offering 99 percent of original storage capacity after 5,000 full drain cycles of service. For ship propulsion, the liquid metal battery sells at a higher price than the iron-air battery and offers the possibility of extended sailing range on a super-size container ship. Possible routes for mega-size ships could include Hong Kong – Singapore, Manila – Singapore and Sydney – Wellington with numerous coastal routes along North and South America.

North American Service

Seawaymax ships are built with a 78-foot beam by up to 700 feet in length, while Upper Great Lakes ships are built with a 105-foot beam by 1000 foot length. A Seawaymax ship carrying nine rows by nine columns of forty-foot containers holding liquid metal batteries would offer 324 megawatt-hours or 434,000 horsepower-hours of energy. Alternatively, it would carry 100 hours of 1.5 MW or 2000 horsepower in fully recharged iron-air batteries. Given the speed restrictions along the St. Lawrence River and Seaway, a ship powered by iron-air batteries could actually operate some types of freight service and passenger tourist service.

Ships designed to sail the Upper Great Lakes could carry 12 widths by 15 lengths of containers could offer some four megawatts (5,300 horsepower) for 100 hours in iron-air batteries, with the ship sailing at a maximum speed of 10 knots. Alternatively, the same space would offer 965,000 horsepower-hours in more expensive liquid metal batteries.

Both iron-air and liquid metal battery technologies incur much lower initial capital cost than the lithium battery technologies being introduced to short-distance and hybrid maritime vessel propulsion. While the iron-air battery would be suitable for short-distance sailing, the liquid metal technology might be suitable for longer-distance operation.

Flow Batteries

Much progress has occurred in increasing the energy storage density of vanadium flow redox batteries the use liquid-state positive and negative electrolyte. As of early 2021, the energy storage density was 40 watt-hours per liter with long-term potential of approaching 70 watt-hours per liter. An insulated 40-foot shipping container could hold up to 64,000 liters of electrolyte in two sections and provide up to 2.5 MWh of electrical energy. A full-size container ship could carry 20 widths by 18 rows of such containers and hold up to 920 MWh of power, enough to sustain a ship of 40,000 horsepower for up to 30 hours at 24 knots, or up to 700 nautical miles.

Slowing the ship to 12 knots at 6,000 horsepower could sustain propulsion for up to 200-hou s and provide a theoretical 2,000 nautical miles of possible operating range. A future version of the battery holding 70 Wh/l would offer 4.5 MWh per container or 1,600 MWh of energy, which could sustain 24 knots at 40,000 horsepower for up to 50 hours and cover over 1,000 nautical miles in distance. A Seawaymax ship could carry 80 energy containers at 40 Wh/l and have over 200 MWh of energy available, for up to 50 hours of propulsion at 10 knots.

While vanadium flow batteries can be recharged during an extended length layover at port, there is an option that involves pumping out the depleted liquid electrolyte from the ship to shore-based holding tanks before pumping in fully recharged positive and negative liquid electrolyte. The company Vanadium Corp Resources Inc. is planning to lease liquid electrolyte to ship operators with the option of being able to exchange spent electrolyte for freshly recharged electrolyte during layover at port.

Alaska Cruises

The Alaska cruise between Seattle/Vancouver and Juneau attracts a significant segment of the international tourist market. Most of the almost 900 nautical mile distance is essentially a river cruise where ships sail through scenic channels. The sailing distance is within the range of developing battery technologies and especially if the cruise ship industry were willing to invest in the development of a power station near Juneau. The power station would recharge ship batteries during the layover period at Juneau. Cruise ships would require two container levels of energy storage to provide for both propulsion and onboard hotel power.

Double levels of 400 containers each of low-cost iron-air batteries should sustain both ship propulsion and hotel power requirements between Seattle and Juneau, while double levels of containers (800) of liquid metal batteries would sustain propulsion and hotel power over longer voyages. A single level of insulated containers (400) of present generation vanadium flow batteries would sustain propulsion and hotel power between Vancouver and Juneau.


While the iron-air battery incurs the lowest investment cost, it also offers the lowest energy storage density and shortest operating range of grid-scale batteries adapted to maritime propulsion. The liquid metal battery offers higher storage density and greater operating range at a higher initial price. Both batteries require extended layover to recharge the onboard long-life batteries. Ships propelled by flow batteries can exchange spent liquid electrolyte for recharged liquid electrolyte during short layover periods at port. Depending on expected operating range, there is scope to adapt otherwise stationary service grid-scale batteries to mobile service.

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