The development of battery electric maritime propulsion has involved using lithium battery technology that incurs high cost per kilowatt hour. An alternative liquid metal battery technology that incurs much lower cost per kilowatt-hour offers competitive application for short-distance propulsion along inland waterways.
Introduction
Efforts and initiatives aimed at reducing carbon emissions have prompted the maritime sector to develop battery-powered vessels for ferry and tugboat service. While the present cost of lithium battery technologies is currently around $400 per kilowatt-hour ($/kW-hr), future costs are expected to decline to $250 to $300/kW-hr between 2030 and 2035. A competing liquid metal battery technology developed by MIT for stationary application incurred a prototype limited-production cost of $180/kW-hr. Full scale production versions of the liquid metal battery are expected to drop to below $35/kW-hr between 2030 and 2035, with both stationary and mobile applications.
The liquid metal battery is housed in a container that measures eight feet wide by 10 feet long and 10 feet tall, with storage capacity of 1,000 kilowatt-hours that can be delivered at up to a steady 250 kW over four hours. The container weighs up to 60,000 pounds. Repeated deep cycle discharge of small-scale versions of the liquid metal battery technology has revealed minimal fade over 20,000 full-depth cycles, with the battery having withstood 100,000 deep drain cycles. The battery combines low initial cost combined with extended usable service life.
On the Water
The coupling of standard size barges into trains (tows) provides possible application for short distance propulsion using liquid metal battery technology. While it is possible to install liquid metal battery containers into the construction of a tug, it is also possible for standard-size barges built to 195 feet length by 35 feet width to carry the weight of multiple liquid metal battery containers that each weigh 60,000 pounds. An empty barge typically weighs 280,000 pounds and with water density at 62.4 pounds were cubic foot, would cause the barge to displace 8 inches depth of water.
The combination of barge weight plus that of 40 battery containers (40 x 60,000 = 2.4 million pounds) spread in an array on the barge would displace water to a depth of 6 feet 4 inches. An array of 48 battery modules on the barge would displace water to a depth of 7 feet 5 inches, while an array of 52 battery modules would displace 8 feet of water depth which is the typical navigation depth of barges that operate along American inland waterways. The barge would require a cover for reasons of weather protection and water that could splash into the container area.
Onboard Energy Storage
Each battery container would hold 1,000 kW-hour of electrical energy. A barge carrying 40 battery containers would offer 40,000 kW-hour of energy, or up to 10,000 kW (13,400 horsepower) over four hours duration.
Some of the largest tows along the Mississippi River have involved barges coupled 9 lengthwise by up to 7 abreast for total of 63 barges, pushed by a tug of 10,000 horsepower (7,460 kW). If the last row of barges were battery barges offering 280,000 kW-hour to 364,000 kW-hour of energy, a battery tug could push the tow for between 35 hours and 45 hours. The less powerful tugs on the inland waterway develop 3,500 Hp (2,610 kW) to 4,200 Hp (3,200 kW), allowing a battery barge converted to a tug to push a tow of barges for 12 to 14 hours.
Deeper Waterways
Battery electric tugs built to 40 feet wide by 220 feet long could push barges of containers from the Port of Newark to a container terminal at the Port of New York. A new container terminal is being built east of Montreal, on the south side of the Lower St. Lawrence River. There may be scope for battery electric tugs to each push a barge laden with containers between Port of Montreal’s new terminal and a container terminal on the south side of the island of Montreal. Battery electric tugs could shuttle barges of containers between terminals at Port of Singapore and nearby Malaysia.
The St. Lawrence River and Seaway allows a navigation depth of over 20 feet. A vessel weighing 360,000 pounds and built to the dimensions of a Mississippi standard barge could carry 72 battery containers, displace water to a depth of 11 feet and hold 72,000 kW-hour of energy. It could carry 144 battery containers on 2 levels and displace water to a depth of just over 21 feet while holding 144,000 kW-hour of energy.
Future Power Generation
Political efforts to convert the commercial transportation sector toward greater use of electrical propulsion would require construction of multiple new power stations in several nations, with nuclear conversion being the prime candidate. Development of new power stations requires massive investment over the period of at least a decade and longer. Progress is slow and steady in the area of energy conversion using tidal currents and ocean waves. While the cost of solar power and wind power conversion has been declining, there is emphasis on making newer versions of both technologies more recyclable.
Conclusions
Unlike electric road transportation vehicles that require expensive lithium-ion battery technology to travel for multiple hours, electrically powered large-scale maritime transportation vehicles may use large grid-scale batteries that offer up to 20,000 full-depth discharges that when in full-scale production, incur a cost of around 1/10th per kilowatt hour of lithium-ion battery technology. Some lithium iron phosphate batteries can deliver up to 3,000 full-depth discharges. Lithium-ion batteries are best being rarely fully depleted and can achieve 1,500 cycles when repeatedly discharged from 100% down to 20%, or up to 3,000 cycles when repeatedly discharged from 100% down to 50%.
In tug barge operation along inland waterways, the battery barges would be coupled at the back of the main tow and sail in the hydraulic shadow of the barges coupled immediately ahead, thereby reducing parasitic hydraulic drag. As batteries approach depletion, the battery barges might be exchanged at prearranged locations for barges carrying fully recharged batteries. In battery electric maritime operation, the liquid metal battery, the iron air battery, the aluminum air battery and large-scale lithium-ion batteries would each occupy distinctive market niches within the electrically powered maritime sector.
