While some large, short-haul maritime vessels use battery-electric propulsion, the useful working life of electro-chemical batteries is comparatively short and involves high cost-of-replacement. But battery technology is available in many forms, and there are exciting possibilities.
The first non-human propelled submarines built in the U.S. during the late 1800s were propelled by steam batteries, that is, they stored propulsive energy in insulated high-pressure tanks of steam compressed to the liquid state. A piston engine drove the propeller.
Today, large-scale versions of this technology are still possible using multiple insulated spherically-shaped pressure vessels holding compressed steam at 150 atmospheres pressure (2,200psi). The solar thermal power industry has developed several heat-of-fusion compounds and mixtures capable of converting water to superheated steam capable of driving turbine engines, with the premium storage material being lithium compounds.
But, the demand for lithium by the medical sector, lubricant sector and electro-chemical battery sector has raised its price. An alternative non-lithium thermal storage material involves a mixture of sodium chloride and magnesium chloride that melts in excess of 400⁰C (750⁰F) and can raise superheated steam.
A maritime vessel power by high-temperature thermal batteries would require access to a coastal thermal power station to recharge its energy supply. Depending on size and power output, a thermal battery ship or tug-barge could provide an operating range of 200 to 500 nautical miles.
A cheaper option may be possible in tropical regions involving maritime vessels that may require an operating range of perhaps 100 nautical miles between a mainland and offshore island. An insulated barge towed by a tug may carry a massive amount of ice. The heat sink to produce the ice may be located offshore at deep levels of sea, with water temperature near 5⁰C. Even at tropical locations, water-saturated porous rock some 100 to 200 meters below ground surface can be as cool as 10⁰C or 40⁰F, reducing the amount of energy to convert water to ice.
A large-scale steam-vacuum cooling system can operate on waste steam and provide a water temperature of 5⁰C, as can new-generation heat-driven absorption refrigeration technology. Closed-cycle ammonia-turbine engines aboard a tugboat can generate propulsive power between the surface temperature of tropical seawater (35⁰C or 95⁰F). The ice in the towed barge, perhaps sufficient for the tug to propel an articulated barge coupled ahead of it, between two nearby coastal ports or between a mainland and an offshore island.
Despite low propulsive efficiency, the energy storage technology involves low cost, long service life and very low replacement cost.
The propulsive technology is based on ocean thermal energy conversion, proposed to generate electric power offshore of Hawaii and India, using the difference in seawater temperature between the surface and 300-metres depth. In maritime propulsion, a tug pushing a revenue barge and towing an insulated ice barge may arrive at a port, exchange ice barges and push a different revenue barge to another port.
A large fleet of ice barges could sustain the operation of a small fleet of tugs that source propulsive power from the surface temperature of seawater.