Quantifying Unmanned Undersea Vehicle Range Improvement Enabled by Aluminum-Water Power System

Aluminum is an attractive energy storage material for underwater propulsion because of its high density and strongly exothermic reaction with seawater. However, the degree to which an aluminum-seawater power system could outperform other systems has remained unknown because of uncertainties about volume and energy costs associated with the balance of plant. This work addresses this problem by developing a thermodynamic model for a complete Rankine-cycle propulsion system based ob the aluminum-seawater reaction and combining this with a scaling methodology for inferring the system's effective energy density. The results show that replacing battery-based power systems with aluminum combustion based ones could increase range/endurance by factors of four to ten over competing technologies. Overall system efficiency is maximized by adjusting the water mass flow to fuel mass flow ratio so as to control the temperature and quantity of steam. Although increasing the amount of combustion byproduct, hydrogen, improves the performance of the turbine, the thermodynamic cost of compressing the hydrogen can be very high. As a result, developing a compact device for achieving an isothermal compression of waste hydrogen is necessary to fully realize the energy density advantage of the aluminum fuel.