Potential of Organic Rankine Cycles for Unmanned Underwater Vehicles

Abstract

Steam Rankine cycles are typically employed as the power system for Unmanned Underwater Vehicles, but with the problem of low system efficiency. In this paper, Organic Rankine Cycles are proposed as an alternative, where the required output power is of the order of 10 kW. The working conditions and associated sizing constraints for power cycles operating at underwater environments are first detailed. The small-scale axial turbine is specifically designed for different operating conditions and incorporated into the system thermodynamic model. Using the established thermodynamic model for turbine and heat exchangers, various organic fluids are scrutinized to maximize system efficiency and to ensure the sizing constraint as encountered at underwater environment. Numerical results show the high-temperature dry fluid with trans-critical cycles can largely enhance system efficiency. The system efficiency of 25.32% and 23.01% is obtained using Cyclohexane and Toluene, respectively, while the sizing constraints are also satisfied. This corresponds to the increase of 6.57%–8.87% in terms of system efficiency compared to conventional steam Rankine cycles. Parameter studies are also performed to study the influence of pinch temperatures on system performance. The work provides the insight into the potential application of organic Rankine cycles for underwater vehicles.