Thermodynamic Analysis of a Novel Designation of a Cascade Waste Heat Recovery Cycles for 100 MW Nuclear-Powered Vessels

The decarbonization of maritime transport demands propulsion and onboard energy systems that simultaneously achieve ultra-low emissions, high power density, and robust operational reliability. Nuclear propulsion, particularly when coupled with small modular reactors (SMRs), offers a compelling pathway due to its near-zero operational emissions and exceptional energy density; however, the efficient utilization of high-grade nuclear thermal energy under shipboard constraints remains a critical challenge. To address this issue, this study presents a comprehensive thermodynamic and exergy-based assessment of a novel cascaded waste heat recovery (WHR) architecture designed for a 100 MW class nuclear-powered vessel, integrating a supercritical carbon dioxide (sCO2) cycle with downstream steam Rankine (SRC) and Kalina cycles. Detailed process modeling is performed using Aspen HYSYS to quantify energy and exergy performance at the component and system levels. The proposed cascade exploits the complementary thermodynamic characteristics of each cycle, enabling staged recovery of high-, medium-, and low-grade heat from the nuclear secondary loop. Results indicate that while the sCO2 cycle dominates gross power generation due to its high power density and favorable high-temperature performance, its net efficiency is constrained by substantial compression work and associated auxiliary losses. In contrast, the SRC and Kalina cycles exhibit significantly higher energy efficiencies, demonstrating superior suitability for medium- and low-temperature waste heat utilization. Through thermodynamic synergy, the integrated sCO2–SRC–Kalina configuration achieves an overall energy efficiency of 20.73%, representing a substantial improvement over a standalone sCO2-based WHR system. Exergy destruction analysis reveals that system irreversibilities are primarily concentrated in heat exchangers, particularly the primary heat exchanger interfacing the nuclear heat source with the sCO2 loop, whereas turbomachinery contributions are comparatively minor. These findings highlight heat exchanger design optimization and improved temperature matching as the most effective pathways for further performance enhancement. Overall, this study demonstrates that multi-cycle cascade integration provides a viable and high-efficiency solution for nuclear marine energy recovery, with strong implications for advanced hydrogen energy systems and other high-performance power generation applications requiring compactness, scalability, and thermodynamic robustness.