Development of numerical code for an in-depth energy, exergy, exergoeconomic (3-E) assessments, and sensitivity analysis of NS Savannah marine propulsion: A pre-optimization-focused approach

The maritime sector heavily relies on heavy fuel oil as its primary energy source for ship propulsion, which has a significant adverse impact on the environment, underscoring the urgent need for clean, eco-friendly propulsion alternatives. This paper aims to develop an in-depth thermoeconomic modeling of a real-world nuclear marine propulsion system, employing the nuclear ship Savannah as the baseline benchmark. Given the limited availability of data, this study utilizes innovative ideas to precisely characterize the thermodynamic properties and energy flows within the nuclear propulsion, enabling a comprehensive exergoeconomic performance assessment. Upon validating the developed nuclear ship Savannah propulsion model, an extensive analysis is undertaken from the energy, exergy, and exergoeconomic viewpoints, combining the principles of the first and second laws of thermodynamics with the specific exergy costing technique. The nuclear propulsion model is subsequently integrated with both local sensitivity analysis and global sensitivity analysis to examine how output variables respond to variations in different design parameters. Four key thermoeconomic performance indexes of nuclear propulsion including the energy efficiency, exergy efficiency, propulsion power, and total product exergy cost rate are considered as system output variables. The study employs a one-at-a-time approach for local sensitivity analysis and utilizes the variance-based Sobol method for global sensitivity analysis. Within the local sensitivity analysis framework, a novel indicator, termed “dispersion sensitivity index” is introduced to precisely quantify the overall sensitivity of outputs to inputs. This is subsequently compared with the total sensitivity index obtained from the global sensitivity analysis. The energy analysis demonstrates that the high-pressure and low-pressure steam turbines achieve mechanical power outputs of 6.881 MW and 8.209 MW, respectively, with the overall nuclear propulsion efficiency determined to be 26.18 %. The high-pressure steam generator is identified as the primary source of exergy destruction, with a value of 7161.03 kW, while the condensers exhibit the lowest exergy efficiency, around 30.68 %. Additionally, the exergoeconomic evaluation highlights that the high-pressure steam generator bears the highest exergy costs for both fuel and product, at $1490.20 and $1600.70 per hour, respectively, and the highest total operational cost of $110.60 per hour. The sensitivity analysis reveals that the steam flow rate at the high-pressure turbine inlet exerts the greatest influence on energy efficiency, exergy efficiency, and propulsion power with total sensitivity index of 28.5 %, 42.2 %, and 50.34 %, respectively. Conversely, the heat transfer surface area of high-pressure steam generator has the most significant effect on total product exergy cost rate, with a substantial total sensitivity index of 59.34 %. The integration of sensitivity analysis with exergoeconomic modeling of nuclear marine propulsion enables the identification of critical design parameters that have a substantial impact on system performance. This approach facilitates targeted improvements in energy and exergy efficiency, propulsion power, and economic costs. Furthermore, the analysis assesses system robustness by evaluating the variability of outputs with respect to parameter changes, thereby prioritizing optimization efforts to achieve maximum efficiency and cost-effectiveness while minimizing the need for trial-and-error iterations.