Selection criteria for a pressure exchanger for energy recovery in submarines and exergy analysis by numerical methods

During submarine operations, requirements such as stealth, extended submerged endurance, and low noise levels are of critical importance. Conventional submarines are typically equipped with diesel electric propulsion systems, where the power required for underwater operations is supplied by energy stored in batteries. Consequently, the duration of submarine missions is limited by the energy storage capacity of the batteries. Air-independent propulsion (AIP) systems allow energy stored in various forms to be utilized for submarine propulsion without dependence on atmospheric oxygen. Since hydrocarbon or alcohol-based fuels are often used in AIP systems, catalytic reactions in the reformer generate carbon dioxide as a byproduct. The carbon dioxide produced must be discharged outside the pressure hull in a silent, safe, and economical manner. Considering the operational constraints, dissolving the carbon dioxide in seawater at low pressure and discharging it outside the pressure hull is considered the most suitable solution. The pressure exchanger is an innovative energy recovery device, consisting of a rotating rotor, housing, and connection ports, which enables direct energy transfer between high and low-pressure fluids and enhances the overall system efficiency. While this system is commonly used as an energy recovery device in seawater desalination processes, in this thesis, a pressure exchanger is investigated for its applicability in transferring the pressure of seawater taken into the pressure hull at different operational depths to a carbon dioxide dissolving unit operating at up to 7.2 bar, and subsequently pumping the dissolved CO₂ back outside at the working depth. In this study, the selection criteria for the pressure exchanger system were evaluated in terms of technical suitability and energy efficiency, and the optimal configuration was determined. Computational Fluid Dynamics (CFD) analyses were then performed for the selected, high-efficiency, and maintenance-free pressure exchanger, and these analyses were repeated for different submarine operating depths. The technical feasibility of integrating such a system into a submarine was discussed based on the CFD results, and an exergy analysis was conducted to assess the thermodynamic performance of the pressure exchanger. As a result, the study demonstrates the energy recovery potential and applicability of the proposed system for submarine operations.