Thermal System Modeling and CoSimulation with AllElectric Ship

Thermal System Modeling and Co-Simulation with All-Electric Ship Hybrid Power System Ruixian 1 Fang , Wei 1 Jiang , Jamil 1 Khan , Roger Dougal 2 Departments of Mechanical Engineering 1 and Electrical Engineering 2, University of South Carolina, Columbia, SC The main goal of the present work is to comprehensively model thermal plant on board of future all-electric ship at the system-level to resolve the dynamic interactions between the hybrid power system and thermal system. Specific objectives are: • Integrate an existing Solid Oxide Fuel Cell / Gas Turbine hybrid electrical power model with the ship cooling system model on the same Virtual Test Bed (VTB) platform. • study co-simulation issues between the coupled electrical and thermal systems such as start-up, system control, etc. • Investigate dynamic responses of the coupled thermalelectrical systems under a step change of the service load to reveal important system interactions. • Outline a typical portion of such a configuration for the whole ship systems on VTB platform and serves as the baseline for future All Electric Ship simulation. Detailed SOFC/GT hybrid power subsystem Detailed zonal thermal subsystem System Configuration Objectives Two sets of SOFC/GT hybrid engine subsystems are used for power generation. The high quality heat source exhaust from the SOFC stack is channeled to the gas turbine to produce extra power. After power conversion, the electrical power generated by the SOFC stack is sent to a common electrical bus for allocation. The synchronous gas turbine generator provides extra power to the same DC bus after power generation and conversion. Through flexible distribution and switching architecture, the common electrical bus can supply electrical power to both non-propulsion and propulsion electrical loads and instantly redistribute power as necessary. Figure on the right illustrates the configuration of the zonal freshwater cooling subsystem implemented in the co-simulation. The pumps supply the circulation flow through the freshwater loop and the seawater loop respectively. For the freshwater loop, system will distribute flow into each of the eight PCM cabinets internally based on the fluid system characteristic. The seawater loop is configured as an open-loop in this cosimulation. It will be a closed centralized loop in the ship’s whole cooling system. For propulsion, electrical power from the bus is sent to motor to drive the propellers. Non-propulsion power includes ship’s service load and electric auxiliaries. The interaction between the electrical system and thermal system is implemented through a thermal port on each power consumption component. The losses resulting from the efficiency calculation in each electrical component serve as the forcing function for thermal system. The heat load from the electrical components such as the converters is transferred to the heat sink module in thermal system. Heat sink temperature is considered to be the same as the electrical component being cooled. Simulation Results Gas turbine validation The responses of the PCMs heat losses to the step change of the ship’s service load are shown in Figure (A)-(F). All those figures are obtained from VTB simulation plotting directly, with the Y-axis representing the characteristic concerned and the X-axis representing the time in seconds. Fuel Combustor Bleeding compressor Turbine inlet In the SOFC stack, electrical energy is produced along with the heat generation during the electro-chemical reactions at the electrodes. The produced electrical energy is then supplied to the switchboard. For further energy extraction, un-reacted high– temperature gases are channeled to the combustor for a complete combustion there. The exhaust gas from the power turbine passes through two heat exchangers to preheat fuel mixture and compressed air for maximum utilization of the residual heat. • N = 11427 rpm, • Mass the dark lines in Figure (C), (D) and (E) represents the power converters heat generation, while the grey lines in those figures represents the heat dissipation by their corresponding heat sinks. flow rate for compressor = 21. 018 kg/s. Characteristic curves near design point were extracted and put into VTB model. To obtain 1 -15 bar air pressure, a two-compressor with an intercooler design is chosen to satisfy the operation conditions. The compressed air is then channeled to the cathode of the fuel cell. The high temperature gas from the combustor expands through the two-shaft gas turbines whereby mechanical power is generated. Because of the symmetric arrangement of components in both the power generation subsystem and the propulsion, the values of temperature and heat load of those symmetric PCMs, such as PCM 1 and PCM 4, are always the same. The gas turbine is validated by comparing VTB model results with Gas. Turb commercial simulation software. Boundary conditions for the VTB model match those for Gas. Turb: compressor inlet conditions, air bleed, design point of shaft speed, etc. Design point: Design Point Validation Compressor • Pressures Outlet pressure error 2%, • Outlet temperature error 9% • Error caused by the assumption of ideal compression. Turbine • Outlet pressure error 4% • Outlet temperature error 8% Shaft power error 4% Note: Gas. Turb uses Generic fuel, while VTB assumes methane for these comparisons, we adjust the methane flow rate to match the compressor exhaust temperature. The heat load received by each heat sink comes from each individual electrical component. In this example co-simulation, only the heat losses from those power converters are dissipated into thermal subsystem. The interaction with thermal system is through thermal port on each PCM. Any loss resulting from the efficiency calculation is supposed to be the forcing function for thermal subsystem. These losses are computed from their instantaneous component through power values multiplication with loss coefficients between 2% and 5%. Off- Design Point validation Off-design validation • Same engine settings • Different operating point. • N = 9999 rpm 10% below design point. Compressor • Outlet temperature consistent Gas. Turb with • Outlet pressure error 42% • Error caused by the modeling method of the characteristic curve Turbine • Outlet temperature error 15% • Outlet pressure error 9% Shaft power error 8% Thermal subsystem responses to the step change of the service load PCM temperature variations Conclusion / Future work This work presented an integrated approach for system level thermo-electrical co-simulation. An example simulation by integrating the SOFC/GT hybrid power generation subsystem, power distribution subsystem, propulsion subsystem with a zonal thermal subsystem on VTB platform has been implemented. Both steady state and dynamic simulations are performed. With a step change of the ship’s service load, the transient responses of the heat losses and temperatures of the power converters and the performances of thermal plant are analyzed in detail. The comprehensive analytical models and the system-level co-simulation methodology provided in this paper lead to an improved understanding of thermal management of large scale complex systems. As next stage for such a co-simulation, it’s necessary to enhance the level of detail represented in thermal plant, the power distribution system, and the power conversion system. Such as developing a heat generation model of electronic power converter leg at the switching-averaged detail level, with temperature dependent parameters and averaged heat loss calculations; and Incorporating more sophisticated control system onto thermal plant.