Development and Analysis of a Hybrid Solid Oxide Fuel Cell Microturbine System

Hybrid solid oxide fuel cell microturbine (SOFC-MT) systems present opportunities for improvement over conventional systems, including high electric efficiency, cogeneration, and the potential for low carbon emissions. Hybrid systems require stringent control, however, and competing systems (including non-hybrid SOFC systems) currently generate power reliably and efficiently. In order to advance toward commercialization, hybrid systems need to adopt a control strategy that maintains safe and efficient operation, while also exhibiting favorable exergetic and economic performance. The present work investigates the SOFC stack's dynamic response to step changes in control variables, as well as the hybrid and non-hybrid systems' energetic, exergetic, economic, and environmental performances. The numerical, 1-D, SOFC stack model developed herein allows for simulations on multiple timescales. An equivalent circuit combines the fuel cell's irreversiblities with the charge double layer. The hybrid and non-hybrid models integrate the SOFC stack model with the balance-of-plant component models, evaluating the energy and exergy flows through each component. Finally, the techno-economic model calculates the hybrid and non-hybrid systems' levelized costs of electricity (LCOEs). Manipulating the current density is found to be the most effective way to control the fuel cell stack's power, giving rise to instantaneous power changes without restricting the fuel cell stack's fuel utilization. The charge double layer negligibly influences the fuel cell stack's behavior during normal operation, even during proportional-integral control. During baseload operation, the hybrid system model exhibits an LCOE of 8.7 cents/kWh, and the non-hybrid system exhibits an LCOE of 11.9 cents/kWh. The hybrid system also operates at higher electric and exergetic efficiencies (58% (HHV) and 64%, respectively) than the non-hybrid system (44% (HHV) and 51%, respectively). The non-hybrid system cogenerates greater thermal energy than the hybrid system, however, yielding a fuel cost that is on par with that of the hybrid system. Both systems meet the EPA's proposed carbon pollution standard for new combustion turbines of 0.50 kg CO2/kWh. Hybrid systems demonstrate the potential to save fuel and money. Continued development of these systems, particularly focused on improving the system's dynamic behavior and minimizing cost, is warranted. Investment in hybrid systems will likely become viable in the future.