Thermodynamic Design Analysis of a Solid Oxide Fuel Cell Gas Turbine Hybrid System for High-Altitude Applications

Abstract

A systems-level analysis and design iteration of a liquid hydrogen fueled 50 kW class solid oxide fuel cell gas turbine (SOFC/GT) hybrid auxiliary power unit for high-altitude unmanned aerial vehicles is performed with the intention of investigating system configuration, system parameters and system design constraints on system efficiency. The analysis is performed using thermodynamic models where the sizing of components is allowed to vary. Effects of system pressure ratio, oxygen utilization, and current density on the system design are examined. It was found that an optimal effective pressure ratio exists, indicating that a SOFC/GT hybrid system operating at high altitude should not necessarily be pressurized to one standard atmosphere or higher. It was discovered that the major design constraints for SOFC/GT system operating at high altitude is the large preheating requirement of the liquid hydrogen and cold ambient air at high altitude. To alleviate preheating challenges and increase system efficiencies, five SOFC/GT hybrid systems were developed for high altitude operation. The most efficient system achieved a thermodynamic efficiency greater than 65% at the design power of 50 kW, indicating that SOFC/GT hybrid systems can provide substantial efficiency increases for long endurance unmanned aerial vehicles.