Understanding the Thermodynamic Possibilities and Limitations of the Solid-oxide Fuel Cell, Gas Turbine Double Cycle

A demand for high efficiency electricity generation has led to the development of power plants that combine a solid-oxide fuel cell with a gas turbine engine. The thermodynamically optimal configurations and limitations of this cycle are yet to be fully understood. This study addresses this issue using a novel method for engine optimization, denoted the “attractor” methodology. This method essentially tracks the effect every device in the engine architecture has on the ultimate system irreversibility. To maximize efficiency, this quantity should be minimized. The attractor was developed for optimization of gas turbine architectures and is extended here for use with solid-oxide fuel cells. A series of device orderings and optimal operating points are then motivated through the attractor analysis and confirmed as optimal through parametric analyses. Fuel cell length is a key variable in determining the best architecture. For common 0.5 m fuel cell tubes, the maximally efficient architecture examined is CXinFXBTXout with a possible LHV efficiency of 75%. For a fuel cell with sufficient length to guarantee near-complete fuel utilization at 18 bar pressure, the best system changes to (CI)nCXinFxBTXout with a possible LHV efficiency of 79%. These efficiencies are not only higher than any existing double cycle, but also higher than Mitsubishi Heavy Industries’ reported efficiency of their solid-oxide fuel cell, gas turbine, steam turbine triple cycle.