Hybrid solid oxide fuel cell-gas turbines and triple combined cycles approaching 85% efficiency

Hybrid cycles combining Solid Oxide Fuel Cells (SOFCs) with gas turbine cycles can achieve a net electric efficiency significantly higher compared to conventional combustion-based cycle. This work proposes and compares three different hybrid cycles. The systems are modelled using a software developed by researchers of the Department of Energy of Politecnico di Milano, and the performance is optimized through sensitivity analyses on the compression ratio, the primary fuel/air ratio, the secondary fuel/air ratio, the SOFC single-pass fuel utilization factor and the SOFC voltage, centred on state-of-the-art parameters for modern SOFC technologies. The results show that the triple combined cycle can achieve an electric efficiency of 84.4% with a pressure ratio of 15, while the regenerative hybrid gas cycle reaches an efficiency of 80.7% in correspondence of a pressure ratio equal to 16. However, the main limitation of these power cycles is the specific capital cost (in the range between 1536–1582 €/kWel), primarily associated with the fuel cell modules. The analysis, based on the average natural gas price of 2023 and assumptions for SOFC cost deriving from a GW-scale production volume, reveals that the Cost of Electricity of a hybrid cycle is close to that of the reference H-class combined cycle (71 €/MWh). However, the economic advantage of the hybrid cycles increases with the fuel price, making them a very interesting option for high cost green fuels, such as green hydrogen.