Pressurised Solid Oxide Fuel Cells: From Electrode Electrochimstry to Hybrid Power Plant System Integration

Abstract

Hybrid power plants consisting of a gas turbine and solid oxide fuel cells (SOFC) promise high electrical efficiencies if both components are directly coupled and the SOFC is operated at elevated pressure. This thesis covers the different aspects of this topic ranging from pressure influences on electrochemistry at the electrodes to operating strategies of a hybrid power plant. The influence of pressure on SOFC performance is investigated theoretically and experimentally. Experiments are carried out using a test rig that allows for characterisation of SOFC stacks at pressures up to 0.8 MPa. Polarisation curves and electrochemical impedance spectra are used for evaluations. In addition to experimental investigations an SOFC stack model is developed based on an existing electrochemistry modeling framework. The stack model is experimentally validated and used for a theoretical analysis of pressure effects. Results show that Nernst potential increases with increasing pressure causing a higher open circuit voltage. Furthermore, gas diffusion is enhanced with increasing pressure and the charge transfer reaction is facilitated due to higher adsorption rates of reactants at the electrode surfaces. These effects significantly improve SOFC performance. At constant operating conditions and efficiency an increase in SOFC power density of up to 83 % is measured. If power density is kept constant, electrochemical efficiency is improved by up to 14 %. Results generally show that pressure influence is stronger at low pressures up to 0.5-1 MPa and weakens towards higher pressures. The influence of pressure on formation of nickel oxide and solid carbon is investigated. An analytical evaluation of the nickel oxidation propensity shows that nickel oxidation is more likely to occur at higher pressures because the equilibrium partial pressure of oxygen in the anode gas increases. However, further investigations are necessary as electrochemical oxidation of nickel is not considered in this study. Carbon deposition is another degradation mechanism that can decrease the performance of an SOFC system. It was investigated via thermodynamic simulations using the software package Cantera. Thermodynamic equilibrium of gas mixtures with different oxygen to carbon ratios is calculated showing that the aptitude for carbon deposition is highly pressure dependent. Carbon deposition should be avoidable if oxygen to carbon ratio is kept above 2 within conditions that are relevant for hybrid power plants. The developed stack model is integrated into an existing validated gas turbine model that is extended to include further SOFC system components. A system operating strategy is presented that is based on a gas turbine control. Operating conditions of the SOFC are not directly controlled. A sensitivity analysis is carried out showing that the power ratio between gas turbine and SOFC is the most important parameter in order to achieve a high electrical efficiency. Other parameters like the number of SOFC stacks as well as gas and heat recirculation rates are of less importance. Thermal losses can significantly reduce electrical efficiency if they occur downstream of the recuperator. Finally, the operating range of a hybrid power plant based on the proposed system control is investigated. It is found that high electrical efficiencies above 60 % (based on the HHV) are achievable within an electrical power range from 310 to 670 kW if gas turbine speed and SOFC electrical power are adjusted. A further reduction in electrical power output of the power plant is possible but will result in a significant drop in electrical efficiency.