• Energy Research

Abstract Hybrid power plants consisting of solid oxide fuel cells (SOFC) and a gas turbine (GT) can play an essential role in the future energy scenario due to the expected high electrical efficiency, fuel flexibility and good part-load performance. A demonstration SOFC/GT hybrid power plant is being setup in Stuttgart with state of the art, commercially available electrolyte supported cell (ESC) stacks and its operation is being simulated by means of a overall system model. However, the model used in this paper, in contrast to most models in literature, accounts for heat transfer based on actual geometries and materials. In the present study, the system model is integrated with a set of sub-models that predict the heat losses of the components of the hybrid power plant with a feasible computational speed. This allows for an improved prediction of the operating range as well as for the prevention of undesired operating conditions. The results of the simulations of the stationary operation of the hybrid power plant with varying heat losses are shown and discussed. Operating limitations are analyzed as well as system performance. It is shown that it is possible to operate the hybrid power plant from design power output to 30% of it. A system electrical efficiency higher than 0.55 considering the fuel’s higher heating value is maintained throughout the entire range. Further design choices and developments could lead to an improvement of this condition. In addition, an adiabatic assumption can lead to about 4 percentage points overestimation of electrical efficiency and reduces the high power operating range by about 10%. This approach opens up a new perspective on the simulation of this type of power plant.

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 contribution discusses various aspects of the pressure influences on electrochemistry at the electrodes to operating strategies of a hybrid power plant. The influence of pressure on SOFC performance has been investigated theoretically and experimentally. Experiments are carried out using a test rig that allows for characterization of SOFC stacks at pressures up to 0.8 MPa. Performance 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. As expected, 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. 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 thatnickel oxidation is more likely to occur at higher pressures because the equilibrium partial pressure of oxygen in the anode gas increases. 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.