Efficiency Optimization of SOFC Subject to Degradation and Minimum Power Constraint

Solid Oxide Fuel Cell (SOFC) is a developing energy conversion technology featuring high efficiency, power density, durability, and fuel compatibility. With these advantages, SOFCs are combined frequently with heat engines such as gas turbines, for higher system efficiency and power density. In practice, SOFCs are often required to deliver certain power. Considering economical aspects, power generation efficiency is critical to the running cost of energy conversion systems. However, high power and high efficiency are likely reached under different operating conditions. For example, to meet peak power demands, higher operating currents and fuel flowrates are necessary, while SOFCs are generally more energy efficient at lower current densities and fuel flowrates. Hence, the ideal operating condition is the solution to a power-constrained efficiency optimization problem. Moreover, SOFCs degrade as they operate, introducing gradual changes to SOFC characteristics. Thus, the optimization needs to be solved dynamically. To analyze the above-mentioned problem, a 10 cm by 10 cm planar SOFC was repeatedly tested for polarization characterizations and electrochemical impedance spectra. A 2-dimentional multi-physics model was developed and calibrated using both polarization and impedance data. To account for the lower open circuit voltage at lower fuel flowrates, an equivalent leakage current is included in the model. The model was thereafter employed for efficiency optimization given power constraints. At each required power output, the optimal fuel flowrate and the corresponding voltage, current, and efficiency was calculated. The electrical efficiency peaks globally at around 0.1 Standard Liter per Minute (SLM) hydrogen flowrate, while at higher power, the optimal efficiency is reached at fuel utilizations between 65% and 90%. Accounting for degradation in terms of growing ohmic resistance and decreasing electrode exchange current densities, the optimal efficiency is reached at lowering fuel utilization and lowering voltage, which may result in local oxidization of the anode. Moreover, an easy and straight-forward way was proposed to estimate the optimal fuel flowrate given polarization data. Further studies involving the safety constraints will be carried out in the future. Figure 1