Analyzing a micro-solid oxide fuel cell system by global energy balances

A simple method to predict the thermal characteristics of a micro-SOFC system is presented in this study. The basic design requirements for a thermally self-sustaining operation at a stack temperature of about 550 °C are assessed. Based on steady-state global energy and mass balances, the influence of the electrical efficiency, the overall air-to-fuel ratio and the heat losses on the operating temperature is discussed. It was found that at high electrical efficiencies and, hence, low heat release rates, a recuperator is needed to achieve the desired operating temperature. At lower electrical efficiencies, in contrast, disposing the released heat becomes an issue and an efficient cooling of the stack is required. Whether a recuperator or additional cooling components are necessary also depends on the electrical power output, the stack size and the thermal insulation specifications. The threshold between cooling and recuperator mode is of special interest, since this operating point allows a simple design of the thermal system without recuperator and only a minimum of air to be supplied to the system. This threshold efficiency, which is the maximal electrical efficiency that allows a thermally self-sustaining operation without recuperator, is above 50% under adiabatic operating conditions. In a non-adiabatic system, the threshold efficiency is reduced to about 45% in a 20 Wel and to about 20% in a 2.5 Wel system even with a state-of-the art thermal insulation. The considered thermal insulations dominate the system volume. Therefore, the ability of heat loss minimization is highly dependent on the targeted volumetric power density of the system.

► Steady-state thermal design for butane fueled micro-SOFC systems.
► Development of constant temperature operating maps to identify possible thermal design configurations.
► Case study for electrical power outputs between 2.5 W and 20 W with link to geometry parameters.
► Operating temperature of 550 °C achievable by a state-of-the-art thermal insulation.