Thermodynamic and Exergy Analysis of Reversible Solid Oxide Cell Systems
SOCs are devices able of converting electrical energy into chemical fuels, through electrolysis, and chemical fuels into electricity, through electrochemical oxidation.
Although a SOC can theoretically work in both modes, the same can not be directly said for a system. Basically, high conversion system efficiencies largely depends on the addition of specific bottoming cycles, usage of specific components or specific operative conditions.
For instance, it is expected that a SOEC system requires a large heat transfer equipment to bring liquid water from ambient (15 °C - 25 °C ) to the cell operative temperature (700 °C - 800 °C). Conversely, SOFC major requirements would concern gas and air pre-heating. Looking at pressurized systems, while SOFC systems are mainly based on gas compression, electrolysis includes compression of liquid water to a large extent. Although the same process is performed, gas and liquid compression have a completely different energy demands for different equipment (pump and compressor).
In literature, reversible systems have not yet been widely investigated. Consequently, there is the need to address possible configurations and evaluate their conversion performances.
The main goal of the present work is to address differences and similarities among simple SOFC and SOEC systems, based on hydrogen oxidation and steam electrolysis.
In order to evaluate which are important variables in determining SOFC and SOEC conversion efficiencies, how do variables affect system efficiencies, and which are crucial auxiliary components in a SOFC or SOEC system, the present work proposes a thermodynamic analysis. Both energetic and exergetic point of views are adopted to estimate system and components performances.
Systems are modelled with the flowsheet programming tool Aspen Plus. Since this software does not include a built-in model of SOFC and SOEC, these are devised ad hoc.
Scope of the analysis is to identify useful operative regimes for a reversible system, comparing SOFC and SOEC system based on the same variables.
Temperature, pressure, reactant composition, utilization factor and current density emerge as key variables that determine system efficiency.
SOC systems are studied at atmospheric (~ 1 bar) and high pressure conditions (10 bar), as well as at different current densities. For a 90% inlet molar fraction of hydrogen and steam for SOFC and SOEC mode respectively, a fuel cell stack efficiency of ~44% at 800 °C, with a power output of 12 MW is achieved. At atmospheric conditions, overall system energy and exergy efficiencies are ~40% and ~41% in SOFC mode. However, at 10 bar, for the SOFC system, auxiliary compression work is larger than produced. Thus in absence of a bottoming cycle like a Gas Turbine (SOFC-GT systems have been shown to be highly efficient) the pressurisation does not result into a feasible system.
Current densities and stack area obtained in the SOFC mode are used as inputs for SOEC simulations. Power requirements of SOEC are 18273 kW (~ 1bar) and 19890 kW (10 bar), with voltages equal to 1.142 V and 1.243 V, at atmospheric and pressurized conditions respectively.
The corresponding atmospheric SOEC configuration energy and exergy efficiencies of 75.35% and 85.54% are obtained. A similar energy efficiency (76.50%) but lower exergy efficiency (81.44%) is observed for the pressurized SOEC system. Higher exergy losses in the pressurized case are because of the higher current density leading to higher exergy losses within the SOEC. Another contribution appears to be from the wasted fraction of input exergy linked to a higher system outlet temperature and pressure. Heat recovery especially of the water stream also contributes to the exergy losses.
SOEC pressurization can anyway be an advantage from a system point of view. For instance in case of liquid fuel production, syngas pressurization could be replaced by pressurized electrolysis, enhancing overall efficiency. This is due to the lower energy intensity associated with liquid water pressurization, compare to gas.
Looking at system operation and obtained efficiencies, it is feasible to design a reversible SOFC / SOEC system. Overall, multiplying SOFC and SOEC efficiencies, total reversible system energy efficiency is around 30%. This value could be increased with system modifications such as varying the SOFC current density, or introducing SOFC anode and cathode recirculation.