Thermodynamic analysis of a solid oxide co-electrolysis cell system for its optimal thermal integration with external heat supply

A solid oxide co-electrolysis cell system is expected to play a more crucial role in the field of energy and environment under the pressure of carbon emission regulation. Based on its high-operating temperature and ability to decompose steam and carbon dioxide simultaneously, solid oxide co-electrolysis cell can provide highly efficient and economic means of carbon conversion to synthetic gas. However, most of the previous studies are focused on the performance of a stack itself without investigating the key features of a thermodynamic system. Thus, in this study, we develop a solid oxide co-electrolysis cell thermodynamic system model which incorporates design variables and empirical correlations of a stack and several balance-of-plant components as well as fluid characteristics of incoming working fluids. The basic concept of a solid oxide co-electrolysis cell system is designed and integrated with an oxy-fuel combustion power cycle from which high-quality heat and high-purity carbon dioxide are supplied. 72 different system layouts are suggested, each of which has unique thermal integration regarding external heat supply and exhaust gas utilization method. Using temperature-heat transfer diagram analysis, the system layouts that are thermodynamically infeasible are screened out. After screening out similar layouts once again, 11 candidate system layouts are chosen and investigated in detail. It can be evidenced that using minimal amount of external heat is key to increase the system efficiency. Moreover, immediate branching of external heat flux into two main-heating heat exchangers as soon as it is discharged from the steam generator shows the least pressure drop in the main heat exchanger for air due to its short heat exchange path, minimizing the parasitic power. The optimal solid oxide co-electrolysis cell system layout shows the energy efficiency of 37.31% and the exergy efficiency of 23.97%. This study suggests the key idea of thermal integration of solid oxide co-electrolysis cell system integrated with potential high-quality heat and carbon dioxide sources, which provides effective and reliable basis on which system demonstration and actual operation can be performed.