Model-based evaluation of the integration of solid oxide fuel cells and electrolysis cells for high purity oxygen production

• Main Author: Taher, Mohamed Asaad Asaad
• Other Authors: Brandon, Nigel
• Published: Imperial College London 2017
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.739643

Oxygen is used for a wide range of applications, with a globally projected production capacity of 1.8 million tonne per day in 2020. Depending on the economic range and the required purity, various methods are used to extract oxygen. Conventionally, cryogenic air separation is used for the large to medium production scale, characterised by high purity oxygen and relatively low energy consumption, whilst pressure swing adsorption (PSA) is widely used for the small-scale production, with lower oxygen purity and higher energy consumption. A high-efficiency system for high purity oxygen production based on the integration of solid oxide- fuel and electrolysis cells (SOFC and SOEC) was first proposed by Iora and Chiesa in 2009. However, the lack of a detailed methodology and the novelty of such a system necessitated a system-level energy analysis with an emphasis on the SOFC and SOEC to understand the nature of thermal and electrical coupling between them. Here, the initial feasibility of the system has been evaluated considering the lumped-parameter modelling of the SOFC, SOEC and balance of plant. A system energy consumption that is significantly less than that of PSA systems was predicted, and a significant contribution of the stack energy consumption to the overall system energy consumption was observed, suggesting the need for a thorough examination of the electrochemical models. Therefore, the parameter estimation technique has been implemented to validate the electrochemical models based on a 5-cell stack and a single repeating unit SOEC experimental data. A good agreement was obtained between the experimental and model-predicted cell potential across all operating conditions, and key electrochemical parameters were estimated with confidence. The validated electrochemical model has then been integrated into a newly-developed onedimensional model of a planar SOFC-SOEC stack to further improve the predictions of the stack and system performance. Significant contributions of experimental validation and distributed modelling on enhancing the predictions of the stack model were observed. The advantages of the system over PSA systems in terms of energy efficiency and oxygen purity were confirmed. A potential design point of the system was selected via a techno-economic study, revealing an extremely low contribution of the electricity cost to the total cost of production. An adequate thermal integration at both the stack and system levels were demonstrated at the design point.