Conceptual design for critically-high power generation efficiency by using multi-stage solid oxide fuel cells (SOFCs) or proton-conducting ceramic fuel cells (PCFCs)

Abstract

Recently we have developed the conceptual design for realizing a critically-high electrical efficiency with solid oxide fuel cells (SOFCs). SOFCs are promising electrochemical devices that enable the highest fuel-to-electricity conversion efficiencies under high operating temperatures. The concept of multi-stage electrochemical oxidation using SOFCs has been proposed and studied over the past several decades for further improving the electrical efficiency. However, the improvement is limited by fuel dilution downstream of the fuel flow. Recently we developed and reported a conceptual design that has a potential to realize a critically-high fuel-to-electricity conversion efficiency of up to as high as 85% (LHV, gross DC), in which a high-temperature multi-stage solid oxide fuel cells (SOFCs) is combined with a proton-conducting solid oxide electrolyte. Switching a solid electrolyte material from a conventional oxide-ion conducting material to a proton-conducting material under the high-temperature multi-stage electrochemical oxidation mechanism has proven to be highly advantageous for the electrical efficiency. The DC efficiency of 85% (LHV) corresponds to a net AC efficiency of approximately 77% (LHV), where the net AC efficiency refers to the transmission-end AC efficiency. This evolved concept will yield a considerably higher efficiency with a much smaller generation capacity than the state-of-the-art several tens-of-MW-class most advanced combined cycle (MACC). In the conceptual design the proton-conducting electrolyte was assumed to have a protonic transport number of 1. However, the protonic transport number of the proton-conducting solid oxide electrolyte depends on the material and operating conditions such as temperature, oxygen partial pressure, kinds of fuel and so on, and would affect the electrical efficiency. Therefore, to realize the critically-high electrical efficiency by using the multi-stage electrochemical oxidation, proton-conducting solid oxide having a high ionic transport number should be developed.