A solid oxide fuel cell- sCO2 Brayton cycle hybrid system

Abstract

New technologies are being developed to produce electricity cleaner and more efficient. A promising technology among these is the solid oxide fuel cell (SOFC). It electrochemically converts chemical energy into electricity. This process is highly efficient and several types of fuel are suitable. Furthermore, the SOFC operates at a high temperature, thus producing high quality excess heat which can be converted into electricity in a thermodynamic power cycle to increase the efficiency. Commonly this is done by a directly coupled gas turbine (GT).
The supercritical carbon dioxide (sCO2) Brayton cycle has recently received attention for its potential as a next generation power cycle. It combines the advantages of the steam Rankine cycle and air Brayton cycle. So far, two heat sources are mainly considered for this cycle: Nuclear and concentrated solar power (CSP).

The aim of this study is to investigate the potential of integrating a SOFC with a sCOs Brayton cycle. A thermodynamic model of the SOFC- sCOኼ Brayton cycle hybrid system (SSHS) is developed to explore and analyze different concepts that effect the integration of both systems. Methane is converted to syngas in an indirect internal reforming (IIR) setup. The steam required for this process is either fed by a heat recovery steam generator (HRSG) or supplied by recirculating
anodic exhaust gas. Both options are considered. Recirculating the exhaust of the cathode is another options that is explored and analyzed. Two sCO2 cycle setups are analyzed in combination with the SOFC system: A simple recuperative
cycle and a recompression cycle.
Different setups of the SSHS are compared on efficiency, complexity of the system and size of the exchangers. For comparison, a directly coupled solid oxide fuel cell (SOFC)- GT hybrid system is considered as well.

It is found that the recompression cycle in combination with SOFC system is more efficient than the simple recuperative cycle but significantly increases the complexity of the heat exchanger network, recirculating cathodic air decreases the size of the heat exchangers and increases the efficiency and supplying steam through a HRSG decreases the efficiency. Compared to a directly coupled SOFC-GT system the SSHS is a significantly more complex system. However, it does not require a pressurized SOFC since the sCO2 Brayton cycle is indirectly coupled
to the SOFC. The most efficient setup of the SSHS, combining the recompression cycle with cathode recirculation, has a higher LHV efficiency than the directly coupled SOFC- GT hybrid system, 66.58% over 62.38%. This setup of the SSHS is rather complex though. Other setups of the SSHS show efficiencies similar to that of the directly coupled SOFC- GT hybrid system.
A promising result, but the practical feasibility of the SSHS is something that should be carefullyconsidered in future research and practice.