Performance analysis of hybrid systems incorporating high temperature fuel cells and closed cycle heat engines at part-load operation

This work presents a comparative analysis of hybrid systems based on molten carbonate fuel cells and making use of closed-cycle externally-heated bottoming systems. Two options are considered: reciprocating (Stirling) engines and supercritical carbon dioxide turbines. These two engines share the common feature of working on closed cycles with pure fluids (H2 and CO2 respectively) but, at the same time, they differ in their internal structure: Stirling engines make use of volumetric machinery whereas the supercritical carbon dioxide gas turbine (SCO2) is composed by turbomachinery. In both cases, the working fluid is subjected to very high pressures and temperatures in the range of 50–200 bar and 40–650 °C.A brief description of both bottoming systems is provided in the first sections of this article along with their expected performance in on-design and off-design (part-load) operation. The analysis of each system is therefore split into three stages. First, the most relevant features of the models of performance are discussed. Then, a comparison is shown for on-design operation aiming to evaluate the maximum efficiency attainable by the proposed engines. Finally, an analysis of off-design operation is presented.The final section of paper concludes that hybrid systems based on atmospheric fuel cells and externally heated closed-cycle bottoming engines have the potential to outperform conventional pressurised fuel cell plus gas turbine hybrids while avoiding the demanding operating conditions on the fuel cell topping system of the latter configuration.

► Hybrid systems comprise a topping high temperature fuel cell and bottoming engine.
► Hybrids using closed-cycle heat engines achieve close to 60% efficiency.
► Hybrids using closed-cycle heat engines allow safer operation of the fuel cell.
► Hybrids using supercritical CO2 turbines outperform any other configuration known.