Potential of the Supercritical Carbon Dioxide Cycle in High Temperature Fuel Cell Hybrid Systems

by José M. Muñoz de Escalona, Ricardo Chacartegui, & David Sánchez
Thermal Power Group, University of Seville
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The supercritical carbon dioxide cycle yields a superb performance for intermediate temperature power systems. This reference temperature is typically found halfway from the higher temperatures where conventional gas turbines achieve 40%+ efficiencies and the lower ones where steam turbines are usually employed. This work presents a system where the particular features of the SCO2 cycle can be fully exploited in conjunction with a high temperature fuel cell.

Molten Carbonate Fuel Cells (MCFC) have been proposed for distributed generation given their very high efficiency and low environmental impact. These electrochemical devices operate at constant temperature (600-650 ºC) and generate electricity by oxidising hydrogen electrochemically. In other words, there is no combustion process and, therefore, no NOx emissions. Nevertheless, in spite of the very high chemical to electrical energy conversion (of around 50%), there is still a substantial amount of energy in the form hot exhaust gases that is not used. This is the underpinning idea of fuel cell hybrid systems: a further conversion of waste heat to mechanical energy by means of a gas turbine cycle.

Numerous research works have been developed up to date in the topic of hybrid systems, the vast majority of which make use of conventional hot air turbines or conventional combustion turbines where the combustor is substituted for a fuel cell. In all cases, the rather low exhaust temperature of the cell, which is very similar to the temperature at turbine inlet, affects the bottoming system efficiency negatively (turbine work decreases rapidly whereas compressor work remains constant).

This work presents the benefits of using a SCO2 bottoming cycle in lieu of a conventional hot air turbine. The better performance of the carbon dioxide cycle at lower maximum temperatures yields an upsurge in hybrid system efficiency of almost 10%, approaching the noteworthy 60% value.

This proposal, which is based initially on Molten Carbonate Fuel Cells, is extendable to other high temperature fuel cells like Solid Oxide Fuel Cells (SOFC).