The thermodynamics of chemical looping combustion applied to the hydrogen economy

Adoption of the hydrogen economy (HE) is one means by which industrial economies can reduce point source CO2 emissions. At its simplest, H2 is generated centrally using a primary energy source to split water; the H2 is then transmitted to end users, thereby ‘carrying’ energy from the central plant to, say, a motor vehicle. Assuming the primary energy input to drive the system comes from fossil fuels, carbon capture at these plants is required to reduce the specific CO2 emissions of the system to the minimum. However, an additional thermodynamic advantage of the HE is often ignored, as it facilitates a rise in second law efficiency in the utilisation of fossil fuels. The HE can be viewed as an open-loop, chemical looping combustion (CLC) system, with H2 as the oxygen carrier. In CLC systems, entropy recirculation leads to a reduction in the reversible reaction temperature; in the HE this results in a rise in the efficiency of both H2 producing and H2 consuming devices. In consequence, the second law efficiency of internal combustion engines burning H2 is increased for a given peak cycle temperature. For fuel cells, with notionally higher thermal efficiency than internal combustion (IC) engines, the percentage gain in second law efficiency is even more pronounced. A process flow analysis allowing for likely irreversibilities shows that combining a CLC plant and a fleet of fuel cells, the overall efficiency of the system equals 40.8%, exceeding the performance of competing fuel powered technologies.