Second Law viability of upgrading waste heat for thermochemical hydrogen production

A thermodynamic analysis is presented to upgrade “waste heat” with chemical heat pumps for hydrogen production by thermochemical water decomposition. Low-grade waste heat refers to low-temperature heat that typically has limited or no utility in practical applications. In this paper, low-grade thermal energy is upgraded by releasing heat at successively higher temperatures in exothermic reactors of salt/ammonia and MgO/vapor chemical heat pumps. If waste heat is partly recovered to produce electricity by a heat engine, then this electricity can be used to drive compressors that pressurize vapor in the chemical heat pumps and supply heat for high-temperature processes in a copper–chlorine thermochemical cycle of hydrogen production. A Second Law analysis of the system is examined and results for the coefficient of performance (COP) are presented for the heat pumps.