(210c) Assessing the Efficiency Limits for Hydrogen Production by Thermochemical Cycles

The present revival in interest in thermochemical cycles as a means of producing hydrogen has generated a number of publications which present new flowsheet variations for existing processes as well as additional cycles for consideration. In all of these, the overall process efficiency is a key parameter, although there remains some variability in the way in which this parameter is defined. Variations in process flowsheets and a lack of consistency in the way in which process efficiency is used can lead to significant uncertainties over the relative values of different cycles and suggests the adoption of a more consistent approach. The value of a Carnot efficiency for determining the limiting efficiency of a thermal process producing work is taken for granted in power engineering and a similar approach for thermochemical cycles would aid in the assessment of the maximum efficiencies available, irrespective of the means of implementation and any subsequent flowsheet variations. The work presented will set out a basis for representing such a maximum possible efficiency, based on a step by step examination of each stage of the cycle in terms of its free energy change and heat requirements. The maximum thermodymamically allowed internal heat transfer is used to minimise external heat input and an overall efficiency is calculated based on the residual free energies and heat requirements, including the recombination of hydrogen and oxygen. Residual free energies include the thermodynamically calculated values associated with chemical changes and species separation and may be combined consistently with other work terms such electrical work associated with hybrid cycles allowing a more general comparison of thermochemical cycles to be made. In this way, the thermochemical cycle is viewed as a mechanism for converting heat into work, as represented by stored chemical free energy. It is demonstrated that for the special case of a simple thermochemical cycle consisting of heating, decomposition, cooling and recombination, an identity with Carnot efficiency is achieved when internal heating and cooling requirements are exactly matched. Since this condition is generally not achieved, maximum efficiencies are less than Carnot, based on the highest and lowest reservoir temperatures. The work presents a numerical analysis of a number of thermochemical cycles, some of which have no previous quoted efficiency values, whereas others have some efficiency data available based on particular flowsheet representations. A number of these have been drawn from the set of 'screened' cycles presented by Besenbruch et al in 2000 with some additional higher temperature cycles included. These include the better known cycles such as Westinghouse, S-I and UT3 as well as less well studied ones such as zinc oxide, nickel ferrite and iron/chlorine. HSC Chemistry v5 has been used to quantify the chemical reactions, phase changes and sensible heat changes. The examination of the maximum theoretical efficiency of cycles in this way provides a useful starting point for any comparison between cycles and also allows identification of the strengths and weaknesses within cycles. The effect of reservoir temperatures has also been explored, since these affect the relative contributions of work and heat for some of the decomposition stages involved. For higher temperature cycles, with potentially higher efficiencies, this can also show the effect of operating at lower, non-optimal temperatures, enabling comparisons with cycles optimised for these lower temperatures.