Comparative studies of Cu-Cl Thermochemical Water Decomposition Cycles for Hydrogen Production

Hydrogen is a potential alternative that has been viewed as a promising fuel with some unique characteristics that makes it an ideal energy carrier that can satisfy all energy needs. Achieving the energy –source goal of... [ view full abstract ]

Hydrogen is a potential alternative that has been viewed as a promising fuel with some unique characteristics that makes it an ideal energy carrier that can satisfy all energy needs. Achieving the energy –source goal of hydrogen requires its production from a non-fossil fuel source and on a large scale. Thermochemical water decomposition is an emerging technology for large-scale production of hydrogen. About 200 thermochemical cycles have been identified but the copper–chlorine thermochemical cycle is regarded as one of the most promising approaches. A 3-step, 4-step and 5-step cycles of Cu-Cl system for thermochemical water decomposition cycles have been studied extensively but there is need for comparative studies that can aid in decision making on the most effective and efficient step cycle especially with respect to the second law efficiency of the process.

This research focus on thermodynamic analysis of the copper chlorine cycles. The cycles were simulated using Aspen Plus software. All thermodynamic data for all the chemical species was defined from literature and the reliability of other compounds in the simulation was compared with some other sources such as HSC chemistry software. The 5-step Cu–Cl cycle consist of five steps; hydrolysis, decomposition, electrolysis, drying and hydrogen production. The 4-step cycle combines the hydrolysis and the drying stage of the 5-step cycle to eliminate the intermediate production and handling of copper solids. The simulation of the 3-step cycle is presented in Figure 1. An optimisation procedure was conducted for the 3-step case. The optimisation objective was the exergy efficiency. Bootstrap aggregated neural network (BANN) was used to enhance model accuracy and prediction.

The results of the analysis gave 59.64%, 44.74% and 78.21% for the 5-step cycle, 4-step cycle and 3-step cycle respectively.  Parametric studies were conducted and possible efficiency improvement of the cycles were found to be between 59.57-59.67%, 44.32-45.67% and 23.50-82.10% for the 5-step, 4-step and 3-step respectively.

The results from the parametric analysis of the simulated process will assist ongoing efforts to understand the thermodynamic losses in the cycle, to improve efficiency, increase the economic viability of the process and to facilitate eventual commercialization of the process.