Simulation of the copper–chlorine thermochemical cycle

Abstract

The global fossil reserves are dwindling and there is need to find alternative sources of energy. With global warming in mind, some of the most commonly considered suitable alternatives include solar, wind, nuclear, geothermal and hydro energy. A common challenge with use of most alternative energy sources is ensuring continuity of supply, which necessitates the use of energy storage. Hydrogen has properties that make it attractive as an energy carrier. To efficiently store energy from alternative sources in hydrogen, several methods of hydrogen production are under study. Several literature sources show thermochemical cycles as having high potential but requiring further development. Using literature sources, an initial screening of thermochemical cycles was done to select a candidate thermochemical cycle. The copper-chlorine thermochemical cycle was selected due to its relatively low peak operating temperature, which makes it flexible enough to be connected to different energy sources. Once the copper-chlorine cycle was identified, the three main copper-chlorine cycles were simulated in Aspen Plus to examine which is the best configuration. Using experimental data from literature and calculating optimal conditions, flow sheets were developed and simulated in Aspen Plus. The simulation results were then used to determine the configuration with the most favourable energy requirements, cycle efficiency, capital requirements and product cost. Simulation results show that the overall energy requirements increase as the number of steps decrease from five-steps to three-steps. Efficiencies calculated from simulation results show that the four and five-step cycles perform closely with 39% and 42%, respectively. The three-step cycle has a much lower efficiency, even though the theoretical calculations imply that the efficiency should also be close to that of the four and five-step cycles. The five-step reaction cycle has the highest capital requirements at US$370 million due to more equipment and the three-step cycle has the lowest requirement at US$ 275 million. Payback analysis and net present value analysis indicate that the hydrogen costs are highest for the three-step cycle at between US$3.53 per kg for a 5-10yr payback analysis and the five-step cycle US$2.98 per kg for the same payback period.