Impact of Process Parameters on Thermochemical Redox Cycle Performance

Abstract

Hydrogen offers an enormous potential for energy storage and as energy carrier for the transportation sector. So far nearly all hydrogen is produced from natural gas, which is a reasonable source of hydrogen in the near term, as it offers modest benefits and lower costs than most other sources. The main target for the future is to develop a carbon-free production path which allows the production of hydrogen renewably at competitive costs. Water is an attractive Hydrogen source. The production of hydrogen by direct water splitting is technically hardly feasible due to the required temperatures of more than 2000°C. In thermochemical cycles the water splitting step is separated into a combination of steps, which enable the water splitting at significantly lower temperatures. These thermochemical cycles make use of redox material which integrates oxygen into its structure in a water splitting step producing hydrogen and which gets reduced in a second high temperature step to be recycled. The high temperature heat which is necessary to run the process can be provided by point focusing concentrated solar systems. Different materials are studied to identify suitable candidates for the redox material. Amongst the material side the main challenges are the reaction kinetics, temperature stability, oxygen uptake capacity, and partial pressure dependency. For the examination experimental set-ups are used like commercially available “Thermogravimetric analysis” and laboratory set-ups, which have been specially designed for the investigation of the redox material behaviour under realistic boundary conditions. In the presentation the experimental set-ups used to analyse redox material will be introduced. Especially the reduction of the redox material with its high temperatures and its requirements on the gas stream conditions is a challenging step of the process. By focusing on this step, relevant parameters for the optimization of the process will be analysed. As will be shown the optimization of this step is crucial for the improvement of the whole process. It will be also shown that the importance of the different process parameters like heat recovery, temperature, pressure, oxygen partial pressure, residence time for the overall process performance highly depends on the concept and design of the solar interface.