Influence of the Type of Oxygen Carriers on the Performance of a Hybrid Solar Chemical Looping Combustion System

A thermal analysis of a hybrid solar chemical looping combustion (Hy-Sol-CLC) system is presented to identify the energetic performance of various combinations of fuel and oxygen carriers. Three fuels, namely, natural gas, carbon monoxide, and hydrogen, are assessed in combination with the oxides of five metals, namely, Co, Cu, Fe, Mn, and Ni as oxygen carrier. Of primary importance is the heat of the fuel oxidation reaction due to its relation to chemical solar thermal energy storage. Also assessed are their solar share, the fraction of stored energy relative to the total input energy (from both the fuel and the concentrated solar energy), the fuel conversion efficiency, the system Carnot efficiency, the metal oxides energy density, and the volumetric heat capacity. The calculations show that, from the assessed pairs, only CoO/Co, NiO/Ni, and Fe2O3/Fe3O4 can be utilized in Hy-Sol-CLC systems working with natural gas. The calculations also show that the highest system Carnot efficiency is achieved with Co, followed by Ni and Fe, while the highest solar share is achieved by Fe. Nevertheless, on balance, the nickel-based OC particles, employing NiO/Ni as the active metal oxide, are judged to achieve the best performance for Hy-Sol-CLC systems using natural gas as the fuel. It is also shown that carbon monoxide cannot be used in the Hy-Sol-CLC system because the CO oxidation reaction with all of the considered metal oxides is exothermic. Furthermore, hydrogen can only be used with the Fe2O3/FeO and FeO/Fe pairs. The volumetric heat capacities of these oxygen carriers, together with NiAl2O4, Al2O3, SiO2, TiO2, and ZrO2, are also reported as an indicator for the cost of storage.