Abstract
Energy storage is important for the application of the discontinuous and unstable renewable energy. The CaO/Ca(OH)2 thermochemical system of high energy density and little heat lose is promising for thermal energy storage, yet the heat and mass transfer processes related to dehydration reaction needs further improvement and the regulation manner related to the hydration reaction needs further investigation. In this work, a hybrid indirect-direct reactor of the CaO/Ca(OH)2 system is proposed with the physicochemical model developed to study the dehydration and hydration processes under different operating and structural conditions, and five indicators are defined to evaluate the overall reaction performance. During the dehydration process, the reactant is indirectly heated by the electrical heater at the reactor outside wall and the reaction proceeds in radial direction which is mainly limited by the poor heat transfer, leading to low energy storage efficiency of 6.1 %. The increase of reactant thermal conductivity and application of high-thermal-conductivity porous channel greatly enhance the heat transfer leading to significant reduction of reaction time and increase of energy storage efficiency up to 25.63 %. During the hydration process, the gas mixture directly contacts and reacts with solid reactant and the reaction proceeds in axial direction with reaction region of “U” shape which is mainly restricted by the reaction equilibrium. The thermal power density and reaction temperature of the hydration process can be regulated by the inlet mass flow rate and the inlet steam mass fraction respectively. The increase of inlet mass flow rate and inlet steam mass fraction leads to higher heat exchange efficiency at the same reaction extent and shorter hydration reaction time. The present study demonstrates the superiority of the indirect-direct type reactor and the performance of which can be further improved by optimizing the geometrical and operating conditions.
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