Coupled heat transfer and chemical kinetics in a calcium oxide/hydroxide fixed bed thermochemical energy storage reactor

Abstract

Among energy storage technologies, thermochemical heat storage despite its unique advantages, remains at a nascent state due to varied technical challenges. One of the key unknowns in the otherwise well-studied calcium oxide/hydroxide chemistry, is a lack of understanding of the possible output power profiles. Here, we investigate the theoretical power output from a fixed-bed, modular, honeycomb-geometry thermochemical energy storage (TCS) reactor based on a calcium oxide/hydroxide, using finite element modeling of heat transfer coupled with reaction kinetics. We calculate the extent of reaction and temperature profiles in a lattice of honeycomb cells that make up the bed, while varying heat transfer related parameters. We find that the size of the unit cell can limit energy discharge, for example, increasing the time to completion for the reaction to ∼7 hours as the cell size increases to 6 cm, even though the reaction kinetics is fast at ∼2 mins. The time constant of the transient power profile increases by a factor of ∼4 as the cell size increases by a factor of ∼3, when considering cells in the nominal size range 2–6 cm. Besides cell size, we investigate how the power profile is affected by the following parameters: the thermal conductivity of the bed, the initial temperature, the material of the cell wall and the heat transfer coefficient of external flows. In particular, we identify the combination of parameters that can vary the power profile on demand to operate the same reactor as either a thermal capacitor or alternately, a thermal battery. This work informs future design possibilities with oxide-hydroxide, solid–gas reaction TCS and provides insight into critical heat transfer parameters.