Modeling and Identification of the Dehydration Process in a CaO/Ca(OH)2-based Heat Storage System

Abstract

Thermochemical energy storage systems are considered a loss free, environmentally friendly solution for storage of renewable energy. This work focuses on a specific application based on CaO/Ca(OH)2 for heat storage. During the endothermic dehydration of Ca(OH)2, heat is stored in the chemical bonds. By bringing CaO and H2O together in the exothermic hydration process, the stored heat is recovered. In this work, we set up a mathematical model to describe the dehydration process. The model covers the reaction driven by heaters in the reactor and the mass flow of gaseous water along a pressure gradient to a heat exchanger, where it condenses. Working towards model based control of the reactor system, the model equations are derived using enthalpy and mass balances as well as reaction kinetics. This yields dynamic equations for the reactor temperature, the water mass in the condenser and the conversion. Three unknown parameters related to heat transfer are identified via an optimization problem. For three data sets we apply a cross validation scheme, where two data sets are used for identification and the third one for model validation. All combinations of data sets in the cross validation were investigated and evaluated with the root mean square error and the qualitative model behavior. Although the identification of a distinct, globally optimal parameter set is challenging, the best parameter set is identified and can be used for future investigations and controller design.