Thermochemical energy storage for concentrated solar power

This thesis is focused on the study of a new way of storing thermal energy coupled with already mature CSP technologies in order to enhance their competitiveness in the energy market. Currently the main solutions for existing CSP plants, as thermal energy storage, are based on sensible heat storage technology made using molten salt at high temperature. The alternative way studied in this work is instead based on the exploitation of chemical bonds. The heat is not simply stored but is used to drive an endothermic reversible chemical reaction where the products act as the actual energy storage. The inverse exothermic reaction is then the energy releasing step. This process, called chemical looping, permit to store thermal energy for theoretically infinite time without any losses during the storage period. The system performance and behaviour of the process are strongly affected by the chosen chemical reaction. The focus of this work is on the chemical looping based on calcium hydroxide. The chemical looping process is then composed by two different reactions: the dehydration of the hydroxide, that is the endothermic reaction, and the hydration of the oxide, that is the exothermic one. Their characteristics show that they are suitable for medium-low temperature processes (150 – 600 °C). This range of temperature is high enough to permit the exploitation of a Rankine-Hirn cycle with superheated steam for power production. The effect of reaction temperature and pressure are evaluated in order to enhance the TES performance and its integration with the power production system. Many system modifications are hypothesized in order to improve the system efficiency and the pinch point analysis has been used to see how the management of the available heat sources should be done. The direct integration of a steam turbine in the discharge process of the TES system has been studied in order to enhance the overall performance of the system. It has also been studied the exploitation of ORC, with toluene or cyclopentane as working fluids, for the power generation.