With the growing concerns regarding climate change and the need to move towards sustainable energy sources and stable energy supply, energy storage systems have become one of the main focuses of research. This thesis addresses this issue through the numerical modeling of a thermocline energy storage system (TES) integrated with internal electric resistances, designed for Concentrated Solar Power (CSP) and Photovoltaic (PV) hybrid plants. This research is done as a part of an ongoing project of Italian National Agency for New Technologies, Energy and Sustainable Economic Development (ENEA). The introduction describes the main environmental and technical issues that the energy sector faces.