OPTS project aims at developing a new Thermal Energy Storage (TES) system based on single tank configuration using stratifying Molten Salts (MS, Sodium/Potassium Nitrates 60/40 w/w) as heat storage medium at 550°C maximum temperature, integrated with a Steam Generator (SG), to provide efficient, reliable and economic energy storage for the next generation of trough and tower plants.The three-year experimental program will be focused to the full development of the integrated system (TES-SG) up to demonstration level. The SG, with natural recirculation of the MS, can be either positioned directly into the tank (Pool-type) or as an external shell-and-tube once-through SG (Loop-type) with piping system and pump.Since the objective is to develop a new TES-SG concept for large-scale CSP plants, i.e. 50 MWe, the experimental concept has to be verified in a relevant scale (at least 12.5 MWth), maintaining the same main thermo-fluid-dynamic parameters as the full scale system. This test section will be designed, built, and tested. Basic experimental, modelling and engineering activities will be carried out to support the design and optimization of the proposed technology. These activities include assessment of MS heat transport properties under relevant representative conditions for the TES-SG system.Moreover, design, optimization and cost-benefit assessment for a full-scale TES-SG system (125 MWth/50MWe) will be done in the framework of the OPTS project coupled with a CSP (tower or trough) plant.