###### Abstract  
Spain's energy transition poses the dual challenge of managing renewable curtailment and enhancing the competitiveness of concentrated solar power (CSP) technologies. This study evaluates the suitability of replacing molten salts with solid particles for energy storage and, additionally, explores the storage of surplus electricity from grid in Carnot batteries. Four scenarios were analyzed using a Gemasolar-type plant model: each storage medium was studied with and without the integration of curtailed electricity. The solar field was modeled with SAM (System Advisor Model), while curtailment data from Red Electrica de Espana (2016-2021) quantified the available surplus. Results show that solid particles lead to 7.4% higher annual electricity production compared to molten salts, mainly due to improved power cycle efficiency. The integration of curtailment increased output further, with the solid particle Carnot battery scenario achieving the highest performance (up to 19.0% sun-to-electricity efficiency and 69.7% capacity factor). However, round-trip efficiency for curtailment storage was limited (-25-27%), and although solid particles showed lower LCOE (levelized cost of energy) than salts (192 vs. 211 USD/MWh), the Carnot battery increased costs. These findings suggest that while solid particles offer clear advantages, the economic viability of Carnot batteries remains constrained by current cost and operational limitations. capcaptionxcolor=black, outer=0.5 em  
Among renewable energy technologies, solar energy has been identified as one of the most promising options, particularly in its shortwave photovoltaic (PV) and longwave concentrated solar power (CSP) modalities [5,6]. Notably, CSP technology stands out as a key option due to its ability to deliver clean and flexible energy, supported by efficient thermal energy storage systems [7]. Thanks to its high levels of direct solar irradiation, Spain is positioned as a potential leader in the development and deployment of these technologies [8, 9, 10], making the country a strategic location for CSP implementation. Despite its inherent advantages, the LCOE for CSP remains higher than that of other renewable sources such as PV and wind power [11]. Consequently, reducing these costs is a major focus for both the scientific community and industry. One of the most promising strategies to achieve this goal is increasing the operating temperature in solar thermal plants, as several studies have demonstrated its potential to enhance efficiency and lower operational costs [12,13]. On the other hand, the growing penetration of renewable energy has led to an increase in electricity surpluses due to the intermittent nature of some technologies [14,15]. It is estimated that, by 2030, electricity curtailment in Spain could range between 7.5 and 34 TWh\({}_{\text{e}}\)[16,17]. This trend has spurred the development of various energy storage solutions, with batteries emerging as one of the most widely adopted options [18,19]. In this context, thermal energy storage using molten salts, as implemented in modern CSP plants, and solid particles, which stand out for their advantages in thermal efficiency and operational flexibility [20,21]. Beyond the technical analysis, the insights from this study hold direct relevance for energy policy and strategic planning in Spain. The National Integrated Energy and Climate Plan (PNIEC) emphasizes not only the massive deployment of renewable generation but also the need for flexible and dispatchable technologies to ensure system reliability. In this context, CSP plants equipped with thermal storage--especially when adapted to recover curtailed electricity--can provide both long-duration storage and valuable grid inertia. Moreover, the integration of Carnot batteries could support industrial electrification strategies by enabling the use of surplus renewable electricity for high-temperature thermal applications.