Assessment of Battery-Integrated Hybrid Wind–Solar Plants: A Spanish Case Study

The increasing penetration of variable renewable energy sources requires flexible solutions to ensure system stability and economic efficiency. In this context, this study presents a comprehensive assessment of hybrid plants combining wind farms (WF) and photovoltaic (PV) systems integrated with battery energy storage systems (BESS), using the Casetona project in Spain as a real-world study. Three configurations (PV + WF + BESS, PV + BESS, and WF + BESS) are evaluated based on 2024 operational data combined with simulation tools. Under the assumptions of this study (2024 data, Spanish market), the results indicate that WF generation outperforms PV, mainly due to higher capacity factors and better alignment with high-price periods, while PV output is affected by price cannibalization. Under current Spanish market conditions and at the assumed BESS cost (236 €/kWh), energy arbitrage is not economically viable, yielding negative net present value across all configurations. In contrast, participation in automatic frequency restoration reserve services provides higher revenues under current Spanish market conditions, with the WF + BESS configuration achieving the best performance. From the perspective adopted in this study, the sustainability analysis reveals that the hybrid system enables annual greenhouse gas emissions reductions between 13,695 and 49,195 tCO2,eq, depending on the displaced generation source. Although BESS does not directly reduce emissions, it enhances renewable integration, reduces curtailment, and improves grid flexibility. The results also highlight the importance of regulatory frameworks and market design in determining the economic viability of storage systems. While the quantitative results are specific to the case study and sensitive to regulatory parameters, this study provides a comprehensive and transferable methodology for evaluating hybrid renewable systems with storage, supporting informed decision-making in the transition toward low-carbon and resilient energy systems.