Spain’s Fundación Ciudad de la Energía (Ciuden), a government energy research foundation, has completed operational testing of a 1 MW/8 MWh vanadium redox flow battery (VRFB) system at its Cubillos del Sil technology center, which it claims is the largest vanadium flow battery in Europe dedicated to applied research.
Ciuden, a body under the Spanish Ministry for Ecological Transition and the Demographic Challenge, has completed testing of a VRFB system at its technology development center in Cubillos del Sil, northwestern Spain.
The 1 MW/8 MWh system includes a dedicated 100 kW/800 kWh module for R&D testing under controlled conditions. The contract was awarded to Spanish company CYMI for €6.4 million ($7.4 million), incorporating technology from South Korean company H2 Inc.
The VRFB system joins two storage technologies already deployed at the site in 2025: a sodium sulfur battery (NaS) with a capacity of 1 MW/5.8 MWh and a lithium-ion system with a capacity of 600 kW/1.3 MWh. The three systems are hybridized with a 2.2 MWp solar plant, bringing the total storage capacity to almost 15 MWh – enough to absorb the solar power plant’s daily production during periods of high generation. The installation also includes two electrolyzers that will be commissioned at the end of 2025: a 300 kW proton exchange membrane (PEM) unit and a 250 kW solid oxide electrolysis cell (SOEC) operating at high temperature.
Ciuden said the VRFB system offers more than 15 hours of storage autonomy, making it the longest-lasting battery system available in Spain for demonstration-scale R&D testing. The technology utilizes the redox reaction of vanadium in four oxidation states in liquid electrolytes stored in independent tanks, enabling a service life of more than 20 years and modular capacity expansion by decoupling power and energy levels.
The primary objective of the project is to generate technical and operational data to support industrial scalability, analyze efficiency, degradation, integration with renewable energy sources and response to different load profiles.
Ciuden said the multi-technology configuration – which combines solar generation, three different storage chemistries and two types of electrolyzers – represents a unique European-scale experimental environment for validating advanced storage and green hydrogen solutions.
The initiative is funded under the NextGenerationEU program within the Spanish Recovery, Transformation and Resilience Plan (PRTR).
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