The Fundación Ciudad de la Energía (CIUDEN), which is under the Ministry for Ecological Transition and the Demographic Challenge (MITECO), has recently finished the operational tests of its energy storage facility that uses sodium-sulfur (NaS) technology
/MADRID, SPAIN, August 29, 2025, 11:30 CET, Renewable Market Watch™/
This facility is part of CIUDEN’s green hydrogen production and energy storage project. The sodium-sulfur (NaS) battery energy storage system functions efficiently at a temperature of 305°C. Upon receipt, the system underwent a series of comprehensive tests, including a cold start-up assessment to ensure the proper functioning of each individual unit—focusing on heating and temperature control mechanisms. Additional evaluations, referred to as hot tests, operational tests, and performance tests, were conducted on the entire system to gauge its overall reliability.
This battery system boasts a maximum nominal charge/discharge capacity of 1,000 kW and 750 kW, while the minimum nominal stored energy is 5,800 kWh (5.8 MWh). The results from the operational, commissioning, and performance tests confirmed that the system operates within the specified limits outlined in the tender documentation.
The project was awarded through a tender process to the Spanish company CYMI (Control y Montajes Industriales, part of the COBRA IS group) with a base budget of €4,840,000. As the integrating company, CYMI has implemented an energy storage system at the Technology Development Centre of CIUDEN in Cubillos del Sil. The batteries for this system were manufactured by the Japanese company NGK, with BASF being the European distributor.
This facility will work alongside other energy storage systems to store renewable energy from a 2.1 MWp solar photovoltaic plant. It will power two types of electrolyzers: one using polymer membrane (PEM) technology and the other utilising high-temperature solid oxide electrolysis cell (SOEC) technology for the production of green hydrogen. The facility’s objectives include experimenting with and self-consuming energy, evaluating the performance and efficiency of the technology, assessing its compatibility with power generation systems, optimising energy management, improving supply security, reducing costs, and testing various usage scenarios.
The green hydrogen production and energy storage project, initiated by the Fundación Ciudad de la Energía, is a significant initiative funded by the European Union’s Recovery, Transformation, and Resilience Plan (PRTR) as part of the Next Generation EU program. This project is designed to gather comprehensive technical data on multiple advanced technologies associated with hydrogen production and energy storage at an industrial scale.
Through systematic experimentation and analysis, the project seeks to identify and establish optimal operating conditions for these technologies. The insights gained from this research will play a crucial role in enhancing the efficiency and effectiveness of green hydrogen production. Additionally, the findings aim to support the broader goal of decarbonising various industrial sectors, ultimately contributing to a sustainable energy future and the reduction of greenhouse gas emissions.
How is the sodium-sulfur battery energy storage system working?
A sodium-sulfur battery energy storage system consists of modules that contain the batteries in which energy is stored. This technology relies on the electrochemical charge and discharge reactions that occur within the batteries, involving a positive electrode (cathode) composed of molten sulfur (S) and a negative electrode (anode) composed of molten sodium (Na). The electrodes are separated by a solid ceramic material called sodium beta alumina, which acts as the electrolyte and allows only positively charged sodium ions to pass through.
To maintain the electrodes in a molten state, the battery’s temperature must be kept within the range of 300 to 340°C, necessitating the use of separate heaters. The main advantages of this technology include its large storage capacity due to high energy density, long service life, resistance to high temperatures, low cost of sodium sulfide, and the availability of necessary raw materials.
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Image Credit: CUIDEN
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