The present invention relates to energy storage systems and reactors useful in such systems. Inventive reactors comprise a reaction vessel defining an inner volume and a compensation element, whereby said inner volume is filled with a fixed bed that is free of cavities and that comprises particles of formula (I), FeOx (I), where 0≤x≤1.5; said compensation element is adapted to adjust said inner volume. The reactors are inherently explosion proof and thus suited for domestic use. The systems are useful for compensating long-term fluctuations observed in production of renewable energy.