Energy storage is the main challenge for a deep penetration of renewable energies into the grid to overcome their intrinsic variability. Thus, the commercial expansion of renewable energy, particularly wind and solar, at large scale depends crucially on the development of cheap, efficient and non-toxic energy storage systems enabling to supply more flexibility to the grid. The Ca-Looping (CaL) process, based upon the reversible carbonation/calcination of CaO, is one of the most promising technologies for thermochemical energy storage (TCES), which offers a high potential for the long-term storage of energy with relatively small storage volume. This manuscript explores the use of the CaL process to store Concentrated Solar Power (CSP). A CSP-CaL integration scheme is proposed mainly characterized by the use of a CO 2 closed loop for the CaL cycle and power production, which provides heat decoupled from the solar source and temperatures well above the ~550ºC limit that poses the use of molten salts currently used to store energy as sensible heat. The proposed CSP-CaL integration leads to high values of plant global efficiency (of around 45-46%) with a storage capacity that allows for long time gaps between load and discharge. Moreover, the use of environmentally benign, abundantly available and cheap raw materials such as natural limestone would mark a milestone on the road towards the industrial competitiveness of CSP.

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The present work is focused on thermochemical energy storage (TCES) in Concentrated Solar Power (CSP) plants by means of the Calcium-Looping (CaL) process using cheap, abundant and non-toxic natural carbonate minerals. CaL conditions for CSP storage involve calcination of CaCO 3 in the solar receiver at relatively low temperature whereas carbonation of CaO is carried out at high temperature and high CO2 concentration to use the heat of reaction for power production by means of a CO2 closed power cycle. Under these conditions, large CaO particles derived from limestone to be used in industrial processes are rapidly deactivated due to pore plugging, which limits the extent of the reaction. This is favored by the relatively small pores of the CaO skeleton generated by low temperature calcination, the large thickness of the CaCO3 layer built upon the CaO surface and the very fast carbonation kinetics. On the other hand, pore plugging at CaL conditions for CSP storage does not limit carbonation of CaO derived from dolomite (dolime). Dolime is shown to exhibit a high multicycle conversion regardless of particle size, which is explained by the presence of inert MgO grains that allow the reacting gas to percolate inside the porous particles

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Thermochemical energy storage in Concentrated Solar Power plants by means of the Calcium-Looping process is a promising novel technology that would allow for a higher share of renewables. A main benefit of this technology is the use of widely available, non-toxic and environmentally friendly calcium carbonate minerals as raw materials to store energy. Efficient integration of the Calcium-Looping process into Concentrated Solar Power plants involves the endothermic calcination of CaCO3 in the solar receiver while the exothermic carbonation of CaO is carried out at high temperature under high CO2 partial pressure. The heat released by this reaction is carried out by the excess CO2 and employed for power generation by means of a closed CO2 cycle. This works explores the multicycle Calcium-Looping performance of naturally occurring CaCO3 minerals such as limestone, chalk and marble for thermochemical energy storage in Concentrated Solar Power plants. Despite their similar composition (almost pure CaCO3), these minerals exhibit a significant difference in their Calcium-Looping multicycle activity, which may be attributed to differences in particle size and microstructure. Pore plugging at the Calcium-Looping conditions for thermochemical energy storage tested in our work is a main limiting mechanism on the multicycle CaO carbonation activity.