One of the energy policy goals of the European Union is to achieve decarbonization of the power sector by 2050. To achieve this target, a new power generation system that can increase considerably the thermal to electric conversion efficiency is envisaged: Brayton cycle with supercritical carbon dioxide (s-CO2) as the working fluid. The project COMPASsCO2 focuses on the efficient and reliable integration of storable solar energy provided by concentrated solar power systems into s-CO2 Brayton cycle[1].
Concentrated Solar Power (CSP) systems use large mirror areas to concentrate solar radiation onto a receiver and provide this energy to a heat transfer medium (thermal oil and molten salt mainly). In this project, fine ceramic particles are envisaged as the storage and heat transfer medium. These solid particles allow higher process temperatures compared to molten salts (>600°C). Once heated, the particles may be stored in an insulated tank and / or used to heat a secondary working fluid (s-CO2)[2] .
The solid particle receiver concept requires the identification of materials that will survive repeated thermal cycles at high temperatures (up to 1000°C). The particle selection should match an adequate functionality in the CSP collector as well as in the heat exchanger. Previous studies revealed that a class of ceramic materials, known as proppants, may offer desirable combinations of properties to be used in CSP[3] . Proppants are originally developed to keep an induced hydraulic fracture open during, or following a fracturing treatment. In general, proppants are developed to exhibit high degrees of sphericity (to permit high flowability) and high strength to avoid failure to crushing. Consequently, ceramic proppants can be a suitable option due to their high thermophysical and also to their optical properties (dark color).  However, proppants are not stable under oxidizing conditions and exhibit a color change, and consequently a reduction in solar absorptance, when heated in air for several hours at 700°C or more[2, 4]. Moreover, due to the market demand decline, most of the proppants suppliers have closed production.
In the frame of the European project COMPASsCO2, new ceramic particles specifically designed for CSP tower system with focus on a particles/ s-CO2 heat exchanger are developed by Saint-Gobain. Solutions based on two different processes (fusion and granulation) have been selected. These new ceramic media have been characterized, tested and compared to proppants as potential new materials for CSP plants particularly in terms of mechanical, thermal, optical properties and durability. 
 

ACKNOWLEDGMENTS
This project has received funding from the European Union’s Horizon 2020 Research and Innovation Action (RIA) under grant agreement No. 958418.