INDUSTRY TRACK: High Performance Materials for Solar Supercritical CO2 Plants with Solid Particles as Heat Transfer and Storage Medium

INDUSTRY TRACK: The project COMPASsCO2 covers a large set of topics related to materials in concentrating solar thermal (CST) application, particularly for industrial decarbonization and heat storage. As this project is coming to its end, this work summarizes the main results from the development, testing, and modelling of advanced materials designed to enhance CST systems.

One part of the project has been devoted on developing and testing new solid particles and their coatings for use as heat transfer and thermal storage materials. Solid particle systems offer promising advantages for future CST systems by enabling higher operating temperatures and considerable cost reductions compared to molten salts. In these systems, concentrated solar radiation is directly absorbed by dark solid particles, significantly reducing the need for high-temperature alloys and high absorptance coatings in receivers, which are prone to durability issues. and ensure high receiver efficiency. Sintered bauxite particles, commonly used in the oil and gas industry, were evaluated for use in solar towers, but their declining solar absorptance over time presents a challenge. The research task is to develop new particles with high solar absorptance, minimum optical ageing over time and high heat capacity for a cost equal or lower to the above-mentioned proppants (about 1€/kg). In response, seven different types of particles were produced and tested within the framework of COMPASsCO2 project, including state-of-the-art options (such as Ultraprop Sintered Bauxite, Interprop and BauxLite) and novel compositions (granulated and fused). In addition, three coatings techniques (chemical deposition coating by CIEMAT, pigment coating in Resonance Acoustic Mixer by DLR and pack cementation coating by DECHEMA) were developed and evaluated.

The project also investigated the development and testing of new alloys to be used as bulk material or coatings for the application in particles / supercritical CO2 (sCO2) heat exchanger (HEX) tubes. While the exterior surfaces of the HEX tubes should withstand erosion, thermal cycling, oxidation, and creep in air, the interior surfaces must be resistant to corrosion and creep in sCO2. Due to the complexity of the operating conditions, the material choice is limited to high-alloyed austenitic steels and Ni-based alloys, which offer sufficient creep strength and corrosion resistance. Beyond commercial materials, novel chromium superalloys (Cr-NiAl bulk and Cr-Cr3Si coating), with promising erosion and corrosion resistance, high melting points, low price and exceptional oxidation resistance were developed, produced and tested. State-of-the-art and novel materials were tested and compared in laboratory set-ups that simulated the above mentioned conditions.

The targeted industrial application is a solar tower power plant that uses a sCO2 Brayton cycle, operating at extreme conditions of 700°C and 260 Bar, to achieve the highest thermal efficiency of 49%. The knowledge gained from this project is not only applicable to the case above, but can also contribute to the decarbonization of other industrial applications that require materials beyond the current state of the art.

Advanced computer models, including AI-tools, have been used to assess the aging of the particles and the tubes under the conditions mentioned and also for fast screening and characterization of potential innovative alloys combination. These methods will be summarized in this work.