High performance thermal concrete with bauxite residue for energy storage applications.

The objective of this project is to research and develop new concrete mixtures customized for high-temperature thermal energy storage applications, such as bauxite residue (BR) combined with calcium-aluminate cement (CAC) and olivine aggregates from Norway. BR is a waste product from the alumina production process composed mainly of iron oxides, giving it a red color, and silica. BR represents a major waste-problem and this project has aimed at providing a positive way of utilizing BR within the context of large-scale energy storage. BR is highly alkaline in nature, and initial experiments using refined and dried BR as a dry powder additive resulted in enhanced thermal properties compared to ordinary CAC concretes. An important and surprising result is the increased strength for CAC concrete with BR added after high-temperature treatments, which points to the fact that BR is chemically reactive with the concrete at higher temperatures. This is contrary to BR added to ordinary concrete which normally results in lower strength. A challenge for using untreated BR from the aluminum production is the high alkalinity, which results in strict handling procedures for mixing and casting processes. Handling and processing methods developed in the project enables addition of untreated BR to CAC concretes, however the resulting thermal properties have not met expectations even with using refined and less alkaline BR. Further activities in the project have focused on alternative reactive and inactive micro-scale additives for CAC concretes, where MgO has demonstrated the best thermal and mechanical properties. A particular difficulty with the CAC-concrete is that it exhibits quite significant shrinkage during curing and heating. Alternative designs for the thermal elements have been developed in to deal with this problem whereby the shrinkage will be of less importance for the interaction between heat exchangers and concrete. This idea has now been submitted for patenting. New knowledge about concrete mixing and casting has been developed and new ways of characterizing such properties have been established. SINTEF Byggforsk has carried out the material testing the mechanical properties of the different concretes up to 595 degrees. Connected with this, the project has established new techniques for deriving corrected elastic moduli based on force-displacement curves from standard concrete cubes and cylinders. Thermo-mechanical cycling and thermal stress testing has been performed with good success for a laboratory pilot storage at the laboratories of Energigune in Spain. The efficiency of the thermal batteries has been compared with numerical simulations carried out for the test cases. It was found that the correlation between theory and practice was very good. EnergyNest has cooperated with Sibelco Nordic AS regarding the development of the new concrete mixes. Norsk Hydro ASA have provided the BR and they have also performed thermal characterization of material samples at their Herøya laboratories. It has been a great inspiration and motivation for everyone involved in the project to be part of research and development for a new technology with great importance for the society in connection with the green transition and the goal of electrification of the industry.

Prosjektet har med sitt fokus på storskala termisk energilagring søkt å utforske og teste nye betong-typer med forbedrede egenskaper for varmeledning, varmekapasitet og bestandighet ved høy temperatur. En målsetting har vært å kunne gjøre bruk av bauxitt-avfall (betegnet rødslam eller bauxite residue=BR) kombinert med kalsium-aluminat sement (CAC) og tilslaget er norsk olivin-stein. Metode og blandingsforhold utviklet i prosjektet muliggjør tilsetting av ubehandlet BR i CAC-betong, men sammenlignet med alkaliredusert raffinert BR er den forventede gevinsten ved økte termiske egenskaper lavere. Ny kunnskap om produksjonsmetoder og materialets egenskaper samt karakteriseringsmetoder har blitt utviklet i prosjektet. Resultatene har både vitenskapelig verdi og har et vesentlig potensial for storskala, kommersiell anvendelse som forventes å gi en vesentlig samfunnsmessig betydning for det grønne skiftet, og målet om elektrifisering av industri og CO2 reduksjon.

The idea of the research project is to create new concrete mixtures customized for high-temperature thermal energy storage applications using bauxite residue (BR), among other dispersed active or inactive micro-scale filler material, combined with calcium-aluminate cement (CAC) and olivine aggregates. BR is a waste product from the alumina production process, and contains many minerals and active components that may be very well suited for high performance thermal concrete. Sibelco Nordic is the worlds largest producer of refractory concretes based on CAC using olivine mined and sourced in Åheim, Norway. In particular the high content of hematite, alumina and silicon oxide in BR in combination with the very good thermal properties of olivine is expected to enhance the thermal conductivity, heat capacity and thermal integrity of targeted new concrete mixtures up to 600 °C, and even beyond. The new concrete mixture will be used for the Thermal Battery elements and modules of the EnergyNest large-scale energy storage technology.