Citation: Rahjoo, M.; Goracci, G.; Gaitero, J.J.; Martauz, P.; Rojas, E.; Dolado, J.S. Thermal Energy Storage (TES) Prototype Based on Geopolymer Concrete for High-Temperature Applications. Materials 2022, 15, 7086. https:// doi.org/10.3390/ma15207086 Academic Editors: Xiaohu Yang, Kamel Hooman and Hubert Rahier Received: 15 September 2022 Accepted: 10 October 2022 Published: 12 October 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). materials Article Thermal Energy Storage (TES) Prototype Based on Geopolymer Concrete for High-Temperature Applications Mohammad Rahjoo 1, * , Guido Goracci 1 , Juan J. Gaitero 2 , Pavel Martauz 3 , Esther Rojas 4 and Jorge S. Dolado 1,5, * 1 Centro de Física de Materiales, CSIC-UPV/EHU, Paseo Manuel de Lardizábal 5, 20018 Donostia-San Sebastián, Spain 2 TECNALIA, Basque Research and Technology Alliance (BRTA), Parque Tecnológico de Bizkaia, Astondo Bidea, Edif. 700, 48160 Derio, Spain 3 Považská Cementáre ˇ n Cement Plant (PCLA), Ulica Janka Král’a, 01863 Ladce, Slovakia 4 Plataforma Solar de Almería (PSA-CIEMAT), Av. Complutense 40, 28040 Madrid, Spain 5 Donostia International Physics Center (DIPC), Paseo Manuel de Lardizabal 4, 20018 Donostia-San Sebastián, Spain * Correspondence: m.rahjoo@csic.es (M.R.); j.dolado@ehu.eus (J.S.D.); Tel.: +34-943-01-8772 (J.S.D.) Abstract: Thermal energy storage (TES) systems are dependent on materials capable of operating at elevated temperatures for their performance and for prevailing as an integral part of industries. High- temperature TES assists in increasing the dispatchability of present power plants as well as increasing the efficiency in heat industry applications. Ordinary Portland cement (OPC)-based concretes are widely used as a sensible TES material in different applications. However, their performance is limited to operation temperatures below 400 ◦ C due to the thermal degradation processes in its structure. In the present work, the performance and heat storage capacity of geopolymer-based concrete (GEO) have been studied experimentally and a comparison was carried out with OPC-based materials. Two thermal scenarios were examined, and results indicate that GEO withstand high running temperatures, higher than 500 ◦ C, revealing higher thermal storage capacity than OPC-based materials. The high thermal energy storage, along with the high thermal diffusion coefficient at high temperatures, makes GEO a potential material that has good competitive properties compared with OPC-based TES. Experiments show the ability of geopolymer-based concrete for thermal energy storage applications, especially in industries that require feasible material for operation at high temperatures. Keywords: cement; concrete; geopolymer high-temperature TES; OPC; thermal energy storage 1. Introduction One of the important factors in intermittent renewable power sources, such as con- centrated solar power (CSP) and solar heat for industrial processes (SHIP), and in waste heat recovery industries is optimal dispatch. A power dispatch optimization method in the thermal industry is the implementation of thermal energy storage (TES) systems. In general, TES assists in enhancing foreseeability, capacity, and managing the state of gen- eration, as well as distribution in energy. High-temperature TES applications sometimes face infrastructure constraints as they require apparatus, materials, and heat transfer fluids capable of working at high-temperature regimes, up to ~1000 ◦ C[1]. The stored amount of heat in sensible TES (Q S (J)) is proportional to the mass of storage material (m (kg)), its heat capacity (C p (J/kg ◦ C)), and the difference between the storage material final and initial temperature (ΔT ( ◦ C)), Equation (1). Therefore, the maximum achievable thermal storage capacity is vastly reliant on achievable ΔT [2]. Q s = m·C p ·ΔT (1) Materials 2022, 15, 7086. https://doi.org/10.3390/ma15207086 https://www.mdpi.com/journal/materials