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Six supplementary cementitious materials (SCMs) were identified to be incorporated in concrete exposed to high-temperature cycling conditions within the thermal energy storage literature. The selected SCMs are bauxite, chamotte, ground granulated blast furnace slag, iron silicate, silica fume, and steel slag. A microstructural characterization was carried out through an optical microscope, X-ray diffraction analysis, and FT-IR. Also, a pozzolanic test was performed to study the reaction of SCMs silico-aluminous components. The formation of calcium silica hydrate was observed in all SCMs pozzolanic test. Steel slag, iron silicate, and ground granulated blast furnace slag required further milling to enhance cement reaction. Moreover, the tensile strength of three fibers (polypropylene, steel, and glass fibers) was tested after exposure to an alkalinity environment at ambient temperature during one and three months. Results show an alkaline environment entails a tensile strength decrease in polypropylene and steel fibers, leading to corrosion in the later ones. ; Funding This work was partially funded by the Ministerio de Ciencia, Innovación y Universidades de España (RTI2018-093849-B-C31—MCIU/AEI/FEDER, UE and RTI2018-093849-B-C32—MCIU/AEI/FEDER) and by the Ministerio de Ciencia, Innovación y Universidades—Agencia Estatal de Investigación (AEI) (RED2018-102431-T). This work is partially supported by ICREA under the ICREA Academia programme. This study was partially funded by AEI—Spanish Ministry of Science, Innovation and Universities (PCI2020-120695-2/AEI/10.13039/501100011033 and PCI2020-120682-2/AEI/10.13039/501100011033 through PCI call). Acknowledgments The authors would like to thank the Catalan Government for the quality accreditation given to their research groups (2017 SGR 1537 and 2017 SGR 118). GREiA and DIOPMA are certified agents TECNIO in the category of technology developers from the Government of Catalonia. The authors also thank the companies that provided the material to make possible this experimental research: Gestión Medioambiental de Neumáticos S.L., Arciresa, EDERSA—Masaveu Industria and Promsa—Megasa.

The development of new materials to store thermal energy in a passive building system is a must to improve the thermal efficiency by thermal-regulating the indoor temperatures. This fact will deal with the reduction of the gap between energy supply and energy demand to achieve thermal comfort in building indoors. The aim of this work was to test properties of novel PCM/wood composite materials developed at Riga Technical University. Impregnation of PCM (phase change material) in wood increases its thermal mass and regulates temperature fluctuations during day and night. The PCM used are paraffin waxes (RT-21 and RT-27 from Rubitherm) and the wood used was black alder, the most common wood in Latvia. The PCM distribution inside wood sample has been studied as well as its thermophysical, mechanical and fire reaction properties. Developed composite materials are promising in the field of energy saving in buildings. ; The research leading to these results has received funding from the European Commission Seventh Framework Programme (FP/2007-2013) under grant agreement n° PIRSES-GA-2013-610692 (INNOSTORAGE) and from the European Union's Horizon 2020 research and innovation program under grant agreement No 657466 (INPATH-TES). The authors would like to thank the Catalan Government for the quality accreditation given to their research group DIOPMA (2014 SGR 1543). This work has been partially funded by the Spanish government (ENE2015-64117-C5-2-R (MINECO/FEDER)). Dr. Camila Barreneche would like to thank Ministerio de Economia y Competitividad de España for Grant Juan de la Cierva FJCI-2014-22886.

This paper presents the importance of the thermal losses in the performance evaluation of thermal storage systems. In order to reinforce this statement, an evaluation of a pilot plant whose size is sufficiently representative for the extrapolation of results at larger scales has been carried out. The evaluation of the heat losses of a molten salt pilot plant with 8.1 MWhth built in Spain by Abengoa is presented. While the storage materials development has attracted a lot of attention from the researchers, the performance of a two-tank storage system has not been evaluated in detail. The relevance of the design of conventional systems such as insulation, mechanical assembly or foundation, are found to be the key for the feasibility of a TES system. Different performance scenarios were performed and based on experimental results, decisions for reengineering of the pilot plant could be taken to improve commercial storage plants. ; The research leading to these results has received funding from Spanish goverment (Fondo tecnológicoIDI-20090393, ConSOLida CENIT 2008-1005). The work is partially funded by the Spanish government (ENE2011-28269-C03-02, ENE2011-22722). The authors would like to thank the Catalan Government for the quality accreditation given to their research group GREA (2014 SGR 123) and research group DIOPMA (2014 SGR 1543).

To be able to extend the operation of a solar power plant (CSP) up to 15 h, thermal energy storage (TES) is necessary. But TES also provides more versatility to the plant and makes its reliance during operation hours more dependable. On the other hand, due to the different CSP configurations, a broad spectrum of storage technologies, materials and methods is needed. Sensible and latent heat storage are known technologies in CSP, but thermochemical storage (TCS) is still very much at laboratory level. Nevertheless, TCS has de advantage of nearly no losses during storage and very good volumetric energy density. This review summarizes and compares the different TCS that are today being investigated. Those systems are based in three redox reactions, sulfur-based cycles, metal oxide reduction–oxidation cycles, and perovskite-type hydrogen production, and metal oxide non-redox cycles due to their similarity. This review shows that all these cycles are promising, but none of them seems to have all the characteristics necessary to become the only one storage system for CSP. The main conclusion of the review is that the calcium carbonate is the cycle with most experimentation behind it to infer that it could be viable and should thus be attempted at a research plant scale once a reactivation cycle can be designed; and the manganese oxide cycle, while less developed, is fundamental enough to be a suitable application for desert climates over the rest of the water-frugal or even water-avoiding cycles. ; The work was partially funded by the Spanish government (project ENE2011−22722). The authors would also like to thank the Catalan Government for the quality accreditation given to their research group GREA (2014 SGR 123) and DIOPMA (2014 SGR 1543). The research leading to these results has received funding from the European Union׳s Seventh Framework Programme (FP7/2007−2013) under Grant agreement n° PIRSES-GA−2013−610692 (INNOSTORAGE).

At this moment, the global energy consumption in buildings is around 40% of the total energy consumption in developed countries. Thermal energy storage (TES) is presented as one way to address this energyrelated problem proposing an alternative to reduce the gap between energy supply and energy demand. One way to store energy is using thermochemical materials (TCM). These types of materials allow accumulating energy through a chemical process at low temperature, almost without heat losses. In addition, it is a stable way to perform the heat storage and TCM can be implemented for seasonal storage or/and long term storage. This study compares the cyclability, from the thermophysical point of view, CaCl2 which follows a chemical reaction and zeolite which follows a sorption process to be used as TCM for seasonal/long term storage. The main results show that the chemical reaction TCM is more energy-efficient than the sorption TCM. The CaCl2 calculated energy density is 1.47 GJ/m3 , being the best option to be considered to be used as TCM, even though the dehydration process of the zeolite is simpler and it occurs at higher temperatures its calculated energy density is only 0.2 GJ/m3 . ; The research leading to these results has received funding from the European Commission Seventh Framework Programme (FP/ 2007-2013) under grant agreement No ENER/FP7/1295983 (MERITS) and under grant agreement no. PIRSES-GA-2013-610692 (INNOSTORAGE). Furthermore, the work is partially funded by the Spanish government (ENE2011-28269-C03-02 and ENE2011- 22722), and the European Union by Cost Action TU0802. The authors would like to thank the Catalan Government for the quality accreditation given to their research groups GREA (2014 SGR 123) and DIOPMA (2014 SGR 1543).

Thermochemical materials (TCM) are proposed for thermal energy storage as one of the future options to achieve lower energy consumption in buildings and other industrial applications, as well as to store energy from solar energy. In this study, the thermophysical properties of two TCM, CaCl2 and zeolite, are determined with TGA and DSC and samples are cycled 4 times with TGA. Results show that the material with the highest energy density is the salt, CaCl2. Moreover, both materials under study present noble cyclability. ; The work is partially funded by the Spanish government (ENE2011-28269-C03-02 and ENE2011-22722). The authors would like to thank the Catalan Government for the quality accreditation given to their research group GREA (2009 SGR 534) and research group DIOPMA (2009 SGR 645). Furthermore, the work was partially funded by European Union (Cost Action TU0802).

Microencapsulated Phase Change Materials (MPCM) are widely used in active and passive systems for thermal energy storage. To evaluate the strength of a proper shell/PCM system, comparisons were performed between laboratory-prepared MPCM samples produced by in situ polymerization with a phase change temperature of 50 C and a particle size of around 1–2 m with tetracosane as PCM, and polystyrene (PS) and poly (methyl methacrylate) (PMMA) as shells. Evaluation of mechanical performance was performed for different samples by means of Atomic Force Microscopy (AFM) at different temperatures (23 C and 60 C) and with different encapsulation ratios (1:3 and 1:1, shell:core) in order to compare their properties with the PCM below and above its phase change. Evaluations of the Effective Young's modulus (E) and deformation properties were performed for both types of MPCM. For an encapsulation mass ratio of 1:3, PS has better mechanical properties because, when increasing the temperature, the E decreases less than with PMMA. In the comparison between PS/tetracosane systems with different encapsulation mass ratios (1:3 and 1:1), E values were higher for the 1:3 encapsulation mass ratio at both temperatures under study. This means that, in terms of mechanical and thermal properties, the best combination core/shell/encapsulation mass ratio is PS/tetracosane/1:3. ; The work was partially funded by the Spanish government (ENE2015-64117-C5-2R). The research leading to these results received funding from the European Union's Seventh Framework Program (FP7/2007–2013) under grant agreement No. PIRSES-GA-2013-610692 (INNOSTORAGE) and from the European Union's Horizon 2020 research and innovation program under grant agreement No. 657466 (INPATH-TES). The authors would like to thank the Catalan Government for the quality accreditation given to their research group DIOPMA (2014 SGR 1543).

Today, thermal energy storage materials are proposed as a promising solution to increase the energy efficiency in building sector and to reduce the total energy demand because building sector accounts up to 34% of total energy consumption. Under this situation, phase change materials (PCM) are well-considered as materials to store energy allowing high energy densities (between 50 and 600 MJ/cm3). Available materials to be used as PCM for building application in literature were added to a database for to be used with CES Selector software. More than three hundred PCM whit phase change temperatures between −50 °C and 150 °C and considering commercial and non-commercial PCM were listed and classified by their properties and the values available of the materials have been introduced in this database. The main objective of this study is to generate a PCM database and draw on it in order to facilitate the selection of the most suitable PCM depending on the building application. ; The research leading to these results has received funding from the European Commission Seventh Framework Programme (FP/ 2007-2013) under grant agreement no. ENER/FP7/1295983 (MERITS) and under grant agreement no. PIRSES-GA-2013-610692 (INNOSTORAGE). Furthermore, the work is partially funded by the Spanish government (ENE2011-28269-C03-02 and ENE2011-22722). The authors would like to thank the Catalan Government for the quality accreditation given to their research groups GREA (2014 SGR 123) and DIOPMA (2014 SGR 1543).

This paper presents a comprehensive review of all correlations and experimental studies available in the literature to determine the heat transfer coefficient of supercritical CO2 flowing in heat exchangers. The different applications in which it is used are also reviewed and discussed. The correlations obtained from extensive experimental measurements are presented for different geometries (horizontal, vertical and inclined tubes, closed-loop circular pipes, and mini-channels) and dimensions. The review shows that there is a lack of a unique universal correlation for each geometry, suggesting the need for more work in this area. ; The work partially funded by the Spanish government (ENE2015-64117-C5-1-R (MINECO/FEDER) and ENE2015-64117-C5-3-R (MINECO/FEDER)). The authors would like to thank the Catalan Government for the quality accreditation given to their research group (2014 SGR 123). This project has received funding from the European Commission Seventh Framework Programme (FP/2007-2013) under Grant agreement N°PIRSES-GA-2013-610692 (INNOSTORAGE) and from the European Union's Horizon 2020 research and innovation programme under grant agreement No 657466 (INPATH-TES), and the funds received by the Royal Society of New Zealand. Alvaro de Gracia would like to thank Ministerio de Economia y Competitividad de España for Grant Juan de la Cierva, FJCI-2014-19940.

Thermal energy storage (TES) in molten salts is the storage dominating technology in solar power applications today. In two-tank molten salt storage systems energy density ranges from 30 to 70 kW h/m3 are achievable. The salt material used is a binary system, composed of 60% NaNO3 and 40% KNO3. In the 8 MWhth pilot plant built and tested by Abengoa, the storage tanks were made of steel A516Gr.70 using the Appendix M of code API 650 for their design. A specific testing device was developed to evaluate thermo-mechanical properties, and a study was conducted in order to evaluate tensile properties of A516Gr.70 specimens under operation conditions for the hot tank at the pilot plant that is in contact with molten salts at 380 °C. Results confirmed the outcomes of the work: the reduction of the yield limit, elongation before fracture, and Young modulus at 380 °C after having been 5 min immersed in molten salts. Moreover, after a creep-test simulating operating 7 days conditions during, an additional reduction of the yield limit was measured. ; The research leading to these results has received funding from Spanish government (Fondo tecnológico IDI-20090393, ConSOLida CENIT 2008-1005). The work is partially funded by the Spanish government (ENE2011-28269-C03-02, ENE2011-22722, ENE2015-64117-C5-1-R, and ENE2015-64117-C5-2-R). The research leading to these results has received funding from the European Union's Seventh Framework Programme (FP7/2007-2013) under grant agreement n° PIRSES-GA-2013-610692 (INNOSTORAGE) and from the European Union's Horizon 2020 research and innovation programme under grant agreement No 657466 (INPATH-TES). The authors would like to thank the Catalan Government for the quality accreditation given to their research groups GREA (2014 SGR 123) and research group DIOPMA (2014 SGR 1543). Dr. Camila Barreneche would like to thank Ministerio de Economía y Competitividad de España for Grant Juan de la Cierva FJCI-2014-22886.

Thermal energy storage (TES) is recognised as a key technology for further deployment of renewable energy and to increase energy efficiency in our systems. Several technology roadmaps include this technology in their portfolio to achieve such objectives. In this paper, a first attempt to collect, organise and classify key performance indicators (KPI) used for TES is presented. Up to now, only KPI for TES in solar power plants (CSP) and in buildings can be found. The listed KPI are quantified in the literature and compared in this paper. This paper shows that TES can only be implemented by policy makers if more KPI are identified for more applications. Moreover, close monitoring of the achievements of the already identified KPI needs to be carried out to demonstrate the potential of TES. ; The work is partially funded by the Spanish Government (ENE2011-28269-C03-02, ENE2011-22722). The authors would like to thank the Catalan Government for the quality accreditation given to their research group GREA (2014 SGR 123) and research group DIOPMA (2014 SGR 1543). The research leading to these results has received funding from the European Union's Seventh Framework Programme (FP7/2007-2013) under grant agreement n° PIRSES-GA-2013-610692 (INNOSTORAGE).

Phase change materials are a promising strategy to reduce energy consumption in a wide range of applications including the building sector. Many studies have been done to evaluate the impact of PCM on thermal properties of building materials, however there exists little information on the influence of PCM on other properties of the support materials. This knowledge is necessary to determine the feasibility to apply and use building materials containing PCM. In this paper, the effect of the addition of different percentages of microencapsulated phase change material on the properties of two commercial single layer mortars has been studied. Physical and thermal properties as well as fire reaction have been evaluated. ; The work is partially funded by the Spanish Government(ENE2011-28269-C03-02 and MAT2011-26410). The authorswould like to thank the Catalan Government for the quality accredi-tation given to their research group GREA (2014 SGR 123), researchgroup DIOPMA (2014 SGR 1543) and research group GICITED (2014SGR 1298).The work leading to this invention has received fundingfrom the European Union's Seventh Framework Programme(FP7/2007-2013) under grant agreement no. PIRSES-GA-2013-610692 (INNOSTORAGE)

Phase change materials are capable of storing and releasing energy in the form of heat in determined temperature ranges, so to increase a building"s thermal inertia, stabilize its indoor temperatures and reduce its energetic demand. Therefore, if we used these materials we could have more energetically efficient buildings. Nevertheless, are these materials most appropriate to be used in buildings? Could the incorporation of phase change materials in buildings with concrete structures be generalized? This article aims to carry out a review of these phase change materials from construction professionals" points of view, study their applications for buildings with reinforced concrete structures and the key points for these applications, draw conclusions and provide recommendations useful for all professionals within the sector who are considering the application of these materials. ; Revisión crítica del uso de materiales de cambio de fase en edificios con estructuras de hormigón armado. Los materiales de cambio de fase son capaces de almacenar y liberar energía en forma de calor en un determinando rango de temperaturas, y así aumentar la inercia térmica de un edificio, estabilizar las temperaturas en el interior y reducir la demanda energética. En consecuencia, si utilizáramos estos materiales podríamos tener un parque de edificios más eficientes energéticamente. No obstante, ¿estos materiales son apropiados para usarse en edificios? ¿Se podría generalizar la incorporación de materiales de cambio de fase en edificios con estructuras de hormigón? Este artículo tiene como objetivos hacer una revisión del estado del arte de estos materiales de cambio de fase desde el punto de vista de los profesionales de la construcción, estudiar las aplicaciones en edificios con estructuras de hormigón armado y los puntos clave para estas aplicaciones, extraer conclusiones y recomendaciones útiles para los profesionales del sector que se planteen la utilización de estos materiales. ; This research was partially funded by the Spanish government (ENE2011-28269-C03-02). The authors would like to acknowledge the support received from the Concrete Technology Research Group of Barcelona Tech (UPC). The authors would also like to thank the Catalan Government for the quality accreditation given to the research group GREA (2009 SGR 534) and the research group DIOPMA (2009 SGR 645).

Postmortem tests were performed over components removed from the TES-PS10 pilot plant after almost four years of continuous operation being exposed to solar salts under representative commercial operation conditions. Accordingly, corrosion performance of plates sections extracted from both storage tanks and samples cut from the hot molten salts pump were evaluated. Corrosion damage extension, oxides layers morphology, and corrosion products chemistry were analyzed by X-ray diffraction, energy dispersive X-ray spectrometry, optical microscopy, and SEM. In addition to materials compatibility with molten salts, mechanical tests were carried out over the storage tanks plates sections. Although the corrosion damage extension over the plates extracted from storage tanks was low and the mechanical properties were according to the standard, some interesting observations were seen. For example, a sample extracted from the hot storage tank and exposed to intermittent contact with molten salts showed some areas characterized by the growth of a non-protective oxide scale. Moreover, localized corrosion with slight penetration through the base material was identified for all storage plates samples under evaluation. On the other hand, the molten salt pump parts under study showed adherent and uniform oxides layers without detecting corrosion–erosion phenomena. Some cavities were found in the discharge elbow, and these imperfections were associated to a bad metallurgical quality during casting. Summarizing, corrosion phenomena suffered by the components under evaluation could be corrected in future plants by applying the lessons learned discussed in this study. As conclusion, materials selection analyzed within this postmortem evaluation is valid from the corrosion point of view to be used in the design of commercial TES systems in PTC plants. ; The research leading to these results has received funding from Spanish government (Fondo tecnológico IDI-20090393, ConSOLida CENIT 2008-1005). The work is partially funded by the Spanish Government (ENE2011-28269-C03-02, ENE2011-22722, ENE2015-64117-C5-1-R, and ENE2015-64117-C5-2-R). The authors would like to thank the Catalan Government for the quality accreditation given to their research group GREiA (2017 SGR 1537) and research group DIOPMA (2017 SGR 138). GREiA and DIOPMA are certified agents TECNIO in the category of technology developers from the Government of Catalonia.

The increasing role of concentrated solar power (CSP) within the renewable energy portfolio is attributed to the possibility of integrating thermal energy storage (TES) systems. Then, CSP technology has become one of the most interesting clean options to deliver dispatchable power on demand. Nowadays, commercial facilities use high quality solar salts (60%:40% NaNO3 and KNO3 by weight) as storage medium due to the attractive properties of this fluid to be applied under CSP operation conditions. Taking into account that CSP installations are designed with really large TES systems containing tens of thousands of tons, the use of lower quality nitrates salts would reduce the molten salts inventory cost and finally the investment cost of the CSP storage systems at commercial scale. The most important drawback of selecting low quality nitrates salts for high temperature CSP applications is the corrosion impact produced by impurities. Accordingly, chlorides have been identified in the state of the art as the impurity with higher effect over corrosion. This work is focused on A516 Gr70 carbon steel corrosion performance evaluation under high-chlorides content nitrates salts (1.2% and 3% by weight) at 400 °C. In addition, the feasibility of using the proposed low purity mixtures with current CSP facilities materials selection is analyzed. Results reported within this study show the critical effect of chloride content over corrosion mechanism producing lack of adherence between base metal and oxides layers in addition to corrosion products delamination and internal cracking. Then, the use of A516 Gr70 carbon steel is rejected for a long term design under solar salts containing chlorides content in the range 1.2–3% by weight being necessary a higher corrosion resistant materials selection. An improved materials selection focused on higher corrosion resistance alloys is discussed. ; The research leading to these results has received funding from Spanish government (Fondo tecnológico IDI-20090393, ConSOLida CENIT 2008-1005). The work is partially funded by the Spanish government (ENE2011-28269-C03-02, ENE2011-22722, ENE2015-64117-C5-1-R (MINECO/FEDER), and ENE2015-64117-C5-2-R (MINECO/FEDER)). The authors would like to thank the Catalan Government for the quality accreditation given to their research group GREA (2014 SGR 123) and research group DIOPMA (2014 SGR 1543).