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Phase change materials (PCM) are able to store thermal energy when becoming liquids and to release it when freezing. Recently the use of PCM materials for thermal energy storage (TES) at high temperature for Concentrated Solar Power (CSP) technology has been widely studied. One of the main investigated problems is the improvement of their low thermal conductivity. This paper looks at the current state of research in the particular field of thermal conductivity enhancement (TCE) mechanisms of PCM to be used as TES. This work considers a numerical approach to evaluate the performance of a group of TCE solutions composed by particular configurations of two of the principal TCE systems found on the literature: finned pipes and conductive foams. The cases are compared against a single PCM case, used as reference. Three different grades of graphite foams have been studied, presenting a charge time 100 times lower than the reference case for the same capacity. For fins two materials are analyzed: carbon steel and aluminum. The charge times of fin cases are from 3 to 15 times faster, depending on the amount and type of material employed. The internal mechanisms are analyzed to understand the results and locate possible improvement. ; The research leading to these results has received funding from CDTI Thesto (ITC-20111050) innterconecta Thesto). The work partially funded by the Spanish government (ENE2011-28269-C03-02 and ENE2015-64117-C5-1-R). The authors would like to thank the Catalan Government for the quality accreditation given to the research group GREA (2014 SGR 123). The research leading to these results has received funding from the European Union's Seventh Framework Programme (FP7/ 2007e2013) 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).

Thermal energy storage (TES) is seen today as a key technology to reduce the existing gap between energy demand and energy supply in many energy systems. There are, currently, three well known methods to store thermal energy and they are: sensible heat storage (SHS), latent heat storage (LHT) and thermochemical heat storage. Every method has its own thermophysical requirements for the mediums of storage, such as thermal stability, high enthalpy of phase change or reaction, high heat capacity and suitable temperature of the thermal phenomenon for a respective application, among others. In this regard, the composition of materials usually needs to be modified in order to improve their performance or to reach a determined requirement. As a consequence, the costs of potential TES materials to be applied in renewable energy systems are too high to compete with traditional systems using fossil fuels. On the other hand, several wastes and by-products from the nonmetallic mining, such as salt hydrates and double salts, are available without any application but accumulating in the mining sites. This is the case for astrakanite (Na2SO4·MgSO4·4H2O) and lithium carnallite (LiCl·MgCl2·7H2O) with no current application, and potassium carnallite (KCl·MgCl2·6H2O) used as a supplementary raw material to obtain KCl. Since the costs of these materials are close to zero, they were characterized as TES materials taking into account the properties required for the three methods of storage. Results showed that astrakanite and potassium carnallite have potential to be applied as thermochemical material at lowmedium temperature (< 300 °C). Also, a dehydrated product obtained from astrakanite showed potential to be applied as phase change material (PCM) at high temperature, from 550 °C to 750 °C. Nevertheless, lithium carnallite did not show potential to be applied as TES material due to it low thermal stability, presenting partial decomposition below 200 °C. ; The work at the University of Antofagasta was supported by CONICYT/FONDAP N° 15110019, FONDECYT N°1170675 and partially, by ERANet-LAC 2nd Joint Call, ERANET-LAC 2015–2016, project ELAC2015/T06-0988. Veronica Mamani Barreneche would like to thank CONICYT for her doctorate scholarship CONICYT N° 21150145. The authors from the University of Lleida and the University of Barcelona would like to thank the Catalan Government for the quality accreditation given to their research group GREA (2014 SGR 123) and DIOPMA (2014 SGR 1543), respectively. The work was partially funded by the Spanish Government (ENE2015-64117-C5-1-R (MINECO/FEDER)). This work 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). Dr. Camila Barreneche would like to thank Ministerio de Economia y Competitividad de España for grant Juan de la Cierva, FJCI-2014-22886.

Salt hydrates are an appealing option to be used as sorption materials in thermal energy storage (TES). In this work, strontium bromide and magnesium sulphate have been selected as one of the most promising salt hydrates since they present high energy storage density (>130 kWh/m3) and efficiency (>20%). One of the main drawbacks of sorption materials rely on control the hydratation-dehydratation process but there are other parameters that can modify this behaviour as the corrosive potential of these salts in contact with the container material selected for the application. Hence, four different metal container materials, specifically stainless steel, copper, aluminium, and carbon steel have been tested in SrBr2$6H2O and MgSO4$7H2O hydrate salts, during 100 h at dehydratation conditions. After the gravimetric and micrograph analysis carried out via scanning electron microscopy (SEM) study, only carbon steel is not recommended for this application in contact with SrBr2$6H2O, obtaining a corrosion rate of 0.038 mm/year, with a metallographic corrosion layer thickness of 25.2 mm. Aluminium, copper and stainless steel showed a better corrosion resistance also in SrBr2$6H2O and MgSO4$7H2O with corrosion rates below 0.008 mm/year. ; This work was partially funded by the Ministerio de Ciencia, Innovación y Universidades de España (RTI2018-093849-B-C31 - MCIU/AEI/FEDER, UE). The authors would like to thank the Catalan Government for the quality accreditation given to their research group GREiA (2017 SGR 1537). GREiA is a certified agent TECNIO in the category of technology developers from the Government of Catalonia. This work is partially supported by ICREA under the ICREA Academia programme.

Thermal energy storage represents a crucial element to increase solar power dispatchability. Within sensible heat storage in solid media, concrete is considered a low-cost alternative to be further developed and therefore, this has been addressed in this paper. Four concrete dosages were designed, combining each type of considered cement, ordinary Portland and calcium aluminate cement, with each type of considered aggregate, silicocalcareous and a steel slag. Thermo-mechanical properties of concrete were studied before and after 10 thermal cycles from 290 ◦C to 700 ◦C. Maximum operating temperature and heating rates were selected accordingly to the targeted application, a concentrating solar power (CSP) tower plant. At macro-level, results show thermal cycle stability of concrete with steel slag aggregate in both cement types. On the contrary, at micro-level, the petrography analysis shows the lack of bonding between steel slag aggregate and the cement paste. In contrast, concrete mixtures containing silico-calcareous aggregates collapse after thermal cycling. ; This work was partially funded by the Ministerio de Ciencia, Innovación y Universidades de España (RTI2018-093849-B-C31 - MCIU/AEI/FEDER, UE) and by the Ministerio de Ciencia, Innovación y Universidades - Agencia Estatal de Investigación (AEI) (RED2018-102431-T). The authors at University of Lleida would like to thank the Catalan Government for the quality accreditation given to their research group (2017 SGR 1537). GREiA is certified agent TECNIO in the category of technology developers from the Government of Catalonia. This work is partially supported by ICREA under the ICREA Academia programme Also, the authors appreciate the collaboration of the companies "Ciments Molins", "PROMSA", "Sika", and "GLS Prefabricats" for the material supplied in this research.

This paper addresses the optimal design of desalination plants that integrate reverse osmosis, a Rankine cycle, parabolic trough solar collectors and thermal energy storage (TES). A multi-objective mixed-integer nonlinear programming model (MINLP) is developed to model such an integrated system and optimize its design and operating conditions according to economic and environmental metrics. The model considers the simultaneous minimization of cost and environmental impact given a specific water demand to be fulfilled. The environmental performance is quantified via life cycle assessment (LCA) principles. Particularly, the CML 2001 methodology, a widely used LCA-based framework, is used to assess the impact, enabling the identification of the main sources of damage across the entire life cycle of the plant. The capabilities of our method are illustrated through its application to a case study considering weather data in Tarragona (Spain). We show that coupling seawater desalination with solar collectors and thermal energy storage leads to significant environmental savings at a marginal increase in cost. ; The authors would like to acknowledge financial support from the Spanish Government (DPI2008-04099, ENE2011-28269-C03-03, ENE2011-22722, CTQ2009-14420-C02) and to thank the Catalan Government for the quality accreditation given to their research groups SUSCAPE and GREA (2009 SGR 545, 2009 SGR 534).

In the present study, geometry optimization of a phase change material (PCM) heat storage system is presented. The existing PCM-fins heat exchanger system works at the back side of a solar receiver in order to minimize the effect of the solar radiation fluctuations inside the cavity. As initially designed, the system does not accomplish the expected design purposes and thus optimization is needed. Optimization is usually time-consuming and some algorithms need a starting point, therefore one suitable method is geometrical optimization which aims to find the optimal shape of a system for a given criteria and providing a rough optimal geometry. Here, constructal theory, 'point to volume', is applied to find the optimum shape factor of the elemental volume of the presented PCM-heat exchanger. With this methodology, an optimum ratio of the PCM and fin width and length is found and beyond that the method is extended to 'surface to volume' problem. Results have been numerically validated using a CFD software and demonstrate that it gives a very good approximation of the real optimum which can be used as initial configuration for further optimization through CFD simulation or other optimization methods that require a starting point. ; The author Aran Solé would like to thank the Societat Economica Barcelonesa Amics del Pais (SEBAP) for the funds that made possible her research stay. The authors would like to thank Jean-Marie Mancaux for his help and Jinqiu Shen for her contribution in the work. The authors would like to thank the Catalan Government for the quality accreditation given to their research group GREA (2014 SGR 123). GREA is certified agent TECNIO in the category of technology developers from the Government of Catalonia. The work is partially funded by the Spanish government (ENE2015-64117-C5-1-R (MINECO/FEDER)). The research leading to these results has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 657466 (INPATH-TES). Aran Solé would like to thank Ministerio de Economía y Competitividad de España for Grant Juan de la Cierva, FJCI-2015-440 25741.

Deep Reinforcement Learning (DRL) proved to be successful for solving complex control problems and has become a hot topic in the field of energy systems control, but for the particular case of thermal energy storage (TES) systems, only a few studies have been reported, all of them with a complexity degree of the TES system far below the one of this study. In this paper, we step forward through a DRL architecture able to deal with the complexity of an innovative hybrid energy storage system, devising appropriate high-level control operations (or policies) over its subsystems that result optimal from an energy or monetary point of view. The results show that a DRL policy in the system control can reduce the system operating costs by more than 50%, as compared to a rule-based control (RBC) policy, for cooling supply to a reference residential building in Mediterranean climate during a period of 18 days. Moreover, a robustness analysis was carried out, which showed that, even for large errors in the parameters of the system simulation models corresponding to an error multiplying factors up to 2, the average cost obtained with the original model deviates from the optimum value by less than 3%, demonstrating the robustness of the solution over a wide range of model errors. ; The authors would like to thank the Catalan Government for the quality accreditation given to their research group (2017 SGR 1537). GREiA is certified agent TECNIO in the category of technology developers from the Government of Catalonia.

The implementation of organic phase change materials (PCMs) in several applications such as heating and cooling or building comfort is an important target in thermal energy storage (TES). However, one of the major drawbacks of organic PCMs implementation is flammability. The addition of flame retardants to PCMs or shape-stabilized PCMs is one of the approaches to address this problem and improve their final deployment in the building material sector. In this study, the most common organic PCM, Paraffin RT-21, and fatty acids mixtures of capric acid (CA), myristic acid (MA), and palmitic acid (PA) in bulk, were tested to improve their fire reaction. Several flame retardants, such as ammonium phosphate, melamine phosphate, hydromagnesite, magnesium hydroxide, and aluminum hydroxide, were tested. The properties of the improved PCM with flame retardants were characterized by thermogravimetric analyses (TGA), the dripping test, and differential scanning calorimetry (DSC). The results for the dripping test show that fire retardancy was considerably enhanced by the addition of hydromagnesite (50 wt %) and magnesium hydroxide (50 wt %) in fatty acids mixtures. This will help the final implementation of these enhanced PCMs in building sector. The influence of the addition of flame retardants on the melting enthalpy and temperatures of PCMs has been evaluated. ; The research leading to these results has received funding from the European Commission Seventh Framework Programme (FP/2007-2013) under grant agreement number 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 groups GREA (2014 SGR 123), DIOPMA (2014 SGR 1543) and GICITED (2014 SGR 1298), and also the MINECO for the project BIA2014-52688-R, ENE2015-64117-C5-1-R, ENE2015-64117-C5-2-R, and ENE2015-64117-C5-3-R. Alvaro de Gracia and Camila Barreneche would like to thank Ministerio de Economia y Competitividad de España for Grant Juan de la Cierva, FJCI-2014-19940 and FJCI-2014-22886, respectively.

Relocatable, transportable or off-site constructed lightweight buildings typically undergo sharp indoor temperature fluctuations in the heating and cooling seasons due to the lack of sufficient thermal mass in their envelopes, resulting in high energy consumption to provide the zone with comfort temperature. The application of phase change materials has been suggested as a promising solution to control the indoor thermal condition in buildings. This work is an attempt to support the application of PCM technology in lightweight relocatable buildings as a passive alternative to save energy under different weather conditions. The numerical results highlighted the potential of using PCM-enhanced gypsum boards in lightweight buildings to increase the energy performance during both heating and cooling seasons in arid and warm temperate main climate areas. ; The authors from Chile acknowledge to CONICYT/FONDAP No. 15110019. This work has been 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). The research leading to these results has received funding from the European Union's Seventh Framework Programme (FP7/2007-2013) under grant agreement no. PIRSES-GA-2013-610692 (INNOSTORAGE) and from the European Union's Horizon 2020 research and innovation programme under grant agreement No 657466 (INPATH-TES). Alvaro de Gracia would like to thank Ministerio de Economía y Competitividad de España for Grant Juan de la Cierva, FJCI-2014-19940. Paula Marin thanks fellowship CONICYT-PCHA/doctorado nacional/2015-21151359.

High temperature corrosion is one of the most important issues for materials selection, structure design and service life prediction of engineering parts that are exposed to high temperature environments. The prevention of high temperature corrosive attacks on materials plays a critical role in aspects such as reliability, quality, safety and profitability of any industrial sector associated with high temperature process and in the study case, concentrated solar power (CSP) market, using inorganic molten salts as storage material. The aim of this research is the development of corrosion tests through conventional gravimetric techniques focussed on thermal energy storage (TES) materials as well as electrochemical impedance spectroscopy (EIS) analysis for storage systems monitoring. To mitigate corrosion, different alumina forming austenitic (AFA) alloys were exposed to the most promising molten salts to be proposed for the new generation of CSP plants, nitrate, carbonate and chloride molten salts. Ni base alloys showed a better behaviour in the corrosive environments and electrochemical impedance tests revealed the formation of a protective layer in the first 100h. The gravimetric results and the scanning electron microscopy (SEM) analyses that were performed, confirmed the excellent behaviour of the steels tested in nitrate salts reducing the corrosion in these promising novel molten salts compared with the solar salt currently used in commercial CSP. On the other hand, carbonate and chloride molten salts showed a higher corrosion rate in the materials proposed, especially chloride molten salts. ; The authors would like to acknowledge the financial support provided by CONICYT/FONDAP 15110019 "Solar Energy Research Center" SERC-Chile and GIZ "Programa de pasantía en el extranjero en tecnologías de concentración solar para investigadores". The work is partially funded by the Spanish government (ENE2015-64117-C5-1-R (MINECO/FEDER)). The authors would like to thank the Catalan Government for the quality accreditation given to their research group GREiA (2017 SGR 1537). GREiA is certified agent TECNIO in the category of technology developers from the Government of Catalonia.

According to the International Energy Agency (IEA) the primary energy production was increased by 49% and CO2 emissions by 43% over the past 20 years. Furthermore, 30 - 40% of total energy consumption in developed countries comes from the private sector (housing and offices). Thermal energy storage (TES) is proposed as one way to improve the gap between energy consumption and energy supply. TES systems can store energy as sensible heat, latent heat or chemical reaction. Phase change materials (PCM) are extensively studied materials for thermal energy storage as latent heat because these materials have high phase change enthalpy in a wide variety of phase change temperature. Nowadays it is quite difficult to have all the reported information about PCM properties, not only the thermophysical ones but other relevant properties, even though there are different compilations. CES Selector is software used in material selection, besides being a materials database it permits to include new materials introducing their properties, as well as to classify them in several groups, to plot the data and combine graphs production with the material selection under the desired criteria. More than 300 substances used as PCM and reported in the literature were introduced in a new database created using CES Selector. Thermophysical properties as melting temperature (°C) and melting enthalpy (kJ·kg-1) have been plotted and PCM selections under different criteria were carried out. This database can be completed in the future with other relevant properties either than thermal such as chemical properties (durability in different media etc), reported density or viscosity at a given temperature or even cost of each material. Analyzing these plots it is observed that some PCM are represented occupying a wide range of a given thermophysical property. This is the case of CaCl2·6H2O: data came from 7 sources and the occupied area in the plot is the highest one from all the PCM reported. On the other hand, some other PCM are represented with smaller dots associated with a lower dispersion in the property values and this size may possibly be attributed to different reasons: scarce sources for experimental data or low dispersion among several experimental reported values. These results emphasize that there is a lack of standardization for the measurement of the PCM thermophysical properties. ; The work is partially funded by the Spanish government (ENE2011-28269-C03-02) and the European Union (NMP4-SL-2010-260056). The authors would like to thank the Catalan Government for the quality accreditation given their research group GREA (2009 SGR 534) and research group DIOPMA (2009 SGR 645).

In recent years, thermal energy storage (TES) systems using phase change materials (PCM) have been widely studied and developed to be applied as solar energy storage units for residential heating and cooling. These systems performance is based on the latent heat due to PCM phase change, a high energy density that can be stored or released depending on the needs. PCM are normally encapsulated in containers, hence the compatibility of the container material with the PCM has to be considered in order to design a resistant container. Therefore, the main aim of this paper is to study the corrosion effects when putting in contact five selected metals (aluminium, copper, carbon steel, stainless steel 304 and stainless steel 316) with four different PCM (one inorganic mixture, one ester and two fatty acid eutectics) to be used in comfort building applications. Results showed corrosion on aluminium specimens. Hence caution must be taken when selecting it as an inorganic salt container. Despite copper has a corrosion rate range of 6e10 mg/cm2 yr in the two fatty acid formulations tested, it could be used as container. Stainless steel 316 and stainless steel 304 showed great corrosion resistance (0e1 mg/cm2 yr) and its use would totally be recommended with any of the studied PCM. ; 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 research group DIOPMA (2014 SGR 1543). Aran Sole would like to thank the Departament d'Universitats, Recerca i Societat de la Informacio de la Generalitat de Catalunya for her research fellowship. 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)