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The degradation of Ni-based superalloys in molten salts is closely related with the evolution of corrosion scales. In this work, the evolution of corrosion scales, the characteristics of the scale/alloy matrix interface associated with the corrosion mechanisms and the corrosion kinetics of a Ni-based superalloy Ni–20Cr–18W in molten salts were investigated. The outer surface of the corrosion scales was composed of numerous flakes. From the cross-sectional view, the scales gradually presented a layered structure and its formation can be ascribed to the morphology evolution of the inclusions inserted within the scales. It was found that the corrosion progress was a cooperative process involving in both oxidation and sulfuration, and the element of sulfur (S) took the precedence to penetrate into the matrix and resulted in an obvious sulfuration zone nearby the scales/alloy matrix interface. The corrosion activation energy of the investigated alloy was calculated to be 125.27 ​kJ/mol. This work is expected to work out the structural evolution of the corrosion scales and the characteristics of the scale/matrix interface associated with the corrosion mechanisms for Ni-based superalloys.

MgO-based materials possess favorable thermodynamic properties that enable them to be used as CO2 sorbents in the temperature range of 300–550 °C — at least theoretically. MgO-based sorbents have seen a boost in activity since the discovery of molten alkali metal salt promoters, and yet, they practically remove CO2 from a gas stream efficiently only at very high pressures (p > 10 bar), rendering them suitable for precombustion, but not for postcombustion CO2 capture schemes. Most of the current performance assessments rely on measurements of the transition between MgO and MgCO3 (or Mg-double carbonates) rather than the ability of the sorbents to actually remove CO2 from a gas stream, which may be misleading regarding their practical use and application. Further, reliable thermodynamic data for sorbents involving Mg-double carbonates is missing, although these materials have large potential for industrial CO2 capture applications.

[EN]A multi-objective and multi-parametric optimization of a Pumped Thermal Electricity Storage system based on Brayton cycles is presented by the calculation of different Pareto fronts and the associated Pareto optimal sets for energetic and design analysis, respectively. A large range of internal and external irreversibilities and the thermodynamic properties of the storage media are taken into account. The analysis shows that the heat capacity of the working fluid and the heat capacity of the storage media should be the same in the contact with the hot reservoirs and in the contact with the cold reservoir in the heat pump, but in the contact with the cold reservoir for the heat engine the ratio should be 0.33, this offers information regarding the mass flow increasing significantly the achievable values for the round-trip efficiency, power output and the heat engine efficiency in the discharge process. Optimal values are given in terms of the degree of irreversibilities in the system and a comparison is made with extreme cases of infinite and minimum sizes for the storage system. Round-trip efficiencies in the so-called optimum scale/mass-flow-ratio design point exhibits noticeably larger values compared to previously reported results including the so-called endoreversible limit, where no internal irreversibilities are considered and where the improvement can achieve 49% over the endoreversible case in the most ideal scenario. Explicit numerical values of the maximum round trip efficiency, power output, and efficiency are given for a broad range of both internal and external irreversibilities., Agencia Nacional de Investigación e Innovación (ANII): Fondo Sectorial de Energía (Uruguay), under contract FSE-1-2018-1-153077 Universidad de Salamanca, Spain, under contracts 18.KB.YF/463A.C.01 and 0218463AB01.

This study evaluates the proposal of a concrete storage tank as molten salt container, for concentrating solar power applications. A characterization of the thermal and mechanical properties including compression resistance, density, thermal conductivity and chemical degradation were evaluated in a pilot plant storage tank in contact with solar salt (60%NaNO3 + 40%KNO3) during 120 hours at 565◦C. XRD and SEM analysis were carried out after the molten salt test, observing a penetration of salts in the concrete about 14.5 cm. The chemical and mechanical results in the concrete showed a good behaviour and resistance after the exposure test. Nevertheless, the presence of molten salt in the concrete changed its thermal properties, increasing the density and the thermal conductivity in the concrete zones closer to the molten salt reservoir. The research leading to these results has received funding from the Ministerio de Ciencia, Innovación y Universidades de España in the project THAS@L (RTC-2015-3385-3). We would like to thank Abengoa Thermal Storage Team for their comments and suggestions. 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 the University of Lleida would like to thank the Catalan Government for the quality accreditation given to her 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.

The use of a binary mixture of solar molten salts (60 wt% NaNO3 and 40 wt% KNO3) as a heat transfer medium for the production of a solar fuel by the thermochemical conversion of biomass is investigated in the present paper. Thermochemical conversion can be a route for converting the surplus solar irradiation via the direct contact of nitrate molten salts and biomass into storable chemical fuel. Traditional fixed-bed pyrolysis and molten salts pyrolysis have been carried out under an inert atmosphere at a temperature of 500 degrees C. The composition of the permanent gases and the bio-oil produced has been analyzed along with the temperature profiles inside the reactor. Two distinctive pathways have been observed: an endothermic process in the case of traditional fixed-bed pyrolysis and an exothermic process in the case of molten salt pyrolysis. An attempt has been made to identify possible causes for such differences. The financial support of the Spanish Ministry of Economy and Competitiveness from project ENE2014-54942-R is gratefully acknowledged by the authors.

The ‘Solar salt’ (60% NaNO3-40% KNO3, wt. %) is the most used heat transfer and storage material in high temperature CSP systems. The main drawback is its high melting temperature of 228 °C, which requires extra-energy to keep it in the liquid state and avoid damage to pipes at low temperatures. Multi-component salts are combinations of different cations and anions. The difference in size of the ions hinders the crystallization of the material and provides lower melting temperatures. Multi-component salts are considered in this study to replace simpler combinations, such as binary and ternary eutectic mixtures. Herein, we report on two novel six-component nitrates with a melting temperature of 60–75 °C and a thermal stability up to ~500 °C under a linear heating program in N2 atmosphere. Properties such as the thermal conductivity in solid and molten state, heat capacity and vibrational spectra were evaluated. The study of the thermal behaviour of these materials using differential scanning calorimetry was insufficient, hence alternative and complementary techniques were used, such as: the three-omega technique, optical transmission and Raman spectroscopy. Multi-component salts were found to solidify as amorphous solids even at slow cooling rates and water was found to behave as a catalyst of crystallization., The Catalan Institute of Nanoscience and Nanotechnology (ICN2) acknowledges support from the Severo Ochoa Program (MINECO, Grant SEV-2017-0706) and funding from the CERCA Programme/Generalitat de Catalunya. Funding from the Spanish Ministry MINECO/FEDER: FIS2015-70862-P PHENTOM and MAT2015-68394-R NaCarFLOW are also acknowledged. M.-C. L acknowledges support from the Ministry of Science and Technology in Taiwan, MOST 104-2221-E-009-147-MY3.

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.

The use of molten salt-based nanofluids as heat transfer fluids or thermal energy storage materials to increase the efficiency of Concentrated Solar Power plants has gained attention due to the use of the renewable energies against Global Warming. One of the issues of interest is the colloidal stability of the nanoparticles dispersed in ionic media like molten salts. In this work a new experimental set-up to measure the particle size distribution of molten salt-based nanofluids by means of Dynamic Light Scattering was developed. The colloidal stability of silica and Al/Cu nanoparticles dispersed in solar salt (NaNO3-KNO3) was experimentally measured for the first time. Silica nanoparticles were dispersed in water, calcium nitrate tetrahydrate and solar salt, and the formation of micrometrical agglomerates was observed when molten salts were used as base fluid due to the high ionic strength of the medium and the reduced Debye length. The influence of the nanoparticle composition was proved to be also important. For the Al/Cu metal alloy nanoparticles the agglomerates formed were smaller than for silica. Besides, even though both nanoparticles settle after 4 h in static conditions, only Al/Cu nanoparticles recover the initial particle size distribution when they are mechanically redispersed

In this review, we summarise the current status and new trends in concentrating solar power (CSP) technology, analysing the technology cost and their evolution during the last years, with special focus on thermal storage. Moreover, we have carried out a comprehensive review of the molten salts used and proposed in CSP commercial plants. Nitrates, nitrites, chlorides, and carbonates are presented, including their corrosion aspects with common alloys as well as the different possibilities available in the literature to replace them (ternary and quaternary nitrate molten salt) or improve them (addition of nanoparticles). Finally, we have proposed the key factors for a successful new generation of CSP plants, with special focus on high-temperature molten salts (carbonates and chloride blends) and the main important materials requirements for CSP system components. The research leading to these results has received funding from the Spanish government (ENE2015-64117-C5-1-R (MINECO/FEDER)). The authors 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. Angel G. Fernández acknowledges the financial support provided by CONICYT/FONDAP 15110019 “Solar Energy Research Center” SERC-Chile. Dr. Aran Solé thanks Ministerio de Economía y Competitividad de España for Grant Juan de la Cierva, FJCI-2015-25741. Dr. Eduard Oró thanks Ministerio de Economia y Competitividad de España for Grant Juan de la Cierva IJCI-2016-30568. This work was authored in part by the National Renewable Energy Laboratory, operated by Alliance for Sustainable Energy, LLC, for the U.S. Department of Energy (DOE) under Contract No. DE-AC36-08GO28308. Funding provided by U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Solar Energy Technologies Office, Concentrating Solar Power. The views expressed in the article do not necessarily represent the views of the DOE or the U.S. Government. The U.S. Government retains and the publisher, by accepting the article for publication, acknowledges that the U.S. Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for U.S. Government purposes.

Commercial buildings or shopping malls are characterized by large thermal and electrical energy consumptions with high variability of energy demand. Therefor e, there is a large interest to explore novel renewable energy generation systems for these applications. A novel flexible configuration of biomass - fired CHP system with organic Rankine cycle (ORC) is here proposed and applied to the case study of Sainsbur y’s supermarkets in the UK . The proposed configuration adopts a molten salt (MS) circuit to transfer heat from the biomass furnace to the ORC plant. A thermal Energy Storage (TES) is proposed to improve the flexible operation of the plant and reduce the si ze of the biomass boiler . Molten salts have been preferred to thermal oil as they have no fire risks and low environmental impact and can be used as medium for a Two Tank TES with a “direct heating” scheme. Th e plant has been analysed using real input dat a of biomass boiler installed, conversion efficiency and heat demand from the store. The model is informed by hourly energy costs and electricity feed in tariff in order to d efine optimal size and operation of the bottoming ORC for the specific case study of large commercial energy end user in the UK. The results show that the use of thermal storage in a biomass - fired ORC plant can improve the boiler efficiency and reduce the biomass consumption in thermal - load following operating mode and increase the inve stment profitability.

Renewable energies are main players to ensure the long-term energy supply. Solar power plants with thermal energy storage (TES) are one of the available renewable technologies which have more potential. Nowadays, there are still several aspects in the design and operation of these power plants which need to be improved, such as the correct operation of some specific instrumentation, the compatibility between TES materials and storage tanks materials, and operational process strategies. This paper presents the acquired experience during the design, start-up, and operation of a kWh scale pilot experimental facility built at the University of Lleida (Spain) together with Abengoa Research (Spain) in 2008. The versatility of this facility has allowed simulating real working conditions and therefore testing different TES systems, TES materials, solar power plant components, and operational strategies focused on TES for temperatures up to 400 °C. In the present paper, the authors show the lessons learnt at pilot and present the main problems and limitations encountered, and give advices of this experimental set-up to extrapolate the data to real plant, to provide solutions to technical problems and reduce the cost of commercial plants. The authors want to thank specially to Dr. Eduard Oró (today at Institut de Recerca d’Energies de Catalunya, Spain) and Dr. Antoni Gil (today at Massachusetts Institute of Technology, USA) for their help in the design, start-up and operation of the pilot plant facility. The research leading to these results has received funding from Spanish government through CDTI granted projects (Fondo tecnológico IDI-20090393, ConSOLida CENIT 2008-1005) and from Abengoa Solar NT. The work is partially funded by the Spanish government (ENE2008-06687-C02-01/CON, ENE2011-22722, ULLE10-4E-1305, and 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 GREA (2014 SGR 123). GREA is a certified agent TECNIO in the category of technology developers from the Government of Catalonia. This project has received funding from the European Commission Seventh Framework Programme (FP/2007-2013) under Grant agreement NºPIRSES-GA-2013-610692 (INNOSTORAGE) and from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 657466 (INPATH-TES). Laia Miró would like to thank the Spanish Government for her research fellowship (BES-2012-051861). Jaume Gasia would like to thank the Departament d'Universitats, Recerca i Societat de la Informació de la Generalitat de Catalunya for his research fellowship (2017FI_B1 00092).

The selection of a proper heat transfer fluid (HTF) is a key factor to increase the efficiency of concentrated solar power plants and therefore, to reduce their internal associated CAPEX (capital expenditures of developing and constructing a plant, excluding any grid-connection charges) and OPEX (operating expenditures from the first year of a project's operation). This paper presents a comparative study of two commercial HTF which are widely used in different industries and CSP plants: thermal oil Therminol VP-1 and silicone fluid Syltherm 800. First, the authors theoretically studied the properties of both HTF based on the data given by the manufactures. Afterwards, the authors experimentally perform the comparison in a two-tank molten salt thermal energy storage pilot plant built at the University of Lleida (Spain). The study is focused on the plate heat exchanger of the facility during several charging processes with a counter flow arrangement. Results from both studies showed that, for the same working conditions, Therminol VP-1 is the best candidate for the above-mentioned purposes due to its higher heat transfer, lower thermal losses and lower power consumption associated to the HTF pump. However, it presents problems a low crystallization point, which should also be considered. The research leading to these results has received funding from Spanish goverment (Fondo tecnológico IDI-20090393, ConSOLida CENIT 2008-1005) and from Abengoa Solar NT. The work is partially funded by the Spanish government (ENE2008-06687-C02-01/CON, ENE2011-22722, ULLE10-4E-1305 and 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 GREA (2014 SGR 123). This project has also received funding from the European Commission Seventh Framework Programme (FP/2007-2013) under Grant agreement NºPIRSES-GA-2013-610692 (INNOSTORAGE) and from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 657466 (INPATH-TES). Laia Miró would like to thank the Spanish Government for her research fellowship (BES-2012-051861). Jaume Gasia would like to thank the Departament d’Universitats, Recerca i Societat de la Informació de la Generalitat de Catalunya for his research fellowship (2017FI_B1 00092). The authors would like to thank Dr. Eduard Oró from Catalonia Institute for Energy Research (Spain) and Dr. Antoni Gil from Massachusetts Institute of Technology (USA) for their help during the initial stages of the experimentation.

At present, two-tank molten salt storage systems are the established commercially available concept for solar thermal power plants. Due to their very low vapour pressure and comparatively high thermal stability, molten salts are preferred as the heat transfer fluid and storage medium. Therefore, the development of alternative, more cost-effective concepts is an important step in making thermal energy storage more competitive for industrial processes and solar thermal applications. The paper gives an overview of various storage concepts such as thermocline, floating barrier or embedded heat exchanger that have been developed in recent years. In this context, a description of functionality, a summary of the technical specification and the state of development of each concept is given. The latter includes the next steps and open questions, respectively. Additionally, further research topics, such as the stability of material selections or the functionality of necessary components are presented. In order to answer many of the open questions, a new molten salt test facility called “Test facility for thermal energy storage in molten salts (TESIS)” is under construction at DLR. This facility has two separate sub-systems, which can be operated independently from each other. One system is designed to investigate new single tank storage concepts focusing on the thermocline-filler concept. The other system is a flexible test section for molten salt process technology and component tests, such as valves, solar receiver tubes and instrumentation. Both sub-systems can operate at temperatures up to 560 °C, using nitrate salt as the storage and heat transfer media. Experimental results on the compatibility of filler materials and molten salt will be presented. In addition, the current construction and development status of the facility will be reported. Finally, an overview of current research topics and an outlook related to the test facility is presented.

Despite the fact that there are some commercial concentrated solar power plants worldwide, there is currently a lack of experimental reports about the operational characteristics of this type of plants. Therefore, a two-tank molten salts thermal energy storage (TES) pilot plant at the University of Lleida (Spain) was used to analyze charging and discharging processes under real conditions. In this facility, 1000 kg of molten salts are used as TES material and Therminol VP-1 is used as heat transfer fluid (HTF). This facility is equipped with measurement equipment which allows an exhaustive analysis of the processes. In this study, the fact of varying the flow arrangement in the heat exchanger (parallel and counter flow arrangements) and the temperature difference between the molten salts and the HTF have been studied and discussed in terms of temperature profiles, energy and power stored/released from/to both HTF and molten salts, efficiencies and effectiveness. The best working conditions found were counter flow arrangement with a temperature grading of about 65 °C. The research leading to these results has received funding from Spanish government (Fondo tecnológico IDI-20090393, ConSOLida CENIT 2008-1005) and from Abengoa Solar NT. The work is partially funded by the Spanish government (ENE2008-06687-C02-01/CON, ENE2011-28269-C03-02, ENE2011-22722 and ULLE10-4E-1305). The authors would like to thank the Catalan Government for the quality accreditation given to their research group GREA (2014 SGR 123). This project has also 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). Laia Miró would like to thank the Spanish Government for her research fellowship (BES-2012-051861). Jaume Gasia would like to thank the Departament d’Universitats, Recerca i Societat de la Informació de la Generalitat de Catalunya for his research fellowship (2016FI_B 00047).

Carbon was deposited on a mild steel cathode during electrolysis in the molten mixture of Li2CO3 and K2CO3 (mole ratio: 62:38) under CO2 or mixed N2 and CO2 atmospheres at 3.0–5.0 V and 540–700 °C. In a three-electrode cell, cyclic voltammetry was applied on a platinum working electrode to study the reduction and deposition processes. A two-electrode cell helped correlate electrolysis variables, e.g. temperature and voltage, with the deposition rate, current efficiency, and properties of the deposited carbon powders. High current efficiency (>90%) and deposition rate (>0.11 g cm−2 h−1) were achieved in the study. Elemental analysis of the electro-deposits, following washing with HCl solutions (2.3–7.8 mol L−1), showed carbon as the dominant element (75–95 wt.%) plus oxygen (5–10 wt.%) and small amounts of other elements related to materials of the electrolytic cell. Thermogravimetry detected fairly low onset combustion temperatures (310–430 °C), depending on the electrolysis and acid washing conditions. Amorphous and various nanostructures (sheet, rings and quasi-spheres) were revealed by electron microscopy in carbon samples deposited under different process conditions. The specific surface area of the carbon deposited at 5.0 V and 540 °C was as high as 585 m2 g−1. An analysis of the energy consumption suggests several ways for efficiency improvement so that the electrolytic carbon from CO2 will become commercially attractive.

International audience; With the aim of preparing mullite, reactions between aluminum sulfate and silica in appropriate proportions and molten sulfate media M2SO4 (M=Na and/or K) were performed at different temperatures. The powders obtained were characterized by XRD, FT-IR, SEM and TEM. The reactivity was the same in Na2SO4 and (K,Na)2SO4 media. The best results in terms of yield (98.3%) and weight of mullite produced (95%) were obtained in Na2SO4 at 950 °C. The mullite phase exhibits an acicular morphology (75×0.75 μm) and a specific surface area close to 20 m2/g. In K2SO4 medium, a potassium alumino silicate is formed as well as mullite.