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Compatibility of a post-industrial ceramic with nitrate molten salts for use as filler material in a thermocline storage system
Applied Energy 109 (2013) 387–393
Contents lists available at SciVerse ScienceDirect
Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Compatibility of a post-industrial ceramic with nitrate molten salts for use as filler material in a thermocline storage system Nicolas Calvet a,b,⇑, Judith C. Gomez b, Abdessamad Faik a, Vladimir V. Roddatis a, Antoine Meffre c, Greg C. Glatzmaier b, Stefania Doppiu a, Xavier Py c a b c
CIC Energigune, Albert Einstein 48, 01510 Miñano (Álava), Spain National Renewable Energy Laboratory, 1617 Cole Blvd, 80401 Golden CO, USA PROMES CNRS, UPR8521, Université de Perpignan, Rambla de la Thermodynamique, Tecnosud, 66100 Perpignan, France
h i g h l i g h t s " An innovative ceramic is considered as thermal energy storage material. " The ceramic is tested in direct contact with heat transfer fluids in static state at 500 °C during 500 h. " Two kinds of molten salts are used: Solar salt (NaNO3, KNO3) and the ternary nitrate salt (NaNO3, KNO3, Ca(NO3)2). " The ceramic is compatible with Solar salt. " The ceramic reacts with the ternary nitrate salt.
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Article history: Received 21 August 2012 Received in revised form 15 November 2012 Accepted 29 December 2012 Available online 8 February 2013 Keywords: Concentrated solar power (CSP) Thermal energy storage (TES) Molten salt thermocline Filler materials Ceramic Asbestos containing waste (ACW)
a b s t r a c t This paper demonstrates the potential of a post-industrial ceramic commercially called CofalitÒ as a promising, sustainable, and cheap filler material in a molten salt direct thermocline storage system. This ceramic, which comes from industrial treatment of asbestos containing waste, demonstrates relevant properties to store thermal energy by sensible heat up to 1100 °C and is very inexpensive. In the present study, the compatibility of this ceramic with two different molten salts—the conventional binary Solar salt and a promising ternary nitrate salt also called HITEC XL—is tested at medium temperature (500 °C) under static state. The objective is to develop a molten salt thermocline direct storage system using low-cost shaped ceramic as filler material. It should significantly decrease the cost of parabolic trough storage systems and simultaneously increase the efficiency of the plants by producing superheated steam at higher temperature. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Since the first oil crisis of the 1970s, concentrated solar power (CSP) generation has become a promising alternative to conventional electricity production. Contrary to other renewable energy technologies like photovoltaic or wind power, CSP offers the advantage of thermal energy storage (TES), which allows for electricity production on demand and independently of the solar resource. The most widespread CSP technology is the parabolic trough system, where parabolic mirrors are used to concentrate sunlight onto an absorber tube located at the focus line. A heat transfer fluid (HTF) circulates in thousands of meters of lighted ⇑ Corresponding author at: CIC Energigune, Albert Einstein 48, 01510 Miñano (Álava), Spain. Tel.: +34 945 297 108. E-mail address: [email protected] (N. Calvet). 0306-2619/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apenergy.2012.12.078
pipes to collect the thermal energy and then transfer it to a steam generator via a heat exchanger. This superheated steam is later used by a turbine to produce clean electricity in the power block. Research and industry efforts are now focusing on developing new, more efficient TES systems [1,2] using latent heat or thermochemical reactions. Nowadays, all commercial CSP plants with storage, use sensible heat storage systems. The most mature technology is the conventional two-tank molten salt storage system [3] like in the 50-MWe ANDASOL 1 plant near Granada in Spain. Two different fluids are used in this system. The first one is the HTF, typically synthetic oil commercially called Therminol VP-1Ò, which circulates in the absorber tubes in the solar field to collect the thermal energy. The second one is molten salt, which exchanges the thermal energy with the HTF via an oil-to-salt heat exchanger to store it during sunny periods or discharge it during cloudy periods and at night. The molten salt is separated in two different tanks: a
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Nomenclature
DT Tm wt.%
temperature difference (K) melting temperature (°C) weight percentage
Abbreviations ACW asbestos-containing waste BSED back scattered electron detector CSP concentrated solar power
‘‘hot’’ tank dedicated to the storage step (386 °C), and a ‘‘cold’’ tank dedicated to the discharge step (292 °C). In the case of ANDASOL 1, 28,500 tons of nitrate salts called ‘‘Solar salt’’ are used as an alternative to the solar resource for 7.5 h at nominal power. Using synthetic oil as HTF presents a significant advantage—its low freezing point (around 13 °C for Therminol VP-1Ò)—but it also presents three major drawbacks. First, this so-called ‘‘solar oil’’ is in fact a synthesized mixture of two very flammable compounds (73.5 wt.% oxide diphenyl and 26.5 wt.% diphenyl) with a strong potential eco-toxic impact. Second, this oil presents a vapor pressure higher than atmospheric pressure above 200 °C which requires expensive large pressure vessels. And finally, this oil cannot exceed a maximum temperature of 393 °C at the risk of being degraded, which restrains the maximum operating temperature of the superheated steam and by consequence limits the efficiency of the turbine. As parabolic trough technology can easily heat the HTF up to 500 °C, a potential extra DT of 100 °C to superheat the steam is theoretically lost. Replacing the oil with another fluid, stable up to 500 °C, would present a double advantage: first by producing 100 °C higher superheated steam and second by increasing the sensible heat storage capacity. In fact, the storage capacity is directly proportional to the temperature difference between the charge and the discharge step (DT). Increasing the DT by a factor of two, compared to a conventional plant like ANDASOL 1, would either double the storage capacity using the same amount of material or use half of the quantity for the same storage capacity. In order to reach these objectives, a possible alternative to synthetic oil is to use molten salt as both the HTF and the storage media in an active direct storage system. The main advantage is the thermal stability of the salt at high temperature. For example, the conventional Solar salt is stable up to around 600 °C. Another advantage relates to the global cost of the TES system. First, the molten salt itself is roughly half the cost of the conventional HTF. The price of molten salt is estimated between 400 € and 900 € per ton (depending on the composition) compared to about 1650 € per ton [4] for the synthetic oil Therminol VP-1Ò. Second, the elimination of the expensive oil-to-salt heat exchanger (4.1 Million € [9]) in an active direct storage system also significantly reduces the global price of the system. In the past, molten salt was demonstrated as a HTF in two central receiver power plants. The first was in the 1980s in the THEMIS power tower [5] in France; the second was in the 1990s in the Solar TWO power tower [6] in the United States. In the first case, the salt was a mixture of 53 wt.% KNO3, 40 wt.% NaNO2, and 7 wt.% NaNO3 commercially called HITECÒ Heat Transfer Salt. In the second case, Solar salt (60 wt.% NaNO3, 40 wt.% KNO3) was used. Today, a similar molten salt direct system using Solar salt is operating at the first commercial power tower, GEMASOLAR, which was inaugurated in June 2011 in Seville, Spain and has 15 h of storage [7]. The main drawback of using molten salts as HTF is their high freezing point: 142 °C for HITECÒ and 220 °C for Solar salt. In order to prevent irreversible damage caused by the solidification of the liquid salt in the
EDX ESEM HTF MWe TES XRD
energy dispersive X-ray spectroscopy environmental scanning electron microscope heat transfer fluid mega watts electric thermal energy storage X-ray diffraction
pipes, the molten salts are kept above their crystallization temperatures. In the THEMIS project, parallel steam lines were used. In the Solar TWO project, electric tracing was preferred, as is also currently used in GEMASOLAR. Still, in either case, a significant quantity of energy is lost by preventing the freezing of the liquid salt. Even more complex is the case of parabolic trough plants because of the huge length of their solar collectors. One experiment in progress at the ARCHIMEDE project in Sicily, Italy [8] has recently demonstrated that molten salt could be used as a HTF in parabolic troughs with 6.4 km of absorber tubes. Electric tracing is used to prevent freezing in the case of cloudy spells, and a drain-back system can drain all the pipes in long periods without sun. This first large-scale demonstration plant shows that direct molten salt storage could be feasible and should be further developed for parabolic trough plants but some additional work is still needed to reduce the freezing point of molten salts. A potential solution to reduce the energy lost by preventing the solidification of the HTF is to formulate new compositions of salts that have lower crystallization temperatures [17]. Several papers mention a calcium, sodium, and potassium nitrate mixture commercially called HITEC XL as a promising molten salt candidate (Tm 120 °C) [9–11]. This salt was used with success in different prototypes of the so-called thermocline storage systems, for instance, at Sandia National Laboratories in the United States. The thermocline system uses a single tank instead of two, which could reduce the cost of the storage system up to 35% compared to the conventional two-tank system [16]. The ‘‘cold’’ fraction of the salt is located at the bottom of the tank, while the ‘‘hot’’ salt fraction is at the top. The natural stratification between the cold and the hot liquid is called a thermocline. Another advantage of the thermocline system is the possibility to use inexpensive solid filler materials [10,15] such as sand or rocks, in order to reduce the quantity of expensive salt by up to 80% in the tank. In this case, one supplementary heat exchange is necessary between the solid materials in direct contact with the HTF. Many papers in literature are dedicated to the modeling of packed-bed molten salt thermocline systems [18–20]. Among the different studied parameters, the thermal conductivity of the filler materials (e.g., for conventional fillers <5 W m 1 K 1) play an important role. A potential increase of this effective thermal conductivity induces a decrease of the TES system discharging efficiency [20,21]. This is due to a higher conduction between solid fillers which increases the thermocline layer and reduce the useful part of the storage. One possible solution to enhance the heat transfer between the fillers and the HTF without increasing it between the solids themselves is to increase the solid material specific heat exchange surface. This could be possible by giving a particular shape to the filler materials. Post-industrial ceramic resulting of the vitrification of hazardous industrial waste such as asbestos containing waste (ACW), commercially called CofalitÒ [12], are obtained as a molten form during their industrial production and can be directly shaped at the exit of the furnace without
N. Calvet et al. / Applied Energy 109 (2013) 387–393 Table 1 Two compositions of prepared and used nitrate salts. No.
Commercial name
#1 #2
HITEC XL [10] Solar salt
Components (wt.%) Ca(NO3)2
NaNO3
KNO3
Melting temperature Tm (°C)
42 0
15 60
43 40
118 [11] 220 [4]
any additional energy consumption. A direct molten salt thermocline storage system using these low-cost recycled shaped ceramics as filler material is considered in this paper. This concept should potentially use 8 times less molten salt than conventional plants like ANDASOL 1 with the same storage capacity. One primary parameter to demonstrate the feasibility of this TES system is the absolutely imperative compatibility of the ceramic and the molten salt in the range of operating temperature (300–500 °C). Indeed, in the packed-bed TES system, the ceramic will be always in direct contact with the molten salt entering into the tank at a maximum temperature of 500 °C during the charge. The risk is that the salt reacts with the ceramic and destroys it partially or even entirely. In literature, many papers are dealing with corrosion aspect between molten salts and steel container. But few papers are dedicated to corrosion tests between molten salts and solid thermal energy storage materials. Sandia National Laboratory has proofed the compatibility between a mixture of quartzite and sand and HITEC XL ternary salt [5]. In our previous work [14] the incompatibility of CofalitÒ has been proofed with some sulphates, phosphates and carbonates. However no corrosion has been detected with the eutectic mixture of sodium and potassium nitrates (50 wt.% NaNO3, 50 wt.% KNO3) at 270 °C during 75 h. In this new complementary study the compatibility of the ceramic was tested with two other different nitrate salts: the conventional Solar salt (60 wt.% NaNO3, 40 wt.% KNO3) and the promising HITEC XL (42 wt.% Ca(NO3)2, 15 wt.% NaNO3, 42 wt.% KNO3). The tests were performed during 500 h at 500 °C which is the potential maximum working temperature of parabolic troughs. The experimental results are discussed in this paper. 2. Materials and methods
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nitrates. Table 1 summarizes the two tested salt compositions. The salt mixtures were prepared using commercially available, 99.0%pure (Alfa Aesar) salt powders. In the case of HITEC XL, because the calcium nitrate is only commercially available in a tetrahydrated form (Ca(NO3)24H2O), the included water must be boiled off before mixing all the components together in order to obtain the desired composition. The tetra-hydrated calcium nitrate was heated up in a furnace at 300 °C for 24 h to completely remove the water and thereby to produce anhydrous calcium nitrate (Ca(NO3)2). Because of the hydrophilic character of calcium nitrate (to avoid water absorption during the weighing), all preparations and precise measurements were made under dry nitrogen in a glove box and sealed in a ceramic container. Each preparation was thoroughly mixed for 40 min using a commercial mixer, the SFM-3 Desk-Top high-speed vibrating ball miller from MTI Corporation. 2.1.2. CofalitÒ as filler materials The selected thermocline tank filler material, CofalitÒ, is a lowcost inert ceramic industrially produced in France by the INERTAM Company by high-temperature (1500 °C) plasma treatment of asbestos-containing waste (ACW). Depending on the cooling rate, the material is obtained as a ceramic or a glass form. This study is focused on the ceramic form which was proved to be the more appropriate for TES applications as a part of the SOLSTOCK French national program [12–14]. CofalitÒ is a calcium magnesium iron alumino-silicate with different impurities (e.g., Cr, Cu, Zn, Mn, etc.) depending on the origin of the waste used. This advanced ceramic presents relevant properties to store thermal energy as sensible heat on whole the range of temperatures of CSP plants from 200 to 1100 °C. The advantages of using this ceramic instead of other fillers such as sand or rocks are multiple: a lower price, the use of recycled material rather than new raw material, which allows to avoid a depletion of natural resources (regarding the large amount of materials needed), the lack of conflict of use, the ability to directly shape ceramic into efficient heat exchanger/storage modules with little embodied energy during the industrial waste treatment.
2.1. Materials 2.1.1. Molten salts as HTF The different HTFs used in this study are nitrate salts such as the conventional Solar salt, a binary mixture of sodium and potassium nitrates currently used in commercial CSP plants, and the promising HITEC XL—a ternary mixture of sodium, potassium and calcium
This last point is especially important, as it could permit to increase the heat exchange specific surface between the HTF in direct contact with the storage modules in the thermocline tank and to solve the problem of relatively low thermal conductivity of the ceramic material (around 2 W m 1 K 1 [12]) leading to low-power TES system.
Fig. 1. Pictures of (a) a post-industrial ceramic sample in a crucible, (b) covered with powder salt before the test, and (c) covered with molten salt during the test.
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Fig. 2. Pictures of (a) a reference sample that did not undergo any test in salt, (b) the sample #1 tested in HITEC XL, (c) and the sample #2 tested in the Solar salt.
Fig. 3. ESEM images of the cut of (a) the reference sample and (b) the sample #2 tested in the Solar salt after 500 h of corrosion test.
Fig. 4. XRD patterns of the ceramic reference sample (top) and the sample tested in Solar salt (bottom). The vertical bars indicate the Bragg reflections, the top row corresponds to the Wollastonite phase and the bottom row corresponds to the Augite phase.
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Fig. 5. ESEM images of the surface of (a) the reference sample, (b) the sample #1 tested in HITEC XL and (c) the sample #2 tested in Solar salt after 500 h of corrosion test.
2.2. Methods 2.2.1. Corrosion test The ceramic samples used in this study were graciously provided by the INERTAM Company and prepared by the PROMES CNRS Laboratory. They were token-like pieces 25 mm in diameter and 3 mm in thickness cut from the same core drilling of the more dense part of an industrial CofalitÒ ingot. The samples were placed at the bottom of ceramic crucibles (Fig. 1a). The basic compatibility test method was performed at the National Renewable Energy Laboratory and consisted of soaking the sample in a salt and proceeding with thermal treatment in a furnace under air at constant temperature of 500 °C for 500 h. The salt, initially in powder form (Fig. 1b), was melted at this temperature and perfectly covered the ceramic sample during the thermal treatment (Fig. 1c). After the thermal treatment, the sample were removed from the salt and cleaned in hot water to eliminate some potential salt residues (these nitrates are soluble in water).
2.2.2. Structural and microstructural characterization Structural and microstructural analysis using X-ray powder diffraction (XRD) and environmental scanning electron microscopy (ESEM), respectively, were performed on the CofalitÒ samples after corrosion test to be compared with a reference sample that did not undergo any thermal treatment in salt. The raw sample surface (upper surface of the token in direct contact with the salt during the corrosion test) as well as the internal structure was analyzed. For the latest, specimens for SEM were prepared by cutting a thin layer (perpendicularly to the surface) of the samples then embedded in ATM KEM 90 resin. Finally specimens were polished with
Fig. 6. ESEM image of the cut of the sample #1 tested in HITEC XL after 500 h of corrosion test.
1 lm diamond suspension. All samples were imaged using the SEM Quanta 200 FEG operated in low vacuum mode at 30 kV using Back Scattered Electron Detector (BSED). Structural analysis was performed with an XRD D-8 Advance X-ray diffractometer (BRUKER Company).
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Fig. 7. ESEM image of the perpendicular cut of the sample #1 tested in HITEC XL after 500 h of corrosion test and corresponding EDX maps.
3. Experimental results and discussions The Fig. 2 shows pictures of the surface of three samples: the reference sample that did not undergo any thermal treatment in salt (Fig. 2a), the sample #1 tested 500 h in HITEC XL (Fig. 2b) and the sample #2 tested 500 h in Solar salt (Fig. 2c). The surface morphology of the reference sample and of the sample #2 tested in Solar salt is very similar, which is clearly not the case for the sample #1 tested in HITEC XL. This is confirmed by the ESEM images of the surface of the three samples showed in Fig. 5. 3.1. Compatibility CofalitÒ/Solar salt The Fig. 3 shows ESEM images of the cut of the reference (Fig. 3a) to be compared with the cut of the sample #2 tested in Solar salt (Fig. 3b). The internal crystal structure (morphology and size of grains) is similar in both cases. Two main phases can be
identified (dark grey and bright grey zones) which correspond to the two usual phases of the CofalitÒ ceramic already identified in precedent works [12–14]: Wollastonite (CaSiO3) (dark grey) and Augite ((Ca, Mg, Al, Fe)(SiO3)2) (bright grey) with various substitutions. The nature of these two phases was confirmed by XRD analysis in the case of the tested ceramic samples as presented in Fig. 4. In the two ESEM images (Fig. 3a and b) some impurities of metal oxides insertions (small white areas) as well as some closed porosity (black areas) and some micro-cracks (black lines) were also identified. In case of corrosion, the perpendicular cut of the sample usually permit the clear identification of a different corroded layer at the surface of the sample as already demonstrated in precedent works [14]. On the surface of the sample #2 tested in the Solar salt, no corrosion layer was observed after 500 h of test at 500 °C. The unchanged internal structure and the absence of corroded layer show that the CofalitÒ ceramic is thermo-chemically stable in Solar salt.
Fig. 8. XRD pattern of the reference sample (bottom) and of the ceramic sample #1 tested in HITEC XL (top) after 500 h of corrosion test. The vertical bars indicate the Bragg reflections, the top row corresponds to the Wollastonite phase, the middle row corresponds to the Augite phase and the bottom row corresponds to the Ca2SiO4 phase.
N. Calvet et al. / Applied Energy 109 (2013) 387–393
3.2. Compatibility CofalitÒ/HITEC XL The Fig. 5 shows ESEM images of the surface of the raw reference sample (Fig. 5a) to be compared with the surface of the sample #1 tested in HITEC XL (Fig. 5b). The aspect of the two surfaces is totally different which clearly shows that one modification on the surface of the material occurred during the thermal treatment in the ternary salt. A structure in form of ‘‘agglomerate needles’’ covering almost all the surface of the sample is well observed and confirmed this modification of the sample surface. The cut of the sample #1 tested in HITEC XL (Fig. 6) is shown in the following ESEM micrograph. The internal structure is similar than in the reference sample. No traces of elements resulting from the salts have been observed by energy dispersive X-ray spectroscopy (EDX). It confirms the stability of the internal structure of the material and that the modification of structure was only limited to the surface which was in direct contact with the salt. The cut of the sample #1 in Fig. 7 demonstrates clearly a presence of thin layer with a thickness of about 10 lm at the surface of the sample. The EDX mapping shows that the elements composing this layer are calcium, silicon and oxygen. In order to obtain information about the mechanism of corrosion, a XRD was performed to identify the nature of the new layer. Fig. 8 shows the diffractograms of the sample #1 after the corrosion test in comparison to the raw reference material. Contrary to the first case the diffractogram of the sample #1 presents the appearing of new extra peaks marked by stars in the figure at 2h around 32.5 and 42. These peaks were identified as calcium silicate Ca2SiO4 resulting from the reaction of the Ca(NO3)2 of the salt with the Wollastonite phase CaSiO3. It seems that this layer is formed at the beginning of the thermal treatment and acts as a protective layer because there was no penetration of salts observed inside the material along the numerous micro-cracks. Some supplementary work must be done to confirm that this layer does not grow with time and thermal cycles and also to determine the consequences of this layer on the material behaviors as well as a potential salt degradation or modification. 4. Conclusions A corrosion test has been performed on ceramic samples, from industrial treatment of asbestos containing waste, immersed for 500 h at 500 °C in two kinds of nitrate salts, the binary sodium, potassium nitrate and the ternary calcium, sodium, potassium nitrate. The post-treatment microstructure analysis had permitted to conclude that: (i) no corrosion layer can be observed for the sample tested in the binary salt, and (ii) a 10 lm thin layer of calcium silicate was formed at the surface of the sample tested in the ternary salt, which shows that the calcium present in the salt reacted with the ceramic. The CofalitÒ ceramic can be used in direct contact with the conventional Solar salt. This compatibility must be confirmed in dynamic state with a moving molten salt trough a packed bed of ceramic. A laboratory scale molten salt thermocline system will be constructed in order to validate this approach using the CofalitÒ as filler material with the Solar salt. Supplementary work is needed to conclude about the compatibility with HITEC XL. Acknowledgments The work at CIC Energigune was supported by the Department of Industry, Innovation, Commerce and Tourism of the Basque gov-
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ernment through the funding of the ETORTEK CIC Energigune-2011 research program. The work at NREL was supported by the U.S. Department of Energy under Contract No. DE-AC36-08-GO28308. The work at the PROMES CNRS laboratory was supported by the French government through the funding of the ANR research programs: ANR SESCO (No. ANR-09-STOCK-E-09-03). The authors would like to acknowledge the INERTAM/EuroPLASMA Company (France) for providing samples of CofalitÒ ceramic. Finally, they want to thank Devonie McCamey and Dr. Anne K. Starace for their help in editing this document. References [1] Gil A, Medrano M, Martorell I, Lázaro A, Dolado P, Belén Zalba, et al. State of the art on high temperature thermal energy storage for power generation. Part 1— concepts, materials and modellization. Renew Sustain Energy Rev 2010;14:31–55. [2] Medrano M, Gil A, Martorell I, Potau X, Cabeza LF. State of the art on hightemperature thermal energy storage for power generation. Part 2—case studies. Renew Sustain Energy Rev 2010;14:56–72. [3] Herrmann U, Kelly B, Price H. Two-tank molten salt storage for parabolic trough solar power plants. Energy 2004;29:883–93. [4] Kearney D, Herrmann U, Nava P, Kelly B, Mahoney R, Pacheco J, et al. Assessment of a molten salt heat transfer fluid in parabolic trough solar field. Trans ASME 2003;125:170–6. [5] Cosar P, Etievant CI, Pouget-Abadie X. La centrale électrosolaire THEMIS. Revue générale de thermique Août-Septembre 1978, Fr 200–201. [6] Speidel PJ, Kelly BD, Prairie MR, Pacheco JE, Gilbert RL, Reilly HE. Performances of the solar two central receiver power plant. J Phys 4 France 1999;9. [7] Burgaleta JI, Arias S, Ramirez D. Gemasolar, the first tower thermosolar commercial plant with molten salt storage system. In: Proceedings of SOLARPACES 2011, Granada, Spain, September 20–23, 2011. [8] Donatini F, Zamparelli C, Maccari A, Vignolini M. High efficiency integration of thermodynamic solar plant with natural gas combined cycle. 1-4244-0632-3/ 07; 2007. [9] Brosseau D, Kelton JW, Ray D, Edgar M, Chisman K, Emms B. Testing of thermocline filler materials and molten-salt heat transfer fluids for thermal energy storage systems in parabolic trough power plants. J Sol Energy Eng 2005;127(109). [10] Pacheco JE, Showalter SK, Kolb WJ. Development of a molten-salt thermocline thermal storage system for parabolic trough plants. In: Proceedings of ASME 2001, solar forum 2001, solar energy: the power to choose, April 21–25, 2001, Washington, DC. [11] Gomez JC, Calvet N, Starace AK, Glatzmaier GC. Ca(NO3)2–NaNO3–KNO3 molten salt mixtures for direct thermal energy storage systems in parabolic trough plants. J Sol Energy Eng 2013;135:021017-2. [12] Py X, Calvet N, Olivès R, Meffre A, Echegut P, Bessada C, et al. Recycled material for sensible heat based thermal energy storage to be used in concentrated solar thermal power plants. J Sol Energy Eng 2011;133:031008-1. [13] Faik A, Guillot S, Lambert J, Véron E, Ory S, Bessada C, et al. Thermal storage material from inertised wastes: evolution of structural and radiative properties with temperature. Sol Energy 2012;86:139–46. [14] Guillot S, Faik A, Rakhmatullin A, Lambert J, Veron E, Echegut P, et al. Corrosion effects between molten salts and thermal storage material for concentrated solar power plants. Appl Energy 2012;94:174–81. [15] St Laurent SJ, Kolb WJ, Pacheco JE. Thermocline thermal storage tests for largescale solar thermal power plants. Sandia National Laboratories, Report SAND2000-2059C; 2000. [16] Solar Thermocline Storage Systems: Preliminary Design Study. EPRI, Palo Alto, CA; 2010, 1019581. [17] Peng Q, Ding J, Wei X, Yang J, Yang X. The preparation and properties of multicomponent molten salts. Appl Energy 2010;87–9:2812–7. [18] Flueckiger S, Yang Z, Garimella SV. An integrated thermal and mechanical investigation of molten-salt thermocline energy storage. Appl Energy 2011;88:2098–105. [19] Qin FGF, Yang X, Ding Z, Zuo Y, Shao Y, Jiang R, et al. Thermocline stability criterions in single-tanks of molten salt thermal energy storage. Appl Energy 2012;97:816–21. [20] Xu C, Wanga Z, He Y, Li X, Bai F. Sensitivity analysis of the numerical study on the thermal performance of a packed-bed molten salt thermocline thermal storage system. Appl Energy 2012;92:65–75. [21] Bayón R, Rojas E, Rivas E. Effect of storage medium properties in the performance of thermocline tanks. In: Proceedings of SOLARPACES 2012 conference, Marrakech, Morocco September 11–14, 2012.
Contents lists available at SciVerse ScienceDirect
Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Compatibility of a post-industrial ceramic with nitrate molten salts for use as filler material in a thermocline storage system Nicolas Calvet a,b,⇑, Judith C. Gomez b, Abdessamad Faik a, Vladimir V. Roddatis a, Antoine Meffre c, Greg C. Glatzmaier b, Stefania Doppiu a, Xavier Py c a b c
CIC Energigune, Albert Einstein 48, 01510 Miñano (Álava), Spain National Renewable Energy Laboratory, 1617 Cole Blvd, 80401 Golden CO, USA PROMES CNRS, UPR8521, Université de Perpignan, Rambla de la Thermodynamique, Tecnosud, 66100 Perpignan, France
h i g h l i g h t s " An innovative ceramic is considered as thermal energy storage material. " The ceramic is tested in direct contact with heat transfer fluids in static state at 500 °C during 500 h. " Two kinds of molten salts are used: Solar salt (NaNO3, KNO3) and the ternary nitrate salt (NaNO3, KNO3, Ca(NO3)2). " The ceramic is compatible with Solar salt. " The ceramic reacts with the ternary nitrate salt.
a r t i c l e
i n f o
Article history: Received 21 August 2012 Received in revised form 15 November 2012 Accepted 29 December 2012 Available online 8 February 2013 Keywords: Concentrated solar power (CSP) Thermal energy storage (TES) Molten salt thermocline Filler materials Ceramic Asbestos containing waste (ACW)
a b s t r a c t This paper demonstrates the potential of a post-industrial ceramic commercially called CofalitÒ as a promising, sustainable, and cheap filler material in a molten salt direct thermocline storage system. This ceramic, which comes from industrial treatment of asbestos containing waste, demonstrates relevant properties to store thermal energy by sensible heat up to 1100 °C and is very inexpensive. In the present study, the compatibility of this ceramic with two different molten salts—the conventional binary Solar salt and a promising ternary nitrate salt also called HITEC XL—is tested at medium temperature (500 °C) under static state. The objective is to develop a molten salt thermocline direct storage system using low-cost shaped ceramic as filler material. It should significantly decrease the cost of parabolic trough storage systems and simultaneously increase the efficiency of the plants by producing superheated steam at higher temperature. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Since the first oil crisis of the 1970s, concentrated solar power (CSP) generation has become a promising alternative to conventional electricity production. Contrary to other renewable energy technologies like photovoltaic or wind power, CSP offers the advantage of thermal energy storage (TES), which allows for electricity production on demand and independently of the solar resource. The most widespread CSP technology is the parabolic trough system, where parabolic mirrors are used to concentrate sunlight onto an absorber tube located at the focus line. A heat transfer fluid (HTF) circulates in thousands of meters of lighted ⇑ Corresponding author at: CIC Energigune, Albert Einstein 48, 01510 Miñano (Álava), Spain. Tel.: +34 945 297 108. E-mail address: [email protected] (N. Calvet). 0306-2619/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apenergy.2012.12.078
pipes to collect the thermal energy and then transfer it to a steam generator via a heat exchanger. This superheated steam is later used by a turbine to produce clean electricity in the power block. Research and industry efforts are now focusing on developing new, more efficient TES systems [1,2] using latent heat or thermochemical reactions. Nowadays, all commercial CSP plants with storage, use sensible heat storage systems. The most mature technology is the conventional two-tank molten salt storage system [3] like in the 50-MWe ANDASOL 1 plant near Granada in Spain. Two different fluids are used in this system. The first one is the HTF, typically synthetic oil commercially called Therminol VP-1Ò, which circulates in the absorber tubes in the solar field to collect the thermal energy. The second one is molten salt, which exchanges the thermal energy with the HTF via an oil-to-salt heat exchanger to store it during sunny periods or discharge it during cloudy periods and at night. The molten salt is separated in two different tanks: a
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Nomenclature
DT Tm wt.%
temperature difference (K) melting temperature (°C) weight percentage
Abbreviations ACW asbestos-containing waste BSED back scattered electron detector CSP concentrated solar power
‘‘hot’’ tank dedicated to the storage step (386 °C), and a ‘‘cold’’ tank dedicated to the discharge step (292 °C). In the case of ANDASOL 1, 28,500 tons of nitrate salts called ‘‘Solar salt’’ are used as an alternative to the solar resource for 7.5 h at nominal power. Using synthetic oil as HTF presents a significant advantage—its low freezing point (around 13 °C for Therminol VP-1Ò)—but it also presents three major drawbacks. First, this so-called ‘‘solar oil’’ is in fact a synthesized mixture of two very flammable compounds (73.5 wt.% oxide diphenyl and 26.5 wt.% diphenyl) with a strong potential eco-toxic impact. Second, this oil presents a vapor pressure higher than atmospheric pressure above 200 °C which requires expensive large pressure vessels. And finally, this oil cannot exceed a maximum temperature of 393 °C at the risk of being degraded, which restrains the maximum operating temperature of the superheated steam and by consequence limits the efficiency of the turbine. As parabolic trough technology can easily heat the HTF up to 500 °C, a potential extra DT of 100 °C to superheat the steam is theoretically lost. Replacing the oil with another fluid, stable up to 500 °C, would present a double advantage: first by producing 100 °C higher superheated steam and second by increasing the sensible heat storage capacity. In fact, the storage capacity is directly proportional to the temperature difference between the charge and the discharge step (DT). Increasing the DT by a factor of two, compared to a conventional plant like ANDASOL 1, would either double the storage capacity using the same amount of material or use half of the quantity for the same storage capacity. In order to reach these objectives, a possible alternative to synthetic oil is to use molten salt as both the HTF and the storage media in an active direct storage system. The main advantage is the thermal stability of the salt at high temperature. For example, the conventional Solar salt is stable up to around 600 °C. Another advantage relates to the global cost of the TES system. First, the molten salt itself is roughly half the cost of the conventional HTF. The price of molten salt is estimated between 400 € and 900 € per ton (depending on the composition) compared to about 1650 € per ton [4] for the synthetic oil Therminol VP-1Ò. Second, the elimination of the expensive oil-to-salt heat exchanger (4.1 Million € [9]) in an active direct storage system also significantly reduces the global price of the system. In the past, molten salt was demonstrated as a HTF in two central receiver power plants. The first was in the 1980s in the THEMIS power tower [5] in France; the second was in the 1990s in the Solar TWO power tower [6] in the United States. In the first case, the salt was a mixture of 53 wt.% KNO3, 40 wt.% NaNO2, and 7 wt.% NaNO3 commercially called HITECÒ Heat Transfer Salt. In the second case, Solar salt (60 wt.% NaNO3, 40 wt.% KNO3) was used. Today, a similar molten salt direct system using Solar salt is operating at the first commercial power tower, GEMASOLAR, which was inaugurated in June 2011 in Seville, Spain and has 15 h of storage [7]. The main drawback of using molten salts as HTF is their high freezing point: 142 °C for HITECÒ and 220 °C for Solar salt. In order to prevent irreversible damage caused by the solidification of the liquid salt in the
EDX ESEM HTF MWe TES XRD
energy dispersive X-ray spectroscopy environmental scanning electron microscope heat transfer fluid mega watts electric thermal energy storage X-ray diffraction
pipes, the molten salts are kept above their crystallization temperatures. In the THEMIS project, parallel steam lines were used. In the Solar TWO project, electric tracing was preferred, as is also currently used in GEMASOLAR. Still, in either case, a significant quantity of energy is lost by preventing the freezing of the liquid salt. Even more complex is the case of parabolic trough plants because of the huge length of their solar collectors. One experiment in progress at the ARCHIMEDE project in Sicily, Italy [8] has recently demonstrated that molten salt could be used as a HTF in parabolic troughs with 6.4 km of absorber tubes. Electric tracing is used to prevent freezing in the case of cloudy spells, and a drain-back system can drain all the pipes in long periods without sun. This first large-scale demonstration plant shows that direct molten salt storage could be feasible and should be further developed for parabolic trough plants but some additional work is still needed to reduce the freezing point of molten salts. A potential solution to reduce the energy lost by preventing the solidification of the HTF is to formulate new compositions of salts that have lower crystallization temperatures [17]. Several papers mention a calcium, sodium, and potassium nitrate mixture commercially called HITEC XL as a promising molten salt candidate (Tm 120 °C) [9–11]. This salt was used with success in different prototypes of the so-called thermocline storage systems, for instance, at Sandia National Laboratories in the United States. The thermocline system uses a single tank instead of two, which could reduce the cost of the storage system up to 35% compared to the conventional two-tank system [16]. The ‘‘cold’’ fraction of the salt is located at the bottom of the tank, while the ‘‘hot’’ salt fraction is at the top. The natural stratification between the cold and the hot liquid is called a thermocline. Another advantage of the thermocline system is the possibility to use inexpensive solid filler materials [10,15] such as sand or rocks, in order to reduce the quantity of expensive salt by up to 80% in the tank. In this case, one supplementary heat exchange is necessary between the solid materials in direct contact with the HTF. Many papers in literature are dedicated to the modeling of packed-bed molten salt thermocline systems [18–20]. Among the different studied parameters, the thermal conductivity of the filler materials (e.g., for conventional fillers <5 W m 1 K 1) play an important role. A potential increase of this effective thermal conductivity induces a decrease of the TES system discharging efficiency [20,21]. This is due to a higher conduction between solid fillers which increases the thermocline layer and reduce the useful part of the storage. One possible solution to enhance the heat transfer between the fillers and the HTF without increasing it between the solids themselves is to increase the solid material specific heat exchange surface. This could be possible by giving a particular shape to the filler materials. Post-industrial ceramic resulting of the vitrification of hazardous industrial waste such as asbestos containing waste (ACW), commercially called CofalitÒ [12], are obtained as a molten form during their industrial production and can be directly shaped at the exit of the furnace without
N. Calvet et al. / Applied Energy 109 (2013) 387–393 Table 1 Two compositions of prepared and used nitrate salts. No.
Commercial name
#1 #2
HITEC XL [10] Solar salt
Components (wt.%) Ca(NO3)2
NaNO3
KNO3
Melting temperature Tm (°C)
42 0
15 60
43 40
118 [11] 220 [4]
any additional energy consumption. A direct molten salt thermocline storage system using these low-cost recycled shaped ceramics as filler material is considered in this paper. This concept should potentially use 8 times less molten salt than conventional plants like ANDASOL 1 with the same storage capacity. One primary parameter to demonstrate the feasibility of this TES system is the absolutely imperative compatibility of the ceramic and the molten salt in the range of operating temperature (300–500 °C). Indeed, in the packed-bed TES system, the ceramic will be always in direct contact with the molten salt entering into the tank at a maximum temperature of 500 °C during the charge. The risk is that the salt reacts with the ceramic and destroys it partially or even entirely. In literature, many papers are dealing with corrosion aspect between molten salts and steel container. But few papers are dedicated to corrosion tests between molten salts and solid thermal energy storage materials. Sandia National Laboratory has proofed the compatibility between a mixture of quartzite and sand and HITEC XL ternary salt [5]. In our previous work [14] the incompatibility of CofalitÒ has been proofed with some sulphates, phosphates and carbonates. However no corrosion has been detected with the eutectic mixture of sodium and potassium nitrates (50 wt.% NaNO3, 50 wt.% KNO3) at 270 °C during 75 h. In this new complementary study the compatibility of the ceramic was tested with two other different nitrate salts: the conventional Solar salt (60 wt.% NaNO3, 40 wt.% KNO3) and the promising HITEC XL (42 wt.% Ca(NO3)2, 15 wt.% NaNO3, 42 wt.% KNO3). The tests were performed during 500 h at 500 °C which is the potential maximum working temperature of parabolic troughs. The experimental results are discussed in this paper. 2. Materials and methods
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nitrates. Table 1 summarizes the two tested salt compositions. The salt mixtures were prepared using commercially available, 99.0%pure (Alfa Aesar) salt powders. In the case of HITEC XL, because the calcium nitrate is only commercially available in a tetrahydrated form (Ca(NO3)24H2O), the included water must be boiled off before mixing all the components together in order to obtain the desired composition. The tetra-hydrated calcium nitrate was heated up in a furnace at 300 °C for 24 h to completely remove the water and thereby to produce anhydrous calcium nitrate (Ca(NO3)2). Because of the hydrophilic character of calcium nitrate (to avoid water absorption during the weighing), all preparations and precise measurements were made under dry nitrogen in a glove box and sealed in a ceramic container. Each preparation was thoroughly mixed for 40 min using a commercial mixer, the SFM-3 Desk-Top high-speed vibrating ball miller from MTI Corporation. 2.1.2. CofalitÒ as filler materials The selected thermocline tank filler material, CofalitÒ, is a lowcost inert ceramic industrially produced in France by the INERTAM Company by high-temperature (1500 °C) plasma treatment of asbestos-containing waste (ACW). Depending on the cooling rate, the material is obtained as a ceramic or a glass form. This study is focused on the ceramic form which was proved to be the more appropriate for TES applications as a part of the SOLSTOCK French national program [12–14]. CofalitÒ is a calcium magnesium iron alumino-silicate with different impurities (e.g., Cr, Cu, Zn, Mn, etc.) depending on the origin of the waste used. This advanced ceramic presents relevant properties to store thermal energy as sensible heat on whole the range of temperatures of CSP plants from 200 to 1100 °C. The advantages of using this ceramic instead of other fillers such as sand or rocks are multiple: a lower price, the use of recycled material rather than new raw material, which allows to avoid a depletion of natural resources (regarding the large amount of materials needed), the lack of conflict of use, the ability to directly shape ceramic into efficient heat exchanger/storage modules with little embodied energy during the industrial waste treatment.
2.1. Materials 2.1.1. Molten salts as HTF The different HTFs used in this study are nitrate salts such as the conventional Solar salt, a binary mixture of sodium and potassium nitrates currently used in commercial CSP plants, and the promising HITEC XL—a ternary mixture of sodium, potassium and calcium
This last point is especially important, as it could permit to increase the heat exchange specific surface between the HTF in direct contact with the storage modules in the thermocline tank and to solve the problem of relatively low thermal conductivity of the ceramic material (around 2 W m 1 K 1 [12]) leading to low-power TES system.
Fig. 1. Pictures of (a) a post-industrial ceramic sample in a crucible, (b) covered with powder salt before the test, and (c) covered with molten salt during the test.
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Fig. 2. Pictures of (a) a reference sample that did not undergo any test in salt, (b) the sample #1 tested in HITEC XL, (c) and the sample #2 tested in the Solar salt.
Fig. 3. ESEM images of the cut of (a) the reference sample and (b) the sample #2 tested in the Solar salt after 500 h of corrosion test.
Fig. 4. XRD patterns of the ceramic reference sample (top) and the sample tested in Solar salt (bottom). The vertical bars indicate the Bragg reflections, the top row corresponds to the Wollastonite phase and the bottom row corresponds to the Augite phase.
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Fig. 5. ESEM images of the surface of (a) the reference sample, (b) the sample #1 tested in HITEC XL and (c) the sample #2 tested in Solar salt after 500 h of corrosion test.
2.2. Methods 2.2.1. Corrosion test The ceramic samples used in this study were graciously provided by the INERTAM Company and prepared by the PROMES CNRS Laboratory. They were token-like pieces 25 mm in diameter and 3 mm in thickness cut from the same core drilling of the more dense part of an industrial CofalitÒ ingot. The samples were placed at the bottom of ceramic crucibles (Fig. 1a). The basic compatibility test method was performed at the National Renewable Energy Laboratory and consisted of soaking the sample in a salt and proceeding with thermal treatment in a furnace under air at constant temperature of 500 °C for 500 h. The salt, initially in powder form (Fig. 1b), was melted at this temperature and perfectly covered the ceramic sample during the thermal treatment (Fig. 1c). After the thermal treatment, the sample were removed from the salt and cleaned in hot water to eliminate some potential salt residues (these nitrates are soluble in water).
2.2.2. Structural and microstructural characterization Structural and microstructural analysis using X-ray powder diffraction (XRD) and environmental scanning electron microscopy (ESEM), respectively, were performed on the CofalitÒ samples after corrosion test to be compared with a reference sample that did not undergo any thermal treatment in salt. The raw sample surface (upper surface of the token in direct contact with the salt during the corrosion test) as well as the internal structure was analyzed. For the latest, specimens for SEM were prepared by cutting a thin layer (perpendicularly to the surface) of the samples then embedded in ATM KEM 90 resin. Finally specimens were polished with
Fig. 6. ESEM image of the cut of the sample #1 tested in HITEC XL after 500 h of corrosion test.
1 lm diamond suspension. All samples were imaged using the SEM Quanta 200 FEG operated in low vacuum mode at 30 kV using Back Scattered Electron Detector (BSED). Structural analysis was performed with an XRD D-8 Advance X-ray diffractometer (BRUKER Company).
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Fig. 7. ESEM image of the perpendicular cut of the sample #1 tested in HITEC XL after 500 h of corrosion test and corresponding EDX maps.
3. Experimental results and discussions The Fig. 2 shows pictures of the surface of three samples: the reference sample that did not undergo any thermal treatment in salt (Fig. 2a), the sample #1 tested 500 h in HITEC XL (Fig. 2b) and the sample #2 tested 500 h in Solar salt (Fig. 2c). The surface morphology of the reference sample and of the sample #2 tested in Solar salt is very similar, which is clearly not the case for the sample #1 tested in HITEC XL. This is confirmed by the ESEM images of the surface of the three samples showed in Fig. 5. 3.1. Compatibility CofalitÒ/Solar salt The Fig. 3 shows ESEM images of the cut of the reference (Fig. 3a) to be compared with the cut of the sample #2 tested in Solar salt (Fig. 3b). The internal crystal structure (morphology and size of grains) is similar in both cases. Two main phases can be
identified (dark grey and bright grey zones) which correspond to the two usual phases of the CofalitÒ ceramic already identified in precedent works [12–14]: Wollastonite (CaSiO3) (dark grey) and Augite ((Ca, Mg, Al, Fe)(SiO3)2) (bright grey) with various substitutions. The nature of these two phases was confirmed by XRD analysis in the case of the tested ceramic samples as presented in Fig. 4. In the two ESEM images (Fig. 3a and b) some impurities of metal oxides insertions (small white areas) as well as some closed porosity (black areas) and some micro-cracks (black lines) were also identified. In case of corrosion, the perpendicular cut of the sample usually permit the clear identification of a different corroded layer at the surface of the sample as already demonstrated in precedent works [14]. On the surface of the sample #2 tested in the Solar salt, no corrosion layer was observed after 500 h of test at 500 °C. The unchanged internal structure and the absence of corroded layer show that the CofalitÒ ceramic is thermo-chemically stable in Solar salt.
Fig. 8. XRD pattern of the reference sample (bottom) and of the ceramic sample #1 tested in HITEC XL (top) after 500 h of corrosion test. The vertical bars indicate the Bragg reflections, the top row corresponds to the Wollastonite phase, the middle row corresponds to the Augite phase and the bottom row corresponds to the Ca2SiO4 phase.
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3.2. Compatibility CofalitÒ/HITEC XL The Fig. 5 shows ESEM images of the surface of the raw reference sample (Fig. 5a) to be compared with the surface of the sample #1 tested in HITEC XL (Fig. 5b). The aspect of the two surfaces is totally different which clearly shows that one modification on the surface of the material occurred during the thermal treatment in the ternary salt. A structure in form of ‘‘agglomerate needles’’ covering almost all the surface of the sample is well observed and confirmed this modification of the sample surface. The cut of the sample #1 tested in HITEC XL (Fig. 6) is shown in the following ESEM micrograph. The internal structure is similar than in the reference sample. No traces of elements resulting from the salts have been observed by energy dispersive X-ray spectroscopy (EDX). It confirms the stability of the internal structure of the material and that the modification of structure was only limited to the surface which was in direct contact with the salt. The cut of the sample #1 in Fig. 7 demonstrates clearly a presence of thin layer with a thickness of about 10 lm at the surface of the sample. The EDX mapping shows that the elements composing this layer are calcium, silicon and oxygen. In order to obtain information about the mechanism of corrosion, a XRD was performed to identify the nature of the new layer. Fig. 8 shows the diffractograms of the sample #1 after the corrosion test in comparison to the raw reference material. Contrary to the first case the diffractogram of the sample #1 presents the appearing of new extra peaks marked by stars in the figure at 2h around 32.5 and 42. These peaks were identified as calcium silicate Ca2SiO4 resulting from the reaction of the Ca(NO3)2 of the salt with the Wollastonite phase CaSiO3. It seems that this layer is formed at the beginning of the thermal treatment and acts as a protective layer because there was no penetration of salts observed inside the material along the numerous micro-cracks. Some supplementary work must be done to confirm that this layer does not grow with time and thermal cycles and also to determine the consequences of this layer on the material behaviors as well as a potential salt degradation or modification. 4. Conclusions A corrosion test has been performed on ceramic samples, from industrial treatment of asbestos containing waste, immersed for 500 h at 500 °C in two kinds of nitrate salts, the binary sodium, potassium nitrate and the ternary calcium, sodium, potassium nitrate. The post-treatment microstructure analysis had permitted to conclude that: (i) no corrosion layer can be observed for the sample tested in the binary salt, and (ii) a 10 lm thin layer of calcium silicate was formed at the surface of the sample tested in the ternary salt, which shows that the calcium present in the salt reacted with the ceramic. The CofalitÒ ceramic can be used in direct contact with the conventional Solar salt. This compatibility must be confirmed in dynamic state with a moving molten salt trough a packed bed of ceramic. A laboratory scale molten salt thermocline system will be constructed in order to validate this approach using the CofalitÒ as filler material with the Solar salt. Supplementary work is needed to conclude about the compatibility with HITEC XL. Acknowledgments The work at CIC Energigune was supported by the Department of Industry, Innovation, Commerce and Tourism of the Basque gov-
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ernment through the funding of the ETORTEK CIC Energigune-2011 research program. The work at NREL was supported by the U.S. Department of Energy under Contract No. DE-AC36-08-GO28308. The work at the PROMES CNRS laboratory was supported by the French government through the funding of the ANR research programs: ANR SESCO (No. ANR-09-STOCK-E-09-03). The authors would like to acknowledge the INERTAM/EuroPLASMA Company (France) for providing samples of CofalitÒ ceramic. Finally, they want to thank Devonie McCamey and Dr. Anne K. Starace for their help in editing this document. References [1] Gil A, Medrano M, Martorell I, Lázaro A, Dolado P, Belén Zalba, et al. State of the art on high temperature thermal energy storage for power generation. Part 1— concepts, materials and modellization. Renew Sustain Energy Rev 2010;14:31–55. [2] Medrano M, Gil A, Martorell I, Potau X, Cabeza LF. State of the art on hightemperature thermal energy storage for power generation. Part 2—case studies. Renew Sustain Energy Rev 2010;14:56–72. [3] Herrmann U, Kelly B, Price H. Two-tank molten salt storage for parabolic trough solar power plants. Energy 2004;29:883–93. [4] Kearney D, Herrmann U, Nava P, Kelly B, Mahoney R, Pacheco J, et al. Assessment of a molten salt heat transfer fluid in parabolic trough solar field. Trans ASME 2003;125:170–6. [5] Cosar P, Etievant CI, Pouget-Abadie X. La centrale électrosolaire THEMIS. Revue générale de thermique Août-Septembre 1978, Fr 200–201. [6] Speidel PJ, Kelly BD, Prairie MR, Pacheco JE, Gilbert RL, Reilly HE. Performances of the solar two central receiver power plant. J Phys 4 France 1999;9. [7] Burgaleta JI, Arias S, Ramirez D. Gemasolar, the first tower thermosolar commercial plant with molten salt storage system. In: Proceedings of SOLARPACES 2011, Granada, Spain, September 20–23, 2011. [8] Donatini F, Zamparelli C, Maccari A, Vignolini M. High efficiency integration of thermodynamic solar plant with natural gas combined cycle. 1-4244-0632-3/ 07; 2007. [9] Brosseau D, Kelton JW, Ray D, Edgar M, Chisman K, Emms B. Testing of thermocline filler materials and molten-salt heat transfer fluids for thermal energy storage systems in parabolic trough power plants. J Sol Energy Eng 2005;127(109). [10] Pacheco JE, Showalter SK, Kolb WJ. Development of a molten-salt thermocline thermal storage system for parabolic trough plants. In: Proceedings of ASME 2001, solar forum 2001, solar energy: the power to choose, April 21–25, 2001, Washington, DC. [11] Gomez JC, Calvet N, Starace AK, Glatzmaier GC. Ca(NO3)2–NaNO3–KNO3 molten salt mixtures for direct thermal energy storage systems in parabolic trough plants. J Sol Energy Eng 2013;135:021017-2. [12] Py X, Calvet N, Olivès R, Meffre A, Echegut P, Bessada C, et al. Recycled material for sensible heat based thermal energy storage to be used in concentrated solar thermal power plants. J Sol Energy Eng 2011;133:031008-1. [13] Faik A, Guillot S, Lambert J, Véron E, Ory S, Bessada C, et al. Thermal storage material from inertised wastes: evolution of structural and radiative properties with temperature. Sol Energy 2012;86:139–46. [14] Guillot S, Faik A, Rakhmatullin A, Lambert J, Veron E, Echegut P, et al. Corrosion effects between molten salts and thermal storage material for concentrated solar power plants. Appl Energy 2012;94:174–81. [15] St Laurent SJ, Kolb WJ, Pacheco JE. Thermocline thermal storage tests for largescale solar thermal power plants. Sandia National Laboratories, Report SAND2000-2059C; 2000. [16] Solar Thermocline Storage Systems: Preliminary Design Study. EPRI, Palo Alto, CA; 2010, 1019581. [17] Peng Q, Ding J, Wei X, Yang J, Yang X. The preparation and properties of multicomponent molten salts. Appl Energy 2010;87–9:2812–7. [18] Flueckiger S, Yang Z, Garimella SV. An integrated thermal and mechanical investigation of molten-salt thermocline energy storage. Appl Energy 2011;88:2098–105. [19] Qin FGF, Yang X, Ding Z, Zuo Y, Shao Y, Jiang R, et al. Thermocline stability criterions in single-tanks of molten salt thermal energy storage. Appl Energy 2012;97:816–21. [20] Xu C, Wanga Z, He Y, Li X, Bai F. Sensitivity analysis of the numerical study on the thermal performance of a packed-bed molten salt thermocline thermal storage system. Appl Energy 2012;92:65–75. [21] Bayón R, Rojas E, Rivas E. Effect of storage medium properties in the performance of thermocline tanks. In: Proceedings of SOLARPACES 2012 conference, Marrakech, Morocco September 11–14, 2012.