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Material aspects of Solar Salt for sensible heat storage
Applied Energy 111 (2013) 1114–1119
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Material aspects of Solar Salt for sensible heat storage Thomas Bauer a,⇑, Nicole Pfleger b, Nils Breidenbach b, Markus Eck b, Doerte Laing b, Stefanie Kaesche c a
Institute of Technical Thermodynamics, German Aerospace Center (DLR), Linder Höhe, 51147 Köln, Germany Institute of Technical Thermodynamics, German Aerospace Center (DLR), Pfaffenwaldring 38-40, 70569 Stuttgart, Germany c Materials Testing Institute University of Stuttgart (MPA), Pfaffenwaldring 32, 70569 Stuttgart, Germany b
h i g h l i g h t s Thermophysical data of Solar Salt reviewed and consistent data identified. Overview of corrosion aspects for steels in molten alkali nitrate salts given. Kinetic differences for nitrite and oxide formation for decomposition identified. Thermal stability improvement for increased oxygen partial pressure determined.
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Article history: Received 11 February 2013 Received in revised form 19 April 2013 Accepted 24 April 2013 Available online 20 May 2013 Keywords: Molten salt Corrosion Nitrite formation Oxide formation Thermophysical properties
a b s t r a c t For sensible thermal energy storage (TES) in liquids in the temperature range from 250 °C to 550 °C, a mixture of 60 wt% sodium nitrate (NaNO3) and 40 wt% potassium nitrate (KNO3), known as Solar Salt, is commonly utilized. At the time of writing, TES technology for concentrating solar power is the major application. Although commercial systems have been demonstrated, there are still several material aspects to be investigated. In this paper we address thermophysical properties and metallic corrosion, as well as thermal decomposition processes. The paper reviews temperature dependent thermophysical properties of Solar Salt. Deviations among the authors of these properties were small for the density (±1.5%), medium for the heat capacity (±7%) and large for thermal diffusivity and thermal conductivity values (±15%). The paper gives an overview of the various aspects of steel corrosion in molten alkali nitrate salts. From literature data, four steel type categories mainly depending on the temperature range are defined. The paper presents thermal stability examinations of Solar Salt and NaNO3 by isothermal labscale tests and thermal analysis measurements. Salt analysis in the isothermal test showed a steadily increasing oxide level at a constant nitrite to nitrate ratio. The result shows that there are kinetic differences in the first decomposition process with nitrite formation and the second decomposition process with oxide formation. The impact of the partial oxygen pressure on the decomposition temperature was examined by thermogravimetric measurements. Measurements show an improved stability limit for higher partial oxygen pressures. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Solar thermal power plants are a key technology for electricity generation from renewable energy resources. Thermal energy storage (TES) is indispensable for solar thermal power plant applications. It makes it possible to meet the intermediate load profile with dispatchable power, a benefit that has a high value to power utilities and that gives concentrating solar power (CSP) technology an edge over photovoltaic and wind power. Most commonly three types of TES systems are distinguished. These are sensible heat, la⇑ Corresponding author. Tel.: +49 (0) 2203 601 4094. E-mail addresses: [email protected] (T. Bauer), nicole.pfl[email protected] (N. Pfleger), [email protected] (N. Breidenbach), [email protected] (M. Eck), [email protected] (D. Laing), [email protected] (S. Kaesche). 0306-2619/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apenergy.2013.04.072
tent heat and chemical heat. Sensible heat storage systems utilize an increase or decrease of the storage material temperature in solids (e.g., ceramics) or liquids (e.g., molten salt). Latent heat storage is connected with a phase transformation of the storage material. The materials are called phase change materials (PCMs) and they typically undergo a physical phase change from solid to liquid and vice versa. High-temperature PCM-storage developments aim for the so-called direct steam generation CSP plants using water/ steam as the two-phase heat transfer fluid in the absorber [1]. The third type, thermochemical heat storage is based on reversible thermochemical reactions. The energy is stored in the form of chemical compounds created by an endothermic reaction and it is recovered again by an exothermic backward reaction. For CSP, the dissociation of hydroxide salts to oxides and steam has been investigated [2].
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The two tank molten salt system is a cost-effective sensible storage technology that is to-date commercially available. Commercial large-scale examples are parabolic trough plants using thermal oil as a heat transfer fluid with an indirect storage concept (e.g., Andasol plants). Other examples are power tower plants using molten salt as a heat transfer fluid with a direct storage concept (e.g., Solar Two Project, Gemasolar plant) [3,4]. Current research aims for the reduction of the relatively expensive molten salt inventory in the storage system. Here, compatible low-cost filler materials and thermal stratification (or thermocline) phenomena in a single tank are examined [5–8]. Most commonly a molten salt mixture with 60 wt% sodium nitrate (NaNO3) and 40 wt% potassium nitrate (KNO3) is utilized [9]. This mixture is usually known as Solar Salt. In this paper we present experimental results of recent material investigations and critical literature reviews of NaNO3 and Solar Salt with respect to thermophysical properties, thermal stability and metallic corrosion.
2. Materials and methods The thermal stability and decomposition of nitrate salts was examined by two methods. These dynamic and static stability tests refer to open-system type measurements in oxygen–nitrogen atmosphere at ambient pressure. Static measurements at constant temperatures were performed. It is well known that molten nitrates decompose to nitrites. The thermal dissociation is reversible and the equilibrium reaction can be written for NaNO3 as shown in Eq. (1) [10–16].
1 NaNO3ðlÞ () NaNO2ðlÞ þ O2ðgÞ 2
ð1Þ
Kinetic data of the oxidation and the decomposition of different alkali metal nitrates have been reported. They include KNO3/KNO2, KNO3–NaNO3/KNO2–NaNO2 and NaNO3/NaNO2 [10–16]. The time to reach equilibrium depends on parameters such as the reaction direction (decomposition or oxidation), the atmosphere and the experimental setup. A typical experimental setup reported in the literature utilizes bubbling of gas through the melt. At higher temperatures the equilibrium is generally reached faster compared to lower temperatures. The reported periods range from days at low temperatures to tens of minutes at high temperatures. We examined the kinetics of the nitrite formation in the NaNO3 melt in the temperature range 450–550 °C in a forced convection oven. For all experiments the sample was technical NaNO3 with a weight of 110–170 g (BASF type U, min. 99.0%). Open crucibles made of glazed ceramics (450 °C) and dense aluminum oxide (500 °C and 550 °C) were used. The size of the glazed ceramic crucible was 150 ml. Experiments at higher temperatures resulted in high salt creeping rates with unacceptable mass losses. Also, at high temperatures the glazed surface may be attacked by molten nitrate salt. Hence, results with glazed ceramic crucible at high temperatures were excluded. On the other hand, aluminum oxide crucibles showed a suitable compatibility and lower creeping rates at higher temperatures. The dimensions of the cylindrical aluminum oxide crucibles were: outer diameter 50 mm, inner diameter 46 mm and height 75 mm, one front face open. The temperature of the salt was monitored by a mineral insulated thermocouple (Inconel 600, Class 1) within the melt. Synthetic air with a rate of 100 ml min1 and 600 ml min1 was purged through the melt via a stainless steel tube. The setup included a tube within the oven for air preheating (Inconel, length 400 mm, inner diameter 2 mm, outer diameter 3 mm). This gas supply increased the decomposition rate and ensured a uniform distribution of the melt. The nitrite concentration was monitored by taking samples of the melt periodically. For nitrite analysis, the solidified samples of the melt
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were dissolved in deionized water. The nitrite (NO 2 ) concentration was determined by a photospectrometric standard method (Analytik Jena AG, type Spekol 1100, 4500-NO 2 Colorimetric Method). We examined the kinetics of oxide formation in Solar Salt at 550 °C in synthetic air atmosphere in an open type system. A self-assembled stainless steel autoclave system with a vertical heated tube as test chamber was utilized (inner tube diameter 55 mm and inner tube height 170 mm). Solar Salt was prepared from pro analysis grade salt supplied by Merck (minimum purity 99.0%). The material of the crucible inside the chamber was dense aluminum oxide (outer diameter 50 mm, inner diameter 46 mm and height 75 mm). The temperature of the salt was monitored and controlled by a mineral insulated thermocouple (Inconel 600, Class 1) within the melt. Synthetic air with a rate of 120 ml min1 was purged through the melt via an Inconel tube. The oxide concentration was monitored by taking samples of the melt periodically. For oxide analysis, samples with a mass of 0.86 g were dissolved in about 20 ml deionized water. The examined oxides dissolved in water form an alkaline solution. An acid–base titration with 0.01 mol l1 HCl solution was performed (Mettler Toledo FiveEasy™ FE20). In addition to static measurements, dynamic measurements with a heating ramp were performed by thermogravimetry (TG) using a commercial system (Netzsch STA449). The major parameters were the salt composition, partial oxygen pressure and the heating rate. This approach led to a large number of TG measurements. For all measurements, the thermal decomposition temperature refers to the temperature with a mass loss of 3 wt% compared to the base line [17]. The TG-signal is less accurate for lower mass losses. For higher mass losses, the creeping of the melt due to the high wettability is a critical aspect. In order to minimize creeping, the maximum temperature of the heating ramp was individually adapted. Each measurement aimed for a mass loss of slightly more than 3 wt% based on the expect result. In some cases, high mass losses occurred and the sample holder was cleaned by water. The TG-baseline stability was checked on a regular basis to ensure that no salt contamination effects occurred. The atmospheres were pure nitrogen or an oxygennitrogen mixture. The total gas flow rate was kept constant for all experiments at 100 ml min1. The heating rates ranged from 0.5 K min1 to 10 K min1. KNO3 and NaNO3 were purchased from Merck (purity minimum 99.99%) and used without further purification. Solar Salt was prepared by mixing, melting (air atmosphere at 350 °C for 30 min), quenching and grinding. The sample mass of about 20 mg was checked before and after the TG measurement with a microbalance to confirm mass losses. Directly prior to the measurement the salt was dried at 150 °C for 2 h in synthetic air within the TG-system. Platinum–rhodium crucibles without a lid were used. Reference measurements in aluminum oxide crucibles confirmed the decomposition temperatures. To describe the dependency of the decomposition temperature on the relative partial oxygen pressure an empirical model was used. This model was fitted to experimental values using a non-linear regression algorithm. Measurement results were extrapolated to equilibrium conditions using the fitted curves of the 0.5 K min1 and 2 K min1 results and a linear equation.
3. Results and discussion 3.1. Thermophysical properties of Solar Salt Previous work at DLR focused on the thermophysical properties of NaNO3 [18]. In the present paper, literature values of the density, heat capacity, thermal diffusivity and thermal conductivity of Solar Salt are reviewed. Previous authors focused mainly on single thermophysical property values. In the presented work, several
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literature values were verified by the correlation k = a q cp, where k is the thermal conductivity, a is the thermal diffusivity, q is the density and cp is the heat capacity. Fig. 1 plots density values from different authors [19–25]. These measurements refer to mixtures with a composition that is close to Solar Salt. Hence, data were interpolated to obtain the Solar Salt density. Bradshaw showed that the density of salt mixtures composed of LiNO3, NaNO3, KNO3, Ca(NO3)2 can be calculated by molar volume addition of the single salts [19]. For the NaNO3 density, Janz reported newer values which are about 1% lower compared to the older values [18]. This lower values can explain the lower values obtained by molar volume addition (Fig. 1). In general, the literature values of the density show the lowest scattering compared to other thermophysical properties. The deviation is less than ±1.5% compared to recommended data by Janz in 1972 [20]. Ichikawa determined a value of 140 J mol1 K1 for the heat capacity of the single salts KNO3, LiNO3 and NaNO3 [26]. Rogers measured a value of 142 J mol1 K1 for the heat capacity of the complete system KNO3–NaNO3 [27]. With the assumption of a constant molar heat capacity, it is possible to calculate the heat capacity of Solar Salt with the weight fraction m and the heat capacity cp of the two salts KNO3 and NaNO3 (cp,ges = cp,1 m1 + cp,2 m2). Direct measurements of the heat capacity of Solar Salt are limited. Hence, we calculated Solar Salt values from single salt values and the mass fractions. Fig. 1 shows these calculated results from authors which reported NaNO3 and KNO3 values [28–31]. Bradshaw and Zavoico reported also heat capacity values of Solar Salt [12,25]. We calculated a temperature dependent average heat capacity using these data. The average heat capacity increases only slightly with temperature. Literature values lie within ±5% compared to the calculated average heat capacity. Older values from Gustafsson are an exception.
Fig. 1. Temperature dependent thermophysical data of Solar Salt [19–43].
Fig. 2 shows the thermal conductivity of the KNO3–NaNO3 system dependent on the composition. In general, work on this dependency is limited. McDonald and Bloom claim that the conductivities of the mixtures drop compared to the two single salts [32,33]. However, most literature values show an almost linear dependency on the composition (Fig. 2) [33–35]. Hence, in order to calculate the thermal conductivity and thermal diffusivity of Solar Salt, the presented work assumes a linear dependency on the molar composition and measurement values from literature of the single salts. In a second step, the average thermal conductivity of Solar Salt was obtained. This average thermal conductivity includes values from nine previously calculated literature sources (Fig. 1)[32–40]. The resulting average thermal conductivity increases with temperature and the uncertainty is rather large. The maximum deviation from the average thermal conductivity line is ±15%. The calculated average thermal conductivity agrees closely with data reported by Zavoico [25]. Finally, the average temperature dependent thermal diffusivity was calculated by the equation a = k/(q cp). Fig. 1 shows this averaged thermal diffusivity as a dashed line. Literature values of the thermal diffusivity of Solar Salt could not be identified. Fig. 1 plots calculated values using experimental single salt results from literature and a linear interpolation with the Solar Salt molar fraction [30,41–43]. Fig. 1 shows that thermal diffusivity increases more strongly with the temperature compared to the thermal conductivity. The maximum deviation from the average value is also ±15% as for the thermal conductivity. 3.2. Literature review of metallic corrosion The molten alkali nitrate salts constitute in combination with the metallic parts of solar power plants a corrosion system with the molten salt acting as an electrolyte comparable to an aqueous electrolyte. But whereas the corrosion mechanisms of metals in numerous aqueous electrolytes are well established and well understood there still exists a lack in knowledge concerning the corrosion mechanisms of metals in molten salts. For understanding the ongoing corrosion mechanism it is important to know the nature of the reactions leading for example to oxide layer formation and/or unwanted metal dissolution which causes obvious damaging. Another point of interest when talking about corrosion problems of steels in nitrate melts is the need for further information about the question if there is evidence for the occurrence of stress corrosion cracking (SCC) in molten salts or other environmentally induced cracking mechanisms. In the following a short survey about the most interesting corrosion aspects for steels in molten alkali nitrate salts will be given. A schematic drawing of the corrosion system with its three constituents together with additional possible influencing parameters for each of the constituents is shown below. Fig. 3 gives a good impression on the big variety of possible factors which all have to be kept in mind when selecting a proper
Fig. 2. Literature values of the thermal conductivity of the system KNO3–NaNO3 at 340 °C except data at 315 °C from Omotani [32–35].
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material-environment-stress-combination for structural parts in solar power plants. From the figure it can be seen that not only the interaction of the material with the environment leading to corrosion mechanisms as uniform corrosion or pitting (summarized as ‘‘corrosion’’ in Fig. 3) plays an important role but also the additional interaction with existing stresses causing SCC in a prone material. The types of steel used in combination with molten nitrate salts can be divided in four categories mainly depending on the temperature range of application. In literature a survey for material selection is given [44,45]: Low alloyed carbon steel (6400 °C). Cr–Mo steel (6500 °C) (Cr-content up to about 9 wt%). Stainless Cr–Ni steel (6570 °C) (with and without alloying elements as Mo, Nb, Ti). Ni-alloys (6650 °C) (i.e. Alloy 800). In literature the main discussed issues in concern with the medium are chloride impurities and nitrate decomposition. Especially, chloride impurities are of major importance since they can degrade adhesion of the oxide scale to metal surface and cause spalling of the oxide layer making corrosion attack of the base material possible [45–47]. A further well studied and often discussed point is the chemical stability of the nitrate melts which has a strong influence on corrosivity of the medium. One significant reaction in the interesting temperature range up to 600 °C is the nitrate/nitrite equilibrium (Eq. (1)). It is by now not clear which exact influence the nitrate decomposition has on the corrosion mechanism. Corrosion is also enhanced by trace moisture in the melt [47]. Chromium is soluble in molten nitrates and can preferentially be solved from the surface scale leading to a degradation of the protectiveness of the oxide layer. Nickel and iron do not form soluble species [10,48]. As mentioned above stresses are of major importance in terms of the possible occurrence of stress corrosion cracking of steels in contact with molten nitrate melts. In general, it is said that three prerequisites are necessary for the occurrence of SCC:
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Stresses can arise from mechanical or thermal loading or exist as residual stresses within the material due to processing (rolling, bending, surface finishing, welding). Up to know there is not much information found in literature on this topic in concern to steels used in contact with nitrate melts. In [44] the occurrence of SSC is described. 3.3. Thermal stability of Solar Salt and NaNO3 Mass losses with gas evolution of alkali metal nitrate salts may occur due to three mechanisms: nitrite formation in the melt and oxygen release (1), alkali metal oxide formation in the melt and nitrogen/nitrogen oxide release (2) and vaporization of the nitrate salts (3) [9,49]. Static and dynamic open-system type experiments with nitrogen–oxygen atmospheres were performed. Fig. 4 plots static NaNO3 measurements at the three temperatures 450 °C, 500 °C and 550 °C in synthetic air. The figure shows the nitrite formation depending on the time with the salt temperature as a parameter. Additional experiments with increased air flow from 100 ml min1 to 600 ml min1 at 500 °C are also shown. For the increased gas flow in the first experiment at the beginning only a small increase of the decomposition rate was observed. Both experiments at 500 °C (100 ml min1 and 600 ml min1) indicate a very similar molar NO 2 =NO3 ratio in equilibrium. A simple empirical exponential growth model was fitted to the measurement data with 100 ml min1 using non-linear regression techniques. The model uses two parameters and can approximately describe the reaction kinetics. The three model curves show that the temperature level not only affects the amount of NO 2 in equilibrium but also the decom-
Susceptible material, Critical environment, Stress. All of these prerequisites can be fulfilled in the here considered corrosion system at least for some of the described materials. Fig. 4. Experimental kinetic results of nitrite formation in NaNO3 in contact with synthetic air.
Fig. 3. Schematic drawing of the corrosion system.
Fig. 5. Experimental kinetic results of oxide formation in Solar Salt in contact with synthetic air at 550 °C.
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bility of NaNO3 and Solar Salt improves with increased partial oxygen pressures. Results were extrapolated to equilibrium conditions (heating rate of 0 K min1). For NaNO3 and equilibrium conditions, results show a decomposition temperature of 525 °C in synthetic air and 542 °C in oxygen at atmospheric pressures. For Solar Salt and equilibrium conditions, the values are 529 °C (synthetic air) and 562 °C (oxygen). As a consequence, measurements show that the thermal stability of Solar Salt is higher compared to NaNO3. It can be also seen that the stability of Solar Salt (Fig. 7) depends more strongly on the partial oxygen pressure than NaNO3 (Fig. 6). For example, for NaNO3 the stability difference between synthetic air and oxygen is 17 K (542 °C minus 525 °C). For Solar Salt, about double the value is obtained (33 K).
Fig. 6. Experimental thermogravimetry results of the thermal stability of NaNO3 depending on the partial oxygen pressure and the heating rate (0.5–10 K min1).
Fig. 7. Experimental thermogravimetry results of the thermal stability of 40 wt%KNO3–60 wt%NaNO3 depending on the partial oxygen pressure and the heating rate (0.5–5 K min1). position rate of NO 3 to NO2 . The equilibrium at 550 °C was quickly reached after several tens of hours. On the other hand, at 450 °C the time constant was much longer with several hundreds of hours. As discussed in the previous paragraph, the primary decomposition reaction with nitrite formation in the melt and oxygen release is well examined. The secondary decomposition reaction with alkali metal oxide formation and nitrogen/nitrogen oxide is less understood. The chemistry of these oxygen species is complex and some of the evidence is in conflict. In literature the formation of oxide, superoxide and peroxide is discussed [16,49]. Fig. 5 refers to measurements of Solar Salt at 550 °C in an open type system with synthetic air atmosphere. The figure plots not only the NO 2/ NO 3 ratio (left hand axis) but also the equivalence point of the titration (right hand axis). It can be seen that the NO 2 /NO3 ratio reaches equilibrium after a few 10 h as opposed to the volume of titration which increases steadily. It can be concluded that although the NO 2 /NO3 ratio reaches equilibrium, the oxide level reaches no equilibrium in this experiment. Work at Sandia aims for oxygen-stabilized Solar Salt up to 650 °C [50]. In the present experimental work the impact of the partial pressure of oxygen on the thermal stability was examined. Figs. 6 and 7 plot results of dynamic TG-measurements of NaNO3 and Solar Salt depending on the partial oxygen pressure. It can be seen that the heating rate has a strong impact on the observed decomposition temperature. Higher heating rates result in measurement results with higher decomposition temperatures. Also, all measurement series show that higher partial oxygen pressures lead to higher decomposition temperatures. In other word, the sta-
4. Conclusions Reliable thermophysical properties are important for the modeling and dimensioning of molten salt storage systems. For example, incorrect heat capacity values may lead to oversized two-tank molten salt systems. Hence, consistent thermophysical data presented in this paper can improve modeling results and increase the accuracy in sizing of thermal energy storage systems. Temperature dependent thermophysical data of the density q, heat capacity cp, thermal diffusivity a, and thermal conductivity k from various authors were compared. Representative thermophysical properties of Solar Salt and correlations between properties of the single salt and Solar Salt were presented. Results show that these properties differ in terms of the uncertainty. Density values show the lowest scattering. Heat capacity data vary up to ±7% compared to the average value. Thermal diffusivity and thermal conductivity values showed the strongest variations with maximum deviations of ±15% compared to average values. To handle molten salts in TES systems it is mandatory to understand the corrosion behavior of steels used for tanks and components. For reliable long-term application structural parts of modern solar power plants need to have good corrosion resistance up to the thermal stability limit of molten nitrate salts. Corrosion resistance of the material strongly depends on restricting the environmental conditions and the stresses to limits where the chosen material shows suitable corrosion resistance. This can only be achieved in a reproducible manner when the corrosion mechanism is well understood. In already existing power plants corrosion in different structural parts occurred and shows the need for further fundamental corrosion studies. In addition new lower melting salt mixtures might bring about new corrosion aspects. The thermal stability of NaNO3 and Solar Salt has been experimentally studied. For the dynamic TG-measurements a thermal stability limit of about 530 °C in synthetic air at atmospheric pressure in an open system has been obtained. This value is lower compared to the previously reported value of 565 °C [3]. Measurements refer to a mass loss of 3 wt%. It should be considered that different mass loss definitions will result in other stability limits. Static measurements of Solar Salt at 550 °C in synthetic air in an open type system show that the primary decomposition reaction with nitrite formation reaches equilibrium within the experimental time frame. On the other hand, this measurement indicates that the secondary decomposition reaction with oxide formation and nitrogen release reaches no equilibrium within the experimental time frame. Hence, further work focuses on the long-term thermal stability limit of Solar Salt to examine the different decomposition mechanisms. Also, we have measured the influence of the partial pressure of oxygen on the thermal stability. Our results confirm a stabilizing effect by increased oxygen partial pressure in open-system type experiments with nitrogen-oxygen atmospheres. These results are particularly important for power tower plants. These
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plants aim for temperature levels in the thermal energy storage system near the thermal stability limit of Solar Salt to improve the economics and the plant efficiency. For large-scale process technology aspects, a molten salt storage test facility is in the planning stage at DLR. The start-up is scheduled for 2014. The facility will be used for the investigation of innovative storage concepts as well as components and corrosion testing. Another focus will be the examination of the salt chemistry in different atmospheres on a system level. Acknowledgements We express our thanks especially to Ulrike Kröner and Markus Braun for the experimental part and data analysis in this paper. References [1] Laing D, Bauer T, Breidenbach N, Hachmann B, Johnson M. Development of high temperature phase-change-material storages. Appl Energy 2013;109: 497–504. [2] Schaube F, Koch L, Wörner A, Müller-Steinhagen H. A thermodynamic and kinetic study of the de- and rehydration of Ca(OH)2 at high H2O partial pressures for thermo-chemical heat storage. Thermochim Acta 2012;538:9–20. http://dx.doi.org/10.1016/j.tca.2012.03.003. [3] Pacheco JE. Final test and evalution results from the solar two project. Albuquerque (NM): Sandia National Laboratories. Report No.: SAND20020120; 2002. [4] Dunn RI, Hearps PJ, Wright MN. Molten-salt power towers: newly commercial concentrating solar storage. Proc IEEE 2012;100:504–15. http://dx.doi.org/ 10.1109/jproc.2011.2163739. [5] Yang Z, Garimella SV. Molten-salt thermal energy storage in thermoclines under different environmental boundary conditions. Appl Energy 2010;87:3322–9. http://dx.doi.org/10.1016/j.apenergy.2010.04.024. [6] Flueckiger S, Yang Z, Garimella SV. An integrated thermal and mechanical investigation of molten-salt thermocline energy storage. Appl Energy 2011;88:2098–105. http://dx.doi.org/10.1016/j.apenergy.2010.12.031. [7] 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. http://dx.doi.org/10.1016/ j.apenergy.2011.12.057. [8] Calvet N, Gomez JC, Faik A, Roddatis VV, Meffre A, Glatzmaier GC, et al. Compatibility of a post-industrial ceramic with nitrate molten salts for use as filler material in a thermocline storage system. Appl Energy 2013;109:387–93. [9] Carling RW, Kramer CM, Bradshaw RW, Nissen DA, Goods SH, Mar RW, et al. Molten nitrate salt technology development – development status report. Livermore (CA): Sandia National Laboratories; 1981. Report No.: SAND80-8052. [10] Nissen DA, Meeker DE. Nitrate/nitrite chemistry in sodium nitrate–potassium nitrate melts. Inorg Chem 1983;22:716–21. http://dx.doi.org/10.1021/ ic00147a004. [11] Bartholomew RF. A study of the equilibrium KNO3/KNO2(l) + ½ O2(g) over the temperature range 550–750 °C. J Phys Chem 1966;70:3442–6. http:// dx.doi.org/10.1021/j100883a012. [12] Bradshaw RW, Carling RW. A review of chemical and physical properties of molten alkali nitrate salts and their effect on materials used for solar central receivers. Albuquerque (NM): Sandia National Laboratories; 1987. Report No.: SAND87-8005. [13] Kust RN, Burke JD. Thermal decomposition in alkali metal nitrate melts. Inorg Nucl Chem Lett 1970;6:333–5. [14] Paniccia F, Zambonin PG. Redox mechanisms in an ionic matrix. III. Kinetics of the reaction nitrite ion + molecular oxygen = nitrate ion in molten alkali nitrates. J Phys Chem 1973;77:1810–3. http://dx.doi.org/10.1021/ j100633a018. [15] Sirotkin GD. Equilibrium in melts of the nitrates and nitrites of sodium and potassium. Russ J Inorg Chem 1959;4:1180–2. [16] Plambeck JA. Sodium nitrate – potassium nitrate. In: Bard AJ, editor. Encyclopedia of electrochemistry of the elements, vol. X. New York: Marcel Dekker; 1976. p. 189–232. [17] Raade JW, Padowitz D. Development of molten salt heat transfer fluid with low melting point and high thermal stability. ASME J Sol Energy Eng 2011;133:031013. http://dx.doi.org/10.1115/1.4004243. [18] Bauer T, Laing D, Tamme R. Characterization of sodium nitrate as phase change material. Int J Thermophys 2011;33:91–104. http://dx.doi.org/10.1007/ s10765-011-1113-9. [19] Bradshaw RW. Effect of composition on the density of multi-component molten nitrate salts. Albuquerque (NM): Sandia National Laboratories; 2009. Dez. Report No.: SAND2009-8221. [20] Janz GJ, Krebs U, Siegenthaler HF, Tomkins RPT. Molten salts: volume 3, nitrates, nitrites, and mixtures, electrical conductance, density, viscosity, and surface tension data. J Phys Chem Ref Data 1972;1:581–746.
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[38] Santini R, Tadrist L, Pantaloni J, Cerisier P. Measurement of thermal conductivity of molten salts in the range 100–500 °C. Int J Heat Mass Transfer 1984;27:623–6. http://dx.doi.org/10.1016/0017-9310(84)90034-6. [39] Kitade S, Kobayashi Y, Nagasaka Y, Nagashima A. Measurement of the thermal conductivity of molten KNO3 and NaNO3 by the transient hot-wire method with ceramic-coated probes. High Temp-High Press 1989;21:219–24. [40] Turnbull AG. The thermal conductivity of molten salts, II theory and results for pure salts. Aust J Appl Sci 1961;12:324. [41] Ohta H, Ogura G, Waseda Y, Suzuki M. Thermal diffusivity measurements of molten salts using a three-layered cell by the laser flash method. Rev Sci Instrum 1990;61:2645–9. [42] Kato Y, Furukawa K, Araki N, Kobayashi K. Thermal diffusivity measurement of molten salts by use of a simple ceramic cell. High Temp-High Press 1983;15:191–8. [43] Knothe W. Entwicklung einer Apparatur zur Messung der Wärmeleitfähigkeit von Schmelzen [dissertation] Aachen (Germany): RWTH-Aachen; 1985 [in German]. [44] Moore R, Vernon M, Ho CK, Siegel NP, Kolb JK. Design considerations for concentrating solar power tower systems employing molten salt. Albuquerque (NM): Sandia National Laboratories; 2010. Report No.: SAND2010-6978. [45] Bradshaw RW, Goods SH. Corrosion of alloys and metals by molten nitrates. In: Sequeira CAC, editor. High temperature corrosion in molten salts. Trans Tech Publ. Ltd.; 2003. [46] Goods SH, Bradshaw RW. Corrosion of stainless steels and carbon steel by molten mixtures of commercial nitrate salts. J Mater Eng Perform 2004;13:78–87. [47] Kearny D, Kelly B, Herrmann U, Cable R, Pacheco J, Mahoney R, et al. Engineering aspects of a molten salt heat transfer fluid in a through solar field. Energy 2004;29:861–70. [48] Carling RW, Bradshaw RW. Review of the characterization of molten nitrate salt for solar central receiver applications. Proc 21st Intersoc Energy Conv Eng Conf, vol. 2. San Diego, US: American Chemical Society; 1986. p. 834–8. [49] Stern KH. High temperature properties and thermal decomposition of inorganic salts with oxyanions. Boca Raton: CRC-Press; 2001. [50] Bradshaw R, Cordaro J, Jayaraman S, Sun A, Siegel N, Gill D et al. Advanced heat transfer fluid development. Sandia National Laboratories. Presentation DOE CSP Program Annual Review. Contract No.: DE-AC04-94AL85000; 2010.
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Material aspects of Solar Salt for sensible heat storage Thomas Bauer a,⇑, Nicole Pfleger b, Nils Breidenbach b, Markus Eck b, Doerte Laing b, Stefanie Kaesche c a
Institute of Technical Thermodynamics, German Aerospace Center (DLR), Linder Höhe, 51147 Köln, Germany Institute of Technical Thermodynamics, German Aerospace Center (DLR), Pfaffenwaldring 38-40, 70569 Stuttgart, Germany c Materials Testing Institute University of Stuttgart (MPA), Pfaffenwaldring 32, 70569 Stuttgart, Germany b
h i g h l i g h t s Thermophysical data of Solar Salt reviewed and consistent data identified. Overview of corrosion aspects for steels in molten alkali nitrate salts given. Kinetic differences for nitrite and oxide formation for decomposition identified. Thermal stability improvement for increased oxygen partial pressure determined.
a r t i c l e
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Article history: Received 11 February 2013 Received in revised form 19 April 2013 Accepted 24 April 2013 Available online 20 May 2013 Keywords: Molten salt Corrosion Nitrite formation Oxide formation Thermophysical properties
a b s t r a c t For sensible thermal energy storage (TES) in liquids in the temperature range from 250 °C to 550 °C, a mixture of 60 wt% sodium nitrate (NaNO3) and 40 wt% potassium nitrate (KNO3), known as Solar Salt, is commonly utilized. At the time of writing, TES technology for concentrating solar power is the major application. Although commercial systems have been demonstrated, there are still several material aspects to be investigated. In this paper we address thermophysical properties and metallic corrosion, as well as thermal decomposition processes. The paper reviews temperature dependent thermophysical properties of Solar Salt. Deviations among the authors of these properties were small for the density (±1.5%), medium for the heat capacity (±7%) and large for thermal diffusivity and thermal conductivity values (±15%). The paper gives an overview of the various aspects of steel corrosion in molten alkali nitrate salts. From literature data, four steel type categories mainly depending on the temperature range are defined. The paper presents thermal stability examinations of Solar Salt and NaNO3 by isothermal labscale tests and thermal analysis measurements. Salt analysis in the isothermal test showed a steadily increasing oxide level at a constant nitrite to nitrate ratio. The result shows that there are kinetic differences in the first decomposition process with nitrite formation and the second decomposition process with oxide formation. The impact of the partial oxygen pressure on the decomposition temperature was examined by thermogravimetric measurements. Measurements show an improved stability limit for higher partial oxygen pressures. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Solar thermal power plants are a key technology for electricity generation from renewable energy resources. Thermal energy storage (TES) is indispensable for solar thermal power plant applications. It makes it possible to meet the intermediate load profile with dispatchable power, a benefit that has a high value to power utilities and that gives concentrating solar power (CSP) technology an edge over photovoltaic and wind power. Most commonly three types of TES systems are distinguished. These are sensible heat, la⇑ Corresponding author. Tel.: +49 (0) 2203 601 4094. E-mail addresses: [email protected] (T. Bauer), nicole.pfl[email protected] (N. Pfleger), [email protected] (N. Breidenbach), [email protected] (M. Eck), [email protected] (D. Laing), [email protected] (S. Kaesche). 0306-2619/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apenergy.2013.04.072
tent heat and chemical heat. Sensible heat storage systems utilize an increase or decrease of the storage material temperature in solids (e.g., ceramics) or liquids (e.g., molten salt). Latent heat storage is connected with a phase transformation of the storage material. The materials are called phase change materials (PCMs) and they typically undergo a physical phase change from solid to liquid and vice versa. High-temperature PCM-storage developments aim for the so-called direct steam generation CSP plants using water/ steam as the two-phase heat transfer fluid in the absorber [1]. The third type, thermochemical heat storage is based on reversible thermochemical reactions. The energy is stored in the form of chemical compounds created by an endothermic reaction and it is recovered again by an exothermic backward reaction. For CSP, the dissociation of hydroxide salts to oxides and steam has been investigated [2].
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The two tank molten salt system is a cost-effective sensible storage technology that is to-date commercially available. Commercial large-scale examples are parabolic trough plants using thermal oil as a heat transfer fluid with an indirect storage concept (e.g., Andasol plants). Other examples are power tower plants using molten salt as a heat transfer fluid with a direct storage concept (e.g., Solar Two Project, Gemasolar plant) [3,4]. Current research aims for the reduction of the relatively expensive molten salt inventory in the storage system. Here, compatible low-cost filler materials and thermal stratification (or thermocline) phenomena in a single tank are examined [5–8]. Most commonly a molten salt mixture with 60 wt% sodium nitrate (NaNO3) and 40 wt% potassium nitrate (KNO3) is utilized [9]. This mixture is usually known as Solar Salt. In this paper we present experimental results of recent material investigations and critical literature reviews of NaNO3 and Solar Salt with respect to thermophysical properties, thermal stability and metallic corrosion.
2. Materials and methods The thermal stability and decomposition of nitrate salts was examined by two methods. These dynamic and static stability tests refer to open-system type measurements in oxygen–nitrogen atmosphere at ambient pressure. Static measurements at constant temperatures were performed. It is well known that molten nitrates decompose to nitrites. The thermal dissociation is reversible and the equilibrium reaction can be written for NaNO3 as shown in Eq. (1) [10–16].
1 NaNO3ðlÞ () NaNO2ðlÞ þ O2ðgÞ 2
ð1Þ
Kinetic data of the oxidation and the decomposition of different alkali metal nitrates have been reported. They include KNO3/KNO2, KNO3–NaNO3/KNO2–NaNO2 and NaNO3/NaNO2 [10–16]. The time to reach equilibrium depends on parameters such as the reaction direction (decomposition or oxidation), the atmosphere and the experimental setup. A typical experimental setup reported in the literature utilizes bubbling of gas through the melt. At higher temperatures the equilibrium is generally reached faster compared to lower temperatures. The reported periods range from days at low temperatures to tens of minutes at high temperatures. We examined the kinetics of the nitrite formation in the NaNO3 melt in the temperature range 450–550 °C in a forced convection oven. For all experiments the sample was technical NaNO3 with a weight of 110–170 g (BASF type U, min. 99.0%). Open crucibles made of glazed ceramics (450 °C) and dense aluminum oxide (500 °C and 550 °C) were used. The size of the glazed ceramic crucible was 150 ml. Experiments at higher temperatures resulted in high salt creeping rates with unacceptable mass losses. Also, at high temperatures the glazed surface may be attacked by molten nitrate salt. Hence, results with glazed ceramic crucible at high temperatures were excluded. On the other hand, aluminum oxide crucibles showed a suitable compatibility and lower creeping rates at higher temperatures. The dimensions of the cylindrical aluminum oxide crucibles were: outer diameter 50 mm, inner diameter 46 mm and height 75 mm, one front face open. The temperature of the salt was monitored by a mineral insulated thermocouple (Inconel 600, Class 1) within the melt. Synthetic air with a rate of 100 ml min1 and 600 ml min1 was purged through the melt via a stainless steel tube. The setup included a tube within the oven for air preheating (Inconel, length 400 mm, inner diameter 2 mm, outer diameter 3 mm). This gas supply increased the decomposition rate and ensured a uniform distribution of the melt. The nitrite concentration was monitored by taking samples of the melt periodically. For nitrite analysis, the solidified samples of the melt
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were dissolved in deionized water. The nitrite (NO 2 ) concentration was determined by a photospectrometric standard method (Analytik Jena AG, type Spekol 1100, 4500-NO 2 Colorimetric Method). We examined the kinetics of oxide formation in Solar Salt at 550 °C in synthetic air atmosphere in an open type system. A self-assembled stainless steel autoclave system with a vertical heated tube as test chamber was utilized (inner tube diameter 55 mm and inner tube height 170 mm). Solar Salt was prepared from pro analysis grade salt supplied by Merck (minimum purity 99.0%). The material of the crucible inside the chamber was dense aluminum oxide (outer diameter 50 mm, inner diameter 46 mm and height 75 mm). The temperature of the salt was monitored and controlled by a mineral insulated thermocouple (Inconel 600, Class 1) within the melt. Synthetic air with a rate of 120 ml min1 was purged through the melt via an Inconel tube. The oxide concentration was monitored by taking samples of the melt periodically. For oxide analysis, samples with a mass of 0.86 g were dissolved in about 20 ml deionized water. The examined oxides dissolved in water form an alkaline solution. An acid–base titration with 0.01 mol l1 HCl solution was performed (Mettler Toledo FiveEasy™ FE20). In addition to static measurements, dynamic measurements with a heating ramp were performed by thermogravimetry (TG) using a commercial system (Netzsch STA449). The major parameters were the salt composition, partial oxygen pressure and the heating rate. This approach led to a large number of TG measurements. For all measurements, the thermal decomposition temperature refers to the temperature with a mass loss of 3 wt% compared to the base line [17]. The TG-signal is less accurate for lower mass losses. For higher mass losses, the creeping of the melt due to the high wettability is a critical aspect. In order to minimize creeping, the maximum temperature of the heating ramp was individually adapted. Each measurement aimed for a mass loss of slightly more than 3 wt% based on the expect result. In some cases, high mass losses occurred and the sample holder was cleaned by water. The TG-baseline stability was checked on a regular basis to ensure that no salt contamination effects occurred. The atmospheres were pure nitrogen or an oxygennitrogen mixture. The total gas flow rate was kept constant for all experiments at 100 ml min1. The heating rates ranged from 0.5 K min1 to 10 K min1. KNO3 and NaNO3 were purchased from Merck (purity minimum 99.99%) and used without further purification. Solar Salt was prepared by mixing, melting (air atmosphere at 350 °C for 30 min), quenching and grinding. The sample mass of about 20 mg was checked before and after the TG measurement with a microbalance to confirm mass losses. Directly prior to the measurement the salt was dried at 150 °C for 2 h in synthetic air within the TG-system. Platinum–rhodium crucibles without a lid were used. Reference measurements in aluminum oxide crucibles confirmed the decomposition temperatures. To describe the dependency of the decomposition temperature on the relative partial oxygen pressure an empirical model was used. This model was fitted to experimental values using a non-linear regression algorithm. Measurement results were extrapolated to equilibrium conditions using the fitted curves of the 0.5 K min1 and 2 K min1 results and a linear equation.
3. Results and discussion 3.1. Thermophysical properties of Solar Salt Previous work at DLR focused on the thermophysical properties of NaNO3 [18]. In the present paper, literature values of the density, heat capacity, thermal diffusivity and thermal conductivity of Solar Salt are reviewed. Previous authors focused mainly on single thermophysical property values. In the presented work, several
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literature values were verified by the correlation k = a q cp, where k is the thermal conductivity, a is the thermal diffusivity, q is the density and cp is the heat capacity. Fig. 1 plots density values from different authors [19–25]. These measurements refer to mixtures with a composition that is close to Solar Salt. Hence, data were interpolated to obtain the Solar Salt density. Bradshaw showed that the density of salt mixtures composed of LiNO3, NaNO3, KNO3, Ca(NO3)2 can be calculated by molar volume addition of the single salts [19]. For the NaNO3 density, Janz reported newer values which are about 1% lower compared to the older values [18]. This lower values can explain the lower values obtained by molar volume addition (Fig. 1). In general, the literature values of the density show the lowest scattering compared to other thermophysical properties. The deviation is less than ±1.5% compared to recommended data by Janz in 1972 [20]. Ichikawa determined a value of 140 J mol1 K1 for the heat capacity of the single salts KNO3, LiNO3 and NaNO3 [26]. Rogers measured a value of 142 J mol1 K1 for the heat capacity of the complete system KNO3–NaNO3 [27]. With the assumption of a constant molar heat capacity, it is possible to calculate the heat capacity of Solar Salt with the weight fraction m and the heat capacity cp of the two salts KNO3 and NaNO3 (cp,ges = cp,1 m1 + cp,2 m2). Direct measurements of the heat capacity of Solar Salt are limited. Hence, we calculated Solar Salt values from single salt values and the mass fractions. Fig. 1 shows these calculated results from authors which reported NaNO3 and KNO3 values [28–31]. Bradshaw and Zavoico reported also heat capacity values of Solar Salt [12,25]. We calculated a temperature dependent average heat capacity using these data. The average heat capacity increases only slightly with temperature. Literature values lie within ±5% compared to the calculated average heat capacity. Older values from Gustafsson are an exception.
Fig. 1. Temperature dependent thermophysical data of Solar Salt [19–43].
Fig. 2 shows the thermal conductivity of the KNO3–NaNO3 system dependent on the composition. In general, work on this dependency is limited. McDonald and Bloom claim that the conductivities of the mixtures drop compared to the two single salts [32,33]. However, most literature values show an almost linear dependency on the composition (Fig. 2) [33–35]. Hence, in order to calculate the thermal conductivity and thermal diffusivity of Solar Salt, the presented work assumes a linear dependency on the molar composition and measurement values from literature of the single salts. In a second step, the average thermal conductivity of Solar Salt was obtained. This average thermal conductivity includes values from nine previously calculated literature sources (Fig. 1)[32–40]. The resulting average thermal conductivity increases with temperature and the uncertainty is rather large. The maximum deviation from the average thermal conductivity line is ±15%. The calculated average thermal conductivity agrees closely with data reported by Zavoico [25]. Finally, the average temperature dependent thermal diffusivity was calculated by the equation a = k/(q cp). Fig. 1 shows this averaged thermal diffusivity as a dashed line. Literature values of the thermal diffusivity of Solar Salt could not be identified. Fig. 1 plots calculated values using experimental single salt results from literature and a linear interpolation with the Solar Salt molar fraction [30,41–43]. Fig. 1 shows that thermal diffusivity increases more strongly with the temperature compared to the thermal conductivity. The maximum deviation from the average value is also ±15% as for the thermal conductivity. 3.2. Literature review of metallic corrosion The molten alkali nitrate salts constitute in combination with the metallic parts of solar power plants a corrosion system with the molten salt acting as an electrolyte comparable to an aqueous electrolyte. But whereas the corrosion mechanisms of metals in numerous aqueous electrolytes are well established and well understood there still exists a lack in knowledge concerning the corrosion mechanisms of metals in molten salts. For understanding the ongoing corrosion mechanism it is important to know the nature of the reactions leading for example to oxide layer formation and/or unwanted metal dissolution which causes obvious damaging. Another point of interest when talking about corrosion problems of steels in nitrate melts is the need for further information about the question if there is evidence for the occurrence of stress corrosion cracking (SCC) in molten salts or other environmentally induced cracking mechanisms. In the following a short survey about the most interesting corrosion aspects for steels in molten alkali nitrate salts will be given. A schematic drawing of the corrosion system with its three constituents together with additional possible influencing parameters for each of the constituents is shown below. Fig. 3 gives a good impression on the big variety of possible factors which all have to be kept in mind when selecting a proper
Fig. 2. Literature values of the thermal conductivity of the system KNO3–NaNO3 at 340 °C except data at 315 °C from Omotani [32–35].
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material-environment-stress-combination for structural parts in solar power plants. From the figure it can be seen that not only the interaction of the material with the environment leading to corrosion mechanisms as uniform corrosion or pitting (summarized as ‘‘corrosion’’ in Fig. 3) plays an important role but also the additional interaction with existing stresses causing SCC in a prone material. The types of steel used in combination with molten nitrate salts can be divided in four categories mainly depending on the temperature range of application. In literature a survey for material selection is given [44,45]: Low alloyed carbon steel (6400 °C). Cr–Mo steel (6500 °C) (Cr-content up to about 9 wt%). Stainless Cr–Ni steel (6570 °C) (with and without alloying elements as Mo, Nb, Ti). Ni-alloys (6650 °C) (i.e. Alloy 800). In literature the main discussed issues in concern with the medium are chloride impurities and nitrate decomposition. Especially, chloride impurities are of major importance since they can degrade adhesion of the oxide scale to metal surface and cause spalling of the oxide layer making corrosion attack of the base material possible [45–47]. A further well studied and often discussed point is the chemical stability of the nitrate melts which has a strong influence on corrosivity of the medium. One significant reaction in the interesting temperature range up to 600 °C is the nitrate/nitrite equilibrium (Eq. (1)). It is by now not clear which exact influence the nitrate decomposition has on the corrosion mechanism. Corrosion is also enhanced by trace moisture in the melt [47]. Chromium is soluble in molten nitrates and can preferentially be solved from the surface scale leading to a degradation of the protectiveness of the oxide layer. Nickel and iron do not form soluble species [10,48]. As mentioned above stresses are of major importance in terms of the possible occurrence of stress corrosion cracking of steels in contact with molten nitrate melts. In general, it is said that three prerequisites are necessary for the occurrence of SCC:
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Stresses can arise from mechanical or thermal loading or exist as residual stresses within the material due to processing (rolling, bending, surface finishing, welding). Up to know there is not much information found in literature on this topic in concern to steels used in contact with nitrate melts. In [44] the occurrence of SSC is described. 3.3. Thermal stability of Solar Salt and NaNO3 Mass losses with gas evolution of alkali metal nitrate salts may occur due to three mechanisms: nitrite formation in the melt and oxygen release (1), alkali metal oxide formation in the melt and nitrogen/nitrogen oxide release (2) and vaporization of the nitrate salts (3) [9,49]. Static and dynamic open-system type experiments with nitrogen–oxygen atmospheres were performed. Fig. 4 plots static NaNO3 measurements at the three temperatures 450 °C, 500 °C and 550 °C in synthetic air. The figure shows the nitrite formation depending on the time with the salt temperature as a parameter. Additional experiments with increased air flow from 100 ml min1 to 600 ml min1 at 500 °C are also shown. For the increased gas flow in the first experiment at the beginning only a small increase of the decomposition rate was observed. Both experiments at 500 °C (100 ml min1 and 600 ml min1) indicate a very similar molar NO 2 =NO3 ratio in equilibrium. A simple empirical exponential growth model was fitted to the measurement data with 100 ml min1 using non-linear regression techniques. The model uses two parameters and can approximately describe the reaction kinetics. The three model curves show that the temperature level not only affects the amount of NO 2 in equilibrium but also the decom-
Susceptible material, Critical environment, Stress. All of these prerequisites can be fulfilled in the here considered corrosion system at least for some of the described materials. Fig. 4. Experimental kinetic results of nitrite formation in NaNO3 in contact with synthetic air.
Fig. 3. Schematic drawing of the corrosion system.
Fig. 5. Experimental kinetic results of oxide formation in Solar Salt in contact with synthetic air at 550 °C.
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bility of NaNO3 and Solar Salt improves with increased partial oxygen pressures. Results were extrapolated to equilibrium conditions (heating rate of 0 K min1). For NaNO3 and equilibrium conditions, results show a decomposition temperature of 525 °C in synthetic air and 542 °C in oxygen at atmospheric pressures. For Solar Salt and equilibrium conditions, the values are 529 °C (synthetic air) and 562 °C (oxygen). As a consequence, measurements show that the thermal stability of Solar Salt is higher compared to NaNO3. It can be also seen that the stability of Solar Salt (Fig. 7) depends more strongly on the partial oxygen pressure than NaNO3 (Fig. 6). For example, for NaNO3 the stability difference between synthetic air and oxygen is 17 K (542 °C minus 525 °C). For Solar Salt, about double the value is obtained (33 K).
Fig. 6. Experimental thermogravimetry results of the thermal stability of NaNO3 depending on the partial oxygen pressure and the heating rate (0.5–10 K min1).
Fig. 7. Experimental thermogravimetry results of the thermal stability of 40 wt%KNO3–60 wt%NaNO3 depending on the partial oxygen pressure and the heating rate (0.5–5 K min1). position rate of NO 3 to NO2 . The equilibrium at 550 °C was quickly reached after several tens of hours. On the other hand, at 450 °C the time constant was much longer with several hundreds of hours. As discussed in the previous paragraph, the primary decomposition reaction with nitrite formation in the melt and oxygen release is well examined. The secondary decomposition reaction with alkali metal oxide formation and nitrogen/nitrogen oxide is less understood. The chemistry of these oxygen species is complex and some of the evidence is in conflict. In literature the formation of oxide, superoxide and peroxide is discussed [16,49]. Fig. 5 refers to measurements of Solar Salt at 550 °C in an open type system with synthetic air atmosphere. The figure plots not only the NO 2/ NO 3 ratio (left hand axis) but also the equivalence point of the titration (right hand axis). It can be seen that the NO 2 /NO3 ratio reaches equilibrium after a few 10 h as opposed to the volume of titration which increases steadily. It can be concluded that although the NO 2 /NO3 ratio reaches equilibrium, the oxide level reaches no equilibrium in this experiment. Work at Sandia aims for oxygen-stabilized Solar Salt up to 650 °C [50]. In the present experimental work the impact of the partial pressure of oxygen on the thermal stability was examined. Figs. 6 and 7 plot results of dynamic TG-measurements of NaNO3 and Solar Salt depending on the partial oxygen pressure. It can be seen that the heating rate has a strong impact on the observed decomposition temperature. Higher heating rates result in measurement results with higher decomposition temperatures. Also, all measurement series show that higher partial oxygen pressures lead to higher decomposition temperatures. In other word, the sta-
4. Conclusions Reliable thermophysical properties are important for the modeling and dimensioning of molten salt storage systems. For example, incorrect heat capacity values may lead to oversized two-tank molten salt systems. Hence, consistent thermophysical data presented in this paper can improve modeling results and increase the accuracy in sizing of thermal energy storage systems. Temperature dependent thermophysical data of the density q, heat capacity cp, thermal diffusivity a, and thermal conductivity k from various authors were compared. Representative thermophysical properties of Solar Salt and correlations between properties of the single salt and Solar Salt were presented. Results show that these properties differ in terms of the uncertainty. Density values show the lowest scattering. Heat capacity data vary up to ±7% compared to the average value. Thermal diffusivity and thermal conductivity values showed the strongest variations with maximum deviations of ±15% compared to average values. To handle molten salts in TES systems it is mandatory to understand the corrosion behavior of steels used for tanks and components. For reliable long-term application structural parts of modern solar power plants need to have good corrosion resistance up to the thermal stability limit of molten nitrate salts. Corrosion resistance of the material strongly depends on restricting the environmental conditions and the stresses to limits where the chosen material shows suitable corrosion resistance. This can only be achieved in a reproducible manner when the corrosion mechanism is well understood. In already existing power plants corrosion in different structural parts occurred and shows the need for further fundamental corrosion studies. In addition new lower melting salt mixtures might bring about new corrosion aspects. The thermal stability of NaNO3 and Solar Salt has been experimentally studied. For the dynamic TG-measurements a thermal stability limit of about 530 °C in synthetic air at atmospheric pressure in an open system has been obtained. This value is lower compared to the previously reported value of 565 °C [3]. Measurements refer to a mass loss of 3 wt%. It should be considered that different mass loss definitions will result in other stability limits. Static measurements of Solar Salt at 550 °C in synthetic air in an open type system show that the primary decomposition reaction with nitrite formation reaches equilibrium within the experimental time frame. On the other hand, this measurement indicates that the secondary decomposition reaction with oxide formation and nitrogen release reaches no equilibrium within the experimental time frame. Hence, further work focuses on the long-term thermal stability limit of Solar Salt to examine the different decomposition mechanisms. Also, we have measured the influence of the partial pressure of oxygen on the thermal stability. Our results confirm a stabilizing effect by increased oxygen partial pressure in open-system type experiments with nitrogen-oxygen atmospheres. These results are particularly important for power tower plants. These
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plants aim for temperature levels in the thermal energy storage system near the thermal stability limit of Solar Salt to improve the economics and the plant efficiency. For large-scale process technology aspects, a molten salt storage test facility is in the planning stage at DLR. The start-up is scheduled for 2014. The facility will be used for the investigation of innovative storage concepts as well as components and corrosion testing. Another focus will be the examination of the salt chemistry in different atmospheres on a system level. Acknowledgements We express our thanks especially to Ulrike Kröner and Markus Braun for the experimental part and data analysis in this paper. References [1] Laing D, Bauer T, Breidenbach N, Hachmann B, Johnson M. Development of high temperature phase-change-material storages. Appl Energy 2013;109: 497–504. [2] Schaube F, Koch L, Wörner A, Müller-Steinhagen H. A thermodynamic and kinetic study of the de- and rehydration of Ca(OH)2 at high H2O partial pressures for thermo-chemical heat storage. Thermochim Acta 2012;538:9–20. http://dx.doi.org/10.1016/j.tca.2012.03.003. [3] Pacheco JE. Final test and evalution results from the solar two project. Albuquerque (NM): Sandia National Laboratories. Report No.: SAND20020120; 2002. [4] Dunn RI, Hearps PJ, Wright MN. Molten-salt power towers: newly commercial concentrating solar storage. Proc IEEE 2012;100:504–15. http://dx.doi.org/ 10.1109/jproc.2011.2163739. [5] Yang Z, Garimella SV. Molten-salt thermal energy storage in thermoclines under different environmental boundary conditions. Appl Energy 2010;87:3322–9. http://dx.doi.org/10.1016/j.apenergy.2010.04.024. [6] Flueckiger S, Yang Z, Garimella SV. An integrated thermal and mechanical investigation of molten-salt thermocline energy storage. Appl Energy 2011;88:2098–105. http://dx.doi.org/10.1016/j.apenergy.2010.12.031. [7] 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. http://dx.doi.org/10.1016/ j.apenergy.2011.12.057. [8] Calvet N, Gomez JC, Faik A, Roddatis VV, Meffre A, Glatzmaier GC, et al. Compatibility of a post-industrial ceramic with nitrate molten salts for use as filler material in a thermocline storage system. Appl Energy 2013;109:387–93. [9] Carling RW, Kramer CM, Bradshaw RW, Nissen DA, Goods SH, Mar RW, et al. Molten nitrate salt technology development – development status report. Livermore (CA): Sandia National Laboratories; 1981. Report No.: SAND80-8052. [10] Nissen DA, Meeker DE. Nitrate/nitrite chemistry in sodium nitrate–potassium nitrate melts. Inorg Chem 1983;22:716–21. http://dx.doi.org/10.1021/ ic00147a004. [11] Bartholomew RF. A study of the equilibrium KNO3/KNO2(l) + ½ O2(g) over the temperature range 550–750 °C. J Phys Chem 1966;70:3442–6. http:// dx.doi.org/10.1021/j100883a012. [12] Bradshaw RW, Carling RW. A review of chemical and physical properties of molten alkali nitrate salts and their effect on materials used for solar central receivers. Albuquerque (NM): Sandia National Laboratories; 1987. Report No.: SAND87-8005. [13] Kust RN, Burke JD. Thermal decomposition in alkali metal nitrate melts. Inorg Nucl Chem Lett 1970;6:333–5. [14] Paniccia F, Zambonin PG. Redox mechanisms in an ionic matrix. III. Kinetics of the reaction nitrite ion + molecular oxygen = nitrate ion in molten alkali nitrates. J Phys Chem 1973;77:1810–3. http://dx.doi.org/10.1021/ j100633a018. [15] Sirotkin GD. Equilibrium in melts of the nitrates and nitrites of sodium and potassium. Russ J Inorg Chem 1959;4:1180–2. [16] Plambeck JA. Sodium nitrate – potassium nitrate. In: Bard AJ, editor. Encyclopedia of electrochemistry of the elements, vol. X. New York: Marcel Dekker; 1976. p. 189–232. [17] Raade JW, Padowitz D. Development of molten salt heat transfer fluid with low melting point and high thermal stability. ASME J Sol Energy Eng 2011;133:031013. http://dx.doi.org/10.1115/1.4004243. [18] Bauer T, Laing D, Tamme R. Characterization of sodium nitrate as phase change material. Int J Thermophys 2011;33:91–104. http://dx.doi.org/10.1007/ s10765-011-1113-9. [19] Bradshaw RW. Effect of composition on the density of multi-component molten nitrate salts. Albuquerque (NM): Sandia National Laboratories; 2009. Dez. Report No.: SAND2009-8221. [20] Janz GJ, Krebs U, Siegenthaler HF, Tomkins RPT. Molten salts: volume 3, nitrates, nitrites, and mixtures, electrical conductance, density, viscosity, and surface tension data. J Phys Chem Ref Data 1972;1:581–746.
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