Evaluation of reaction characteristics of Na2S2O3.5H2O for thermochemical energy storage

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ScienceDirect Materials Today: Proceedings 17 (2019) 239–245

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ICAMEES2018

Evaluation of reaction characteristics of Na2S2O3.5H2O for thermochemical energy storage Ankammarao Padamurthya, Jalaiah Nandanavanama,*, Parameshwaran Rajagopalana a

Department of Mechanical Engineering, Hyderabad Campus, BITS-Pilani, Hyderabad, 500078, Telangana, India

Abstract In the present study, calorimetric investigations are performed on Na2S2O3.5H2O to determine the dissociation temperature(s), decomposition temperature and reaction enthalpies due to dehydration and hydration, using the DTG-60H instrument. Towards this, heating and single cycle (comprising of sequential dehydration and hydration reactions) tests are performed. The heating tests are performed to measure the dissociation and decomposition temperatures by heating the material’s sample for a temperature up to 600 °C at a heating rate of 5/ 10/ 20 °C/min. The single cycle tests are performed at different hydration temperatures (30 and 50 °C) and durations (30, 60 and 120 min). The results indicate that the dissociation and decomposition temperatures increase with heating rate. The heating tests also helped in determining the material’s reaction enthalpy due to dehydration. Reaction enthalpy due to hydration increases with its duration and decreases as its temperature increases. For the single cycle with 120 min duration and 30 °C temperature of hydration, the calculated energy storage densities are respectively 1.90 and 1.81 GJ/m3. These values are far superior to that of materials used for conventional thermal energy storage methods. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on Advanced Materials, Energy & Environmental Sustainability, ICAMEES2018 Keywords: Na2S2O3.5H2O; Differential Thermogravimetric Analysis; Dehydration; Hydration; Reaction Enthalpy

* Corresponding author. Tel.: +91-40-66303514; fax: +91-40-66303998. E-mail address: [email protected]

2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on Advanced Materials, Energy & Environmental Sustainability, ICAMEES2018

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Nomenclature DTA DTG TCES TCM TG-DSC

differential thermal analysis differential thermogravimetric thermochemical energy storage thermochemical material thermogravimetric-differential scanning calorimetry

1. Introduction Fossil fuels have been the major source of energy supply across the world and currently support as much as 80% of the world’s energy demand [1]. Ever increasing human population and their lifestyle leads to a non-stop growth in the global energy demand. This upsurge in energy demands has escalated the utilization of fossil fuels, thus leading to their drastic depletion. Increased use of fossil fuels is also a prime contributor to the greenhouse gas emissions and thereby making a serious threat to the ecosystem and environment. This situation called for identifying viable and suitable alternate energy sources. Acknowledging this need and condition, scientists and nations have started exploring on the alternates, including the renewable energy sources (RES). If used effectively, the renewable energy sources like solar, wind, hydropower, geothermal shall address the current global energy demands significantly. Notably, solar energy is available abundantly across the globe. Being in its primitive and thermal form, it is often considered as a choice for heating and drying applications. Its diluted and non-uniform availability across the seasons, diurnal cycles and geographical locations stand as a major constraint in using for various heating applications. Development of suitable energy storage technologies vis-à-vis materials shall help using the solar energy effectively and thereby address the present day energy demand-supply gap amicably. Thermal energy, be it solar or industrial waste heat, has been stored conventionally in either sensible or latent form. These (sensible/ latent) energy storage methods are reasonably matured and plenty of studies on them can be found in the open literature [2]. An emerging and promising technique under the thermal energy storage methods is thermochemical energy storage (TCES) method. When compared to the conventional energy storage methods, the TCES offers several benefits such as high energy storage density, negligible heat losses, long term energy storage and long distance transport possibility [3]. In the TCES method, thermal energy is stored in chemical form during the charging period and retrieved in thermal form during the discharging period [4]. The commonly investigated charging and discharging reactions respectively are dehydration and hydration (salty hydrates, metal hydroxides), decarbonation and carbonation (carbonates), reduction and oxidation (metal oxides), and desorption and adsorption (zeolites, silica gel, activated carbons, etc.). The materials investigated in TCES method are commonly referred to as thermochemical materials (TCMs). Different groups of TCMs were investigated in the past, namely salt hydrates [5,6], metal hydroxides [7,8], metal oxides [9], carbonates [10], zeolites [11,12], etc. intended for various applications such as space/water heating and power generation. Among them, salt hydrates are the most commonly considered TCMs, for several reasons that include better energy storage density, non-corrosive, non-poisonous and compliance to the temperatures of energy sources like industrial waste heat and solar energy for charging reactions [13]. Several salt hydrates were investigated for water heating and chemical heat pump applications using theoretical and experimental analyses [5,6]. Based on the experimental results, they identified SrBr2.6H2O and LaCl3.7H2O as potential candidates for the said applications. Sharma et al. [14] investigated sodium cation based salt hydrates for solar heat storage applications. They concluded that ‘the ratio of activation energy to the percentage of mass loss due to dehydration’ shall stand as a criterion in deciding the material’s suitability for thermal energy storage applications. Van Essen et al. [13] investigated four salt hydrates namely CaCl2.2H2O, MgCl2.6H2O, Al2(SO4)3.18H2O and MgSO4.7H2O for seasonal energy storage applications using a TG-DSC analyzer and an experimental prototype. MgCl2.6H2O was noted to exhibit high temperature lift among the tested salt hydrates under practical conditions and hence treated as a promising candidate for energy storage applications. Barreneche et al. [15] performed calorimetric

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and cyclability studies on Calcium chloride and zeolite for long term thermal energy storage applications and found that the energy storage density of Calcium chloride (1.47 GJ/m3) was ten times that of the zeolite (0.2 GJ/m3). Selected salt hydrates (SrBr2.6H2O, LaCl3.7H2O, MgSO4.7H2O and MgCl2.6H2O) were investigated by Padamurthy et al. [16] for the TCES applications. SrBr2.6H2O was found to offer high dehydration and hydration reaction enthalpies against the other materials tested. Mixed salt hydrates (CaCl2/MgCl2, CaCl2/MgSO4 and MgCl2/MgSO4) were investigated by Rammelberg et.al [17] to determine the material’s cycle stability and reaction enthalpies. They concluded that CaCl2/MgCl2 combination exhibited higher cyclability compared to the other tested mixed salt hydrates. From this literature study, it is understood that material’s reaction enthalpies and cyclability play an important role in developing a compact and efficient TCES system. Further, no significant calorimetric investigations were reported on Sodium Thiosulfate Pentahydrate (Na2S2O3.5H2O). The current work aims to undertake different thermal characterization tests on the Na2S2O3.5H2O pellets with 99+% purity, an Alfa Aesar product. 2. Methodology To understand the material’s reaction kinetics and suitability for TCES applications, it is subjected to different temperature programs illustrated in Fig.1 using the SHIMADZU’s differential thermogravimetric analyzer (DTG60H). In the past, TGD-9600, a thermobalance working on similar principle was used for thermal characterization of magnesium hydroxide based composite materials [18,19]. In the present study, the desired reaction(s) of a temperature program are executed by making suitable arrangements to the DTG-60H instrument, as illustrated in Fig.2. During a heating/ dehydration process only nitrogen gas is provided. For the purpose of hydration, water vapor generated in a thermal bath is supplied along with the nitrogen gas. To prevent moisture entry into the DTG60H instrument, the gas mixture (i.e. water vapor and nitrogen) is filtered through a glass wool. a

b

Fig. 1. Temperature programs to evaluate/ characterize Na2S2O3.5H2O (a) heating up to 600 °C; (b) single cycle.

Fig. 2. Experimental setup for material’s thermal characterization.

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In order to determine the (i) dissociation temperature(s) and (ii) decomposition temperature of the anhydrous material, the sample is heated up to 600 °C at a heating rate of 5, 10 and 20 °C/min, as illustrated in Fig.1(a). To determine the reaction enthalpies during the dehydration and hydration periods, a single cycle comprising of dehydration and hydration processes is performed. Keeping the dehydration temperature program constant, tests are performed for different hydration periods (30/ 60/ 120 min) and temperatures (30/ 50 °C), as shown in Fig.1(b). During the dehydration process, the material is heated up to 150 °C temperature and maintained the same for 30 min, to determine the material’s suitability for using industrial waste heat and solar energy [20]. 3. Results and Discussions 3.1. Heating To determine the dissociation and decomposition temperatures of Na2S2O3.5H2O, its sample is heated at different heating rates as outlined in the Fig.1(a). The temperature at which a water molecule leaves the material is taken as dissociation temperature. The point at which the anhydrous material begins to decompose is taken as decomposition temperature. These temperatures for salt hydrates are understood to be affected by the heating rate [5]. Fig.3(a) shows the simultaneous thermogravimetric-differential thermal analysis curves for Na2S2O3.5H2O heated up to 600 °C at 5 °C/min heating rate. The material’s dissociation temperatures are determined on the basis of mass present against the original mass of the sample in terms of molecular weight. The first H2O molecule is noticed to detach at 44.1 °C and the rest sequentially at 54.6, 62.5, 71.6 and 131.8 °C temperatures. On further heating, the anhydrous material shows a noticeable mass drop at 302.4 °C, suggesting that the material begins to decompose thereafter. Reaction enthalpy due to dehydration is determined from the simultaneously derived differential thermal analysis (DTA) curve. The reaction enthalpy due to dehydration is estimated to be 1590 J/g. Fig.3(b) shows the differential thermogravimetric (DTG) curves of Na2S2O3.5H2O for different heating rates. Changes in mass during the initial periods of heating is shown in the inset. The dissociation and decomposition temperatures are noticed to increase with heating rate. The reaction enthalpies due to dehydration for different heating rates are compiled and provided in Table 1. Energy storage density of a material is normally taken as the product of reaction enthalpy and its gravimetric density [15]. In the current study, the gravimetric density of Na2S2O3.5H2O is taken as 1690 kg/m3 [21]. a

b

Fig. 3. (a) DTG and DTA curves for 5 °C/min heating rate; (b) DTG curves for different heating rates. Table 1. Results for Na2S2O3.5H2O at different heating rates. Heating rate Dissociation temperature corresponding (°C/min)

to the last H2O molecule (°C)

Decomposition temperature (°C)

Reaction enthalpy due Energy storage density due to dehydration (J/g)

to dehydration (GJ/m3)

5

131.8

302.4

1590

2.68

10

187.3

316.4

1750

2.95

20

198.0

352.4

1640

2.77

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For the limited heating rates tested, the reaction enthalpy due to dehydration is observed to be high for 10 °C/min heating rate. The role of heating rate on material’s reaction enthalpy can be better understood if tests with fine increments in heating rate are undertaken. The authors opine that a complete evaluation of material’s reaction kinetics including the (i) dissociation temperatures, (ii) reaction enthalpy due to dehydration, and (iii) decomposition temperature is helpful in (a) deciding the material’s suitability and (b) developing a system/ prototype towards the effective use of energy sources like industrial waste heat or solar energy. 3.2. Single cycle To determine the reaction enthalpy due to hydration process, single cycle tests consisting of sequential dehydration and hydration reactions are performed for the temperature program shown in Fig.1(b). Keeping the dehydration temperature program constant, tests are performed for different hydration periods (30/ 60/ 120 min) and temperatures (30/ 50 °C).

Fig. 4. DTG-DTA curves of a single cycle with 120 min hydration at 30 °C.

Fig.4 shows the simultaneous thermogravimetric-differential thermal analysis curves for a single cycle with 120 min hydration at 30 °C temperature. As noted from the thermogravimetric (mass) curve, the material becomes completely anhydrous on heating to 150 °C, a temperature well above the dissociation temperature corresponding to the last H2O molecule of Na2S2O3.5H2O. During this heating process, the energy in heat form is stored in the material in chemical form and the same is determined on the basis of peaks noted from the simultaneously derived DTA curve. On initiating the water vapor supply, the material begins to hydrate and shows up a simultaneous gain in its mass. The dehydration and hydration reaction enthalpies of this single cycle are respectively 1126 and 1012 J/g. The dehydration enthalpy of this single cycle is comparatively low against the corresponding value of a complete heating process done up to 600 °C. This suggests that the reaction enthalpies are influenced by the duration and the temperature to which the reactions are performed. To understand these influencing parameters, particularly on hydration enthalpies, single cycle tests with different hydration temperatures (30/ 50 °C) and durations (30/ 60/ 120 min) are performed and the results are shown in Fig.5. The gain in mass during the hydration at 30 °C is observed to increase with duration, and hence the reaction enthalpies. When the hydration temperature is increased to 50 °C, the gain in mass is meager even for longer durations, and hence the reaction enthalpies are very low. This tendency can be attributed to the adsorption affinity of the sorbate (water vapor) with sorbent (material) for the given temperature.

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a

b

Fig. 5. (a) Mass changes; (b) Reaction enthalpies of a single cycle at different hydration temperatures and durations.

4. Conclusions In the present work, Na2S2O3.5H2O is tested on DTG-60H instrument to determine (i) water molecules’ dissociation temperatures, (ii) material’s decomposition temperature, (iii) reaction enthalpies due to dehydration and hydration and thereby energy storage densities. Based on this study, following conclusions are drawn.  The dissociation and decomposition temperatures are found to increase with heating rate. For the tested heating rates, dissociation temperature of the last water molecule is around 130 °C and above, thus permitting to use industrial waste heat or solar energy.  Heating tests at different heating rates help to determine the material’s reaction enthalpy due to dehydration and thus its energy storage density.  Reaction enthalpy due to hydration increases with its duration and decreases as its temperature increases. This is attributed to the (i) time availability for reaction and (ii) adsorption affinity for the given temperature.  For the single cycle with 120 min duration and 30 °C temperature of hydration, the reaction enthalpies due to dehydration and hydration are respectively 1126 and 1012 J/g and the associated energy storage densities are respectively 1.90 and 1.81 GJ/m3. These values are far superior to that of materials used for conventional thermal energy storage methods. References [1] Key World Energy Statistics 2017, International Energy Agency, Available: ; [Accessed on 01.12.2018]. [2] Lefebvre D, Tezel FH, A review of energy storage technologies with a focus on adsorption thermal energy storage processes for heating applications. Renewable & Sustainable Energy Reviews 2017; 67:116–25. [3] Aydin D, Casey SP, Chen X, Riffat S, Novel “open-sorption pipe” reactor for solar thermal energy storage. Energy Conversion Management 2016; 121:321–34. [4] Yan J, Zhao CY, Experimental study of CaO/Ca(OH)2 in a fixed-bed reactor for thermochemical heat storage. Applied Energy 2016; 175:277–84. [5] N’Tsoukpoe KE, Schmidt T, Rammelberg HU, Watts BA, Ruck WKL, A systematic multi-step screening of numerous salt hydrates for low temperature thermochemical energy storage. Applied Energy 2014; 124:1–16. [6] Richter M, Habermann E, Siebecke E, Linder M, A systematic screening of salt hydrates as materials for a thermochemical heat transformer. Thermochimica Acta 2017; 659:136-150. [7] Kato Y, Takahashi R, Sekiguchi T, Ryu J, Study on medium-temperature chemical heat storage using mixed hydroxides. International Journal of Refrigeration 2009; 32:661–6. [8] Ishitobi H, Uruma K, Ryu J, Kato Y, Durability of Lithium Chloride-Modified Magnesium Hydroxide on Cyclic Operation for Chemical Heat Pump. Journal of Chemical Engineering of Japan 2012; 45:58–63. [9] Block T, Schmücker M, Metal oxides for thermochemical energy storage: A comparison of several metal oxide systems. Solar Energy 2016; 126:195–207.

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