Investigation Into Sro/Srco3 for High Temperature Thermochemical Energy Storage

Solar Energy 160 (2018) 85–93

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Solar Energy

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Investigation into SrO/SrCO3 for high temperature thermochemical energy T storage ⁎ Elham Bagherisereshki1, Justin Tran1, Fuqiong Lei, Nick AuYeung

School of Chemical, Biological, and Environmental Engineering, Oregon State University, Corvallis, OR 97331, USA

ARTICLE INFO ABSTRACT

Keywords: Global energy needs are continuously increasing while fossil fuels remain an uncertain resource. With a growing Solar thermochemistry population and increasing demand for energy, alternative energy is being pursued to power the future. Solar thermochemical Concentrated solar power (CSP) is a promising method of converting solar energy into electricity and works in Solar thermochemical energy storage conjunction with thermal energy storage (TES) to allow for power generation beyond on-sun hours. One method Carbonation cycles of TES is thermochemical energy storage (TCES), which is based on storing chemical energy via reversible

reactions. An SrO/SrCO3 carbonation cycle offers high temperature heat (ca. 1200 °C), leading to higher effi- ciencies. The carbonation reaction was further investigated to determine the effects of particle size, temperature,

partial pressure of CO2, and heat treatment temperature. Unfortunately, high temperatures cause materials to sinter, resulting in a decrease in reactivity over multiple cycles. The use of an inert diluent may help to inhibit

sintering by acting as a physical barrier between the particles. Stored energy density of SrO/SrCO3 systems

supported by CaSO4 and Sr3(PO4)2 was investigated for multiple cycles of 1150 °C exothermic carbonation followed by 1235 °C decomposition. At 25 and 50 wt%, Sr3(PO4), stable energy densities of roughly 500 ± 0.05 kJ/kg are achieved. In addition, it was found that the initial moisture content of the material aﬀects performance of the material over several cycles due to a change in particle size. This behavior was thoroughly investigated and is useful for future work in TCES involving carbonation cycles.

1. Introduction energy storage subsystems is reduced with greater volumetric energy density. TCES of on-sun thermal energy is achieved when a reactive Concentrated solar power (CSP) is one promising method of con- system absorbs thermal energy and proceeds with a reversible chemical verting clean solar thermal energy into electricity which avoids the use reaction. In a time of power demand, the reverse reaction is then in- of fossil fuels and the anthropogenic greenhouse effect (Müller- itiated and energy is released, thus recovering stored chemical energy Steinhagen and Trieb, 2004; Voorthuysen et al., 2005). CSP in con- for use in a power cycle (Stekli et al., 2013; Rhodes et al., 2015). junction with thermal energy storage (TES) can increase the utilization Different TCES systems have been classified according to their re- of solar energy by enabling plant operators to generate electricity be- action family. Metallic hydrides, carbonates, hydroxides, redox system, yond normal on-sun hours; thus, CSP can be a more efficient and cost- ammonia system and organic system such as CH4/H2O, CH4/CO2 are effective technology than photovoltaic (PV) technologies that directly candidates for high-temperature thermochemical heat energy storage. convert solar energy directly into electricity. Thermochemical energy Among these candidates, the ammonia dissociation and synthesis is the storage (TCES) is an emerging type of TES system based on a reversible most developed system which has been tested at the pilot–scale, while reaction that offers greater volumetric and gravimetric energy density most of other systems have only been tested on a laboratory scale. than latent or sensible energy storage. Efficiency of a power generation Moreover, the dehydration/hydration of the Ca(OH)2/CaO couple has system depends on the temperature at which heat is available and the been studied as a potential system for energy storage, but additional temperature level at which the reject heat can be disposed according to studies and projects must be done to overcome different barriers hin- the second law of thermodynamics and Carnot efficiency. Thus, high dering scale-up (Pardo et al., 2014). efficiency is reached by energy release at high temperature that is at- CaO precursors are cheap and widely available, which make car- tainable with TCES systems. Moreover, the capital cost of thermal bonation/decomposition cycling of CaCO3/CaO suitable for a variety of

⁎ Corresponding author. E-mail address: [email protected] (N. AuYeung). 1 Denotes equal authorship. https://doi.org/10.1016/j.solener.2017.11.073 Received 19 November 2017; Accepted 26 November 2017 Available online 22 December 2017 0038-092X/ © 2017 Elsevier Ltd. All rights reserved. E. Bagherisereshki et al. Solar Energy 160 (2018) 85–93

Nomenclature X fractional conversion

Glossary Greek symbols

3 ΔH° standard enthalpy of reaction, kJ/mol ρ density, kg/m ΔG° standard state gibbs free energy change of reaction, kJ/ mol Subscripts K equilibrium constant, atm P partial pressure, atm s solid R gas constant, 8.314 J/mol·K g gas T temperature, K eq equilibrium Δm mass change, g rxn reaction 0 initial mi mass after the initial dehydration step, g M molecular weight, g/gmol t at time t uses such as thermochemical energy storage, pre-combustion and post- parameters still need to be investigated in order to produce SrO-based combustion CO2 capture (Valverde et al., 2014; Edwards and Materić, materials which demonstrate stable energy density over many cycles. 2012; Wang et al., 2011; Qin et al., 2012; Liu et al., 2010). The re- Compared to other TCES candidate chemistries, there are some versible calcium looping cycle between CaO and CaCO3 occurs between signiﬁcant advantages in energy storage using the SrCO3/SrO system. 650 and 900 °C, at atmospheric pressure (Edwards and Materić, 2012; The reaction does not involve any catalyst and there are no side reac-

Qin et al., 2012). tions or side products. CO2 is neither flammable nor corrosive, and moreover, it is non-toxic in low concentration (Rhodes et al., 2015; CaCO↔+ CaO CO , ΔHkJmol °= 178 / 3(ssgr ) ( ) 2( ) xn (1) Randhir et al., 2014). Strontium carbonate has been studied previously as a reference material for Differential Thermal Analysis (DTA) CaO-based sorbents exhibit a great capacity for CO2 adsorption, however like most sorbents, this capacity eventually reduces with the (Charsley et al., 1993; Robbins et al., 1995). The thermal decomposi- α β number of carbonation/calcination cycles (Wang et al., 2011). tion of orthorhombic ( -SrCO3) and hexagonal ( -SrCO3) strontium carbonate polymorphs has been studied by non-isothermal thermo- An energy storage system similar to CaO/CaCO3 based on the reversible carbonation/decomposition of SrO/SrCO , has been introduced gravimetric analysis (TGA). The reaction was then modeled using the 3 fi “ by Arlt and Wasserscheid (2011). Kissinger equation and a kinetic function tting method. The kinetic triplet” of the activation energy, frequency factor and solid state kinetic

SrCO3(ssgr )↔+ SrO( ) CO 2( ) , ΔHkJmo °=xn 234 / l (2) model was determined. It is shown that the transformation of α-SrCO3 to β-SrCO causes the activation energy of the process to decrease from Under atmospheric pressure, the cyclic carbonation/decomposition 3 255 to 227 kJ/mol and changes the reaction mechanism from interface occurs at high temperatures (≈1200 °C). Such high-quality heat is controlled growth to a process with zero or decreasing nucleation rate suitable for high efficiency, combined cycle power generation, which (Ptáček et al., 2014). has the potential to translate into more competitive solar electricity The thermodynamics of the system greatly depend on the partial prices. The endothermic decomposition of SrCO occurs at a tempera- 3 pressure of CO (Rhodes et al., 2015). Fig. 1 illustrates the equilibrium ture greater than the equilibrium temperature (ΔG° = 0 kJ/mol at 2 rxn partial pressure of CO , P , during the carbonation-decomposition 1175 °C). The reverse exothermic carbonation of SrO occurs at a tem- 2 CO2,eq reaction shown in Eq. (2) at temperature between 900 °C and 1175 °C. perature below the decomposition temperature and the released heat is Calculation of the equilibrium partial pressure is equivalent to the used for power generation. calculation of the equilibrium constant K assuming first order reaction A TCES subsystem based on carbonation cycles of either CaO or SrO (Dou et al., 2010). could be integrated in the context of a solar thermal power plant in a variety of methods. Given the need to calcine large amounts of solids, a system that has solids moving in and out of a directly-irradiated reactor 2 as has been demonstrated previously would likely be the first logical 1.8 Carbonation choice (Meier et al., 2005, 2006). Decarbonated solids could be stored SrCO SrO + CO 3 in beds until times of demand. The CO2 could be captured in a sub- 1.6 2 stance that has low energy of solution (e.g. ionic liquids or deep eutectic 1.4 solvents), so that it can be easily released. The discharge carbonation reaction can be carried out in a fluidized bed. It is anticipated that 1.2

ﬀ (atm)

design of an e ective unit would require a creative approach on how to q e

, 1

elegantly harness the heat of reaction from the exothermic carbonation 2 process. Heat generated here would then be taken via heat transfer ﬂuid O C 0.8 to the power block, where power is generated. P The common problems of such storage schemes are sintering and 0.6 loss of surface area due to high-temperature involvement which slows 0.4 down the reaction rate. The energy density of the system supported by Decomposition zirconia-based sintering inhibitors was investigated by Rhodes et al. 0.2 SrCO SrO + CO (2015). In this investigation, samples of SrO with inert support material 3 2 of yttria-stabilized strontium zirconate show energy densities of 1500, 0 900 950 1000 1050 1100 1150 1200 1430 and 1260 MJ/m3 after 10, 15 and 45 cycles, respectively. How- Temperature ( ° C) ever, zirconate is relatively expensive, leading to the need for other supports. In each cycle, the carbonation was conducted at 1150 °C and Fig. 1. Equilibrium for the SrO-SrCO3-CO2 system under stoichiometric conditions at the decomposition at 1235 °C. Many operational and material synthesis various temperatures.

86 E. Bagherisereshki et al. Solar Energy 160 (2018) 85–93

⎡ Δ()GT°rxn ⎤ Table 1 K ==PCO2 ,eq exp − ⎣ RT ⎦ (3) Structural properties of SrO sorbent. Sorbent Specific surface Pore volume Average pore In Eq. (3), PCO2,eq is the equilibrium partial pressure of CO2 from the area (m2/g) (ml/g) diameter (Å) decomposition of SrCO3 in a closed system, R is gas constant, and T is temperature. ΔG°rxn(T) is the standard state Gibbs free energy change Strontium oxide 4.343 0.02457 226.3 for the reaction at temperature T, calculated from tabulated thermochemical data provided by Barin (1995). The trend in Fig. 1 suggests 2. Experimental that increasing the partial pressure of CO2 at a constant temperature or decreasing the temperature at a constant partial pressure of CO would 2 2.1. Material preparation result in more capture of CO2 by SrO. At high temperatures, sintering of strontium oxide occurs. Sintering SrO (Alfa Aesar) was heated in a muffle furnace at 1500 °C for 8 h is the physical aggregation of crystals that lead to increased particle under air to ensure the formation of large grains. SrO powder absorbs size. This reduces the surface area of the sorbent particles, leading to a moisture from the air and quickly transforms into strontium hydroxide decrease in reactivity. In order to prevent sintering, an inert material Sr(OH) or a hydrate during the material preparation process. Thus, the with greater temperature stability may be added to prevent the crystals 2 mass ratio of SrO is unknown at the beginning of the kinetic study. from aggregating. Polymorphic spacers have been used to inhibit sin- During an initial dehydration step at 1200 °C in TGA, SrO is purified tering for calcium looping. Calcium silicate Ca SiO was used as an 2 4 due to water loss. The minimum mass reached after dehydration was inhibitor since it undergoes a crystal structure change between 660 and taken to be the actual mass of SrO. The resulting powder was sieved to 930 °C, between carbonation and calcination of CaO/CaCO . The idea is 3 prepare samples in different particle size ranges: 25–53, 53–75, that as the inhibitor changes crystal structures, it preserves space be- 75–106 µm. The specific surface area of the material was determined tween particles. The material is shown to be stable for 15 cycles when with the BET method using Micromeritics ASAP 2020. Table 1 gives the diluted with a polymorphic spacer (Zhao et al., 2014). specific surface area, pore volume, and average pore diameter of the Polymorphic spacers have not been studied with the carbonation prepared SrO. and decomposition of SrO/SrCO . Since SrO/SrCO occurs at higher 3 3 Following heat treatment, samples were tested using D8-Discover X- temperatures, other polymorphic materials are needed to potentially ray Diffractometer to determine the XRD spectra. The identity of the synthesize sintering resistant sorbents. Two potential materials that can sample was determined with the spectra and ensured that no reaction be used with SrO/SrCO are calcium sulfate CaSO and strontium 3 4 occurred between the sample and inhibitor. The spectra ran between 20 phosphate Sr (PO ) . Calcium sulfate was found to make a monazite- to 3 4 2 and 70 2θ and compared to the spectra for strontium oxide, calcium barite-type transition at around 1230 °C (Fujii et al., 2016). This tran- sulfate, and strontium phosphate (see Supplementary Information, Fig. sition occurs right before decomposition occurs for SrCO , making it a 3 S8). Due to the hygroscopic nature of SrO, the spectra for Sr(OH) and potential sintering inhibitor. Furthermore, phosphates were in- 2 its hydrates contains many small peaks throughout the spectra. Major vestigated due to their known polymorphic behavior. Calcium phos- peaks were used to confirm the identity of the sample. phate Ca3(PO4)2 undergoes a β-toα-type transition between 1140 and 1200 °C, past the SrO carbonation temperature of 1150 °C (Maciejewski et al., 2008). Also, Sr (PO ) is believed to undergo a similar transition 3 4 2 2.2. Temperature studies, particle size studies, partial pressure studies as calcium phosphate since both materials are similar (Zhai et al., 2010). Initial temperature studies, particle size studies, partial pressure With phase transitions between the carbonation and decomposition studies were conducted via thermogravimetric analysis (TGA) using a temperature of SrO/SrCO , CaSO and Sr (PO ) are hypothesized to 3 4 3 4 2 TA Instruments SDT Q600. For each experiment, approximately 40 mg inhibit sintering by maintaining particle size at high temperatures, as of material was measured into an alumina crucible. As it is mentioned shown in Fig. 2. earlier, SrO samples were heated at 1200 °C for 2 h under an inert

SrO With No Spacer

noitanobraC noitisopmoceD

OrShserF OCrS 3 Sintered SrO

SrO With Polymorphic Spacer

Polymorphic Spacer Polymorphic Phase Carbonation Transition Decomposition

Fresh SrO SrCO 3 SrCO 3 Porous SrO

Fig. 2. SrO is carbonated to SrCO3 and then decomposed to SrO. Without an inhibitor, SrO sinters after repetitive use. To reduce sintering, a polymorphic spacer is added to separate particles and push them back between cycles.

87 E. Bagherisereshki et al. Solar Energy 160 (2018) 85–93

fl fi atmosphere (N2) with a ow rate of 40 sccm in the rst step. The weight at time t, and MCO2 and MSrO are molecular weights of SrO and temperature was then decreased at 5 °C/min to the carbonation tem- CO2 equal to 103.62 and 44.01 g/gmol, respectively. perature, in the range 900–1150 °C and stabilized around 10 min before introducing CO2 with the flow rate of 40 sccm to initiate the exothermic 3.1. Effect of particle size carbonation step. Fig. 3 represents the mass and temperature change during a typical experiment with 40 mg of a sample at 1100 °C. All the A series of experiments was carried out to determine the effects of carbonation reactions were performed in a similar fashion under iso- the particle size of SrO on carbonation reaction at 1100 °C and 1 bar of ff thermal conditions. Similarly, the e ect of partial pressures of CO2 CO2 pressure. Fig. 4 shows that the particle size of SrO varying from less (0.1–1 bar) has been investigated at 900 °C and 1000 °C. than 25 µm to more than 106 µm had almost no effect on the carbonation behavior. The carbonation behavior of SrO is independent of 2.3. Inhibitor studies and investigation into hydration effect particle size. Therefore, the kinetic experiments were performed by using particles with a diameter between 53 and 75 µm. The lack of Inhibitor and hydration studies were conducted via thermogravi- changes may be the result of dehydration step at 1200 °C, during which metric analysis (TGA) and differential scanning calorimetry (DSC) using the particle size could equalize due to the high temperature. a Netzsch STA449 F5 Jupiter. A correction file was run prior to each experiment to correct for any phase changes that the alumina crucible 3.2. Effect of temperature may undergo. The correction file was done at the same conditions as experiments, with the only exception being that no sample was loaded. Fig. 5 presents the fractional conversion versus time for carbonation Each experiment was performed with approximately 40 mg of material of SrO at different carbonation temperatures ranging from 900 °C to loaded into an alumina crucible. 1150 °C with 1 bar CO2 pressure. The carbonation reaction progression SrO samples were heated at 1200 °C for 2 h under an inert atmo- can be divided into two stages: a rapid kinetically-controlled carbona- sphere (Ar) with a flow rate of 40 sccm in the initial dehydration step tion stage primarily on the surface of the sample; followed by a sluggish for experiments. The temperature was then decreased to the carbona- diffusion-controlled regime that takes place as CO2 diffuses through a tion temperature of 1150 °C. CO2 began flowing at 10–25 sccm began layer of SrCO3 formed on the surface in the second stage. Final con- upon reaching 1150 °C while inert flow was reduced to 5–20 sccm. The version is close to completion at temperatures higher than 900 °C but total flow of CO2 and Ar remained constant at 30 sccm during carbo- the complete conversion of SrO (X = 1) is never reached over the nation. The system remained isothermal for 90 min to ensure carbo- timeframes studied due to diffusion limitations through the product nation of the SrO. layer and sintering. The temperature was then increased to 1235 °C for decomposition of Although carbonation at lower temperatures should theoretically fi the newly formed SrCO3. The system remained isothermal for 30 min to have higher nal extent of reaction from thermodynamics, it is inter- ensure complete decomposition of strontium carbonate. Carbonation esting to note that carbonation at higher temperature resulted in and decomposition were repeated for 10–30 cycles to see the effects of greater conversion after the given amount of time. Though these ob- sintering over time. The temperature was increased and decreased at a servations are in contrast to thermodynamic predictions, they agree rate of 20 °C/min. well with previous work with CaO carbonation (Rouchon et al., 2013; A series of experiments were carried out to determine the effects of Bhatia and Perlmutter, 1983). This is likely due to greater solid phase pretreatment temperature, carbon dioxide flow rate, and various weight diffusional transport of CO2 through the SrCO3 product layer at higher percent of strontium oxide with a polymorphic spacer. Energy densities temperature. Slow diffusion at lower temperature makes the ultimate were taken as kJ/kg rather than MJ/m3 due to inconsistencies when conversion practically unachievable within reasonable time frames. determining the densities of samples. Densities were measured using Fig. 5 also displays an apparent lag in carbonation upon commen- the mass after the initial dehydration step and the volume, which was cing CO2 flow. Strangely, this effect is noticed to a greater extent at as estimated using a caliper. Volumetric energy density was calculated temperature increases, reaching nearly 10 min for the 1150 °C trial. To using Eq. (4) and gravimetric energy density was calculated using Eq. rule out a potential “induction” time for the reaction, a fixed bed study (5). was carried out which showed that no induction time exists (see Supplementary Information). Thus, this behavior is likely attributed to MJ Δ··Δmρ Ho Energy Density ⎡ ⎤ = rxn,25° C poor gas mixing in the TGA/DSC. The greater lag times at higher 3 ⎣ m ⎦ MmCO2· i (4) temperatures could be due to less thermodynamic driving force for

kJ Δ·ΔmHo Energy Density ⎡ ⎤ = rxn,25° C 1200 CO Starts ⎢ ⎥ 2 ⎣ kg ⎦ MmCO2· i (5) 1100 45 1000 where Δm is the mass change, ρ is the density, ΔHrxn,25° C is the standard 900 heat of reaction, MCO is the molecular weight of CO2, and mi is the 2 40

C) 800 mass after the initial dehydration step. ° Error was calculated using propagation of error, with the major 700 ’ error coming from the instrument s balance. The instrument contains an 600 error of 0.05 μg. This value was used to calculate the error in energy

500 Weight (mg) density and a safety factor was applied to account for any oversights. 400 30 Temperature ( 300 3. Results and discussion 200 Under Nitrogen Plots of fractional conversion versus time were obtained from the 100 0 20 measured mass changes: 0 50 100 150 200 250 300

mmt− 0 MSrO Time (min) XSrO = m0 MCO (6) 2 Fig. 3. Typical experiment on TGA showing initial dehydration step following by upward

mass increase corresponding to carbonation (carbonation at 1100 °C under 1 bar in CO2). where m0 is the initial SrO weight after complete dehydration, mt is SrO

88 E. Bagherisereshki et al. Solar Energy 160 (2018) 85–93

1 carbonation, as decomposition becomes increasingly favorable as temperature increases. 0.9

0.8 3.3. Eﬀect of partial pressure of CO2

0.7 The eﬀect of CO2 pressure on carbonation was investigated by trials 0.6 at 0.1, 0.25, and 1 bar CO2. In the situations where the CO2 was not

O 100%, N2 was used as the balance gas. Other parameters such as par- r 0.5 S ticle size (53–75 µm), and initial mass (∼40 mg) were held constant. X Each run was started with a fresh sample of heat-treated SrO. Fig. 6 0.4 shows the effect of CO2 partial pressure on carbonation at 900 °C and 0.3 1000 °C, respectively. PCO2 has a more dramatic effect at 1000 °C, ff < 25 m whereas at 900 °C the e ect is minimal. This can be explained by the 0.2 m 25 - 53 thermodynamics, as the PCO2,eq increases with temperature. 53 - 75 m 0.1 75 - 106 m > 106 m 3.4. Effect of heat treatment 0 0 10 20 30 40 50 60 Each sample is pretreated by placing it within a muffle furnace for Time (min) eight hours to form large grains. Typically, samples are treated at

Fig. 4. Isothermal kinetic curves of SrO carbonation at 1100 °C under CO2 pressure of 1500 °C due to preserve furnace lifetime. Since energy density is de- 1 bar for different particle size. pendent on the post analysis sample density, different temperatures of heat treatment were investigated. Three different treatment tempera- 1 tures were looked at: 1400 °C, 1500 °C, and 1525 °C. All samples were strontium oxide with no spacer. Fig. 7 shows the result of the three 0.9 samples as well as an untreated sample over 10 cycles, where energy 0.8 density is calculated based on the total mass change of the sample during carbonation during the cycle. 0.7 At higher temperatures of heat treatment, the energy densities of the sample increase. After the first cycle, the energy density is lowest when 0.6 the sample is treated at 1400 °C and highest for the untreated sample.

O 0.5 After multiple cycles, sintering is experienced for the untreated sample r

S and the sample treated at 1525 °C since the energy density decreases X 0.4 after each cycle. However, the two samples treated at 1400 and 1500 °C

0.3 appear to be stable for 10 cycles. The untreated sample initially has the highest energy density – up to

0.2 900 ° C 2000 kJ/kg. However, the energy density quickly decreased as more 1000° C cycles are performed. This comes as a result of the untreated sample 0.1 ° 1100 C having ﬁne particle sizes. Small particles cause the sample to sinter 1150 ° C 0 more rapidly, leading to the need of heat treatment. 0 10 20 30 40 50 60 70 Interestingly, the sample treated at 1400 °C shows an increase in Time (min) energy density before it begins to stabilize at cycle 7, which could be

due to limitations in diﬀusion of CO2. Overall, treating samples at Fig. 5. Isothermal kinetic curves of SrO carbonation under CO2 pressure of 1 bar. 1500 °C shows the most promise, since the material appears to be stable

0.7 1

0.9 0.6 0.8

0.5 0.7

0.6 0.4 O r O

r 0.5 S S X

0.3 X 0.4

0.2 0.3

0.2

0.1 1 bar 1 bar 0.25 bar 0.1 0.25 bar 0.1 bar 0.1 bar 0 0 0 10 20 30 40 50 60 0 10 20 30 40 50 60 Time (min) Time (min) (a) (b)

Fig. 6. Measured isobaric kinetic curves of SrO carbonation at 900 °C (a) and 1000 °C (b).

89 E. Bagherisereshki et al. Solar Energy 160 (2018) 85–93

2500 900

Untreated SrO SrO Heat Treated at 1500 ° C SrO Heat Treated at 1400 ° C 50 wt% Sr (PO ) 3 4 2 SrO Heat Treated at 1500 ° C 800 ) 25 wt% Sr (PO4 2 SrO Heat Treated at 1525 ° C 3 15 wt% Sr (PO ) 2000 3 4 2 700

600 1500 500

400 1000

300 Energy Density (kJ/kg) Energy Density (kJ/kg) 500 200

100

0 0 123 4 5 6 7 8910 11 0 0 123 4 5 6 7 8910 11 Cycle Cycle Fig. 7. Gravimetric energy density of samples of strontium oxide heat treated at diﬀerent Fig. 9. Gravimetric energy density of samples with diﬀerent ratios of Sr3(PO4)2. Each temperatures. Each test used particles between 53 and 75 μm. sample was heat treated at 1500 °C and each test used particles between 53 and 75 μm. Pure SrO is used as a reference. 900 ° SrO Heat Treated at 1500 C 1600 50 wt% CaSO 800 4 50 wt% Sr (PO ) 3 4 2 1400 700

600 1200

500 1000

400 800

300 600

Energy Density (kJ/kg) 200

Energy Density (kJ/kg) 400

100 200 0 12 10 11 0 3 4 5 6 7 89 0 5 10152025 Cycle 0 30 Cycle Fig. 8. Gravimetric energy density of strontium oxide paired with CaSO4 or Sr3(PO4)2.

Each sample was heat treated at 1500 °C and each test used particles between 53 and Fig. 10. Gravimetric energy density of SrO with 25 wt% Sr3(PO4)2 over 10 and 30 cycles. 75 μm. Pure SrO is used as a reference. Both samples were treated the same. Each sample was heat treated at 1500 °C and each test used particles between 53 and 75 μm. over 10 cycles. However, it is possible that the sample will experience sintering after 10 cycles due to the lack of a spacer. diluted sample. Lower amounts of inhibitor will help increase the energy density; however, lower amounts of inhibitor could result in sintering.

3.5. Effect of inhibitor addition The Sr3(PO4)2 sample also remains stable through 10 cycles. The energy density of the system with Sr3(PO4)2 is greater than the system Polymorphism in the temperature of interest (1000–1300 °C) was with CaSO4 by about 200 kJ/kg. SrO with Sr3(PO4)2 has a high initial fi investigated for both CaSO4 and Sr3(PO4)2. CaSO4 appears to decom- energy density but it decreases after the rst two cycles. Afterwards, the pose at 1220 °C, whereas Sr3(PO4)2 appears to remain stable in the sample stabilizes and is competitive with the pure SrO sample, as both temperatures of interest (see Supplementary Information Figs. S6 and have energy densities between 500 and 600 kJ/kg. S7). Given these results, polymorphism was therefore not deemed to be Although the pure SrO sample has comparable energy density to the the driving force behind any sintering inhibition. diluted sample and appears to be stable, it is speculated that longer Each sintering inhibitor was initially tested at 50 wt% to determine testing will result in sintering due to the high temperatures and the lack if sintering could be hindered with the presence of the inhibitor. Weight of any type of inhibitor. Furthermore, since 1) SrO diluted with percent was chosen as a basis due to its simplicity in calculations and Sr3(PO4)2 has a higher energy density than doping the material with measurements. In Fig. 8, the reaction appears to be stable through 10 CaSO4; and 2) CaSO4 likely decomposes at temperatures of interest; ff cycles with polymorphs. This reveals that the Sr3(PO4)2 may work as a Sr3(PO4)2 was further investigated at di erent mass ratios with SrO. sintering inhibitor. Sr3(PO4)2 was further investigated at three different compositions:

The CaSO4 sample remains stable throughout the 10 cycles. 15, 25, and 50 wt%. At lower percentages of Sr3(PO4)2, there is more However, the energy density is relatively low compared to SrO with no reactive SrO that will increase the energy density. The downside is that inhibitor. This is a result of a lower percentage of reactive SrO in the there is less spacer, increasing the chance for sintering. Fig. 9 shows

90 E. Bagherisereshki et al. Solar Energy 160 (2018) 85–93

1600 42 Test 1 Test 2 1400 Test 3 40 Test 3

1200 38

1000 36

800 34

32 600 Weight (mg)

Energy Density (kJ/kg) 400

28 Test 1 200 Test 2 Test 3 26 Test 3 0 0 123 4 5 6 7 8910 11 0 20 40 60 80 100 120 140 160 180 Cycle Time (min)

Fig. 11. Gravimetric energy density of SrO heat treated at 1500 °C for four separate trials. Fig. 13. TGA Data for the four diﬀerent tests during the initial dehydration step. Tests 1 Tests 1 and 2 were conducted within 24 h of heat treatment and tests 3 and 4 were and 2 show that the sample was not highly hydrated, whereas tests 3 and 4 were highly conducted one month after heat treatment. Each test used particles between 53 and hydrated. 75 μm.

Table 2 how different weight ratios of Sr3(PO4)2 to SrO affect energy density. Summary of experiments with percent hydration to validate results. The energy density is higher for 25 wt% than it is for 50 wt% at the Sample % Hydration Figure reference end of the 10 cycles. The lowest ratio of Sr3(PO4)2 of 15 wt% shows evidence of sintering, showing a similar trend as the pure sample. From Untreated SrO 42.5% 7 results shown in Fig. 9,itisdifficult to make a strong recommendation SrO heat treated at 1400 °C 0.6% on whether or inhibitor addition helps the overall energy density or not. SrO heat treated at 1500 °C Test 1 2.2% SrO heat treated at 1525 °C 7.5% However, one sample (25 wt% Sr3(PO4)2) was found to be appeared to SrO heat treated at 1500 °C Test 1 2.2% 8 actually increase in energy density as cycles progressed. The energy 50 wt% CaSO4 19.5% density of the sample with 25 wt% Sr3(PO4)2 shows a gradual decrease 50 wt% Sr3(PO4)2 11.1% during the first two cycles and then increases in the subsequent cycles. SrO heat treated at 1500 °C Test 1 2.2% 9 This causes it to overcome the energy density of 50 wt% at cycle 7. The 50 wt% Sr3(PO4)2 11.1% 25 wt% Sr (PO ) 1.6% increase in energy density may be due to mass transfer limitations of 3 4 2 15 wt% Sr3(PO4)2 2.3% carbon dioxide, and further testing is needed to be done to see if the 25 wt% Sr3(PO4)2 10 cycles 1.6% 10 increase in energy density is consistent. It could also be due to hydra- 25 wt% Sr3(PO4)2 30 cycles 22.7% tion effects which are discussed later in this paper. SrO heat treated at 1500 °C Test 1 2.2% 11 SrO heat treated at 1500 °C Test 2 1.2% SrO heat treated at 1500 °C Test 3 26.0% 3.6. Cycleability SrO heat treated at 1500 °C Test 4 27.1%

The 25 wt% Sr3(PO4)2 sample was tested for 30 cycles to see if the sample remains stable for longer periods. Fig. 10 shows the results from sample as it becomes hydrated due to its hygroscopic nature, so further the 30 cycle test. investigation was done. For the diluted sample, the energy density starts at 1450 ± 0.05 kJ/kg for the 30 cycle test, greater than the 10 cycle test 3.7. Eﬀect of moisture absorption by 1000 kJ/kg. The energy density is not consistent between the 30 cycles as the lowest energy density is 400 ± 0.05 kJ/kg, much lower Following the 30 cycle test, it is believed that as the sample absorbs than the starting point. The trend for the 30 cycle test resembles the moisture from the atmosphere, the particles change in size. This was trend seen by the untreated pure SrO sample in Fig. 7. The diﬀerence tested by running replicates of the 1500 °C heat treated sample. Two between trends may be attributed to a change in particle size in the more tests were conducted from the sample that came from the same

Fig. 12. Distribution of particle sizes for a sample of SrO that has been stored for one month after heat treatment and initially being sieved to 53–75 μm. Distribution of new particle sizes are from 25 to 53, 53 to 75, and 75 to 106 μm.

25 – 53–75 75 – 106

91 E. Bagherisereshki et al. Solar Energy 160 (2018) 85–93 batch as the ﬁrst test, but has been stored for one month. Another test was 25 wt%. The percent hydration of the sample prior to testing has an was run from a fresh sample that was heat treated within 24 h of eﬀect on particle size and sintering. When samples are less hydrated, testing. Results are found in Fig. 11. they are more likely to be stable compared to highly hydrated samples.

Two trends are seen in Fig. 11: 1) energy density quickly decreases Sr3(PO4)2 in conjunction with a low moisture content shows inhibition with cycles and 2) energy density increases during the first two to three of sintering. Pretreating SrO at higher temperatures increased energy cycles and then stabilizes. The former is seen with samples that are density up to 1500 °C. Temperatures above that cause samples to ex- tested a month after heat treatment and the latter is seen with samples perience sintering. As the importance and benefits of storing thermal that are tested within 24 h of heat treatment. When a sample is stored energy are realized, this research effort may pave the way to a new type for prolonged periods of time, it absorbs moisture from the air and of efficient and useful thermochemical energy storage system. becomes strontium hydroxide or a hydrate. A sample was re-sieved after being stored for a month and it was Acknowledgements found that the particle sizes changed. The sample was originally sieved and stored to be between 53 and 75 μm. When it was re-sieved, the We would like acknowledge the gracious support of funding from particles ranged from 25 to 106 μm, as seen in Fig. 12. the U.S. Department of Energy SunShot Initiative (Award DE- The amount of moisture absorbed by the SrO can be determined EE0006534) and the Pete and Rosalie Johnson Summer Internship from the initial dehydration step during tests (Fig. 12). The percent Program. hydration for the four tests was determined by the following equation: Δm Appendix A. Supplementary material %Initial Hydration =×100% mi (7) Fixed bed studies to clarify lag in carbonation response, inhibitor From Fig. 13, tests 3 and 4 are shown to have been more hydrated studies in TGA/DSC, and XRD characterizations are included as than tests 1 and 2. The samples used for tests 1 and 2 were 2.2 and 1.2% Supplementary Information. Supplementary data associated with this initially hydrated, respectively. The samples used for tests 3 and 4 were article can be found, in the online version, at http://dx.doi.org/10. 26 and 27% initially hydrated, respectively. 1016/j.solener.2017.11.073. At high percentages of hydration (above 20%), it is hypothesized that the particle size of SrO changes, allowing it to initially be more References reactive yet sinters after multiple cycles. Hydration was reevaluated for previous tests to confirm results. Summary of the results can be found in Arlt, W., Wasserscheid, P., 2011. Storing and transporting energy, comprises e.g. using Table 2. one-component system of A and B in a chemical reaction under heat absorption and The only tests where there were large discrepancies between per- removal of carbon dioxide to form an energy-rich compound B. Barin, I., 2005. Thermochemical data of pure substances. VCH. cent hydration is the 30 cycle test. The sample was 20% more hydrated Bhatia, S.K., Perlmutter, D.D., 1983. Effect of the product layer on the kinetics of the CO2- than the sample used for the 10 cycle test. This test will be repeated lime reaction. AIChE J. 29, 79–86. with samples that are low in moisture to ensure accuracy. All other Charsley, E.L., Earnest, C.M., Gallagher, P.K., Richardson, M.J., 1993. Preliminary round- robin studies on the ICTAC certified reference materials for DTA - Barium carbonate comparative results are validated. and strontium carbonate. J. Therm. Anal. 40, 1415–1422. Results where energy density showed a slight increase over cycles Dou, B., Song, Y., Liu, Y., Feng, C., 2010. High temperature CO2 capture using calcium fi – were all low in moisture content (< 2%). The increase of energy den- oxide sorbent in a xed-bed reactor. J. Hazard. Mater. 183, 759 765. Edwards, S.E.B., Materić, V., 2012. Calcium looping in solar power generation plants. 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