Investigation Into Sro/Srco3 for High Temperature Thermochemical Energy Storage
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Solar Energy 160 (2018) 85–93 Contents lists available at ScienceDirect Solar Energy journal homepage: www.elsevier.com/locate/solener 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 re- versible 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.