Screening of thermochemical systems based on solid-gas reversible reactions for high temperature solar thermal energy storage

Renewable and Sustainable Energy Reviews 64 (2016) 703–715

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Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser

Screening of thermochemical systems based on solid-gas reversible reactions for high temperature solar thermal energy storage Laurie André, Stéphane Abanades n, Gilles Flamant Processes, Materials, and Solar Energy Laboratory, PROMES-CNRS, 7 Rue du Four Solaire, 66120 Font-Romeu, France

art ic l e i nf o

a b s t r a c t

Article history: Received 18 August 2015 Received in revised form 20 June 2016 Accepted 25 June 2016

A viable way to manage the inherently intermittent availability of solar energy in concentrated solar power plants is to store solar energy during on-sun hours to be able to use it later during off-sun hours, enabling on-demand electricity delivery. Thermochemical heat storage systems present some noteworthy advantages when compared with latent and sensible heat storage, namely (i) high energy storage density because the storage capacity by unit of mass or volume corresponding to the reaction enthalpy is generally high, (ii) heat storage at room temperature and long term energy storage because the products can be cooled and stored at room temperature without energy losses as heat can be stored indefinitely in chemical bonds, (iii) facility of transport because solid materials can be transferred over long distances, (iv) constant restitution temperature providing constant heat source because exothermic reactions are carried out at sufficiently high temperatures to generate electricity in constant conditions and therefore to produce a constant power. This paper presents an overview of the different potential thermochemical systems based on reversible solid-gas reactions operating at high temperatures and a screening of suitable materials that are interesting candidates in the 400–1200 °C range for thermochemical heat storage in concentrated solar power systems. The most promising materials belonging to the metal oxides, hydroxides, and carbonates solid-gas systems are selected for experimental validation and further investigations. & 2016 Elsevier Ltd. All rights reserved.

Keywords: Thermal energy storage Thermochemical heat storage Concentrated solar power Reversible reactions Solid-gas systems

Contents 1. 2. 3.

4.

n

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 704 Thermochemical heat storage systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 704 Potential candidates for thermochemical heat storage based on solid-gas reversible reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 705 3.1. Metal sulfates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 705 3.2. Metal carbonates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 706 3.3. Metal hydroxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 707 3.4. Pure metal oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 708 3.4.1. Cobalt oxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 708 3.4.2. Manganese oxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 708 3.4.3. Barium oxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 708 3.4.4. Copper oxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 709 3.5. Mixed oxide and perovskite systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 709 3.5.1. Mixed oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 709 3.5.2. Perovskites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 709 Selection of candidate systems for TES application and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 710 4.1. Selection based on defined required criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 710 4.2. Determination of transition temperatures and experimental validation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 710 4.3. Results and discussion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 711 4.4. Overview of future research trends in thermochemical energy storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 712

Corresponding author. E-mail address: [email protected] (S. Abanades).

http://dx.doi.org/10.1016/j.rser.2016.06.043 1364-0321/& 2016 Elsevier Ltd. All rights reserved.

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5. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 713 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 714 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 714

1. Introduction Most of the world energy sources come from fossil fuels. However, the use of fossil fuels in combustion processes produces greenhouse gases such as carbon dioxide in quantities that cannot be consumed or balanced naturally. In addition, the natural reserves of fossil fuels are gradually being depleted. Consequently, it has become necessary to search for new renewable pathways to produce energy. To address this matter, several options have been exploited, such as hydraulic energy, wind energy, waste-to-energy, solar energy and many others. Concentrated solar power (CSP) associated to thermal energy storage (TES) can be used as an environmentally friendly way to produce electricity. This paper focuses on thermochemical solid-gas systems applied to solar energy conversion and storage, and thus far, many systems have been studied in order to fulfill nowadays requirements for sustainable production [1–5]. In fact, solar energy represents an abundant renewable source of energy and technologies using solar light and/or heat, for example solar thermal energy or photovoltaics, have been developed. Moreover, a more efficient way to use solar energy is to store some of this energy to use it during off-sun hours and then be able to keep a constant production for industrial processes. Thermal energy storage is a key feature of CSP plants to match the intermittent energy source supply with the variable electricity demand, improving energy generation dispatchability. Three main routes have been explored in this optic and they are latent heat storage, sensible heat storage and thermochemical heat storage. Latent heat storage and sensible heat storage are thermophysical processes. Latent heat storage systems make use of phase change materials and mainly focus on a solid to liquid phase transition partly due to the additional difficulty of storing gas when working with liquid to gas phase change. Sensible heat storage generally relies on molten nitrate salts as heat storage media but also as heat transfer fluids in CSP applications with direct storage systems. If an indirect storage system is used, the energy is transferred from the heat transfer fluid to the storage medium through a heat exchanger. Sensible heat storage is being developed commercially, in contrast to latent and thermochemical heat storage. Though, one drawback of sensible heat storage is its requirement for high quantities of storage media, which can become costly when adapted to large scale systems. Thermochemical heat storage has shown specific advantages over the two other paths [4,6,7] (Table 1) and has been scarcely investigated for the past fifty years. For example, it shall be possible to reach higher energy storage densities and longer storage duration with the thermochemical approach. The main current challenge

consists in developing thermochemical cycles with high conversion rate, rapid reaction kinetics and materials performance stability upon cycling. This review paper presents a screening of potential candidates for thermochemical energy storage using thermochemical cycles based on reversible solid-gas reactions.

2. Thermochemical heat storage systems The concept of thermochemical cycles was first postulated in 1966 by Funk and Reinstorm [8], and can be used for thermochemical heat storage applications. Thermochemical heat storage systems present the advantages, over latent and sensible heat storage, to achieve higher energy storage densities thanks to high enthalpies of reaction, to show suitability for large-scale application, and to enable long storage duration and long-range transport at ambient temperature [9]. By the means of materials storage in solid state at room temperature, the transportation of the materials in which the energy is stored as chemical bonds is facilitated. Furthermore, the exothermic reactions release heat at a constant restitution temperature and allow a consistency in the generation of electricity through industrial processes which, combined with the storage of solar energy, may provide a constant power production. Heat is released at a constant temperature during reversible chemical reaction, which provides a constant heat source. A thermochemical cycle can thus be selected to match the required turbine operating temperature. Thermochemical cycles find applications in diverse fields such as the production of renewable fuels through water-splitting or CO2-splitting reactions to generate hydrogen or syngas [10] and thermal energy storage when the exploitation of the heat effects of the reactions for the storage of solar heat is favored over the production of chemical fuels. Thermochemical cycles are qualified as such because of the reagents being recycled. Such a cycle makes use of the heat produced in a solar receiver during on-sun operation to power an endothermic chemical reaction that should be completely reversible, thereby enabling the complete recovery of the thermal energy via the reverse reaction taking place during off-sun operation (Fig. 1). During the heat charge, a compound A(s) is heated up using CSP and decomposes into the products B(s) and C(g) through an endothermic reaction (Fig. 2). The B(s) product stores the thermal energy converted into chemical energy as chemical bonds. B(s) can be isolated from the gas C(g) in order to be stored indefinitely as a stable solid material with minimal environmental effect. It can be advantageously cooled and stored at room temperature, which simplifies the

Table 1 Comparison of TES types [4,6]. Storage type Gravimetric energy density (kWh kg
1 of material) Maturity Storage Period Transport Technology Disadvantage

Sensible  0.02–0.03 kWh kg

Latent
1

of material

Industrial scale Limited (thermal losses) Small distance Simple Significant heat loss over time (depending on the level of insulation). Large volume needed

 0.05–0.1 kWh kg

Thermochemical
1

of material

Pilot scale Limited (thermal losses) Small distance

 0.5–1 kWh kg
1 of reactant

Laboratory scale Theoretically unlimited Distance theoretically unlimited Medium Complex Significant heat loss over time (depending on the level High capital cost. Technically complex of insulation). Corrosivity of the material. Low heat conductivity

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– High enthalpy of reaction (enabling high storage density). – Suitable thermodynamics and fast enough kinetics of the reaction for both the charge and discharge steps. – Long-term stable and superior thermo-physical and mechanical properties. – High availability of the material at low cost to adapt for largescale applications. – Environmentally-friendly and non-toxicity of the material. – Absence of undesirable side reactions and by-products.

Fig. 1. Thermochemical solar heat storage concept considered for concentrated solar power.

A theoretical approach based on thermodynamic chemical equilibrium, using data obtained from HSC Chemistry software, offers a first overview of candidate materials for thermal energy storage (Fig. 3) by providing an estimation of the reaction enthalpy and turning temperature of the chemical systems. The turning temperature is defined as the temperature for which the standard Gibbs free enthalpy reaches zero. The main targeted materials are metal sulfates, carbonates, hydroxides, and oxides. Fig. 3 provides an overview of the different thermochemical systems for TES application as a function of their theoretical operating temperature and expected energy storage density. Above 400 °C, metal oxides (based on Mn, Fe, Co, Cu, Ba), alkaline earth metal carbonates and hydroxides (of Ca, Sr, Ba), but also metal sulfates that feature the highest energy storage densities, appear to be potentially attractive for solar energy storage, according to Fig. 3. A survey of the different candidate materials that are applicable to thermochemical heat storage has been conducted. 3.1. Metal sulfates Metal sulfates have been studied for application as heat storage media for solar energy [14]. Metal sulfates are potential candidates for thermochemical heat storage because they exhibit high reaction enthalpies and can be suitable for operating with concentrated solar energy since their operating temperatures are comprised between roughly 900 °C and 1400 °C (Table 2). Two possible reaction routes are considered and are presented below [15]:

Fig. 2. Operating principle of reversible thermochemical solid-gas reactions for TES application.

problems of materials for storage tanks, and there is no need for insulation, and no corrosive fluid or high temperature solid to be handled. Thermochemical systems can store energy during long period of time without energy losses. During the discharge, B(s) is again brought into contact with the gas C(g) at lower temperature to release the energy stored through an exothermic reaction. The reversible reactions can be used to transport thermal energy. Since the products can be stored at room temperature, they can be transferred to long distances. It has to be mentioned that when using thermochemical heat storage, the heat transfer fluid may also be a reactant (typically air for metal oxides, enabling open loop operation). The objectives of this review are the screening of the possible reversible solid-gas thermochemical reactions for high-temperature solar energy storage in concentrated solar power systems and the selection of the most suitable candidates with respect to relevant criteria.

3. Potential candidates for thermochemical heat storage based on solid-gas reversible reactions To be eligible as candidates and to be compatible with CSP plants, the materials have to fit a set of few criteria [6,11–13], for instance: – Complete reversibility of the reaction and cycling stability. – High reaction temperature (turning temperature in the range of ca. 400–1200 °C).

MSO4 þHeat2MOþ SO3(g)

(1)

MSO4 þHeat2MOþ SO2(g) þ½ O2(g)

(2)

According to the thermodynamic data summarized in Table 2, the decomposition of metal sulfates generating SO2(g) and O2(g) seems suited for solar energy storage thanks to high energy storage densities and temperatures of decomposition ranging from 875 °C to 1140 °C. Previous work has been done in the field of metal sulfates for thermal energy storage applications, thus decomposition as well as reverse reaction of metal sulfates have been investigated [15–19]. Tmar et al. [15] proposed reversible reactions employing sulfates as storage media for solar energy. The decomposition of sulfates of Al, Cu, Co, Fe, Mg, Ni, and Zn was studied. The kinetics and temperatures of metal sulfates decomposition were also studied by Tagawa and Saijo [20]. They mentioned the formation of intermediate compounds such as oxysulfates or oxides that are first obtained at a higher oxidation state and then reduced to obtain the final oxide during the reaction of decomposition. Such observation was also noticed in other researches. For example, Ibanez et al. [21] showed that the presence of an intermediate which can be formed during the decomposition of a metal sulfate has to be considered. They studied the thermal decomposition of pure ZnSO4 between 900 °C and 980 °C. It was concluded that the decomposition proceeds in two steps by first decomposing to an oxysulfate before decomposing to ZnO. Other reactions involving metal sulfates hydrates [22] for thermal energy storage applications, but at lower temperature, have been considered such as metal salts hydration/dehydration [23–25]. Recently, zeolite composites coated with an optimum percentage of

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Fig. 3. Survey of candidate materials for thermochemical heat storage: gravimetric energy storage density versus theoretical temperature. Table 2 Turning temperature and heat storage capacity of metal sulfates considering the possible generation of either SO3(g) or SO2(g) and O2(g). Chemical material

Temperature (°C) (SO3)

Reaction enthalpy (kJ/mol) (SO3)

Gravimetric storage density (kJ/kg) (SO3)

Temperature (°C) (SO2 þ ½O2)

Reaction Enthalpy (kJ/mol) (SO2 þ ½O2)

Gravimetric Storage density (kJ/kg) (SO2 þ½O2)

MgSO4/MgO MnSO4/MnO FeSO4/FeO CoSO4/CoO CuSO4/CuO ZnSO4/ZnO CdSO4/CdO NiSO4/NiO

1360 1215 1165 1130 930 1315 1270 920

248.703 257.04 247.502 230.177 202.312 187.634 249.442 321.37

2066.2 1702.2 1629.3 1485 1267.5 1162 1196 2076

1140 1065 1030 1005 875 1080 1095 995

348.766 357.827 346.318 329.564 300.373 285.514 354.504 222.755

2897.5 2369.7 2279.78 2126.3 1881.9 1768.2 1700.5 1439.4

15 wt% of MgSO4 have been investigated for thermochemical heat storage at lower temperatures, such as 150 °C [26] and 265 °C [27], using the enthalpy of reaction of the hydration/dehydration of the material. Although MgSO4 does not seem to be able to use its full sorption capabilities, it shows promising results for this application, with a storage capacity up to 1090 J/g [27] depending on the support. Yan et al. [28] studied the stability and the decomposition mechanisms of CaSO4 mixed with oxides such as SiO2, Al2O3 or Fe2O3 at high temperatures. They state that the decomposition rate of CaSO4 is increased with the addition of Fe2O3 and the range for the decomposition temperature is reduced, while the addition of SiO2 or Al2O3 lowers the initial decomposition temperature. These studies show the potential of using sulfates as thermochemical energy storage media. However, the use of sulfates introduces a critical issue of corrosion which has to be taken into account when choosing the materials for the storage system. For that reason, these compounds were not considered further in the experimental validation of the screened materials during this study. 3.2. Metal carbonates Metal carbonates have mainly been proposed and studied as sorbent materials in the context of CO2 capture. The capture and

storage of carbon dioxide has become of prime interest in order to reduce the concentration of greenhouse gases in the atmosphere and as such, literature data on the thermal behavior of sorbents (especially calcium oxide-based sorbents) during carbonation/calcination looping cycles is abundant. Metal carbonates are also potential candidates for thermochemical heat storage with some of them showing remarkable high reaction enthalpy and energy storage density [6,11,12,29,30]. Theoretical temperatures of calcination of carbonates and thermodynamic data are presented in Table 3 in order to offer a first estimation of their capacity when considering thermochemical heat storage application. The two-step thermochemical reactions considered for metal carbonates are as follow: MCO3 þHeat-MOþCO2(g)

(3)

MOþCO2(g)-MCO3 þ Heat

(4)

Regarding mitigation of CO2 emissions via CO2 capture, the attention directed at this environmental issue contributed in multiplying the researches on the thermal decomposition of carbonates, among which CaCO3, SrCO3, MgCO3 and BaCO3 present the highest energy densities [6,11,12,29,30]. The kinetics of the decomposition reaction of these carbonates has been studied [31] while the possible irreversibility of these decompositions has been mentioned

L. André et al. / Renewable and Sustainable Energy Reviews 64 (2016) 703–715

Table 3 Turning temperature and heat storage capacity of metal carbonates.

707

Table 4 Turning temperature and heat storage capacity of metal hydroxides.

Chemical material

Temperature (°C) Reaction enthalpy (kJ/mol)

Gravimetric energy density (kJ/kg)

Chemical material

Temperature (°C) Reaction Enthalpy (kJ/mol)

Gravimetric energy density (kJ/kg)

CaCO3/CaO SrCO3/SrO MgCO3/MgO BaCO3/BaO CdCO3/CdO ZnCO3/ZnO PbCO3/PbO

885 1220 300 1555 290 120 310

1656.8 1370.65 1174 836.6 560.1 544.9 314

Ca(OH)2/CaO Mg(OH)2/MgO Be(OH)2/BeO Mn(OH)2/MnO Sr(OH)2/SrO Ba(OH)2/BaO Ni(OH)2/NiO Zn(OH)2/ZnO Cd(OH)2/CdO

515 265 70 190 755 1005 70 55 125

1352 1333 1191 754 728.3 545.47 516 498.96 409.4

165.832 202.349 98.984 165.096 96.573 68.326 83.906

[32,33]. Feng et al. [34] performed a screening of candidates for CO2 capture such as carbon-based sorbents and hydrotalcite-like compounds, which operate at temperatures below 300 °C [35], and various metal oxides. They considered CaO to be the most interesting candidate for capturing CO2 at high temperature in terms of thermodynamic properties. However, it suffers from a well-known issue of loss-in-capacity, which means that its capacity for carbon capture decays dramatically when cycles of carbonation/calcination are repeated [34,36,37]. Strategies, which have been reviewed by Liu et al. [38], have been investigated in order to overcome the lossin-capacity issue. Recently, extensive research work has been conducted for enhancing the cyclic performance of CaO-based sorbents through either improving the performance of natural minerals, such as steam hydration and pretreatment or modification of CaO sorbents by some techniques such as doping and synthesis. Potential solutions to the loss-in-capacity problem can be identified: hydration during or after calcination is found to be effective in recovering the capacity of natural minerals and mixing with stabilizing agents can produce highly effective synthetic sorbents. Nikulshina et al. [10] used calcium hydroxide as CO2 sorbent to overcome the kinetic limitations of the reaction involving the CaCO3/CaO pair, though this implies a three step reaction path:

100.177 77.745 51.276 67.072 88.581 93.462 47.846 49.609 59.952

was also tested recently by Rhodes et al. [59]. The sintering of SrO over several carbonation steps was addressed by using 40 wt% of SrO supported on yttria-stabilized zirconia. This way, the material remained stable over 15 cycles, with energy storage density stabilized at approximately 1400760 MJ m
3. Degradation of the capacity of the material is however noticed after about 45 cycles. Magnesium carbonate has also attracted attention for the capture of CO2 and its reactivity has been studied [60,61] but for applications at lower temperature [62–64]. The effect of the calcination temperature on the synthesis of mesoporous MgO has been studied and lower calcination temperature was shown to favor larger specific surface area and to improve the capacity of the material to adsorb carbon dioxide [65]. Metal carbonates show potential to store and release heat via reversible reactions and they are thus considered to be worthy for further investigation in this study. 3.3. Metal hydroxides

Ca(OH)2 þCO2-CaCO3 þH2O

(5)

The dehydration reaction of metal hydroxides can lead to high heat storage efficiency and places these chemicals as candidates for thermochemical heat storage. The two-step thermochemical reactions considered for metal hydroxides are as follow:

CaCO3-CaO þCO2

(6)

M(OH)2 þ Heat-MO þH2O(g)

(8)

CaO þH2O-Ca(OH)2

(7)

MOþH2O(g)-M(OH)2 þHeat

(9)

Kotyczka-Moranska et al. [39] compared different methods to enhance CaO-based sorbents for the capture of CO2 and present the most important factors to take into account in order to influence the CO2 absorption capacity by CaO. Following the interest shown to calcium oxide for the capture of CO2, the influence of various parameters such as particle size, CO2 concentration, gas temperature, initial calcination temperature [40], presence of steam (H2O) which would be beneficial to the system [41], was investigated. The capacity to capture and release carbon dioxide was also investigated with various calcium-based sorbents or additives, to inhibit agglomeration and sintering of particles and thereby improve stability and durability of sorbents, such as CaZrO3 [42–44], Zr-stabilized CaO [45], Ca12Al14O33 [46–48], Ca-Al-CO3/TiO2 composite [49], CaTiO2/Nano-CaO [50], CaO-MgAl2O4 [51], CaO-SiO2 [52], La2O3 [53], Y2O3 [54], but also MgO whose presence lowers the CO2 capture capacity compared to pure calcium oxide [55,56] but also lowers the decline in CaO activity at 20–26 wt% MgO [40,57]. Besides, SrCO3 was tested for CO2 sorption by Miccio et al. (2015) [58] and combined with another sorbent, apatite. In their study, they investigate calcination/carbonation of commercial SrCO3 in Ar(50%)-CO2(50%) at 1100 °C and 1200 °C. The material shows a loss in regeneration over several cycles. They highlight the sintering of the material at 1200 °C by using SEM analysis. The potential of the SrCO3/SrO couple for thermochemical energy storage application

The theoretical turning temperatures and thermodynamic data are presented in Table 4. The temperatures of the reactions range from 70 °C to 1005 °C, though, a majority of the reaction temperatures are too low for high temperature CSP application (Table 4). Ca(OH)2/CaO system shows the highest enthalpy of reaction, followed by Mg(OH)2/MgO. The reversible reaction of dehydration/ rehydration of Ca(OH)2 has attracted attention, because it is a cheap material and it features a high enthalpy of reaction, and therefore it has already been studied for potential application in energy storage [66,67]. Schmidt et al. [68] performed a test with large quantities of Ca(OH)2 and achieved reversible conversion of 77% of the material. Azpiazu et al. [69] achieved up to 20 cycles before the material loses effectiveness due to carbonation. Schaube et al. [70] studied the thermodynamics and kinetics of the reversible reaction of dehydration/rehydration of Ca(OH)2 at lab scale and observed cycling stability over 100 cycles. They obtained full conversion with an enthalpy of reaction of 104.4 kJ/mol at an equilibrium temperature of 505 °C for an H2O partial pressure of 1 bar. The effect of additives on the reaction rate and the decomposition temperature of Ca(OH)2 was discovered early and studied for thermochemical energy storage applications [71]. Pardo et al. [72] reached promising energy densities by combining Al2O3 powder with Ca(OH)2 powder and experimented this process in a fluidized bed reactor. Yan and Zhao [73] worked on

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the study of the micro-mechanism of chemical heat storage using Ca(OH)2/CaO as such and doped with Li or Mg, and showed that the time and temperature needed for the dehydration reaction of Ca(OH)2/CaO can be lowered with Li doping. Magnesium hydroxide also presents a high enthalpy of reaction. Its thermal decomposition mechanism has been investigated by L’vov et al. [74]. Yukitaka et al. [75] stated that this material could not be dehydrated and store heat in the range of 200–300 °C. To address this issue, they worked on mixed metal hydroxides, such as magnesium hydroxide and nickel hydroxide, which were capable of storing heat in a packed bed reactor in the same range of temperature. Shkatulov et al. [76] recently worked on doped magnesium hydroxide, with best results using LiNO3 doping, and calcium hydroxide, with best results using KNO3 doping, to tune their reaction temperature and enhance their kinetics. Zamengo et al. [77,78] worked on a Mg(OH)2-expanded graphite composite and achieved a better thermal conductivity and an enhancement of the reaction rate than when using pure Mg(OH)2. Given the proven interest of hydroxides for low temperature TES, their study needs to be extended to appraise their applicability to high temperature CSP plants. 3.4. Pure metal oxides Various metal oxides have been studied for thermochemical heat storage applications [79] due to their high gravimetric storage density, which is important for lowering the necessary amount of reactant involved in large-scale process. The two-step thermochemical redox reactions considered for metal oxides are as follow: MO(ox) þHeat-MO(red) þ½ O2 (g)

(10)

MO(red) þ½ O2 (g)-MO(ox) þHeat

(11)

They also possess a wide range of operating temperatures starting from 145 °C until up to 1700 °C (Table 5), which can help adapting the process to several kinds of CSP technologies. Metal oxides with higher transition temperatures are excluded for TES (e.g., TiO2, Fe3O4, NiO, ZnO, SnO2, Nb2O5, Ga2O3, MoO3, WO3…). Thermochemical heat storage based on redox reactions using metal oxides also presents the advantage of directly using air as heat transfer fluid. Without the necessity to store the heat transfer fluid, it becomes possible to work with an open loop system, in contrast to metal sulfates or carbonates that require a closed-loop operation (using sulfur/carbon dioxide as heat transfer fluid) and hydroxides that require steam generation. Wong et al. [79] investigated the potential of several metal oxides to be used for thermochemical heat storage and mentioned that only a few have the required properties: Co3O4, BaO2, Mn2O3, CuO, Fe2O3, Mn3O4 and V2O5, with the precision that cobalt oxide shows the best re-oxidation kinetics. 3.4.1. Cobalt oxide Cobalt oxide is one of the most promising and studied materials for thermochemical heat storage even though it has drawbacks such as being expensive and potentially carcinogenic. Because of its promising TES potential, cobalt oxide has been tested in reactors, such as directly irradiated rotary kiln and powder-coated honeycomb reactors [80,81]. Neises et al. [80] tested the reduction and reoxidation of cobalt oxide powder for 30 cycles in a solar-heated rotary kiln and in air atmosphere at about 900 °C, with the reduction starting at 820 °C. A storage capacity of about 400 kJ/kg was achieved per cycle with only half of the material being reduced due to insufficient mixing. Agrafiotis et al. [82,83] achieved about 30 redox cycles with rigid, porous, foams entirely made of Co3O4, with the foam retaining its integrity and stoichiometric redox

Table 5 Turning temperature and heat storage capacity of metal oxides. Chemical material

Temperature (°C) Reaction Enthalpy (kJ/mol)

Gravimetric energy density (kJ/kg)

Rh2O3/Rh2O V2O5/V2O4 Co3O4/CoO Mn3O4/MnO CuO/Cu2O Li2O2/Li2O Fe2O3/Fe3O4 BaO2/BaO MgO2/MgO Cr5O12/Cr2O3 PtO2/PtO PbO2/PbO Sb2O5/SbO2 Mn2O3/Mn3O4 UO3/U3O8

970 1710 935 1700 1115 145 1360 880 205 105 420 295 325 915 670

982.1 970.9 816.1 850.6 810.2 745.9 485.6 432.6 380.6 279.1 276.5 230.9 210.4 190.1 123

249.276 176.594 196.532 194.63 64.446 34.225 232.613 73.258 21.432 126.17 62.802 55.234 68.064 90.038 35.232

performance after the cycles, as opposed to pellets which exhibit cracks after a few cycles. The reduction temperature for the Co3O4/CoO pair is confirmed to be about 885–905 °C. They also state that, when using foams or pellets made entirely of Co3O4, all the amount of oxide used is exploited for the thermochemical reaction. Karagiannakis et al. [84] demonstrated that flow-through pellets made of cobalt oxide show better reaction kinetics than powder of cobalt oxide due to better heat transfer characteristics of the structured material. They observed very good cycling stability upon a few cycles and noticed the beginning of the reduction step at 930 °C and the beginning of the oxidation upon cooling at 880 °C with a gravimetric energy storage density of 495 kJ/kg for the powdered cobalt oxide and 515 kJ/kg for the pellet. 3.4.2. Manganese oxide The same experiment was conducted with manganese oxide as pellets and the same improvement was observed [84]. Considering the Mn2O3/Mn3O4 couple, the reduction step of manganese oxide was observed in the range of 920–1000 °C and the notably slow reoxidation was observed in the range of 850–500 °C with a gravimetric energy storage density of 110 kJ/kg. It is specified that the reoxidation happens in two steps, the first one being during the cooling and in between 700–500 °C and the second one being during the re-heating and in the range of 500–850 °C [84]. As Mn2O3/Mn3O4 is a promising metal oxide redox pair for thermochemical heat storage, Carillo et al. [85] tested the durability of this material over 30 oxidation-reduction cycles performed by thermogravimetry. They enlightened the necessity to pay attention to the initial particle size of these oxides since it influences the kinetics and the thermodynamics of the reaction. Especially, smaller particles would contribute in lowering the oxidation temperature, but it would also hinder the diffusion of O2 by favoring the sintering of the material. 3.4.3. Barium oxide The BaO2/BaO system was studied in order to evaluate its potential for TES. Bowrey and Jutsen [86] performed 5 oxidation/ reduction cycles with BaO powder with no degradation of the material, and recorded a maximum conversion of 93% for the oxidation. The maximum heating temperature used for this reaction was 850 °C and a low heating rate (8 °C/min) was used in order to avoid the crusting of the surface. This system was also investigated by Fahim and Ford [87], emitting the possibility of BaO2/BaO to be an interesting candidate for chemical energy storage. However, more work needs to be done to improve the reversibility of the system.

L. André et al. / Renewable and Sustainable Energy Reviews 64 (2016) 703–715

3.4.4. Copper oxide The CuO/Cu2O thermochemical redox cycle was examined by Hänchen et al. [88] for the separation of oxygen from inert gases. The range of temperature for the reduction of CuO to Cu2O is situated between 1030 °C and 1134 °C, suggesting that this system can be suitable for applications associated with advanced CSP concept. When studying the reaction kinetics of CuO/Cu2O, it was observed that the reduction step is faster than the oxidation step. The issue of material sintering during the reduction step at high temperatures is also mentioned. Alonso et al. [89] also studied the CuO/Cu2O pair, in air and in argon, using a rotary solar reactor. The reduction conversion was 80% in argon and 40% in air. For the oxidation in air, only 9% conversion was achieved. It is also mentioned that the temperature at which the oxygen is released is 300 °C higher in air (800 °C) than in argon (500 °C). They also performed several cycles and noticed a loss in the fraction of active material during each new cycle. Due to their ability to operate in open loop with air as working fluid, multivalent metal oxide redox cycles represent an attractive option with high potential for TES. 3.5. Mixed oxide and perovskite systems 3.5.1. Mixed oxides More complex systems are being investigated in order to enhance the properties or overcome the drawbacks of already interesting candidates for thermochemical heat storage. Generally, secondary oxide incorporation can be employed to increase anion vacancies concentration and enhance mass transfer. Differences in oxidation state and atomic size result in charge imbalances and lattice strain. The increased lattice vacancies density leads to higher oxygen mass transfer through the lattice. Further than doping compounds, research progresses in mixed metal oxides [90,91] and perovskites [92]. Basically, the long term redox performance of materials can be improved by inhibiting grain growth with secondary oxide additions. The redox cycle using hercynite was used for the production of H2 or CO but it was studied by Ehrhart et al. [91] for potential thermochemical heat storage application. The reaction proceeds as follow: (Co,Ni)Fe2O4 þ3 Al2O3 þHeat2(Co,Ni)Al2O4 þ2 FeAl2O4 þ ½ O2. Motohashi et al. [93] studied the capacity of layered cobalt oxide system: REBaCo4O7 þ δ, where RE can be Dy, Y, Yb or Lu, to store and release O2, though this reaction occurs at temperatures below 400 °C, and in 1 atm O2. In addition, they mentioned to take notice of the O2 intake/release behavior of the material depending on the chosen rare earth metal used in the system. Ceria-zirconia mixed oxides were also investigated as materials for oxygen storage. Kim et al. [94] prepared ceria-zirconia mixed oxides through hydrothermal synthesis in supercritical water and proposed that this method could enhance the thermal stability and oxygen storage capacity of the material, due to its morphology, compared to the mixed oxides prepared by the coprecipitation method. Tin oxide doped with alkaline earth metal was investigated as a potential oxygen storage material [95]. Among several alkaline earth metals tested, the synthesized Ba-doped SnO2 hollow nanospheres were tested at 600 °C and presented a high oxygen storage capacity and a good thermal stability. Carillo et al. [90] compared the heat storage capacity of pure materials such as Co3O4 and Mn2O3 to that of mixed oxides such as Mn3
xCoxO4. They concluded that mixed metal oxides either show equal or lower performances than pure metal oxides and Co3O4 is again emphasized as a potential material for thermochemical heat storage. Block et al. [96] investigated the effect of iron oxide mixed with cobalt oxide for high temperature TES application and show that both addition of iron oxide to cobalt oxide or addition of cobalt oxide to iron oxide reduces the enthalpies of reaction compared to those of the pure oxides. However, they estimated that 10 mol% iron oxide doped cobalt oxide still shows high enthalpy of reaction and possesses higher

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reduction/oxidation reversibility than pure cobalt oxide. The reversibility of the redox reactions of cobalt oxide based flow-through pellets, with the addition of alumina, ceria, iron oxide, manganese oxide, silicon carbide or zirconia, was also tested by Pagkoura et al. [97] in air flow and in the range of 800–1000 °C. While 25 wt% ceria doping improved the reaction kinetics of cobalt oxide, it did not improve the low thermo-mechanical stability of the structured material. Cobalt oxide doped with 10–20 wt% of alumina or iron oxide presented a good thermo-mechanical stability for 10 redox cycles while maintaining the high performances of cobalt oxide for thermochemical heat storage. Carillo et al. [98] further studied the performances of manganese oxide for thermochemical energy storage and sought to improve it with the addition of iron oxide to the material. The incorporation of iron oxide did not allow avoiding the sintering encountered with manganese oxide, however it increased the heat storage density of the material and it stabilized and enhanced its oxidation rate over 30 redox cycles. The material presenting the fastest and the most stable oxidation reactions in their study was Mn2O3 doped with 20 mol% Fe. Carillo et al. [99] also considered FeCu co-doping in manganese oxide to narrow the thermal hysteresis loop for the manganese oxide system (temperature gap of ca. 200 °C between the reduction and oxidation temperatures). They showed that the reduction temperature can be diminished by Cu incorporation whereas oxidation temperature is increased through Fe doping. The difference between reduction and oxidation temperature was decreased from 225 °C, for the un-doped Mn oxide, up to 81 °C for the sample containing 20 mol% Fe and 5 mol% Cu. However, the addition of 5% Cu lowered the reduction and oxidation rates probably due to the formation of segregated mixed Mn–Cu spinel. 3.5.2. Perovskites Perovskites structured metal oxides present a high oxygen mobility and the ability to release and incorporate oxygen during cyclic endothermic reduction and exothermic oxidation. Their oxidation/ reduction reaction kinetics can be improved by A-site alkaline earth metal or B-site transition metal cations substitutions in the structure in order to create oxygen vacancies, which results in more accessible oxygen absorption sites for the re-oxidation of the material. Further work on perovskites may lead to potential application to TES [100]. Masunaga et al. [101] investigated, as oxygen sorbents for the pressure swing adsorption process, Fe-based oxides with double perovskite composition and found that Sr0.5Ba0.5FeO3
δ shows the highest oxygen sorption capacity at 450 °C. Thermogravimetric analysis (TGA) measurements showed that these materials have a good oxygen sorption/desorption cycle stability. The amount of sorbed oxygen at 500 °C increased with the increase of the tolerance factor for these Fe-based structures. Shen et al. [102] studied the SrCo1
xFexO3
δ perovskite as oxygen carrier and they stated that the Co-doping improves the oxygen desorption of the material. Compared to two other materials, La0.1Sr0.9Co0.5Fe0.5O3
δ and Sr0.5Ca0.5Co0.5Fe0.5O3
δ, SrCo1
xFexO3
δ showed a better stability and regeneration capacity over sorption/desorption cycles, switching from air and CO2 atmosphere for adsorption and desorption, at 750 °C. Lin et al. [92] worked on Sr0.5Ca0.5Co0.5Fe0.5O3 as CO2 sorbent at high temperature. They observed a different behavior of the material towards the sorption of carbon dioxide below and above 750 °C. Below 750 °C, the sorption kinetic rate is dependent on the temperature and CO2 pressure, while at temperatures higher than 750 °C the equilibrium CO2 sorption capacity decreases with the rising temperature and the decreasing of CO2 pressure. Gupta et al. [103] studied the CO2 capture capacity of nano-CaFeO2.5 for capturing CO2 from hot exhaust gases. They noticed no decay in CO2 capture capacity over 30 fast carbonation/calcination cycles, in CO2 atmosphere at 500 °C. The material can start being regenerated in air at 650 °C and begins recapturing CO2 at 450 °C.

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Recently, LaxSr1-xCoyMn1-yO3-δ and LaxSr1
xCoyFe1
yO3
δ were studied by Babiniec et al. [104] as thermochemical energy storage media, with optimal redox activity obtained for a La content of x¼0.3. Galinsky et al. [105] considered Ca1
xSrxMnO3 and Ca1
xBaxMnO3 oxygen carriers. While it seems that the incorporation of Ba in CaMnO3 was ineffective, the Sr-doped CaMnO3 oxygen carrier, on the other hand, was able to sustain over 100 isothermal redox cycles at 850 °C. CaBxMn1
xO3
δ (B¼ Al, Ti) was recently studied by Babiniec et al. [106] who showed the highest reaction enthalpy capacity for perovskites reported to date for thermochemical energy storage application, with a reaction enthalpy of 390 kJ/kg. In addition, a series of Ba and Sr containing perovskites with Fe, Co and Mn incorporated on the B-site was investigated by Zhang et al. [107] to identify the most attractive ones regarding oxygen storage capacity and reaction reversibility. The results revealed that Ba induced systems show enhanced O2 exchange capacity (in 20% O2 up to 1050 °C) compared with Sr induced systems, and Co-based perovskites show the highest O2 exchange capacity and reaction enthalpies for solar thermochemical energy storage. Mixed oxides and perovskites with increased oxygen exchange capabilities thus constitute a significant alternative for high-temperature TES applications. Research is needed in this field to propose optimized thermochemical energy storage systems adapted to CSP.

material safety data sheets of the products. PbO2, PbCO3, CdCO3, Be(OH)2 and UO3 are not further considered because of toxicity issues. Sulphates are also not included in this selection due to the toxicity and corrosiveness of gaseous sulphur oxides released during decomposition. Concerning Cr2O12, Li2O2, MgO2 and Sb2O5, the transition temperatures are too low for efficient heat recovery (Table 5). Regarding hydroxides and carbonates, the following compounds: MgCO3, ZnCO3, Mg(OH)2, Mn(OH)2, Ni(OH)2, Zn(OH)2 and Cd(OH)2, are also eliminated due to the low processing temperatures (Tables 3 and 4). For PtO2 and Rh2O3, raw materials and processing costs are too high for being viable in large-scale applications. The remaining chemical candidates are: CaCO3, SrCO3, BaCO3, Ca(OH)2, Sr(OH)2, Ba(OH)2, Co3O4, Fe2O3, CuO, V2O5, BaO2, Mn2O3 and Mn3O4. Among this last selection, Co3O4 and Sr(OH)2 are more expensive than the others and Co3O4 is suspected to be carcinogenic but they are kept in the selection. Finally, the melting point of V2O5 (690 °C) is low and well below the transition temperature, which likely makes this material impractical for performance repeatability during reversible redox reactions. Barium peroxide (BaO2) also features a low melting point (450 °C), which may be unsuitable for the re-oxidation of BaO.

4. Selection of candidate systems for TES application and discussion

For the selected candidates, the transition temperatures estimated from thermodynamics were validated via experimental measurements in TGA. The Ellingham diagram for considered species was used as a first assessment of the required conditions for complete reactions. In the case of metal oxides, the use of air as heat transfer fluid is intended. Thus, the RT.ln(PO2) line corresponding to a reacting gas (O2) with partial pressure of 20% is also included in the diagram for determining the transition temperatures in the representative atmosphere composition (the same partial pressures for steam and CO2 are considered for comparison). The transition temperature decreases when decreasing the partial pressure of the reacting gas (Figs. 4–6). The theoretical transition temperatures obtained in the corresponding atmospheres are given in Table 6. Note that the phase change occurring below the transition temperature for vanadium and barium oxides is inconvenient for TES applications. Besides, the theoretical transition temperature of Mn3O4/MnO is too high

Several candidates are selected among the stoichiometric chemical materials presented previously according to their high gravimetric energy storage density and to the temperature the reaction requires. Mixed oxides or non-stoichiometric redox reactions are not considered in this selection. 4.1. Selection based on defined required criteria To be suitable for high temperature CSP application, the minimum threshold decomposition temperature being considered is 400 °C (thus compatible with parabolic trough or solar tower systems). The selected potential candidates have yet to meet criteria including low cost and non-toxicity. Toxicity towards the environment is first considered and the information is based on the

4.2. Determination of transition temperatures and experimental validation

Fig. 4. Ellingham diagram for selected metal carbonates.

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Fig. 5. Ellingham diagram for selected metal hydroxides.

for practical interest in heat storage for CSP. This redox pair will not further be taken into account as candidate in this selection. For the retained compounds, an experimental study was conducted (Tables 7 and 8) to determine the transition temperatures and the reaction reversibility as a function of the gaseous atmosphere (20% O2 for oxides, 20% CO2 for carbonates balanced with inert gas). Thermogravimetric analysis (TGA) of metal compounds was carried out in a Netzsch STA 449 F3 System. Non-isothermal runs with nominal heating and cooling rates of 20 °C/min and 10 °C/min were used for reactions characterization (using 10 sccm of O2 or CO2 flow in 40 sccm Ar as the carrier gas). While complete reduction usually takes place whichever the compound, re-oxidation kinetics are extremely slow for some compounds and strategies for improving the reactions reversibility thus need to be proposed and tested (e.g., doping, stabilization with inert support, controlled synthesis for tailored morphology,…). During heating, the sample mass loss corresponds to the thermal reduction step releasing O2 (or calcination releasing CO2). Upon cooling, the material is re-oxidized back to the initial stoichiometry and regains some of its lost weight, which characterizes the reaction reversibility. The temperatures of reduction and re-oxidation are usually different as the result of a hysteresis phenomenon due to kinetic limitations (Fig. 7). Heat flow absorbed and released by the sample, as monitored by combined DSC, can also be evidenced during the cyclic process. The endothermic and exothermic peaks occurring during weight changes are the signals illustrating the capacity for heat storage and recovery during TES. Re-oxidation, as measured by weight increase, is more sensitive to sample cooling rate than reduction because of reaction kinetic limitations. This is because the solid/gas reaction is controlled mainly by mass transfer (oxygen diffusion at the oxide surface and through the oxide lattice). The use of fast cooling rates thus reduces the amount of time the sample is maintained at suitable temperatures for fast kinetics, which leads to lowered re-oxidation yield. Reaction kinetics is highly dependent on the materials thermal stability and sintering that reduces the available surface area for the solid/gas reaction. 4.3. Results and discussion TGA results show rapid full thermal reduction or decomposition upon heating for all the compounds. Complete weight recovery is observed for CoO oxidation to Co3O4 (Fig. 7), in agreement with

previous studies [9,79]. In contrast, Mn3O4 (Fig. 8a and b) and Cu2O (Fig. 9), do not undergo complete re-oxidation. The reaction temperatures obtained experimentally for each metal oxide are listed in Table 7. It can be noticed that the synthesis method greatly affects the reoxidation yield of manganese oxide powder, as previously observed by Carillo et al. [85] who pointed out the effect of the morphology on the reactivity of Mn2O3/Mn3O4. The commercial powder does not reoxidize significantly (Fig. 8a), while the synthesized manganese oxide power (Fig. 8b) is able to regain O2 even if the re-oxidation is not total (total reduction and 93% re-oxidation yield). The influence of the oxidation temperature is also highlighted in the case of Cu2O (Fig. 9), the highest re-oxidation yield being observed at 950 °C. The two last oxidation reactions at 950 °C in Fig. 9 show the repeatability of oxidation/reduction cycles of copper oxide even though the re-oxidation was not total with respect to the first reduction (total reduction and 88% re-oxidation yield). During previous experiments with copper oxide, Hänchen et al. [88] noted a faster reduction step of the material compared to its oxidation step, also affected by the concentration of O2. In air, the temperatures of reduction and oxidation of Cu2O powder in TGA were found to be 1075 °C and 1000 °C respectively, and Cu2O re-oxidation to CuO reached 92% conversion for one cycle. During tests in packed bed, material sintering was evidenced, which led to a loss in capacity. Recently, Alonso et al. [89] tested the suitability of the CuO/Cu2O couple for thermochemical energy storage in a rotary kiln reactor. They noticed a reduction in air taking place at higher temperature (300 °C difference) than a reduction in Ar. Reduction was not complete (about 80% in argon and 40% in air) and re-oxidation was also low, reaching about 9%. These results show the impact of the experimental conditions, sintering effect and morphology of the material on its reactivity and stability. Fe2O3 was successfully reduced to Fe3O4 with a reaction start at 1361 °C in a 20% O2 atmosphere, and at 1145 °C in Ar. Even though the re-oxidation of Fe3O4 was not complete, it reached a conversion of 92%, which shows the potential of iron oxide to be used as thermochemical energy storage material. Fe2O3/Fe3O4 redox pair was previously studied in a three-step cycle for hydrogen production, presenting a reduction temperature of 1300 °C under inert atmosphere at atmospheric pressure [108]. The kinetics of Fe3O4 oxidation in air, after reduction, were studied by Monazam et al. [109] between 750 °C and 900 °C, showing a higher conversion rate as temperature raised, reaching about 80% at 900 °C.

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Fig. 6. Ellingham diagram for selected metal oxides.

Table 6 Theoretical transition temperature of selected metal compounds in 20% of reacting gas. Category

Chemicals

Transition temperature (°C)

Oxides

BaO2/BaO Co3O4/Co2O3 Mn2O3/Mn3O4 CuO/Cu2O Fe2O3/Fe3O4 Mn3O4/MnO CaCO3/CaO SrCO3/SrO BaCO3/BaO Ca(OH)2/CaO Sr(OH)2/SrO Ba(OH)2/BaO

775 790 820 1025 1290 1575 785 1090 1365 445 625 820

Carbonates

Hydroxides

Fig. 7. Co3O4/CoO reduction/oxidation reaction hysteresis.

Table 7 Reaction temperatures of metal oxides (only the temperature of reaction start is quoted). Chemicals

Co3O4

CuO

Fe2O3

Mn2O3

Reduction temperature (°C)

890 (20% O2 ) 772 (Ar) 875

1018 (20% O2) 958 (Ar) 1009

1361 (20% O2) 1145 (Ar) 1345

903 (20% O2) 710 (Ar) 733

Re-oxidation temperature (°C)

Table 8 Reaction temperatures of metal carbonates (only the temperature of reaction start is quoted). Chemicals

BaCO3

CaCO3

SrCO3

Calcination temperature (°C)

1025 (Ar)

Carbonation temperature (°C)

None

807 (20% CO2) 670 (Ar) 766

1122 (20% CO2) 873 (Ar) 1006

Regarding carbonates (Table 8), decomposition of BaCO3 in 20% CO2 is not observed during TGA up to 1400 °C (inert atmosphere is then required to lower the decomposition temperature, Fig. 4). The temperature range for BaCO3 decomposition is estimated to be between 1155 °C and 930 °C under nitrogen atmosphere [33]. Experimental results show a temperature of 1025 °C under Ar for the

beginning of BaCO3 powder decomposition but carbonation is negligible. Pure CaCO3 and SrCO3 do not show complete reversible reactions during calcination/carbonation cycles, since the carbonation step is partial due to the carbonate shell formation at the particles surface [34–37,58,59], then hindering CO2 diffusion to the core. Experimental results show that pure CaCO3 powder starts to release CO2 during calcination at 807 °C in 20% CO2, which is close to the theoretical transition temperature of CaCO3/CaO (Table 6). Similarly, the start of the calcination step of pure SrCO3 powder occurs at 1122 °C experimentally, which is consistent with the theoretical transition temperature of SrCO3/SrO (Table 6). Mixing of the carbonates with stabilizing inert matrix can be used to inhibit sintering and to improve materials stability [42– 65]. The measured transition temperatures are close to that predicted by theoretical thermodynamics except for BaCO3/BaO. Among the selected pure compounds, Co3O4 appears to be the best suited based on the complete reversibility and faster reaction kinetics, consistently with previous studies [80–84], which are key factors in determining whether the material is practical for efficient solar heat storage application [6,11–13]. 4.4. Overview of future research trends in thermochemical energy storage In this paper, a state of the art of the chemical reactions and materials with the highest potential for high-temperature

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Fig. 8. Thermo-gravimetric analysis of Mn2O3/Mn3O4 reduction/oxidation reaction: a. commercial manganese oxide powder, b. synthesized manganese oxide powder.

Fig. 9. Thermo-gravimetric analysis of CuO/Cu2O reduction and oxidation at 1050 °C, 1000 °C and 950 °C in 20% O2.

(400–1200 °C) thermochemical heat storage in CSP plants was presented. Most of the described systems were only evaluated and tested on laboratory scale so far. The energy density of such thermochemical systems is usually 5 to 10 times higher than sensible and latent heat storage systems. Reversible solid-gas thermochemical systems appear to be the most promising way for long-term solar thermal energy storage. Indeed, reaction products can be stored at ambient temperature without any loss of thermal energy during storage and thus, both storage period and transport distance are theoretically unlimited. Concerning the targeted characteristics when developing such systems, the process must be reversible with a constant conversion rate and without performance degradation after a large number of cycles to avoid a decrease in the heat storage capacity of the material. Another research area is the optimization of the temperature difference between charging/discharging steps. The objective is to diminish the

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temperature difference between both steps to improve the process efficiency and facilitate the process control. Laboratory-scale studies have demonstrated the feasibility of several reaction systems for TES application. The systems involving metal oxides are the most simple for short-term implementation in CSP plants because air can be directly used and the technical hurdles associated with the heat transfer fluid management are relieved thanks to open loop operation with air. Research is underway on modification of redox properties of metal oxides (transition temperatures, kinetics…) thanks to the synthesis of mixed oxides, study of synthesis effect on the morphology of the material and on its reaction efficiency (for improving stability and reaction reversibility), and development of ion conducting oxides (e.g., perovskites) that may become attractive candidates for thermochemical energy storage. While metal oxide systems could become rapidly the most mature technology for high temperature thermochemical energy storage application, the systems based on metal carbonates also show practical interest as they have been extensively developed in the field of post-combustion CO2 capture (especially CaO-looping concept). The CaO system has been extensively studied for CO2 capture purposes where lime may be used in a cyclic process to first remove CO2 from flue gas (carbonate formation), while subsequent calcination of the formed carbonate produces a pure CO2 stream suitable for direct use or storage. These reactions can further be included in a heat storage process as lime meets the required criteria since it is abundant in nature, cost-effective and also possesses high energy storage potential. The main challenge of such a system, particularly when based on natural lime materials, is the rapid cycle-tocycle loss of activity. To this direction, the different strategies in order to overcome this phenomenon can directly be applied to maintain the charging/discharging system efficiency during solar energy storage (e.g., use of composite materials or stabilizing additives). Finally, hydroxides and sulfates are also worth to be studied as they may represent suitable thermochemical systems for heat storage thanks to their high energy storage density at the expense of additional constraint linked to corrosiveness of evolved gas products used as heat transfer fluid, which may represent a serious barrier for running the thermodynamic power generation cycle of the CSP plant. At present, scale-up is also an important step to address for future development of the technology. Large-scale experiments and pilot demonstration are necessary to prove the feasibility of the thermochemical energy storage systems for both short and longterm storage and to integrate them in the whole solar power plant. Thermal energy storage integrated with CSP plants is currently a commercially demonstrated and relatively low cost solution capable of storing enough energy for several hours of operation when the solar resource is not available. When dealing with thermochemical heat storage involving reversible solid-gas reactions, higher storage temperatures can be accessible and the cost of solar power systems with thermal energy storage will inherently be reduced through an increase in power cycle operating temperature that will enable more efficient electricity production in next generation CSP plants.

5. Conclusion A list of potential materials, showing high energy storage capacity, for thermochemical solar heat storage has been established. A selection with respect to requirements such as suitable range of reaction temperature for CSP applications and non-toxicity leads to the following compounds: CaCO3, SrCO3, BaCO3, Ca(OH)2, Sr(OH)2, Ba(OH)2, Co3O4, Fe2O3, CuO, BaO2, and Mn2O3. The cost of the raw materials or the cost of their synthesis will have to be considered for large scale process. Further experimental investigations showed that Co3O4 is the most suited raw material given the fast reaction kinetics and complete reaction reversibility. Optimization of materials reactivity is

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required for the other species by using e.g. doping strategies, controlled synthesis techniques for tailored morphology, stabilization with inert materials to alleviate effects of sintering, etc. The reaction kinetics and the thermodynamic properties of the selected materials need to be characterized further, as well as their performance stability over successive cycles, in order to confirm the suitability of the chosen materials for TES application. Then, the design and testing of adapted solar reactor concepts will also be required to demonstrate the feasibility of materials processing in solar reactor prototypes during solar heat charging and discharging. Furthermore, raw materials were addressed here but recent researches concerning the efficiency of hybrid materials are proving promising and need to be explored.

[24]

[25]

[26]

[27]

[28]

[29]

Acknowledgements This study was performed in the framework of the STAGE-STE European project (FP7, project N° 609837).

[30]

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