Development on Thermochemical Energy Storage Based on CaO-Based Materials: A Review

Abstract

The intermittent and inconsistent nature of some renewable energy, such as solar and wind, means the corresponding plants are unable to operate continuously. Thermochemical energy storage (TES) is an essential way to solve this problem. Due to the advantages of cheap price, high energy density, and ease to scaling, CaO-based material is thought as one of the most promising storage mediums for TES. In this paper, TES based on various cycles, such as CaO/CaCO₃ cycles, CaO/Ca(OH)₂ cycles, and coupling of CaO/Ca(OH)₂ and CaO/CaCO₃ cycles, were reviewed. The energy storage performances of CaO-based materials, as well as the modification approaches to improve their performance, were critically reviewed. The natural CaO-based materials for CaO/Ca(OH)₂ TES experienced the multiple hydration/dehydration cycles tend to suffer from severe sintering which leads to the low activity and structural stability. It is found that higher dehydration temperature, lower initial sample temperature of the hydration reaction, higher vapor pressure in the hydration reactor, and the use of circulating fluidized bed (CFB) reactors all can improve the energy storage performance of CaO-based materials. In addition, the energy storage performance of CaO-based materials for CaO/Ca(OH)₂ TES can be effectively improved by the various modification methods. The additions of Al₂O₃, Na₂Si₃O₇, and nanoparticles of nano-SiO₂ can improve the structural stabilities of CaO-based materials, while the addition of LiOH can improve the reactivities of CaO-based materials. This paper is devoted to a critical review on the development on thermochemical energy storage based on CaO-based materials in the recent years.

1. Introduction

The demand for energy is rapidly increasing with rising human population around the world. Global consumption of energy is expected to expand by 57% from 2002 to 2025 [1,2]. As the dominant energy source, fossil fuels account for more than 85% of the energy in demand at present. However, the heavy use of fossil fuels has caused a lot of problems, such as global warming and energy security issues [3]. Scientists have proposed that limiting global warming to 2 °C is very crucial to prevent serious negative climate change, but it is impossible to achieve this goal if use of fossil fuels continues without restriction [4]. The use of renewable energy is a good way to control the adverse effects of the global greenhouse. It is reported that if the renewable energy accounting for more than 50% of the total energy by 2028, global warming can be limited to <2 °C [4]. Solar energy as alternative energy is one of the most promising renewable energies and the largest energy source of the world, which does not pollute the environment or lead to global warming. Its clean and inexhaustible characteristics encourage the widely use of solar energy via concentrated solar power (CSP), central heating, and distributed power generation [5,6,7]. However, the intermittent and inconsistent nature of solar radiation throughout a day’s cycle results in the discontinuous operation of CSP plants, which can lead to serious imbalances between energy supply and energy demand. This is the main problem which restricts the development of CSP plants. The development of thermochemical energy storage (TES) provides an effective solution to solve this problem which has attracted considerable attention [8]. The coupling of TES and CSP plants can realize the continuous operation of units by storing excessive solar energy in the daytime and releasing the stored heat at night to maintain the continuous operation of the CSP plants, as shown in Figure 1 [9].

In general, a TES system mainly consists of a heat transfer device, a storage medium, and a sealing system. According to different energy storage mediums, TES is divided into three different types: sensible heat storage (SHS), latent heat storage (LHS), and thermochemical heat storage (THS). SHS uses a traditional approach to heat the material directly by heat transfer. LHS mainly stores latent heat of fusion produced in the process of material phase transformation. It should be noted that LHS mainly depends on molten salt and several other phase change materials. SHS and LHS have been widely used in the industrial production. However, low energy densities of materials and high heat loss have seriously hindered the further development of SHS and LHS. Different from SHS and LHS, THS uses reversible chemical reactions of the various chemical substances to realize heat storage and utilization. There are many materials and methods for THS which have been reported in the references [10,11,12,13,14,15,16,17,18,19,20,21,22]. Figure 2 presents the heat storage densities of some commonly used TES materials. It shows that silica gel, zeolite, and CaCl₂·H₂O have higher energy density than the other materials. In fact, the three materials all belong to THS. Furthermore,

Table 1. Turning temperature and energy densities of different heat storage mediums.

Type of TES	Material	Turning Temperature (°C)	Energy Density(MJ/m3)	Reference
SHS	Silicone oil	300–400	189	[23]
SHS	Nitrite salts	250–450	548	[23]
SHS	Nitrate salts	265–565	898	[23]
SHS	Carbonate salts	450–850	1512	[23]
SHS	Liquid sodium	270–530	287	[23]
LHS	Sodium nitrite	270	373	[24]
LHS	Sodium nitrate	307	389	[23]
LHS	Potassium nitrate	333	561	[25]
LHS	Sodium carbonate	854	701	[26]
THS	FeO/CO2	180	2600	[27]
THS	CaO/H2O	500	3000	[23]
THS	CaO/CO2	800–900	4400	[23]
THS	NH4HSO4/NH3	467	3082	[28]
THS	SrO/CO2	1108	3948	[28]

shows more different energy storage materials belonging to three kinds of heat storage mediums. It indicates that THS has higher energy density and lower heat loss than SHS and LHS. The materials used for THS have lower cost and higher environmental value. Thus, THS is more promising for energy storage than SHS and LHS.

THS can achieve the seasonal energy storage without special insulation measures. According to different materials, there are many kinds of THS systems as shown in Figure 3. In general, the selection of THS system needs to meet the following factors [29]: high enthalpy, large energy storage density, temperature, and pressure within the allowable range of equipment conditions, high endothermic/exothermic rates, high energy storage efficiency, no by-products, no pollution and corrosion, and low cost of materials. Among these THS systems, many studies have proven that CaO-based material is one of the most promising storage mediums due to the abundant and cheap resources, high energy density and easy operation in the industrial applications, etc. [9,30,31,32]. In this paper, the latest research progresses and applications of CaO-based materials for TES are introduced in detail.

2. CaO/CaCO₃ and CaO/Ca(OH)₂ TES

2.1. CaO/CaCO₃ TES

CaO/CaCO₃ TES is based on the multiple carbonation/calcination cycles of CaO, which have been widely studied as an effective way to capture CO₂. Heat storage using this way has attracted widespread attention in the recent years. As shown in Equation (1), the endothermic process is the calcination of CaCO₃ to produce CaO and CO₂, which converts solar energy into chemical energy in CaO. Baker et al. [33] pointed out that high calcination temperature was beneficial for the decomposition of CaCO₃. However, too high a temperature above 950 °C would aggravate the sintering of CaO-based sorbents [34]. In addition, the carbonation temperature between CO₂ capture and TES is significantly different. Grasa et al. [35] and Chen et al. [36] proposed that carbonation temperature had an important effect on carbonation conversion of CaO. The carbonation temperature for TES is higher than that of CO₂ capture. The carbonation reaction of CaO with CO₂ is an exothermic process, as illustrated in Equation (2).

$${\mathrm{CaCO}}_3\to {\mathrm{CaO}+\mathrm{CO}}_2\Delta \mathrm{H}=178\mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}$$

(1)

$${\mathrm{CaO}+\mathrm{CO}}_2\to {\mathrm{CaCO}}_3\Delta \mathrm{H}=-178\mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}$$

(2)

CaO/CaCO₃ TES is widely studied for the high temperature energy storage because of the relatively high heat transfer temperature. Figure 4 shows the schematic diagram of CaO/CaCO₃ TES. Q_in is the heat energy for the decomposition of CaCO₃ into CaO and CO₂ int the calcination process, which is obtained from solar radiation at the temperature of 850–950 °C. Then CaO reacts with CO₂ in the carbonation process, which releases the heat energy (Q_out). Q_out can be used to maintain the operation of CSP plants at night.

There are many studies on the coupling of CaO/CaCO₃ reversible reaction and power plants. One of the most extensive studies is coupling CaO/CaCO₃ system with CSP plants, mainly including industrial production of lime by solar chemical reactor [38,39] and TES based on CaCO₃-looping process [30,32,37,40,41,42,43,44,45,46]. Meier et al. [39] reported that using solar energy instead of combustion of fossil fuels to provide the required heat for the calcination of CaCO₃ could reduce the CO₂ emission by 40% in conventional cement plants. In addition, CaO/CaCO₃ TES system has been studied in other types of power plants (e.g., wind power plants) [47] and integrated gasification combined cycle (IGCC) plants [48]. The reverse peak characteristics of wind energy make the abandoned wind phenomenon serious. Coupling CaO/CaCO₃ TES with biomass power plants to accommodate wind power is one way to solve this problem [47]. IGCC technology has great potential to realize power generation and carbon capture and storage (CCS) simultaneously [49]. However, the temperature and flow constraints have severely limited the flexibility of the IGCC technology with CCS. To solve this problem, Vandersickel et al. [48] proposed a CaO/CaCO₃ cycle and CO₂ storage system to ensure the temporary increase in IGCC peak power output.

As for the development of CaO/CaCO₃ TES in CSP plants, Edwards et al. [40] proposed a new process consisting of a solar calciner, a pressurized fluidized bed combustor (PFBC) carbonator, and a gas turbine as illustrated in Figure 5. CaCO₃ was firstly decomposed into CaO and CO₂ in the solar calciner. CaO and CO₂ were then stored in their corresponding reservoirs. When heat was required, CaO and CO₂ were transferred to the PFBC carbonator where the heat was recovered from the carbonation reaction and CaCO₃ was generated again. The balance calculations of heat and mass showed that the highest power generation efficiency could be obtained when the carbonation activity of sorbent was above 15%. The highest carbonation activity of sorbent was 20–40%. When the carbonation activity of sorbent carbonated at 800–900 °C under the pressure of 2.8–9.1 bar was in the range of 20–40%, the power generation efficiency could reach 40–46% [30].

Obermeier et al. [42] believed that the storage densities of TES systems were very important for the industrial applications. They thought the effective storage density was mainly influenced by carbonation temperature, heat recovery in exothermic reaction, and reaction conversion. Chacartegui et al. [32] designed a CaO/CaCO₃ TES integration system consisting of CO₂ closed loop and power production, as shown in Figure 6. Despite the different types of the reactors, the basic heat storage process and power generation were the same as those proposed by Edwards et al. [30]. In this integration system, when the outlet pressure of turbine was 3.2 bars and the carbonation temperature was 875 °C, the power generation efficiency could reach above 45%. Besides, they pointed out that the first-law and second-law efficiencies of this integration system under the practical conditions were 40–46% and 43–48%, respectively.

In order to solve the problem of uneven distribution of sunlight intensity on the earth, Müller et al. [41] proposed a new energy transport substances (ETS) system based on alkaline earth metal compounds as energy carrying compounds. They studied the effects of chemical carriers such as CaO and MgO, the process scheme and the feature of CaO/CaCO₃ ETS system. They found that the CO₂ mitigation potential could reach 40–50%, assuming that electricity generation in Germany was completed by the CaO/CaCO₃ ETS system instead of burning lignite [41].

2.2. CaO/Ca(OH)₂ TES

CaO/Ca(OH)₂ TES is based on the hydration/dehydration reactions of CaO. Equation (3) represents the dehydration process of Ca(OH)₂ and the required heat for the endothermic process is provided by solar energy. After the dehydration, CaO and water vapor are formed in the range of 410–550 °C. Equation (4) depicts the hydration process of CaO to release a lot of heat. The exothermic reaction can occur easily and quickly at room temperature and almost all CaO can be converted to Ca(OH)₂. These properties significantly reduce the threshold of heat release and contribute to improve the energy conversion efficiency [10]. Figure 7 presents the schematic diagram of CaO/Ca(OH)₂ TES [50,51,52]. In CSP plants, the required heat for the decomposition of Ca(OH)₂ is mainly from solar energy. It can also come from the excess electricity generated during low peak period of power consumption [47]. CaO and water vapor are generated from the decomposition of Ca(OH)₂ in the solar dehydrator, which are stored in CaO and water storage tanks, respectively. During this process, the heat energy becomes the chemical energy in CaO. When the heat is needed, CaO and water vapor are transported to the hydrator from their corresponding storage tanks and react with each other to release heat. The chemical energy is then converted into the heat energy in the hydration process. At night, the released heat in the hydration process can heat feed water in the coal-fired boiler to reduce the consumption of coal and then maintain the normal power generation of CSP plants at night. The reactivities of hydration and dehydration are generally described by the hydration conversion (X_Hy) and the dehydration conversion (X_Dehy), as presented in Equations (5) and (6), respectively [53].

$${\mathrm{Ca}(\mathrm{OH})}_2\to {\mathrm{CaO}+\mathrm{H}}_2\mathrm{O}\Delta H=104.4\mathrm{kJ}/\mathrm{mol}$$

(3)

$${\mathrm{CaO}+\mathrm{H}}_2\mathrm{O}\to {\mathrm{Ca}(\mathrm{OH})}_2\Delta H=-104.4\mathrm{kJ}/\mathrm{mol}$$

(4)

$$X_{\mathrm{Hy}}=\frac{56\times m_{{\mathrm{H}}_2\mathrm{O}}}{18\times m_{\mathrm{CaO}}}\times 100\%$$

(5)

$$X_{\mathrm{Dehy}}=\frac{74\times m_{{\mathrm{H}}_2\mathrm{O}}}{18\times m_{{{}_{\mathrm{Ca}(\mathrm{OH})}}_2}}\times 100\%$$

(6)

The different reaction kinetics models for both hydration reaction of CaO and dehydration reaction of Ca(OH)₂ according to the references are presented in

Table 2. Kinetics models of hydration/dehydration reactions of CaO reported in references.

Reference	Method	Results
[67]	Thermogravimetric analysis (TGA) Multiple rate scanning Mechanism function	The non-isothermal decomposition kinetics equation of Ca(OH)2: $\frac{d\alpha }{dt}=(\frac{A}{\beta })\exp (-\frac{E}{RT})f(\alpha )$ The thermal decomposition kinetics model of Ca(OH)2 is a contraction of the cylinder model. Mechanism function: $\mathrm{G}(\alpha )=1-{(1-\alpha )}^{1/2},f(\alpha )=2(1-\alpha {)}^{1/2}$
[58]	High pressure TGA	CaO hydration rate (R): $R=\frac{1}{d_{\mathrm{p}}^{0.11}}\times 0.0069\exp (-\frac{8400}{RT})\times (p_{{\mathrm{H}}_2\mathrm{O}}-p_{{\mathrm{H}}_2\mathrm{O}}^{*})$
[68]	Combining constructal approach and exergy analysis Lagrange multipliers method	Exergy diffusivity can be defined according to the parameters of the reactor. Global exergy destruction ($e{x}_{\mathrm{d}}$):$e{x}_{\mathrm{d}}=z{q}^2$
[69]	TGA	Hydration rates: $\frac{d{X}_{\mathrm{Hy}}}{dt}=k_{\mathrm{Hy}}(T)\cdot (v_{{\mathrm{H}}_2\mathrm{O}}-v_{\mathrm{eq}})f(X_{\mathrm{Hy}})$ Dehydration rates: $\frac{d{X}_{\mathrm{Dehy}}}{dt}=k_{\mathrm{Dehy}}(T)\cdot (v_{\mathrm{eq}}-v_{{\mathrm{H}}_2\mathrm{O}})f(X_{\mathrm{Dehy}})$
[70]	Clausius–Duhem inequality Numerical analysis	A macroscopic model of multicomponent compressible gas flow through porous solids is derived theoretically. The interphase coupling is established by the interaction of mass, momentum and energy.
[71]	TGA Scanning electron microscope (SEM) analysis	Kinetics of hydration reaction promoted by vapor pressure An anti-Arrhenius behavior and “blocking effect” was observed.
[59]	TGA SEM analysis	Kinetic equations of the dehydration: X < 0.2: $\frac{dX}{dt}=1.9425\times {10}^{12}\exp (-\frac{187.88\times {10}^3}{RT})\cdot {(1-\frac{P}{P_{\mathrm{eq}}})}^3\cdot (1-X)$ X > 0.2: $\frac{dX}{dt}=8.9588\times {10}^9\exp (-\frac{162.62\times {10}^3}{RT})\cdot {(1-\frac{P}{P_{\mathrm{eq}}})}^3\times 2{(1-X)}^{0.5}$ Kinetic equations of the hydration: Teq − T ≥ 50 K: $\begin{array}{l}\frac{dX}{dt}=13945\times \exp (\frac{-89.486\times {10}^3}{RT})\cdot (\frac{P}{P_{\mathrm{eq}}}-1{)}^{0.83}\times 3(1-X)\\ \times {\left[-\ln (1-X)\right]}^{0.666}\end{array}$ Teq − T < 50 K: $\begin{array}{l}\frac{dX}{dt}=1.0004\times {10}^{-34}\exp (\frac{53.332\times {10}^3}{T})\cdot (\frac{P}{{10}^5}{)}^6\\ \times (1-X),P>P_{\mathrm{eq}}\end{array}$

. The hydration kinetics of CaO with water and water vapor were both studied at room temperature [54,55] and at high temperature [56,57,58], respectively. Compared to room temperature, high hydration temperature is closer to the actual conditions of the exothermic reaction [59]. Lin et al. [58] discovered that the hydration conversion of CaO to Ca(OH)₂ achieved 100% at hydration temperature of 1023 K under high vapor pressure 3.8 MPa. However, the hydration conversion decreased with increasing the hydration temperature further. The result shows that proper high temperature and high pressure can promote the hydration reaction of CaO. The dehydration kinetics were investigated under the different atmospheres such as nitrogen, air, and vacuum [57,60,61,62,63,64]. The result indicates that a certain degree of vacuum can promote dehydration reaction of Ca(OH)₂. In addition, Wang et al. [65] made 3D simulations of CaO/Ca(OH)₂ TES. Nagel et al. [66] analyzed the effect of gas–solid reaction kinetics on models of TES. However, many proposed kinetic models are too complex to be widely used. It is necessary to find simpler and more practical kinetic models for CaO/Ca(OH)₂ TES.

The generation of waste heat and ineffective utilization of low-grade heat cause a serious waste of energy. Upgrading and reusing the waste heat is an effective method for heat utilization. A chemical heat pump (CHP) can increase the temperature level of thermal energy from waste heat and solar heat and store high-density heat based on reversible chemical reactions [72], which is suitable for TES. Lots of studies on fundamental principles and thermodynamic analysis of CHP have been reported and proved the good availability of CHP for TES [73,74,75,76,77,78,79]. CHPs can be classified into adsorption heat pumps and absorption heat pumps. They mainly involve an exothermic adsorption/absorption process and an endothermic desorption process, as shown in Equation (7).

$$\mathrm{A}+\mathrm{B}\leftrightarrow \mathrm{C}$$

(7)

The reversible reaction is carried out at two different temperatures. The forward reaction is exothermic, while the backward reaction is endothermic. In general, a CHPs consist of a synthesis/decomposition reactor which is connected to a condenser or evaporator. The chemical reactions of CHP mainly depend on various chemical substances which play an important role in the endothermic and exothermic processes [80]. The chemical substances based on CaO [81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98] and MgO [99,100] have been widely reported. Lots of studies focus on the CHP based on the hydration/dehydration reactions of CaO/Ca(OH)₂. The synthesis/decomposition reactor of the CaO/Ca(OH)₂ CHP is a hydration/dehydration reactor in which the temperature is higher than that in the condenser/evaporator. Figure 8 shows the schematic diagram of CHP based on CaO/Ca(OH)₂ reactions [94]. In the heat storing process, the heat Q_H,S is stored by the endothermic decomposition of Ca(OH)₂ at high heat storing temperature T_H,S. The released water vapor is transported into another reactor and condenses into liquid water to release the heat Q_L,S at low heat storing temperature T_L,S. In the heat releasing process, liquid water is evaporated into water vapor at low heat releasing temperature T_L,R. The water vapor returns to the high-temperature reactor and reacts with CaO to release the heat Q_H,R which can be used for industrial and domestic energy. The driving force of water vapor flowing between the two reactors is a pressure difference.

The decomposition process and the heat transfer between the two reactors were investigated by Arai and Kanzawa [101], and they proposed an optimal configuration of the reactors. Fujimoto et al. [82,93] simulated the dynamic performance of CaO/Ca(OH)₂ CHP system. Ogura et al. studied the effective application of CaO/Ca(OH)₂ CHP for more than 10 years and they did a lot of fundamental studies, such as heat transfer enhancement [95,98], heat-storing mode [91], mass transfer enhancement [97], and reaction process simulation [84]. In addition, they developed a CaO/Ca(OH)₂ CHP system for low-temperature generation [92,96] and energy circular utilization [83]. The above-mentioned research results indicate that the coefficient of performance (COP) of CaO/Ca(OH)₂ CHP is higher than that of other heat pumps and its operating temperature window is much wider. Thus, it is suitable for TES.

A lot of studies on CSP plants using CaO-based materials for TES have been published [23,51,52,102,103,104,105,106,107,108,109]. Ogura et al. [81] found that the recovered heat from the CaO/Ca(OH)₂ system could heat the air from room temperature to 70 °C. Schmidt et al. [102] designed a 10 kW TES reactor indirectly operated at high temperature for 20 kg Ca(OH)₂, as presented in Figure 9. The reactor was based on plate heat exchanger concept. They found that when the decomposition temperature was 450 °C and the hydration temperature was 550 °C, the overall conversion of the reversible reaction reached to 77%. The physical properties of the samples remained stable after 10 cycles. The maximum thermal power of this reactor could reach 7.5 kWth.

About the stability of CaO/Ca(OH)₂ in the multiple hydration/dehydration cycles, the different researchers have different views. Azpiazu et al. [103] proposed that CaO/Ca(OH)₂ in reversible reactions only kept good activity in the first 20 cycles due to the incomplete reversibility of the dehydration reaction. Schaube et al. [59] found no reversible loss over 100 cycles by the thermogravimetric analysis. As the direct heat transfer has a high requirement for stability of materials and minimizing the material loss caused by pressure drop, Schaube et al. [108] also studied the direct heat transfer performance of CaO/Ca(OH)₂ with heat carriers (nitrogen and water) using a stainless steel resistance heater (fixed bed reactor). The results showed that 60 g Ca(OH)₂ material remained stable and high chemical reactivity over 25 cycles. However, an obvious agglomeration phenomenon was found in the reaction process. In addition, they proposed that the mass flow rate of the heat carrier severely affected the hydration reaction [108].

2.3. Coupling of CaO/Ca(OH)₂ and CaO/CaCO₃ TES

Although the reaction enthalpy of carbonation/calcination cycles of CaO/CaCO₃ is higher than hydration/dehydration cycles of CaO/Ca(OH)₂, the calcination temperature of CaCO₃ is much higher than dehydration temperature of Ca(OH)₂. This means that larger solar heat collection area and more heat resistant materials are required for CaO/CaCO₃ TES, compared to CaO/Ca(OH)₂ TES. Coupling of CaO/Ca(OH)₂ and CaO/CaCO₃ TES is better than individual CaO/CaCO₃ TES, which can efficiently store energy and capture CO₂ simultaneously. Hanak et al. [109] proposed a coal-fired power plant with dual functions of energy storage and CO₂ capture, as shown in Figure 10. In the coal-fired power plant, CO₂ was captured by cyclic calcination/carbonation reactions that occurred in a calciner and a carbonator, respectively. The way of energy storage had three portions: CaO/CaCO₃ energy storage, CaO/Ca(OH)₂ energy storage, and cryogenic O₂ energy storage. During the low peak period of the power consumption, the power plant operated at a load of 40%. The energy was stored by producing liquid O₂ and CaO at the rated capacity. During the high peak period of the power consumption, the power plant operated at the nominal capacity. The stored O₂ and/or CaO was used to release energy to unload the units of air separation and CO₂ compression, which could reduce their heat requirements for the calciner. In the research on the possible combined options of energy storage, they used the energy density (D_v) and the specific energy (D_m) as indicators to measure the capacity of energy storage, which were defined in Equations (8) and (9). Hanak et al. [109] found that CaO/Ca(OH)₂ cycle could increase the energy density and the specific energy of CaO/CaCO₃ storage system by 57.4% and 71.4%, respectively. In addition, they reported that the energy density of the system coupling of CaO/Ca(OH)₂ and CaO/CaCO₃ energy storage was 86%, which was higher than that of conventional CaO/Ca(OH)₂ energy storage system.

$$D_{\mathrm{v}}=\frac{E_{\mathrm{stored}}}{3.6\times m_{\mathrm{media}}}{\rho }_{\mathrm{media}}$$

(8)

$$D_{\mathrm{m}}=\frac{E_{\mathrm{stored}}}{m_{\mathrm{media}}}\times 1000$$

(9)

Many researchers reported that after the hydration/dehydration reaction of CaO, the conversion of CaO to CaCO₃ was improved [110,111,112,113,114,115]. Nikulshina et al. [113,114] proposed that the addition of water vapor could almost double the conversion of CaO to CaCO₃, which led to complete removal of CO₂. They applied a chemically-controlled rate law to explain the high carbonation conversion in the presence of water vapor. It was attributed to an interface consisting of water or OH⁻ on the solid surface that was not covered by CaCO₃. The carbonation reaction of CaO to CaCO₃ is an exothermic reaction. The higher the conversion of CaO to CaCO₃ is, the more heat is released. Thus, the above results indicate that CaO/Ca(OH)₂ TES can improve the performance of CaO/CaCO₃ TES. It can explain why the coupling of CaO/Ca(OH)₂ and CaO/CaCO₃ TES is better than individual CaO/CaCO₃ TES.

3. Effect of Energy Storage Conditions on CaO/Ca(OH)₂ TES

3.1. Effect of Form of Water in Hydration and Dehydration Atmosphere

The form of water in hydration has a great influence on the performance of CaO-based materials in CaO/Ca(OH)₂ TES. Sakellarious et al. [50] reported that, compared with the dehydration temperature when the hydration reaction was carried in pure steam, the required dehydration temperature when the hydration reaction was carried out in distillation water was significantly reduced. This study shows that aqueous water is easier to hydrate with CaO than the vaporous water. For the dehydration reaction, Criado et al. [116] found that for the dehydration under 100% water vapor, the hydration conversion of Ca(OH)₂/Si composite material prepared from CaCO₃ and Na₂Si₃O₇ decreased sharply with the number of hydration/dehydration cycles, but there was no obvious decay in the hydration conversion for the dehydration under air. They explained this phenomenon as the formation of the hydrated silicate in the dehydration process under 100% water vapor, which hindered the hydration of CaO. It shows that the dehydration atmosphere has an important effect on the hydration reaction of CaO.

3.2. Effect of Temperature and Vapor Pressure

Knoll et al. [117] proposed that the hydration temperature of CaO was affected by water vapor concentration. Yan et al. [53] found that the dehydration temperature and the initial sample temperature of the hydration reaction had an influence on the performance of CaO-based materials in CaO/Ca(OH)₂ TES, as follows:

(a)

with increasing the dehydration temperature, the dehydration rate (the ratio of dehydration conversion to time) of Ca(OH)₂ was accelerated and the dehydration conversion was greatly improved;

(b)

when the initial sample temperature of the hydration reaction increased, both the hydration rate (the ratio of hydration conversion to time) and the hydration conversion of CaO decreased sharply.

The above research only focused on the phenomena and regularities of the effect of temperature rather than the mechanism. Shi et al. [118] investigated the effect of calcination temperature and hydration temperature on the rate of heat evolution. Figure 11 showed that the rate of heat evolution increased and the time for reaching the maximum rate was shortened as the hydration temperature increased and the calcination temperature decreased [118]. It indicates that the increase of hydration temperature can promote the hydration reaction at the temperature range of 12–50 °C, but the high calcination temperature hampers the hydration reaction. Shi et al. [118] explained this phenomenon according to the change of CaO microstructure. The complete lattice structure of CaO led to the low free energy of the lattice surface and the low hydration activity of CaO. They thought that when the calcination temperature increased, the lattice deformation extent of CaO decreased which led to the low rate of heat evolution. As for the positive effect of the hydration temperature, they proposed that the increase of the hydration temperature could make CaO get enough energy to overcome the reaction barrier which could increase the rate of heat evolution [118].

Yan et al. [53] studied the effect of vapor pressure on the hydration performance of CaO. They proposed that the existence of air in the fixed bed hydration reactor hindered the contact between CaO particles and water. The hydration conversion increased obviously with the increasing vacuum degree. Therefore, it is important to maintain a certain degree of vacuum in the hydration reaction. They found that when the vapor pressures were 0.18, 0.24, and 0.32 MPa, the hydration conversions of pure CaO were 31.7, 60.9, and 72.8%, respectively. It indicates that higher vapor pressure leads to higher hydration conversion of CaO. In addition, Schmidt et al. [8] pointed out that when water vapor at high pressure was integrated into the Rankine steam cycle, the energy storage efficiency could be increased by 27%.

3.3. Effect of Reactor Type

The thermal conductivity of the reactor directly affects the performance of CaO-based materials in CaO/Ca(OH)₂ TES. For the same temperature and atmosphere, the sintering of CaO-based materials in fixed bed reactors is more likely to occur than that in circulating fluidized bed (CFB) reactors, which leads to the bad thermal conductivity and the low reaction rate. In addition, the fixed bed reactors have many other disadvantages, such as limited application range, low heat transfer capacity, large heat transfer area, high construction cost, and poor material adaptability [119]. Hence recently, many researchers have concentrated on the study of CaO-based materials for TES in CFB reactors [43,119,120,121,122,123,124,125,126]. Criado et al. [119] designed a CFB energy storage system based on CaO/Ca(OH)₂ TES, as shown in Figure 12. In this system, the exothermic process was carried out under the mixer of water vapor and little air at 700–750 K. The endothermic process was carried out under pure water vapor at 813 K. Criado et al. [119] calculated the relevant parameters of this system and pointed out that the maximum heat output of the CFB system was 100 MW_th, the effective energy storage density was about 260 kWh/m³, the net efficiency was 63% and the storage capacity per hour was 390–460 m³. The above-mentioned research demonstrated the superiority of CFB energy storage system.

Pardo et al. [120] investigated the feasibility and energy storage properties of Ca(OH)₂ and Ca(OH)₂-Al₂O₃ composite in a CFB reactor. Under the condition of the endothermic temperature being increased to 480 °C and the exothermic temperature being increased to 350 °C, the average energy storage density of the composite containing 30 wt % Ca(OH)₂ and 70 wt % Al₂O₃ was 60 kWh/m³, which amounted to a pure Ca(OH)₂ energy storage density of 156 kWh/m³ under the ideal operating conditions. It indicates that the addition of Al₂O₃ in CFB reactors can improve the energy storage capacity of CaO-based materials for TES. In addition, Pardo et al. [120] found that the ratio of sensible heat of Al₂O₃ to total stored heat was lower than 20% assuming that the reaction time was 4 h, which indicated that the heat efficiency of the CFB reactor was more than 80%. It was concluded that the long-term high efficiency of the CFB reactor can enhance the feasibility of the hydration/dehydration reactions of Ca(OH)₂-Al₂O₃ composites, although the energy storage density was lower than 260 kWh/m³, which was obtained by Criado et al. [119]. Kribus et al. [127] proposed a principle of multistage solar receiver to minimize the convection loss, including the low-temperature stage and the high-temperature stage, and they reported that for the two-stage system, the temperature of low-temperature stage exit and air exit could reach 750 °C and 1000 °C, respectively.

In conclusion, the performances of CaO-based materials in CaO/Ca(OH)₂ TES in the different reactors such as fixed bed reactor, CFB reactor, or multistage receiver are distinct due to the different thermal conductivities in these reactors. The optimal reactors for TES should be considered in the industrial application.

4. Improvement on Performance of CaO-Based Materials in CaO/Ca(OH)₂ TES

The natural CaO-based materials for TES tend to suffer from a severe sintering after many hydration/dehydration cycles, which leads to low reactivity and structural stability of the materials [103]. This seriously hinders the continuous process of heat storage. Many researchers have reported that the modification methods can effectively improve the energy storage performance of CaO-based materials in CaO/Ca(OH)₂ TES. The CaO-based materials modified with the different additives reported in the references are illustrated in

Table 3. Some additives used for modification of CaO-based materials for TES.

Additives	Whether It Improves Performance of CaO-Based Materials	References
Al2O3	Yes	[50,116,128,130,131,132,133]
LiOH	Yes	[53,129,134]]
Na2Si3O7 or nanoparticles of SiO2	Yes	[116,128,135,136,137]
Mg(OH)2	No	[129]
FeO	No	[128]
MnO	No	[128]

. It should be noted that not all additives are effective for improvement in performance of CaO-based materials in TES. Iguchi et al. [128] studied the hydration performance of CaO-(FeO or MnO) solid solutions. They found that when the content region of FeO or MnO was less than 30 mol %, the hydration performance of CaO-FeO or CaO-MnO solid solutions was almost the same as that of pure CaO, but the the hydration performance became worse when the solute content region was higher. Yan et al. [129] proposed that the addition of Mg(OH)₂ had little effect on the CaO/Ca(OH)₂ TES by analyzing the computational results of the micro-mechanism.

4.1. CaO-Based Materials Modified with Al₂O₃ in CaO/Ca(OH)₂ TES

Sakellarious et al. [50] studied the heat storage properties of synthetic CaO-Al₂O₃ materials in CaO/Ca(OH)₂ TES and compared their heat storage capacities fabricated from various precursors (calcium nitrate, calcium acetate, etc.) by the liquid phase self-propagating high-temperature synthesis (LPSHS). LPSHS processed all precursors in one step and used an organic compound (e.g., citric acid) to promote an explosive reaction and then to produce the desired synthetic material. Compared to pure CaO, Sakellarious et al. [50] found that the increase of the aluminum content in the synthetic materials could improve the macroscopic integrity but reduce the hydration activity of the synthetic materials. In addition, by means of Brunauer–Emmett–Teller (BET) analysis, SEM analysis, and X-ray diffraction (XRD) analysis, they found that Ca₃Al₂(OH)₁₂ was the main hydrated phase, which indicated that the main composition in the CaO-Al₂O₃ materials was Ca₃Al₂O₆. The formation of Ca₃Al₂O₆ led to better macroscopic integrity of the synthetic materials than that of pure CaO. However, CaO was wrapped by Ca₃Al₂O₆, which caused low contact area between CaO and water. It was the main reason why the hydration activity of the synthetic material was lower than that of pure CaO. In addition, according to the experimental results of the macroscopic integrity, Sakellarious et al. [50] found that the hydration reactivity of CaO-Al₂O₃ material prepared from calcium nitrate precursor was higher than that prepared from calcium acetate precursor. When the synthetic material containing mass ratio of CaO/Al₂O₃ of 81:19 (wt %) was prepared from calcium nitrate precursor, it could achieve better heat storage performance. These results were also related to the existence of Ca₃Al₂O₆ in the synthetic material.

Above-mentioned research shows that Ca₃Al₂O₆ is a key to increase the mechanical strength of CaO-based material modified with Al₂O₃. Mohamed et al. [131] proposed a three-dimensional diffusion-controlled reaction kinetic model for the synthesis of Ca₃Al₂O₆. They found that Ca₁₂Al₁₄O₃₃ and CaAl₂O₄ were two mesophases in the synthesis of Ca₃Al₂O₆. Mercury et al. [132] studied the mechanism of synthesis of Ca₃Al₂O₆ with CaCO₃ and Al(OH)₃ and they found that the nucleation and growth of Ca₃Al₂O₆ occurred at 1300 °C. The porous structure of Ca₃Al₂O₆ was found after the synthesis process which was caused by the release of CO₂ and water. Except for Ca₃Al₂O₆, Ca₁₂Al₁₄O₃₃ is another main composition in CaO-based material modified with Al₂O₃. Ruszak et al. [133] found that Ca₃Al₂O₆ was one of the intermediate phases during the synthesis process of Ca₁₂Al₁₄O₃₃. They also proposed that the low temperature synthesis using Ca₅Al₆O₁₄ as the intermediate phase was the best method to synthesize Ca₁₂Al₁₄O₃₃. The above results indicate that the different calcium-aluminate phases such as Ca₃Al₂O₆, Ca₁₂Al₁₄O₃₃, CaAl₂O₄, and Ca₅Al₆O₁₄ were formed in the CaO-based materials modified by Al₂O₃, which depended on the synthesis conditions (e.g., temperature). Therefore, the control of synthesis conditions is essential to obtain Ca₃Al₂O₆ in the CaO-based materials modified with Al₂O₃.

4.2. CaO-Based Materials Modified with LiOH in CaO/Ca(OH)₂ TES

In order to conduct the micro-mechanism analysis of CaO-based material modified with LiOH, Yan et al. [129] used crystal structure models of quantum chemistry based on transition state principle and the first principle. The change of the molecular structure of the transition state obtained by model calculation showed that the dehydration kinetics of Ca(OH)₂ were changed due to the addition of LiOH. The energy barrier for the dehydration of Ca(OH)₂ was reduced from 0.4 to 0.11 ev due to the addition of LiOH, which indicated that the dehydration reaction of Li-doped Ca(OH)₂ could occur at lower temperature than original Ca(OH)₂. Yan et al. [129] found that the addition of Li made the O-H bond of Ca(OH)₂ broken more easily, thereby improving the dehydration reaction rate and heat storage efficiency of Ca(OH)₂. It indicates that the addition of LiOH mainly changes the dehydration reaction kinetics of Ca(OH)₂ and improves its dehydration efficiency.

Yan et al. [53] also investigated the effect of LiOH doping on the dehydration reaction of Ca(OH)₂ in a designed fixed bed reactor. The molar ratio of Li/Ca was 5%. As shown in Figure 13, the water vapor in the experiment was provided by a steam generator. The reactor and the steam generator were controlled and regulated by their respective control cabinet, respectively. They found that the addition of LiOH could accelerate the heat storage process of Ca(OH)₂ and increased the dehydration efficiency of Ca(OH)₂ from 43.5% to 71.1%. However, Yan et al. [134] reported that the addition of LiOH did not affect the specific heat capacity and the reaction enthalpy of the dehydration reaction. According to the non-isothermal decomposition experiment, Yan et al. [134] found that the non-isothermal kinetic process of Ca(OH)₂-LiOH/H₂O synthetic materials was divided into two stages. They attributed this phenomenon to the change of molecular structure of Ca(OH)₂. In general, the interaction force between atoms was related to the distance between atoms. According to the results of quantum chemistry calculation, Yan et al. [134] found that the addition of lithium atoms exactly changed the distance between the atoms, so the non-isothermal decomposition kinetic process of Ca(OH)₂ was changed. The new non-isothermal decomposition kinetic process had a lower energy barrier, so the decomposition of Ca(OH)₂ and the heat storage process were promoted. However, the change of molecular structure of Ca(OH)₂ did not affect the specific heat capacity and the reaction enthalpy of the dehydration reaction.

Although LiOH doping can greatly improve the dehydration efficiency of Ca(OH)₂, the high cost of LiOH should not to be ignored. The price of lithium resources is $4–7 per kilogram [138]. Annual production of lithium resources around the world is maintained at about 32,000 t, while the world average consumption of lithium resources is still increasing by 8% per year [138]. The consumption of lithium as an additive in the modification of CaO-based material is very small [53], so it will not result in high costs.

4.3. CaO-Based Materials Modified with Na₂Si₃O₇ and Nano-SiO₂ in CaO/Ca(OH)₂ TES

The mechanical performance of CaO derived from the natural limestone is poor, which leads to the structural deformation and further results in the deactivation in the performance of CaO-based material during the repetitive hydration/dehydration cycles [69,139,140]. Criado et al. [116] studied the crushing strength and the mechanical stability of CaO-based material modified with Na₂Si₃O₇. The synthetic materials were prepared by putting the calcium precursor (CaCO₃, Ca(OH)₂ or CaO) in the Na₂Si₃O₇ solution, mechanically mixing them together at the ambient temperature, drying the mixed solution and then calcining the materials at 850 °C for 10 min. In the synthesis process, CaO reacted with Na₂Si₃O₇ to generate Na₂CaSiO₄ and Ca₂SiO₄, as shown in Equation (10).

$$5\mathrm{CaO}+\mathrm{N}{\mathrm{a}}_2\mathrm{S}{\mathrm{i}}_3{\mathrm{O}}_7\to \mathrm{N}{\mathrm{a}}_2\mathrm{CaSi}{\mathrm{O}}_4+2\mathrm{C}{\mathrm{a}}_2\mathrm{Si}{\mathrm{O}}_4$$

(10)

Criado et al. [116] found that CaCO₃ was the most suitable precursor for the synthetic materials, because the CaO particles derived from CaCO₃ exhibited the highest crushing strength (>26 N). Criado et al. [135] synthesized an ideal stoichiometric CaO-based material prepared from co-precipitated CaCO₃ and Na₂Si₃O₇ to achieve the complete conversion of Ca into Na₂CaSiO₄ and Ca₂SiO₄ which did not contain free CaO. To determine the effect of the number of the hydration/dehydration cycles, they subjected the stoichiometric sample to 40 cycles under pure water vapor for hydration at 450 °C and for dehydration at 550 °C. Figure 14 showed SEM images of this stoichiometric calcium-based material at the different stages [135]. It was found that the stoichiometric CaO-based material maintained very rich microcellular network structures and bridging formations after 40 hydration/dehydration cycles, as illustrated in Figure 14a,b. It explained why the stoichiometric CaO-based material had high crushing strength. Criado et al. [135] found that the crushing strength of the CaO-based material modified with Na₂Si₃O₇ after 100 cycles was higher than 7 N, while that of unmodified CaO after five cycles was only lower than 2 N. The crushing strength of CaO particles modified by Na₂Si₃O₇ was three times higher than that of unmodified CaO particles. It indicates that the modification by Na₂Si₃O₇ can improve the structural stability of CaO-based materials in CaO/Ca(OH)₂ TES. Criado et al. [135] thought that the high crushing strength was contributed to the existence of complex calcium silicate matrix (Na₂CaSiO₄ and Ca₂SiO₄).

Criado et al. [135] thought that the mechanical stability of the CaO-based materials modified with Na₂Si₃O₇ could be further improved by two methods. The first method was to heat the cured mixture of CaCO₃ and Na₂Si₃O₇ up to 880 °C under pure CO₂ before the calcination of CaCO₃. On the one hand, CaCO₃ could not be decomposed at 880 °C, because the equilibrium temperature for the decomposition of CaCO₃ under pure CO₂ was 900 °C. On the other hand, this method ensured the formation of a hard silicate framework around the unreacted CaCO₃, as shown in Equations (11)–(14):

$${\mathrm{CaCO}}_3+{\mathrm{Na}}_2{\mathrm{Si}}_3{\mathrm{O}}_7\to {\mathrm{Na}}_2{\mathrm{CaSiO}}_4+{2\mathrm{SiO}}_2+{\mathrm{CO}}_2$$

(11)

$$2\mathrm{CaC}{\mathrm{O}}_3+\mathrm{Si}{\mathrm{O}}_2\to \mathrm{C}{\mathrm{a}}_2\mathrm{Si}{\mathrm{O}}_4+2\mathrm{C}{\mathrm{O}}_2$$

(12)

$${\mathrm{CaCO}}_3+2{\mathrm{Ca}}_2{\mathrm{SiO}}_4\to {\mathrm{Ca}}_5{{(\mathrm{SiO}}_4)}_2{\mathrm{CO}}_3$$

(13)

$${\mathrm{Na}}_2{\mathrm{CaSiO}}_4+{2\mathrm{CO}}_2\to {\mathrm{Na}}_2{\mathrm{Ca}(\mathrm{CO}}_3{)}_2+{\mathrm{SiO}}_2$$

(14)

Different from Equation (10), Na₂Ca(CO₃)₂ and Ca₅(SiO₄)₂CO₃ were generated which wrapped around CaCO₃. It was the hard silicate framework around the unreacted CaCO₃ that allowed the Ca(OH)₂ particles to generate with less effects on the silicates layer, so the mechanical stability of the synthetic materials was improved. The second method was to ensure incomplete hydration of CaO. Criado et al. [135] found that shortening the reaction time was a suitable method to improve the mechanical stability. The mechanical stresses of the CaO-based material particles were improved by increasing the degree of CaO converting to Ca(OH)₂, which decreased the mechanical stability. Since the high mechanical stresses had a negative impact on maintaining the mechanical stability, shortening the hydration time may be an effective way to improve the mechanical stability of CaO-based materials.

In addition, Roβkopf et al. [137] found that the addition of small amounts of nano-SiO₂ could prevent agglomeration of CaO particles and stabilize the bulk properties of CaO particles, which led to the good cyclic stability of CaO during the multiple hydration/dehydration cycles. In addition, Roβkopf et al. [136] further found that the mixing intensity of CaO, SiO₂, and water had a great influence on the hydration conversion of CaO. The high mixing intensity increased the hydration conversion of CaO, because the surface of CaO particles accumulated the nanoparticles obtained from the separation of nanostructured agglomerates. In fact, nano-SiO₂ just covered on the surface of CaO particles as the inert supports instead of reacting with CaO. This is different from the result reported by Criado et al. [116,135] in which CaO could react with Na₂Si₃O₇.

5. Conclusions

TES based on CaO-based materials have many advantages, such as low cost, high energy density, and ease of industrial application. CaO/Ca(OH)₂ TES has low temperature requirement for solar collector. Coupling of CaO/Ca(OH)₂ and CaO/CaCO₃ TES system can efficiently store energy and capture CO₂ simultaneously. The performance of CaO-based materials in CaO/Ca(OH)₂ TES have been highlighted in this review. The effects of form of water in hydration and dehydration atmosphere are often in conjunction with other factors, such as different additives and reaction time or temperature in CaO/Ca(OH)₂ TES. The CaO-based materials in CFB reactors and multistage receivers show better heat storage performance in CaO/Ca(OH)₂ TES than that in fixed bed reactors. The method of modification for CaO-based materials in CaO/Ca(OH)₂ TES, such as Al₂O₃, LiOH, SiO₂, and Na₂Si₃O₇ as the additives can improve the reactivity, mechanical strength, and structural stability of CaO-based materials in CaO/Ca(OH)₂ TES. CaO/Ca(OH)₂ TES with high activities of CaO-based materials is a very promising technology to make use of solar energy.