State of the art on the high-temperature thermochemical energy storage systems

Energy Conversion and Management 177 (2018) 792–815

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Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman

Review

State of the art on the high-temperature thermochemical energy storage systems

T

Xiaoyi Chen, Zhen Zhang, Chonggang Qi, Xiang Ling , Hao Peng ⁎

⁎

School of Mechanical and Power Engineering, Nanjing Tech University, No. 30, PuZhu South Road, Nanjing 211816, PR China

ARTICLE INFO

ABSTRACT

Keywords: Thermochemical energy storage systems Cycle life Heat storage performance

Thermal energy storage can provide cost-effective benefits for different commercial fields because it allows heat recycling for use, such as in concentrated solar power plants or metallurgical and steel plants. Compared to traditional sensible and latent energy storage, thermochemical energy storage (TCES) offers a greater possibility for stable and efficient energy generation owing to high energy storage densities, long-term storage without heat loss, etc. The aim of this review was to provide a comprehensive insight into the current state of the art of research on several typical TCES systems at high operation temperatures (673–1273 K). These systems include hydride, metal oxide and organic systems. Each system is discussed in regard to two aspects: cycle life and heat storage performance. A concrete analysis of the current research and development provided in this review exposes that the main challenges of the aforementioned TCES systems rely on robust cycling stability of materials, reliable energy charging and discharging reactors, and a high-efficiency system for widespread commercialization. To bring the TCES system to market, more intensive studies about enhancement of cycle life from micro perspective, charging-discharging behavior in the reactors and development of system integration are still required.

1. Introduction Solar energy is considered a promising solution for environmental pollution and energy shortage because it can result in a significant reduction in greenhouse gas emissions and the use of fossil fuels [1]. It has been estimated from the Britain Petroleum Co. Ltd that concentrated solar power (CSP) plants are expected to be the fastest growing power technology, accounting for 40% of newly constructed power stations per year [2]. However, owing to intermittency and low efficiency, solar energy fails to meet the requirements of viable energy used in industrial plants. Among many methods [3-9], it is extensively believed that thermal energy storage (TES) is a valid and prospective option to overcome the limitations of solar energy. A solar power tower with a molten salt (a type of TES) can increase annual solar power availability by from 25% to 65%. At present, the common methods for TES can be divided into three types: sensible thermal energy storage (STES), latent thermal energy storage (LTES) and thermochemical energy storage (TCES) [10]. STES is the simplest and most mature technology, and has already been used in commercial CSP plants such as PS10 in Spain and Solar One in USA. In addition, LTES, mainly depending on the phase change of the storage materials, presents a higher energy storage density than the former and ⁎

offers a charging-discharging process at a constant temperature [11]. Nevertheless, it still has some disadvantages such as low thermal conductivity, failure in long-term storage, heat loss, etc. In contrast, TCES is recognized as the high potential for stable and efficient energy generation owing to its intrinsic advantages: high energy density (nearly 1000 kJ/L), long term storage, smaller storage volume, and no heat loss among the three types of TES. TCES can be divided into sorption and chemical reactions. The principle of sorption occurs during a reaction, and to occur, at least two components are needed: a sorbent, which is typically a liquid or solid, and a sorbate, which is typically vapor. The principle of the chemical reaction is similar to the sorption, but its reaction temperature in demand is much higher (673–1273 K) than that of the sorption temperature (243–343 K). Therefore, a chemical reaction is widely investigated for high temperature application, such as CSP plants and other industrial processes [12], and conversely sorption is more suitable for low temperature application such as seasonal energy storage and refrigeration. Considering sorption has been reviewed a lot [13–15], we only focus on TCES at high temperature (673–1273 K) in this paper. Currently, the TCES are still limited by some critical drawbacks including great complexity in system configuration, low attainable thermal levels in the actual system, high capital costs, etc. To overcome

Corresponding authors. E-mail addresses: [email protected] (X. Ling), [email protected] (H. Peng).

https://doi.org/10.1016/j.enconman.2018.10.011 Received 17 June 2018; Received in revised form 13 September 2018; Accepted 4 October 2018 0196-8904/ © 2018 Elsevier Ltd. All rights reserved.

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Nomenclature ANU AUD CaL CBC COP CS CSP DFT DSC HWT LTES PF

PR PSD SEM SHP STES TCES TEM TES TGA XRD

Australian National University Australian dollar calcium looping process closed CO2 Brayton cycle coefficient of performance crushing strength concentrated solar power plants density functional theory differential scanning calorimetry Hydrid-Wasserstoff-Technik equipment latent thermal energy storage storage performance factor

pressure ratios particle size distribution scanning electron microscopy specific heating power sensible energy storage thermochemical energy storage transmission electron microscopy thermal energy storage thermogravimetric analysis X-ray diffraction

Greek ϕ

these barriers, considerable academic efforts have been made worldwide. Several papers have reviewed mainly about TCES materials in the recent years. Liu et al. [16] demonstrated that TCES materials has the potential to make the next generation CSP plants viable. Later, Pardo et al. [17] provided advantage and disadvantage of different high temperature TCES materials in detail. Wu et al. [18] reviewed TCES materials based on metal oxides redox cycle in terms of their reaction kinetics, economics and future developments. Prieto et al. [19] compared three redox TCES materials (sulfur based cycles, metal oxide cycles, and perovskite-form structures) in view of their similarity. However, these papers reviewed solely from the perspective on TCES materials, thorough understanding of these systems is also required to be fully discussed. In this paper, we review TES from alternative perspective on TCES systems, mainly focusing on their both cycle life and heat storage performance (Fig. 1). In this paper, a comprehensive review on several typical kinds of TCES systems at high operation temperatures (673–1273 K), including hydride, metal oxide and organic systems, are presented in this paper based on Fig. 1. Each system is discussed in regard to two aspects: cycle life and heat storage performance (reactor and system integration). Their applications are demonstrated in detail as well. Major conclusions and perspectives on the TCES system are subsequently noted at the end of our paper.

diameter

MgH2 system can flexibly operate under a temperature range from 200 to 500 °C and a hydrogen partial pressure range from 1 to 100 bar. The reaction is as follows:

MgH2(s) + H

Mg (s) + H2(g)

H= 75 kJ/mol

(1)

2.1.1. Cycle life Mg/MgH2 as a TCES material suffers from a slow hydrogenation/ dehydrogenation kinetic and poor reversibility that limit their industrial application. Numerous strategies have been employed to overcome these issues, such as nanostructures, addition of alloying, and MgH2-based composite. In order to address the limitation of bulk MgH2, nanostructural materials have been proposed and investigated by many groups [20]. Zaluski et al. [21] investigated the effects of grain size on grain boundaries, internal strain, and chemical disorder during hydride formation. It was found that nanostructural materials could improve the kinetics of hydrogenation, which is attributed to the fact that (a) MgH2 particle reaction surface, materials porosity and grain boundary are increased; and (b) diffusion distance between hydrogen is decreased. From a microscopic perspective, Wagemans et al. [22] studied the role of grain size in the thermodynamic stability of Mg/MgH2 based on ab initio Hartree-Fock and density functional theory (DFT). Their calculation results indicated that a smaller grain size could more readily enable MgH2 desorption at a lower temperature because of a decrease in dehydrogenation bond energy. However, nanostructures may result in a poor sorption property and a reduction in the thermal conductivity. The addition of alloying is an effective and easy-to-handle method to improve the sorption property of Mg/MgH2 and thus increase in cycle life. Borislav et al. [23] compared the cycle behavior of Ni-doped and undoped Mg/MgH2 using Hydrid-Wasserstoff-Technik (HWT) equipment for a high-temperature TCES system. They found that the cycling stability of Ni-doped Mg/

2. Hydride systems 2.1. Metal hydride system Three metal hydride systems have been studied for thermal energy storage: lithium hydride (LiH), calcium hydride (CaH2) and magnesium hydride (MgH2) systems. In this paper, we only focus on MgH2 system for thermochemical energy storage (TCES) because limited attention has been paid to both CaH2 and LiH systems during recent years. Mg/

Fig. 1. Typical kinds of TCES systems at high temperatures (673–1273 K). 793

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MgH2 materials was determined by the hydrogenation-dehydrogenation operation conditions. Particularly, it was under a very mild operation condition where the Ni-doped Mg/MgH2 materials presented stable reversibility over several numbers of cycles and reacted in the high hydrogenation/dehydrogenation rates. In addition, Kumar et al. [24] doped nano Fe into MgH2 via ball milling and also investigated hydrogenation-dehydrogenation kinetics by means of thermogravimetric analysis and differential scanning calorimetry (TGA-DSC). No significant change in the cycling stability of Fe-doped MgH2 after 50 hydrogenation-dehydrogenation cycles was found. However, its activation energy decreased by 30 kJ/mol. Later, Puszkiel et al. [25] used the same method to investigate the hydrogen cycling performance of xMg-Fe (x = 2, 3, and 15) at a constant temperature (375 °C). After 1000 cycles, the 15 Mg-Fe material showed the highest cyclic reaction conversion. While the cyclic reaction conversion of 15 Mg-Fe remarkably decreased during the first 40 cycles, but then more gradually, which might have resulted from the Mg evaporation and the formation of Mg2FeH6. Agglomerates were observed over 1000 cycles using scanning electron microscopy (SEM), X-ray diffraction (XRD) and particle size analyzer (PSA). Moreover, the agglomerates size of 15 Mg-Fe material was within a range of 5–50 μm, which was smaller than that of the other materials. MgH2-based composites, namely combined with other metal hydrides, are another means to overcome the limitation of Mg/MgH2. Liu et al. [26] prepared MgH2-AlH3 composite materials via ball milling. Meanwhile, they investigated the kinetics and cycling stability of these materials. The results showed that their activation energy decreased. Their charging process was divided into two steps: (a) MgH2 and Al forming Mg17Al12; and (b) self-decomposition of the residual MgH2. In addition, the composite materials showed a good cycling stability over 5 h without decay in the cyclic reaction conversion. Quyang et al. [27] studied the cycling stability and kinetics of CeH2.73-MgH2-Ni composite materials, prepared by induction melting. Their reaction temperature was 100 K lower than that of pure MgH2, and the activation energy of CeH2.73-MgH2-Ni decreased to 63 ± 3 kJ/mol during the charging process. This value of activation energy was much lower than that of pure MgH2 (158 ± 2 kJ/mol). However, excellent cycling stability over 500 cycles was also observed via XRD and transmission electron microscopy (TEM), which was attributed to the composite materials effectively suppressing Mg/MgH2 grain growth. In 2010, graphene was awarded the Nobel Prize in Physics owing to its unique two-dimensional (2D) nanostructure, light weight, excellent thermal conductivity and high chemical stability. It is hoped that ultrathin graphene could not only improve the thermal conductivity of MgH2/Mg, but also prevent agglomeration over numbers of cycles. Xia et al. [28] proposed a simple and facile surfactant-free, hydrogenationinduced, solvothermal method to enable MgH2 powder to homogeneously disperse on the surface of graphene. They used TGA to study the cycling stability and thermodynamics of MgH2/Mg. It was found that the cyclic reaction conversion was 98.4% with a 0.43 × 10−3 wt% attenuation ratio per cycle after 30 cycles. Therefore, it is believed that the MgH2-graphene composite material could easily achieve an ultralong cycle life and high thermal conductivity. Singh et al. [29] investigated the effect of graphitic nanofibers on MgH2/H2 using a combination of experiments and DFT calculation. They found that the onset dehydrogenation temperature of graphene-doped MgH2 decreased by 45 °C, which could be explained by electron transfer. The graphene edges (C-Mg-H) could grab more electronic charge from the charge of the Mg-H because of the high electronegativity on the graphene edge. Therefore, the Mg-H bond weakened, and the dehydrogenation energy subsequently decreased. The aforementioned studies mainly focused on the reaction kinetic and cycling stability of Mg/MgH2 with alloying and composite materials using TGA. In addition, Gambini et al. [30] established a numerical model to evaluate the behavior of an Mg/MgH2 TCES system and found that the numerical results agreed well with the experimental results.

2.1.2. Heat storage performance To maximize the industry application of Mg/MgH2 materials, the system should be studied. In general, an Mg/MgH2 TCES system is composed of a reactor, several heat exchangers, Mg/MgH2 storage tanks, and an electric generator. The reactor is the most important part in the Mg/MgH2 TCES system. A fixed bed is usually recognized as the most suitable reactor during the charging and discharging steps, but its poor effective thermal conductivity is notorious. In order to improve the thermal conductivity of the fixed bed, Gambini [31] developed a lumpedparameters model for the Mg/MgH2 reactor, which was conducted to describe the mass and heat transfer of the Mg/MgH2 TCES system. This model can be used for the heat transfer design of the reactor for TES systems. Then, Askri et al. [32] analyzed the heat transfer performance for a 2D Mg/MgH2 reactor, considering the radiative heat transfer. It was found that the radiative effects on hydrogenation conversion were very sensitive. Shen et al. [33] developed a mathematical model to investigate the effects of metal foam and its porosity on heat transfer performance in the Mg/MgH2 reactor. Their results indicated that the addition of metal foam not only promoted exothermic power, but also lead to a 40% reduction in reaction time. In particular, both the reaction rate and output power achieved a peak value when the porosity was 0.96 and the reaction temperature was 620 K. In addition, a variety of novel reactors have been designed to alleviate the poor heat and mass transfer in a fixed bed reactor. Bogdanovic et al. [34] tested a process steam generator, as shown in Fig. 2. It was composed of a cylindrical pressure tank for Mg/MgH2 storage, a helical tube used as a heat recovery system, and a central sinter metal tube for hydrogen flow. Sekhar et al. [35] constructed a prototype for a metal hydride system (Fig. 3) and calculated the coefficient of performance (COP), specific heating power (SHP), and exergy efficiency to predict the performance at different operating temperatures. It was found that the COP and SHP, for a single-stage metal hydride system, increased by approximately 13.3% and 17.6% at a temperature ranging from 393 K to 413 K, respectively. While, the exergy efficiency decreased 5%. A double-stage metal hydride system showed the same results, with an 11.5% and 24% increase in both COP and SHP as well as a 3.3%

Fig. 2. Steam generator process based on MgH2. (Reprinted with permission from [34].) 794

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10 6 5

42

18

52

8

108

35

1

4

7 3

2

9

11

Fig. 3. Schematic of cylindrical reactor (1-Filter, 2-Copper fins, 3-End plate, 4-Outer flange, 5-Metal hydride, 6-Teflon washer, 7-Inner flange, 8-Reactor, 9Thermocouple, 10-Inlet of heat transfer fluid, 11-Outlet of heat transfer fluid). (Adapted with permission from [35].)

reduction in the exergy efficiency [36]. Paskevicius et al. [37] modified the former metal hydride TCES reactor that could operate with 20 g of materials in a 2250-cm3 gas tank, as shown in Fig. 4. They investigated the Mg/MgH2 thermal behavior in the reactor using cold water into the reactor. According to the temperature gradients, it was argued that the environmental heat loss could be determined by the reactor geometries and its structure. Bhouri et al. [38] developed a one-dimensional (1D) finite element model and simulated the charging process of 2LiNH2-1.1MgH20.1LiBH4-3 wt% ZrCoH3 (CxH) and MgH2-LaNi4.3Al0.4Mn0.3 (MeH) in a tubular reactor, as shown in Fig. 5. A dimensionless number was developed to compare the dominance of the reaction kinetics and the heat diffusion between the storage media. The results showed that the thickness of CxH and MeH determined the diffusion of the reaction

heat. In addition, they [39] proposed a conceptual MgH2-Mg(OH)2 TCES system, as shown in Fig. 6, which was a combination of hydrogen storage and TCES. Compared to a traditional Mg/MgH2 reactor, there are two advantages as follows: (a) less mass for the heat storage media and (b) wider range of operating pressure conditions for the MgH2-Mg (OH)2 TCES system. A 2D simulation of the thermal interaction between the two storage media in the reactor was also provided and a shorter hydrogen storage time (0.5 h) could be realized [40]. These studies did not exactly describe the performance of a largescale TCES system, but these studies did help to assess the geometries and design of TCES reactors under realistic operating conditions. The following studies mainly focused on metal hydride system integration. Gambini et al. [41] provided some crucial parameters to evaluate the performance of metal hydride TCES systems, namely the cycle life,

Stainless Steel

Pump

T

Heat Transfer Loop

Hydrogen Gas Store

High Temperature Metal Hydride Bed

Top

Side

T

Heat Engine 1 0 mm

(a)

(b)

Fig. 4. The high temperature metal hydride TCES system. (a) Thermal heat storage prototype system; (b) Cross section profiles of the metal hydride reactor. (Adapted with permission from [37].) 795

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Fig. 5. (a) The cross-section of the combination storage system and (b) the geometry used for computations in the 1D model. (Adapted with permission from [38].)

(a)

step reaction. Storage of Hydride Reaction Heat, Qabs

H2

Mg

MgO Tabs>Tdehy

Tc Qc

Mg(OH)2

MgH2

2.1.3. Summary Detailed recent advances in Mg/MgH2 TCES system have led to the following conclusions: First, the Mg/MgH2 TCES system is generally limited by a slow hydrogenation-dehydrogenation kinetic and a poor reversibility under high temperature and pressure conditions because of its high thermodynamic inertness and sluggish kinetics. To obtain cycle life extension and kinetic enhancement, some important breakthroughs have been achieved via three effective approaches including the construction of a nanostructure, the addition of alloying, and MgH2-based composites. Regarding the construction of a nanostructure, both theoretical calculations and experimental results prove that nanoscales can enlarge MgH2 porosity and the particle reaction surface, and thereby increase the reversibility of the Mg/MgH2 TCES materials. Nevertheless, it is difficult to improve its sorption properties and maintain its original thermal conductivity at the same time using this approach. Based on a nanostructure, the cycling stability can be increased via the addition of alloying or a combination with other metal hydrides. Table 1 summarizes some widely used additives in Mg/MgH2 as well as their characteristics. It is notable that graphene is helpful to improve the cycling stability of Mg/MgH2, but this composite material remains at the lab-scale stage. Overall, these materials could not completely meet the commercial application demands of a TCES system according to the current status of the research progress. Thus, additional research efforts are required to shift to the modification of existing MgH2-based composites from the microcosmic perspective. In particular, theoretical calculations based on molecular dynamics or DFT are able to provide an in-depth understanding of the charging-discharging properties and cycling behavior at a microscale, which can guide the rational design of MgH2-based materials with a stable cycle life. Second, a fixed bed as an Mg/MgH2 reactor has been widely investigated for a long time because of its low manufacturing cost and easy to industrial enlargement. However, the Mg/MgH2 particle cannot be completely heated in a fixed-bed reactor, arising from its low thermal conductivity, and therefore it is difficult for them to react with one another. Therefore, most research is focusing on heat performance analysis coupling the chemical reaction with heat transfer. The results have found that the addition of fins and metal foams can effectively improve heat conduction inside the fixed-bed reactor, and accordingly

H2O(g)

H2 Absorption Reaction

Dehydration Reaction

Exothermic

Endothermic

Condensation

(b) H2

Release of Hydration Reaction Heat, Qhyd

Mg

MgO Tdes
MgH2 H2 Desorption Reaction Endothermic

Mg(OH)2 Hydration Reaction Exothermic

H2O(g) Tv Qv H2O(l)

Evaporation

Fig. 6. Operating principle of magnesium hydride reactor using magnesium hydroxide as heat storage media: (a) Absorption of H2/Dehydration of Mg (OH)2/Condensation of H2O and (b) Desorption of H2/Hydration of MgO/ Evaporation of H2O. (Adapted with permission from [39].)

thermal input and output, effective attainable temperature levels, and efficiency. Ward et al. [42,43] proposed a steam power plant (Fig. 7) with Na3AlH6-NaMgH2F and NaMgH3-NaAlH4 as reacants. It was found that the new composite materials could promote cycle life and thermal conductivity, and thus obtained a 5% promotion in exergy efficiency. In addition, the system with Na3AlH6-NaMgH2F cost less than 30 $/kWth, which is 11% lower than that of the other material (NaMgH3-NaAlH4). Bao et al. [44] simplified the metal hydride TCES system, which is composed of a metal hydride reactor, a shut-off valve, and a hydrogen storage tank. Models of the charging process and discharging process were developed. As a result, the outlet temperature during the discharging process increased with a reduction in the flow rate of the heat transfer fluid. Additionally, the outlet temperature for a multi-step reaction showed less fluctuation and was more stable than that of the one796

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Sun available Sun not available Steam power plant Heat In

Heat Out Turbine

Solar Concentration system

H2

Heat Out

H2

Heat In

HT metal hydride

Steam generator

Condenser

LT metal hydride Pump

Fig. 7. Steam power plant. (Adapted with permission from [42,43].)

field during recent years owing to its reversibility and high energy density. The reaction is written as Eq. (2). Considering its mature kinetic studies based on the Haber-Bosch process, we only review the heat storage performance of ammonia TCES system here.

Table 1 Summary of some widely used additives in Mg/MgH2. Material

Method

Ni

Ball milling

Fe

Ball milling

AlH3

Ball milling

CeH2.73-Ni

Induction melting

Graphene

SolvethermalBall milling

Characteristic ● Activation energy ● Cycling stability ● Hydrogenation/ dehydrogenation rate ● Activation energy ● Agglomerate size ● Activation energy ● Cycling stability ● Two steps dehydrogenation ● Activation energy ● Cycling stability ● Cycling stability ● Thermal conductivity

Reference [23]

2NH3(g) + H

N2(g) + 3H2(g)

H = 66.9 kJ/mol

(2)

2.2.1. Heat storage performance In 1999, the ammonia reversible reaction was proposed by Australian National University (ANU) for a TCES system (Fig. 8) and an attempt was made to measure thermodynamic data in the reactor, which was used for an initial assessment of the theoretical limits on thermal efficiency and work production [45]. After feasibility verification, a techno-economic analysis was performed and demonstrated that a capital investment of AUD 180 million was required for a CSP plant based on an ammonia TCES system. Moreover, this system could reach an 18% solar-to-electric conversion efficiency and an 80% capacity factor, leading to reduction in electricity costs (less than AUD 0.25 per kWh) [46]. Later, a 2D pseudo-homogeneous model was developed in a packed-bed catalytic reactor [47,48], which was validated with experimental results in a 1-kW ammonia synthesis reactor. This model not only showed that a higher operating pressure resulted in higher thermal output, but it could also contribute to scaling the reactor up. In 2001, an experimental investigation was conducted concerning the effect of operational parameters on thermal output [49]. Among most of the factors, three parameters (pressure, mass flow, and inlet gas composition) were found to have a great impact on thermal output. In addition to these operational parameters, the average temperature of the reactor walls was also dominant in achievable output. Then, an exergy analysis was completed for a 30 MPa isobaric system in order to maximize the thermal output in the system, and it was found that the major exergy destruction occurred in two locations [50]: the

[24,25] [26] [27] [28,29]

promote exothermic power. On the other hand, several novel reactors (Figs. 2, 3 and 5) have been proposed and demonstrated that they are valid to overcome the limitations of heat and mass transfer in the fixedbed reactors (such as 13.3% and 17.6% promotion of the COP and SHP for Fig. 3). However, these reactors are still at the prototype design stage. In addition, a few numerical studies of these reactors have attempted to assess the heat diffusion and mass transfer performances, while its validation is ignored. Thus, it is necessary to investigate the heat and mass transfer in these reactors based on practical experimental results to rationalize the numerical studies. Finally, research regarding the integration of an Mg/MgH2 TCES system and CSP plant remains at the stage of conceptual process design. Thus far, a few system integrations based on the Mg/MgH2 reversible reaction have been proposed. However, these system integrations are too simple to fully take advantage of reaction heat which would allow for a compact heat recovery exchanger network for a reduction in the energy penalty. In other words, the performance of the system integrations based on Mg/MgH2 TCES must be improved in the future, particularly, using compact heat exchangers and highly efficient solar collectors. The main challenges of the Mg/MgH2 TCES system are summarized in Table 2.

Table 2 Challenges of the Mg/MgH2 TCES systems. Name Mg/MgH2

2.2. Ammonia system Ammonia synthesis with an iron catalyst has been used for chemical fertilizer production at high temperatures (673–973 K) and high pressure (10–30 bar) for 120 years. This reaction has extended to the TES 797

Energy density ● 580 kWh/m ● 0.8 kWh/kg

Challenges 3

● Sintering ● Low thermal conductivity ● High operating pressure ● System integration design ● Economic assessment

Related technology ● H2 production

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Ammonia Synthesis (Exothermic Reactor) Ammonia Dissociation (Endothermic Reactor)

Heat Exchangers

H2/N2 gas

Power Generation (Steam Cycle)

liquid NH3

Separation and Storage Fig. 8. An ammonia-based TCES system. (Adapted with permission from [45].)

exothermic reactor and the counter-flow heat exchanger between the ingoing and outgoing reactants. After optimization, the reactor could achieve an exergy efficiency of 71% in theory, namely of conversion efficiency of approximately 20% from solar power to electricity [51]. After decades of research at ANU, a reliable system consisting of a 15kW reactor and a 20-m2 dish solar concentrator was successfully constructed (Fig. 9). Iron-based catalysts were used in the ammonia system to maximize the potential for electrical power. Finally, a 53% of energy storage efficiency for this system was achieved [52]. During recent years, Chen et al. proposed an ammonia-based TCES system integrated with a supercritical steam power block [53]. The system was composed of a short reactor and long reactor. The short reactor was only used for ammonia synthesis and the other was employed to produce supercritical steam. Later, they built a long reactor with a steam tube in the centre and a porous bed of iron catalyst around the tube [54]. The reactor was first experimented to heat steam at ∼26 MPa from ∼350 °C to ∼650 °C based on an ammonia synthesis reaction for a supercritical steam power block. Then, models were developed to describe temperature distributions and ammonia yield in a concentric-tube ammonia synthesis reactor during the steam heating process [55]. The model clarified the relation between reaction kinetics Heat exchanger

and heat transfer. Heat transfer was limited by a decreasing reaction rate. Furthermore, increasing the heat transfer coefficients led to improved performance, but resulted in reaction rate limitation. In 2018, they attempted to minimize the volume of reactor wall material to decrease initial capital costs because the wall material (a high-temperature, creep-resistant, nickel-based alloy material) was estimated to be the highest priced in the whole ammonia TCES system [56]. It was found that the smaller diameter of the reactors and the faster mass flow rates contributed to less material volume. 2.2.2. Summary Detailed recent advances in an ammonia-based TCES system have led to the following conclusions: First, ammonia synthesis is a mature technology based on the Haber-Bosch process, which can provide sufficient reference for the improvement of cycle life. However, current catalytic technology cannot achieve a total conversion of both the forward and reverse reactions, which leads a reduction in the thermal energy absorbed and released in commercial application as compared to calculation in theory. In the past, Haber et al. focused on the effect of thermodynamic parameters and types of catalysis on ammonia synthesis to identify the

insulation

Feed header

Product header

675mm

Focal plane of dish

Water-cooled Lambertian shield

Cavity radiation shield Reactor tube (length 500 mm) 200mm 600mm Ring support structure

(a)

(b) 2

Fig. 9. (a) The cavity receiver with 15 kW solar ammonia dissociation reactor and (b) its assembly on ANU’s 20 m dish. (Adapted with permission from [52].) 798

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optimal reaction conditions, but the micro reaction mechanism remains unclear. Thus, further investigation is required to improve the conversion of both the forward and reverse reactions from the micro perspective in order to propose a novel catalysis for overcoming this drawback. Second, the ammonia-based TCES is usually connected with a dish concentrator to produce steam for power generation because the dish can provide a circumferentially homogenous solar flux profile. A large dish concentrator (489 m2) with an ammonia-based TCES system is now technically achievable, but conventional industry practice suggests very slow ramp rates for the ammonia synthesis process, which results in a problem of intermittency between the ammonia-based TCES system and electricity demand. Thus, some innovative technologies need to be developed in the future to make the system more flexible. Finally, the ammonia TCES system proposed by ANU can achieve 20% of conversion efficiency from solar power to electricity in theory (namely 71% of exergy efficiency), but only 53% of energy storage efficiency for this system can be reached in practice. On the other hand, a novel idea of system coupling with a supercritical steam power block was recently proposed, which is apparently better than superheated steam for power generation as suggested by ANU. However, this idea is still at the lab-scale experimental stage, mainly focusing on the reactor design. Thus, parameter analysis and their energy efficiency in this novel power system is required to be developed for efficiency improvements in the energy production, as well as taking the experimentally charging and discharging characteristic into account in the future. The main challenges of the ammonia TCES system are summarized in Table 3.

value. However, a remarkable reduction in the hydration capacity of the composite materials occurred over 500 cycles because of the formation of hydrated silicates as shown via XRD analysis [61]. Sakellarious et al. [62] prepared a mixed calcium oxide-alumina composition using synthetic materials (calcium nitrate and calcium acetate). The latter led to materials with higher surface areas, which resulted in higher hydration/dehydration performance. However, a considerable reduction in hydration/dehydration capacity for the composite materials was identified except for a 0.11 and 0.19 M Al/Ca ratio. Kariya et al. [63] combined Ca(OH)2 with vermiculite using the impregnation method, and subsequently investigated the kinetics of the composite material. Vermiculite is a highly porous, chemically stable, and low-cost material. The hydration rate of this composite material (0.62 × 10−2 s−1) was less than that of the pure Ca(OH)2 during the first cycle, but its cycling stability was superior to that of the pure Ca (OH)2 over 15 cycles, which could be explained by the reaction constant and reaction order. Notably, the reaction order is more dominant than the reaction constant during the hydration process based on the grain model. Following the core shell principle, Afflerbach et al. [64] developed semipermeable ceramic materials, which were porous and closed shell throughout, wrapping around the CaO/Ca(OH)2. More stable reversibility over several hydration-dehydration cycles could be achieved by transferring water steam via a porous ceramic shell. At a low steam partial pressure, the experimental results also showed that the weight of the semipermeable ceramic materials was only 56–58% of the weight of the pure CaO/Ca(OH)2, and the volume was 33% of the unmodified CaO/Ca(OH)2 storage material. From an atomic perspective, Yan et al. [65] investigated CaO/Ca (OH)2 with Li or Mg doping based on DFT and transition state theory. The results indicated that the effect of Mg-cation doping on the heat storage process was negligible. In contrast, the time required for the Lidoped CaO/Ca(OH)2 charging process was less than that for undoped CaO/Ca(OH)2, which could by the (a) modified crystal structure and symmetry of the transition state, (b) the reduction in the energy barrier of the heat storage process from 0.40 eV to 0.11 eV, and (c) the more readily broken OH bonds of Ca(OH)2. The experimental results were well in accordance with the analysis of the DFT [66]. Xu et al. [67] simulated the agglomeration behavior of CaO/Ca(OH)2 using the molecular dynamic method. They concluded that the higher the temperature, the more expansion and grain agglomeration. Meanwhile, the larger particle diameters and separation distances, the slower the grain agglomeration rate. On the other hand, Yan et al. [68] found that CO2 had a negative effect on the CaO/Ca(OH)2 TCES system. It was found that CO2 could react with CaO/Ca(OH)2 under a dry condition, leading to a slight amount of CaCO3 generation, which could result in agglomeration and sintering. When the reaction occurred under a wet condition, the side effect was worse in that the amount of CaCO3 increased by 10.7% and 28.7%, respectively. Furthermore, this effect was aggravated with an increasing number of cycles. The aforementioned studies were mainly conducted using TGA. However, during recent years, the cycling behavior of CaO/Ca(OH)2 TCES systems has gradually been carried out in lab-scale reactors. Roβkopf et al. [69] investigated the agglomeration behavior in a small

3. Metal oxide systems 3.1. Hydroxide system There are two main hydroxide systems used for TCES applications, including MgO/Mg(OH)2 and CaO/Ca(OH)2. In this paper, we only focus on the CaO/Ca(OH)2 TCES system. One reason is the similarity of both reaction mechanisms and the energy density (approximately 0.39 kWh/kg [17]). The other reason is that MgO/Mg(OH)2 is more readily decomposed at a relatively low temperature (≈330 °C), which is beyond the scope of this work, while operating temperatures of the CaO/Ca(OH)2 TCES system is from 623 to 1173 K and its steam partial pressure is from 0 to 2 bar. The reaction is as follows:

Ca(OH) 2(s) + H

CaO(s) + H2 O(g)

H = 104 kJ/mol

(3)

3.1.1. Cycle life During recent years, considerable works regarding the reaction kinetics and cycle life have been performed. Ervin [57] was the first to use the CaO/Ca(OH)2 reaction for a thermal energy storage plant. Schaube et al. [58] indicated that the reaction enthalpy was measured as 104.5 kJ/mol at 505 °C and an H2O partial pressure up to 1 bar, in which its reversibility remained stable over 100 cycles. Criado et al. [59] also investigated the reaction mechanism of CaO/Ca(OH)2 in three aspects: (a) temperature, (b) partial steam pressure, and (c) particle size. The experimental data were well in accordance with a shrinking core model for both the charging and discharging processes. However, these studies noted the intrinsic drawback of the CaO/Ca (OH)2 TCES system: poor mechanical properties resulting from particle attrition. This limitation leads to reduction in chemical activity and agglomeration. In response, several novel composite materials have been proposed. Criado et al. [60] offered a new composite material to improve the mechanical and reactivity properties. It was found that the lowest crushing strength (CS) value could be reached (CS > 26 N) over 200 cycles, when the molar Ca/Si was 4.8–6.2 at a particle size of 36–63 μm. Hydration conversion also accordingly achieved the peak

Table 3 Challenges of the ammonia TCES system. Name NH3/H2

799

Energy density ● 745 kWh/m3 ● 1.09 kWh/kg

Challenges ● ● ● ●

Gases safety High operating pressure Incomplete reaction Economic assessment

Related technology ● H2 production

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lab-scale reactor, as shown in Fig. 10. The results revealed that the addition of small amounts of SiO2 could help to prevent agglomeration over 8 cycles and the time required for both the hydration and dehydration processes was considerably reduced. Later, they also studied the composite materials of which Ca(OH)2 particles acting as guest particles were attached to the SiO2 surface in an indirect operating pilot-scale thermochemical reactor (Fig. 11) and an addition of up to 30 wt% of SiO2 [70]. The side reaction product was observed during their experiments (Ca5(SiO4)2(OH)2), leading to a stabilization of the surface structure at the expense of a capacity loss of the thermochemical reactor. Based on kinetic parameters from TGA and a single crossflow factor, Rouge et al. [71] derived a standard KL bubbling reactor model that took the operation mode into account in both the dynamic and steady state. Thus, this model could exactly describe average hydration conversion in the fluidized bed. In particular, the model-predicted molar mass of the outlet steam was in good agreement with the experimental results. Most recently, Criado et al. [72] further demonstrated the validity of the KL bubbling reactor model. They experimented with the reaction behavior of the hydration-dehydration cycles based on this model, and pointed out that rapid dehydration kinetics resulted in remarkable emulsion-bubble mass-transfer resistances.

However, this factor made it more difficult for reaction gas to escape from the rector, where the remaining gas increased the probability of a slow reaction or reverse reaction. In contrast, a highly porous bed reactor offered a desired reaction rate but with a lower energy output. As for size, it was suggested to avoid an increase in size because increasing the height of the reactor limited the steam flow owing to the pressure drop. In addition, stretching the width further reduced the heat transfer in the reactor because of the poor thermal conductivity of the material property. Azpiazu et al. [80] analyzed the thermal behavior of hydrationdehydration cycles in a fixed-bed reactor, equipped with fins to promote heat transfer and composed of Cu. The experimental results showed that heat generated from the modified reactor could increase the water temperature from 0 to 33.3 °C, and the heat efficiency was improved by 72.5%. However, poor cycling stability was observed over 20 cycles because of carbonation below a temperature of 900 °C. When increasing the temperature to 1000 °C, the side reaction was effectively reduced. The aforementioned studies describe the heat storage performance of a CaO/Ca(OH)2 TCES system in a fixed-bed reactor from various aspects. These included operating conditions, reactor size, chargingdischarging characteristics, etc. While, Schauble et al. [81] indicated that an upscaling of a fixed-bed reactor was considered uneconomical owing to its poor thermal conductivity and large pressure drop in the fixed bed. Thus, it was argued that a fluidized bed was more suitable as a reactor. Pardo et al. [82] investigated 1.93 kg CaO/Ca(OH)2 with inert particles (SiO2, Al2O3 and SiC) in a fluidized-bed reactor at 480 °C for a hydration reaction and 350 °C for a dehydration reaction. The inert particles played a major role in the powder fluidization and sensible heat recovery. The experiment found that 30 wt% Ca(OH)2 with 70 wt% Al2O3-B was the most appropriate proportion, which amounted to an energy density of 60 kW·h/m3 Ca once the reactant and the easyto-fluidized particles were separated. Moreover, the sensible heat of the Al2O3 powder accounted for less than 20% of the total stored thermal energy (15 kW·h/m3) during the 4-h charging process. Criado et al. [83] proposed a conceptual process design of a CaO/Ca (OH)2 TCES system using fluidized-bed reactors for a reference case with a heat output of 100 MWt, as shown in Fig. 15. A mass and heat transfer model was developed. As a result, an effective energy density of 260 kWh/m3 and net process efficiency of 63% were obtained with reasonable activities of the solids.

3.1.2. Heat storage performance The reactor as an important container determines the heat storage performance of a CaO/Ca(OH)2 TCES system to a large extent. A fixed bed reactor, until now, has been the most commonly studied for the CaO/Ca(OH)2 TCES system. Schaube et al. [73] made a comparison between a CaO/Ca(OH)2 TCES reactor with indirect and direct heat transfer based on the Finite Element Method. They found that the thermal performance of the reactor with direct heat transfer was superior to that with indirect heat transfer. Later, they built a type of reactor with direct heat transfer and conducted an experiment. The heat output and the particle reaction rate were found to be the two dominant limiting factors for thermal flux, after comprehensively assessing the kinetic parameters of the CaO/Ca (OH)2 reaction, cycling stability, heat and mass transfer, and so on [74]. However, Schmidt et al. [75] designed a reactor with an indirect heat transfer, as shown in Fig. 12. In their experiment, 77% of the cyclic storage conversion was obtained without degradation over 10 cycles. Meanwhile, two modes were successfully conducted in the reactor, including peak power and nominal power. The peak power occurred when the air outlet temperature could be maintained above 450 °C for 35 min, releasing nearly 7.5 kWth of thermal power. While, the nominal power released 3 kWth for 75 min. Then, they attempted to use 2.4 kg CaO/Ca(OH)2 at a low water vapor pressure in this reaction bed (Fig. 12) [76]. CaO/Ca(OH)2 under this reaction condition demonstrated an increase in process integration possibilities, as well as the overall thermal capacity of the CaO/Ca(OH)2 TCES system. The experimental results showed that the CaO/Ca(OH)2 reaction rate was very sensitive to small changes in the reaction conditions. Most recently, they [77] proposed a CaO/Ca(OH)2 TCES system in a CSP plant (Fig. 13), where its storage efficiency of up to 87% could be reached. Yan et al. [78] analyzed the charging-discharging characteristics of CaO/Ca(OH)2 in a fixed bed reactor (Fig. 14). During the heat storage process, a higher dehydration temperature not only increased the heat storage rate, but also led to a higher heat storage efficiency (47% at 510 °C and 65% at 540 °C). During the heat release process, a high hydration conversion could be reached by reducing the initial temperature and increasing vapor pressure at the same time. In particular, the conversion was 31.7%, 60.9% and 72.8% under vapor pressures of 0.18 MPa, 0.24 MPa, and 0.32 MPa, respectively. Ranjha et al. [79] developed a 3D model to analyze the heat and mass transfer in a fixed-bed reactor. It was found that two factors had an effect on the heat and mass transfer: porosity and size (height and width). A lower bed porosity could increase the energy density.

Fig. 10. (a) Lab-scale reactor, photographic image and (b) position of powder bed and thermocouple. (Adapted with permission from [69].) 800

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strengthen the heat transfer (equipping with fins or the addition of metal foam) have been developed for the fixed-bed reactor. But pressure drop and mass transfer barriers remain a significant limitation in a large-scale fixed-bed reactor. In light of these drawbacks, it has been argued that a fluidized-bed reactor is a better alternative for a CaO/Ca (OH)2 TCES system, benefiting from sufficient interaction between heat fluid and solid particles. However, a serious drawback of back mixing inevitably occurs in a large-scale fluidized-bed reactor, which can increase the temperature difference between the top and bottom of the reactor, and thereby lead to an inadequate CaO/Ca(OH)2 reaction. At present, this limitation is insufficiently discussed because most studies have focused on the solid behavior (CaO/Ca(OH)2) during the chargingdischarging process at a lab-scale test. Moreover, an investigation of thermal behavior is rarely involved. Therefore, advanced efforts should be directed at large-scale reactor design and operation for future industrial application of the CaO/Ca(OH)2 TCES system. Finally, the research regarding system integration of CaO/Ca(OH)2 TCES is at an initial stage. During a typical discharging process, the released thermal energy can directly transfer heat with unreacted water vapor, resulting in superheated or supercritical steam for electricity generation. This is among the advantage of the CaO/Ca(OH)2 TCES system. Therefore, it is important and interesting to both feasibly and economically design industrial waste heat recovery systems for renewable energy power plants or other industrial plants, using compact heat exchangers, an advanced CaO/Ca(OH)2 TCES system, and more efficient expanders, in order to satisfy climate and energy targets. The main challenges of the CaO/Ca(OH)2 TCES system are summarized in Table 4.

Fig. 11. An indirect operating pilot-scale thermochemical reactor. (Adapted with permission from [70].)

3.1.3. Summary Detailed recent advances in the CaO/Ca(OH)2 TCES system have led to the following conclusions: First, a poor mechanical property is considered a major problem leading to decay in cycle life. The addition of inert materials (SiO2, Al2O3, vermiculite, etc.) is among the methods to overcome this limitation, but the attenuation of the cycle stability does not disappear. Some research has attempted to explain the reason why the addition of inert materials is conducive to increasing the cycle stability from a micro perspective, while it is not sufficient to associate the factor of inert materials into CaO/Ca(OH)2 with an enhancement of cycle life. Thus, further research is required to explore reasonable descriptors to identify the relationship between modified CaO/Ca(OH)2 intrinsic properties and cycle life, which shows the role of inert materials during the charging-discharging process. In addition, CaO/Ca(OH)2 TCES is a dynamic coupling technique of chemical reaction and heat transfer. Therefore, additional research efforts must concentrate on the variation in thermodynamic parameters (such as reaction enthalpy, energy density, specific heat, etc.) after adding the inert materials. Second, a fixed bed has thus far been mainly investigated for use in a CaO/Ca(OH)2 TCES reactor. The fixed-bed reactor is extensively used in industrial applications because of its ease of production and operation. However, the heat transfer is notorious for its poor effective thermal conductivity. To overcome this drawback, different methods to

3.2. Carbonate system Among many carbonate salts, CaO/CaCO3 is the most promising in the field of TCES systems because of its high energy density, earth abundance, non-toxicity, and so on. Although MgO/MgCO3 and PbO/ PbCO3 have also been proposed for the TCES system, controversy persists because of the high refractoriness of MgO/MgCO3 and toxicity of PbO/PbCO3. Therefore, this paper only focuses on CaO/CaCO3. Its operational condition is a temperature ranging from 973 and 1273 K with a CO2 partial pressure of from 0 to 10 bar.

CaCO3(s) + H

CaO + CO2(g)

H = 178 kJ/mol

(4)

Fig. 12. Top: heat exchanger plate used as reactor; bottom left: storage material filled into the frame; bottom right: filter plater to encase the reaction bed. (Reprinted with permission from [76].) 801

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Fig. 13. Process design of an indirectly heated thermochemical reactor in a CSP plant. (Reprinted with permissiion from [77].) 6 M

5

4

2

Table 4 Challenges of the CaO/Ca(OH)2 TCES system.

13

6

P M

2

P

T

3

Name

Energy density

Challenges

Related technology

9

CaO/Ca (OH)2

3 10

1

11 7

● 693 kWh/m3 ● 0.49 kWh/kg

● Agglomeration and sintering ● Low thermal conductivity ● Reactor design ● System integration design ● Economic assessment

T

12 8

Fig. 14. Fixed bed reactor (1-steam generator, 2-pressure gauge, 3-k-type thermocouple thermometer, 4-pipe, 5-flow control valve, 6-steam mass flowmeter, 7-heating tube, 8-vacuum pump, 9-high-temperature and high-pressure reactor, 10-electric heating system, 11-thermal insulation material, 12-heating storage materials, 13-outlet). (Adapted with permission from [78].)

temperature carbonation and calcination cyclic runs. It was found that the initial cycle was always 100%, but the cyclic conversion significantly decreased after several cycles because of the loss of pore volume. Benitez-Guerrero et al. [85] studied the multicyclic stability of different CaCO3 minerals (limestone, chalk, and marble). Despite similar composition (nearly pure CaCO3), marble and chalk provided a higher cyclic conversion as compared to limestone during the first cycle.

3.2.1. Cycle life Barker [84] proposed CaO/CaCO3 applied to the field of TCES technology. A cycling examination was performed during 40 high-

a)

b)

(7) H20(V)

(12) H20(V)

(4)

(14) H20(V)

H20(V) (10)

H20(V)

(3)

● Heat pump

(13) H20(V)

CaO

Ca(OH)2

(11)

(5)

QIN

QOUT CaO

CaO

Ca(OH)2

(1)

(8)

CaO (2) H20(V)

Ca(OH)2

Ca(OH)2 (9)

(6) H20(V)

H20(V)

Fig. 15. Process design of a CaO/Ca(OH)2 TCES system during (a) the hydration stage (discharge) and (b) the dehydration stage (charge). (Adapted with permission from [83].) 802

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However, after 20 cycles, the marble showed a most remarkable deactivation, followed by chalk, while the attenuation ratio for limestone was the lowest, which resulted from the differences in particle size and microstructure. Later, they conducted the same investigation for limestone, calcitic marble, and dolomite. It showed that pore-plugging resulted in a remarkable decrease in the natural CaCO3 minerals over a large number of cycles [86,87]. However, pore plugging is not a limiting mechanism in the case of dolomite arguably because of the presence of inert MgO domains, which help the diffusion of CO2 into the inner pores of the CaO particles. Thus, the addition of inert materials and a reduction in particle size are viable and promising options for improving the cycling stability of the CaO/CaCO3 TCES system. Lu et al. [88] prepared Li2SO4-doped CaO/CaCO3 using a simple wet solution impregnation method and found that the samples with an optimized content of 3.0–5.0 wt% Li2SO4 retained a 51% cyclic conversion over 11 calcination-carbonation cycles, which was higher than the 27.3% of pure CaO/CaCO3. The difference is attributed to an increase in the pore size and macro-pore proportion of the samples caused by a certain Zener pinning force, which was used to describe the additional resistance of grain boundary migration after Li2SO4 doping. Benitez-Guerrero et al. [89] prepared a relatively inexpensive, abundant and benign CaCO3/SiO2 TCES composites using rice husk and calcium nitrate based on a bio-template method and tested their cycling stability in the TGA. Their experimental results showed that the shortage of pore plugging could be relieved after addition of SiO2. Later, Chen et al. [90] measured thermodynamics properties of these CaCO3/SiO2 TCES composites in the DSC. A slightly negative effect on heat storage capacity was found because of SiO2, but 20% enhancement of specific heat capacity was also obtained in their paper. Wu et al. [91] suggested a type of nano CaO/Al2O3 composite material, which provided an increased cyclic conversion (68.3%) after 50 cyclic runs of high-temperature carbonation and calcination, arising from a newly formed substance of (CaO)12(Al2O3)7 at 800 °C, and thereby an increase in pore size of the materials. They also found that calcination and carbonation occur at a more rapid rate for the nano CaO/Al2O3 as compared to the micro CaO/Al2O3. This effect was also found by Benitez-Guerrero et al. [92]. In particular, 5% Al2O3 is the most appropriate molar ratio because increasing the content of Al2O3 can reduce the theoretical energetic efficiency, leading to a lower number of active materials [93]. Aihara et al. [94] used two preparation methods (the alkoxide and powder methods) to mix CaO/CaCO3 with the addition of calcium titanium. The results indicated that the composite materials prepared using the alkoxide method during the calcination and carbonation were 2.4 and 1.8 times more, respectively, than those prepared using the powder method. In addition, the novel materials effectively prevented grain agglomeration and improved the cycling stability. Then, Wang et al. [95] further investigated TiO2-coated nano CaCO3. A compact factor was developed and was used to quantify the coating compactness. It was found that the value of the attenuation ratio reached a minimum when the content of TiO2 ranged from 8% to 12% with a compact factor in a range of 0.9–1.3. In addition, for the addition of inert materials, the hydration of CaO is an alternative method to periodically regenerate the CaCO3 materials while maintaining its cycling stability at a high level. Its principle is that the CaO core expands and then fractures, as a result of the greater volume of Ca(OH)2 per molar than that of CaO, when CaO hydration occurs. Then, a further carbonation reaction can easily occur owing to an increase in both the surface and porosity of CaO available. Valverde et al. [96] performed an in situ XRD analysis on the hydration steam of CaO/CaCO3 and found that the calcination rate could be observably enhanced by H2O at very low concentrations. In addition, the reactivity and crystal structure of the reformed CaO remained nearly the same as the original, which meant that the cycling stability of CaO was not impaired.

7

8 5

2 1

ΔP 9 3

4

10 8

6

Fig. 16. Solar fluidized bed reactor (1-fluid bed, 2-concentrated solar rays, 3gas distributor consisting of glass, iron or zirconia beads, 4-grid, 5-transparent silica tube, 6-gas inlet, 7-gas outlet, 8-thermocouples, 9-reflectors, 10-pressure loss measurement). (Adapted with permission from [100].)

Blamey et al. [97] developed a shrinking core model to express the kinetic mechanism of CaO/CaCO3 in the presence of H2O using a TGA analyser at different hydration temperatures of 473, 573, and 673 K. The numerical model fitted data from the experiment and showed that increasing the hydration temperature resulted in an increase in the hydration rate. Subsequently, they further investigated the reactivation of CaO using steam in a lab-scale fluidized-bed reactor [98]. The results showed that the hydration extent was deeply affected by the calcination temperature and hydration temperature. Increasing both the calcination temperature and hydration temperature could lead to a higher hydration extent and thereby carbonation conversion following hydration increased linearly with hydration extent. Notably, the hydration was more suitable in a fixed-bed reactor rather than a fluidized-bed reactor because a considerable proportion of energy storage materials might escape from the reactor owing to the fluidized environment [99]. 3.2.2. Heat storage performance The reactor plays an important role in heat storage performance. In 1999, Shimizu et al. [100] proposed a calcium looping (CaL) process based on a fluid-bed reactor (Fig. 16), and showed an electricity efficiency of 46.6% during the first cycle and 42.6% during the second cycle steam turbines. Then, in 2001, Kato et al. [101] analyzed the temperature distribution of a CaO/CaCO3 reaction system in a packed bed reactor (Fig. 17). It was observed that approximately 800–900 kJ/ kg of heat was stored and the temperature increased to 997 °C during the carbonation process. A solar rotary kiln reactor (Fig. 18) was developed by Meier et al. [102] and reliably operated for more than 100 h in 2004. After many years of studies [103,104], the dual fluidized bed reactor has been widely adopted as the most appropriate for the CaL process because it avoids the limitation of heat transfer capacity in the fixed beds to enable energy charging and discharging steps very easily to match the thermal power involved in typical large-scale energy storage systems. In addition, large floor space and solid particle friction 803

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could be a serious drawback in a large-scale rotary kiln reactor for a TCES system because they make it necessary to increase the cost of whole system. On the other hand, the CaL process used for CO2 capture has been extensively investigated as a potentially clean and low-energy-penalty system during recent years, whereas an energy storage system based on the CaL process remains in the conceptual stage. Thus, some valuable conclusions can be learned from the CaL process based on CO2 capture, although thermodynamic conditions to achieve a high TCES global efficiency during the CaL process are radically different than those for CO2 capture [105-110].

9 10

8

1 2 3

4

7

6

11

● Models of an integrated CaL process and CSP can help to predict the effect of different variations (sorbent residence time, CO2 inlet molar ratio, and fluidization velocity) on the carbonation degree and efficiency, sorbent loss by elutriation, and the thermal power demand/produced in the reactor. ● Taking full use of the heat from the solid and gaseous streams

5 Fig. 17. Solar fixed bed reactor (1-reactor bed, 2-reactor tube, 3-reactor vessel, 4-insulation, 5-balance, 6-heating controller for joule heating around reactor tube, 7-vacuum pump, 8-CO2 bomb, 9-pressure regulation system, 10-thermocouples, 11-personal computer). (Adapted with permission from [101].)

2

4

1

7

5

6

(a)

3

(b) Fig. 18. Solar rotary kiln (a) reactor structure (1-refractor tube, 2-power inlet, 3-power outlet, 4-insulator, 5-axis of the kiln, 6-concentrated solar rays, 7-water cooled metallic shell) and (b) reactor photographs. (Adapted with permission from [102].) 804

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leaving the calciner can effectively contribute to higher energy efficiencies, smaller overall system size, and lower energy penalties. ● Coal-fired power plants with solar aided CO2 capture systems are better for energy recovery performance than solar aided coal-fired power plants with a CO2 capture system, in which the efficiency penalty of 13.44% for the former is lower than that for the latter (13.57%). ● The CaO/CO2 ratio and make-up flow of solids are important parameters in the CO2 capture system. Carbonation conversion increases as the molar ratios of CaO/CO2 increase. However, the cost of the system increases as well. In addition, increasing the make-up flows results in an enhancement of the carbonation extent, but this requires more heat and CaO at the calciner.

(PR = 3) between carbonator and turbine outlet.[114]. 3.2.3. Summary Detailed recent advances in the CaO/CaCO3 TCES system have led to the following conclusions: First, the cycling stability of the CaO/CaCO3 decreases with an increase in the number of cycles, as a result of pore plugging in the calcium oxide and agglomeration of the calcium carbonate. Two effective methods are usually used to overcome the limitation of the CaO/CaCO3: the addition of inert materials and the hydration of CaO. The key of the former means is to production of some strain that creates some additional resistance at the grain boundary migration after adding inert materials. This means is easily implemented and has been studied for many years; however, no composite material has been made commercially available thus far. This is because current studies regarding the addition of inert materials have mainly focused on a cyclic experiment using TGA and theoretical research, rather than cyclic behavior over a 1000 cycles. Meanwhile, hydration is the other promising means to recover the activity of CaO after several cycles. Studies during recent years have successfully achieved high carbonation efficiencies using the hydration of CaO under wet conditions, but the wet mixtures tend to agglomerate without desiccation at an industrial scale. However, the hydration method requires more capital costs to construct more reactors. Therefore, it is difficult to separate which means is better because each has its own merits and disadvantages. Second, the reactor design and research regarding the CaO/CaCO3 TCES system are rarely involved. Fortunately, lessons can be learned from the CaL based technology for CO2 capture, which has been extensively investigated as a potential technology for low-energy penalties during recent years. Among the various high-temperature reactors, the dual fluidized bed is considered an appropriate reactor for both the calcination and carbonation processes because of its greater performance in heat and mass transfer, lower floor space, and lower capital costs. Although there is a similarity between the technologies based on CO2 capture and the CaO/CaCO3 TCES systems in many respects, such

Based on these suggestions, Edwards et al. [111] developed a novel CSP plant system with calcium looping (Fig. 19) and derived heat and mass balance for an energy storage system. The results indicated that higher CaO activity levels increased power efficiencies, but also led to a smaller storage volume. However, power efficiencies were calculated as only 40–50% with carbonation activities of 15–40%. To further increase the power efficiencies of the TCES system, the Chacartegui group [112] investigated the effect of CaO conversion, turbine pressure ratio, turbine outlet pressure, and carbonator temperature on the energy storage performance based on Fig. 20. Three points can be summarized from their study: (1) a decreasing value in CaO conversion results in less heat loss in the calciner; (2) a high pressure allows the temperature at the turbine outlet and carbonator inlet to decrease; and (3) increasing the temperature at the turbine inlet enables higher plant efficiency. Later, they [113] compared several system configurations and concluded that a TCES system coupled with a closed CO2 Brayton cycle (CBC) presented better performance as compared to a steam reheat Rankine cycle and a supercritical CO2 Brayton power cycle. A comprehensive discussion regarding the method of power generation was also provided and a TCES system based on CaL using (CBC) could reach a maximum power efficiency of approximately 45% at pressure ratios

Fig. 19. Flow diagram of the CaL solar power plant. (Adapted with permission from [111].) 805

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CaO

CaO

CaO storage

Q Input

Q Released

CaCO3/CaO FCaO,unr+FCaCO3,crb

CALCINER CaCO3(s) → CaO(s) + CO2(g) ΔHro=+178kJ/mol

HEAT EXCHANGER NETWORK

CaCO3/CaO storage

CO2

HEAT EXCHANGER NETWORK

TUR

CO2 storage

CARBONATOR CaO(s) + CO2(g) → CaCO3(s) ΔHro=-178kJ/mol

CO2

COMP

Fig. 20. Conceptual CSP-CaL integration for TCES. (Adapted with permission from [113].)

as reactor type and reaction kinetics, there are other concerns that need to be considered before TCES industrial application. In particular, the thermodynamic conditions of the CaO/CaCO3 TCES system differ from those of CO2 capture in reaching high carbonation efficiency during the CaL process. This system involves carbonation under relatively low temperature and high CO2 partial pressure, and conversely calcination under a low CO2 concentration and high temperature. Thus, additional efforts should be made to clarify the physical and chemical processes inside the fluidized-bed for the CaO/CaCO3 TCES system to provide theoretical guidance for further reactor design. Finally, knowledge regarding the integration of the CaO/CaCO3 TCES system and CSP plant is not sufficient, although a few systems based on the CaL process have been developed. From the aforementioned studies, a CaO/CaCO3 TCES system coupled with CBC is the most promising because it can theoretically reach a maximum power efficiency of around 45%. This value is higher than that for CaO/CaCO3 TCES with a steam reheat Rankine cycle and a supercritical CO2 Brayton power cycle. However, it is clear that the current research only separately places emphasis on theoretical analysis and power efficiency optimization in the overall system. Thus, further research is required to investigate the characteristics of the CBC integrated into a TCES system in combination with simulations and experiments, as well as economic assessment. This technology not only abandons traditional water vapor as a medium, but also can result in a higher power efficiency. When this technology realizes industrialization, it could lead to a technological revolution in the field of thermal power generation, which is not only significant for CSP plants, but also is of a very important value in the steel industry and other renewable energies. The main challenges of the CaO/CaCO3 TCES system are summarized in Table 5.

[115], who derived an activation energy of 247.21 kJ/mol and 58.07 kJ/mol for the Co3O4 decomposition and CoO oxidation, as well as an Arrhenius constant of 1.968 and 1.506, respectively. There was no byproduct and no decay in cycling stability over 10 cycles. However, its toxicity and cost was controversial. Therefore, a combination with less harmful and inexpensive metal oxides might be advantageous. Agrafiots et al. [116] added metal elements (Ni, Mg, Na and Cu) into the cobalt oxide and investigated their charging and discharging process using a TGA-DSC method. The addition of Na changed the CoO microstructure without an increase in redox performance, while no significant variation occurred for Ni, Mg and Cu cobaltates, as compared to the pure Co3O4. Later, they mixed Co3O4/CoO and Mn3O4/Mn2O3 powders to study the thermal cycling stability of the mixture. It was demonstrated that both powders could be reduced and oxidized in complementary temperature ranges, thus extending the temperature operational window of the whole storage cascade [117]. Carrillo et al. [118] further showed a quantitative relation between both pure (Mn2O3 and Co3O4) and mixed oxides (Mn3-xCoxO4). It was concluded that Mn-doped cobalt oxides (x ≥ 2.79) showed a better performance in terms of kinetics, energy capacity, and cycle stability, compared to those of the Co-doped manganese oxides. This is because Co3+ for the Mn3-xCoxO4 with low x value (0.02 ≤ x ≤ 0.025) substituted Mn3+ on the manganese sesquioxide lattice, leading to thermal stabilization of the Mn2O3 and a progressive loss of cycling stability. Pagkour et al. [119] prepared cobalt oxide-alumina and cobalt oxide-iron oxide composites. These novel composite materials showed a higher redox performance than that of the pure Co3O4/CoO. Then, Andre et al. [120] elucidated that increasing the content of Fe led to a decrease in the redox activity and energy capacity of the Co3O4/CoO. Whereas, increasing the content of Fe could contribute to a 15% increase in the reaction rate, reversibility and cycling stability of Mn2O3/ Mn3O4, and a slight promotion of energy storage capacity. It was notable that an approximately 10% iron oxide was identified as having the most optimal energy storage capacity and it was beneficial in terms of microstructural stability because the material structure did not manage

3.3. Redox system The redox system is also among the most promising means of converting heat from CSPs to chemical energy, and it is generally based on oxidation-reduction reactions. In this paper, we only focus on the oxide redox pair Co3O4/CoO in a TCES system because Co3O4/CoO shows great performance in terms of reaction rate, energy storage capacity and cycling stability among Co3O4/CoO, MnO2/Mn2O3, CuO/Cu2O, Fe2O3/FeO, Mn3O4/MnO and V2O5/VO2 [18]. The reaction is as follows:

2Co3 O4(s) + H

6CoO(s) + O2(g)

H = 205 kJ/mol

Table 5 Challenges of the CaO/CaCO3 TCES system. Name

(5)

CaO/CaCO3

3.3.1. Cycle life Co3O4/CoO redox cycles have been used for many years in chemical looping combustion, where the oxide is chemically reduced using a fuel and then re-oxidized in air, allowing for a cleaner (no NOx), and lower temperature fuel combustion. Recently, this oxide cycle was proposed as a TCES system for a CSP plant and investigated by Muroyama et al. 806

Energy density ● 437 kWh/m3 ● 0.39 kWh/kg

Challenges ● Agglomeration and sintering ● Low thermal conductivity ● Reactor design ● System integration design ● Economic assessment

Related technology ● CO2 capture ● Heat pump

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to maintain macro-structural integrity with an increasing the number of cycles and the reduction/oxidation reaction temperature was remarkably altered [121].

3.3.3. Summary Detailed recent advances in the Co3O4/CoO TCES system have led to the following conclusions: First, a combination of Co3O4/CoO and other metal oxides (mainly manganese oxide and iron oxide) not only can reduce the cost and toxicity of Co3O4/CoO, but also are helpful in improving the reaction, kinetics, and cycling stability as compared to their single cation metal oxide. However, different reaction temperatures are a drawback for these mixed oxides, which can result in partial reduction properties. For example, it is required to heat above the higher reduction temperature, that of Mn2O3, during the day. Similarly, the Mn3O4 oxidation reaction initially first occurs at much lower temperature than that of the CoO oxidation during the night. In other words, these mixed oxide reactions are more complex. Thus, the relation between the reactions of binary oxide systems (Co-Mn, Co-Fe) should be further clarified in the future. Second, the charging and discharging processes of the Co3O4/CoO TCES system mainly occur in the fixed-bed reactor, where Co3O4 are coated cordierite honeycombs. This porous structure can increase the heat dissipation efficiency. When an experiment is conducted using TGA, this system shows a repeatable and sustainable cycling stability over several numbers of reduction-oxidation runs. Its structural integrity does not diminish with a number of cycles using TGA, but the Co3O4-based honeycomb is subjected to severe deformation as the scale of the experiment increases. Therefore, more intensive research is required of experiments of large-scale Co3O4/CoO TCES systems to overcome this limitation. On the other hand, the effect of crucial parameters, namely the porosity of the honeycombs, various boundary temperatures, and reaction rate on exothermic power and storage performance remain unknown. Finally, knowledge of the system integration of the Co3O4/CoO TCES system is insufficient. Although integration of Co3O4/CoO TCES and Air Brayton power cycle has been proposed, an understanding of the following aspects regarding this system remain lacking. First, the O2 gas is cooled from the reaction temperature to the normal temperature without taking advantage of reaction waste heat. This results in thermal energy waste. Second, aims of the development of the TCES system are to save energy and reduce energy consumption. However, energy consumption based on the system integration of the Co3O4/CoO TCES system remains unknown. Thus, future investigation is recommended to optimize the heat exchanger networks for energy consumption minimization. On the other hand, the system integration proposed based on Co3O4/CoO TCES system is very minor. More system integrations need to be developed, such as the integration of Co3O4/CoO and compressed air energy storage. The main challenges of the Co3O4/CoO TCES system are summarized in Table 6.

3.3.2. Heat storage performance Karagiannakis et al. [122] investigated the effect of porous structure (Fig. 21) such as honeycombs on heat storage performance. The results showed that pure Co3O4 extruded honeycomb showed the highest heat dissipation efficiency but suffered from severe deformation upon multicyclic operation, appearing notably swollen. However, Agrafiotis et al. [123] obtained different conclusions that the shaped structure of silicon carbide foams or honeycombs were coated with Co3O4, which showed a repeatable, quantitative, and cyclic reduction-oxidation behavior over 100 consecutive cycles and maintained their structural integrity. The difference was attributed to the former experiment occurring in a reactor, whereas the lab scale (ϕ 25 mm) was much larger than the latter. Later, they [124] compared the structural integrity between the porous structure and a pellet of Co3O4/CoO using TGA. It was found that the foams showed satisfactory structural integrity with a stoichiometric redox performance, but the pellets presented cracks even after only two cycles. Additionally, the thermal effects of the mixed oxide with porous structure materialized into a temperature increase in the air stream exiting the cascade [125]. To determine the thermal effects of the CoO oxidation reaction clearly and accurately, Tescari et al. [126] established a pilot-scale redox-based TCES system (Fig. 22), which was composed of inert honeycomb supports (cordierite) coated with 88 kg of cobalt oxide, and used a storage performance factor (PF) to evaluate how each experiment approaches the ideal behavior over the redox-oxidation cycles. The experimental result indicated that this TCES system can provide an approximately two-fold heat capacity (47 kW h) increase as compared to the sensible TES in the same volume (25.3 kW h). Singh et al. [127] developed a 2D, axisymmetric numerical model, which agreed well with the experimental results obtained from a 74 kW hth-capacity prototype reactor (Fig. 23). The numerical results showed that the solid temperature in the reactor showed a parabolic temperature distribution with the distance of the reactor centre increasing as a result of the reaction propagation during the charging and discharging process. These research studies provide guidance for the design and simulation of a Co3O4/CoO TCES system. Schrader et al. [128] performed a thermodynamic analysis of an Air Brayton cycle with a Co3O4/CoO redox reaction (Fig. 24). The results showed that the maximum value is 44% of cycle efficiency at 30 bar and 26% at 5 bar outlet compressor pressures. Exergy destruction was observed in the reactor during the heating reactants process via concentrated solar irradiation. A parametric study of the reactor design was also conducted in which the optimal parameters maximized the conversion from Co3O4 to CoO (0.94) and with a 0.76 absorption efficiency, reaching a 1385 K particle outlet temperature and 1572 K flow temperature [129]. Ströhle et al. [130] combined the thermochemical and sensible TES systems, and studied the effects of gas-solid contacting pattern on the integration of the storage system into the CSP plant. They developed a reactor model in a parallel configuration of the TES system and found that the packed bed was more appropriate than the other reactors because the axial temperature gradient in the packed-bed reactor was very near the temperature at the outlet of the power block, leading to low exergy destruction between the two heat transfer steams. The other reason was that the device required for fluidized-bed reactor was sophisticated, including a higher void fraction, the need for cyclones, and an extra transport disengaging zone. Additionally, only 14% of thermochemical heat was converted in the packed-bed reactor as compared to 30% in the fluidized-bed reactor, but the total energy density of both the sensible and thermochemical heat using the packed-bed reactor was 11% higher than that using the fluidized-bed reactor.

Fig. 21. Porous structure of fresh Co3O4. (Reprinted with permission from [122].)

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Air Blower Air Storage pilot system Solar Thermal Storage (Julich Germany)

Environment O2 sensor

Flowmeter

Filter

Gas Burner Cooling blower

Natural Gas

Air Environment Fig. 22. Schematic of the complete installation consisting of (from the top left side following the flow line) blower, heat storage of the STJ, flowmeter, gas burner, reactor chambers, oxygen sensor, particles filter and blower for air cooling. (Adapted with permission from [126].)

4. Organic system

of 1000 °C. In addition, the energy storage performance reached its maximum at the bed porosity of 0.7–0.8 and a reactor diameter of 40 mm. On the other hand, methane reforming with carbon dioxide presents better qualities for a TCES system than methane reforming with steam, which can be attributed to solar driven CO2 methane reforming storing an extra 20% of concentrated solar energy to chemically bonded energy and a solar-to-electricity efficiency that is up to 42% [137]. The reaction for methane reforming with CO2 is as follows:

4.1. Methane reforming Methane reforming with steam or carbon dioxide is extensively used for the industrialization of H2 production. These reactions have also been studied for heat transportation [131–133], and thus have been found to be suitable for TCES to collect concentrated solar energy via a high-temperature endothermic process. However, both studies of TCES have been lacking mainly because of their fatal drawbacks: side reactions. The reaction for methane steam reforming is as follows:

CH 4(g) + H2 O(l) + H

CO(g) + 3H2(g)

H = 250 kJ/mol

CH 4(g ) + CO2(g ) + H

CO2(g) + H2(g)

H = 41.2 kJ/mol

H = 247 kJ/mol

(8)

The side reaction with CO2 is:

(6)

CO2(g ) + H2(g ) + H

The side reaction for methane steam reforming as follows:

CO(g) + H2 O(l) + H

2CO(g ) + 2H2(g )

CO(g ) + H2 O(g )

H=

41.2 kJ/mol

(9)

This reaction occurs at temperatures between 973 and 1133 K and an absolute pressure of 3.5 bar. Wang et al. [138] provided theoretical guidance to describe the thermochemical kinetic and heat transfer of the methane reforming process with CO2 using a finite volume method and a local thermal non-equilibrium model. The numerical results showed that the thermochemical storage efficiency of methane reforming with CO2 could reach an optimal value when the porosity of the metal foam reactor (Fig. 25) was 0.66 and the CH4/CO2 ratio was 0.67 [139]. Lu et al. [140] simulated and analyzed the thermochemical characteristics of methane reforming in a tubular packed reactor (Fig. 26) with carbon dioxide using a laminar finite-rate model and Arrhenius expression, which resulted in a maximum thermochemical storage efficiency of 47.2% with a methane conversion of 76.8%, and a total energy efficiency of 70%. The simulation results agreed well with the experimental results. Additionally, they also identified the importance of sensible heat and heat loss during the total energy storage process. A year later, they redesigned the reactor (Fig. 27), which consisted of five tubes with an outer diameter of 30 mm and an inner diameter of 26 mm. In addition, 74.8% methane conversion, 19.7% thermochemical storage efficiency and 28.9% total energy efficiency was obtained in a methane-based TCES system integrated with a solar dish. It was also found that the peak values of the concentrated solar energy flux and heat loss were in the middle region of reactor; however, that of net heat flux and reaction kinetic rate were in the front region, because

(7)

Methane steam reforming occurs under condition in which the temperature ranges from 873 to 1223 K and the pressure between 20 and 150 bar. In 1975, Kugelers et al. [134] first proposed an idea that the thermal energy from nuclear plants could be stored and transferred via the reaction of methane steam reforming. Then, they offered and designed a conceptual process flow and calculated its energetic efficiency at approximately 65%. After many years without surveys, Wang et al. [135] developed a steam methane reforming model (modified-Sutherland model) to evaluate thermochemical performance in a porous medium reactor (Fig. 25). This model offered a higher calculation accuracy when calculating the thermal conductivity of a gas mixture. They found that increasing the heating time could result in increase in both dimensionless thickness and temperature of the thermal equilibrium region. Additionally, they also found that solar irradiance is an important parameter, determining temperature distribution and reaction extent. Thermochemical reaction rates increases with solar irradiance increases. Later, Yuan et al. [136] investigated the effect of operational condition and reactor structures on the energy storage performance of steam methane reforming in a tubular reactor (Fig. 26), and found that thermochemical energy storage efficiency achieved a maximum of 35.6% as compared to the sensible energy storage efficiency of 36.8%, and thereby a total energy efficiency of 38.3% at an outlet temperature 808

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achieve high methane-based TCES efficiency is radically different than that for H2 production. The former involves three processes, charging (endothermic reaction), storage, and discharging (exothermic reaction), that require more consideration than the only endothermic reaction of H2 production. For a methane-based TCES system, the main drawback is that the products (CO and CO2) from methane reforming easily react with reactants (CO2 and H2O) during the charging and discharging processes. These reactions can disturb the energy storage performance. Thus, it is impossible to ignore the investigation of cycle life for a methane-based TCES system because of the mature technology for H2 production. Finally, there are three techno-economic assessments required to promote the scale-up of a methane-based TCES system. These include the capital cost of the main components of the system; variability in the future economic climate, and, most importantly, a comparison to a conventional CSP plant with molten salt. The main challenges of the organic TCES system are summarized in Table 7.

Air (in)

Air (out)

(a)

5. Application TCES systems can be used for power generation and heat recovery system in high temperature industries. The state of the art for TES in the power plants is to use solid to liquid phase change materials such as molten salts [10], which are limited to temperatures of around 600 °C. However, the TCES system can have higher specific energy storage, and theoretically store the heat at higher temperatures (> 600 °C), allowing for more efficient electricity production. In an idea process, the TCES system can be charged by solar power during the day. Then, thermal energy is stored via molecular bond formation. When needed, the system can release heat in time. The released heat provides a higher load factor for the CSP plants to keep a stable electricity output. For example, the Australian National University is experimenting with a solar driven closed-loop system, operating on a paraboloid dish concentrator [52]. Besides, the TCES systems based on metal hydroxides, in which water (steam) reacts with a metal oxide (e.g. CaO), are especially interesting for steam power generation applications [77]. In industrial processes, waste heat is often generated, then dissipating in the environment and thereby most energy is wasted. By coupling with a TES system, the waste heat of a large area can be merged with an internal district heating system and the temperature can be increased through the use of heat exchangers for normal district heating grids or process steam generation [142]. While many industrial processes accompany chemical reactions, which inspire to combine the waste heat recovery systems integrated with the desired endothermic reaction. For example, iron and steel industry is one of the most energyintensive industries [12]. Water quenching is a traditional heat recovery technology, which uses cold water to cool down slag so as to achieve the desired glassy by-products. However, this technology consumes a huge amount of water and fails to recover the sensible heat of slag. Among some other heat recovery technologies, coupling with a TCES system is a good choice to save energy and reduce water consumption. In recent years, the high-quality TES technologies for heating and power generation, as well as high efficient heat recovery system become more and more important for our life with the number of renewable energy applications increasing. With regard to the mature twotank molten salt at present, it is demonstrated that a CSP plant with an energy storage has a decrease in supplemental fuel usage by 43%, and an increase in solar share by 47% [143], overall energy efficiency by 18.48% [144], as well as vital economic profits by 120% [145]. Several studies argue that an addition of TES has a large potential in economic development, although CSP capital cost increases from $4.2/W (no storage) to $8.6/W (7.5 h storage) [146,147]. On the other hand, for waste heat recovery, the addition of TES is also identified as a

(b) Fig. 23. Cordierite honeycomb reactor. (a) structure of the reactor (Adapted with permission from [127]) and (b) picture of the pilot-scale reactor chamber. (Reprinted with permission from [126].)

of the high temperature and high reactant fraction [141]. 4.2. Summary Detailed recent advances in the methane-based TCES system have led to the following conclusions: Primarily, according to state-of-the-art studies, the maximum methane conversion could reach 76.8% in a reactor, leading to 70% energy efficiency, while the maximum methane conversion could reach 74.8% in the whole system, leading to 28.9% total energy efficiency. A minor fraction of unreacted CH4 circulates in the system because of uneven thermal distribution, therefore allowing for a reduction in the energy efficiency. This suggests that the performance of the methane-based TCES system could be improved by optimizing the heat recovery exchanger network. It is also recommended that the reactor structure is redesigned to enhance heat transfer in the reactor for high energy efficiency in the future. In addition, a high CH4 conversion, as it has come to be known, helps to increase the energy storage performance. Although methane reforming with water and CO2 has been extensively investigated as a prospectively low-carbon alternative to the use of the commercial ammonia based technology for H2 production, the reaction process to 809

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. Qcool Environment

. Qsolar

T0,p0,yO 0,yN 0 2

2

. Qloss

Cooling

O2 at T0,p0 O2 at Tturbine,pcomp

Solar Reactor Cold Storage

Co3O4,CoO at Trecover

CoO at Treactor

Re-Oxidizer

Air Air at T0,p0

O2-depleted Air at Tturbine,pcomp

at Tcomp,pcomp

Comp

. Wvac

. Qpump

Vacuum Pump

O2 at T0,p0

Hot Storage O2-depleted Air

Turbine

at Texhaust,p0

. Wturbine

. Wcomp

Fig. 24. A flow diagram of the Air-Standard Brayton cycle with an integrated two-step solar TCES cycle based on Co3O4/CoO redox reactions is depicted with relevant heat and work flows into and out of the system. (Adapted with permission from [128].)

by providing supplemental electrical power without burning additional fossil, especially CaO/CaCO3 and CH4/CO2 TCES systems; (c) many policies on tax reduction or government financial subsidy for waste heat recovery and renewable energy are carried out all over the world, such as 0.91 €/y for the utilization of waste heat in China. However, TCES systems used in waste heat recovery may have better advantage than the renewable energy system for the time being. In fact, it is urgent and imperative for action in energy saving and reduction in carbon emission of traditional industrial plants, resulting from terrible environmental issues day by day. Moreover, the capital cost for addition of waste heat recovery in an industrial plant is much less than that for a newly-constructed CSP power station.

Table 6 Challenges of the Co3O4/CoO TCES system. Name CoO/Co3O4

Energy density

Challenges

● 295 kWh/m3 ● 0.24 kWh/kg

Related technology

● Material safety and cost ● System integration design ● Environmental impacts ● Economic assessment

/

z

6. Conclusion

Products outlet

Incoming sunlight

The TCES is a promising method for efficient heat storage owing to its high energy density, long-term storage without heat loss, less storing volume in the same heat capacity, and so on. The main objective for using TCES systems is to develop compact and low cost systems to recover waste heat in industrial plants, or to overcome dispatchability between solar energy and power demands. Review demonstrated that:

Porous reactor

Reactants inlet

y

● Cycling stability is a significantly important parameter that determines service life and its suitability as a thermochemical energy storage system. Until now, the cycling stability of materials extensively investigated for the TCES system decreases with the number of cycles owing to different reasons, such as the poor mechanical property for CaO/Ca(OH)2, side reactions for the organic system, etc. A series of methods (e.g. nanostructure for Mg/MgH2, and binary oxide system for Co3O4) have been proposed to improve the cycling life. However, cycle stability at present is far from requirement of commercial scale application. ● The reactor plays an important role in the establishment of a reliable energy charging and releasing energy process. The heat storage performance is remarkably contingent on heat transfer in the reactor. Currently, the reactor of the ammonia system is the most mature. This contributes not only to its mature kinetic studies based on Haber-Bosch process, but to the reactor studies on the ammonia TCES system by famous research institution (ANU) for 40 years. In addition, Mg/MgH2 and CaO/CaCO3 TCES reactors can learn some

Dish collector

x Fig. 25. Schematic diagram of the porous medium thermochemical reactor with a parabolic dish solar collector. (Adapted with permission from [135].)

significant role in energetic and economic saving [142,148], such as an increase in plant efficiency up to 65% [149], a decrease in CO2 emissions by 22% [150], and its energy costs drop to 15 €/MW h [151]. Thus, TCES as the most promising system among three key techniques of TES represents an enormous prospect in commercial applications of both fields of renewable energy and waste heat recovery. In our opinion, its commercial feasibility is attributed to three factors as follows: (a) TCES systems can dramatically boost economic profit potential in both fields; (b) TCES systems can effectively reduce carbon emissions 810

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(a) Furnace

Catalyst bed

x

30mm

26mm

Quartz tube

Inlet zone,La=200mm

Lca=200mm Heating zone,Lh=400mm

Outlet zone,La=200mm

(b) Fig. 26. The tubular reactor ((a) reactor photographs and (b) structure of reactor). (Adapted with permission from [136].)

experience (e.g. fluidization behavior of solid particles or reactor structure) from technologies of H2 production and CO2 capture respectively. While, the rest studies, for both CaO/Ca(OH)2 and CoO/ Co3O4 TCES reactors, are too little, mostly focusing on fixed-bed reactors, and their experimental results are barely satisfactory for the time being. ● System integration is the other factor to determine heat storage performance. Thus far, some TCES system integrations are proposed. These systems theoretically allow renewable energy power plant to be at relatively high energy efficiencies and low environmental impact. However, the lab-scale experiments of these systems are rarely involved.

clarify cycling attenuation mechanism from the micro perspective and offer a direction for desired materials. ● For reactors, the research is relatively limited compared to that of cycling stability. It is suggested that more reactor types should be developed to meet TCES system requirements because of poor effective thermal conductivity in fixed bed and back mixing in fluidized bed. In addition, their physical and chemical processes in these new types of reactors should be investigated, in combination with simulation and experimental studies, to clarify the relationship between heat transfer and chemical reaction in the reactors. In this way, the method to control the charging-discharging rate can be further developed. ● For system integration, the current knowledge is insufficient. Therefore, more efforts may concentrate to (a) further optimize these system integrations to reach higher level in energy efficiency; (b) validate the feasibility of theoretical research via the experiments in practice; and (c) comprehensively investigate influence of TCES systems on local economy and environment.

Outlook: ● For cycle stability, attention at the molecular or atomic level has been lacking in previous papers, although the experimental observations of these composite materials have been published extensively. Further research is required to explore reasonable descriptors to identify the relationship between intrinsic properties and cycle life of these TCES, which shows the role of inert materials during the charging-discharging process. Then, these descriptors can

Finally, there is every hope that these strategies can boost TCES systems to be scientific and commercial breakthrough in the near future. 811

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Rcactor Gas chromatograph

Solar dish CO2

CH4

(a)

(b) Fig. 27. Schematic diagram and experimental system of thermochemical storage system ((a) schematic diagram and (b) experimental system). (Adapted with permission from [141].) Table 7 Challenges of the organic TCES systems. Name CH4/H2OCH4/ CO2

Energy density

Challenges 3

● 7.8 kWh/m ● 7.7 kWh/m3

● Side reactions ● Incomplete reaction ● System integration design ● Environmental impacts ● Economic assessment

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Related technology ● H2 production

Acknowledgements The authors acknowledge the financial support provided by the National Natural Science Foundation of China (Grant No. 51576095 and 51776095), Postgraduate’s Innovation Project of Jiangsu Province (Grant No. KYCX18_1090 and No. KYCX18_1089), Undergraduate’s Innovation Project of Jiangsu Province (Grant No. 201810291194T) and the Young Science Leaders Project of Jiangsu Province. References [1] Chu S, Majumdar A. Opportunities and challenges for a sustainable energy future. Nature 2012;488(7411):294–303. [2] Britain Petroleum Co. Ltd. BP statistical review of world energy. http://www.bp. com/zh_cn/china/reports-and-publications/_bp_2017-_.html; 2017. [3] Peng H, Shan XK, Yang Y, et al. A study on performance of a liquid air energy storage system with packed bed units. Appl Energy 2018;211:126–35. [4] Peng H, Yang Y, Li R, et al. Thermodynamic analysis of an improved adiabatic compressed air energy storage system. Appl Energy 2016;183:1361–73. [5] Xiong Y, An S, Xu P, et al. A novel expander-depending natural gas pressure

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