Dark calcium carbonate particles for simultaneous full-spectrum solar thermal conversion and large-capacity thermochemical energy storage

Solar Energy Materials & Solar Cells 207 (2020) 110364

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Dark calcium carbonate particles for simultaneous full-spectrum solar thermal conversion and large-capacity thermochemical energy storage Hangbin Zheng a, Chao Song a, Chuang Bao a, Xianglei Liu a, *, Yimin Xuan a, Yongliang Li b, Yulong Ding b a b

School of Energy and Power Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing, 210016, China Birmingham Centre for Energy Storage, School of Chemical Engineering, University of Birmingham, Birmingham, B15 2TT, UK

A R T I C L E I N F O

A B S T R A C T

Keywords: Calcium carbonate Thermochemical energy storage Full-spectrum Solar energy Cyclic stability

Possessing nontoxicity, high thermochemical energy storage density, and good compatibility with supercritical CO2 thermodynamic cycles, calcium carbonate (CaCO3) is a very promising candidate in storing energy for nextgeneration solar thermal power plants featured with high temperature over 700 � C. However, CaCO3 particles are usually white with little absorption of sun light, inhibiting their application in efficient volumetric solar energy conversion systems. In this paper, dark CaCO3 particles are designed by doping with Cu, Fe, Co, and Cr elements based on sol-gel procedures. For particles doped with only Cu elements, the solar absorptance in the visible range is improved prominently while that in the near-infrared does not change so much. By further adding Cr elements, full-spectrum absorption of solar energy is achieved with a value as high as 73.1%, but the energy storage density decreases rapidly with cycling. By incorporating Mn or Al elements, the cyclic stability is enhanced greatly. For binary-doped particles with Cu and Mn, the energy storage density achieving 1952 kJ kg 1 after 20 cycles, which is 84% higher than that of pure CaCO3 particles. Additionally, the average solar absorptance is still considerable with a value of ~60% after 20 cycles. This work guides the design of highefficiency, large-capacity, and stable thermochemical energy storage particles for simultaneous solar thermal conversion and high-temperature thermochemical energy storage.

1. Introduction Challenges of increasing energy demand, environmental pollution, and non-renewability of fossil fuels are driving people to put more attention on sustainable energy sources. Among various renewable en­ ergy sources, solar energy has advantages of abundance, wide distri­ bution, and being free of pollution. Besides the conversion of solar energy into electricity based on photovoltaic (PV) cells, collecting sun­ light into heat and then converting thermal energy into electric power through different thermodynamic cycles are also crucial to deal with energy and environmental issues. Solar energy has intermittent and unstable characteristics due to changes of weather, seasons, and so on. Therefore, the thermal energy storage plays a vital role in solar energy utilization [1–8]. The developing trend of solar thermal power plants goes to high temperature and light efficiency [9–14]. It is predicted that the inlet temperature of working fluids into turbines will be higher than 700 � C for the future third generation solar thermal power plants, so that

the efficiency can be higher than 40% [15,16]. The sensible thermal storage technology based on molten salt has advantages of simple principles, mature technologies, and rich material sources, so it is the most widely used and the only commercially applied thermal storage method so far [17–19]. However, molten salt suffers from thermal decomposition and corrosion issues when the temperature is higher than 550 � C [20,21]. Recently, thermochemical energy storage techniques have attracted increasing attention due to possessing higher storage density and lower heat losses [22–29]. Metal sulfates are promising candidates owing to its high reaction enthalpies, but they suffer serious corrosion issues, resulting in poor safety in thermochemical heat storage. The dehydra­ tion reaction of metal hydroxides can lead to high heat storage effi­ ciency, but the reaction temperature of most of them is too low to be applied in next-generation solar power systems [30]. Metal oxide redox cycles have become an attractive choice because they possess wide operating temperatures and can directly use air as working fluid [9,12].

* Corresponding author. E-mail address: [email protected] (X. Liu). https://doi.org/10.1016/j.solmat.2019.110364 Received 17 August 2019; Received in revised form 13 December 2019; Accepted 15 December 2019 Available online 26 December 2019 0927-0248/© 2019 Elsevier B.V. All rights reserved.

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Fig. 1. The CaCO3/CaO energy storage cycle system.

However, most metal oxides have low energy storage density with a value lower than 1000 kJ kg 1 [13]. Therefore, it is necessary to seek appropriate thermochemical materials with high temperature reactions, good safety, and high energy storage density. Many researchers identi­ fied that CaCO3 is one of the most promising candidates in storing en­ ergy for next-generation solar thermal power plants featured with high-temperature working conditions over 700 � C [31,32]. The main advantages of CaCO3 as an energy storage material are due to the following properties: (i) environmentally benign and inexpensive, (ii) higher energy storage density, (iii) higher reaction temperature, and (iv) good compatibility with supercritical CO2 cycles since the reactant CO2 is also the working fluid in the power cycle [33–38]. The CaCO3/CaO energy storage cycle system is shown in Fig. 1. The solar light irradiates into the receiver through the heliostat field and CaCO3 particles undergo decomposition as they absorb solar thermal energy directly or indirectly in the calciner. The products CaO and CO2 enter the corresponding storage tanks, respectively. Later, they are combined in an exothermic reaction on demand in a reaction chamber to resynthesize CaCO3 and recover the stored solar energy. The energy of reaction chamber is transferred to the thermal cycle working fluid, such as CO2, through the heat exchanger, which in turn drives the turbine generator to work and output electrical energy. The heat released by carbonation reaction can raise the working fluid temperature up to over 700 � C, so it can meet the high temperature requirements of the next-generation solar thermal power plants to improve the efficiency [15,16]. Capturing solar irradiation efficiently is a significant issue in the CaCO3/CaO energy storage cycle system. In a conventional solar receiver, solar energy is usually absorbed by indirect ways based on surface coatings and then converted to working materials [39–44]. As for indirect light absorption method, large amounts of heat loss occur because the heat is mainly concentrated on the surface coating. More­ over, the photothermal conversion surface coating may suffer stability issues at high temperature conditions. Besides, there exit severe radiant heat loss and large thermal resistance between surface coating and CaCO3 particles [45,46]. To overcome the above issues, the volumetric solar conversion is a promising method, for which particles can absorb the irradiation directly [45–52]. Tedious heat transfer processes are thus avoided, leading to a high efficiency for solar thermal conversion and storage. Nevertheless, CaCO3/CaO particles are white, and they barely absorb solar light, prohibiting their applications in volumetric solar conversion systems [45,46]. Therefore, it is desired to design dark par­ ticles with good solar thermal conversion properties. Adding appro­ priate absorption-enhanced materials may be a good choice, and typical representatives are graphite and carbon nanotubes, but they suffer sta­ bility issues at high temperature conditions. Some metal oxides have been employed to improve solar absorption capacities [45,46].

However, previous studies focused on the solar absorption properties of planar CaCO3 substrates while particles are more promising candidates in simultaneous volumetric solar thermal conversion and storage. The cyclic stability is another common issue that should be taken into ac­ count for thermochemical energy storage materials. CaCO3 particles tend to gather together, leading to severe sintering at the decomposition temperature. This is because the Tammann temperature of CaCO3 par­ ticles, at which sintering is initiated, is lower than its decomposition temperature [53,54]. Many methods have been developed to improve the calcium-based cyclic performance recently [55–58]. For example, Chen et al. synthesized CaO-based materials by the sol-gel method with manganese nitrate tetrahydrate (Mn(NO3)2⋅4H2O) precursors to solve the decay problem and improve the sintering resistance [59]. Azimi et al. prepared the synthesized sorbents by adding aluminum nitrate (Al (NO3)3⋅9H2O) using sol-gel process to enhance the fluidizability and cyclic performance [60]. Nevertheless, little attention was paid to simultaneously achieve full-spectrum solar absorption and good cyclic stability for CaCO3 particles. This paper aimed to design dark CaCO3 particles with full-spectrum solar thermal conversion, and high temperature thermochemical energy storage with high energy storage densities and good cycling stabilities. Here, we prepare doped CaCO3 particles via doping absorptionenhanced materials (Cu, Fe, Co, and Cr elements) by a citrate sol-gel method. Material characterization is carried out with X-ray diffraction (XRD), scanning electron microscopy (SEM) and spectrophotometry based on powder samples. Energy storage density is obtained from thermogravimetric analysis (TGA). A model to calculate the optical absorption properties of doped CaCO3 particles is proposed by combining effective medium theory and finite-difference time-domain analyses. The simulation results agree well with experimental mea­ surements. By making white CaCO3 particles dark via doping appro­ priate materials, a high average solar absorptance of 73.1% is obtained. The cyclic stability of particles doped with different elements is checked, and adding Mn or Al elements is found to greatly improve the cyclic stability. Compared with pure CaCO3 particles, solar absorptance, cyclic stability, and energy storage density are all successfully improved. 2. Theory and experimental method 2.1. Materials All the chemical regents of analytically pure grade are used directly in the experiment without further purification. Citric acid monohydrate (C6H8O7⋅H2O), copper (II) nitrate trihydrate (Cu(NO3)2⋅3H2O), cobalt (II) nitrate hexahydrate (Co(NO3)2⋅6H2O), chromium (III) nitrate non­ ahydrate (Cr(NO3)3⋅9H2O), manganese nitrate solution (Mn(NO3)2, 2

Solar Energy Materials and Solar Cells 207 (2020) 110364

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Fig. 2. Schematics of doped CaCO3 particles’ preparation based on sol-gel procedures.

50%), and ethanol were purchased from NANJING Reagent Co.,Ltd., China. Calcium nitrate tetrahydrate (Ca(NO3)2⋅4H2O), ferric nitrate nonahydrate (Fe(NO3)3⋅9H2O), aluminum nitrate nonahydrate (Al (NO3)3⋅9H2O), and nitric acid (HNO3, 20%) were obtained from Chengdu Chron Chemicals Co., Ltd., China.

heating rate of 10 � C/min and doped-particles are produced with a molar ratio of CaCO3 to CuO of 100 to 10 (denoted as Ca:Cu ¼ 100 : 10). CaCO3 without any doping is produced using the similar procedure and named pure CaCO3. Other unitary-doped particles with Ca:Cu ¼ 100 : 1 and Ca:Cu ¼ 100 : 20 are prepared according to similar processes only by changing the mass of Cu(NO3)2⋅3H2O to 0.2416 g and 4.832 g, respectively. Furthermore, binary-doped particles with Cu and Fe, Co, or Cr elements, denoted as Ca:Cu:Co ¼ 100 : 5 : 5, Ca:Cu:Cr ¼ 100 : 5 : 5, or Ca:Cu:Fe ¼ 100 : 5 : 5, are synthesized by further doping precursors Co(NO3)2⋅6H2O (1.455 g), Cr(NO3)3⋅9H2O (2.0 g), or Fe(NO3)3⋅9H2O (2.02 g). Particles with Ca:Cu:Mn ¼ 100 : 5 : 5 and Ca:Al:Cu:Fe ¼ 100 : 20 : 5 : 5 are synthesized to enhance the cyclic stability in this work, which are obtained by doping precursors Mn(NO3)2 (1.15 ml) or Fe (NO3)3⋅9H2O (2.02 g) and Al(NO3)3⋅9H2O (7.50 g) according to similar processes. Table 1 shows main characterization methods and compo­ nents corresponding to different synthetic samples.

2.2. Synthetic methods Doped particles are prepared based on a sol-gel procedure as shown in Fig. 2. First, the unitary-doped particles with Cu element are prepared by sol-gel method. Pre-calculated amounts of C6H8O7⋅H2O (21.014 g), doping material precursor Cu(NO3)2⋅3H2O (2.416 g), and calcium pre­ cursor Ca(NO3)2⋅4H2O (23.615 g) are first added into a beaker at room temperature. Then, deionized water (H2O, 100 ml) and ethanol (C2H5OH, 25 ml) are added into the vessel and a mixer continuously stirred till all materials are entirely dissolved. At the same time, Nitric acid (HNO3, 15 ml) is added in order to adjust the PH of the solution to 1–2. Afterwards, the solution is put into a water bath which has an electrically heated magnetic stirring device at 85 � C for 3 h to obtain wet gel samples. Subsequently, wet gel samples are auto-combusted in a furnace at 300 � C for 1 h. When the combustion process is finished, the loose powder is calcined at 850 � C for 2 h with a heating rate of 10 � C/ min based on preliminary tests. Finally, calcined samples are carbonated with the reaction atmosphere (100% CO2) at 800 � C for 3 h with a

2.3. Theory and methods 2.3.1. Effective medium theory (EMT) models and finite-difference timedomain (FDTD) method Herein, a computational model is being developed to figure out the mechanism of spectral absorptance enhancement. The schematic of composite particles, including CaCO3 and absorption-enhanced

Table 1 Main characterization methods and components corresponding to different samples. Molar ratio

Characterization methods

Components

SEM

XRD

Spectrophotometry

DSC-TGA

Pure CaCO3 Ca:Cu ¼ 100 : 1

✓

✓

✓ ✓

✓

CaCO3

Ca:Cu ¼ 100 : 10

✓

✓

✓

✓

CaCO3, CuO, Cu2þ1O

✓

CaCO3,Ca10Cr6O24(CO3),Cu2þ1O

Ca:Cu ¼ 100 : 20

✓

Ca:Cu:Co ¼ 100 : 5 : 5 Ca:Cu:Cr ¼ 100 : 5 : 5

✓

✓

✓

✓

✓

✓

Ca:Cu:Fe ¼ 100 : 5 : 5

✓

Ca:Cu:Mn ¼ 100 : 5 : 5

✓

Ca:Al:Cu:Fe ¼ 100 : 20 : 5 : 5

✓

CaCO3, CoO, Cu2þ1O

✓

✓

✓

✓ ✓

3

CaCO3, Ca2Fe2O5, Cu2þ1O

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Fig. 3. A 3-D FDTD computational model.

materials, is shown in Fig. 2. CaCO3 is usually white with little ab­ sorption of sun light. By doping with metal elements, the particles exhibit high solar absorptance as being demonstrated experimentally. To unveil the underlying mechanism of enhanced absorption, we pro­ pose a simulation model based on finite-difference time-domain ap­ proaches. For theoretical simulation of optical absorption properties, we only consider unitary-doped particles. For binary- or ternary-doped particles, the material composition is very complex, and it is difficult to obtain their dielectric functions. Nevertheless, the mechanism of spectral absorptance enhancement is applicable to both unitary- and binary-doped particles. Obtaining the effective dielectric functions of doped particles is the first step to solve electromagnetic Maxwell equations. As shown in Fig. 2, the doped particle is composed of the first component and the second component. To simplify the theoretical analysis, composite particles are treated as being homogeneous, and their effective dielectric functions can be calculated by EMT [61–65]. Different EMT models have been proposed since a century ago. Here, both Maxwell-Garnet (MG) based and Bruggeman (BR) based EMTs are considered. The dielectric functions of MG-based EMT should satisfy the relation [66]:

εMG ε2 ε1 ε2 ¼ f1 εMG þ 2ε2 ε1 þ 2ε2

scattered from one to the other, interactions between particles should be considered in this system [68]. The interaction of particles can be simulated by the FDTD method [69,70]. This method is an explicit time marching algorithm to solve governing Maxwell’s equations on dis­ cretized spatial grids, which has been widely used owing to considering the scattering of light [71]. The absorption per unit volume is calculated ! ! ! from the divergence of the Poynting vector ( P ): Pabs ¼ 12 realðr ⋅ P Þ. abs The normalized absorbed power can be expressed as: P’abs ¼ PPsource , where

Pabs is the power absorption (W/m3) and Psource is the source power (W). In order to make sure the light scattered completely, the simulated domain is set as 1 � 1 � 400 ​ μm3 . The mesh size of 0:25 ​ nm is used. The perfectly matched layer (PML) boundary conditions are employed at top and bottom surfaces along the z-direction which produce zero re­ flections. At four surfaces in the x- and y-directions, the periodic boundary is applied. 2.3.2. The filling rate of the randomly dispersed particles system In this study, optical absorption properties of unitary-doped particles are investigated experimentally. In our measurement of light reflectance (R), the weight of unitary-doped particles is 5 g, and they are put into the quartz cuvette (r ¼ 2.5 cm) with a height of 4.7 mm. The filling rate fr of the randomly dispersed particles system can be measured according to equation:

(1)

According to BR based EMT, the effective permittivity εBR is given as [67]: f1

ε1 εBR þ

εBR

1 ð 1 3

ε

εBR Þ

þ f2

ε2 εBR þ

εBR

1 ð 2 3

ε

εBR Þ

¼1

fr ¼

(2)

ρsample msample ¼ ρCaCO3 πr2 hρCaCO3

(3)

where msample is the mass of sample, r and h are the radius and height of the sample, respectively, when measuring the spectra using quartz cuvette. ρCaCO3 is the density of CaCO3 with a value of 2.71 g/cm3. Ac­ cording to Eq. (3), fr of CaCO3 is calculated to be around 20%. Since the amount of dopant is small, fr of unitary-doped particles is assumed to be same with that of pure CaCO3.

where f1 and f2 are the volume fraction of different components. ε1 and ε2 are corresponding permittivity of different components, and εMG is the effective permittivity of doped particles. The following assumptions are made for the calculation of absorption properties of doped particles. Particles are dispersed randomly in air and do not overlap as shown in Fig. 3. The energy irradiates from the light incident plane and propa­ gates along the z-direction. It will be absorbed, transmitted, scattered by particles when the light irradiates to simulated domain. The reflectance and transmittance can be monitored by the reflectance calculation plane and transmittance calculation plane, respectively. Because the light can

2.3.3. The average solar absorptance The reflectance of samples is measured in a spectrophotometer. The absorption can be derived from: A ¼ 1 R. To verify the absorption performance of different doped particles, the average solar absorptance 4

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Fig. 4. (a) XRD of pure and unitary-doped particles, (b) SEM of unitary-doped particles (Ca:Cu ¼ 100:10).

(Aave ) is used as the figure of merit [72]: Z 2500nm AðλÞIðλÞdðλÞ Aave ¼ 300nm ; Z 2500nm IðλÞdðλÞ

DN ¼ (4)

where λ is the incident wavelength and IðλÞ is the intensity of AM1.5 solar irradiance. The average solar absorptance of pure and doped CaCO3 particles is calculated by Eq. (4).

2.4. Characterization Morphology features of samples are characterized by a field emission scanning electron microscopy (Zeiss Merlin, Carl Zeiss company, Ger­ many), with operation at 5 kV. The Ultraviolet–Visible–Near infrared (UV–Vis–NIR) spectra of prepared samples are measured on a spectro­ photometer (Agilent Cary 5000, Agilent Technologies, Inc., USA) at room temperature with an integrating sphere. An empty quartz cuvette is used to record the base line of measurements. The crystalline structure of doped particles is determined by X-ray diffraction (smartlab9, Rigaku Corporation, Japan) using Cu Kα radiation. The thermochemical energy storage density of samples is evaluated using thermogravimetric anal­ ysis and differential scanning calorimetry (Simultaneous DSC-TGA SDT 650, TA Instruments, USA).

2.3.4. The reaction efficiency and energy storage density The cyclic stability is one of the most important characteristics of thermochemical storage materials. In the TGA test, prepared samples are calcined at 700 � C in an atmosphere of 100% N2 for 15 min and then 50% CO2 and 50% N2 are put in to react for 10 min at 700 � C. During the experiment, the total gas flow rate is set at 200 ml min 1. The reaction efficiency is defined as [73]: mCarb; N mCal;N WCaO ; ⋅ mCal;N ⋅f WCO2

(6)

where ΔHr is the reaction molar enthalpy of the decomposition reaction ( 178 kJ mol 1).

300nm

xcon ¼

mCarb mCal;N ΔHr ⋅ ; mCal;N WCO2

(5)

where mCal;N and mCarb;N are particles’ mass before and after carbonation over N cycles, respectively. f is the mass fraction of CaO and can be m m WCaO , where m0 is the initial mass of the sample expressed as f ¼ 0mCal;NCal;N ⋅W CO

3. Results and discussion 3.1. Characteristics of different doped particles

2

before the cycles. WCaO , WCO2 are molar masses of CaO and CO2, respectively. The energy storage density DN for CaO, which stands for the actual amount of energy stored per kilogram synthetic samples, is defined as [74]:

In order to investigate the positive effect and the mechanism on absorption enhancement of CaCO3 by doping with absorption-enhanced materials (Cu, Fe, Co, and Cr elements), XRD patterns of doped particles

Fig. 5. (a) Refractive indexes and (b) extinction coefficients of CaCO3, CuO, and unitary-doped particles with the molar ratio of Ca:Cu ¼ 100 : 10. 5

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Fig. 6. Comparison of (a) Refractive indexes and (b) extinction coefficients of unitary-doped particles obtained from different EMT models.

obtained by sol-gel procedure are measured. In Fig. 4 (a), it is illustrated that CaCO3 is the only product from precursors of Ca(NO3)2⋅4H2O. As for unitary-doped particles, CaCO3, CuO, and Cu2þ1O are main components from precursors of Ca(NO3)2⋅4H2O and Cu(NO3)2⋅3H2O as shown in Fig. 4 (a). For binary-doped particles obtained by furthering adding precursors of Fe(NO3)2⋅9H2O, Co(NO3)2⋅6H2O, or Cr(NO3)2⋅9H2O, components are more complex and are shown in Table 1. The microstructure evolution of unitary-doped particles is observed by a field emission scanning electron microscopy (FESEM). Shapes and sizes are shown in Fig. 4 (b). Particle diameters are mainly distributed between 0.4 μm and 0.8 μm through the SEM image as shown in Fig. 4 (b). To simplify the theoretical analysis, the shape of particles can be approximated to spherical and the size is chosen as 0.6 μm. Above

parameters are utilized to simulate absorption properties of randomly dispersed particles system in the FDTD model. 3.2. Improved extinction coefficients of unitary-doped particles The solar spectral absorption of doped particles plays a vital role in determining the efficiency of solar thermal storage systems. However, CaCO3 has extremely poor extinction coefficients at wavelengths ranging from 300 to 2500 nm [75]. Doping different absorptance-enhanced materials into the composites can form different components as shown in Table 1. Unitary-doped particles are obtained by adding Cu elements and the main components are CaCO3, CuO, and Cu2þ1O. Cu2þ1O is regard as Cu2O with metal excess defects [76]. Since

Fig. 7. The spatial absorptance distribution of randomly packed particles of (a) pure CaCO3 and unitary doped particles with molar ratio of dopants as (b) Ca:Cu ¼ 100 : 1, (c) Ca:Cu ¼ 100 : 10 at λ ¼ 500nm. 6

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Fig. 8. Comparison between measured and simulated absorptance spectrum for unitary-doped particles with molar ratio of (a) Ca:Cu ¼ 100 : 1, (b) Ca:Cu ¼ 100 : 10.

dielectric functions of CuO and Cu2O are similar, for simplifying theo­ retical analysis, we only consider two components, i.e., CaCO3 and CuO, in unitary-doped particles. As is shown in Fig. 5 (b), extinction co­ efficients of CuO selected from Palik’s work decrease gradually in 300–1100 nm wavelengths range, but are significantly higher than those of CaCO3 [75]. The volume fraction of CuO is 3.3% which can be derived from the molar ratio of Ca:Cu ¼ 100 : 10. Extinction coefficients of unitary-doped particles with the molar ratio of Ca:Cu ¼ 100 : 10 are calculated by BR-based EMT and shown in the insert picture of Fig. 5 (b). As can be seen in Fig. 5 (b), extinction coefficients of unitary-doped particles with the molar ratio of Ca:Cu ¼ 100 : 10 are larger compared with CaCO3, while are lower than those of the CuO. The same result also appears on the refractive index. This indicates that extinction co­ efficients can be effectively improved due to the addition of CuO. For particles with a lower level of doping, extinction coefficients and refractive indexes decrease, but still have remarkable improvement over those of CaCO3. In addition, the BR-based EMT and MG-based EMT have almost the same refractive indexes and extinction coefficients as shown

in Fig. 6 (a) and (b). 3.3. The optical absorption of the random dispersed particles Light scattered from one particle can be absorbed by other particles, so interactions have to be considered. The stochastic dispersion model is closer to particles’ system with scattering. It is more reasonable to simulate optical absorption properties of particles with a random dispersion system rather than a single particle. The filling rate (fr ) of the randomly dispersed particles system is set to a value of 20% according to the experimental result in section 2.3.1. Spectral characteristics of doped particles at two volume fractions are compared with those of CaCO3 particles. The wavelength-dependent normalized absorbed power of random dispersion system in a partial plane of the x-z axis is given in Fig. 7. The absorption per unit volume is calculated according to methods in section 2.2. For CaCO3 particles, the normalized absorbed power is essentially negligible because its extinction coefficient is almost zero as shown in

Fig. 9. (a) Photographs of CaCO3 particles, unitary-doped particles, and binary-doped particles. Absorption performance comparison between different particles, (b) absorptance spectrum, (c) average solar absorptance. 7

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absorption-enhanced materials changes with a molar ratio of CaCO3 to CuO from 100:1 to 100:10 to 100:20, the averaged absorptance in­ creases gradually from 36.5% to 44.7%–49.5%. Due to poor absorption in the near-infrared range, unitary-doped particles still have a very limited average solar absorptance. By further adding another dopant, the average solar absorptance is prominently improved. For example, the averaged absorptance of particles doping with Cu and Fe elements with Ca:Cu:Fe ¼ 100 : 5 : 5 is higher than that of particles doping with Cu elements with Ca:Cu ¼ 100 : 10. Replacing Fe element by Co or Cr can further enhance the average solar absorptance. As can been seen in Fig. 9 (a), particles doped with Ca:Cu:Cr ¼ 100 : 5 : 5 are dark obvi­ ously, like the color of carbon black powders. The highest absorptance of 73.1% is obtained for binary-doped particles containing Cu and Cr ele­ ments with Ca:Cu:Cr ¼ 100 : 5 : 5 due to its capability of full-spectrum absorption as shown in Fig. 9 (b). 3.5. Effects of doping absorption-enhanced materials on the reaction efficiency and energy storage density Besides solar absorption properties, we also would like to investigate the reaction efficiency and thermochemical energy storage density of CaCO3 with different additions of absorption-enhanced materials. The reaction efficiency and energy storage density measured from TGA are shown in Fig. 10. As shown in Fig. 10, pure CaCO3 particles fabricated based on sol-gel method have a high reaction efficiency (over 80%). Particles doping with Cu elements possess a lower reaction efficiency than that of pure CaCO3 in 1st cycle. Similar results also occur for par­ ticles doped with Cu and Co or Cu and Cr elements. Although doping Cu, Co, or Cr elements can improve the spectral absorptance, the reaction efficiency decreases. That probably because the existence of dopants might inhibit the transport of reactants CO2. As a result, the energy storage density has a prominent decrease from 2490 kJ kg 1 for pure CaCO3 particles, to 1690 kJ kg 1 and 1832 kJ kg 1 for particles doping with Co and Cr elements, respectively. Therefore, it is difficult to achieve both high average solar absorptance and high reaction efficiency or energy storage density. On the other hand, as for particles doping with Cu and Fe elements, their reaction efficiency is higher compared with pure CaCO3 particles as can be seen in Fig. 10, although the solar absorptance (48.24%) is lower compared with particles doping with Cu and Cr elements. The reaction efficiency of particles doping with Cu and Fe elements is 9.83% higher than that of pure CaCO3 particles probably owing to their porous structures and small sizes, as discussed in the next section. However, the percentage of CaCO3 decreases due to existence of dopants, as a result, the energy storage density of doped particles still has slight decrease. Overall, doping appropriate materials can enhance the average solar absorptance tremendously but at the expense of decreased energy storage density to some extent. Nevertheless, the en­ ergy storage density of most doped CaCO3 particles is more than 2000 kJ kg 1, which is still much larger than that of conventional molten salt, as shown in Fig. 10 [77]. Therefore, both high thermal energy storage density and solar absorptance can be obtained by doping with appro­ priate materials into CaCO3 particles.

Fig. 10. The reaction efficiency and energy storage density in 1st cycle with different additions of absorption-enhanced materials. Reaction efficiency and energy storage density are marked with red and blue colors, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 7 (a). As shown clearly in Fig. 7 (b), CuO/CaCO3 unitary-doped particles with the molar ratio of Ca:Cu ¼ 100 : 1 have a much higher absorptance across the whole simulation region. With increasing molar ratio, the total absorptance is further improved. Also note that the penetration depth of solar energy is reduced and energy is mainly absorbed around the top as shown Fig. 7 (c). Fig. 8 presents the absorption of CuO/CaCO3 unitary-doped particles with different molar ratios theoretically and experimentally. It is clearly seen that the agreement between measured absorption and that ob­ tained by the FDTD method is good, further validating that the simu­ lation model is reliable and feasible. However, obviously, CuO/CaCO3 unitary-doped particles mainly absorb visible region, while the ab­ sorption in near-infrared is rather weak, impeding the realization of fullspectrum solar energy absorption. 3.4. Full-spectrum absorption via binary-doped To further make full use of solar spectral irradiation in near infrared region, binary-doped particles are prepared with different absorptionenhanced materials (Cu, and Fe, Co, or Cr elements) in the same molar ratio. The molar ratio of doped particles is Ca:Cu:x ¼ 100 : 5 : 5, where x is the doping element. Fig. 9 (a) shows two kinds of doped particles with different doping elements compared with CaCO3. The prepared particles via doping with Cu element with Ca:Cu ¼ 100 : 10 appear blacker than CaCO3. By further adding binary elements like Cu and Cr elements, the sample shows a darker color, providing an efficient solution for full-spectrum absorption. The absorptance of CaCO3 and doped particles is presented in Fig. 9 (b). When particles doping with Cu and Fe elements, their absorptance in visible light is even worse than that of unitary-doped particles (CaCO3 and CuO), but better than that of CaCO3 particles. For particles doping with Cu and Co elements, the absorptance is strongly enhanced in 300–2500 nm wavelengths region compared to unitary-doped particles. This assessment is a clear indica­ tion of the absorption-enhanced effect of particles doping with Cu and Co elements. Particles doping with Cu and Cr elements have an extended absorptance in the wavelength range of 300–1100 nm where solar irradiance is strongest. The average solar absorptance of different doped particles is shown in Fig. 9 (c). As illustrated schematically in Fig. 9 (c), the averaged ab­ sorption of CaCO3 particles is close to zero. As the proportion of doping

3.6. Cyclic stability characteristics To check the cyclic stability of different kinds of particles, samples’ weight varying with time obtained via STD 650 is shown in Fig. 11. The cyclic performance of pure CaCO3 fabricated by sol-gel method has a remarkable reduction after 20 cycles with the energy storage density decreasing from 2490 kJ kg 1 to 1061 kJ kg 1 as can be seen from in Figs. 11 (a) and Fig. 12. Initially, fresh CaCO3 particles possesses porous structures as shown in Fig. 13 (a1), but exhibit severe sintering after 20 cycles as demonstrated in Fig. 13 (a2). This is because the Tammann temperature of CaCO3 particles, at which sintering is initiated, is lower than its decomposition temperature. That explains the deterioration of energy storage density of pure CaCO3 over 20 cycles. In Fig. 11 (b), the 8

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Fig. 11. Samples’ weight varying with time obtained via STD 650. (a) Pure CaCO3; doped particles with (b) Ca:Cu ¼ 100:10; (c) Ca:Cu:Cr ¼ 100:5:5; (d) Ca:Cu:Fe ¼ 100:5:5; (e) Ca:Cu:Mn ¼ 100:5:5; (f) Ca:Al:Cu:Fe ¼ 100:20:5:5. (g) Energy storage density varying with cycling.

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Although, doping CaCO3 with Cu elements can improve the absorptance efficiently, unfortunately it also reduces the energy storage density. Similar results are found in binary-doped particles containing Cu and Cr elements as can be seen in Figs. 11 (c) and Fig. 13 (c1) and (c2). Binarydoped particles containing Cu and Fe elements can form more fluffy structures with smaller particle sizes as shown in Fig. 13 (d1). As a result, the energy storage density achieves values as high as 2275 kJ kg 1 at the first cycle as shown in Fig. 12. Again, severe sintering reduces the porosity prominently as shown in Fig. 13 (d2), leading to a fast drop of energy storage density to 631 kJ kg 1 at the 20th cycle. For the purpose of improving the cyclic stability, doping particles with Mn or Al elements is considered. As can be seen clearly in Fig. 11 (e), the cyclic stability of particles with Ca:Cu:Mn ¼ 100 : 5 : 5 is very good. The energy storage density decreases by only about 10% after 20 cycles as shown in Fig. 12. Good porosity remains after 20 cycles as shown in Fig. 13 (e1) and (e2), indicating that particles doping with Mn elements can effectively inhibit sintering of CaCO3. That explains the energy storage densities still being as high as 1952 kJ kg 1 after 20 cycles for binary-doped particles containing Mn and Cu elements. Alternatively, we find that doping Al elements also helps improve the cyclic stability. As denoted in Fig. 11 (f), the thermogravimetric per­ formance of particles with Ca:Al:Cu:Fe ¼ 100 : 20 : 5 : 5 almost does not change with cyclic times. As a result, the energy storage density de­ creases by only a slight value of 17 kJ kg 1 over 20 cycles as shown in Fig. 12. Sizes of particles and also the porosity features remain almost the same over 20 cycles, as shown in Fig. 13 (f1) and (f2). That is why doping Al elements can enhance the cyclic stability. It is also interesting to note that although the percentage of CaCO3 decreases by about 8.3% by further adding Al elements to particles with Ca:Cu:Fe ¼ 100 : 5 : 5, the energy storage density at the 20th cycle increases greatly from 631 to 1749 kJ kg 1 due to the excellent cyclic stability. The cyclic performance in terms of spectral absorption is also

Fig. 12. Comparison of the energy storage density at both 1st and 20th cycles.

cyclic performance of the unitary-doped particles containing Cu ele­ ments with Ca:Cu ¼ 100 : 10 shows a significant drop in the first five cycles, and then approaches a stable value. However, its energy storage density is only 237 kJ kg 1 after 20 cycles owing to the even more severe sintering as can be seen from the comparison in Fig. 13 (b1) and (b2).

Fig. 13. Scanning electron microscope images of different samples with (a1,a2) Pure CaCO3; doped particles with (b1,b2) Ca:Cu ¼ 100:10; (c1,c2) Ca:Cu:Cr ¼ 100:5:5; (d1,d2) Ca:Cu:Fe ¼ 100:5:5; (e1,e2) Ca:Cu:Mn ¼ 100:5:5; (f1,f2) Ca:Al:Cu:Fe ¼ 100:20:5:5 before and after 20 cycles. 10

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Formal analysis, Funding acquisition. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 51820105010). The support from the China National Key Research and Development Plan Project (No. 2018YFA0702300) is also appreciated. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.solmat.2019.110364. References Fig. 14. Comparison of spectral absorptance and average absorptance (insert picture) between 1st and 20th cycles.

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analyzed over 20 carbonation/calcination cycles. Herein, the absorp­ tance of sample with Ca:Cu:Mn ¼ 100 : 5 : 5 is compared between the 1st and 20th cycles. As shown in Fig. 14, the optical absorptance does not have a heavy dependence on the cycling number. The solar average absorptance is 58.45% after 20 cycles, which decreases only by 1.83% compared with the 1st cycle as shown in the inserted picture of Fig. 14. This is not surprising given that the material constituent does not change with cyclic times. As a result, the effective dielectric function of doped particles will not change. Considering that particle sizes will increase due to possible sintering phenomenon, the optical absorption properties of doped particles may vary with cyclic times to some extent, but the dependence is not as heavy as that for energy storage density. 4. Conclusion In summary, dark CaCO3 particles made of CaCO3 and Cu, Fe, Co, Cr, Mn or Al containing elements are synthesized, aiming to achieve fullspectrum solar thermal conversion and high-temperature thermochem­ ical energy storage with high densities and good stabilities. Effective medium theory combined with finite-difference time-domain methods is employed to obtain absorption spectrum of randomly dispersed parti­ cles. Simulated results have a good agreement with experimental mea­ surements. It is found that doping CaCO3 with Cu materials can improve the absorption in the visible range prominently while absorptance in the infrared band remains low. Further doping with Co or Cr elements helps enhance the infrared absorptance, leading to a full-spectrum solar ab­ sorption with a value of 73.1%. Nevertheless, the energy storage density has a quick drop with cycle times. By further doping Mn or Al elements, excellent cyclic stabilities are achieved, and the energy storage density remains as high as 1952 kJ kg 1 after 20 cycles, which is much higher than that of pure CaCO3 particles (1061 kJ kg 1). This work paves the way for the design of high-performance thermochemical energy storage materials for next-generation solar power systems. Author contribution Hangbin Zheng: Conceptualization, Methodology, Investigation, Software, Writing-Original draft preparation. Chao Song: Data curation, Chuang Bao: Visualization. Xianglei Liu: Supervision. Yimin Xuan: Writing-Review & Editing, Investigation, Funding acquisition, Valida­ tion. Yongliang Li: Writing-Reviewing and Editing. Yulong Ding: 11

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