Solar Thermal Storage Materials Research Articles

Simultaneous Cycloadditions in the Solid State via Supramolecular Assembly

AbstractChemical reactions conducted in the solid phase (specifically, crystalline) are much less numerous than solution reactions, primarily due to reduced motion, flexibility, and reactivity. The main advantage of crystalline‐state transformations is that reactant molecules can be designed to self‐assemble into specific spatial arrangements, often leading to high control over product regiochemistry and/or stereochemistry. In crystalline‐phase transformations, typically only one type of reaction occurs, and a sacrificial template molecule is frequently used to facilitate self‐assembly, similar to a catalyst or enzyme. Here, we demonstrate the first system designed to undergo two chemically unique and orthogonal cycloaddition reactions simultaneously within a single crystalline solid. Well‐controlled supramolecular self‐assembly of two molecules containing different reactive moieties affords orthogonal reactivity without use of a sacrificial template. Using only UV light, the simultaneous [2+2] and [4+4] cycloadditions are achieved regiospecifically, stereospecifically, and products are obtained in high yield, whereas a simultaneous solution‐state reaction affords a mixture of isomers in low yield. Application of dually‐reactive systems toward (supra)molecular solar thermal storage materials is also discussed. This work demonstrates fundamental chemical approaches for orthogonal reactivity in the crystalline state and highlights the complexity and reversibility that can be achieved with supramolecular design.

Simultaneous Cycloadditions in the Solid State via Supramolecular Assembly.

Chemical reactions conducted in the solid phase (specifically, crystalline) are much less numerous than solution reactions, primarily due to reduced motion, flexibility, and reactivity. The main advantage of crystalline-state transformations is that reactant molecules can be designed to self-assemble into specific spatial arrangements, often leading to high control over product regiochemistry and/or stereochemistry. In crystalline-phase transformations, typically only one type of reaction occurs, and a sacrificial template molecule is frequently used to facilitate self-assembly, similar to a catalyst or enzyme. Here, we demonstrate the first system designed to undergo two chemically unique and orthogonal cycloaddition reactions simultaneously within a single crystalline solid. Well-controlled supramolecular self-assembly of two molecules containing different reactive moieties affords orthogonal reactivity without use of a sacrificial template. Using only UV light, the simultaneous [2+2] and [4+4] cycloadditions are achieved regiospecifically, stereospecifically, and products are obtained in high yield, whereas a simultaneous solution-state reaction affords a mixture of isomers in low yield. Application of dually-reactive systems toward (supra)molecular solar thermal storage materials is also discussed. This work demonstrates fundamental chemical approaches for orthogonal reactivity in the crystalline state and highlights the complexity and reversibility that can be achieved with supramolecular design.

Form-Stable Composite Phase Change Materials Based on Porous Copper-Graphene Heterostructures for Solar Thermal Energy Conversion and Storage.

Solar-thermal energy conversion and storage technology has attracted great interest in the past few decades. Phase change materials (PCMs), by storing and releasing solar energy, are able to effectively address the imbalance between energy supply and demand, but they still have the disadvantage of low thermal conductivity and leakage problems. In this work, new form-stable solar thermal storage materials by impregnating paraffin PCMs within porous copper-graphene (G-Cu) heterostructures were designed, which integrated high thermal conductivity, high solar energy absorption, and anti-leakage properties. In this new structure, graphene can directly absorb and store solar energy in the paraffin PCMs by means of phase change heat transfer. The porous structure provided good heat conduction, and the large surface area increased the loading capacity of solar thermal storage materials. The small pores and superhydrophobic surfaces of the modified porous G-Cu heterostructures effectively hindered the leakage issues during the phase change process. The experimental results exhibited that the thermal conductivity of the prepared form-stable PCM composites was up to 2.99 W/(m·K), and no leakage took place in the solar-thermal charging process. At last, we demonstrated that the PCM composites as an energy source were easily integrated with a thermoelectric chip to generate electric energy by absorbing and converting solar energy.

Preparation and thermal shock resistance of solar thermal storage ceramics from high calcium and high iron steel slag

To guarantee the efficiency of solar thermal power generation, the solar thermal storage material is required to have excellent thermal shock resistance to withstand the process of long-term thermal cycles. In this study, solar thermal storage ceramics were prepared using high calcium and high iron steel slag as the main raw material, adding cordierite and using the pressureless sintering method. The effects of cordierite addition on the physical properties, phase composition, microstructure, thermal storage and thermal shock resistance of the samples were investigated by XRD, SEM and so on. The results show that the addition of cordierite promotes densification and improves thermal shock resistance. The optimal level of addition is 10 %. When the temperature is 1240 °C, the bulk density and bending strength of the B5 (10 wt% cordierite) are 3.03 g cm−3 and 92.52 MPa, respectively. The bending strength of B5 after 30 thermal shock cycles is 97.06 MPa, which is 4.91 % higher than that before 30 thermal shock cycles. The thermal storage density is 867.95 kJ kg−1. This work not only analyzes the feasibility of this material for high-temperature service but also provides a new way for high-value utilization of high calcium and high iron steel slag.

Thermal shock resistance and dimensional stability of zirconia‐corundum‐mullite composites for solar thermal storage

AbstractZirconia‐corundum‐mullite composites were prepared from andalusite, ZrO2, and α‐Al2O3 and evaluated for application as solar thermal storage materials in this study. The effects of ZrO2 concentration (0–25 wt%) and sintering temperature(1500–1650°C) on the bending strength, phase composition, microstructure, dimensional stability, and thermal shock resistance of the samples were investigated. The results showed that sample C2 (containing 10 wt% ZrO2) sintered at 1600°C exhibited the best properties. A portion of Y2O3 existed in ZrO2, which excited a small amount of t‐ZrO2 in the sample after sintering, while the other portion of Y2O3 was separated from ZrO2 and participated in the formation of a high‐temperature liquid phase. The liquid phase reduced the dimensional stability of the sample. However, the liquid phase could also promote sintering, reducing the optimal sintering temperature, with lower sintering temperatures increasing the dimensional stability of the sample. Once again, the C2 sample exhibited the highest thermal shock resistance, with a strength loss of 2.76 MPa after 30 thermal shock tests. The thermal storage density(25–1100°C) of C2 was 1121 kJ/kg. In summary, sample C2 exhibits a high degree of dimensional stability and thermal shock resistance and can thus be used as a reliable solar thermal storage material.

Molecular dynamics analysis of thermophysical properties of Na2CO3-K2CO3 binary molten salts in temperature range of 1125–1475 K

Molten carbonates are highly promising solar thermal storage materials. In this paper, the molecular dynamics (MD) method was used to study the thermal properties of binary molten salt mixtures of Na2CO3-K2CO3 as a function of temperature and composition. Their thermal storage mechanisms were analyzed from the perspectives of microstructure and atomic diffusion. It was confirmed that temperature and various components have significant effects on the density, thermal conductivity and shear viscosity of binary molten salts. Based upon the MD data, the correlation for the temperature-thermal properties-component with the correlation coefficient of higher than 0.98 was obtained. In addition, the radial distribution function (RDF) showed that the change in thermal properties was the result of variation in distance between the ionic clusters. Finally, based on the guidance of material composition design strategy, a coefficient that could reflect the synergistic effect between the binary molten salts was proposed for the first time. The results showed that the synergy coefficient reached its highest point for the molten salt molar ratio of 58:42, and at this point, the positive synergy was most obvious. This study can provide theoretical guidance for the design and enhancement in the performance of molten carbonates as thermal storage materials.

Effect of metallic magnesium on enhanced specific heat capacity of chloride molten salts for solar thermal storage applications

The NaCl–CaCl 2 based composite is a novel high-temperature heat transfer and solar thermal storage material with enhanced specific heat capacity by dissolving metallic magnesium in the molten salt. However, the enhancement mechanism and the interactions between Mg atoms and various ions in the molten NaCl–CaCl 2 are not clear. In this work, the interfacial effects of local coordination structures around the nano Mg particle and the bonding characteristics between Mg atoms and molten salt ions were studied from both the chemical and physical points of view by using the ab-initio molecular dynamics simulations. The results indicate that [MgNa m ] n + and [MgCa x ] y + clusters are formed at the interface according to the analysis of calculated Mayer bond orders. A compressed ion layer with higer ion densities of Na + and Cl − ions is formed around the nano Mg particle. Moreover, a calculated specific heat capacity of 1.387 J g −1 K −1 is obtained with an enhancement of 25.98% for the Mg/NaCl–CaCl 2 system, which is mainly resulted from the formed compressed ion layer and decreased potential energy due to the interfacial effect. And a theoretical thermal energy storage density with an enhancement of 29.59% is achieved for the Mg/NaCl–CaCl 2 composite at 873 K compared to the pure molten salt. ● Bonding interactions and microstructure evolutions at the interface are revealed. ● The doped Mg atoms can react with Na + and Ca 2+ ions instead of Cl − ions. ● A compressed ion layer of Na + and Cl − ions is formed around the nano Mg particle. ● Specific heat capacity of 1.387 J g −1 K −1 of the composite is enhanced by 25.98%.

In-situ synthesis of nitride whiskers-bonded SiAlON–Al2O3 ceramics for solar thermal storage by aluminothermic nitridation of coal-series kaolin

Nitride whiskers-bonded SiAlON–Al2O3 ceramics used for solar thermal storage were in-situ synthesized by aluminothermic nitridation of coal-series kaolin, an abundant solid waste in China. Mechanisms on the aluminothermic nitridation and in-situ synthesis of Si3N4 and SiAlON whiskers were studied by TG-DSC, XRD, SEM, TEM, etc. The effect of sintering temperature on the phase evolution, microstructure and properties of the ceramics was investigated, and the thermophysical properties were compared to those of the available solar thermal storage materials. Results indicated that Si3N4 and SiAlON whiskers could be in-situ synthesized simultaneously at 1300 °C via Vapor–Solid mechanism. Increasing sintering temperature decreased the amount of Si3N4 whiskers, but increased that of SiAlON whiskers, which transformed to be columnar SiAlON grains at 1550–1600 °C. By increasing the sintering temperature, the enhancing densification and the formation of the columnar SiAlON grains were in favor of improving the strength of the ceramics, and the increasing SiAlON content decreased the thermal expansion coefficient of the ceramics to 5.73 × 10−6 °C−1. Compared with other thermal storage materials, the advantages of the SiAlON–Al2O3 ceramics are their relatively higher thermal conductivity (6.34 W/m/K, at room temperature), higher specific energy density (2941 kJ/m3/K, at 900 °C), and better high-temperature thermal stability, making them competent for high-temperature thermal storage uses.

Preparation of Cordierite-mullite Ceramics for Solar Thermal Storage

We developed cordierite-mullite composite ceramic materials to package and encapsulate PCM, and presented a preparation process from raw materials of kaolin, talc and alumina. The properties and microstructre of cordierite-mullite composite ceramic were studied. Due to the strengthening effects of mullite, the sample C2 (80 wt% of cordierite and 20 wt % of mullite) sintered at 1 420 °C possessed excellent physical properties. Determined by X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) analysis, cuboid-shaped cordierite crystals and needle-like or long quadrilateral-prism mullite crystals with staggered patterns were found, which endowed the composites preferable mechanical strength. After 30 cycles of thermal shock (room temperature to about 1 100 °C, air-cooled), the sample presented superior thermal shock resistance, which is suitable to be applied as solar thermal storage materials.

A High Energy Density Azobenzene/Graphene Oxide Hybrid with Weak Nonbonding Interactions for Solar Thermal Storage

Incorporating photochromic chromophores into polymer composites provides the possibility of a reversible photoswitch of the intrinsic properties of these materials. In this paper we report a route to attach azobenzene (AZO) moiety covalently to graphene oxide (GO) to create chromophore/graphene oxide (AZO-GO) hybrid, in which GO is both part of the chromophore and the template. Due to the high grafting density of AZO moiety and the low mass of the novel structure, the hybrid is a potential solar thermal storage material with high energy density of about 240 Wh·kg−1. It is found that C-H···π interaction between the cis-AZO chromophores and the aromatic rings of the substrate induces collective electronic modifications of GO at critical percentage of cis-isomers and reduce the thermal barrier of π-π* transition of the chromophores directly, which results in two sections of first-order reactions during the photoisomerization of trans- to cis-hybrid and also thermally stabilizes the cis-hybrid. Our findings demonstrate that high-performance AZO–GO hybrid can be manipulated by optimizing intermolecular nonbonding interactions.

Synthesis and properties of microencapsulated stearic acid/silica composites with graphene oxide for improving thermal conductivity as novel solar thermal storage materials

Phase change materials (PCM) have stable operation temperature and large storage capacity. However, PCM have problems of leakage and low thermal conductivity. For improving performances of PCM, stearic acid (SA) was encapsulated in silica shell by sol-gel method to form microencapsulated phase change materials (MPCM), and graphene oxide (GO) was attached to surface of silica by a self-assembly process to form GO@MPCM, so as to further improve performances of MPCM. The morphology of MPCM was observed through a scanning electronic microscope (SEM). The chemical structure and crystal phase of MPCM were measured by Fourier transformation infrared spectroscope (FT–IR) and X–ray diffractometer (XRD). Raman spectrometer was used to further verify that GO was attached to the MPCM. Thermogravimetric analyzer (TGA) analysis confirmed that MPCM have good thermal stability. Thermal properties of MPCM were measured by Differential scanning calorimeter (DSC), where melting temperature and latent heat of MPCM2 is 67.78 °C and 179.29 J/g. The melting temperature of GO@MPCM is similar to that of MPCM2, and the melting latent heat of GO1 @MPCM and GO2@MPCM is 146.72 J/g and 134.42 J/g, respectively. Besides, thermal conductivity of MPCM with GO is higher than that of pure SA.

A high energy, reusable and daily-utilization molecular solar thermal conversion and storage material based on azobenzene/multi-walled carbon nanotubes hybrid

Developing molecular solar thermal storage materials based on the reversible isomerization is an important strategy for future efficient solar energy storage and conversion. A new hybrid system composed of azobenzene chromophore (AZO) and multi-walled carbon nanotubes (MCNTs) nano-templates, is designed and prepared via an organosilane as high-energy molecular solar thermal storage material. External high functional density and bundling effect of azobenzenes on the surface of multi-walled carbon nanotubes enable the intermolecular interactions between neighbouring azobenzenes to bring about a conspicuous increase in energy density (77.1Whkg−1) and half-lifetime (14h), respectively increased 92.5% and 2 order of magnitudes relative to the pure azobenzene molecule. Simultaneously, AZO-MCNTs nano hybrid maintains ultrastable and reusable stored energy ability for 30 cycles. Our results demonstrate this molecular AZO-MCNTs can practically palliate the daily periodicity of sunlight by storing the solar energy during the day and releasing as heat at night, which can be applied as highly customizable energy storage and conversion technology.

Preparation and performance study of cordierite/mullite composite ceramics for solar thermal energy storage

AbstractThe employment of solar energy in recent years has reached a remarkable edge. It has become even more popular as the cost of fossil fuel continues to rise. Energy storage system improves an adjustability and marketability of solar thermal and allowing it to produce electricity in demand. This study attempted to prepare cordierite/mullite composite ceramics used as solar thermal storage material from calcined bauxite, talcum, soda feldspar, potassium feldspar, quartz, and mullite. The thermal physical performances were evaluated and characterized by XRD, SEM, EPMA, and EDS. It was found that the optimum sintering temperature was 1280°C for preparing, and the corresponding water adsorption was 11.25%, apparent porosity was 23.59%, bulk density was 2.10 mg·cm−3, bending strength was 88.52 MPa. The residual bending strength of specimen sintered at 1280°C after thermal shock of 30 times decreased to be 57 MPa that was 36% lower than that before. The thermal conductivity of samples sintered at 1280°C was tested to be 2.20 W·(m·K)−1 (26°C), and after wrapped a PCM (phase change materials) of K2SO4, the thermal storage density was 933 kJ·kg−1 with the temperature difference (ΔT) ranged in 0‐800°C. The prepared cordierite/mullite composite ceramic was proved to be a promising material for solar thermal energy storage.

Azobenzene/graphene hybrid for high-density solar thermal storage by optimizing molecular structure

A large capacity storing solar energy as latent heat in a close-cycle is essentially important for solar thermal fuels. This paper presents a solar thermal molecule model of a photo-isomerizable azobenzene (Azo) molecule covalently bound to graphene. The storage capacity of the Azo depending on isomerization enthalpy (ΔH) is calculated based on density functional theory. The result indicates that the ΔH of Azo molecules on the graphene can be tuned by electronic interaction, steric hindrance and molecular hydrogen bonds (H-bonds). Azo with the withdrawing group on the ortho-position of the free benzene shows a relatively high ΔH due to resonance effect. Moreover, the H-bonds on the trans-isomer largely increase ΔH because they stabilize the trans-isomer at a low energy. 2-hydroxy-4-carboxyl-2′,6′,-dimethylamino-Azo/graphene shows the maximum ΔH up to 1.871 eV (107.14 Wh kg-1), which is 125.4% higher than Azo without functional groups. The Azo/graphene model can be used for developing high-density solar thermal storage materials by controlling molecular interaction.

Advanced research in molecular solar thermal storage materials

Development and application of solar energy is the most promising strategy to solve the problem of energy crisis. Molecular solar thermal storage which is a novel technology of solar energy conversion shows the great potential for large-scale application in the field of solar storage. This conversion based on isomerization realizes the transformation from the stable state with lower energy to the higher-energy metastable state under the light irradiation, storing solar energy in the chemical bonds due to the energy level gap and releasing energy as heat under the appropriate conditions. This paper reviews the thermal storage mechanism and key performance indexes of the molecular solar thermal storage materials as well as the common molecules and the particular materials with novel nano-structures, providing certain theoretical basis for studying and developing new solar thermal storage materials with high densities of thermal storage and long-term half-lives.

The development of metal hydrides using as concentrating solar thermal storage materials

Metal hydrides high temperature thermal heat storage technique has great promising future prospects in solar power generation, industrial waste heat utilization and peak load regulating of power system. This article introduces basic principle of metal hydrides for thermal storage, and summarizes developments in advanced metal hydrides high-temperature thermal storage materials, numerical simulation and thermodynamic calculation in thermal storage systems, and metal hydrides thermal storage prototypes. Finally, the future metal hydrides high temperature thermal heat storage technique is been looked ahead.

High-energy, stable and recycled molecular solar thermal storage materials using AZO/graphene hybrids by optimizing hydrogen bonds.

An important method for establishing a high-energy, stable and recycled molecular solar heat system is by designing and preparing novel photo-isomerizable molecules with a high enthalpy and a long thermal life by controlling molecular interactions. A meta- and ortho-bis-substituted azobenzene chromophore (AZO) is covalently grafted onto reduced graphene oxide (RGO) for solar thermal storage materials. High grafting degree and close-packed molecules enable intermolecular hydrogen bonds (H-bonds) for both trans-(E) and cis-(Z) isomers of AZO on the surface of nanosheets, resulting in a dramatic increase in enthalpy and lifetime. The metastable Z-form of AZO on RGO is thermally stabilized with a half-life of 52 days by steric hindrance and intermolecular H-bonds calculated using density functional theory (DFT). The AZO-RGO fuel shows a high storage capacity of 138 Wh kg(-1) by optimizing intermolecular H-bonds with a good cycling stability for 50 cycles induced by visible light at 520 nm. Our work opens up a new method for making advanced molecular solar thermal storage materials by tuning molecular interactions on a nano-template.

In-situ synthesis of a cordierite-andalusite composite for solar thermal storage

An innovative thermal storage material, in-situ synthesised cordierite combined andalusite (Al2O3·SiO2), was prepared via semi-dry pressing followed by pressureless sintering. The samples were mainly composed of andalusite (the main raw material), with in-situ synthesised cordierite added to improve the thermal shock resistance. The results indicate that the formula L4 (70% andalusite, 13% talcum, 13% kaolin and 4% γ-Al2O3) sintered at 1400°C has the best thermal stock resistance, physical properties and chemical properties. The L4 sample can withstand 30 thermal shock test cycles without cracking (wind cooling from 1100°C to 25°C), and the bending strength after the thermal shock cycle test increased 26.20% rather than decreasing. Other properties are as follows: apparent porosity: 22.95%; water absorption: 10.71%; bulk density: 2.14g/cm3; bending strength: 92.74MPa; coefficient of thermal expansion: 4.11×10−6/°C; specific heat capacity: 0.95kJ·(kgK)−1; thermal conductivity coefficient: 0.58×10−2cm2s−1; and thermal conductivity: 1.17W·(mK)−1. The examination of the phase composition indicates that cordierite, mullite, sillimanite, cristobalite and α-quartz are the main phase elements. Analysis of the microstructure shows that the cordierite crystals were formed during the sintering process and were dispersed evenly in the mullite crystals, which gave the samples a favourable thermal shock resistance. It was concluded that the ceramic material, a combination of cordierite and andalusite, is an excellent candidate for solar thermal storage material due to its high thermal shock resistance, temperature resistance and strength.