High-temperature Energy Storage Research Articles

Performance of a novel single-tubular ammonia-based reactor driven by concentrated solar power

Because of high concentrating ratios of Concentrated Solar Power Technologies (CSP), and maturity of industrial ammonia synthesis process, the high-temperature ammonia thermochemical energy storage based on CSP technology has a promising potential. As a prototype of a tubular ammonia reactor under un-uniform heat flux originating from a sunlight spot or line focusing, it is assumed that catalyst particles packed in it is isotropic, and the heat transfer and mass diffusion in the axial direction of the tube were ignored since the length-diameter aspect ratio of the tube is high. Consequently, employing the local thermal non-equilibrium method, a 3-dimensional model for an ammonia decomposition reaction in porous media was developed with semi-perimeter un-uniform heat flux. For further research, the diameter of catalyst particles, the porosity of packed particles and inflow modes were recognized as important factors. The results show that the particular axial or radial distributions of the porosity significantly affect the reaction characteristics. Especially, the distribution of radial linear reduction of porosity increases the ammonia conversion rate and reduced maximum wall temperature. At the same time, the smaller catalyst particles and the inflow mode of one-side in and one-side out all contribute to improving the performance of the tubular reactor. In general, the following measures can effectively improve the reactor performance: choose a smaller catalyst particle diameter, the porosity of catalyst particles packed decreases linearly along the radial direction from the center of the tube, and the inflow mode in the tube is one-side in and one-side out.

Influence of rheological behavior of molten sodium sulfate on adherent heterogeneous surface on performance of high-temperature thermochemical energy storage

Molecular dynamics simulations were carried out to investigate the influence of the rheological behavior of molten sodium sulfate on the adherent heterogeneous surface on the performance of high-temperature thermochemical energy storage. It was found that the molten salts occurred to migrate on a nanoscale adherent heterogeneous surface due to the energy gradient of the heterogeneous surface. Molten salts underwent three spreading stages during the entire migration process. Poor wettability of the molten salt on the CaO surface came out in the climb spreading stage, and good wettability came out in the fall spreading stage. More importantly, the motion of molten salt caused an additional enhancement effect of potential energy at the interface compared to the pure CaO surface. The enhancement mechanism was the kinetic energy caused a more violent collision of ions between the molten salt and the solid surface, which lead to a local overheat at the interface to promote the ionization of the surface. The structural analysis illustrated that the calcium oxide ions of the heterogeneous surface appear stronger fluidization compared with the calcium oxide ions of the pure calcium oxide surface. The enhancement effect worked on the CaO based adsorbent rather than the inert solid supports.

Flexible epoxy film: Moderate crosslinking enhances the high temperature energy storage performance

Flexible epoxy film: Moderate crosslinking enhances the high temperature energy storage performance

Reduction of iron–manganese oxide particles in a lab-scale packed-bed reactor for thermochemical energy storage

A packed-bed reactor heated with an IR furnace is designed and fabricated for high-temperature thermochemical energy storage (TCES) studies using 0.5–1.0 mm particles composed of iron–manganese oxide with a molar ratio of Fe/Mn 2:1 (Fe67) as the storage material. An axisymmetric, two-dimensional, transient model accounting for heat transfer (conduction, convection, and radiation), mass transfer, and thermochemical reaction kinetics is developed to describe the reduction reaction of Fe67 in the packed-bed reactor. The model and experiments are in good agreement, indicating that the model is useful for providing a fundamental understanding of the process.

Enhanced High-Temperature Dielectric Properties of Poly(aryl ether sulfone)/BaTiO3 Nanocomposites via Constructing Chemical Crosslinked Networks.

Heat-resistant and crosslinked polymers/ceramic composites have been prepared and investigated for enhancing high-temperature dielectric properties to adapt the development of advanced electric and electronic systems. Here, a series of crosslinkable heat-resistant poly(arylene ether sulfone)s (DPAES) with large dipole units of -SO2 - are designed and synthesized as matrix, which are blended with BaTiO3 (BT) nanoparticles to fabricate crosslinked polymer composites for boosting high-temperature dielectric properties. The results show that BT/c-DPAES possess great dielectric stability at measured frequency and temperature. Meanwhile, the discharged energy density and efficiency of BT/c-DPAES composites are higher than that of BT/DPAES at high temperatures, e.g., 10 vol% BT/c-DPAES has a discharged energy density of 1.7 J cm-3 and efficiency of 73%, increasing by 42% and 128% in contrast to BT/DPAES, respectively. The enhanced high-temperature energy storage properties can be attributed to the construction of a crosslinked polymer network, reducing leakage current density of composites.

Interface-modulated nanocomposites based on polypropylene for high-temperature energy storage

Polymer dielectrics with excellent energy storage properties at elevated temperatures are highly desirable in the development of advanced electrostatic capacitors for harsh environment applications. However, the state-of-the-art commercial capacitor dielectric biaxially oriented polypropylene (BOPP) has limited temperature capability below 105 ​°C. Here we report the interface modulation of a polypropylene (PP)-based nanocomposite that leads to substantially improved capacitive performance at elevated temperatures. The embedded nanoparticles are functionalized with a layer of polypropylene-graft-maleic anhydride (PP-g-mah) that is well miscible with the PP matrix. The PP-g-mah moieties not only contribute to the suppression of electrical conduction at high temperature by offering deep energy traps, but also benefit the improvement in dielectric constant due to the polar molecular element, which are proved by both the experimental results and computational simulation. The local deep traps introduced by the modulated interface are directly detected and quantitatively probed by the in-situ characterization using Kelvin probe force microscopy, further validating the rationale of the present approach. The resultant polymer nanocomposites display a discharged energy density of 1.66 ​J/cm3 and a charge-discharge efficiency of >90% at 400 ​MV/m and 120 ​°C, 615% that of the pristine PP film at the same conditions. The reported nanocomposites by interface modulation can be used to reduce the volume and weight of the capacitors and to eliminate the auxiliary cooling systems applied in the harsh environment.

Sandwich-structured polymers with electrospun boron nitrides layers as high-temperature energy storage dielectrics

Most dielectric polymers have unsatisfactory energy storage characteristics at high temperature. In this work, hexagonal boron nitride (h-BN) layers with different thickness playing roles in thermal conduction and carrier blocking are designed and transferred on both sides of polycarbonate (PC) films. They existing in the aggregation status of h-BN plates are fabricated via electrospinning technology. It is found that the thermal conductivity of the composite films is improved due to the existence of h-BN layers. The composite film with layer thickness of 1 μm (BN-1) has low leakage current density, high breakdown strength and excellent high-temperature energy storage characteristics. The energy storage density of BN-1 is 5.52 J/cm3 under 500 MV/m electric field at 100 ℃, which is 15.10% higher than that of pure PC. At the same time, the density and efficiency stability of BN-1 are superior to pure PC in the temperature range of RT-100 ℃. Finally, the operation status of composite capacitor in practical applications is simulated by using the dynamic data concerning thermal conductivity and leakage current of composite films at different temperatures. The outstanding operation status in composite capacitor indicates that a promising, scalable and affordable route to high-temperature dielectric is provided.

Thermal performance of a binary carbonate molten eutectic salt for high-temperature energy storage applications

Molten carbonate eutectic salts are promising thermal storage and heat transfer fluid materials in solar thermal power plant with the feature of large specific heat capacity, wide operating temperature range and little corrosive. The high-temperature properties of molten carbonates should be determined accurately over the entire operating temperature for energy system design. In this paper, molecular dynamic simulation is used to study temperature and component dependence of microstructures and thermophysical properties of the binary carbonate molten salt. Negative linear temperature dependence of densities and thermal conductivities of binary mixtures of different components is confirmed with respect to the distances of ion clusters. Besides, positive linear temperature dependence of self-diffusion coefficient is also obtained. When temperature is constant, densities and thermal conductivities of binary mixtures are linearly related with components. Self-diffusion coefficients of CO32− firstly increase and then decrease with increasing mole fraction of Na2CO3. The temperature-thermophysical properties-composition correlation formulas are obtained, and the database of thermophysical properties of molten carbonate salts over the entire operating temperature is complemented, which will provide the essential data for heat transfer and storage system design, operation, and optimization in CSP.

Porous MgO-stabilized CaO-based powders/pellets via a citric acid-based carbon template for thermochemical energy storage in concentrated solar power plants

The reversible CaO/CaCO3 carbonation reaction (CaL) is one of the most promising candidates for high-temperature thermochemical energy storage (TCES) in concentrated solar power plants (CSP). Here, a sacrificial citric acid-based carbon template was developed to produce high-performance CaO-based sorbents to mitigate the progressive deactivation with sequential carbonation-calcination cycling. The carbon template was formed through in situ pyrolysis of citric acid in a simple heating process under nitrogen. After a secondary calcination step in air, a stable porous MgO-stabilized nano-CaO powder was generated and achieved high long-term effective conversion due to its resistance to pore plugging and sintering. By dry mixing citric acid with limestone-dolomite mixtures, this procedure can also be applied to synthesize MgO-stabilized CaO pellets via an extrusion–spheronization route, which resulted in comparably stable and effective conversion as the optimized CaO powder. Additionally, the considerable mechanical strength of MgO-stabilized CaO pellets should enable their realistic application in fluidized bed reactors. Thus, this simple, cost-effective and easily-scalable synthesis technique appears to have great potential for CSP-TCES under high temperature operation.

Research progress of polymer based dielectrics for high-temperature capacitor energy storage

Dielectric capacitors are widely used in modern electronic systems and power systems because of their advantages of fast charge discharge speed and high-power density. Nowadays, the new products related to renewable energy, such as hybrid electric vehicles, grid connected photovoltaic power generation and wind turbines, downhole oil, gas exploration, etc., put forward higher requirements for the energy storage capabilities of dielectric capacitors in elevated-temperature. In this review, the research progress of the polymer-based dielectrics for high-temperature capacitor energy storage in recent years is systematically reviewed to offer benefits for further study. Firstly, the physical mechanism of energy storage of dielectric materials is introduced, and several conduction mechanisms of dielectric materials are summarized and analyzed; then, several strategies to improve the high-temperature energy storage performance of polymer dielectrics are presented, including the nanocomposite modification and design of layer-structured polymer composites, and the molecular structure design and chemical crosslinking treatment of dielectric polymer. Finally the scientific and technological problems in the application of dielectric polymer and their nanocomposites for high-temperature capacitor energy storage are discussed, and a possible research direction in the future is prospected.

The effect of diameter size of single-walled carbon nanotubes on their high-temperature energy storage behaviour in ionic liquid-based electric double-layer capacitors.

We investigated the effect of the diameter size of single-walled carbon nanotubes (SWCNTs), on their high-temperature energy storage behavior in an electric double layer capacitor (EDLC) using the ionic liquid triethyl(2-methoxyethyl) phosphonium bis(trifluoromethylsulfonyl)imide (P222(2O1)-TFSI). We used four SWCNT samples with diameter sizes ranging from 0.8 to 5 nm, and evaluated their electrochemical charge storage behavior through galvanostatic charge/discharge and electrochemical impedance spectroscopy (EIS). We found that for the SWCNTs with small average diameter of 1 nm, the value of the electrode capacitance measured at a current density of 5 mA g−1 increased from 15.8 at room temperature to 27.5 F g−1 at 150 °C, and the value measured at a current density of 80 mA g−1 increased from 14.0 at room temperature to 22.1 F g−1 at 150 °C. The larger diameter samples on the other hand did not show any significant change in their capacitance with temperature. We calculated the size of the interstitial tube spaces from the Raman spectra of the samples, and used density functional theory (DFT) calculations to estimate the sizes of the cation and anion of the electrolyte. The obtained results suggest that the temperature-induced changes in the electrolyte properties improved the ion accessibility into the otherwise constrained space inside the small diameter SWCNTs, while the spaces inside the larger SWCNTs already provided easily accessible storage sites hence good performance at room temperature, making the increase in temperature of little to no effect on the charge storage performance in such SWCNTs.

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

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 next-generation 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 high-efficiency, large-capacity, and stable thermochemical energy storage particles for simultaneous solar thermal conversion and high-temperature thermochemical energy storage.

Enhancing thermal stability of P(VDF-HFP) based nanocomposites with core-shell fillers for energy storage applications

Improving thermal stability of the ceramic-polymer based nano-composited electrostatic capacitors is the key element to their practical applications in harsh environment. In this paper, Fe3O4 @BaTiO3 particles with thermal conductive core and high-k shell were prepared and used as fillers to improve the thermal stability and high temperature energy storage density of poly-(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)) based composited films. Compared to the pure polymer, the films with 10 wt% fillers showed enhanced temperature stability at the range of 40–120 °C. The high-temperature (120 °C) average breakdown strength reached at 1615 kV/cm with ~170% higher than the pure P(VDF-HFP) (970 kV/cm). The discharged energy storage density (120 °C) was 1.68 J/cm3 (1750 kV/cm), which was enhanced up to 1150% comparing to that of pure P(VDF-HFP) (0.146 J/cm3 at 1300 kV/cm). The increased thermal conductivity and the Internal Barrier Layer Capacitor (IBLC) effects at the conductive-insulating interfaces contribute to the enhanced features. The results in the present work explored an effective way to prepare high temperature dielectric nanocomposites for harsh applications in energy storage field.

Long term thermostable supercapacitor using in-situ SnO2 doped porous graphene aerogel

Thermally stable, long-cycle energy storages that are capable of operating at high temperatures are attracting much attention due to their importance in enhancing the thermal safety and durability of the electronic devices and further extending the application scope. Here, we develop high-performance and thermostable supercapacitors operating under high temperatures, which are fabricated by integrating SnO2-doped graphene aerogel electrodes and ionic liquid-based composite electrolytes. The tiny SnO2 nanoparticles are bonded chemically on the graphene sheets by in-situ formation without additional annealing, which initiates the electrochemical synergistic effect at high-temperatures. Also, the addition of fumed silica nanoparticles into the composite electrolytes not only improves the ionic conductivity of the polymer matrix but also enhances thermal stability. As a result, the excellent combination of the porous aerogel electrode and the composite electrolyte improves interfacial stability and enables the high-performance supercapacitor with a high specific capacitance of 541 F g−1 and an energy density of 160 Wh kg−1 at 125 °C. Further, the supercapacitor devices exhibit extremely long-term stability without significant capacitance loss after 10,000 cycles, even under high temperatures. This study provides a rational strategy for high-performance, long-cycle life, and high-temperature energy storage systems that supply electric energy in harsh environments.

Wettability of molten sodium sulfate salt on nanoscale calcium oxide surface in high-temperature thermochemical energy storage

Molecular dynamics simulations were carried out to originally investigate wettability of molten sodium sulfate salt on nanoscale calcium oxide surfaces at high temperature and micro-mechanisms on the molten salt promoting the performance of thermochemical energy storage. It was found that in terms of the molten sodium sulfate salt, the nanoscale calcium oxide surface was hydrophilic, and became more and more hydrophilic with increasing temperatures. More importantly, average diffusion coefficients and radial distribution functions of various ions illustrated that the molten sodium sulfate activated the calcium ions and the oxygen ions on the calcium oxide surface, resulting in weak fluidization of the surface at a temperature below the melting temperature. This promoted potential energy of the atom interaction at the contact interface. Additionally, it was observed that the sodium sulfate transited from partial coverage to complete coverage on the spherical nanoscale calcium oxide surface. Higher temperatures induced better coverage performances due to the decreasing surface tension of the molten salt. Therefore, the molten salt formed a screen on the surface to effectively prevent the calcium oxide particles from sintering. This study provides a theoretical foundation to further utilize the molten salt to promote the performance of the thermochemical energy storage.

A novel multiwalled LiF@GO@SiO2 microcapsule with high phase change temperature

High temperature phase change materials (PCMs)-encapsulated microcapsules with large heat exchange area for addressing specific energy storage have increasing application potentials in many fields. Herein, a novel hybrid multiwalled microcapsule LiF@GO@SiO2 with high phase change temperature was designed and fabricated, which could overcome strong corrosion and volume expansion of lithium fluoride (LiF) during heat storage process. The multiwalled structure of microcapsule contained three layers, consisting of volume expansion buffer layer polydopamine (PDA), anti-corrosion leakage-proof layer (GO) and heat-resist strength layer (SiO2), respectively. Volume expansion buffer layer would be vanished during thermal cycling, and GO as well as SiO2 layers played important roles in protecting molten LiF from leakage. A series of tests including SEM, SEM-EDS, FTIR, XPS, XRD and TGA revealed that three-layer shells were sequentially coated on the surface of core material LiF successfully. Besides, DSC results via quantitatively analyzing, indicated that LiF@GO@SiO2 microcapsules had high phase change temperature of 848 °C, encapsulation ratio of 82.0% and latent melting heat of 894.5 J/g. In addition, encapsulation ratio of the microcapsule had a minor deflection within 5% after 10 times loops by calculating and comparing corresponding enthalpies, which displayed prepared microcapsule could be used repeatedly in high temperature energy storage. Overall, our results opened new avenues for the utilization of high temperature PCMs and their packaging technique.

High-temperature thermochemical energy storage – heat transfer enhancements within reaction bed

Ca(OH)2/CaO reversible reaction system has high potentials to be used for high-temperature thermal energy storage. Endothermic dehydration of Ca(OH)2 and exothermic hydration of CaO can be carried out at high temperatures compatible with most of the concentrated solar power applications. One of the challenges in using Ca(OH)2/CaO is the slow rate of heat transfer inside the solid reaction beds. Heat transfer enhancement techniques are introduced in a circular fixed bed heated indirectly through heat exchanger walls. Two reactor configurations are studied: one in which the heat transfer fluid (HTF) flows through an outer annular shell and another in which the HTF flows through a central pipe. Dehydration and rehydration processes are simulated with and without fins attached to the heat exchanger wall extending into the reaction bed. Heat and mass transport equations coupled with reaction kinetics are solved numerically. The designed heat transfer enhancements (HTE) within the reaction beds resulted in a significant decrease in overall conversion time during the energy storage and discharge. It is shown that the reactor with an outer annular shell as the HTF channel has better performance compared to the reactor with internal pipe flow configuration for HTF.

Experimental investigations on the thermal stability of Na2CO3–K2CO3 eutectic salt/ceramic composites for high temperature energy storage

In this study, eutectic salt/ceramic composites with MgO serving as supportingmaterial and Na2CO3–K2CO3 eutectic salt acting as thermal absorbing materials were prepared by the form-stable technique. As additives, effects of both Kaolin and graphite on the thermal stability of composites were investigated. The microstructures of the composites before and after thermal cycling tests were detected by X-ray diffraction and scanning electron microscope techniques. The thermal stability and thermal properties including latent heat, specific heat capacity, thermal conductivity, etc. were measured using differential scanning calorimetry, laser thermal instrument and thermogravimetric analysis, etc. The results showed the composite phase change materials had good thermal reliability and chemical stability. No obvious damages were found in the samples with thermal cycles reaching 1500 times. Kaolin can improve the stability of the sample by restraining the volatilization of molten salt, but it can reduce the phase transition enthalpy of the composite. In comparison with kaolin, carbon can improve thermal cycle life due to its favorable shock resistance. But it suffers severe oxidation at high temperature, which might be a possible reason for its decay in thermal cycling life. These studies suggest the thermal life of the composite can be controlled by adding proper materials.

Excellent electric field-induced strain with high electrostrictive and energy storage performance properties observed in lead-free Bi0.5(Na0.84K0.16)0.5TiO3-Ba(Nb0.01Ti0.99)O3-BiFeO3 ceramics

In this research, ceramics with a high electromechanical strain based on the composition (1-x)[0.97Bi0.5(Na0.84K0.16)0.5TiO3-0.03Ba(Ti0.99Nb0.01)O3]-xBiFeO3 or (1-x)[BNKT-0.03BNbT]-xBFO (with x = 0, 0.03, 0.05, 0.07, and 0.09 mol fraction) were investigated. All compositions were synthesized via a conventional solid-state mixed oxide method. The effects of the addition of BFO on the phase structure, dielectric, ferroelectric, and piezoelectric properties and electric field-induced strains behavior of BNKT-0.03BNbT ceramics were systematically studied in this work. X-ray diffraction (XRD) analysis revealed that all ceramics exhibited a pure perovskite structure. The coexistence of rhombohedral (R) and tetragonal (T) phases was observed for all compositions. The electric field-induced strain increased with increasing BFO content and attained a maximum value of Smax = 0.65% and d*33 = 1161 pm/V (@40 kV) for the composition x = 0.05 at room temperature (RT). The bipolar strain behavior exhibited minimal asymmetry and a low driving electric field. The composition also exhibited a high electrostrictive response (Q33 ∼ 0.039 m4/C2). In addition, the x = 0.09 composition showed excellent energy storage properties, with an energy storage density of 0.91 J/cm3 at 125 °C, high normalized energy storage density (∼0.14 μC/mm2), thermal stability (7% for 75-150 °C) and energy storage efficiency (η = 87% @125 °C). These results suggest that the (1-x)[BNKT-0.03BNbT]-xBFO ceramics are promising as lead-free piezoelectric material candidates, and are also suitable for both actuator and high temperature energy storage applications.

Structural, thermally stable dielectric, and energy storage properties of lead-free (1 − x)(Na0.50Bi0.50)TiO3 − xKSbO3 ceramics

Effect of substitution and external applied static electric field on the structural and dielectric properties for lead-free (1 − x)(Na0.50Bi0.50)TiO3 − xKSbO3 (0 ≤ x ≤ 0.06) polycrystalline ferroelectric ceramics, fabricated via a modified sol–gel method, were investigated. Structural analysis of synchrotron radiation X-ray diffraction data confirmed the rhombohedral R3c phase for all unpoled samples. After poling, the tetragonal P4bm phase appeared with the rhombohedral phase in all the substituted samples. In poled samples, the phase fraction of the rhombohedral phase suppressed from ~ 93 (for x = 0.03) to ~ 87% (for x = 0.06), while tetragonal phase fraction increased from ~ 7 to ~ 13% as a function of substitution. The high-temperature dielectric analysis confirmed the reduction in depolarization temperature with increasing substitution. Also lattice disorder creates a plateau type dielectric anomaly, which leads to thermally stable dielectric constant ~ 2970 ± 10% (200–390 °C) and ~ 2830 ± 10% (125–400 °C) for x = 0.03 and 0.06 samples, respectively. Ferroelectric measurements showed that ambient temperature ferroelectric properties are improved for x = 0.03 composition with an observed remnant polarization (2Pr ~ 53.4 µC/cm2) and coercive field (2Ec ~ 94.7 kV/cm) as compared to parent NBT compound (2Pr ~ 44.7 µC/cm2, 2Ec ~ 124.5 kV/cm). In addition, at high-temperature, antiferroelectric like ordering enhances the recoverable energy density ~ 0.73 J/cm3 (efficiency ~ 72.3%) for x = 0.06 samples as compared to parent NBT (recoverable energy density ~ 0.05 J/cm3, efficiency ~ 2.4%). These improvements in electrical properties were correlated with structural changes as a function of composition and temperature. Obtained properties suggest that substituted samples might be a suitable candidate for high-temperature stable capacitors (operating temperature > 200 °C), ferroelectric, and energy storage applications.