Stored Energy Density Research Articles

Development and Characterization of Concrete/PCM/Diatomite Composites for Thermal Energy Storage in CSP/CST Applications

Thermal energy storage (TES) systems for concentrated solar power plants are essential for the convenience of renewable energy sources in terms of energy dispatchability, economical aspects and their larger use. TES systems based on the use of concrete have been demonstrated to possess good heat exchange characteristics, wide availability of the heat storage medium and low cost. Therefore, the purpose of this work was the development and characterization of a new concrete-based heat storage material containing a concrete mix capable of operating at medium–high temperatures with improved performance. In this work, a small amount of shape-stabilized phase change material (PCM) was included, thus developing a new material capable of storing energy both as sensible and latent heat. This material was therefore characterized thermally and mechanically and showed increased thermal properties such as stored energy density (up to +7%, with a temperature difference of 100 °C at an average operating temperature of 250 °C) when 5 wt% of PCM was added. By taking advantage of these characteristics, particularly the higher energy density, thermal energy storage systems that are more compact and economically feasible can be built to operate within a temperature range of approximately 150–350 °C with a reduction, compared to a concrete-only based thermal energy storage system, of approximately 7% for the required volume and cost.

Structural, electrical, and thermal features of polyimide composites filled with semiconductive MXene sheets

Polymer-based dielectrics are widely applied in the fields of electronics and electrical power systems due to the high breakdown strength, excellent flexibility, and unique self-healing capability. However, the low stored energy density and unsatisfactory heat management of polymer dielectrics hinder the development of polymer-based film capacitors. In this research, artificial composites with improved capacitive energy storage and thermal conductivity are fabricated by blending the two-dimensional semiconductive MXene sheets with a polyimide (PI) matrix. Remarkably, a PI based composite with 1 wt. % MXene sheets increases its dielectric permittivity from ≈3.27 to ≈3.53 and enhances its discharged energy density from ≈1.93 to ≈2.38 J/cm3 while maintaining its low dielectric loss of <0.02 and relatively high charge–discharge efficiency of >80%. Meanwhile, a high in-plane thermal conductivity of 0.418 W m−1 K−1 is achieved for PI/MXene composites with 5 wt. % MXene. In addition, the distribution of temperature field inside the composites has been investigated by a finite element method. These results represent a strategy in polymer dielectrics to achieve simultaneous high energy density and thermal conductivity, which may also have potential for applications in high temperature environments.

Crystallography and elastic anisotropy in fatigue crack nucleation at nickel alloy twin boundaries

Fatigue crack nucleation at annealing twin boundaries (TBs) within polycrystal nickel-based superalloy René88DT is investigated with a microstructure-sensitive crystal plasticity (CP) model, digital image correlation strain measurements and experimental SEM crack nucleation observations. Strong slip localizations at TBs were experimentally observed and predicted by the CP model, which also showed high predicted geometrically necessary dislocation and corresponding stored energy densities, capturing experimental observations of crack nucleation. In a systematic study, elastic anisotropy was found to drive local elastic constraint and hence resolved shear stress, slip activation, GND density and stored energy density, demonstrating for this reason that TBs are preferential sites for crack nucleation in this alloy. The parent grain / twin pair crystallographic orientation with respect to remote loading was also demonstrated to be key to slip activation parallel to TBs and hence to stored energy density and fatigue crack nucleation, and the range of most damaging parent grain orientations has been identified.

Existence of Weak Solutions for Non-Simple Elastic Surface Models

We consider a class of models for nonlinearly elastic surfaces in this work. We have in mind thin, highly deformable structures modeled directly as two-dimensional nonlinearly elastic continua, accounting for finite membrane and bending strains and thickness change. We assume that the stored-energy density is polyconvex with respect to the second gradient of the deformation, and we require that it grow unboundedly as the local area ratio approaches zero. For sufficiently fast growth, we show that the latter is uniformly bounded away from zero at an energy minimizer. With this in hand, we rigorously derive the weak form of the Euler-Lagrange equilibrium equations.

A finite deformation isogeometric finite element approach to fibre-reinforced composites with fibre bending stiffness

It is a common technique in many fields of engineering to reinforce materials with certain types of fibres in order to enhance the mechanical properties of the overall material. Specific simulation methods help to predict the behaviour of these composites in advance. In this regard, a widely established approach is the incorporation of the fibre direction vector as an additional argument of the energy function in order to capture the specific material properties in the fibre direction. While this model represents the transverse isotropy of a material, it cannot capture effects that result from a bending of the fibres and does not include any length scale that might allow the simulation of size effects. In this contribution, an enhanced approach is considered which relies on the introduction of higher-gradient contributions of the deformation map in the stored energy density function and which eventually allows accounting for fibre bending stiffness in simulations. The respective gradient fields are approximated by NURBS basis functions within an isogeometric finite element framework by taking advantage of their characteristic continuity properties. The isogeometric finite element approach that is presented in this contribution for fibre-reinforced composites with fibre bending stiffness accounts for finite deformations. It is shown that the proposed method is in accordance with semi-analytical solutions for a representative boundary value problem. In an additional example it is observed that the initial fibre orientation and the particular bending stiffness of the fibres influence the deformation as well as the stress response of the material.

A three-dimensional mechanistic study of the drivers of classical twin nucleation and variant selection in Mg alloys: A mesoscale modelling and experimental study

This work presents a detailed investigation of twin inception to identify site of twinning, variant type selected, and the strain to nucleate it within a full 3D reconstructed microstructure obtained using 3D-EBSD. Microstructurally-faithful 3D crystal plasticity analysis provides quantitative insight to show that stored energy density is a key factor (rather than stress or other criteria) which identifies the experimentally observed twin nucleation site (which supersedes twin inception), and the strain necessary to drive nucleation. 3D (as opposed to 2D surface) analysis has been shown to be essential. The critical energy density for twin nucleation was found to be ∼0.015 Jm-2 in Mg alloy AZ31. Further, at the predicted nucleation site, the experimentally observed twin variant is shown to be driven by the local twin resolved shear stress.

Geometric linearization of theories for incompressible elastic materials and applications

We derive geometrically linearized theories for incompressible materials from nonlinear elasticity theory in the small displacement regime. Our nonlinear stored energy densities may vary on the same (small) length scale as the typical displacements. This allows for applications to multiwell energies as, e.g. encountered in martensitic phases of shape memory alloys and models for nematic elastomers. Under natural assumptions on the asymptotic behavior of such densities we prove Gamma-convergence of the properly rescaled nonlinear energy functionals to the relaxation of an effective model. The resulting limiting theory is geometrically linearized in the sense that it acts on infinitesimal displacements rather than finite deformations, but will in general still have a limiting stored energy density that depends in a nonlinear way on the infinitesimal strains. Our results, in particular, establish a rigorous link of existing finite and infinitesimal theories for incompressible nematic elastomers.

An approach to model mechanical damage in particulate composites with viscoelastic matrix

Particulate composites with polymer matrix are used in engineering applications, such as solid propellants, industrial rubbers and road pavements. The durability of such materials critically depend on the characterization of mechanical property degradation. Mechanical damage studies of these composites thus become important. A phenomenological model describing the mechanical damage in polymer matrix is developed in the present work, where the polymer is modelled as a viscoelastic (VE) solid and the damage is characterized by the progressive degradation of storage modulus. This VE model, with two term Prony series approximation of the relaxation modulus, is then coupled with isotropic damage formulation. Two approaches of isotropic damage evolution, viz. explicit and implicit, are implemented accounting the irreversible thermodynamic considerations. The implicit isotropic damage evolution is derived by a dissipation potential function based on the effective elastic stored energy density. An exponential damage evolution, obtained from experiments, is adapted in explicit approach. The analytical VE-damage framework is numerically implemented as stress integration algorithm, and the corresponding simulation results are illustrated. It is seen from the formulation and results that, the damage implicitly affects the relaxation time of the polymer. The stress hardening behaviour occuring immediately after the damage initiation is captured, and the subsequent softening is also demonstrated.

A note about hardening-free viscoelastic models in Maxwellian-type rheologies at large strains

Maxwellian-type rheological models of inelastic effects of creep type at large strains are revisited in relation to inelastic strain gradient theories. In particular, we observe that a dependence of the stored energy density on inelastic strain gradients may lead to spurious hardening effects, preventing the model from accommodating large inelastic slips. The main result of this paper is an alternative inelastic model of creep type, where a higher-order energy contribution is provided by the gradients of the elastic strain and of the plastic strain rate, thus preventing the onset of spurious hardening under large slips. The combination of Kelvin–Voigt damping and Maxwellian creep results in a Jeffreys-type rheological model. The existence of weak solutions is proved by way of a Faedo–Galerkin approximation.

Ferroelectric BT–PVDF Composite Thick Films for Electrical Energy Storage

Hybrid composites have been elaborated by incorporation of BaTiO3 (BT) inorganic nanoparticles into polyvinylidene fluoride (PVDF) polymer. BT–PVDF composite thick films with different volume fractions of BT (0%, 7%, 15%, and 30%) were deposited by spin-coating onto Pt/SiO2/Si substrates. The effects of the BT inorganic content in the PVDF polymeric matrix on the structural, dielectric, ferroelectric, and energy storage properties were investigated at room temperature. The crystal structure of the samples was identified by X-ray diffraction analysis, and their morphology was observed by scanning electron microscopy. Introduction of BT nanoparticles into the PVDF polymer led to a remarkable enhancement of the dielectric constant value and ferroelectric polarization. Additionally, the abnormal dielectric behavior observed for the composite with 30 vol.% BT nanoparticle volume fraction can be explained by the existence of a Debye relaxation domain at low frequency. Moreover, the stored energy density Wc was enhanced from 2.4 J/cm3 for pure PVDF polymer to 9 J/cm3 for the 30 vol.% BT nanoparticle volume fraction. Such BT–PVDF composite thick films are thus promising materials for the manufacture of electrostatic capacitors for electrical energy storage.

Research into storage processes in batteries to create highly efficient reversible hydrogen storage

This paper has experimentally proved that hydrogen accumulates in large quantities in metal ceramic electrodes of alkaline accumulators during their operation. It has been found that hydrogen is not present in the metal ceramic electrodes of alkaline accumulators not yet in operation. It is shown that in alkaline accumulators with a service life of more than five years, the largest amount of hydrogen is present in their electrodes. Hydrogen is found to accumulate in oxide-nickel electrodes of alkaline accumulators, which have hydrogen absorption capacity of 13.4 wt%. Sintered nickel matrix of oxide-nickel electrode of alkaline accumulator was considered. The values of hydrogen absorption in this matrix are 20.2 wt%, the stored energy density is 44.018 kJ/g. The results obtained in the operation are three times higher than the previously obtained data, which are based on the use of traditional methods for reversible metal hydrides, such as magnesium hydride and complex hydrides. In addition, the results obtained exceed the values announced in the criteria for hydrogen storage systems developed by the US Department of Energy.

Heat Stores with a Melting Working Medium for Spacecraft Thermal Control Systems

Heat stores of different design are compared, with a view to identifying the maximum stored energy density and power [1, 2]. The heat store selected has a melting working medium. This design is the most promising for the maintenance of acceptable thermal conditions in spacecraft [3, 4].

Free-standing hierarchical Co@CoO/CNFs/Cu-foam composite based on electrochemical deposition as high-performance supercapacitor electrode

Three-dimensional structural materials with sufficient reactive contact surface to the electrolyte can significantly improve the capacitance performance of supercapacitors. Herein, we report a facile method to fabricate a hierarchical carbon nanofibers (CNFs) modified Cu-foam supported Co@CoO hybrid as a free-standing and binder-free electrode for supercapacitor application. The optimized Co@CoO/CNFs/Cu-foam shows a high specific capacitance of 19.2 F cm−3 at the current density of 50 mA cm−3 and a remarkable capacitance retention of 97.5% after 5000 cycles in a 3-electrode system. A button-type asymmetric supercapacitor was additionally constructed by using Co@CoO/CNFs/Cu foam as the positive electrode and activated carbon electrode as negative electrode to examine its practical application. The stored energy density reached to 180.5 mWh cm−2 with the corresponding power density of 6000 mW cm−2 at a discharging current density of 4 mA cm−2. An excellent cycling life of retention 100% was maintained after 5000 cycles. The high-surface area and conductivity of the CNFs played a positive role as support to highly-dispersed Co@CoO nanoparticles. The pulsed electrodeposition additionally favored the controllable fabrication of Co@CoO nanoparticles with homogeneously distribution and uniform sizes. The hierarchical hybrid electrode with porous architecture eventually allowed good access of electrolyte and efficient charge storage and transfer, which concomitantly improved its overall electrochemical performance.

Influence of substrate temperature on the energy storage properties of bismuth magnesian niobium thin films prepared by magnetron sputtering

Bismuth magnesian niobium (Bi2Mg2/3Nb4/3O7, BMNO) thin films are deposited on Pt/Ti/SiO2/Si substrates at different substrate temperatures by magnetron sputtering. The effect of substrate temperature on the energy storage properties of BMNO thin films is investigated systematically. The X-ray diffraction pattern reveals the thin films are polycrystalline with cubic pyrochlore structure. The dielectric constant of BMNO thin films increases with the substrate temperature, reaches the maximum value of 161 at 700 °C and starts decreasing thereafter. The maximum electrical breakdown strength and recoverable stored energy density are respectively obtained as 3.78 MV/cm and 58.9 J/cm3 when the BMNO thin films are deposited at 700 °C. In addition, the BMNO thin films possess excellent thermal stability of stored energy properties in a wide range of temperatures (-150 °C–200 °C). These results suggest that the BMNO thin films are a promising candidate for lead-free energy storage application in equipment operation under adverse environments.

Fatigue Behavior of Additively Manufactured IN718 with Columnar Grains

Fatigue failure of polycrystalline materials is often dominated by crack initiation processes, which are strongly influenced by salient features existing in the microstructure. Herein, a crystal plasticity framework based on the finite element method is used to quantitatively assess the mechanistic drivers existing in columnar IN718 polycrystals for fatigue crack nucleation. To that end, microstructurally differing polycrystalline samples are subjected to strain‐controlled cyclic loading at ambient conditions to elucidate the microstructural mechanisms of fatigue damage. 2D grain structures obtained from the cellular automata (CA) simulations of the additive manufacturing of IN718 are utilized to construct representative 3D microstructures for use in computational analyses. A criterion related to the stored energy density (SED) is used to predict the scatter in fatigue crack nucleation life. This criterion presents a unique and consistent approach toward predicting fatigue crack initiation at the microstructural scale. It is demonstrated that SED is necessary and sufficient to drive crack nucleation in low cycle fatigue. The results show that grain boundaries are the preferred crack initiation sites.

Numerical modeling of pulse-pumped ultrafast regenerative amplifier with unstable pump laser

A numerical model of a regenerative amplifier based on the Frantz-Nodvik equation with spatially dependent stored energy density in the active medium has been developed. The model was used to investigate the stability of the output energy of the amplifier associated with fluctuations of the pump energy and pump beam position. It was found that, same as in the known case with the fluctuations of the pump energy, the influence of the fluctuations of the pump beam position is much weaker at oversaturated operation of the regenerative amplifier. Opposite to fluctuations of the pump energy, fluctuations of the pump beam position result in highly-asymmetric histogram of the output energy, which is highly skewed to the lower energies.

Microstructural fracture mechanics: Stored energy density at fatigue cracks

This paper addresses the mechanistic drivers of short fatigue crack growth using theory, computational crystal plasticity and experimental test and characterisation. The asymptotic theory shows that the crack tip stored energy density is non-singular and finite and can be related to stress intensity, but unlike the latter, it depends on the crystal Burgers vector and intrinsic slip strength. The computational methods allow the stored energy to be calculated accurately at crack tips and show that good agreement is obtained for static cracks with the theory. The experiments allow the crack tip stored energy to be measured, demonstrating intimate microstructural sensitivity, direct correlation with experimental crack growth variations and good quantitative agreement with both asymptotic theory and computational modelling. Hence a new microstructurally-sensitive fracture mechanics has been presented in the context of short cracks within crystalline materials.

Design of the high performance microbattery with silicene anode

To date, lithium-ion batteries have become one of the most important energy storage devices. Creating lithium ion microbatteries using materials structured at the nanoscale allow you to create solid-state lithium-ion batteries that have the fastest electrodes. In this work, we propose the design of an element of a lithium-ion current source, which has a 10 times higher stored energy density than similar devices that are widely used at present. The developed electrochemical cell has a fully solid-state design. The active material of the anode is silicene, a solid-state electrolyte is used, the cathode is made of complex transition metal oxides, and an ultra-thin insulating pad is used. The presented element of the secondary lithium-ion battery has a theoretical capacity of 3500 mAh g−1 and a high charging speed. The electrochemical cell is characterized by small size, low cost of components and operational safety.

An isogeometric finite element approach to fibre-reinforced composites with fibre bending stiffness

In the modelling of fibre-reinforced composites, it is well established to consider the fibre direction in the stored energy in order to account for the transverse isotropy of the overall material, induced by a single family of fibres. However, this approach does not include any length scale and therefore lacks in the prediction of size effects that may occur from the fibre diameter or spacing. By making use of a generalised continuum model including non-symmetric stresses and couple-stresses, the gradient of the fibre direction vector can be taken into account as an additional parameter of the stored energy density function. As a consequence, the enhanced model considers the bending stiffness of the fibres and includes information on the material length scale. Along with additional material parameters, increased continuity requirements on the basis functions follow in the finite element analysis. The isogeometric finite element method provides a framework which can fulfil these requirements of the corresponding weak formulation. In the present contribution, the method is applied to two representative numerical examples. At first, the bending deformation of a cantilever beam is studied in order to analyse the influence of the fibre properties. An increasingly stiff response is observed as the fibre bending stiffness increases and as the fibre orientation aligns with the beam’s axis. Secondly, a fibre-reinforced cylindrical tube under a pure azimuthal shear deformation is considered. The corresponding simulation results are compared against a semi-analytical solution. It is shown that the isogeometric analysis yields highly accurate results for the boundary value problem under consideration.

Hyperelastic and viscoelastic characterization of hepatic tissue under uniaxial tension in time and frequency domain

In order to create accurate anatomical models for medical training and research, mechanical properties of biological tissues need to be studied. However, non-linear and viscoelastic behaviour of most soft biological tissues complicates the evaluation of their mechanical properties. In the current study, a method for measuring hyperelasticity and viscoelasticity of bovine and porcine hepatic parenchyma in tension is presented.First, non-linear stress–stretch curves resulting from ramp loading and unloading, were interpreted based on a hyperelastic framework, using a Veronda–Westmann strain energy function. Strain-specific elastic moduli, such as initial stiffness EI, were thereupon defined in certain parts of the stress–stretch curves. Furthermore, dissipated and stored energy density were calculated. Next, the viscoelastic nature of liver tissue was examined with two different methods: stress relaxation and dynamic cyclic testing. Both tests yielded dissipated and stored energy density, as well as loss tangent (tanδ), storage modulus (E′), and loss modulus (E′′). In tension, stress relaxation was experimentally more convenient than dynamic cyclic testing. Thus we considered whether relaxation could be used for approximating the results of the cyclic tests.Regarding the resulting elastic moduli, initial stiffness was similar for porcine and bovine liver (EI∼30kPa), while porcine liver was stiffer for higher strains. Comparing stress relaxation with dynamic cyclic testing, tanδ of porcine and bovine liver was the same for both methods (tanδ=0.05−0.25 at 1 Hz). Storage and loss moduli matched well for bovine, but not as well for porcine tissue.In conclusion, the utilized Veronda–Westmann model was appropriate for representing the hyperelasticity of liver tissue seen in ramp tests. Concerning viscoelasticity, both chosen testing methods – stress relaxation and dynamic cyclic testing – yielded comparable results for E′, E′′, and tanδ, as long as elasticity non-linearities were heeded.The here presented method provides novel insight into the tensile viscoelastic properties of hepatic tissue, and provides guidelines for convenient evaluation of soft tissue mechanical properties.