Limited Energy Storage Research Articles

Surface modified zinc ferrite as a carbon-alternative negative electrode for high-energy hybrid supercapacitor

The limited energy storage capacities of carbon-based negative electrodes, which rely on conventional double layer charge storage mechanisms, have limited the electrochemical performances of hybrid supercapacitors (HSCs). The development of nanostructured redox-active materials that serve as alternatives to carbon-based negative electrodes is crucial to overcome the limitations of current HSCs. In this work, a coordination chemistry approach is utilized to tune the nanostructure of zinc ferrite thin films on flexible stainless steel (SS) foil by using several zinc salt precursors, including ZnCl2·6H2O, Zn(NO3)2·6H2O, and ZnSO4·6H2O, while using the same iron precursor (FeCl2·4H2O). Materials prepared with the ZnCl2·6H2O metal salt form porous nanosheets with groundnut-like structures that exhibit maximum specific capacities of 544 mA h/g (454 F/g) at current density of 2.5 A/g. Redox-active solid-state HSCs are assembled by using zinc ferrite as a negative electrode, MnO2 nanoflakes as a positive electrode, and a polymer gel as the electrolyte. The developed solid-state HSC cells show excellent performance as exemplified by a maximum specific capacitance of 123.8 F/g and an energy density of 55.72 Wh/kg, both of which are superior to those of conventional carbon-based symmetric and HSCs.

Managing Risks Faced by Strategic Battery Storage in Joint Energy-Reserve Markets

Securing profits from energy, reserve capacity and balancing markets is critical to ensure the profitability of battery energy systems (BES). However, the intimate connection between offers on these trading floors combined with the limited energy storage capacity of BES renders its scheduling very complex. In this paper, we develop a bilevel optimization problem for strategic participation of a BES in the day-ahead energy-reserve and balancing markets, improving the state-of-the-art by (i) considering the conditional-value-at-risk; (ii) ensuring the real-time feasibility of the obtained day-ahead schedule; (iii) addressing the operational underperformance risk stemming from inaccurate battery modeling. In a case study, we illustrate how the proposed model allows risk-averse BES owners to hedge their day-ahead position without jeopardizing their expected profit, while ensuring the feasibility of their day-ahead schedule.

Superior capacitive energy storage capability in polymer composites induced by polydopamine-coated paraelectric platelets

Dielectric nanocomposites with excellent energy storage capabilities have great potential applications in film energy storage capacitors. However, limited energy storage density (Ue) and poor efficiency (η) of nanocomposites based on the incorporation of the high dielectric constant (er) fillers restrict their practical energy storage application due to low breakdown strengths (Eb) and electric displacement difference (Dmax-Drem) value. Herein, paraelectric SrTiO3 (ST) plates have been successfully synthesized through three-step molten salt process and modified by polydopamine (PDA) nanolayer. The moderate dielectric constant ST has the advantage for increasing the breakdown strength of poly(vinylidene fluoride) (PVDF)-based composites owing to the mitigation of electric field distortion. The ST@PDA platelets bring the highest Dmax-Drem (8.65 μC/cm2) together with enhanced Eb (350 MV/m) to the composites on account of the strong microscopic barrier layers. Consequently, the composite film filled with optimized ST@PDA platelets content (2.5 vol%) endows the maximum Ue of 12.45 J cm−3 at 350 MV/m, which presents the best result when compared to the neat PVDF and dielectric nanocomposites added with diverse ST morphologies including nanoparticles, nanowires, and nanofibers reported previously. This work offers an efficiency strategy utilizing high insulating paraelectric fillers with a plate structure to maximize the improvement in capacitive energy storage performance for dielectric composites.

Energy Accumulation and Dissipation of Coal with Preopening under Uniaxial Loading

Energy accumulation and dissipation play an important role during the entire process of rock failure. Some flaws, such as preexisting holes, will influence energy accumulation and dissipation. In order to investigate the energy evolution of coal specimen with preexisting holes under uniaxial compression through numerical approaches, the particle simulation method was used in numerical simulations. In this paper, the energy evolution of coal specimen was theoretically analyzed, and the influence of different hole arrangement, such as diameter, spacing, angle, and number, on the evolution characteristics of energy was also discussed. At the same time, the arrangement of the artificial boreholes for preventing the rockburst was explored. The results show that, compared with the intact coal specimen, the change of diameter, spacing, angle, and the number of holes weakened the coal specimen’s capacity to store energy and release strain energy. When the diameter, the vertical distance, and relative angle of preexisting holes were 15 mm, 10∼15 mm, and 60°, respectively, the energy storage limit reached optimal value. For arrangement of the artificial boreholes, the diameter, spacing, and angle can be designed on the basis of those optimal values. This study has a guiding significance in designing the arrangement of the artificial boreholes for mitigation of rockburst.

Optimal Edge Computing for Infrastructure-Assisted UAV Systems

The ability of Unmanned Aerial Vehicles (UAV) to autonomously operate is constrained by the severe limitations of their on-board resources. The limited processing capacity and energy storage of these devices inevitably makes the real-time analysis of complex signals – the key to autonomy – challenging. In urban environments, the UAVs can leverage the communication and computation resources of the surrounding city-wide Internet of Things infrastructure to enhance their capabilities. For instance, the UAVs can interconnect with edge computing resources and offload computation tasks to improve response time to sensor input and reduce energy consumption. However, the complexity of the urban topology and large number of devices and data streams competing for the same network and computation resources create an extremely dynamic environment, where poor channel conditions and edge server congestion may penalize the performance of task offloading. This paper develops a framework enabling optimal offloading decisions as a function of network and computation load parameters and current state. The optimization is formulated as an optimal stopping time problem over a semi-Markov process. We solve the optimization problem using Dynamic Programming and Deep Reinforcement learning at different levels of abstraction and prior knowledge of the system underlying stochastic processes. We validate our results in a realistic scenario, where a UAV performs a building inspection task while connected to an edge server.

Silver stoichiometry engineering: an alternative way to improve energy storage density of AgNbO3-based antiferroelectric ceramics

Lead-free dielectric capacitor with high energy storage density is in great demand, but with the challenge of limited energy storage density. In this work, Ag(Nb0.85Ta0.15)O3-x wt% Ag2O (ANTAx) lead-free ceramics with nonstoichiometric Ag2O were fabricated, with the aim of improving energy storage density. The element concentration, phase structure, microstructure, dielectric property, and energy storage performance were investigated. Improved recoverable energy storage density (Wrec) of 4.8 J/cm3 were achieved for ANTA1 ceramics with 1 wt% Ag2O in excess, demonstrating obvious improvement compared with the stoichiometric counterpart. In addition, the ANTA1 ceramics also exhibited highly stable energy storage performance in the temperature range from room temperature to 150 °C, with variations less than 4% and 5% for Wrec and energy storage efficiency, respectively. The good properties may be associated with the dismissing of various defects by adding excess Ag2O. This work demonstrates that silver stoichiometry engineering is an effective method to improve energy storage properties of AgNbO3-based antiferroelectric ceramics.

An adaptive, balanced and energy efficient clustering mechanism for UAV networks

Unmanned aerial vehicles (UAVs) are widely used in many fields, both civilian and military. Mostly mini UAVs are used in civilian applications which are preferred both in terms of affordability and availability. However, these vehicles are insufficient for some applications when they are used alone. This inadequacy is often observed in mini-UAVs having limited capacity in terms of energy storage and payloads. As a solution to this challenge, swarms of networked mini UAVs have been proposed. Thus, the UAVs in such networks are assigned with different tasks to accomplish the overall mission. While such UAV networks have many advantages, they also come with challenges. These challenges include limited on-board energy storage, low power and lossy wireless communication interface, and limited useful payload carrying capability. In this study, a new adaptive, balanced and energy efficient clustering mechanism has been proposed for such UAV networks.

SWIPT-Enabled D2D Communication Underlaying NOMA-Based Cellular Networks in Imperfect CSI

Device-to-Device (D2D) communication is an emerging paradigm which enhances the coverage, capacity, and spectral efficiency of the network using cooperative communication in an underlay of cellular networks. In spite of these advantages, it suffers from large bandwidth and energy loss due to usage of half-duplex relaying and limited energy storage devices. To solve these issues, we integrate the full-duplex (FD) relaying and time splitting simultaneous wireless information and power transfer (SWIPT) technique at the D2D transmitters (DDTs). Moreover, to support the ubiquitous connectivity and reduce the latency, non-orthogonal multiple access is used at the base station. The SWIPT-FD-based DDTs and SWIPT-based cell user equipment (CUE) harvest the energy from the base station, after that the DDTs and CUEs decode the desired signal from the multiplexed signal using successive interference cancellation technique, and forward the desired signal to the D2D user equipment (DUE) to improve their QoS. In this paper, ergodic capacity at the DDTs and DUEs & closed-form expressions of the outage probabilities are derived with imperfect CSI. The results demonstrate that the proposed scheme achieves better performance as compared to that of the existing SWIPT-FD-OMA based scheme.

A New Clustering Routing Protocol for Homogeneous Wireless Sensor Networks Powered by Renewable Energy Sources

The technology of wireless sensor networks (WSNs) is in constant development and it made great progress in many applications. One of the most popular problems in WSNs is the limited energy storage power at every sensor node. This paper aims to propose and develop a new distributed clustering algorithm for energy harvesting wireless sensor networks denoted by DEH-WSN (Energy Harvesting for Distributed Clustering Wireless Sensor Networks Protocol) that relies on matching between clustering and energy harvesting in a distributed topology. DEH-WSN uses initial and residual energy capacity of the nodes to choose cluster heads. Simulation results prove that the proposed method increases network lifetime and the effective throughput.

Enhanced methane hydrate formation in the highly dispersed carbon nanotubes-based nanofluid

The hydrate-based technology has been viewed as a great potential approach for the natural gas storage and transportation, whose commercialization however is challenged by the limited energy storage efficiency and slow formation kinetics. In this work, we used carbon nanotubes (CNTs) as carriers and covalently grafted functional group of -SO3- (similar to the hydrophilic group contained in sodium dodecyl sulfate) onto the nano-carriers surface to prepare a novel promoter, sulfonated carbon nanotubes (SCNTs). The SCNTs nanofluid showed higher dispersity and stability than the pristine CNTs (PCNTs). In the methane hydrate formations, the high methane storage capacity of 142 v/v in the SCNTs nanofluid was achieved, whereas 50–65 v/v was obtained with the oxidized CNTs (OCNTs) or PCNTs as promoter. Interestingly, the larger number of sulfonate groups grafted on CNTs, the shorter hydrate formation period and the smaller hydrate reaction stochasticity were obtained. The formation rate was enhanced from 0.237 mmol/(mol water·min) to 0.413 mmol/(mol water·min) when SCNTs concentration grew from 25 mg/L to 150 mg/L. Additionally, the SCNTs kept excellent recycling performance at ten reuses in the methane hydrate formations, which was meaningful to the actual hydrate-based gas storage and transportation.

Gas chromatography – Mass spectrometry as a preferred method for quantification of insect hemolymph sugars

Insects, due to their small size, have limited energy storage space, but they also have high metabolic rate, so their hemolymph sugars are incredibly dynamic and play a number of important physiological functional roles in maintaining energetic homeostasis. In contrast to vertebrates, trehalose is generally the primary sugar found in insect hemolymph, which is followed by glucose and fructose. Many analytical chemistry methods exist to measure sugars, yet a direct comparison of methods that can measure all three simultaneously, and trehalose in particular, from low sample volumes, are sparse. Using the honey bee as a model, we directly compare the leading current methods of using High Performance Liquid Chromatography (HPLC) with an evaporative light-scattering detector and Gas Chromatography coupled with Mass Spectrometry (GC–MS) to determine which method would be better for measuring trehalose, glucose, and fructose in terms of reproducibility, accuracy, and sensitivity. Furthermore, we injected the enzyme inhibitors trehalozin (a trehalase inhibitor) and sorbose (a trehalase p-synthase inhibitor) to manipulate the trehalose levels in honey bee foragers as a proof of concept that this sugar can be altered independently of hemolymph glucose and fructose levels. Overall the HPLC method was less reproducible for measuring fructose and glucose, and it also had lower sensitivity for measuring trehalose. Consequently, significant differences in trehalose levels within the forager class were only detected with the GC–MS and not the HPLC method. Lastly, using the GC–MS method in the follow up study we found that trehalozin and sorbose causes a significant increase and decrease of trehalose levels respectively, in forager honey bees, independent of the glucose and fructose levels, ten minutes after injection. Taken together, these methods will provide useful tools for future studies exploring the many different physiological functional roles that trehalose can play in maintaining insect energetic homeostasis.

Boosting up the capacitive energy storage performances of lead-free ceramics via grain engineering for pulse power applications

Exploring high-performance ceramics has been a great challenge for advanced electronic devices due to their limited capacitive energy storage densities in high-voltage pulse applications. The refinement of the microstructures can considerably optimize energy storage performances of dielectric ceramics by tailoring dielectric breakdown. Here, two-step sintering was employed to refine the microstructures of the lead-free 2.0 mol% Mn-doped 0.55 (0.94Na0.5Bi0.5TiO3-0.06BaTiO3)-0.45SrTiO3 bulk ceramics to offer the possibility of increasing energy storage performances. Compared with conventional sintering, fine grains by two-step sintering can be obtained, resulting in the obviously enhanced dielectric breakdown due to the increased fraction of grain boundary and the suppressed movement of domain walls. An ultrahigh charge energy storage density of 2.98 J/cm3 with a moderate efficiency (η) of 73.1% can be achieved at 28 kV/mm, accompanying with excellent temperature insensitivity in a wide range from 30 to 180 °C compared with the previous Na0.5Bi0.5TiO3-based derivatives. This work presented here strongly indicates that two-step sintering is a promising way to optimize the microstructural morphology and electrical performances of dielectric ceramics.

Study on Energy Release of Surrounding Rock under the Multiple Unloading Disturbance during Tunnel Excavation

During excavation of deep rock, the release of strain energy plays an important role in geologic hazards caused by excavation. However, in the previous studies, the influence of transient unloading of in situ stress caused by blasting excavation has been ignored, and the blasting excavation of a tunnel is regarded as a single blast process. In this paper, the dynamic adjustment process of strain energy and the energy storage limit of surrounding rocks caused by transient unloading of in situ stress under elastic conditions were firstly analyzed. Then, the brittle-ductile-plastic transition model based on the Hoek–Brown strength criterion was simulated in FLAC3D. Finally, the dynamic release process of strain energy of surrounding rocks caused by multiple unloading disturbances was analyzed during the excavation of 2# Underground Laboratory of Jinping II Hydropower Station employing the newly proposed index, energy release coefficient (ERC). Results show that the strain energy of surrounding rock masses firstly decreases, then increases, next reduces, and finally stabilizes under the transient unloading of in situ stress. In the process of dynamic change of strain energy, when the strain energy exceeds its storage limit, a large amount of strain energy will be released and thus will lead to damage of the surrounding rock masses. Because the cut holes and the first circle of breaking holes are far away from the final excavation boundary, the unloading disturbance to the strain energy of surrounding rock masses is small. Furthermore, the energy release of surrounding rock masses is mainly caused by the unloading of the last circle of breaking holes and perimeter holes, and the closer to the final excavation boundary, the more intense the energy release.

A novel temperature-dependent hardness model for high-temperature structural ceramics

A novel and simple model for determining the temperature-dependent hardness of ceramics based on the concept that the maximum energy storage limit of a ceramic is associated with the material yielding under indentation is developed in this study. This model characterizes the temperature-dependent hardness for the first time by using only the Young's modulus and the melting point of the material without the need for fitting parameters. Good agreement between the predicted results and experimental measurements is achieved for numerous ceramic materials. This model can serve as the starting point for evaluating the temperature dependence of the hardness of ceramic materials.

Decentralized electricity storage evaluation in the Portuguese context

Portugal has made great progress in implementing renewable energy systems (RES) to use its endogenous renewable resources. As the cost of renewable energy generation is decreasing, mainly for photovoltaic energy, a significant increase in its production is expected, in particular at the local and domestic levels. Yet, much investment and development is still needed to fulfil the goals for renewable energy generation defined by the Portuguese government and the European Union, in order to decarbonize energy generation and reduce energy dependence. Besides limitations in the installed capacity, the full potential of existing and future RES is not fulfilled, mainly due to imbalances in supply and demand, resulting from the varying climatic conditions and limited energy storage capacity. Although some investment was made in large scale Energy Storage Systems (ESS), especially pumped hydro, distributed energy storage (DES), in particular for stationary domestic storage appliances, have received little attention from decision makers. When properly defined, designed and implemented, they can contribute to increase the efficiency of existing and future RES and the capacity factor. Thus, in this work the questions regarding the implementation of DES are analysed. The main criteria that have to be considered when selecting the proper storage technology for DES are defined, taking into account information and data from current legislation and/or strategic plans and goals, and the technical and scientific literature, in order to support decision making and policy definition at different levels. The proposed application of the various criteria leads to the conclusion that for DES, electrochemical based ESS are the most adequate, among which Li ion batteries and redox flow batteries, particularly suited for local and/or household applications. The current policies sought out for Portugal are also examined in order to identify which aspects should be improved to promote and increase the relevance of DES in the Portuguese energy mix, and it is concluded that specific policies and support are needed to increase the relevance of decentralized electricity storage systems.

Energy evolution analysis and failure criteria for rock under different stress paths

Triaxial loading and unloading tests on marble specimens under different stress paths were conducted to investigate the characteristics of energy evolution in rock deformation process. Results show that tensile failure occurred in rock specimens under uniaxial compression, while shear failure dominated under triaxial loading and unloading. The energy storage limit of rock specimens under triaxial loading was higher than that under uniaxial compression or triaxial unloading. A nonlinear energy evolution model of rock was established based on the interaction mechanism of energy accumulation and energy dissipation. Results from the theoretical model are in good agreement with the test results. Since the evolution of energy was characterised by bifurcation and chaos, a strain value corresponding to the energy iterative growth factor at the first bifurcation of the energy equation was chosen as the initiation criterion for rock failure. The critical strain accounted for 77%, 72–76%, and 72–81% of the peak strain under uniaxial compression, triaxial loading, and unloading, respectively.

Analysis of energy characteristic and working performance of novel controllable hydraulic accumulator with simulation and experimental methods

Hydraulic accumulator is widely applied in various transmission systems for improving system performance such as installed power reduction, pressure variation absorption and energy efficiency improvement. However, the traditional hydraulic accumulator suffers from two major drawbacks: 1) limited energy storage capacity 2) passively matched system working condition with fixed working mode. To overcome these problems, this study proposed a novel hydraulic accumulator with larger energy storage capacity and high controllability, which mainly comprises a piston accumulator, a gas regulator, and several control valves. First, this paper introduced the working principle of the controllable accumulator and calculated the energy-storage indices. Then, the mathematic model of the controllable accumulator, comprising mechanical, hydraulic and pneumatic model, was established by using appropriate theory. Based on the mathematic model, the control principle of four-working modes and the co-simulation model of the controllable accumulator were presented. Finally, a test rig of the accumulator was carried out to verify its actual function. The research results show that 1) Compared to traditional accumulator, the energy-storage capacity of this novel accumulator can be improved by nearly 1.5 times in this case, which is increased linearly with the volume rate. The energy storage capacity can be further increased with the increment of volume rate, which breaks out the limitation of the traditional accumulator. 2) Using Fuzzy PID algorithm, the precharge pressure, output flow, output pressure and output power in simulation and experiment results can be controlled to match the target working performance indices with high precision. Hence, this novel controllable accumulator can be taken as the high precision hydraulic component and applied in these high-end devices, which shows great superiority when compared to the traditional one. Based on above disadvantages, this novel accumulator is especially suitable for these hydraulic transmission systems with demanded requirements of achieving high accuracy, large power and fast response simultaneously.

Co/CoS nanofibers with flower-like structure immobilized in carbonated porous wood as bifunctional material for high-performance supercapacitors and catalysts

As a kind of green renewable resource from trees, wood has abundant porous structure, which makes it have great potential in the field of energy storage. However, poor conductivity and limited energy storage capacity hinder the application of wood in the field of energy storage. Here, a novel carbonization lignin-free wood (CLFW)@Co-CoS hybrid is designed and fabricated by a combination of vacuum filtration and carbonization. The CLFW@Co-CoS hybrid assembled into wood-based supercapacitor presents a high areal energy density of 610 μWh cm−2, meanwhile maintaining a high areal power density of 192 mW cm−2. There are two main reasons for the excellent capacitance characteristics: one thing is that wood has a natural and abundant porous structure, which provides more space and channels for the storage and transport of electrolyte ions. The other thing is that CoS and Co can provide high capacitance, and numerous new pores are created by the interweaving of CoS nanofibers with flower-like structure. Additionally, Co nanoparticles attached to CoS and wood have more active sites, which endows the hybrid with excellent ORR properties. Thus, this bifunctional material with excellent electrochemical performance and outstanding ORR properties has huge potential in the field of energy storage and catalysis.

Probing Mg Intercalation in the Tetragonal Tungsten Bronze Framework V4Nb18O55.

While commercial Li-ion batteries offer the highest energy densities of current rechargeable battery technologies, their energy storage limit has almost been achieved. Therefore, there is considerable interest in Mg batteries, which could offer increased energy densities in comparison to Li-ion batteries if a high-voltage electrode material, such as a transition-metal oxide, can be developed. However, there are currently very few oxide materials which have demonstrated reversible and efficient Mg2+ insertion and extraction at high voltages; this is thought to be due to poor Mg2+ diffusion kinetics within the oxide structural framework. Herein, the authors provide conclusive evidence of electrochemical insertion of Mg2+ into the tetragonal tungsten bronze V4Nb18O55, with a maximum reversible electrochemical capacity of 75 mA h g-1, which corresponds to a magnesiated composition of Mg4V4Nb18O55. Experimental electrochemical magnesiation/demagnesiation revealed a large voltage hysteresis with charge/discharge (1.12 V vs Mg/Mg2+); when magnesiation is limited to a composition of Mg2V4Nb18O55, this hysteresis can be reduced to only 0.5 V. Hybrid-exchange density functional theory (DFT) calculations suggest that a limited number of Mg sites are accessible via low-energy diffusion pathways, but that larger kinetic barriers need to be overcome to access the entire structure. The reversible Mg2+ intercalation involved concurrent V and Nb redox activity and changes in crystal structure, as confirmed by an array of complementary methods, including powder X-ray diffraction, X-ray absorption spectroscopy, and energy-dispersive X-ray spectroscopy. Consequently, it can be concluded that the tetragonal tungsten bronzes show promise as intercalation electrode materials for Mg batteries.

LMC-based DTC for efficiency improvement of IM drives and their electric vehicle applications

Electric vehicles (EVs) with induction motor (IM) drives require energy optimization due to their limited energy storage capacity. Among the energy optimization methods currently available, a loss model-based controller (LMC) is a satisfactory technique, since it converges rapidly to an optimal operating point. Some of the simplifications applied to LMCs, such as neglecting the core loss and leakage inductances, affect IM drive performance negatively in terms of loss minimization. To overcome this problem, this paper presents an efficient LMC combined with direct torque control (DTC). The proposed method is based on the idea that the magnetizing current is much higher than the core loss current. Using the sum of the stator current and the rotor current instead of the magnetizing current, modelling the losses is simplified and the optimal stator flux is determined easily. A simulation study is carried out to evaluate the performance of the proposed scheme with an EV application during an FTP-75 urban driving cycle. A prototype 3 kW IM drive with a TMS320F28335 digital signal processor (DSP) is built to validate the simulation results. A significant efficiency improvement is obtained at low loads. Both simulation and experimental results demonstrate that the proposed DTC improves efficiency and decreases torque ripple.