Charging Energy Efficiency Research Articles

Piezocatalysis and Piezo‐Photocatalysis: Catalysts Classification and Modification Strategy, Reaction Mechanism, and Practical Application

AbstractPiezoelectric‐based catalysis that relies on the charge energy or separation efficiency of charge carriers has attracted significant attention. The piezo‐potential induced by strain or stress can induce a giant electric field, which has been demonstrated to be an effective means for charge energy shifting or transferring electrons and holes. In recent years, intense efforts have been made in this subject, and the research has mainly focussed on two aspects: i) Alteration of surface charge energy by piezo‐potential in piezocatalysis; ii) the separation of photo‐generated charge carriers and the catalytic activity enhancement of an integrated piezoelectric semiconductor or coupled system composed of piezoelectrics and semiconductors. Systematically summarizing the advances of the above two aspects is helpful in the context of deepening understanding of the relevant issues and developing new ideas for piezoelectric‐based catalysis. In this review, a comprehensive summary on piezocatalysis and piezo‐photocatalysis is provided. The charge transfer behaviors and catalytic mechanisms over a large variety of piezocatalysts and piezo‐photocatalysts are systematically analyzed. In addition, the types of mechanical energy, strategies for enhancing piezocatalysis, and the advanced applications of piezocatalysis and piezo‐photocatalysis are discussed. Finally, the promising development directions of piezocatalysis and piezo‐photocatalysis, such as materials, assembly forms, and applications in the future are proposed.

Effect of initial temperature on electrochemical and thermal characteristics of a lithium-ion battery during charging process

Due to the discharging or long-term placing, the initial temperature of a lithium-ion battery during charging is different, which may lead to various electrochemical and thermal characteristics. An electrochemical-thermal coupled model is developed for a lithium-ion battery and experimentally validated. The local current density, heat generation rate and charging energy efficiency of the battery during charging at different initial temperatures are numerically investigated. The spatial distribution of local current density of the battery cell is non-uniform at different initial temperatures. And the non-uniformity increases with the decreasing of initial temperature, especially in the negative electrode. Initial temperature influences the spatial and temporal distributions of heat generation rate in the battery cell. Most of the heat (over 95%) is generated in the electrodes. And the part of irreversible heat contributes to over 60% of the total heat generation in the negative electrode. The charging energy efficiency changes with the difference of the total irreversible heat and the input charging energy. The charging energy efficiency of the battery increases with the increasing of initial temperature at 0.5 C charging rate, while it will decrease after the initial temperature exceeds 40 °C and 25 °C at 1 C and 2 C charging rate, respectively. So it should use reasonable charging rates at different initial temperatures. These findings give help for the battery management systems design to achieve higher charging energy efficiency.

多段FET 双方向コンバータを備えた エネルギーマネジメント蓄電システム

This paper presents an energy management storage system with a multi-stage FET bidirectional converter. The proposed system is constructed by a bidirectional converter using series connected FETs, and it realizes highly energy management that maximizes battery performance. By calculating the power loss using the converter performance index FOM (Figure of merit), it is shown that the proposed configuration can be possible to reduce the power loss 70% compared to the conventional configuration. In the proposed system, using a power control method that reduces the charging current, the battery charging losses can be reduced 60%. The analysis results show that the proposed system can achieve a charging energy efficiency of 96.9%, and reduce the yearly CO2 emission of 33%.

Identifying rate limitation and a guide to design of fast‐charging Li‐ion battery

AbstractFast‐charging is highly demanded for applications requiring short charging time. However, fast‐charging triggers serious problems, leading to decline in charge acceptance and energy efficiency, accelerated capacity degradation, and safety risk. In this work, a three‐electrode coin cell with a Li metal reference electrode is designed to individually record the potential of two electrodes, and measure the impedance of each electrode by using a power‐optimized graphite‐LiNi0.80Co0.15Al0.05O2 electrode couple. It is shown that regardless of the state‐of‐charge the Li‐ion cell's impedance is contributed predominantly by the cathode, and that the cathode's impedance is dominated by the charge‐transfer resistance. In consistence with the impedance results, polarization of the Li‐ion cell is dominated by the cathode. It is surprised to find that no Li plating occurs on the graphite anode even if the charging rate is increased to 10 C (1 C = 1.30 mA cm−2). The results of this work indicate that low overall impedance with a high cathode‐to‐anode impedance ratio is the key to enabling safe fast‐charging, and that fast‐charging Li‐ion batteries without Li plating on the graphite anode is possible if the cathode and graphite anode are optimistically engineered.image

PSO-based optimization for constant-current charging pattern for li-ion battery

A particle swarm optimization algorithm to search for an optimal five-stage constant-current charge pattern is proposed. The goal is to maximize the objective function for the proposed charge pattern based on the charging capacity, time, and energy efficiency, which all share the same weight. Firstly, an equivalent circuit model is built and battery parameters are identified. Then the optimal five-stage constant-current charge pattern is searched using a particle swarm optimization algorithm. At last, comparative experiments using the constant current-constant voltage (CC-CV) method are performed. Although the charging SOC of the proposed charging pattern was 2.5% lower than that of the CC-CV strategy, the charging time and charging energy efficiency are improved by 15.6% and 0.47% respectively. In particular, the maximum temperature increase of the battery is approximately 0.8 °C lower than that of the CC-CV method, which indicates that the proposed charging pattern is more secure.

The effect of filler morphology on the dielectric performance of polyvinylidene fluoride (PVDF) based composites

The incorporation of linear shaped fillers is thought to be able to effectively enhance the dielectric properties of polymer based composites. However, the effect of electrospinning and subsequent calcination processing on the morphology of the fillers and the dielectric performance of the polymer based composites is still unclear. Herein, we demonstrate that filler illustrating two morphological characteristics at the same time, obtained by selecting optimum electrospun precursor concentration before calcination. It is demonstrated that the final filler morphology varies from linear to intermedia (linear and droplets), to droplets with increasing tetrabutyl titanate concentration in the electrospun solution. For filler consisting both linear and droplets characteristics, a maximum energy density of 12.54 J/cm3 is reached at breakdown strength of 393 kV/mm, with a charge and discharge energy efficiency around 76%. Therefore, this could provide new insight into for the preparation of linear shape based fillers for high performance dielectric polymer composites.

Modeling the effect of two-stage fast charging protocol on thermal behavior and charging energy efficiency of lithium-ion batteries

Abstract To enable fast charging of lithium ion batteries, extensive attention is needed to reduce the heat generation rate to avoid thermal runaway. This work studies the impact of the fast charging protocol on thermal behavior and energy efficiency of a lithium ion battery cell for 30-minute charging with 80% rated capacity. A two-stage constant current (2SCC) charging protocol with different current level combinations is adopted to mitigate the temperature rise leading to thermal runaway and improve the charging energy efficiency, because different charging current will generate different rates of heat. An electrochemical thermal coupled model is developed to predict the heat generation rate, charging energy rate, and temperature rise with experimental validation. The simulation results indicate that the “high-low” charging protocols of the 2SCC fast charge can achieve a lower overall temperature rise and higher charging energy efficiency than both the “low-high” charging protocols and the standard one-stage constant current (1SCC) charging. Compared to the 1SCC charge, the 2SCC charging protocol can improve the overall charging energy efficiency by 1%. Additionally, the impact of the 2SCC charging protocol on the thermal behavior under different initial cell temperatures and the heat transfer coefficients are discussed.

Fast Charging Strategy Based on Electrochemical Model and Dynamic Programming for a Lithium-Ion Battery Cell

Fast charging technology of lithium-ion batteries is critical to commercial application of electrical vehicles (EV). Typically, recharging lithium-ion batteries using standard charging methods takes more than one hour which is considerably longer than refueling of the internal-combustion-engine (ICE) car, roughly 8-10 min. To reduce the charging time, fast charging increases the total current which can adversely affect the cycle life, performance and safety of the battery, such as lithium plating, increased battery temperature, higher heat generation rate, and thermal runaway. The charging energy efficiency can be reduced because of the increased heat generation during fast charging. Therefore, controlling the temperature rise and improving the charging energy efficiency using well-designed fast charging strategies are indispensable for its application in electric vehicles. The objective of this work is to optimize the charging current profile based on electrochemical thermal model and dynamic programming for a 26650 cylindrical lithium iron phosphate (LFP) battery. The proposed model is implemented on a multi-stage constant current (MSCC) fast charging strategy for charging from 0 % to 80 % of its rated capacity within 30 minutes. The goal of the optimization is to maximize the charging energy efficiency or/and minimize the temperature rise and charging voltage. The control variable of various charging current protocols using the C-rate level ranging from 0.6 C-rate and 2.6 C-rate are applied to the MSCC fast charging. The constraints of the optimization include temperature and cell voltage, which are applied to prevent thermal runway and reduce the capacity fade. The optimized charging current profile based on various initial temperatures, ambient temperatures, and heat transfer coefficients will be compared. In addition, the effect of constraints on optimization results will be discussed. To study the effect of fast charging based on the battery dynamics, the electrochemical model first proposed by Newman et al. has been addressed. Even though the electrochemical model is not suitable for online application, the results can serve as a benchmark to evaluate and improve an existing online algorithm for battery fast charging optimization. However, few of those works using electrochemical models discussed the effect of fast charging strategies on battery electrochemical and thermal behavior, as well as charging energy efficiency under various charging current patterns. Based on dynamic programming and electrochemical thermal model, this research proposes a new method to optimize charging current profile under various constraints and different optimization goals, which are useful for fast charging application of lithium-ion batteries on electric vehicles.

Optimal design for sensible thermal energy storage tank using natural solid materials for a parabolic trough power plant

This paper presents numerical investigation of transient behavior and thermal storage capability of a sensible heat storage (SHS) unit designed for storing heat in the temperature range of 523–673 K. The objective of this study is to assess the potential of using two candidate materials as energy storage media found in Jordan. The thermal performance of using these materials in SHS tank is compared against concrete bed. A heat storage unit of cylindrical configuration with embedded charging tubes has been designed. To investigate their heat storage characteristics, a finite element based 3-D mathematical model has been developed using COMSOL Multiphysics 5.1. Numerically predicted results match closely with the data reported in the literature. Performances of the thermal storage bed of capacity of 136.7 MW in 3 h (including charging time, energy storage rate, charging energy efficiency) have been evaluated for the selected three storage materials. In order to optimize the design of energy storage tank, parametric studies are carried out by varying the number of the charging tubes, diameter of charging tubes, fins effectiveness, and storage bed diameter to its height. Simulations results showed that overall thermal performance of these materials using optimal design are satisfactory considering the problems associated with molten salt and concrete bed. More specifically, Dead Sea salt, concrete bed and Basalt rocks are more economically attractive relative to molten salt plant because they eliminate the extra cost associated with freeze protection, no pressurizing problem, no corrosion. Finally, natural stones can operate at higher temperature and do not suffer from cracking due to cyclic charging and discharging.

A high-temperature tolerance solution for positive electrolyte of vanadium redox flow batteries

Vanadium redox flow battery (VRFB) is attractive for energy storage applications, but there still remains a problem of preventing V-precipitation reaction (i.e., V2O5) to provide the thermal stability of electrolyte employed in VRFB. The V2O5 precipitation is accelerated in the positive electrolyte under high temperature, resulting in decreasing the charge capacity and energy efficiency of VRFB. So far, previous supporting materials, which are used to solve such precipitation problem, provide a disadvantage of decreasing voltage efficiency of VRFBs by increasing electrolyte resistance. This paper describes an advanced vanadium-positive electrolyte with high-temperature tolerance for high-efficiency VRFBs, which uses a sodium formate as a supporting material. The sodium formate plays a role as an agent capable of preventing precipitation reaction in the positive electrolyte and it also provides an effect of decreasing a viscosity of the positive electrolyte. The effectiveness of the proposed electrolyte solution is demonstrated through the following experiments: UV–vis spectrometry, viscosity measurement, cyclic voltammetry (CV), VRFB operation and scanning electron microscopy (SEM) analysis. Then, for the performance comparison of high temperature stability, all experiments are carried out at 60°C. Experimental results show that using the sodium formate leads to near 4.5 times increase of diffusion coefficient as compared to conventional electrolyte, and also provides 20.2% higher charge capacity (50th cycle) and 2.27% higher average energy efficiency (50cycles) at the current density of 80mAcm−2. In addition, it appears that the precipitation of vanadium species is not observed in the electrolyte during VRFB operation. Therefore, this paper provides that new direction about effect of the additive in the positive electrolyte and the sodium formate can be considered as a promising additive for high-performance electrolyte of VRFBs.

Micrometer‐Sized RuO2 Catalysts Contributing to Formation of Amorphous Na‐Deficient Sodium Peroxide in Na–O2 Batteries

The results obtained herein demonstrate that the oxygen electrode plays a critical role in determining the morphology and chemical composition of discharge products in Na–O2 batteries. Micrometer‐sized cubic NaO2, film‐like NaO2, and nano‐sized amorphous spherical Na2‐xO2 are characterized as the main discharge products on the surface of reduced graphite oxide (rGO), boron‐doped rGO (B‐rGO), and micrometer‐sized RuO2 catalyst‐coated B‐rGO (m‐RuO2‐B‐rGO) cathodes, respectively. The Na–O2 battery with m‐RuO2‐B‐rGO as the cathode exhibits a much longer cycle life than those with the other cathodes, maintaining an unchanged capacity (0.5 mAh cm‐2) after 100 cycles at a current density of 0.05 mA cm‐2. A good rate capability and deep discharge–charge energy efficiency are also obtained. The excellent electrochemical performance of the battery is attributed to the effect of the micrometer‐sized RuO2 catalyst. Owing to the high affinity of RuO2 for oxygen, the amorphous phase Na2‐xO2 discharge product, which has good electrical contact with the RuO2 particles, can decompose completely under 3.1 V without a sudden voltage jump. Meanwhile, the micrometer‐sized RuO2 catalysts also provide enough active sites and space for the reactions, and effectively minimize the occurrence of side reactions between discharge products and carbon defects.

‘Flat water’ provides a battery boost

A popular approach for developing better batteries and other energy-storage devices is incorporating atoms-thick sheets of solid-state materials. This strategy may hold for liquids, too. Researchers have found that test devices using tungsten oxide electrodes have improved energy-storage kinetics when thin layers of water are confined between crystalline WO3 sheets (Chem. Mater. 2017, DOI: 10.1021/acs.chemmater.6b05485). Hydrated electrodes charge and discharge faster than their anhydrous counterparts, reports a team led by Veronica Augustyn of North Carolina State University. Further, the hydrated material better maintains its charge capacity and energy efficiency at these faster timescales. The intercalated water layers likely lower the energetic barrier encountered by ions moving from liquid electrolytes to solid electrodes, although the team is investigating this hypothesis. Tungsten oxide provides a good model system for these types of fundamental studies, though the material is too heavy for commerc...

Digital Control of a High-Voltage (2.5 kV) Bidirectional DC--DC Flyback Converter for Driving a Capacitive Incremental Actuator

This paper presents a digital control technique to achieve valley switching in a bidirectional flyback converter used to drive a dielectric electroactive polymer-based capacitive incremental actuator. This paper also provides the design of a low input voltage (24 V) and variable high output voltage (0–2.5 kV) bidirectional dc–dc flyback converter for driving a capacitive incremental actuator. The incremental actuator consists of three electrically isolated mechanically connected capacitive actuators. It requires three high-voltage (HV) (2–2.5 kV) bidirectional dc–dc converters to accomplish the incremental motion by charging and discharging the capacitive actuators. The bidirectional flyback converter employs a digital controller to improve the efficiency and charge/discharge speed using the valley switching technique during both charge and discharge processes, without the need to sense signals on the output HV side. Experimental results verifying the bidirectional operation of a HV flyback converter are presented using a 3-kV polypropylene-film capacitor as the load. The energy-loss distributions of the converter are presented when 4- and 4.5-kV HV mosfet s are used on HV side. The flyback prototype with a 4 kV mosfet demonstrated 89% charge energy efficiency to charge the capacitive load from 0 V to 2.5 kV, and 84% discharge energy efficiency to discharge it from 2.5 kV to 0 V.

Enhanced energy storage properties of (1−x)Bi0.5Na0.5TiO3-xBa0.85Ca0.15Ti0.9Zr0.1O3 ceramics

Energy storage properties of (1−x)Bi0.5Na0.5TiO3-xBa0.85Ca0.15Ti0.9Zr0.1O3 ceramics prepared by solid state reaction were first investigated. The results of XRD for all samples show a pure perovskite phase and a pseudo-cubic symmetry. The P-E loops illustrate that the addition of Ba0.85Ca0.15Ti0.9Zr0.1O3 can enhance the breakdown strength, discharge and charge energy density and energy efficiency. The sample with x=0.525 exhibits the highest breakdown strength of 9.35kV/mm, discharge energy density of 0.87J/cm3 and charge energy density of 1.06J/cm3. The sample with x=0.55 exhibits the highest energy efficiency of 82.37%. Four evident current peaks are found in the I-E curves of x=0.475. Two current peaks are the evidence of ferroelectric domain switching for x≥0.50.

Design Considerations for Wireless Charging Systems with an Analysis of Batteries

Three criteria, including charging time, effective charging capacity and charging energy efficiency, are introduced to evaluate the CC (constant current) and CC/CV (constant current/constant voltage) charging strategies. Because the CC strategy presents a better performance and most resonant topologies have the CC characteristic, the CC strategy is more suitable for the design of wireless charging systems than the CC/CV strategy. Then, the state space model of the receiver is built to study the system dynamic characteristics, and the design of nonuse output filter capacitors is proposed, which can improve the system power density and avoid the drop in efficiency caused by capacitor degradation. At last, an electrochemical impedance spectrum (EIS) based analysis method is introduced to validate that the design without output filter capacitors has no effects on the battery characteristics when the charging frequency is higher than 460 Hz. A prototype is fabricated to verify our research results.

Effect of Circular Fins on Latent Heat Storage to Enhance Solar Water Heater, an Experimental Study

In natural circulation solar water heating, the efficiency of energy storage and solar collector can be improved if the water tank is stratified. Employing Phase Change Material (PCM) in the top of the tank will improve the stratification of the energy storage tank. Current paper studies the use of circular fins around PCM geometry with high surface to volume ratio to improve the efficiency of solar water heater. Heating and cooling tests have been performed in a domestic water heater with flat plate collector under real working conditions. Eventually the temperature distribution along the height of the storage tank, charging energy efficiency, collector efficiency have been calculated and compared. Tests have been simulated in CFD software for validating results. Addition of circular fins around PCM produces up to 5% and 36.48% improvement in tank charging energy efficiency, collector efficiency respectively because of improved heat transfer between PCM and water.

Design and characterization of a portable heat storage facility

This paper deals with the experimental investigation of transient behavior and thermal storage capability of a sensible heat storage unit. The former is designed to store solar energy by using sand as a storage medium. A cubic unit has been designed employing three configurations of embedded charging tubes to study the performances of the system. The storage material used in this experimentation (sand) has been characterized by its, grain size, density, and heat capacity. Performances of the thermal storage bed (including charging time, energy stored rate, charging energy efficiency) have been evaluated for selecting the storage material. From our experiments, we found that the number of charging tubes affects significantly the storage performances.

Effect of nafion membrane thickness on performance of vanadium redox flow battery

The performance of vanadium redox ow batteries (VRFBs) using different membrane thicknesses was evaluated and compared. The associated experiments were conducted with Nafion® 117 and 212 membranes that have 175 and 50 μm of thickness, respectively. The charge efficiency (CE) and energy efficiency (EE) of VRFB using Nafion® 117 were higher than those of VRFB using Nafion® 212, while power efficiency was vice versa. In terms of amounts of charge and discharge that are measured in different charging current densities, the amounts in VRFB using Nafion® 212 are more than that in VRFB using Nafion® 117. To further characterize the effect of membrane thickness on VRFB performance, electrochemical impedance spectroscopy (EIS) and UV-vis. spectrophotometer (UV-vis) were used. In EIS measurements, VRFB using Nafion® 117 was more stable than that using Nafion® 212, while in UV-vis measurements, vanadium crossover rate of VRFB usingNafion® 212 (0.0125M/hr) was higher than that of VRFB using Nafion® 117 (0.0054 M/hr). These results are attributed to high crossover rate of vanadium ion in VRFB using Nafion® 212. With these results, vanadium crossover plays more dominant role than electrochemical reaction resistance in deciding performance of VRFB in condition of different membranes.

Sand as a Heat Storage Media for a Solar Application: Simulation Results

This paper deals with the numerical investigation of transient behavior and thermal storage capability of a sensible heat storage unit. The former has a cubic configuration with embedded charging tubes; it is used to store solar energy with sand as a storage media. The system operates in the range of low temperature. To analyze their heat storage characteristics (including the bed temperature, energy stored rate, charging energy efficiency), a finite element based 2-D mathematical model has been developed using COMSOL Multiphysics. The results show that sand has an important thermal inertia. However, it was interesting to develop these storage systems in the region where both sand and solar energy were plentiful.

Use of Frequency-Selective Surface for Suppressing Radio-Frequency Interference from Wireless Charging Pads

Wireless charging is an emerging technology that involves multidisciplinary research. Power converters driving the planar wireless charging pads generate radio-frequency interference (RFI) that can degrade the signal transmission and reception of portable communication devices such as mobile phones. In this paper, the practical evidence of the RFI arising from the switching electronics in a wireless charging pad is provided and the use of a frequency-selective surface (FSS) is proposed for reducing the RFI. The basic principle of using the FSS design, previously used for stealth applications, is explained and practically verified for suppressing the RFI in wireless charging pads. The FSS structures show good RFI blocking performance with negligible effect on the charging energy efficiency. Practical tests have confirmed that the maximum sensitivity of the phone can be improved by 6.5-8.4 dB in GSM900 and by 2.3-6.3 dB in GSM850 when a double-sided FSS is employed. Moreover, the total isotropic sensitivity is improved by 3.3-5.6 dB in GSM900 and 0.2-3.3 dB in GSM850. The proposed FSS can become a low-cost accessory to the wireless charging pads for portable electronics.