The development of flywheel energy storage(FES) technology in the past fifty years was reviewed. The characters, key technology and application of FES were summarized. FES have many merits such as high power density, long cycling using life, fast response, observable energy stored and environmental friendly performance. A single flywheel stored energy of 0.5~130 kW·h in charging or discharging with power of 0.3~3000 kW. The frontier technologies include new materials of flywheel rotor, super-conducting magnetic bearing and high speed motor for FES. The commercial using of FES in power quality and uninterrupted power supply has a niche market share. The fuel saving in 20%~30% was realized in the hybrid power system using FES in vehicles. The FES technology is in a crisis of vehicles industrial application under the pressure from energy-saving and emission-reduction. For the grid application of renewable energy, the single FES stored energy of dozens of kWh should be increased to hundreds of kW·h. The power of FES array should be 10~100 MW and release power long as one hour.

With various appealing advantages over traditional bearings, active magnetic bearings are used successfully in flywheel energy storage systems. In this paper, firstly, their application in flywheel systems around the world were introduced. Then, key technology problems needing to be solved in high-speed flywheels with active magnetic bearings were discussed. At last, the application of active magnetic bearings for a high-speed flywheel system in the future was prospected.

This paper presents the modeling and characteristic analysis of the unknown disturbances for the flywheel energy storage system, considering the property of the three-phase permanent magnet synchronous motor. Firstly, the rotor dynamics mode under the circumstance of the dynamical base is established, and based on this, the model and property of the multi-source disturbances are given in detail. A status detection system is designed and accomplished to gain the essential input parameters for the disturbance. Then, the real-time speed, magnetically bias force and base vibration information can be obtained for the flywheel energy storage system. Finally, the experimental equipment of the magnetically suspended flywheel energy storage system is proposed with the flywheel parameters, which can be used appropriately to demonstrate the disturbance property and its depressing strategy.

To design the active magnetic bearings (AMBs) in a 10 kW·h flywheel energy storage system, the dynamic load capacity operating range of the E-core 12 pole magnetic bearings was analyzed. Through the plot of the dynamic load capacity vs. excitation frequency, the relationship between rotor unbalance excitation and operating dynamical load range was established. Active magnetic bearing electromagnets matched with power amplifiers were designed to get the largest load ability. The technical specification of actuator, and balance quality grade of rotor were determined to provide theoretical basis for the energy storage system general design and the magnetic bearings system design.

Due to the fluctuation and intermittent nature of renewable energy, they massive access will bring new difficulties to the power grid operation. Improving the scheduling of renewable energy by using energy storage technology is the focus of current research. In order to mitigate the bad effect of renewable energy generation on power grid, a layered optimal control method based on flywheel energy storage array is proposed for grid-connected wind-storage microgrid. The upper optimization center establishes the corresponding charge/discharge optimization model according to the power shortfall and the speed of each flywheel, and solves the power reference value of the corresponding flywheel. The lower flywheel controller adopts dual-mode double loop control method to realize the control of flywheel speed and output power. Finally, the effectiveness and feasibility of the proposed control method are validated by MATLAB/Simulink simulation.

A flywheel stores energy was composed of hub and multi-rims made of filament winding with carbon fiber. The hub and rims were subject to axisymmetric centrifugal forces due to rotation and pressure come from the interface of rims. The interface pressure varying with the rotation speed made the stress analysis being very complexity at various radius and condition from stop to the maximal operation speed. In this investigation, based on the assumption of a plane stress, an analytic method for calculating the stresses and displacements distribution of the orthotropic rim is presented respectively. Moreover, the superposition principle was applied to calculate the stresses and displacements. The relatively simple solving process was applied to analyze and design the flywheel, such as choosing the interference fit, evaluating strength of the flywheel, and calculating the maximal speed. The analysis result of the stresses and displacements was carried out for an actual flywheel design example.

For the flywheel energy storage system, the maximum speed of the flywheel energy storage system in experiments is 9300 r/min, which is less than the maximum rated speed of 15000 r/min. This will greatly reduce the maximum energy storage of the flywheel energy storage system. In order to improve rotor dynamics structural design, theoretical formulae analysis, finite element analysis of SAMCEF Rotor are used to study the critical speed distribution and stability of disc rotor and column flywheel rotor with combination of experimental data. The analysis result indicated that the second-order critical speed of the disc flywheel rotor with the same mass and different structure far exceeds the column-type flywheel rotor, and the first-order critical speed of the two types of flywheel is almost the same value. According to the different working speed and energy requirements of the flywheel rotor, different structural solutions should be adopted to ensure the requirements of the operating speed and stable operation.

The configuration and operational principle of the flywheel-based dynamic UPS is introduced firstly, and the AC interconnection topology and structure of the flywheel array are analyzed. The research on the twin trawling charging-discharging experimental method of the flywheel array is presented to solve the problem of the limited power grid capacity and the waster of energy. With the established of the hardware platform, the DC interconnection control logic and the software construction are designed as well the detailed real twin trawling charging-discharging operation processing is presented. Finally, the experimental equipment and the parameters of the magnetically suspended flywheel-based dynamic UPS is described.

The coordinated control strategy of flywheel energy storage array from parallel to the same DC bus is studied in this paper. The change rate of charge state (SOC) under three classical power control strategies is deduced and analyzed. An improved coefficient droop control strategy is put forward. The virtual impedance of improved coefficient droop control strategy is adopted to improve the accuracy of power allocation to solve the static error control problem caused by low voltage of micro-grid. Finally, a micro-grid model with wind farm is established, and the effectiveness of the above control strategy is verified by simulation.

A 1 MW flywheel energy storage array system is proposed according to the operation characteristics and train parameters of urban rail transit to absorb the braking power generated when the train is braking. By comparing different types of regenerative braking energy recovery methods, the necessity of application of flywheel energy storage system in urban rail transit are analyzed. The 1MW array flywheel energy storage system is carried out from the array optimization, security calculation and project implement anticipation based on the test data for the rail transit electrical drive line. The feasibility of 1 MW flywheel energy storage array system applied in urban rail transit is verified.

Flywheel energy storage equipment can be used in vital places, for high power loads in short time duration. Multi-index is used to describe the power quality. The national standard of power quality and several kinds load characteristics are discussed in this paper. An application example of flywheel energy storage equipment was presented in actual engineering, which demonstrated that flywheel energy storage equipment could provide output with excellent power quality.

The flywheel energy storage is used to reduce the power output of the transformer by discharging energy to the power grid when the line load is heavy. FES is useful to reduce the maximum demand value or transformer capacity, depress the negative sequence current of railway and absorb the braking energy generated to save energy. This paper presented the integration structure of the system, converter system, flywheel energy storage device, measurement and control unit. The simulation model of the system is proposed according to the actual substation parameters and the actual measured traction load data. The simulation analysis indicated that the proposed flywheel energy storage system was suitable for peak shaving in electrified railway.

Lithium ion batteries have been widely used in many aspects of the national economy. Unfortunately, with the rapid development of consumer electronics and electric cars, developing high-performance lithium ion batteries with high energy density and high safety has become a hot spot of research. Commercial liquid organic carbonate electrolyte-based lithium ion batteries often suffer from serious safety hazards such as leakage, volatile, combustion and explosion. As compared to inorganic solid lithium batteries, all-solid-state polymer lithium batteries is a relatively ideal solution to solve abovementioned bottleneck issues owing to the facile processibility. In addition, all-solid-state polymer electrolytes, an essential sector of all-solid-state polymer lithium batteries, play a key role in determining the performance of lithium batteries. Up to now, a great deal of efforts have been devoted to the development of high-performance all-solid-state polymer electrolytes. Herein, this paper mainly overviewed the research progress on polyethylene oxide, polysiloxane and aliphatic polycarbonate based on all-solid-state polymer electrolytes. Meanwhile, we also analyzed the technology application and patent layout of all-solid-state polymer lithium batteries by domestic and international well-known companies and research institutes systematically. Finally, we also give the main challenge and perspectives of all-solid-state polymer lithium batteries in terms of binders, lithium salts, advanced all-solid-state polymer electrolytes, interface regulation, manufacturing technique and anodes at the end of this review.

This bimonthly review paper highlights 100 recent published papers on lithium batteries. We searched the Web of Science and found 1706 papers online from Jun. 1,2018 to Jul. 31,2018. 100 of them were selected to be highlighted. Layered oxide and high voltage spinel cathode materials are still under extensive investigations for the structure evolution and modifications. Large efforts were devoted to Si based anode material. There are a few papers related to solid state electrolyte, Li/S battery, Li-air battery, all solid state batteries analyses, theoretical simulations, and modeling.

Sodium acetate trihydrate has been widely studied as an inorganic hydration salt based latent heat storage material. The key challenges in the use of such a material include super cooling, phase separation, and low thermal conductivity. This article reviews the progress to address the challenges. Current methods of resolving the phase separation and supercooling challenges are mainly empirical, involving the search for nucleating agents and thickeners, for which little has been found on the development of theoretical approaches to determine the formulation and dosage. The low thermal conductivity has been addressed through the addition of highly thermally conductive. It is concluded that considerations are needed from both the device and system level for an effective use of sodium acetate trihydrate for thermal energy storage.

This paper reviews manufacturing technologies for cathode materials of lithium ion batteries and discusses the development trend of such materials. Specific attention is paid to the work at Tsinghua University where a novel solid state process was developed. The keys to such a process are an optimized precursor by controlled crystallization. The optimization of the material properties was done from four aspects of crystal cell structure, primary particle structure, secondary particle morphology and the surface chemistry, enabling the products with a controlled size, uniformity, consistency, scale-up and reproducibility. Moreover, the bulk density of the material is high and hence a high energy density of batteries. This technical route of controlling crystallization/solid state reaction has been accepted by the industry, which represents one of the most important contributions to the lithium-ion battery industry.

To study the performance of lithium iron phosphate batteries for hybrid electric vehicles, the Coulomb counting method is used to estimate the battery state of charge (SOC) on the basis of the Thevenin battery model, taking into account of the influence of temperature. The battery model parameters are identified through HPPC tests, the model is built and simulation performed in the Matlab/Simulink environment. The results show that the Thevenin battery model is of a high precision with the simulated and measured end voltage results in close agreement:the average error of the end voltage is 3.6 V, the maximum error is 12.6 V, accounting for 0.79% of the battery rated voltage. The simulated and the measured SOC results are shown to agree within 3%.

Thermodynamic analyses are performed to investigate the influence of varying air compression factor on compressed air storage in a underground rock cavern. Experimental data are used to validate four compression factor calculation methods. The results indicate that the temperature and pressure of compressed air are significantly influenced by the variation of the compression factor. Under cyclic charging and discharging processes, the compression factor, the temperature and the pressure of compressed air increase with increasing charging and discharging times, and eventually show a stablised trend. The calculated compressed air temperature is more sensitive than that of compressed air pressure when the compression factor is considered. The ideal gas assumption can lead to a large errors.

The influence of formation temperature on the performance of lithium-ion batteries is investigated using LiCoO₂ as cathode and graphite as anode. Analyses are performed on the effect of temperature on the formation time, voltage, voltage dropping, and rate of charge. The results show that the formation time is longer with a higher temperature and different formation temperatures give a different extent of influence of temperature on cycle and rate behaviour.

Numerical simulations were performed on a small scale pump turbine device under different flow conditions. The impeller on which the total pressure loss concentrates was optimized using the orthogonal method. A L9(3⁴) orthogonal table was produced with four important design factors for an impeller at three levels. The optimization considered pump head deviation, pump efficiency and turbine efficiency. The results showed that the blade outlet diameter affected significantly the performance of pump turbine. Compared with the original baseline pump turbine, the optimized pump efficiency increased by 1.06% and the optimized turbine efficiency increased by 1.62% under the design conditions. Both the optimized efficiency points under the pump and turbine operating conditions moved toward to a low flow rate due to the augment of the wrap angle.

A 622-type ternary cathode materials (LNCM) was prepared by using molten salt made from a lithium salt based type-622 precursor, 0.24 Li₂CO₃-0.76 LiOH and potassium chloride in an air atmosphere. The effects of the ratio of lithium and potash, reaction time and reaction temperature on the crystal structure of the LNCM were investigated by XRD and SEM. The optimum preparation conditions were found to be from a lithium source to potassium salt ration of 1:5, a precursor to lithium source ratio of 1:1.1, a reaction temperature of 750℃, and a calcination time of 15 hours. LNCM as a cathode material was assembled into a button battery, and a number of electrochemical performance indexes were obtained, e.g. over the 2.7-4.3 V voltage range at 0.1 C rate, the specific capacity of the first discharge cycle was 182.5 mA·h/g, the Coulomb efficiency was 89.1%; over the 2.7-4.5 V voltage range at 0.1 C rate, the discharge specific capacity was 183.2 mA·h/g after 100 cycles, the capacity retention rate was found to be 91.5%.

Lithium ionic conductivity, electronic conductivity of active electrode materials and lithium ionic conductivity of electrolyte materials are closely related to the dynamic behavior of lithium batteries. Therefore, conductivity test and analysis contribute the understanding of electrochemical properties of materials, including direct current method (DC), alternating current impedance (AC impedance), and direct current polarization method (DC polarization). Based on the different conductivity characteristics of electrolyte materials and active electrode materials, this paper introduced the methods, principles, equipments, test procedures and precautions of conductivity test. Besides, the analysis of data was illustrated with specific cases of lithium batteries.