Flywheel Energy Storage System Research Articles

Fault diagnosis of flywheel bearing based on parameter optimization variational mode decomposition energy entropy and deep learning

Flywheel energy storage system is widely used in train braking energy recovery, and has achieved excellent energy-saving effect. As a key component of the flywheel energy storage system, the health of the bearing is greatly significant to realize the effective recovery of train braking energy. The vibration signal of the bearing presents complex nonlinear and non-stationary characteristics, which makes it difficult to diagnose the fault of the bearing. To solve this problem, a fault diagnosis method for bearing of flywheel energy storage system based on parameter optimization Variational Mode Decomposition (VMD) energy entropy is proposed. Firstly, the improved Sparrow Search Algorithm is used to optimize VMD parameters with the dispersion entropy as the fitness value. Then, the original signal is decomposed into a series of intrinsic mode components by using the optimized VMD algorithm, and the energy entropy of each component is calculated to construct the feature vector. Finally, an Inverted Residual Convolutional Neural Network (IRCNN) is used as feature vector input model for fault diagnosis. The experimental results show that the proposed method can effectively extract the bearing fault characteristics and realize accurate fault diagnosis, and the recognition rate reaches 97.5%, which is better than the comparison method.

Model validation of a high-speed flywheel energy storage system using power hardware-in-the-loop testing

Low-inertia power systems with a high share of renewables can suffer from fast frequency deviations during disturbances. Fast-reacting energy storage systems such as a Flywheel Energy Storage System (FESS) can help limit the frequency deviations by injecting or absorbing high amounts of active power, with almost no degradation concerns. But for an accurate evaluation of the benefits of using a FESS in power systems, an accurate and validated model is necessary, which not only reflects its advantages, but also shows the practical limitations of this technology, such as its losses, its auxiliary power requirements, and its limitations during the whole operational range. In this paper, an accurate model for a high-speed FESS is presented, and then experimentally validated by means of Power Hardware-in-the-Loop (PHIL) testing of a full-scale commercial high-speed FESS in several frequency deviation scenarios. The PHIL testing setup allows testing the FESS during extreme and realistic frequency deviation, including the major frequency disturbance of August 9, 2019, in the power system of the UK. In each scenario, the same inputs that are sent to the real FESS are also given to the FESS model, running in real time, for model validation, and an excellent match between the results of the real FESS and its model is observed. Moreover, the PHIL testing results demonstrate the quick response of the FESS, following a frequency deviation, and its compliance with the latest grid code requirements in Germany for frequency support.

Prototype production and comparative analysis of high-speed flywheel energy storage systems during regenerative braking in hybrid and electric vehicles

In conventional EVs and HEVs, only a small part of the vehicle's kinetic energy can be usefully stored during deceleration. Generally, this storage process can be done by providing energy flow to the main battery of the vehicle. Since batteries work with a chemical reaction, they are not suitable for fast charging and discharging required for regenerative braking. In this case, a fast storage system is needed to store the regenerative braking energy in a short time. As a solution, the flywheel energy storage system (FESS) can be offered.In the literature, power transmission of vehicles with integrated FESS is provided by mechanical systems (CVT FESS). These systems are heavy, high cost, large volume, and occupy the rear axle of the vehicle.In the proposed system, a purely electrical power transmission is proposed to store the kinetic energy of the vehicle in FESS. In developed topology, the traction machine of the vehicle is also used as a generator, and the recuperation energy is stored in the electrically driven (M/G) FESS.As a result of the experimental studies, a minimum of 56% energy recovery efficiency was obtained. In addition, it has been shown that the developed system is 30% lighter, occupies 60% less space in volume, is at least 50% more cost-effective, maintenance-free, and has fewer moving parts compared to the CVT FESS.As a result, the addition of a fast-response secondary energy storage system to the electric vehicle battery contributes to the increase in efficiency.

Optimal sizing of a high‐speed flywheel in an electric vehicle using the optimal control theory approach

This study proposes an approach for finding the optimal size of a high-speed flywheel in an energy storage system based on battery-flywheel cooperation, called battery-flywheel energy storage system (BFESS), supplying an electric vehicle. Considering an energy-based model for the flywheel and battery, including the losses, the optimal control theory is utilised as a two-dimensional Pontryagin's minimum principle for solving the problem. The objective function of the flywheel sizing problem includes two terms, in which its maximum interaction with the system from the energy and filtering aspects are taken into consideration by a desirable weighting. While energy interaction is a common task, filtering the fast dynamics of the system such as the harsh conditions for the battery is the key role of the considered lightweight and high-speed flywheel. A trade-off between the energy contribution and filtering is made, targetted for the long-term and short-term dynamics in the drive cycles, respectively. The proposed methodology is presented in detail using some simple dynamics. Also, the sizing is carried out for the ECE-15 urban drive cycle, the WLTC class-3 and FTP-75 drive cycles. The results confirm that by using the determined optimal size of the flywheel energy storage system (FESS), most of the dynamics could be handled from the filtering and energy interaction aspects.

Energy Management and Control of a Flywheel Storage System for Peak Shaving Applications

Peak shaving applications provided by energy storage systems enhance the utilization of existing grid infrastructure to accommodate the increased penetration of renewable energy sources. This work investigates the provision of peak shaving services from a flywheel energy storage system installed in a transformer substation. A lexicographic optimization scheme is formulated to define the flywheel power set-points by minimizing the transformer power limit violations and the flywheel energy losses. Convex functions that represent the flywheel power losses and its maximum power are derived and integrated in the proposed scheme. A two-level hierarchical control framework is introduced to operate the transformer-flywheel-system in a way that handles prediction errors and modelling inaccuracies. At the higher level, a model predictive controller is developed that solves the lexicographic optimization scheme using linear programming. At the lower-level, a secondary controller corrects the power set-points of the model predictive controller using real-time measurements. A software platform has been developed for integrating the proposed controllers in an experimental setup to test their effectiveness in a realistic testbed setting, and the flywheel system characteristics are experimentally identified. Simulation and experimental results validate and verify the modeling, identification, control and operation of a real flywheel system for peak shaving services.

Design and analysis of a flywheel energy storage system fed by matrix converter as a dynamic voltage restorer

This paper presents design, optimization, and analysis of a flywheel energy storage system (FESS) used as a Dynamic Voltage Restorer (DVR). The first purpose of the study was to design a flywheel with a natural resonance frequency outside the operating frequency range of the FESS. The second purpose of the study was to show that a matrix converter structure can be used for a FESS. The matrix converter can bidirectionally convert the power between the grid and the flywheel due to its natural property. The matrix converter needs a special motor/generator design, because of the voltage utilization ratio of the matrix converter. Therefore, a Permanent Magnet Synchronous Motor (PMSM) being compatible with the matrix converter voltage level was designed and optimized as the third purpose of this study. The motor was optimized to achieve low torque ripple and as high torque as possible by using multi-objective optimization algorithm. Input/output voltages of the FESS are analyzed for grid interruption and 50% voltage sag operation conditions. The frequency analysis study was performed by using SolidWorks, the PMSM was designed and optimized by using MAGNET-Infolytica, and all the other results are performed by using MATLAB-Simscape.

Decoupling Control of a Disc-type Rotor Magnetic Bearing

A disc-type rotor magnetic bearing with 3-pole magnet is considered in the paper. The disc-type rotor supported by electro-magnetic forces has potential in industrial application especially in flywheel energy storage systems. However, coupling phenomenon in acting forces makes the balancing control problem in the system challenging. Based on analysing magnetic forces acting on the rotor, the coupling mechanism is identified, and a coordinate transformation is formulated to decouple acting forces. Thanks to the transformation, control design is straightforward for each control channel leading to simple and straightforward control design. The goal of the design is to keep the rotor at its desired equilibrium in the presence of disturbance and parameters variation. The goal can be achieved when the controller is designed in discrete-time domain based on the linearized model, then, the disturbance is compensated by employing the one step delay technique. The validity and robustness of the controller are verified by various numerical simulations in which original nonlinear model is used.

Flywheel energy storage systems: A critical review on technologies, applications, and future prospects

Energy storage systems (ESSs) are the technologies that have driven our society to an extent where the management of the electrical network is easily feasible. The balance in supply-demand, stability, voltage and frequency lag control, and improvement in power quality are the significant attributes that fascinate the world toward the ESS technology. However, being one of the oldest ESS, the flywheel ESS (FESS) has acquired the tendency to raise itself among others being eco-friendly and storing energy up to megajoule (MJ). Along with these, FESS also surpasses the quality of high power density, longer life cycle, higher rate of charge and discharge cycle, and greater efficiency. In this article, an overview of the FESS has been discussed concerning its background theory, structure with its associated components, characteristics, applications, cost model, control approach, stability enhancement, maintenance, and future trends. The FESS structure is described in detail, along with its major components and their different types. Further, its characteristics that help in improving the electrical network are explained. The applications of the FESS have also been illustrated through their economically available prototypes. The economics associated with FESS technology has been projected. A thorough survey of various control strategies of FESS highlighting the merits and demerits of each has been discussed in depth. The stability enhancement and maintenance of the FESS unit have also been enumerated. Further, the article also recommends numerous future work that plays a vital role in the hassle-free operation of FESS for the efficient functioning of the electrical power system.

Coordinated control of VFT and fuzzy based FESS for frequency stabilisation of wind penetrated multi-area power system

This article investigates the effect of coordinated control of variable frequency transformer and fuzzy based flywheel energy storage system for frequency stabilisation of wind penetrated two area power system. Wind turbine generator and fuzzy based flywheel energy storage system are incorporated in two different areas. Variable frequency transformer, set up in series with the tie-line near wind-embedded area, is modelled as a tie-line power flow controller between interconnected areas. Variable frequency transformer introduces the phase angle shift between interconnected areas in accordance with the power demand on account of load and wind speed variations in wind-embedded area, while actual power is supplied from fast acting flywheel energy storage system incorporated in other area. Fuzzy controller for flywheel energy storage system is designed in such way that frequency deviations are mitigated continuously during disturbances and nominal storage capacity of flywheel energy storage system is restored on incessant basis, after every load and wind disturbance is tackled.

Robust Predictive Power Control of N*3-Phase PMSM for Flywheel Energy Storage Systems Application

In this study, a robust predictive power control (R-PPC) method for an N*3-phase permanent magnet synchronous motor (PMSM) is developed in the field of flywheel energy storage systems application, which can effectively improve robustness against inductance parameter mismatch and compensate for the one-beat delay. Firstly, the mathematical model of the N*3-phase PMSM is illustrated, and the topological structure of the N*3-phase PMSM is established. The R-PPC method of the N*3-phase PMSM is then proposed by using the d–q axis current robust predictive control theory. Robustness factors are adopted to modify the current response values in the proposed robust predictive power controller, which can obtain excellent current control performance under the inductance parameter mismatch. Moreover, the next current predicted value is used to replace the current sampled value in the proposed R-PPC method to eliminate the one-beat delay. Finally, comparative simulation and experimental results verify that the proposed R-PPC method can achieve excellent current track performance and smaller torque ripple under both the charge state and discharge state.

Electromagnetic Design of High-Power and High-Speed Permanent Magnet Synchronous Motor Considering Loss Characteristics

The motor is an important part of the flywheel energy storage system. The flywheel energy storage system realizes the absorption and release of electric energy through the motor, and the high-performance, low-loss, high-power, high-speed motors are key components to improve the energy conversion efficiency of energy storage flywheels. This paper analyzes the operating characteristics of the permanent magnet synchronous motor/generator (PMSG) used in the magnetically levitated flywheel energy storage system (FESS) and calculates the loss characteristics in the drive and power generation modes. Based on this, the electromagnetic part of the motor is optimized in detail. Aiming at this design, this paper calculates the loss characteristics of driving and power generation modes in detail, including its winding loss, core loss, rotor eddy current loss and mechanical loss. The calculation results show that the design meets the loss requirements. It can reduce the no-load loss of the permanent magnet synchronous motor at high speed and improve the energy conversion efficiency, which gives this system practical application prospects.

The development of a techno-economic model for the assessment of the cost of flywheel energy storage systems for utility-scale stationary applications

Flywheel energy storage systems are increasingly being considered as a promising alternative to electro-chemical batteries for short-duration utility applications. There is a scarcity of research that evaluates the techno-economic performance of flywheels for large-scale applications. Evaluating the capital cost, levelized cost of storage, and scale factor is crucial to make an informed decision in future development and deployment of the technology. This study, therefore, focuses on developing a bottom-up techno-economic model to design system components and to evaluate the total investment cost and levelized cost of storage of flywheels with a capacity of 20 MW/5 MWh for frequency regulation. Two rotor configurations were considered: composite rotor flywheel and steel rotor flywheel. The total investment costs of the composite rotor and steel rotor flywheel storage systems are $25.88 million and $18.28 million, respectively. The corresponding levelized costs of storage are $189.94/MWh and $146.41/MWh. The model results are highly sensitive to the cost of the rotor material, discount rate, factor of safety, number of cycles per year, and tensile strength of the rotor material. The ranges obtained in the uncertainty analysis for the levelized cost of storage are $122.08-$253.52/MWh and $108.63-$187.64/MWh for the composite rotor and steel rotor flywheel storage systems, respectively.

Electromagnetic design of high-speed permanent magnet synchronous motor for flywheel energy storage system

Flywheel energy storage system (FESS) has significant advantages such as high power density, high efficiency, short charging time, fast response speed, long service life, maintenance free, and no geographical environment restrictions. Motor is the energy conversion core of FESS and plays a significant role on system performance. In this paper, the design features of the motor for FESS are analyzed first. Then, a permanent magnet synchronous motor (PMSM) with a rated speed of 12000 rpm and a rated power of 250 kW is designed. Thirdly, aiming at the key problem of difficult heat dissipation of rotor, an improved rotor structure with “1” rotor permanent magnets layered and divided where placed in tangential and radial directions is proposed, which can reduce eddy current loss by 22.8%. Finally, based on Ansys FEM software, electromagnetic characteristics of the designed motor at no-load and full-load conditions are simulated and analyzed. The results show that the designed electromagnetic scheme meets the expected performance requirements and has guiding significance for prototype.

RT-lab based real-time simulation of flywheel energy storage system associated to a variable-speed wind generator

This paper presents a new simulator used to distribute and execute real-time simulations: the RT-LAB, developed by opal-RT technologies (Montreal, Canada). One of its essential characteristics is the perfect integration with MATLAB/Simulink. The RT-LAB allows the conversion of Simulink models in real time via real-time workshop (RTW) and their execution on one or more processors. In this context, the paper focuses on the RT-LAB real-time simulation as a complement to the MATLAB/Simulink environment, which has been used to perform the simulation of the Flywheel energy storage system (FESS -variable speed wind generation (VSWG) assembly. The purpose of employing a fairly new real-time platform (RT-LAB OP -5600) is to reduce the test and prototype time. This application will be executed on each element of our model that was previously developed under MATLAB/Simulink. The real-time simulation results are observed in the workstation.

Generalized active disturbance rejection controller design: Application to hybrid microgrid with communication delay

In this paper, generalized active disturbance rejection control (GADRC) technique in the presence of communication delay is proposed for load frequency control of a hybrid microgrid system, which is composed of renewable energy sources such as solar and wind input, energy storage devices such as battery energy storage system and flywheel energy storage system as well as fuel cell and diesel generator which are controllable sources of energy. The presence of communication delay involved in transmission of control signals is also considered for controller design. Further, the robustness of proposed scheme is investigated via an elaborate study comprising of varying communication delay, parametric uncertainty, nonlinearities such as generation rate constraint and governor deadband, as well as mismatch in communication delay between the inputs of observer. Finally, the performance indices of the proposed scheme are compared with conventional and fractional order based controller design approaches. The simulation results are a testimony to the efficacy of proposed scheme.

Energy Management and Control System Design of an Integrated Flywheel Energy Storage System for Residential Users

This paper presents the energy management and control system design of an integrated flywheel energy storage system (FESS) for residential users. The proposed FESS is able to draw/deliver 8 kWh at 8 kW, and relies on a large-airgap surface-mounted permanent magnet synchronous machine, the inner rotor of which integrates a carbon-fiber flywheel, leading to a compact and efficient FESS. The proposed energy management system is based on four different operating modes, which are defined and can be selected in accordance with FESS speed and/or user’s preference, while FESS control system is devoted to power/current tracking at both machine- and grid-side converters. The effectiveness of the proposed solutions, as well as the overall energy performance of the proposed FESS, are verified by real-time simulations, which regard different operating conditions and/or realistic scenarios.

Dynamic behavior and stability of energy efficient electro-magnetic suspension of rotors involving time delay

Energy efficient contact less suspension of high-speed rotor shafts in application areas such as power-plants and Flywheel Energy Storage Systems (FESS), can be achieved through Active Magnetic Bearings (AMBs). Inevitably, there exists time delay in sensing the rotor displacement and actual application of control force by electromagnets, thus leading to system instability. This paper therefore analyses stability of AMBs, with constant time delay. FESS is modelled as a rigid rotor-AMB system. Performance of two different control laws, (i) PID and (ii) a recently introduced Four-Element (FE) law, are compared. The governing equations are transformed to frequency domain, to exploit Pade's approximation in modeling time delayed value of feedback signal and stability analysis is carried out by analyzing the system poles. This methodology has been verified using experimental results from the existing literature. Numerical simulation of delay differential equations is also used to validate analytical results. The FE control law shows promise in tolerating delay, as compared to conventional PID controller thus leading to AMB system with higher possible spin speed and relaxed sampling constraints on AMB system instrumentation. One of the prospective applications of this work is in design of flywheel energy storage systems for an electric vehicle.

Essential Enzyme Kinetics: A Textbook for Molecular Life Scientists

Energy efficient contact less suspension of high-speed rotor shafts in application areas such as power-plants and Flywheel Energy Storage Systems (FESS), can be achieved through Active Magnetic Bearings (AMBs). Inevitably, there exists time delay in sensing the rotor displacement and actual application of control force by electromagnets, thus leading to system instability. This paper therefore analyses stability of AMBs, with constant time delay. FESS is modelled as a rigid rotor-AMB system. Performance of two different control laws, (i) PID and (ii) a recently introduced Four-Element (FE) law, are compared. The governing equations are transformed to frequency domain, to exploit Pade's approximation in modeling time delayed value of feedback signal and stability analysis is carried out by analyzing the system poles. This methodology has been verified using experimental results from the existing literature. Numerical simulation of delay differential equations is also used to validate analytical results. The FE control law shows promise in tolerating delay, as compared to conventional PID controller thus leading to AMB system with higher possible spin speed and relaxed sampling constraints on AMB system instrumentation. One of the prospective applications of this work is in design of flywheel energy storage systems for an electric vehicle.

A case study investigation into the risk of fatigue in synchronous flywheel energy stores and ramifications for the design of inertia replacement systems

Flywheels are an attractive energy storage solution for many reasons; high turnaround efficiencies, long cycling lives and high “ramp-up” power rates have all been noted in the literature. Novel flywheel based hybrid energy storage systems have also been suggested by several authors which, due to the inherent partitioning of power sources in the system architecture, provide capacity for flywheels to deliver/receive energy over a comparatively large range of time scales and loading frequencies. Accommodating grid power fluctuations at the millisecond to second time scale is an ever growing problem that almost all grids undergoing de-carbonisation are facing. Synchronous flywheel energy storage systems have the attractive capability of being able to replace “real” (passively controlled) inertia with “real” inertia in a cheap and very robust manner. Flywheel design at the grid scale warrants careful consideration, as for static energy storage applications (i.e. those not used in transportation) the main driving factor is the reduction of manufacturing and material costs. It is paramount that material is used effectively, i.e. it is sufficiently stressed such that the flywheel is not oversized (and therefore expensive) while simultaneously guarding against the likelihood of catastrophic failure during service. Fatigue has the potential to be a serious life limiting mechanism due to fluctuating rotational speeds, however in depth analysis is lacking in the literature. The present work looks to quantify the severity of fatigue in flywheels which re-establish grid inertia by applying fatigue design methods (such as the rainflow cycle counting method and the generalised strain amplitude methods of Ince and Glinka for fatigue lifing) to loading scenarios that represent grid frequency fluctuations. Importantly flywheels are sized based on different limit stress criteria, thereby enabling differing levels of structural capacity usage between designs. For the realistic design cycles considered in the present work (representative of a large scale grid undergoing normal frequency fluctuations) all projected lives are extremely large, suggesting that fatigue is not a limiting factor and that any of the tested design methodologies is viable. Significant improvements in energy density and cost per unit of energy stored may however be achieved if elastic–perfectly-plastic (Tresca based) design criteria are implemented over simple strictly elastic variants. Neglecting containment costs for simplicity, improvements in energy density of ≈74% and cost per unit of energy stored of ≈290% are demonstrated to be achievable.

Analyzing the suitability of flywheel energy storage systems for supplying high-power charging e-mobility use cases

The trend towards increasing the charging power of future e-mobility will challenge existing distribution power systems and raise grid utilization- and connection costs. Flywheel energy storage systems (FESSs) may reduce future power grid charges by providing peak shaving services, though, are characterized by significant standby energy losses. On this account, this study evaluates the economic- and technical suitability of FESSs for supplying three high-power charging electric vehicle use cases. Therefore, we initially investigate the impact of individual charging patterns on the required FESS capacity, the annualized costs, and the FESS efficiency. Based on these correlations, the economic and technical optima of FESS applications are identified for each use case: The supply of electric buses enables a cost-efficient operation at the technical optima of FESSs. In contrast, the economic suitability of FESSs considering electric last-mile delivery trucks or highway fast-charging is restricted to low recharging energy demands and high charging power of electric vehicles. Furthermore, a cost-efficient operation of FESSs at the technical optima requires either a reduction of flywheel costs or an increase of power-based grid utilization charges in the upcoming years.