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THE MODEL OF INTEGRATED STORAGE SYSTEM
T.I. KOSAREVA State University of Aerospace Instrumentation
В работе представлена разработка устройства модели интегрированного накопительного комплекса, включающего синхронный двигатель — генератор с маховиком из стекловолокнистого материала. Двигатель-генератор, маховик, магнитные подшипники в специальном корпусе-резервуаре образуют единую систему.
Modern integrated storage systems (ISS) are based on technology of flywheel and electrical machine. Flywheels are one of the oldest, most common mechanical devises and most promising technologies for replacing conventional lead acid batteries as energy storage systems for a variety of applications. In this paper the advantages of integrated storage system and trends of their progress are discussed and the model of modern energy storage systems for aerospace application is presented.
Presently, developments of flywheel are aimed at increasing its performance so that they can be used as efficient energy storage devices in aerospace power systems. Potential ISS applications include: satellites; space station (a large low earth orbiting (LEO) satellites); planetary probes; aircraft; military vehicles; hybrid and electric vehicles; uninterruptible power supplies. In spacecraft, these advanced aerospace flywheels will be able to store energy more efficiently than rechargeable chemical batteries. Successful development of advanced flywheels for spacecraft will provide significant advantages over today's technologies. In an effort to achieve higher energy and power densities and take advantage of modern composite material and power electronics technologies, many designers have attempted to develop compact flywheel batteries capable of extremely high rotation speeds.
A typical concept of energy storage system for mobile application usually consists of an electrical machine with a flywheel attached. The principal demands for a flywheel energy storage system (FES) are the amount of stored energy and the power density. In general, electrical machine with a flywheel are optimized independently from each other. However, both the power losses in electrical machine and the flywheel’s controllable burst behavior with an acceptable safety reserve within the speed range of the machine, strongly affect the suitability of the design.
In recent years, many countries of the world are engaged in research of high-efficient kinetic storage systems and their main components such us composite flywheel, motor-generator, magnetic bearing and control system. Numerous research programs currently are in progress of development activities for increasing specific energy and life for spacecraft battery and IPACS applications.
The principal demands for a flywheel energy storage system (FES) are the amount of stored energy and the power density.
In general, this system consists of non-contact magnet bearings that provide very low frictional losses, a composite flywheel of high mechanical strength and high energy density, a motor-generator that transfers electrical energy into mechanical and vice versa, and a vacuum chamber that minimizes windbag losses.
The main ways to develop high-efficient energy storage system are: improving the motor-generator performance, appliance magnetic bearing on the base earth-rare magnets and superconductive materials, application of new composite material for
© T.I. Kosareva
Проблемы энергетики, 2006, № 11-12
flywheel, development of integrated with power system complexes. Flywheels require a sophisticated set of power electronics and controls to allow them to be charged during sunlight and discharged during eclipse.
The task for this project was to develop a model of the integrated storage system for using it as a test-bed. The developed model of the integrated storage system involves a synchronous motor-generator unit with a glass-fiber flywheel, the magnetic bearings and the housing that are assembled together and form the system.
The difference between integrated and conventional storage system is that flywheel is combined with the rotor of electrical machine operating both as a motor and generator duty. This enables us to considerably reduce the overall dimensions of the system and to increase its efficiency. There are 3 operating duties of integrated storage system: charging, storing, and discharging. In charging duty electrical machine operates in the motor mode, in discharging it operates as a generator.
The most optimal choice for such a system is a synchronous machine with permanent rare-earth magnets at the external rotor. Rare earth permanent magnet NdFeB is a new kind of magnetic material with excellent magnetic characteristics: high energy product and high coercive force etc. and relatively low cost. Such type of magnetic material saves its characteristic in medium of low temperature.
The task of research work included the calculations of the motor-generator and of the flywheel.
The methods of the project realization are:
- stator winding parameters experimental determination;
- creating the calculation technique that is taking into account all the constraints (limitations) caused by the integrated system design as a whole;
- computer modeling of the rotor magnetic system and magnetic suspension in order to analyze a range of various designs.
The first step in the design process was to establish a set of requirements.
The system performance criteria were: minimum of 50 W-h usable energy, a 1.5 kW charge/discharge power rating, and a maximum operating speed of 12,000 RPM.
The researches began with a calculating electrical synchronous machine. At a first using one of traditional methods for calculating the electrical machine seemed the most right and simple way to solve the task of calculating the parameters of magnetic system, but actually it was not so.
There are a number of peculiarities when calculating an integrated storage system. The major one is stipulated by the necessity to search the best design variant in the limits of technical task.
Thermal restrictions in the electrical machine with permanent magnets are associated with current density of the armature winding ja [1]. For conventional machines with self-ventilation ja = 4 -10 A/mm2.
So the following calculation has been realized: calculation of the electrical machine of 1.5 kW in the generator duty. Operating medium - liquid nitrogen, thus the resistance is 7-8 times less than at room temperature with of electrical losses decreasing. Rated current density at liquid nitrogen is 42.5 A/mm2, thus the motor-generator shall withstand extreme operational for 1 min without winding damage.
One of the most important criteria of any integrated storage system are the specific energy density and the kinetic energy stored. The value of energy density depends on flywheel’s form and kind of material used. Choosing the material was the first stage of flywheel calculation. The two possible variants were the titanium and the glass-fiber.
Further, maximum rotational velocity has an impact on energy storage characteristics of the flywheel more strongly than by its mass. Therefore, a stronger, lighter flywheel may be able to store as much energy and even more than its metallic counterpart. However, the use of composite material has some difficulties in calculating and manufacturing. It was have concluded that the level of stored energy for two flywheel types is approximately equal, but mass of the titanium flywheel will be two times higher for the same dimensions as compared with a glass-fiber flywheel’s ones.
So, the glass-fiber has been chosen because it is the lightest material, cheapest and one of the safest. The calculations of strength of the glass-fiber flywheel and the steel rotor have been carried out.
To find the critical speed of rotation the shaft rigidity calculations have been performed. The calculation is intended to determine critical value of the angular frequency й = й cr , at which the rotation of the disc flywheel, fixed on the steel rotor, becomes unstable.
Conclusion: rated speed of rotation is located far from critical speed of rotation and kinetic storage system may operate without any dangerous of rotor’s steel rim damage. At the last part of project there are performed the calculations of total system losses and the time of discharging mode is defined. As a result of researches the model of integrated storage system, involving synchronous motor-generator with rare-earth permanent magnets and glass-fiber flywheel on rotor has been developed [2, 3].
The developed model of ISS has competitive advantages and performance as compared to others:
1. Efficient motor-generator with practically zero idling losses in liquid nitrogen. Its permanent-magnet rotor is united with a magnetic bearing rotor and flywheel, that spin together in an enclosure.
2. Concentric low weight glass-fiber flywheel.
3. Magnetic bearings, don't depend on lubrication, with practically zero-power axial and radial electronic servos; with a levitated rotor having axial and radial stability and stiffness, spin with no physical contact, and no hysteresis or eddy loss, for practically no idling loss.
4. Light-weight thin-wall enclosure for flywheel and all the rest parts. Unlimited life with no maintenance facilitate housing it in an underground site that can absorb a possible nearly adiabatic explosive burst.
5. Power controller that interfaces motor-generator to dc power circuit, has pulse-width-modulated sinusoidal current control, responsive to dc conduit voltage, rotor feedback, rotor vibration (prevents over-speed, full-energy explosion; monitors reserve energy, etc.). Controller includes integrated safety, and motor-generator control.
Furthermore the calculating technique has been worked out and the results of the field electromagnetic parameters calculation have been checked by computer modeling. The results of R&D works will be used to manufacture a model of the integrated storage system on a magnetic suspension.
Summary
In this project, the concept of the model integrated storage system is presented, that involves a synchronous motor-generator unit with a glass-fiber flywheel. Both components with the bearings and the reservoir are assembled together and form the system. Presently, developments of flywheel are aimed at increasing its performance so that they can be used as efficient energy storage devices in aerospace power systems.
Литература
1. Бут Д.А. Бесконтактные электрические машины. - М.: Высшая школа,
1990.
2. Косарева Т.И. Алгоритм расчета кинетических накопителей энергии с интегрированными синхронными машинами с постоянными магнитами: Сб. трудов Межд. конф. «Гагаринские чтения». - М., 2005 (в печати).
3. Косарева Т.И. Модель высокоэффективного кинетического накопителя
энергии для аэрокосмических объектов (проектирование на основе сопряженного электромагнитного, механического и теплового расчетов в программных комплексах): Сб. докл. Всеросс. конф. творческой молодежи, посвященной Дню авиации и космонавтики, 4-9 апреля 2005 г. - Красноярск: Сибирский гос.
аэрокосм. ун-т им. акад. М.Ф. Решетнева, 2005 (в печати).
