13-04-2012, 12:13 PM
Aerospace Flywheel Development
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INTRODUCTION
Presently, energy storage on the Space Station and satellites is accomplished using chemical batteries; most commonly nickel hydrogen or nickel cadmium. A flywheel energy storage system is an alternative technology that is being considered for future space missions. Flywheels offer the advantage of a longer lifetime, higher efficiency and a greater depth of discharge than batteries. A flywheel energy storage system is being considered as a replacement for the traditional electrochemical battery system in spacecraft electrical power systems. The flywheel system is expected to improve both the depth of discharge and working life by a factor of 3 compared with its battery counterpart. Although flywheels have always been used in spacecraft navigation and guidance systems, their use for energy storage is new. However, the two functions can easily be combined into a single system. Several advanced technologies must be demonstrated for the flywheel energy storage system to be a viable option for future space missions. These include high strength composite materials, highly efficient high speed motor operation and control, and magnetic bearing levitation.
COMPONENTS OF FLYWHEEL SYSTEM
The main components of the flywheel energy storage system are the composite rotor, motor/generator, magnetic bearings, touchdown bearings, and vacuum housing. The flywheel system is designed for 364 watt-hours of energy storage at 60,000 rpm and uses active magnetic bearings to provide a long-life, low-loss suspension of the rotating mass. The upper bearing of the unit is a combination magnetic bearing, providing suspension axially as well as radically. The lower magnetic bearing suspends the shaft in the radial direction only. At each end of the shaft there is also a touchdown bearing. This provides a back up bearing system should the magnetic bearings fail during testing.
CHARGE, CHARGE REDUCTION AND DISCHARGE CONTROL
In charge mode, the flywheel charges at a constant power, constant DC current rate using the excess current from the solar array. The charge control algorithm regulates the acceleration of the flywheel motor so that the DC current is maintained at the commanded set point. There are two components to the controller: the proportional-integral (PI) and the feed-forward (FF). The feed-forward portion uses the DC charging current command and converts it into a motor current command. The measured DC bus voltage and the estimated rotor speed from the back EMF estimation algorithm. The PI portion makes up for any inaccuracies in the relationship and guarantees zero steady state error. Thus fast, accurate performance is achieved with relatively low gains.
AEROSPACE FLYWHEEL CHALLANGES
The primary factor preventing the application of flywheels to long-term energy storage is loss in the bearings. Any mechanical bearing with contact between the stationary and rotating parts will have enough loss to render the system uneconomical one solution to the problem is to use a non-contact active magnetic bearing that employs conventional electromagnets. The rotational loss of such a bearing is 1-10% that of a mechanical bearing under the same operating conditions. The problem, however, is that the bearing itself consumes power, which is dissipated as heat in the copper electromagnets, and the bearing and cooling system power consumption must be included in the calculation of the overall system efficiency.
ADVANTAGES AND DISADVANTAGES
Flywheels are not affected by temperature changes as are chemical batteries, nor do they suffer from memory effect. Moreover, they are not as limited in the amount of energy they can hold. They are also less potentially damaging to the environment, being made of largely inert or benign materials. Another advantage of flywheels is that by a simple measurement of the rotation speed it is possible to know the exact amount of energy stored. However, use of flywheel accumulators is currently hampered by the danger of explosive shattering of the massive wheel due to overload.
APPLICATIONS
In the 1950s flywheel-powered buses, known as gyro buses, were used in Yverdon, Switzerland, and there is ongoing research to make flywheel systems that are smaller, lighter, cheaper, and have a greater capacity. It is hoped that flywheel systems can replace conventional chemical batteries for mobile applications, such as for electric vehicles. Proposed flywheel systems would eliminate many of the disadvantages of existing battery power systems, such as low capacity, long charge times, heavy weight, and short usable lifetimes. Flywheel systems have also been used experimentally in small electric locomotives for shunting or switching.
CONCLUSION AND FUTURE WORK
This paper has presented a new algorithm for regulating the charge and discharge modes of a flywheel energy storage system using a sensor less field orientation control algorithm to provide the inner loop torque control. The algorithm mimics the operational modes presently found in battery systems and would allow the flywheel system to replace batteries on future spacecraft. Experimental and simulation results show the successful control of the flywheel system permanent magnet motor in all modes of operation. Additionally, transition between modes and DC bus voltage regulation during step changes in load was also demonstrated. A future application of flywheel technology is to use flywheels to combine the energy storage and the attitude control functions on a spacecraft. A minimum of four flywheels would be needed to provide three axes of attitude control plus power during eclipse