Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
2001-09-24
2003-09-23
Mullins, Burton (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
C310S052000, C310S074000, C062S259200
Reexamination Certificate
active
06624542
ABSTRACT:
This invention pertains to flywheel energy storage systems and more particularly to a flywheel power source with a heat transfer system for passively cooling the stator of the generator. The flywheel power source achieves an extended generator life and increased power capability with lower costs and higher reliability than previous flywheel systems.
BACKGROUND OF THE INVENTION
Flywheels have been used for many years as energy storage devices. They have often been used as power smoothing mechanisms for internal combustion engines and other kinds of power equipment. More recently, flywheels have been recognized as a very attractive energy storage technology for such electrical applications as uninterruptible power supplies, utility load leveling systems, satellites and electric vehicles.
Modern flywheel energy storage systems convert back and forth between a spinning flywheel's rotational energy and electrical energy. A flywheel energy storage system includes a flywheel, a motor/generator, a bearing system and a vacuum enclosure. The rotating flywheel stores the energy mechanically; the motor/generator converts between electrical and mechanical while the bearing system physically supports the rotating flywheel. High-speed flywheels are normally contained in a vacuum or low pressure enclosure to minimize aerodynamic losses that would occur from atmospheric operation while low speed systems can be operated at atmosphere.
In almost all UPS applications, whether quick discharge type (power ride-through), where discharge time is measured in seconds, or long-term discharge type (power back-up), where discharge time is measured in hours, flywheels directly compete with electrochemical batteries. Power ride-through systems are usually for higher power (>50 kW) applications that can be coupled with a generator set. The flywheel supports the load for the first 30 seconds to 1 minute of the electrical interruption period until the generator can get up to speed or the electrical interruption has ceased. Back-up systems are usually for lower power (<50 kW) applications where the flywheel is expected to support the load for the duration of the electrical interruption period. Two key advantages of flywheels used for electrical energy storage over electrochemical battery systems are longevity and reliability. Electrochemical batteries, in particular, lead acid batteries, have short lifetimes, between two and seven years depending on operating conditions. These batteries require periodic maintenance and can fail unpredictably. In contrast, flywheel energy storage systems are expected to have lifetimes of at least twenty years and it is desirable not to require any maintenance. Such capability can offset the higher initial cost of the flywheel system over batteries by actually becoming cheaper when considered over the life.
All designs of flywheel motor/generators have electrical coil windings for conversion between electrical and magnetic energy used to apply torques to the rotating flywheel. The lifetime and reliability of the motor/generator is directly related to the dielectric insulator life of the insulation on the coil wires. After long-term operation, the dielectric strength of the insulation breaks down causing arcing and shorting. The lifetime of the insulation has been shown to follow a form of the Arrhenius Law with temperature. For a given insulation, the life is cut in half for every 6° C. operating temperature increase. Besides the loss of dielectric breakdown and insulator strength, high temperatures can also physically melt the insulation. One solution to increasing the life of the motor/generator is to use heavier and or higher temperature insulation. Using heavier insulation results in less space for the copper conductors, increasing their resistance and therefore the heat generated by the motor/generator. Heavier insulation also reduces the heat transfer from the coils. A physically larger motor/generator could be used to keep the higher efficiency with the heavier insulation but this significantly increases the cost of the motor/generator. This is especially true in flywheels where the high operating speeds (up to 40 krpm, 666 Hz) requiring use of expensive, very thin laminations to stack up the stator for reduction of eddy currents. Despite the increased costs of using a larger motor/generator, it is not a guaranteed solution to the problem. Size is usually limited by the stress capability of the rotor portion. Likewise, the stator laminations themselves and other metal components, if used, can be a larger source of heat generation through eddy current and hysteresis losses. No changes in the coil insulation can affect this generated heat. Further exacerbating the heat generation and removal from stators in flywheel power sources is that most designs place the stators inside the vacuum container for efficient magnetic coupling. This makes cooling and heat transfer from the flywheel stator much more difficult. High temperatures in the motor/generator can also have an added negative effect on the flywheel system in that the outgassing in the vacuum is strongly related to temperature. Insulation materials, in particular, already exhibit high vacuum outgass rates even at room temperature. Substantial outgassing of stator winding insulation can degrade the vacuum and require costly vacuum renewal steps or shorten the operating life of the flywheel power source.
The second method to extend the life of the flywheel system motor/generator is to actively keep the stator cool. With high power flywheel systems, dissipation of up to several kilowatts of heat power can be necessary. To date, such systems have employed pumped cooling systems. This method is effective in removing the heat but the weak link of the system reliability and longevity becomes the pump. To remove the heat, the liquid is also pumped outside the vacuum chamber. This requires the use of expensive, complicated and potentially unreliable fluid connections and fluid feedthroughs. One such flywheel system using forced liquid convection cooling is shown in FIG.
1
. The flywheel power source
30
is comprised of a high speed flywheel
31
that rotates inside an evacuated chamber
35
within a container
34
to reduce aerodynamic drag. Flywheels can be constructed of composite materials such as carbon fiber/epoxy or of metals such as steel. As shown, the flywheel
31
is constructed with a carbon fiber epoxy rim
32
mounted on a solid metal hub
33
. The flywheel
31
has upper and lower shafts
36
and
37
for journaling the flywheel and for attachment of a motor/generator
40
. The flywheel is supported for rotation using upper and lower bearings
38
and
39
. Flywheel systems typically employ mechanical bearings, magnetic bearings or a combination of the two types.
The motor/generator
40
is comprised of two portions: a rotor
41
that is attached to the flywheel, and a stator
42
that is stationary. The rotor can be a reluctance type motor/generator gear or alternatively permanent magnet type as shown. Any type of motor/generator can be used as long as it does not require brushes that wear. The rotor
41
is surrounded by and cooperates with the stator
42
. The stator contains internal electromagnetic coils (not shown) for electrical-mechanical energy conversion with the rotor
41
. In this case, the stator also includes internal laminations (not shown) for efficient completion of the motor/generator magnetic circuit. The stator
42
, which is sealed inside the vacuum
35
to prevent a loss of vacuum, is cooled by pumping coolant through it. The coolant enters and exits the stator
42
and vacuum container
34
through inlet and outlet feedthroughs
43
and
44
. A separate, and in this case external, pump and or cooler pumps the fluid. The forced convection from fluid is a highly effective method for removing the heat from the motor/generator. Unfortunately as described previously, this type of flywheel system has reduced reliability, and is complicated and expensive.
SUMMARY OF THE INVENTION
The inv
Campbell David R.
Gabrys Christopher W.
Indigo Energy, Inc.
Mullins Burton
Neary J. Michael
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