Electricity: single generator systems – With flywheels or massive moving parts
Reexamination Certificate
2002-10-30
2004-09-07
Lam, Thanh (Department: 2834)
Electricity: single generator systems
With flywheels or massive moving parts
C322S025000, C322S027000, C318S161000, C307S067000
Reexamination Certificate
active
06788029
ABSTRACT:
This invention pertains to flywheel energy storage devices and more particularly to a flywheel device with an output regulator that produces a direct current output to a load, and maintains the direct current output voltage within an allowable range by switching the number of windings that couple to the load as the flywheel slows during a discharging. The invention provides for both higher efficiency, economy and reliability by eliminating the need for a high frequency output switching conversion.
BACKGROUND OF THE INVENTION
Flywheels have emerged as a very attractive energy storage technology for such electrical applications as uninterruptible power supplies, utility load leveling systems, alternative energy generation, satellites and electric vehicles, Flywheel systems convert back and forth between electrical energy and the rotational energy of a spinning flywheel. A flywheel energy storage system includes a flywheel, a motor and generator, a bearing system and a vacuum enclosure. The rotating flywheel stores the energy mechanically; the motor and 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 operation in air at atmospheric pressure.
One typical requirement in the design of flywheel energy storage devices is to provide a near constant output voltage in order to power an electrical load as the flywheel speed slows during discharging. Unfortunately, as the speed of a flywheel slows, the voltage generated for a given generator field strength diminishes. For permanent magnet motor/generators, the field strength is constant so the voltage generated is directly proportional to the speed. Thus, if the flywheel slows to one quarter its full speed, the output voltage drops by a factor of four. Accordingly, the manufacturers of flywheel energy storage devices have used several methods for providing a near constant output voltage.
One such method has been to use an alternator type generator that uses a field coil for generation of the operating magnetic field. As the flywheel slows, the generator maintains a nearly constant output voltage by simply increasing the current to the field coil. This method is very simple, however it does have some drawbacks. The use of an electrically generated field requires a constant power draw and also potentially a magnetic circuit with higher magnetic losses from eddy currents and hysteresis depending on the design. These reduce the efficiency and require use of a larger flywheel if the expected discharge period is lengthy, up to several hours. Other potential drawbacks of alternator generators are significantly larger and heavier construction, smaller magnetic air gaps and generation of higher magnetic destabilizing forces that can make implementation of magnetic bearings, if used, more difficult.
Another prior art approach to the problem is the use of permanent magnet motor/generators with electronic switching conversion. Permanent magnet motor/generators, using permanent magnets to generate the magnetic field for operation, typically offer the highest efficiencies. Unfortunately, as previously explained, the output voltage from the generator falls as the flywheel speed slows. Electronic switching conversion can be used to provide a constant output voltage. One such prior art electronic switching conversion arrangement, shown in
FIG. 1
, is a power system
30
for providing back-up power to a protected load
29
from a flywheel energy storage device using an output DC-DC converter. The power system
30
has input power lines
31
energized from a source of power
39
, such as the power grid, and output power lines
32
to the protected load
29
. Input power in lines
31
is rectified by a rectifier
33
and provided to a DC buss
34
. Power from the DC buss
34
is then provided to the output load
29
via output lines
32
through use of a DC-DC converter
38
. Typical DC-DC converters chop the DC input
34
by switching, and put it back together as a regulated DC output power on lines
32
. Switching of converters usually occurs at high frequencies, around 20 kHz.
In the event of loss of the primary input power from the source
39
, back up power to the protected load
29
, is provided by a flywheel motor/generator
37
driven by a flywheel in the flywheel energy storage device. A motor drive
35
connected to the DC buss
34
converts the DC to synchronous AC in lines
36
to energize the motor/generator
37
to accelerate the flywheel to its normal operating speed. When primary power in the line
31
is interrupted, the motor drive
35
instantly and automatically supplies power back to the DC buss
34
by rectifying the motor/generator AC power in lines
36
. The power provided to the DC buss
34
falls as the flywheel speed is slowed. However, the DC-DC converter
38
converts the varying DC buss voltage
34
to a constant DC output in the lines
32
. A special wide range DC-DC converter can be used to provide constant output voltage
32
during the entire useable flywheel discharge. Unfortunately, switching DC-DC converters typically have efficiencies that range from 75-90% efficiency. Even if the motor/generator has high efficiency, significant energy is lost in the output switching conversion to maintain the constant voltage. A second drawback of power systems with conventional converters is that the high frequency switching reduces the life of the electronics, which can limit the life of the flywheel energy storage device.
A second method for providing a constant output voltage while using a permanent magnet generator is to operate the motor drive in the fourth quadrant. A power system for a flywheel energy storage device using fourth quadrant power conversion of the motor drive inverter to provide output power is shown in FIG.
2
. The power system
40
is comprised of a rectifier
43
that rectifies input power delivered from an input power source
41
, such as a power grid, over lines
48
, and supplies DC power to a DC buss
44
, which is also the output to the load
42
. Back up power is supplied through use of a flywheel motor/generator
47
. A motor drive
45
, connected to the DC buss
44
, converts the DC power to synchronous AC to accelerate the flywheel motor/generator
47
to normal operating speed. During an interruption of primary power
41
, the flywheel motor/generator
47
supplies the output power to the load
42
via lines
46
by reverse conversion from the motor drive
45
. The motor drive
45
is a capable of fourth quadrant operation and hence it can actively slow the flywheel motor/generator
47
and in doing so, it can provide a constant and higher output voltage
42
than the back emf from the motor/generator. High frequency switching similar to that which is employed in the power system of
FIG. 1
is used. Unfortunately, this power system
40
also suffers from similar power losses due to the high frequency switching and has the same life limitation considerations.
Thus, it would be very desirable to have a flywheel energy storage device with a power device that can employ a permanent magnet motor/generator and supply useable DC output power with high efficiency.
SUMMARY OF THE INVENTION
The invention provides a flywheel energy storage device with an output regulator that supplies direct current power to a load with high efficiency. The output voltage tolerated by many loads can be allowed to vary substantially, although not as much as would be encountered over an entire discharge from a flywheel with direct permanent magnet generator output. In telecommunications, one promising applications for flywheels and in many cases an application with a longer term discharge period, the voltage used by downstream equipment has an allowable range. Many DC telecommunications equipment for power of phone lines, wireless, Internet, etc., have embedded DC-DC converters. The DC-DC converters are provided so that the
Lam Thanh
Neary J. Michael
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