Electrical transmission or interconnection systems – Plural supply circuits or sources – Substitute or emergency source
Reexamination Certificate
2001-06-21
2004-11-16
Toatley, Jr., Gregory J. (Department: 2836)
Electrical transmission or interconnection systems
Plural supply circuits or sources
Substitute or emergency source
Reexamination Certificate
active
06819012
ABSTRACT:
This invention pertains to a flywheel energy storage system and more particularly to an energy extraction method and circuit that provides increased energy delivery capacity for a flywheel system by having a higher output efficiency and simultaneously has a longer life and higher reliability.
BACKGROUND OF THE INVENTION
Flywheel uninterruptible power supplies have emerged as an alternative to electrochemical batteries for prevention of power interruptions to critical loads. Electrochemical batteries used in these applications, in particular, valve regulated lead acid batteries, have many undesirable traits. The life of batteries is short, typically between 1 to 7 years depending on the environment and use. They require periodic maintenance and inspection are subject to thermal degradation and can fail unpredictably when required. Lead acid batteries and other types as well are also not environmentally friendly. However, lead acid batteries are relatively inexpensive. Flywheel systems show promise to eliminate the disadvantages of batteries with the expectation of achieving 20 year lives with minimal or no maintenance, temperature insensitivity, previously unachievable reliability while being environmentally benign.
A flywheel energy storage system is shown in
FIG. 1. A
high-speed flywheel
12
stores electrical energy in the rotating inertia of a flywheel. Flywheels can be either constructed of metal or of composite materials. The flywheel is supported for rotation using upper and lower bearings
14
and
15
. The flywheel can be supported on mechanical bearings, magnetic bearings or a combination. An attached motor/generator
16
is used to accelerate and decelerate the flywheel
12
for storing or retrieving energy. Many designs of motor/generators exist and can be employed. Motor/generators can also be made as separate components. To reduce the losses from aerodynamic drag, the interior
13
of the housing
11
surrounding the flywheel
12
is maintained at a low pressure, or for slower flywheels it can be filled with a gas of small molecule size such as helium. The flywheel mechanical unit
17
is electrically connected for operation and conversion of power. Typically, utility power
21
is taken is taken in for input conversion
20
and power is supplied to a critical load
22
through output conversion
19
. A system control
18
provides control for the system
10
.
Regardless of the physical design employed, it is desirable for the flywheel system to both maximize its stored energy capacity and also to maximize its operating life. Such capability can offset the higher initial cost of the flywheel system over batteries by actually becoming cheaper when considered over the system life. One element of flywheel uninterruptible power supplies that deserves particular attention is the power system electronics. Designing electronics for an operating life that is preferably greater than 10-20 years without failures is challenging. Likewise, increasing the efficiency of the power system is preferable for allowing more of the stored energy of the flywheel to be delivered to the load.
One power system configuration that has been used with previous flywheel uninterruptible power supplies is shown at
30
in FIG.
2
. The power system
30
takes in utility power
31
and supplies protected direct current power at the output
32
. For many telecommunications systems such as telephone and wireless, the output voltage
32
required is −48 volts or 24 volts. For other applications, such as high power ride-through for data centers or critical manufacturing, the input and output voltages would be increased. The input power
31
is rectified to a DC bus
34
using a rectifier
33
which can be either controlled or uncontrolled. The DC bus
34
supplies power to a PWM (pulse width modulated) inverter
35
also known as a servo amplifier. The servo amplifier
35
converts the DC current in the bus
34
to synchronous alternating current
36
that provides power to accelerate the flywheel
37
to normal operating speed. When the utility power is operating normally, the DC voltage in the bus
34
is converted to the output voltage
32
using a DC-DC converter
38
. During an interruption in the utility power
31
, energy from the rotating flywheel supplies power to the output
32
by providing power to the DC bus. The inverter provides power to the DC bus instantly and automatically when the utility power is discontinued by antiparallel diodes included with the H-bridge, not shown, inside the inverter. Power automatically flows back and is rectified to the DC bus
34
whenever the generator voltage is greater than the DC bus. As the flywheel speed slows, the voltage to the DC bus
34
drops. The output DC-DC converter
38
maintains the constant output voltage
32
during discharging of the flywheel. The output power can alternatively be alternating current power simply by replacing the output DC-DC converter
38
with an output DC-AC converter or inverter.
Current designs of DC-DC converters typically have efficiencies of between 75-90%. The less than perfect efficiency means that all of the energy stored in the flywheel cannot be effectively used. Such levels of efficiency are acceptable for many applications, however the attention given by customers and potential customers to the cost of energy storage capacity of energy flywheel uninterruptible power supplies, achieving higher efficiency is desirable. The mean time between failure for many converters is only about 12 years, which means a significant portion of flywheel systems will fail before the end of desired operating life.
A second configuration of power system for a flywheel uninterruptible power supply, shown at
40
in
FIG. 3
, is similar to that shown in
FIG. 2
, supplying protected DC output power
42
from utility power
41
. The input power is rectified to a lower voltage DC bus
44
using a switched mode rectifier
43
. The output power
42
is supplied directly from the DC bus
44
. The DC bus is also connected to a PWM inverter
45
that generates synchronous AC to power lines
46
to accelerate the flywheel motor/generator
47
to fully charged operating speed. During an interruption of primary power
41
, the servo amplifier
45
supplies power to the DC bus
44
and output
42
. The output is maintained at a constant voltage during the slowing of the flywheel by using fourth quadrant regenerative operation of the servo amplifier
45
. Fourth quadrant operation uses switching with the internal inductance and capacitance to actively decelerate the flywheel. By actively decelerating the flywheel, the DC bus voltage is boosted to a higher voltage than the generator voltage. The output voltage is thereby maintained constant without the use of an output converter. Unfortunately, as the flywheel slows, the generator voltage drops and the generator current can become excessive if discharged to low speeds as desired for extraction of most of the energy stored in the flywheel. The servo amplifier
45
also operates similarly, having comparable efficiency as a DC-DC converter.
Another method used in previous flywheel systems to maintain a constant output voltage as the flywheel speed is decreased is to use a motor/generator with an external field coil. The field coil is used to create all or some of the magnetic flux of the generator. As the speed decreases, the power to the field coil is increased such that more flux is created, increasing the generator voltage. Some designs use both permanent magnets on the rotor in conjunction with the flux from the field coil. In either case, the requirement of power to generate all of the generator flux or to generate a significant portion of it as would be required for extraction of most of the energy in the flywheel is less efficient than a permanent magnet motor/generator that has a high magnetic flux without the use of any power. Field control designs also have increased areas for hysteresis and eddy current losses due to moving steel portions. Likewise, field control motor/gene
Neary J. Michael
Toatley , Jr. Gregory J.
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