Revolutionizing Power Backup: Exploring the Advantages of Flywheel UPS Systems
Today’s electrical power systems are plagued by a variety of disturbances ranging from short-duration sags, swells, and transients to long-term interruptions. These problems can be caused by motorized equipment within the plant starting and stopping, internal or external faults, or thunderstorm and lightning effects on the utility lines. In addition, power utilities in many areas of the country are operating at or near capacity, resulting in frequent “load shedding” (rolling blackouts).
Studies have shown that approximately 85% of all power events are voltage sags lasting less than 2 sec, with many under 1 sec. A study by Bell Labs found that 87% of downtime is caused by disturbances lasting no more than 0.5 sec.
Aggravating the problem is extensive use of electronic drives on large motors, preventing them from helping to maintain the bus voltage during disruptions. Although electronic systems such as computers are highly susceptible, even electromagnetic contactors can be affected. A disturbance of only a few milliseconds duration can cause the shutdown of one or more individual machines, or an entire plant through a chain-reaction. The resulting cost to a company from downtime, product loss, and equipment damage can be hundreds of thousands of dollars.
Traditional UPS systems
To avoid these problems and ensure uninterrupted operation, more and more facilities are installing some form of uninterruptible power supply (UPS). Long-term needs are typically served by engine-generators. While they can provide power for an unlimited time as long as fuel is available, they require up to several minutes to start and synchronize, making them ineffective in handling the most prevalent short-duration disturbances.
For decades, electrochemical battery UPS systems have been used to handle short disruptions in power. These consist of large banks of rechargeable batteries, a rectifier/charger which provides dc charging current to the batteries when power is available, and inverter electronics that convert the dc voltage to ac to feed the critical load bus when needed. These systems are online and switch to battery power in a matter of milliseconds, and can provide power for up to several hours, which is known as ride-through. While battery systems can provide relatively large amounts of energy, they suffer from several disadvantages including large space requirements, high maintenance, environmental issues, and a limited charge/discharge cycle life.
So is there an option? It would appear that today the answer is yes.
New technology or old idea?
Since the earliest pottery wheels, people have known about the ability of a flywheel to store energy. This fact has been put to use in many machines, from the first steam engine to modern engines of all types, where flywheels provide smooth shaft rotation by eliminating engine pulsation. Sir Isaac Newton formalized this effect in his first law of motion, which states that a mass will tend to maintain its velocity unless acted upon by an external force.
This inertia principle has been used since the 1970s in rotary UPS systems, which consist of a conventional motor-generator with a flywheel installed on the shaft. In operation, the motor takes power from the supply bus, and the generator is always supplying power to the load. During brief interruptions, flywheel energy is used to maintain rotation of the generator and provide uninterrupted power. The rotational speed and generator frequency decay rapidly due to bearing and air friction and the loss of kinetic energy as it is converted to electrical energy. As a result, these systems can recover only a relatively small percentage of the available flywheel energy and are limited to short duration ride-through. They can, however, supply large amounts of power, making them useful for handling short power events on large systems.
The new breed
Today there is a new generation of flywheel UPS systems, known by various names including kinetic battery, electromechanical battery (EMB), or flywheel energy storage system (FESS). They use high-speed flywheels rotating on extremely low-friction bearings in a near-perfect vacuum. They can store large amounts of energy and then deliver it within a few milliseconds when needed.
How they work
Most modern high-speed flywheel energy storage systems (FESS) consist of a huge rotating cylinder supported on a stator (the stationary part of a rotary system) by magnetically levitated bearings. These bearings are permanent magnets that support the weight of the flywheel by repulsion forces and are stabilized with electromagnets.
FESS utilise kinetic energy stored in a rotational mass with low frictional losses. The kinetic energy of the system is described by the equation
Ek = ½ Iω2
where I is the moment of inertia and ω is the angular velocity of the rotating disc; when ω or I increases, the energy of the system increases.
Electric energy input is used to accelerate the rotor up to speed using the built-in motor-generator; the inertia allows the rotor to continue spinning and the resulting kinetic energy is converted to electricity. Energy is discharged by drawing down kinetic energy using the same motor as a generator.
Sophisticated control systems monitor all aspects of operation of the UPS. Flywheel sensors supply information about its rotation, vibration, bearing temperature, and other parameters that provide an early warning of any pending failure.
Key Benefits and Features
- Extended Ride-through up to 2 minutes
- Up to 98% efficient
- Half the space of legacy battery-based UPS
- Field expandable
- Redundant fans and control power units
- Lower cooling requirements
- Lower maintenance and service
- Cost-effective installation
- Color LCD touch-screen display
- Remote monitoring
- Built-in power factor correction
- Generator compatibility
- Dual input and integrated maintenance bypass option
- Seismic provisions – consult factory.
- 20-year design life
- 225kW building blocks expandable to 1.8MW
Flywheel energy storage offers a more sustainable and battery-free UPS solution. As an environmentally friendly, space-saving, and lower total cost of ownership solution, flywheel technology is ideal for applications where no-break transitions to diesel generators or alternative electricity sources are required. This includes data centers, healthcare, industrial/process, broadcast, and campus applications.
