Power Electronics

Hybrid Flywheel System Extends UPS Run Times with Batteries

A hybrid UPS with a fully integrated energy storage system provides power loss protection by combining flywheel and battery technologies. The VDC XEB supports connected loads with clean backup power that is predictable and seamless.

Typically, most all-electronic uninterruptible power supplies (UPSs) will handle loads in the tens of kVA range. Handling hundreds of kVA requires an electromechanical UPS, like a flywheel-based unit. A drawback to the flywheel-based UPS is that it can only provide run times less than one minute. However, you can extend flywheel UPS run times to minutes with the addition of a battery and its associated power control subsystem. That’s exactly what VYCON did when it added batteries to its existing flywheel UPS, yielding the hybrid VDC XEB.

Fig. 1 is a diagram of the VDC XEB. The Automatic Transfer switch normally applies AC utility power to the UPS and activates the Backup Generator when necessary. Under normal conditions, the AC-DC Rectifier supplies DC to the Inverter that provides AC to the load. If there is a short power interruption, the Inverter receives its DC input from the motor/generator that drives the flywheel. If the interruption appears to be longer, the system applies the battery input to the Inverter. The transition to the battery is handled smoothly and seamlessly by the Power Control unit. The Bypass Switch eliminates the UPS and applies power directly to the AC Load; normally this is a maintenance function.

During typical power events, the flywheel alone absorbs 98 percent of power anomalies. For occurrences requiring extra runtime, the VDC XEB intelligently inserts the battery for continuous protection of critical loads. This reduction in battery usage results in fewer battery replacements and extends their useful life.

A contact-free magnetic levitation system and a superior touch-screen control system provides vital information on system performance. Therefore, the VDC XEB can charge and discharge at high rates for countless cycles without degradation throughout its 20-year rated life. And, the combination of the flywheel and batteries dramatically reduces the system’s footprint-saving valuable space for other computing assets.

This power protection system combines the reliability of flywheel energy storage with an advanced Absorbed Glass Mat (AGM) battery to address the needs of users who may require added power backup and redundancy. Adding the battery extends run time from seconds to minutes. The flywheel manages a smart “walk-in” of the battery, eliminating a harsh step-load demand on the battery, which extends its life and improves overall system availability.

The VDC XEB also incorporates smart monitoring of both the flywheel system and batteries. Key parameters of the system are actively monitored and metered to provide real-time status and event recording.

Flywheel Technology

A flywheel energy storage system for high power, short discharge applications is a “mechanical battery” that stores energy kinetically in the form of a rotating mass (the flywheel). When required during a utility outage, the energy stored by the rotating mass converts to electrical energy through the flywheel’s integrated electric generator. With today’s -motor/generator technology, highly efficient flywheel systems can provide consistent, dependable energy for a variety of computer-based systems.

Main components of this flywheel system are:

  • Flywheel
  • High-speed permanent magnet motor/generator
  • Active magnetic bearings
  • Rotor assembly

Energy stored in a flywheel is:

Energy = kMω2 (1)


k = Constant depending on shape of rotating mass

M = Flywheel mass ω = Angular velocity

As seen in Equation (1), flywheel energy is proportional to the square of the angular velocity. That is why the technology trend line is to use a flywheel to store energy at high RPMs. This allows systems with lower weight, smaller footprints, and allows use of full magnetic levitation of the flywheel mass.

The VDC XEB’s flywheel (Fig. 2) employs a steel mass for storage. Steel offers a well-understood, well-supported material that offers virtually no technology or availability risks. The VDC XEB utilizes standard, aerospace grade 4340 steel. The material properties are very well known, available from numerous suppliers and this material is used in many high speed rotating applications. Most important is the integrity of the material can be measured through core samples and ultrasound to assure it complies with the application specifications.

The overall construction of the rotor assembly is a key form of intellectual property, which incorporates steel and magnet materials that upon assembly, behave both mechanically and rotor-dynamically as a single mass. With this design, the rotor assembly is able to maintain balance and structural integrity as it spins through various critical modes up to its maximum spinning speed.

This flywheel can continuously cycle at rates up to once every minute (e.g., 12 seconds discharge, 18 seconds idle, 12 seconds recharge, 18 seconds idle), and even higher for short durations with longer idle periods. Additionally, the motor produces minimal iron losses in the rotor assembly and therefore minimal heating. Other motor technologies, such as switched reluctance machines generate significantly more heat during a discharge, and have difficulty transferring heat from the rotor to the housing due to the vacuum in which the rotor is operating. Taking advantage of this cycling ability, this flywheel system lends itself to applications including rail stations, shipyard cranes, and wind power correction among many others.

Typically, carbon fiber flywheels utilize miles of carbon fiber that is wound on a spindle with an epoxy resin. Imperfections in the process and gaps between the fibers could lead to an “unbalancing” of the wheel over time due to the stresses applied as the wheel is spun from high RPM to low RPM and back again, which occurs during every discharge event. Once the carbon fiber flywheel becomes unbalanced, the entire flywheel module must be replaced - a very costly and time consuming process.

Other flywheel manufacturers use a synchronous reluctance motor that cannot self generate power if a failure occurs in the power electronics. Thus, it requires a back-up supply from a small UPS to provide power to the magnetic bearings if this occurs. In addition, if all power is lost to the magnetic bearings or if a failure occurs to the magnetic bearing controller, of other high-speed flywheels, the rotating flywheel will go on to the touch down bearings at full speed. The flywheel will come to a stop, but it will no longer be suitable for use, it must be replaced completely. In contrast, back-up ceramic bearings can support several drops at full speed and recover normally.

Vycon’s flywheel technology has been used not only in UPS applications, but also in high-cycling, regenerating applications like in electric motors for cranes and electric rail. These applications call on the flywheel to be charged and discharged sometimes 20 times per hour. These applications prove the robustness of utilizing aerospace-grade steel as the preferred flywheel material.

The High Speed Permanent Magnet Motor/Generator design utilizes rare earth magnets to minimize rotor heating and maximize efficiency and reliability. This allows these systems to cycle quickly without overheating, and can therefore be used in difficult applications with high cycling and long life requirements. The motor/generator is rotated at speeds up to 36,750 RPM where the flywheel system will be at a fully charged state. During discharge, the rotor speed decreases to a minimum speed, typically 10,000-12,000 RPM. This speed range is called the discharge range and can be adjusted for more energy or higher cycling depending on the application. The rotor assembly of the flywheel operates in a vacuum provided by an external vacuum pump. Removing air from the rotating area of the motor eliminates all windage losses from the system, thereby increasing electrical efficiency.

The benefit of a permanent magnet type motor generator is twofold:

  • Higher efficiency of the motor generator when charging and discharging allows the high cycling capability of the flywheel.
  • The flywheel can generate its own power to maintain the flywheel levitation even if control power is lost or a failure occurs in the power electronics.

Magnetic Bearings

Magnetic bearings allow the motor rotor assembly to rotate at very high speeds with no physical contact to stationary components, thereby taking advantage of the high efficiencies obtainable with high-speed rotation. Magnetic bearings levitate the rotating assembly through the force of a magnetic field. The magnetic bearing design is based on a combination of permanent magnets, which provide a bias field in the gap, and controlled electromagnets that provide the adjustment and centering of the rotor assembly. The position of the rotor assembly is controlled by the magnetic bearing controller, which feeds rotor position information from position sensors next to the magnetic bearing actuator. The controller in turn adjusts the current into each coil to reposition the rotor within an allowable orbit. By using this patented magnetic bearing technology, the need for maintenance is virtually eliminated as there is no contact points within the flywheel -- no bearings to replace or repack with lubricant. This allows a flywheel with a 20-year life with no bearing maintenance required.

This proprietary design of motor and system controls provides high efficiency and maximizes reliability over the flywheel operating life. The flywheel system controller monitors the performance of the flywheel itself, providing a unique control package that can adjust flywheel power levels as required, prevent failures through warning systems and provide users with discharge data and all operating parameters. Controlling the flywheel rotor, the magnetic bearing control system provides continuous status of operational data such as rotating speed, internal temperature and rotor positioning. With self-diagnostic tools, the flywheel system can proactively prevent failures. For each application, flywheel rotational speed limits are modified for the cycling demands and other specific conditions.

Control panel design provides the user with a graphical description of flywheel storage in real time. The screen provides rotor speed, charged capacity, discharge event history, adjustable voltage settings, RS-232/485 interface, alarm status contacts, soft-start pre-charge from the DC bus and push-button shutdown. Available options include DC disconnect, remote monitoring, Modbus, and SNMP communications and real-time monitoring software.

Minimum Maintenance

The VDC XEB requires minimal maintenance. Because the flywheel utilizes a full levitation system with no mechanical bearing, there is nothing in the flywheel module that needs to be maintained. The unit also utilizes a “medical grade” vacuum pump to maintain a high vacuum level in the flywheel chamber. Because all materials located in a vacuum environment “out-gas” vapor molecules over time, these vapor molecules must be captured and removed to keep the vacuum at the appropriate level in the flywheel chamber. The VDC XEB gathers these vapor molecules in the mineral oil of the vacuum pump. Usually, this mineral oil needs to be replaced after about one year of operation, as the mineral oil has absorbed these contaminants. Replacing the mineral oil “regenerates” the vacuum. The device’s vacuum regeneration procedure takes about 10 minutes to perform and the unit does not have to be taken off-line, thus is available to function if called upon. Because the “out-gassing” rate decreases over time, the frequency of regenerating the vacuum by changing the oil is less. The estimated life of the vacuum pump itself is approximately 8-10 years.

The VDC XEB flywheel is rated at 300kW and, for example, can provide 40 seconds of run time at 100 kW. For higher power requirements and/or longer durations, multiple flywheel units can be run in parallel.

The VDC XEB has a wide operating voltage range. It is UL listed for 400 to 600 VDC, as limited by UL standards, but it can connect at voltages up to 850 VDC.

The sound level from the flywheel module itself is less than 45dBA. Unlike other high-speed flywheels that must be liquid cooled with a water pump, a radiator, fans and the associated plumbing, the VDC is forced air-cooled. The audible noise level when the fans are in operation is approximately 68 dBA at a distance of 3.3 feet.

The VDC XEB uses a bi-directional power converter to either extract or supply energy to the DC bus. The Power Conversion Module uses high-power IGBTs and PWM (Pulse Width Modulation) technology.

Operating Parameters

Operating parameters for the VDC XEB flywheel include:

  • Operating temperature is 4°F to 104°F (-20°C to +40°C) without derating.
  • Voltage Discharge is 380 to 540 VDC
  • Flywheel rotational speed is 14,000 (fully discharged) to 36,750 (fully charged) rpm.
  • At a max power rating of 300kW, the efficiency of the VDC is 99.2% energy efficient.
  • Sealed lead-acid battery (AGM technology) has 10-year rated life at 77°F (full float).
  • DC output ripple is less than 2%.
  • VDC XEB run time is 2.9 minutes with a 300 kVA load at 0.9 power factor

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