When it comes to protecting critical operations from power failures, one can never be too prepared, especially with a world that requires 24/7 operations and availability. By definition being properly prepared for a disaster — whether natural or manmade — means having reliable backup power protection so that operations continue to run smoothly. Every mission critical operation has some sort of power backup in place including uninterruptible power systems (UPSs) and generators. So, it’s not a matter of having backup power, but rather incorporating the right technologies to increase protection levels while at the same time decreasing energy costs and carbon footprint. A word to the wise: not all power protection is created equal. According to a number of published studies, the number one cause of a UPS failing to support the load is battery failure.

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Battery Life
Fly Wheels
Variable Speed Drive
Proper Sizing


The traditional means of guaranteeing a continuous power supply has been the use of lead-acid batteries to bridge the gap between the utility outage and the standby generator start. Although a tried-and-tested solution (no one ever got fired for purchasing batteries), batteries come with a heavy environmental cost. They also require continual and expensive maintenance, are heavy, take up a lot of real estate inside a facility, and are slow to recharge. Battery life is a constant concern (Figure 1), as most battery manufacturers state that battery life can be maintained as advertised for at least four years if, and only if, they are kept at a constant 75°F (requires air conditioning) and experience no excessive cycling.

When one considers the nature of the chemicals used in the batteries, this situation is clearly not ideal. Valve regulated lead-acid (VRLA) batteries recombine hydrogen and oxygen gases given off during charging. The battery’s electrolyte is suspended in either an absorbing glass mat (AGM) or a gel substance (GEL). Due to the chemical reactions during operation, every time the battery is cycled, it diminishes its life. This continual chemical process is problematic when needing a dependable source of power backup. Most batteries for UPSs are arranged in strings and it can take only a single dead cell in a string of typically 240 cells to render the entire string unusable.

UPSs themselves are very dependable; however their batteries are their Achilles heel. The leading cause of a UPS failing to support the load is battery failure. Battery life and availability is impacted by number of times cycled, temperature they are operating, and the level and interval period of maintenance. This is where flywheel energy storage technology comes in.


Spinning energy. The flywheel works like a dynamic mechanical battery that stores energy kinetically by spinning a mass around an axis (Figure 2). Electrical input spins the flywheel rotor up to speed, and a standby charge keeps it spinning 24/7 until called upon to release the stored energy. The amount of energy available and its duration is proportional to its mass and the square of its revolution speed. For flywheels, doubling mass doubles energy capacity, but doubling rotational speed quadruples energy capacity:

E = kMv2

k– Depends on the shape of the rotating mass

M – Mass of the flywheel

v– Angular velocity

In research conducted by the Electric Power Research Institute (EPRI), they concluded that 80% of all utility power anomalies/disturbances last less than two seconds and 98% last less than ten seconds. In the real world, the flywheel energy storage system has plenty of time for the automatic transfer switch (ATS) to determine if the outage is more than a transient and to start the generator and safely manage the hand-off.

From 40 kVA to multi megawatts, flywheel systems are increasingly being used to ensure the highest level of power quality and reliability in a diverse range of applications including harsh loads in industrial applications such as variable-speed drives (VSDs).

During a power event, the flywheel is the primary (first) source of backup power. During a power disturbance or complete power failure from the grid, the flywheel will discharge and immediately support the load until the standby gensets are started and online, typically 20 to 30 seconds.

For applications where longer runtime is required such as synchronizing multiple generators or when input power failure is frequent, the combination of a battery and flywheel (battery hardening) can help improve battery life and UPS system reliability. Additionally, a “walk-in” of the battery is managed by the flywheel, eliminating a harsh step-load demand (Coup de fouet or Whiplash) on the battery, thereby removing one more factor of battery life reduction.


VSDs control the speed of motors, the flow of liquids through pumps, and flow of air in air conditioning/cooling equipment to eliminate the energy waste that is common when operating these devices at a fixed speed. The beauty of  VSD is that they can significantly reduce operational costs by as much as 50%. A small reduction in speed can make a big difference in energy consumption. For example, a centrifugal pump or fan running at 80% speed consumes only half as much energy as a unit running at full speed. This is because the power required to operate a pump or fan changes with the cube of the speed. When there is a power fault, the drive’s control circuit may not be able to handle such an event; a minor voltage sag on the control circuit can cause shutdown.

For critical VSD applications, UPS with batteries were typically placed in front of the VSD to provide backup power. However, using a UPS in this application adds 6% to 8% of inefficiencies due to losses from conversion electronics (AC to DC and back to AC) reducing the efficiency gains from the VSD. UPS batteries are another concern here due to their reliability issues. Interestingly, VSDs have the same rectifier, DC bus, and inverter components as a UPS, therefore flywheels can be placed directly on the VSD instead of requiring the added expense and complexity of placing a UPS in the front of the drive. The flywheel can provide backup power as well as regenerative energy absorption offering a very efficient (99.3%) power solution. Another advantage to using flywheels is that many VSDs are located in harsh environmental environments. High operating temperatures are not a problem as flywheels can operate up to 40°C (104°F).


Normally the sizing of UPSs and flywheels are done based on actual load. Most engineers size the UPS at 30% to 40% larger than the actual load to allow for growth. Once the UPS is sized, the flywheel needs to be sized to the UPS. All UPS ratings are based on kVA and kW numbers, the rating used for power applications is the kW rating. When this kW number is established, this will be labeled as the full load kW rating. For example: 500kVA @.9 power factor (pf) converted to kW (500kVA x the pf (.9) = 450 kW). Additionally, the UPS (inverter) will have efficiency losses as it converts DC to AC power. The inverter efficiency must also be accounted for to determine the total output power the flywheel must support. Using an inverter efficiency of 96% (4% losses), the DC flywheel runtime will be calculated based on 450 kW (UPS at 100% load) plus 4% (inverter losses) or 450 kW plus 19 kW for a total of 469 kW. The kW is the rating used to size the flywheels to assure proper power rating and proper amount of run time requirement. Typically, the flywheel provides 20 to 30 seconds of run time, allowing plenty of time for the generator to start and become the input source during an outage.

The flexibility of flywheels allows a variety of configurations that can be custom-tailored to achieve the exact level of power protection required by the enduser based on budget, space available, and environmental conditions. In any of these configurations, the user will ultimately benefit from the many unique features of flywheel-based power protection systems, including:

• No downtime for regular maintenance (no bearings to replace)

  • High-power density, small footprint
  • Scalable / parallel capability that allows for future expansion
  • Fast recharge (under 150 seconds)
  • No special facility requirements
  • No special / additional cooling required
  • Low maintenance
  • 20-year useful life
  • Simple installation
  • N+1 redundancy options
  • Quiet operation
  • Wide temperature tolerance

Flywheels can operate in harsh environments with no requirement for cooling and they occupy very little space. Moreover, there’s no need for dedicated HVAC and personnel safety equipment. Also, today’s flywheels have very high uptime availability due to their extremely infrequent maintenance needs. Front access to the flywheel further eliminates space issues and opens up installation site flexibility in support of future operational expansions and re-arrangements.

Flywheel implementations comply with the highest international standards for performance and safety, including those from UL and CE. They also incorporate a host of advanced features such as smart monitoring and touch screen control.


Protecting critical systems against costly power outages in an energy efficient, environmentally friendly way, all while providing a low total cost of ownership, has become a priority as electrical engineers and consultants consider the various technologies, cost, and environmental impact.

Double-conversion UPSs paired with flywheels is the next step in greening the power infrastructure.