Today’s data explosion is driving data center operators to implement a variety of solutions to address the ever-increasing appetite for data. AI, telemedicine, self-driving cars, the IoT, and automation — not to mention financial transactions and social media — require increasing data capacity and bandwidth, along with massive energy consumption, to support them.

And now, due to the COVID-19 pandemic, consumers and businesses have quickly moved to data-intensive online meetings, streaming videos, and virtual events to maintain physical distancing. The demand for “always-on” connectivity and continuous content delivery is at its highest level in history, putting tremendous pressure data center infrastructure and personnel.

To this end, the need for reliable and scalable power protection has never been so critical. For large companies, the cost of an outage can reach millions of dollars per hour of downtime. In 2018, the U.S. Department of Energy (DOE) estimated outages were costing the U.S. economy $150 billion annually. Now, with the vast increase of remote workers, those numbers could be much higher.

UPS Systems and Energy Storage

UPS systems have come a long way in efficiency and scalability. While three-phase UPSs for data centers and other mission critical applications have advanced, the energy storage component requires special attention. When there is a power outage, strings of batteries — typically lead-acid type — provide ride-through to a second utility feed or a generator. The problem is that valve-regulated-lead-acid (VRLA) batteries need expensive cooling, ongoing maintenance, frequent replacement, and require a large amount of real estate as well as environmental mitigation and spill containment.

While lithium-ion (li-ion) batteries are an option offered by some UPS manufacturers, lead-acid batteries are still the de facto choice due to their lower initial cost, a widespread understanding of how they operate, required maintenance, and life span. Li-ion batteries do offer some key advantages over lead-acid, including the ability to operate in broader temperature ranges with thousands of charge-discharges cycles over their life span. However, fire safety precautions are a crucial consideration for data center operators. According to The National Fire Protection Association (NFPA) 855 standard, li-ion batteries must include an approved battery management system with thermal runaway management. In addition, there must be 3 feet of clearance around battery cabinets.

Greening the Infrastructure

Data center and facility managers must consider various factors in the quest to increase energy efficiencies and reduce carbon footprint. The question becomes how to implement green technologies without disrupting availability or increasing total cost of ownership (TCO).

This challenge becomes even more crucial when looking at power protection infrastructure. Data center operators need emergency power systems that are highly efficient and have reduced requirements for service, including the ability to conduct preventative maintenance without going into bypass or taking the UPS offline.  Many UPS systems offer flexibility and scalability, allowing users to add additional capability as the power load grows. However, most organizations today are looking to deploy sustainable solutions. A green appraoch to energy storage involves incorporating flywheels into power protection configurations.

Due to their proven reliability, low cost of ownership, small footprint, and favorable environmental characteristics, managers of data centers and other mission critical facilities are reaping the benefits of clean energy storage that flywheels offer.

Real-World Experience

When the UPS batteries at DataBank’s various Tier 3 data centers were coming to the end of their useful life, Danny Allen, DataBank’s vice president of engineering, wanted to find a more reliable, affordable, and sustainable approach to lead-acid batteries. Having previous experience with flywheels at DataBank’s other facilities, Allen was confident in his choice to deploy 36 Vycon VDC XXE 300-kW flywheel systems along with 12 VDC 450-kW XXT models. The flywheels (Figure 1) are paired with Mitsubishi 9900B-750kVA three-phase, online, double-conversion UPS systems.

“The dependable reliability and very low maintenance costs of the Vycon flywheels along with their green advantages make it an easy decision to replace lead-acid batteries,” Allen said.

Another example is Cavern Technologies that has its data center located 125 feet underground in a 3-million-square-foot facility. Fortified by a natural limestone bunker that is three times stronger than concrete, the data center is protected from natural and deliberate disasters, has a consistent 68°F ambient temperature, and is further reinforced by best-in-class biometric and facial recognition security systems. Cavern’s fully redundant infrastructure is designed to meet the specialized power, cooling, and security requirements its customers require to house IT systems that support their mission critical business processes.

During a recent expansion, Mike McDaniel, vice president of facility engineering for Cavern Technologies, was looking for a more reliable and lower maintenance energy storage solution than lead-acid batteries offer.

“All of our critical loads are now protected by four 2N-designed 300-kVA dual-conversion UPSs paired with four of Vycon’s 300-kW VDC flywheels,” said McDaniel. “We still have legacy UPSs that carry many of our customer’s loads, so this is only for customers online within the last two years. Going forward, all new customer loads will be on the flywheel design.”

Flywheel Technology

Designed for high power, short discharge applications, contemporary flywheel systems store kinetic energy in the form of a rotating mass. Magnetic bearings allow the motor assembly to rotate at very high speeds with no physical contact to stationary components, thereby taking advantage of the efficiencies obtainable with high-speed rotation. Vycon’s patented technology incorporates a high-speed motor generator along with active magnetic bearings that levitate and sustain the rotor during operation. A monitoring and control system monitors system operations and performance via a graphic touchscreen panel as well as an advanced communication capability that includes operating parameters, alarm status, and local and remote shutdown.

The flywheel operates like a dynamic (mechanical) battery that stores energy kinetically by rotating a mass around an axis. Electrical input spins the flywheel rotor up to speed while a standby charge keeps it going continuously until called upon to release the stored energy. To minimize bearing power requirements and losses, the flywheel is vertically oriented, thereby only requiring the axial bearing axes to support the full rotor weight. The energy available and its duration are proportional to the flywheel’s mass and the square of its revolution speed: doubling the mass doubles energy capacity, while doubling the rotational speed quadruples energy capacity.

Similar to a battery bank, the systems interface with the DC bus of a UPS, receiving charging current from the UPS itself and providing DC current to the UPS inverter during discharge. Because of this standard DC bus interface, the systems are compatible with most three-phase UPS brands.  The unique magnetic bearing technology eliminates virtually all maintenance, including the need to replace or repack lubricant for a mechanical bearing system. This enables the VDC flywheel to charge and discharge at high rates for countless cycles without degradation throughout its 20-year life.

During a power disruption, the flywheel will provide backup power instantly (Figure 2). When flywheels are used with UPS systems, they provide reliable protection against damaging voltage sags and outages. If a power event lasts longer than 10 or 15 seconds, the flywheel will seamlessly transition to the data center’s engine generator. For longer run times, additional flywheels can easily be integrated.

Jeff Filliez, supercomputer center manager at the University of California San Diego, replaced lead-acid batteries at the Supercomputer Center in a configuration that included a 1-MW 9900C Mitsubishi UPS paired with three 300-kW VDC flywheel units with N+1 redundancy.

“If the load is critical, you can’t repair it on the fly,” he said. “So even if you have 15 or 20 minutes of traditional battery backup, you won’t be able to get back online in that short of time. Quickly transferring the load to the generator just makes sense.”

Another advantage of flywheels is that they can work alongside a facility’s existing UPS lead-acid  or li-ion batteries and absorb charge-discharge cycles to preserve the life of the batteries (aka battery hardening). Every lead-acid battery discharge cycle diminishes its overall lifetime. Moreover, the flywheel provides a redundant source of DC energy storage.

From 40 kW to megawatts, flywheel systems are increasingly being used to ensure the highest level of power quality and reliability in a diverse range of applications. The flexibility of these systems allows a variety of configurations that can be custom-tailored to achieve the exact level of power protection required by the end user based on budget, space, and environmental configurations.


Users of flywheel systems are experiencing huge benefits. By using a flywheel versus a five-minute VRLA battery bank, users can save $100,000 to $200,000 per flywheel deployed. And systems with a contact-free magnetic levitation system — meaning there are no bearings to replace — will save users $10,000 in bearing replacement. These figures don’t include the extra energy savings in reduced cooling, as flywheels can operate up to 104º with no degradation in operating specifications or life expectancy.

Flywheel energy storage is an efficient and environmentally friendly way for data centers to maintain operations.