FM Issue: Flywheel-based UPS Systems

by Todd Kiehn
by July/August 2015 issue

When the power feed from a utility is interrupted, the lights go out, operations are stalled, and productivity plummets. That’s why many facilities have installed critical power equipment to ensure uptime during power disturbances and even small micro-outages. Once engaged, on-site backup generators can operate for several hours, but can take up to 15 seconds to start up and bring power back to the facility. So what happens in the interim?

Uninterruptible power supply (UPS) systems bridge the gap between utility failure and generator start-up, ensuring that there’s never a loss of electricity. When an unplanned outage occurs, an automatic transfer switch (ATS) functions as its name implies—it transfers electrical loads from one power source to another. In a utility outage, if a UPS is present, it provides ride-through power until the generator is ready to assume the load. When the generator comes online, the ATS shifts loads from the UPS to the generator.

UPS systems store energy to provide the ride-through power in a variety of forms. Conventional battery-based UPS use electrochemical energy typically stored in strings of valve-regulated lead acid (VRLA) batteries that are housed in cabinets. These UPS systems can store five to 30 minutes of energy, depending on the facility’s requirements and comfort level with its generator. A small amount of utility power is used to keep the batteries charged at all times.

In recent years, alternatives to battery-based UPS systems have emerged, including one of the oldest energy storage technologies—the flywheel. Due to continuing advances in power electronics, the flywheel is now a mature energy storage technology used in UPS applications to support a critical load during main power interruption.

Like their chemical battery counterparts, flywheel based UPS bridge the time between a utility outage and generator start. Instead of an electrochemical energy storage approach, the solution stores kinetic energy in a spinning mass powered by the most basic principle of physics—an object in motion will remain in motion unless acted upon by an outside force. A flywheel UPS uses a rotor, typically made of steel, as the foundation of a flywheel machine that acts as a motor (converting electrical to kinetic energy to keep the wheel spinning), generator (converting kinetic energy to electrical energy to power the load), and energy storage system.

Once the flywheel is above its minimum discharge speed (about half of its “fully charged” speed), it is capable of supporting a load caused by an electrical interruption and will continue doing so until power to the UPS is restored, either by a standby generator or the return of utility power. At that point, the flywheel resumes charging to replace the stored energy that has been used.

Flywheel systems have benefits that make them worth considering. First, flywheel based UPS products have a lower total cost of ownership than traditional systems. In addition, flywheel UPS manufacturers have put in place a robust UPS topology (system design) that enables the product to withstand more frequent discharges than batteries, which is particularly important where power disturbances are common. A byproduct of this topology is high energy efficiencies compared to a conventional system. Flywheel UPS can also withstand warmer ambient temperatures than battery systems, so cooling costs should be calculated. Finally, flywheel UPS systems have proven to be more reliable than battery based equivalents. Flywheel UPS systems are constantly spinning so the stored energy is known at all times.

A recent study by risk assessment firm MTechnology, Inc., demonstrates a flywheel UPS can reduce the risk of electrical system failure by 80% compared to conventional UPS with batteries in a short utility outage lasting less than 10 seconds. The main driver for this result cited in the study was the improved reliability of the flywheel compared to the battery energy storage.

While UPS systems are an essential element in a backup power strategy, not all are the same. Facility managers should look to alternative solutions such as flywheel UPS to optimize their backup power design.

UPS-systemsKiehn is senior director, product management at Active Power.