Is it again time for the flywheel-based energy storage systems?

Article By : Bill Schweber

Flywheel-based energy storage technology looks very promising "on paper" but has some very difficult issues in practice.

Larger-scale energy storage at the residential, commercial, campus, or even grid level is a challenge to which there’s no definitive and best solution. Options include electrochemical (batteries), potential energy (raised water or weights), hydrogen (via fuel cells), phase-change materials (molten salts), and mechanical work (compressing/decompressing air in huge tanks or caverns) to cite just a few possibilities.

There’s also storage via kinetic energy, usually in the form of flywheels. This is an established technology that looks very promising “on paper” but has some very difficult issues in practice. Nevertheless, despite these challenges and shortcomings, “hope springs eternal” as the cliché says.

A brief background: the underlying principle of the flywheel energy storage system—often called the FES system or FESS—is a long-established basic physics. Use the available energy to spin up a rotor wheel (gyro) via a motor/generator (M/G), which stores the energy in the rotating mass (Figure 1). Electronics is also required for the motor/generator itself, system control, power conversion, and more; the basic rotor assembly is the heart of the system, but there’s lots more needed.

Figure 1 The rotating mass is the heart of the flywheel-based energy storage and recovery system; while that is the most technically challenging part of the system, there is a substantial amount of additional electronics needed. Source: MDPI

When energy is needed due to a power outage or slump, the generator function of the M/G quickly draws energy from that rotor, which of course slows down; the FES system can also be used for planned, longer-term supply in addition to transient or unexpected events. Unlike some other energy-storage approaches, this charge/discharge cycle is repeatable with no apparent wear-out or aging mechanism. What’s not to like?

Challenges and impediments

The initial appeal of FESS designs is strong and varied. In fact, it was tested as the mobile power source for some municipal buses in the 1950s through 1970s, but none worked out and all have been retired. Among the various reasons cited, it’s one thing to start/stop a conventional engine in the shop to check its performance, it’s another to have to spin-up/down a rotor and wait. There was even talk of using FESS in regular cars with a small gas engine to spin-up the rotor even when the car was not moving. However, people were not comfortable with the thought of a spinning rotor in the trunk even with an added safety enclosure, which added dead weight as well. Then there were gyroscopic-induced handling issues.

Despite its first-glance attractiveness, flywheel-based energy storage presents multiple major challenges. The stored energy is proportional to the rotor wheel’s moment of inertia and the square of the rotational speed, so you want that rpm to be pretty high: 50 to 100k rpm is not usual. Due to the obvious material stresses on the rotor, the short- and long-term implications for micro-failures and macro-catastrophe are significant.

The chamber, in which the rotor spins, needs to be an evacuated vacuum since the air-induced friction would overheat the rotor. Conventional ball or other mechanical bearings won’t survive either, so an active closed-loop, magnetic-levitation bearing is used. There are even second-order effects such as a bending moment on the rotor shaft due to the Earth’s rotation, so some installations align the axis of rotation with the Earth’s axis.

Design case studies

There have been some successful installations and also unsuccessful ones. All that kinetic energy in a small space is a potential “explosion” so the entire rotor assembly needs to be in a super-strong enclosure. Controlling spin-up and spin-down of the rotor wheel with its inherent inertia—good for storage metrics but bad for control and safety—means the electronics has to be fairly sophisticated as well.

Even so, bad things happen. In 2011, one widely heralded system from Beacon Power suffered a catastrophic failure of two of its 700 rotor assembles soon after turn-on (Figure 2). The imbalance of the disintegrating rotors triggered the automatic water-cooling system and the water superheated to steam and caused an explosion, which was fortunately contained within the protective enclosure.

Figure 2 Soon after start-up, this 200-unit flywheel storage system suffered a major failure of two of its 7-foot-tall, 3,000-pound flywheels spinning at 16,000 rpm. Source: Times-Union

So where do we go from here? Despite the many challenges and impediments, FESS designs are still attractive with commies claiming innovation and breakthroughs. For example, Revterra, a startup based in Texas, says it has overcome the FESS shortcomings, making flywheels capable of long-term energy storage (Figure 3).

Figure 3 A small-scale demonstration system from Revterra claims to have solved three major problems of the existing flywheel-based systems. Source: Revterra

The company claims advances in the three key areas. First, improved metal and composite materials for the rotors, so it can spin at higher speeds without failure. Second, low-loss motor/generators is based on advanced magnetic-reluctance principles. Third, a passive magnetic-bearing arrangement is based on a high-temperature superconductor (77 K/-196⁰C). In short, it all gets pretty complicated in reality; after all, we were supposed to have superconducting power lines by now, but that didn’t work out.

The Revterra approach sounds promising, but we’ve been there before with FESS. Will FESS designs eventually be among the viable options for short- and long-term energy storage? Will they always be the “next big thing” and “just around the corner” somewhat like the use of nuclear fusion for power?” Will they have a niche in applications of a certain capacity, such as residential or office building, or is their real opportunity at the campus and grid level? Will they continue to have issues which limit their use to a few niche situations, if any?

I don’t know, nor does anyone. Sometimes, good ideas eventually fade away after repeated attempts to make them work; sometimes, they finally do have a breakthrough. I know that all high-density energy-storage approaches have their inherent dangers, and it seems the public is more comfortable with some more than others. Will the thought of a massive rotor spinning nearby at tens of thousands of rpm be unacceptable, even if in an underground, reinforced vault?

Check back in five or 10 years, and the answers may be clearer—or perhaps not.

This article was originally published on EDN.

Bill Schweber is an EE who has written three textbooks, hundreds of technical articles, opinion columns, and product features.

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