Sacramento-based Qnetic has begun low-speed testing of its flywheel energy storage system at its technology center in Shanghai, China. The prototype unit, dubbed Pulsar, represents a continuation of the company’s mechanical storage architecture that is expected to have an initial capacity of 200 kWh and expandable to 500 kWh with the same platform, which would enable four-hour storage at a nominal discharge rate of 125 kW.
Flywheels store energy in rapidly spinning rotors as kinetic motion. Surplus electricity is injected into the system, accelerating the rotor to high speeds. When power is needed, a generator converts kinetic energy back into electricity, slowing the rotor. Advocates for the technology say that flywheels could challenge battery-based energy storage systems (BESS) now dominant for renewables on the grid.
Qnetic reported in September that Pulsar had successfully completed its first 100 continuous charge-discharge cycles with zero faults or capacity degradation. Tested separately, the system’s motor produced 130 kW of power output at 97.3% efficiency with speeds exceeding 11,000 rpm.
Qnetic has established a factory and headquarters in Sacramento where it intends low-volume production of commercial Pulsar units. Pulsar will be evaluated in a pilot program by the Sacramento Municipal Utility District (SMUD) under the auspices of a risk-reduction initiative run by the Electric Power Research Institute. Qnetic aims to be able to start the pilot in the first half of 2027.
Michael Pratt, co-founder and CEO of Qnetic, told pv magazine USA that his company’s implementation of flywheel technology is intended to overcome many of the shortcomings of lithium-ion batteries, particularly in the areas of cycling rates and degradation over time.
“When we were formulating our concept about five years ago, grid-connected lithium-ion batteries in the form of large, containerized systems were just getting into the mainstream,” Pratt said. “It was pretty clear at the time that batteries were too expensive, but the main problem was the degradation issue.”
According to Pratt, the pressing need to integrate renewables on the grid argued for BESS, with its drawbacks, as the best solution then available. Since then, he notes, many billions have been spent trying to improve battery chemistry, but the fundamental issues remain: cycle life and calendar life.
“Lithium-ion batteries are typically doing one cycle per day, and they’re only going to last so many years,” Pratt said. “In addition, the whole time you’re using them, they are getting weaker and weaker. We knew that flywheels had the characteristics to cycle repeatedly on the grid and not lose capacity over time. We also saw that nobody was putting together all the pieces in the way needed for grid storage.”
Historically, flywheels have served as energy storage devices, rapidly delivering high power for short-duration applications measured in minutes or even seconds. Examples include backup power in telecommunications and data centers, or as bridging power until diesel generators come online. Precision control and high output power has also made them suitable for creating tokamaks in fusion energy experiments.
Qnetic’s goal is to produce flywheels suitable for long-duration energy storage on the grid. If four-hour performance is just this side of “long,” it still fulfills many utility requirements. Moreover, flywheel units may be grouped into banks that can be tapped over longer periods of time. Also, discharge rates can be varied to meet demands.
Deployed grid-scale units are buried in the ground with the control machinery accessible through an exposed dome, an arrangement that suggests eggs in a carton. Pratt positions flywheels as safer than lithium-ion batteries from a fire perspective, but the high speeds of the rotors still require consideration in case of “rapid unscheduled disassembly.” He adds that safety measures like making sure the motor stays short of imparting the rotor’s maximum rated speed will keep this from happening.
Competing flywheel maker Amber Kinetics has also developed a version of its 32-kW units for above ground deployment in steel shipping containers.
While Pratt makes arguments for how flywheels overcome some specific drawbacks of BESS – namely cycling and degradation issues – he acknowledges that the mechanical storage technology faces developmental hurdles of its own in order to reach higher capacities and economies of scale.
Some of these are addressed in Pulsar’s architecture. The unit features a carbon fiber rotor rotating in a vacuum on no-contact magnetic bearings, all but eliminating friction. The rotor is kept clear of the central shaft using technology similar to that which enables magnetic levitation trains to travel with no contact with the rails. The assembly is housed in a cylindrical steel container.
A key objective in Pulsar’s development is to increase the strength of the composite rotor, which is made with carbon fiber filaments. The stronger the rotor, the faster it can be made to rotate, and the more kinetic energy stored. With Pulsar’s current configuration, further iteration of the carbon fiber will enable the increase of its capacity to 500 kWh.
“As you build up, you get excellent economies of scale,” Pratt said. “If you double the physical size, you can get eight times the potential energy capacity in that envelope. The second thing you want to do is go fast. You want to make a really strong rotor so that you can spin quickly. So, a really good rotor and going quite large are the keys to getting a low-cost system.”
At the same time, Pratt says there are practical considerations about achieving higher capacity and thus cost-effectiveness through size. One is that the company is essentially breaking new ground with composite rotors. The improved rotor is made using a process called filament winding from carbon fiber, and Pratt says it is not common to manufacture these larger than about two-and-a-half meters in diameter.
“We’re going to cycle the hell out of it, see if anything breaks, and if it does, we’ll fix it,” he said. “Once we’ve gone through that proving process, then we do the first pilots.”
On the whole, Pratt says, all of the fundamental technologies leading to Pulsar are mature and in service. Other than perhaps fabricating a larger carbon fiber rotor, Qnetic is doing a lot of systems integration and control optimization work as opposed to original engineering and research and development. The goal is to achieve MW-scale flywheel units based on a subsequent generation.
If flywheels seem like an exotic energy storage solution, that’s largely because they have generally been relegated to niche applications. A spokesperson for Qnetic told pv magazine USA that grid-connected flywheel pioneer Beacon Power deployed 400 of its 100 kW (25 kWh) units at two utility sites in New York and Pennsylvania between 2011 and 2014 to provide fast-responding frequency regulation. Each location had 200 units with a nameplate capacity of 20 MW (5 MWh), and both are still providing frequency regulation service and generating revenue.
Pratt says flywheel technology has only gotten better and more reliable. He says an asset lifetime in excess of 30 years and the ability to cycle multiple times a day with rapid response and no degradation make flywheels a promising energy storage solution at all scales, including long-duration grid applications.
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