TWO-STROKE ENGINES
evenly around the perimeter. Balancing these systems is essential; since, as Rumney underlines, “centripetal acceleration at these flywheel speeds can be 10,000G or more”. Further, fully composite designs can be completely integrated; high tensile strength carbon fibre rotor/flywheel being a single assembly. It's lighter, and, not to put too fine a point on it, potential failures are better contained. It makes for a much more complex system than the PowerBlade - but getting all these elements right yields a longer energy storage window, with significantly reduced envelope and mass. “Typically this sort of flywheel has an energy storage half-life of several tens of minutes,” explains Rumney: that's a big enough window for a range of 'peaky' consumers. It might not be long before both types of technology start putting their own spin on short-term regeneration and peak shaving applications.
CONSTRUCTION IS LIKEWISE EVOLVING. While steel versions generally tend toward utilising a separate motor to convert spin into electrical energy, others neatly double up the flywheel's role, turning it into a motorgenerator's rotor. These use permanent magnets rather than coil windings for strength and higher power density. Because a high moment of inertia (that is, mass times radius) is no longer the most important feature at very high rotational speeds - seen in the land based and automotive market units - these flywheels can take advantage of either part or full composite construction, embedding the magnetic material
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8 High speed flywheels are already finding a place in both automotive and landside power industries
8 The main components of a high speed flywheel
8 The active heave compensation onboard big drill rigs will add a couple of megawatts to the load on each lift of the string
Photo: NOV
SPIN Other developments utilise another aspect of the flywheel principle. As kinetic energy is proportional to mass times velocity squared, doubling the mass doubles energy storage... but doubling the rotational speed quadruples it. So, increasing the spin speed yields a far more compact unit explains Tim Rumney of Inetic. Imagine a package “less than a 50cm cube with a mass of around just 100kg”, says Rumney, who was involved in a development project exploring flywheels for naval vessels. He added: “The brief included getting it through any doorway on the ship.” The focus wasn't so much about regenerating energy, but using the technology for a typical ESS application: peak shaving the onboard load, with motors 'charging up' the flywheel. The advantages also align neatly with commercial vessels' challenges, especially since, like battery cells, flywheels lend themselves to a modular approach. As a result, “you can pick the amount of energy and power you need and arrange the units in a series or parallel configuration”, Rumney explains, so they can act in concert, or take up the load sequentially. A typical naval application would see half-a-dozen of these modular packages distributed around the ship, making it suitable for managing “short, but large bursts of power inside a particular area” he says, without recourse to huge capacitors or main grid cabling. Further, this makes it's possible to shunt the energy between nearby consumers enabling zonal power management. However, reducing the size in this way requires spinning the flywheel at up to 40,000 or 50,000rpm. Therefore, friction is the enemy: “At that rate, the air drag resistance alone can lose tens of kilowatts of energy if not managed,” explains Rumney. As a result, all high-speed flywheels need to be enclosed in a vacuum. There's also another challenge for developers: the spin creates a considerable gyroscopic effect. As a ship will experience pitch and roll movement, there's a need for “fairly robust bearings” to deal with these generated forces says Rumney, potentially entailing magnetic or low-friction precision systems, though some recent automotive developments have put gimbals beneath their installations.
Photo: Geni
While the active heave draw will still be linked to the wave period, Van den Bos points out that the installation would likely have to be sized for the lifting capacity. Therefore, massive crane vessels would likely require a scaled-up version as two cranes working in tandem can have a combined lifting capacity over 14,000 tonnes. Additional flywheels, (rather than a single, oversized mass) make a neater, more flexible package. Further, Verhoef adds the energy storage capacity can be tailored to suit. If the system is only designed to accommodate the AHC for a few seconds, it could even be installed without the battery, significantly cutting costs.
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