Spin doctors: a new take on peak shaving

Boring into a seabed 3,000m below means the drawworks onboard a drillship – those sizeable winch systems controlling the drill lines – will be responsible for handling thousands of tonnes. But while active heave compensation (AHC) is key to keeping the drill head stationary, “each lift of the string asks a couple of megawatts of the generator load” explains Richard Verhoef of NOV.

An energy storage system could smooth out the power peaks and pick up the wasted kinetic energy – but the question is, what kind of ESS could handle not only the scale, but also the frequency of these cycles?

The more familiar solutions used in the industry weren’t capable of meeting the demands. Firstly, “as a rough rule of thumb, a battery can only absorb around half of its energy capacity in one go”, says Verhoef. Secondly, even scaled to cope with this challenge, the work schedule of a drillship could result in a million AHC-driven cycles in a couple of years. “That’s enough to destroy any battery,” he points out, and it would also likely kill a supercapacitor.

By contrast, there’s nothing better than a flywheel if cycle life is the issue.

However, these are not merely big lumps of metal as there are huge forces to contend with: take the 1,500mm, 3.5 tonne flywheel developed by NOV and set it spinning at 2,000 rpm, and there will be tremendous stresses at its perimeter. Therefore, the manufacturing must be tightly managed.

But the mass is, after all, what makes it work. As Verhoef explains, NOV’s flywheel “can absorb a lot of power – 1.75MW peak – and give out the same amount for around six to ten seconds”.

That may seem like a short period of time, but it has been tailored to tally with the length of demand from the active heave, “which is basically the wave period,” he points out. Admittedly, this differs a little from region to region and the response is affected by the vessel’s shape, but he adds that “generally, it’s around eight seconds.” This brevity also simplifies the technology by, for example, allowing the design to stick with spherical roller bearings instead of specialised magnetic varieties designed to ‘spin out’ the energy retention.

However, there are longer spans of demand such as when raising the drill bit. While it’s theoretically possible to scale up the flywheel to meet these operations, it would entail an unfeasible rise in diameter and motor size.

Therefore, NOV has added a more conventional 450kWh lithium-ion battery system linked by two 1,100 hp motors to create the PowerBlade kinetic energy recovery system. The neat thing about this pairing is that the flywheel can push the power either back to the winch or to the battery where it also acts as a buffer, giving the cells an easier ride. The combination lends the PowerBlade a 3.25MW maximum charge absorption, while it can also belt out a sizeable 4.75MW at peak output.

It is effective: the theoretical figures from a Norwegian vessel show up to 80% energy recovery during active heave compensation. Further, as with other ESS, if incorporated at build, the technology allows resizing the power plant instead of scaling it for peak demand.

Most importantly, it promises to soften wear and tear for much of the onboard kit. “The endless ups and downs on the distribution bus aren’t good for any of the components,” comments Verhoef. This gives you a more stable system.”

The PowerBlade is to be offered as a fully boxed solution in a standard ISO container that can be dropped onto the deck. “The installation philosophy is that it can be fitted between jobs with the pre-wiring carried out during operations and just a day or two allocated for putting this onboard while offshore – so the ship doesn’t have to come into port,” says Verhoef.

OTHER APPLICATIONS

It will likely find a wider audience than the drillships. Guido Van den Bos, business development director for vessel designer GustoMSC, (a NOV subsidiary), has been considering other potential applications. For example, he sees advantages for “large jackups or other semi-submersible vessels.”

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.

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.

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 motor-generator’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 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.