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Wavepower devices use ballscrews to drive generators

06 December, 2021

A Swedish start-up has developed an innovative wave power technology that uses heavy-duty ballscrews to convert the linear up-and-down motion of sea swells into rotational motion that drives a generator.

Wave energy is the largest untapped resource of renewable energy. Capturing the energy provided by the motion of the waves could produce electricity for future generations. But the oceans – which cover two-thirds of the earth's surface – are still largely untapped as a source of energy, with the exception of a few tidal power plants at river estuaries.

This situation could change if a Swedish start-up called Ocean Harvesting Technologies has its way. The company has developed an innovative wave energy converter (WEC), called InfinityWEC, that can harvest energy from the motion of ocean waves

The principle is simple. Ocean swells cause buoys attached to the seabed to rise and fall. This movement is captured by a hydraulic constant-force spring system – a hydraulic cylinder connected to a large gas volume – and two heavy-duty ballscrews. The nuts of ballscrews are connected directly to the cylinder, thus converting the up-and-down linear movement into a rotary motion of the ballscrew spindle that, in turn, drives a generator to produce electrical power.

The spring force of the cylinder provides an almost constant force of 1MN pulling on the buoy, while the two ballscrews, in combination with the generators, deliver a bidirectional force of up to ±1MN. In total, the power take-off in each WEC produces an instantly controllable force of 0-2MN. In this way, it is possible to achieve high power densities, ensuring high energy yields.

In a practical application, the converters would be arranged in clusters of 12, with cables transferring power from each device to a central hub. Numerous devices would be installed together to form an energy “park”.

Ocean Harvesting estimates that with 100MW of installed capacity, electricity from the system would cost around €100 per MWh and that with 5GW of installed capacity it would cost less than €35/MWh.

Dual-function ballscrews: energy generation and damping

Many technology details had to be developed before this elegant concept could be put into practice. For instance, for effective energy generation, it is essential that the buoy aligns optimally – so-called phase control. The system then extracts energy by applying the ballscrew force in the opposite direction to the motion – so-called damping.

A great deal of effort went into developing the algorithm that calculates the ideal buoy position for each stroke of the WEC, and into achieving this motion by controlling the ballscrew force.

“Before the wave rises,” explains Ocean Harvesting CEO, Mikael Sidenmark, “the buoy must be in the ideal position, which is where the ballscrews play a crucial role by providing a phase control force, assisted by the pre-tensioned spring system – i.e. from stored kinetic energy. The damping they exert then follows a calculated force curve to maximise the power output.”

It is also necessary to guarantee the downward return movement of the ballscrew nut – a task performed by the hydraulic cylinder. The cylinder stores part of the energy generated by the buoyancy force and releases it to the ballscrew when it moves down.

In practical applications, arrays of InfinityWEC converters will be arranged to form offshore energy “parks”
Image: Ocean Harvesting

Another innovation is a two-stage end-stop cushion. In heavy seas, this stops the upward movement of the buoy gently. Sidenmark explains the advantages this brings: “In strong wave conditions, the buoy is held submerged through the crest of large waves and returns to produce power as soon as the buoy surfaces again. It never shuts down in survival mode, unlike many other WEC devices, where power production is interrupted during long periods.”

Rack-and-pinion, winch or ballscrew?

Choosing the drive system for the power take-off was a challenge, especially because it has to operate constantly under adverse environmental conditions, and has to be removed from its underwater location to perform maintenance.

“We originally used winch drives in our system, and later rack-and-pinion drives,” Sidenmark recalls. “However, we have found that heavy-duty ballscrews provide a superior combination of force capacity, high velocity, long stroke, high efficiency and a high conversion ratio from linear to rotary motion, at a relatively low cost.”

Ocean Harvesting chose heavy-duty ballscrews from NSK's HTF series, partly because of their long service lives. “The ballscrews are expected to perform 100 million load cycles in 20 years - that's very demanding,” Sidenmark explains.

These heavy-duty ballscrews were originally developed for machine tool applications such as presses and other forming machines. They are also used in injection-moulding machines that rely on electric rather than hydraulic drives. Their ability to withstand high loads while delvering long service lives, means that the ballscrews are also suitable for other infrastructure applications, such as in damping elements that help protect high-rise buildings against earthquakes.

Ocean Harvesting is currently running tests on a 1:10 scale prototype of its InfinityWEC device, and is planning sea trials using a 1:3 scale version in 2023. After this, it will move to full-size wave energy converters to prove their viability.

The company has identified several co-operation partners for the first WEC applications. For example, an oil and gas company is evaluating the possibility of using the wave energy converters to power its offshore installations.

NSK is continuing to design and manufacture heavy-duty ballscrews in the sizes required by Ocean Harvesting.

NSK Europe:  LinkedIn  Facebook

The wave energy converter (left) houses its core components – ballscrews, hydraulic cylinders and a generator – in a sealed and air-conditioned hull, below the sea surface. The cutaway view (right) shows the two ballscrews and hydraulic cylinder with spring accumulator that convert the up-and-down motion of the buoy into rotational movement that drives the generator.
Image: Ocean Harvesting

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