Introduction to InfinityWEC

In 2017, OHT discovered the opportunity to use an infinitely variable transmission (IVT) in combination with a flywheel energy storage, to provide advanced force control features and constant power output in a Wave Energy Converter (WEC). The technology is similar to Kinetic Energy Recovery Systems (KERS) used in vehicles to recover brake energy, but with a high-speed generator connected to the flywheel to output electricity.

This technology has been named InfinityDrive PTO and is the central piece of the InfinityWEC device. The InfinityWEC uses a ball screw actuator system with integrated level adjustment, and a mooring pipe system with a fixed PTO position that eliminates the frequent bending of the power cable from the wave cycles.

Compared to offshore wind power, InfinityWEC is highly cost competitive, offers >10 times higher capacity per used area, has virtually no visual impact and provides a more stable energy supply with less intermittency.

Extracting some energy from the waves with a point absorbing buoy is easy but reaching the full potential is very difficult. This is because, unlike wind, each wave is distinctly different, requiring a highly adaptable PTO force that can be optimized for every individual wave.

The optimal PTO force requires some of the captured energy to be returned again in every wave cycle. A flywheel energy storage is ideal for managing the high number of cycles and high peak power for this application, without any cyclic degradation of the storage capacity.



Wave power faces similar general challenges to those in off-shore wind: Overall cost competitiveness relative to existing mature generation forms, challenges of operating in marine environments, engagement of a dedicated supply chain and grid connection.

The specific challenge with wave power is that waves have slow oscillating motions with high and random variation of size and energy content, which leads to:

  • Difficulties to capture energy efficiently.
  • Large size and high cost of WEC´s.
  • Need for energy storage to meet grid compliance.

InfinityWEC solution

The InfinityWEC has the potential to overcome many of the drawbacks in other WEC technologies that have hindered the wave energy industry from realizing its enormous potential, by providing:

  • Improved power capture and motion control, with mechanical KERS technology providing a fully flexible PTO force. Power output is optimized for every individual wave without exceeding the maximum allowed PTO force, velocity or stroke length. InfinityWEC can benefit from advance control strategies using model predictive control, machine learning and AI in combination with LIDAR technology to predict wave input.
  • Constant output power, and a downsized generator that only have to manage 1/10 of the maximum power used in the PTO force. A high-speed, light-weight generator is connected directly to the flywheel inside the vacuum casing.
  • Ball screw actuation system with separated level adjustment, providing high efficiency and durability, high conversion ratio from linear motion to rotary motion, low weight, and sufficient total stroke to fully submerge the WEC in extreme weather.
  • Static protected route for the power cable, with PTO in a fixed potion on top of a mooring pipe system similar to a riser pipe in oil & gas.
  • Highly competitive to offshore wind, providing lower cost of energy, >20 times higher installed capacity per used sea area, virtually no visual impact and less intermittent power supply.

InfinityDrive PTO force control

  • InfinityDrive PTO uses ratio control of an Infinitely Variable Transmission (IVT) connected to a flywheel to provide a full flexible control force with power going back and forth between the wave and buoy, while keeping a high uni-directional speed of the flywheel.
  • An IVT is a Continuously Variable Transmission (CVT) coupled to a planetary gearbox. The IVT provides continuous variation of the gear ratio, from full negative Overall Transmission Speed Ratio (OTSR), through geared neutral, to full positive OTSR.
  • The IVT gearbox on the left shows a CVT of the type Double roller Full Torodial Variator (DFTV) traction drive, coupled to the epicyclic gear of the IVT system. The DFTV is an improved variation of the Torodial Variator, which is more efficient, lighter, more durable, has a wider ratio spread and can be manufactured at a lower cost.

Mooring and actuation systems

InfinityDrive PTO sits in a fixed position on top of a mooring pipe system, similar to the riser pipes used in oil & gas. The hull oscillates around a steel cylinder, which connects to a flexible composite pipe. The power cable and communication fiber is protected inside the pipes and terminates in a quick connector attached to a seabed foundation. This arrangement provides fast installation and retrieval of the WEC, and eliminates the frequent bending of the power cable from the wave cycles.

A heave system shares the PTO force over four non-rotating ball screws fixed with the heaving hull of the WEC. The ball screws are pulled up and down through the gearbox with the movements of the hull. Rotating ball nuts for each screw is connected to the InfinityDrive PTO unit through a planetary gear arrangement.

A level system connects the PTO platform to the mooring pipe with a single lead screw, providing level adjustment for tidal variation and additional stroke length to fully submerge the buoy to protect it from damage in extreme weather.

Using a separate system for heave motion and level adjustment enables the heave screws to have a shorter stroke length, which is more cost effective. The heave screws are exposed to much higher velocity and 100 million wave cycles in a 25 years design life, compared to the level system exposed to only 20 000 tidal cycles.


  • 500 kW maximum continuous power output.
  • 6 kWh flywheel storage capacity.
  • 10 000 – 20 000 rpm operating speed of the flywheel and generator.
  • 6 meter stroke length of the heave system.
  • 8 meter stroke length of the level system.
  • 2 m/s maximum buoy velocity.
  • 3 MN maximum PTO force.
  • 75% Wave-to-wire efficiency.
  • 75 ton total weight.


OHT´s patent portfolio comprises 6 patent families and is protected world wide. The patents covers all aspects of the InfinityWEC system in different variations. A special focus is how energy storage technologies can be integrated in the power take-off to provide force control features and power smoothing.


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