Technology overview

OHT offers WEC developers and infrastructure partners:

  • A high pressure fluid collection system for WECs with power smoothing and electricity generation in a central tower.
  • A low-cost and efficient energy storage, in the form of a weight that is lifted with a rack and pinion drive inside the tower, providing constant pressure in the collection system and constant power output.
  • PTO functionality which enables wave-by-wave damping force control in buoys. The hydraulic cylinder is replaced by a rack and pinion connected to a variable/multi displacement pump arrangement.
  • Reduced CAPEX and increased annual yield.

The collection tower is generic. Any WEC buoy can be adapted to connect to the hydraulic collection system. The tower can preliminarily be scaled up to 50-100 MW. A commercial size array will include several towers. 

10 MW collection tower 

  • 10 MW continuous (±5%) output power and up to 35 MW peak power input to the hydraulic motors.
  • 12 drive train modules are connected to a common 60 meter long rack and lifts a 500 ton weight in the gravity storage.
  • Each drive module comprises hydraulic radial piston motor, planetary gearbox, rack pinion, flywheel and generator.
  • Target to show > 80% wave-to-wire efficiency.
  • Preliminary scalable up to 50-100 MW for commercial arrays.
  • The tower is a CFG (Crane free gravity) foundation used for offshore wind turbines that can be floated to the installation site. This technology supports installation depth down to approx. 100 meters.
  • The weight in the gravity storage is implemented as a steel container that is filled with ballast when the tower is installed.

Working principles

A central tower collects power from the surrounding buoys in the form of high pressure hydraulic fluid that is pumped from the buoys to the tower through a hydraulic piping system. The flows from all buoys converge in the tower and drives a hydraulic motor. The hydraulic motor is connected to a planetary gearbox and a generator. A floating ring gear of the planetary gearbox is connected to a rack and pinion drive that lifts a large weight. This arrangement provides a constant torque to the drive train in the tower and a constant pressure to the hydraulic collection system. This reduces the cost and increases the efficiency of components in the complete power take-off (PTO).

The generator is operated with a constant speed that is tuned to the sea state to match the output power to the average captured power from the waves. Since the torque is constant while the motor operates with variable speed and the generator operates with constant speed, the weight is lifted when the input power is higher than the output power and vice versa.  

Buoys with hydraulic power take-offs can simply connect the hydraulic cylinder directly to the collection system and remove the on board motor and generator. Also, on board hydraulic accumulators can be significantly reduced in size or removed completely as the energy storage in the tower provides the power smoothing necessary to meet grid quality requirements. Simulations show that using an optimized constant force, instead of a force depending on the level of stored energy in an hydraulic accumulator, will almost triple the annual power capture.

Buoys with mechanical power take-offs e.g. based on a rack and pinion drive or similar can replace the generator with a variable displacement pump. This provides the advantage of adding energy storage to the system without reducing the force control capabilities. Hydraulic accumulators can be used directly in the buoys. The buoys can also be attached to the hydraulic piping system and export hydraulic power to the central tower with the gravity storage and centralized conversion to electricity, which is the most cost effective solution. Certain types of hydraulic motors are much lighter and also more efficient then electrical generators when operating at high peak to average speed and constant pressure. The efficiency of the generator is significantly increased when operated at constant speed and also the frequency converter and other power electronics and power cables can be sized to the average power instead of the peak power.