Power Electronics

Energy Derived from Ocean Waves to Supply Scotland Homes with Utility Power

A system for converting wave energy into electricity employs a staged power conversion system. The system performs hydrodynamic-to-mechanical, mechanical-to-hydraulic, hydraulic to mechanical, and mechanical to electrical energy.

Find a downloadable version of this story in pdf format at the end of the story.

Aquamarine Power will begin deployment of three wave energy converters (WECs) at European Marine Energy Centre (EMEC) in Orkney, Scotland in summer 2011. Three 800 kW devices will be linked to a pair of onshore 1.2 MW hydro-electric turbines. When completed, the WECs near the shore can provide enough energy to power up to 12,000 homes.

The new 2.4MW system will deliver 700% more power than the original WEC, called Oyster 1, that was successfully deployed at EMEC in Orkney 2009 (Fig. 1). The new version, Oyster 2, incorporates design improvements, enabling it to produce more energy while being easier to install and maintain. Fig. 2 shows Oyster 1 in operation.

The basic Oyster WEC consists of an oscillator flap mounted on a sub-frame support structure (Fig. 3). The oscillating action of waves on the flap drives hydraulic pistons to pressurize fresh water, which is then pumped to shore through the high pressure pipelines. At shore, a hydroelectric plant converts the hydraulic pressure into electrical power via a Pelton wheel turbine, which turns an electrical generator tied to the local utility grid.

Design of the Oyster WEC required a balance between capital cost and wave power conversion efficiency. The key wave power conversion parameters are water depth, flap geometry, buoyancy and damping. Its design objective was to maximize captured wave power per ton of device and foundations.

Oyster 1 extracts power from the waves by harnessing the horizontal surge component of the wave motion that produces forces on its flap. Forces are maximized by making the flap penetrate the full water column and minimizing leakage underneath or from overtopping the device. Widening the flap increases the forces approximately in proportion to the square of the flap width. However, as the width increases, the maximum wave force reaches a limit due to phase incoherence across the flap. Water depth reduces the water's horizontal amplitude, so motion increases but overall power reduces. Besides optimizing the power take off, there are other design considerations that affect installation and operating costs:

  • Location of the offshore site and seabed conditions

  • Method of fixing the structure to the seabed

  • Method of installation and removal of the WEC

  • Method of converting the wave energy

  • Where the components of the power take off system are located

  • How it is to be maintained

One challenge was to design a mechanical control system that could accept the water being pumped ashore at variable flow rates and pressures, and then efficiently convert it to mechanical energy. This led to use of a special Pelton wheel turbine directly coupled to a flywheel and a variable speed induction generator.

A Pelton wheel operates most efficiently when running at a constant speed, which requires a steady flow of water at a constant pressure. However, the Oyster WEC produces water that typically changes from no flow to peak flow twice every wave period (7-14 seconds). Therefore, the design had to maintain and control the system pressure. It had to actively control its spear valves and also control the braking torque from the generator. For a set system pressure there is an associated optimal speed for the Pelton wheel. The change in flow rates during a wave period was managed by cycling the effective flow area of the spear valve from fully closed to open and back. (A spear valve changes the flow nozzle size without stopping the turbine.)

As the hydraulic power comes onto the Pelton wheel it will try and speed up, which is controlled by a flywheel that stores some of the energy, increasing the generator braking torque and throttling the flow. Throttling the flow has two effects: it slows down the Pelton wheel and also diverts some of the flow into an onshore accumulator, this in turn slightly increases the system pressure, which increases the water jet velocity to match the increased speed of the flywheel. Wave power is not constant, so every 15 minutes the control system also changes the system pressure to ensure that it achieves a near optimal hydraulic-to-electrical conversion throughout the day.

If the Pelton wheel over speeds, or any of the temperature, vibration or pressure instruments read outside their normal range, the system will initiate a controlled shutdown sequence and dump all hydraulic power. Two valves on the main high pressure line open to dump the flow directly to an underground tank, bypassing the Pelton wheel.

ELECTRICAL SYSTEM

The electrical system obtains mechanical power on the common shaft between the Pelton wheel, generator and the grid connection at EMEC. The electrical energy is transferred to the grid via an 11kV interface at EMEC. The key components of the electrical power system are:

  • 11kV breaker and protection
  • 11kV/400V power transformer
  • Utility grid synchronization
  • Variable Speed Drive
  • Generator and auxiliaries

Oyster 1 power conversion included a variable frequency inverter coupled to an induction generator. The primary purpose of this drive is to match the torque/speed characteristics of the Pelton turbine. Fig. 4 shows the key drive components.

The flywheel is used to store energy, allowing the generated electrical power to be stabilized, and also allows the generator to have a rating of about half the Pelton wheel power rating.

Of the four separate Unidrive modules, two provide the variable speed requirements of the generator with the other two providing the grid interface. One advantage of this modular arrangement is that it provides fault tolerance. If one drive unit should fail, the others can continue to operate at a reduced power until repaired.

With Oyster 2 (Fig. 6.) slated for 2011 deployment, future objectives are to build wave power stations of 20 to 100MW. Research is ongoing to determine the hydrodynamic performance of clusters of flaps in different geometric patterns, and modeling the array of devices with multiple hydraulic inputs to determine the optimum size and spacing of onshore hydroelectric plants. It was always recognized that the first prototype is the hardest to design and operate, as it does not have the benefit of smoothing of the hydraulic flow from multiple units and realizing the economies of scale of the onshore hydroelectric plant.

The future wave energy development program also includes wave-powered desalination by feeding the high pressure water directly into reverse osmosis tubes.

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WHAT IS A PELTON WHEEL?

THE PELTON WHEEL is among the most efficient types of water turbines. The wheel (Fig. 5) extracts energy from the impulse (momentum) of moving water, as opposed to its weight like the traditional overshot water wheel. With Pelton's paddle geometry, when the rim runs at one-half the speed of the water jet, the water leaves the wheel with very little speed, extracting almost all of its energy, allowing a very efficient turbine.

Invention of the wheel by Lester Pelton apparently originated from an accidental observation. Pelton was watching a spinning water turbine when the key holding its wheel onto its shaft slipped, causing it to become misaligned. Instead of the jet hitting the cups in their middle, the slippage made it hit near the edge, rather than stopping the water flow, it was deflected into a half-circle, coming out again with reversed direction. Surprisingly, the turbine now moved faster.

WAVE ENERGY CONVERSION

WHEN A BODY OSCILLATES in still water, it creates waves at the frequency of oscillation. The amplitude of this radiated wave depends on the size of the body and the amplitude of its motion. You can consider wave energy absorption as a destructive interference of this radiated wave with an incident wave when the correct phase relationship is present. Wave energy is proportional to the square of the wave amplitude. Similarly, the reduction in the incident wave amplitude is related to the energy absorbed by the device.

A simple method of absorbing wave surge energy is to use a hinge or bearing to allow the body to oscillate in pitch, which then turns the surge force into a moment, or wave torque. Moreover, a purely surging device has no inherent spring force to aid in tuning and to resist nonlinear drift forces. However pitching devices do, whether it is excess buoyancy for a bottom hinged device, such as Oyster.

The dynamics of bottom hinged pitching flap type WECs tend to be inertia dominated and therefore the natural pitching period is generally well above that of the most common wave energy periods, which is about 5 to 15 seconds.

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