Electrical Power From Marine Sources of Renewable Energy

Introduction to Marine Renewable Energy Devices

Marine renewable energy has been shelved for many years (apart from offshore wind energy) because of the short-sightedness of successive governments in not providing financial support for R & D.

We are fortunate here in Scotland to have a European Marine Energy Centre (EMEC) on the island of Orkney. Since its inauguration it has successfully pioneered many wave and underwater current renewable energy devices.

This is another article on my series on renewable energy devices and is a follow up to a previous marine energy article I wrote a while ago. As things have developed since then I thought it was appropriate to write an article on current devices operating around the British coasts and islands.

We start by recapping the different categories and types of devices used, then examine new and projected capacities of these offshore devices, where they are located and their capacities.

Categories of Marine Renewable Energy Devices

There are three main categories of marine renewable energy devices in use today, around our coasts and between the mainland and the outer islands of the UK.

  • Wind Power
  • Wave Power
  • Tidal Stream Power

Apart from getting larger, wind turbines have not changed greatly since my last article, so we will concentrate on wave power and tidal stream power in the following sections.

Wave Power Devices

As we know marine power has been ignored for a few decades, but The Crown Estate have been in action again, this time allocating areas of the Pentland Firth for installation of energy extraction devices. The Pentland Firth is a narrow stretch of water between the most Northern Scottish Coastline and the Island of Orkney. It is notorious for rip tides and high waves. (Having been through it on many occasions whilst an Engineer at sea I can verify this, especially with the ship in ballast.)

Anyway I digress; waves are produced by the wind blowing across the sea surface, with the various wave power energy devices or absorbers,extracting this energy from or near the surface.

The wind velocity and fetch along with the depth of water combined with the condition of the subsea surface govern the height of the waves, a higher wave height containing more energy.

There are numerous categories of wave absorbers as shown below,

  • Oscillating Wave Surge Converter – a hybrid of a point absorber and an oscillating water column. A recent innovation operating on wave surges in shallow waters.
  • Point Devices – usually buoys and extract the energy by the bobbing action caused by the waves
  • Terminator Devices – positioned vertical to the wave direction using the energy to pump a fluid up a column.
  • Attenuator Devices – long snake-like and multi-sectioned segments float on the top of the waves, and their energy is used to drive hydraulic pumps.

Oyster Wave Surge Converter

This Oscillating Wave Surge Converter was installed and tested at the European Marine Energy Centre on the island of Orkney. There has now been a full scale version built at the offshore construction yard in Ross shire, where I was engineer for many years.

The Oyster device as the name implies opens and closes like an oyster shell operating by wave energy. It is installed near-shore in about 10 meters of water. The top shell is connected to a buoy, which opens and closes against the bottom shell which is fixed to the seabed.

Incorporated in the bottom shell is a hydraulic water cylinder whose ram is connected to the top shell. As the top shell opens and closes, it operates the ram producing high pressure water and pumping it ashore to be used to drive power generators.

Once the water passes through the onshore drive mechanism it is returned in a closed circuit to the oyster hydraulic system.

At present an Oyster Wave Power Device is installed at the EMEC in Orkney where it produces over 300kW of electrical power providing a clean and reliable source of renewable energy to the island’s community.

Now, please read on to see the rest of the wave power devices and, some tidal stream devices along with operational sketches

Wavegen Limpet Oscillating Water Column Device.

The Limpet (Land Installed Marine Power Energy Transformer) is operating on Islay, an island located off the Scottish Coast. It provides the islanders with a combined output of 500kW.

It is an Oscillating Water Column device which uses the power of the waves to produce compressed air in a chamber. The air drives a couple of Wells turbines which incorporate power generators, developed by Professor Wells of Queens, Belfast.

The Wells turbine is designed to turn in the same direction whether it is being rotated by drawing incoming air, or being driven by air produced by the waves.

The Limpet was constructed from concrete in a natural gully on the Islay coast. This location was chosen for its proven ability to operate constantly through supplying large quantities of wave energy regardless of the status of the tide.

As the waves advance into the chamber they push the air through wells turbines, and as the waves recede they draw ambient air through the wells turbine rotating the integrated generators.

If you have ever stood on top of a cliff on the coast with a blow-hole connected to the tide, you will have heard the whoosh of air being expelled as the water level rises and falls due to the wave’s energy. This is the simple innovative operating principle of this Oscillating Water Column Device produced by Voith Hydro Wavegen of Inverness.

Current Projects

They have installed another Limpet in the Faroe Islands and are in the process of developing a larger version of this onshore OWC which will be capable of delivering 100MW of power. The company have their own test centre in Inverness which they use as well as the Orkney Limpet to test various components used in floating offshore OWC devices such as The Osprey. This device consists of a vertical caisson in which the water rises and falls, driving larger Wells Turbine generator units.

Pelamis Wave Energy Converter

This attenuator wave energy device is constructed by the linking of long tubular sections, which are hinged together to form a long snake-like structure from whence it gets its name.

The hinges cause the structure to oscillate on the surface of the sea thereby operating hydraulic rams. The pressurized hydraulic oil is led to accumulators supplying hydraulic motors which rotate and drive a power generator of 250kW. There are usually three units on the structure giving a total power output of 750kW.

The device was designed and constructed by the Pelamis Wave Power Company of Edinburgh. This was the first commercial offshore wave power device to supply the national grid in 2004.

A Pelamis wave farm was commissioned of the Portuguese coast supplying 2.25MW of power, although it has since been decommissioned. However a similar array has been planned for the construction of a wave farm at EMEC in Orkney with an output of 2MW which is funded by the Scottish Executive.

Tidal Stream Devices

There are abundant locations suitable for extraction of this energy in and around the UK and Scottish coastline, and between the mainland and the islands.

Numerous devices have been designed and I have listed the popular ones below.

  • Vertical and Horizontal Turbines – similar to wind turbine operation the currents rotate the blades which drive the power generator.
  • Oscillating Hydrofoil – shaped like the wing of a skate or stingray, this oscillates in the current actuating a hydraulic ram.
  • Venturi Device– this device uses the venturi effect to accelerate the currents through an integral turbo-generator.

Seagen Tidal Stream Turbine

This is an axial turbine which extracts the energy from underwater tidal streams.

The device consists of rotor blades driving a power generator and is supported by a monopile fixed to the seabed. It extracts the energy from the tidal streams in a similar fashion to that of a wind turbine extracting energy from the wind.

Modern devices have two power units consisting of a set of rotors driving an integral power generator. These 600kW power units are mounted on a horizontal crossbeam, the crossbeam being fitted centrally to the vertical monopile. One such device is currently operating in the strong tidal currents of Strangford Lough in Northern Ireland supplying 1.2MW of electrical power to the N.I. Grid.

Sketches of Current Marine Energy Extraction Devices

Pelamis Sea Snake
Seagen Turbine

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