Marine Energy - Power From the Sea (1)

If you know about rogue waves or rip currents, you can imagine how much power is inherent in the otherwise calm waters of the sea. Where is wave power used and how it can be converted to useful energy? That's precisely what great minds of the green revolution are researching now.

For decades man has explored the concept of extracting energy from the waves. However it is only in the last couple of years that these devices have become a more popular form of renewable energy conversion mechanism. Here in Scotland, this is due mainly to grants from the Scottish Office and European Marine Energy Center (EMEC) facility on Orkney, on which the first wave power energy converter produced electricity to the national grid.

There are a number of different classifications of wave energy converters:

• A Point Absorber is a floating structure that extracts energy from the waves by its alternation at or near the surface of the water, e.g. the Edinburgh Duck device.
• Overtopping Devices are floating reservoirs which are anchored to the seabed and partially submerged, e.g. the Wave Dragon device
• A Terminator is vertical to the direction of the waves and is usually onshore or just off the coast, e.g. the Limpet device
• Attenuators are a long, cylindrical, snake-like structures made up of sections hinged together which float on the surface of the sea, e.g. the Pelamis device

In the following sections we shall examine three of these wave energy extraction devices; the LIMPET, an onshore device, and the Pelamis and Dragon, both offshore wave energy devices.

The first section examines the working principles of the LIMPET, developed in Queens University Belfast (my old Uni) being built and tested at Wavegen Inverness, before being installed in the outer islands of Scotland.

The LIMPET device is built into a natural gully on the rocky shoreline of the island of Islay, and was the first such device to supply power to the grid.

It works on the principle of waves rising and falling in a column, which creates a suction and discharge of air to and from the atmosphere. This can be observed at a cliff top where a natural blow hole emerges from the sea. By standing close enough to the edge of the cliff one can hear the sound of air being sucked in and blown out.

However, this device is manmade and incorporated into a natural channel in the rocky shoreline, consisting of a concrete wave inlet, leading to an air chamber. Here the action of the waves rising up the chamber causes the air to be compressed and expelled through two Wells Turbines. This type of turbine was developed by Belfast University and has the unique advantage of rotating in the same direction whether the air is being drawn in or expelled through it. When the waves fall, this causes air to be drawn in through the Wells Turbines, driving the power generators (which are rated at 250kW each), giving a nameplate output of 500kW (See diagram). It is imperative that the waves do not over-fill the chamber, spilling through the turbines; therefore calculations have to be made showing the max wave height expected in worst case scenario. There are no known environmental detrimental effects of this device.

A diagram of a LIMPET is shown below.

The Wave Dragon device is loosely anchored to the seabed and floats on the surface of the sea. It is usually anchored offshore to catch the highest waves. It has two wave reflectors which guide the waves toward a ramp which allows the waves to encroach and fill a reservoir with seawater. The reservoir overflows, spilling though a large pipe housing a Kaplan turbine, a low-head propeller type turbine suitable for this application. The turbine operates a power generator. Several turbine generators can be employed on the one steel or concrete structure with this device.

Manufacturers claim that the constant output of power can be between 20kW and 11MW depending on the area of the reservoir and number of turbine power generators installed. There are no known detrimental effects on the environment.

My interpretation of a Wave Dragon is shown below.

Thie Pelamis device is made up of circular sections, hinged together to form a long snake like structure from which it gets its name. The hinges cause the structure to oscillate on the surface of the sea thereby operating hydraulic rams. Pressurized hydraulic oil is led to accumulators and a reservoir from which hydraulic motors are supplied and rotate to drive a 250kW power generator. There are usually three units on the structure giving a total power output of 750kW.

A Pelamis wave generation field has been tested and commissioned off the Portuguese coast, supplying 2.25MW of power to their grid. A similar array has been planned for the construction of a wave farm of the Orkney coast. The first Pelamis P2 type wave generator arrived at the EMEC (European Marine Energy Centre in Orkney) on 5th November 2011. Pelamis have secured seabed leases for the wave farm that will consist of up to 14 Pelamis machines. These will be located in the wave farm of the Bernera Island coast; being 100-square kilometers in size, is estimated to produce 10MW of power.

The Pelamis device is not expected to have any harmful effect on the environment. Although it does contain hydraulic oil, the manufacturer states that there are only small quantities and a spill would be too minor to cause danger to native marine life. The oil is also biodegradable in salt water so there would be no need for an expensive clean-up effort.

A drawing of a Pelamis is shown below.

Advantages of Wave Power.

• There is a wealth of experience in offshore structures gained in the offshore oil/gas construction.
• Not dependent on fossil fuels
• Non-polluting renewable energy, and it is free
• Very efficient energy conversion: >75% efficient as opposed to fossil fuel power plants <40%

Disadvantages of Wave Power

• Expensive research and development (most require Government Grants)
• Offshore weather conditions are extremely hazardous and highly corrosive to structures.
• Underwater power supply cable from device to shore is expensive, and nearest landfall is often where the power is not needed