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Wave energy generation devices fall into two general classifications, fixed and floating.

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Wave energy generation devices fall into two general classifications, fixed and floating.

Fixed Generating Devices

Fixed generating devices are either built into the shoreline (i.e. on breakwaters), or fixed to the seabed in shallow water. Fixed systems have some significant advantages over floating systems, particularly in the area of maintenance. However, the number of suitable sites available for fixed devices is limited. The Oscillating Water Column and the TAPCHAN as described below are examples of fixed wave energy generation devices.

 

Oscillating Water Column

The Oscillating Water Column (OWC) generates electricity in a two step process. As a wave enters the column, it forces the air in the column past a turbine and increases the pressure within the column. As the wave retreats, the air is drawn back past the turbine due to the reduced air pressure on the ocean side of the turbine (see Figures 1a and 1b). Irrespective of the airflow direction, the turbine (referred to as a Wells turbine, after its inventor) turns in the same direction and drives a generator to produce electricity.

 

 

Figure 1a & 1b Schematic of an Oscillating Water Column

 

OWC technology is in use in the Isle of Islay, Scotland, where a system called LIMPET has been installed since 2000 (see Figures 2a and 2b). This system has a maximum output of 500 kW. It is ideal for locations where there is strong wave energy, such as breakwaters, coastal defences, land reclamation schemes and harbour walls. This form of energy generation is suitable for producing power for the national grid. In the Isle of Islay, the electricity generated is being used to power an electric bus, the first bus in the world to use wave power as its fuel. (Green Energy Works, 2006)

The performance has been optimised for annual average wave intensities of between 15 and 25 kW/m. The water column feeds a pair of counter-rotating turbines, each of which drives a 250 kW generator, giving a nameplate rating of 500 kW. The LIMPET’s design makes it easy to build and install. Its low profile gives low visibility, so it doesn’t intrude on coastal landscapes or views (WaveGen, 2006).

 

 

Figure 2a & 2b The LIMPET Oscillating Water Column front and rear, installed on the Isle of Islay, Scotland.

 

TAPCHAN

TAPCHAN, or tapered channel systems consist of a tapered channel feeding into a reservoir that is constructed on a cliff as shown in Figure 3. The narrowing of the channel causes the waves to increase their amplitude (wave height) as they move towards the cliff face. Eventually the waves spill over the walls of the channel and into the reservoir, which is positioned several metres above mean sea level. The kinetic energy of the moving wave is converted into potential energy as the water is stored in the reservoir. The generation of electricity is then similar to a hydroelectric power plant. The stored water is then fed through a Kaplan turbine.

 

 

Figure 3 TAPCHAN wave energy device
(Copyright Boyle, 1996).

 

The concept of TAPCHAN is an adaptation of traditional hydroelectric power production. With very few moving parts, all contained within the generation system, TAPCHAN systems have low maintenance costs and are reliable. TAPCHAN systems also overcome the issue of power on demand, as the reservoir is able to store the energy until it is required.

Unfortunately, TAPCHAN systems are not suitable for all coastal regions. Suitable locations for TAPCHAN systems must have consistent waves, with a good average wave energy and a tidal range of less than 1m, suitable coastal features including deep water near to shore and a suitable location for a reservoir.

 

WaveRoller

The WaveRoller device is a plate anchored on the sea bottom by its lower part and pivots back and forth. The back and forth movement of bottom waves moves the plate, and the kinetic energy produced is collected by a piston pump. This energy can be converted to electricity either by a generator linked to the WaveRoller unit, or by a closed hydraulic system in combination with a generator/turbine system. WaveRoller is a modular concept, and in practice this means that the plant capacity is formed by connecting a number of production modules into a WaveRoller plant (see Figures 4 and 5). Due to the modular design, the WaveRoller plant can be taken into production gradually, module by module. AW-Energy, the company that is developing the WaveRoller, claim that the modules are also easily maintained and electricity production can continue during unit maintenance 

 

 

Figure 4 A schematic of AW-Energy’s WaveRoller individual unit and farm system

AW-Energy has conducted WaveRoller marine tests in the European Marine Energy Centre (EMEC) in Orkney, Scotland (see Figure 5), which have verified the energy generation potential of bottom waves and the suitability of WaveRoller in converting this energy source into electricity. The results suggest that WaveRoller will be able to leap-frog other ocean energy technologies in terms of performance and economic considerations. WaveRoller is best suited for locations where wave periods are long and the swell is strong. Furthermore, due to the nature of bottom waves, the power levels achieved throughout the year in these locations fluctuate considerably less than for surface wave devices or wind energy. Based on an estimated nominal power output of 13 kW per individual WaveRoller plate, the investment cost amounts to approx. 2100 euros per kW already at the pilot stage 

 

 

Figure 5 The installed WaveRoller marine test in Orkney, Scotland

 

 

Floating Generating Devices

Floating wave energy generation devices are systems that are floating in the ocean, either close to shore or offshore. The following devices are examples of floating generating devices:

 

Pelamis

The Pelamis (see Figure 6) is a semi-submerged, articulated structure composed of sections linked by hinged joints. The motion of these joints is resisted by hydraulic rams, which pump high-pressure oil through hydraulic motors. The motors drive generators to produce electricity. Several devices can be connected together and linked to shore through a single seabed cable. The machine is held in position by a mooring system comprising of a combination of floats and weights, which prevent the mooring cables becoming taut to maintain the Pelamis positioned and allow it to swing head-on to oncoming waves. The 750 kW full-scale prototype is 120m long and 3.5 m in diameter and contains three power conversion modules, each rated at 250 kW. Each module contains a complete electro-hydraulic power generation system (Ocean Power Delivery, 2005).

 

 

Figure 6 Pelamis machine pointing into the waves: it attenuates the waves, gathering more energy than its narrow profile suggests. 

The Pelamis is manufactured by Ocean Power Delivery (OPD) who recently announced the signing of an order with a Portuguese consortium, led by Enersis, to build the first phase of the world's first commercial wave farm. The initial phase will consist of three Pelamis P-750 machines located 5km off the Portuguese coast, near Póvoa de Varim.  The €9 million project will have an installed capacity of 2.25MW, and is expected to meet the average electricity demand of more than 1,500 Portuguese households. Subject to the satisfactory performance of the first stage, an order for a further 25 Pelamis machines (21 MW) is anticipated (Ocean Power Delivery, 2005). This new floating device is one of the success stories of the wave energy industry and seems to have a bright future. In September 2008 the first 3 Pelamis units were installed  2008).

 

Salter Duck

The Salter Duck is another floating wave energy device, like the Pelamis, which generates electricity through the harmonic motion of the floating part of the device, (as opposed to fixed systems, which use a fixed turbine which is powered by the motion of the wave). In these systems, the devices rise and fall according to the motion of the wave and electricity is generated through the motion. The Duck rotates with a nodding motion as the wave passes. This motion pumps a hydraulic fluid that drives a hydraulic motor, which in turn, drives an electrical generator. The Salter Duck (see Figure 7) is able to produce energy extremely efficiently. However its development was stalled during the 1980s due to a miscalculation in the cost of energy production by a factor of 10 and it has only been in recent years that the technology was reassessed and the error identified.

 

 

Figure 7 A schematic of the Salter Duck wave energy conversion device
(Copyright Ramage, 1996).

 

Wave Dragon

The Wave Dragon is essentially an overtopping device that elevates ocean waves to a reservoir above sea level where water is let out through a number of turbines and in this way transformed into electricity (see Figure 8). The Wave Dragon is a very simple construction and has only the turbines as the moving parts, which is useful for operating offshore under extreme forces and fouling. The Wave Dragon is moored in relatively deep water to take advantage of the ocean waves before they lose energy as they reach the coastal area. The device is designed to stay as stationary as possible, simply utilising the potential energy in the water that overtops it. This water is stored temporarily in a large reservoir creating a head, i.e. the difference between the "mean" level of the ocean surface and the water surface in the reservoir. The water is let out of the Wave Dragon reservoir through several turbines generating electricity in a similar way to hydroelectric power plants.

 

 

 

Figure 8 Waves overtopping a test wave generator of the Wave Dragon in Nissum-Bedding, Denmark. (courtesy of Earth Vision).

 

 

Figure 9 Waves overtop the Wave Dragon over the ramp, which is designed to optimise water capture. 

 

The Wave Dragon ramp (shown in Figure 9) can be compared to a beach. The Wave Dragon ramp is very short and relatively steep to minimise the energy loss that every wave faces when reaching a beach. A wave approaching a beach changes its geometry. The special elliptical shape of the ramp optimises this effect, and model testing has shown that overtopping increases significantly. The Wave Dragon is designed to be sited offshore at more than 20 to 30 meters depth to produce between 4 to 11 MW, depending on wave activity (Wave Dragon, 2005).

 

Archimedes Wave Swing

The Archimedes Wave Swing (AWS) generates electricity by drawing energy from sea swells. It is a simple system of connected air chambers utilising a flywheel effect, using the heave of the sea to produce electrical energy (The UN Atlas of the Oceans, 2006).

The AWS consists of two cylinders. The lower cylinder is fixed to the seabed while the upper cylinder moves up and down under the influence of waves (see Figure 10). Simultaneously, magnets, which are fixed to the upper cylinder, move along a coil. As a result, the motion of the floater is damped and electricity is made. The interior of the AWS is filled with air and when the upper cylinder moves downwards, the air inside is pressurised. As a result, a counteracting force is created which forces the upper cylinder to move up again. For long waves, amplification can be up to three times the wave elevation, while this is even more for short waves. Amplification can be compared with the effect of a swing. If one pushes the swing at the right moment, motion will be amplified (Archimedes Wave Swing, 2004).

 

Figure 10 The pre installed 2MW Archimedes Wave Swing before being totally submerged.

 

The Mighty Whale & JAMSTEC (Japan Agency for Marine-Earth Science Technology)

Developments in Japan began with Yoshio Masuda's experiments in the 1940s (JAMSTEC, 1998). These reached significant scales in the 1970s, and since then, a number of prototypes have successfully been tested in Japan. In the 1970s, the wave-energy group at JAMSTEC developed a large scale floating prototype named Kaimei. The device was tested in the Sea of Japan, off the town of Yura in Yamagata Prefecture. Two series of tests were completed, one of these under the auspices of the International Energy Agency. In the early 1980s, JAMSTEC developed a shore-fixed device  for tests near Sanze, also in Yamagata Prefecture. Since 1987, the focus has been on another floating device named the Mighty Whale (see Figure 11). Projected applications for a row of such devices include energy supply to fish farms in the calm waters behind the devices and aeration/purification of seawater. The prototype dimensions were chosen to be 50 m (length) x 30 m (breadth) x 12 m (depth). The design called for it to float at even keel at a draft of 8 m. The Mighty Whale generates electricity when waves enter the 3 air chambers in the front part of the device. The internal water surface moves up and down generating pneumatic pressure, causing the air turbines to spin. This causes the generators connected to the turbines to produce electricity at a maximum output of 110 kW. (JAMSTEC, 1998).

 

Figure 11 The Mighty Whale launch on March 24,1998.

 


PowerBuoyâ„¢

Ocean Power Technologies (OPT) have developed a wave generation system known as the PowerBuoyâ„¢. The system uses an ocean-going buoy to convert wave energy into a controlled mechanical force which drives an electrical generator (see Figure 12). The generated AC power is converted into high voltage DC and transmitted ashore via an underwater power cable. The PowerBuoyâ„¢ incorpate sensors that monitor the performance and surrounding ocean environment. (Ocean Power Technologies, 2005).

 

Figure 12 An artists impression of an OPT power station showing multiple buoys and underwater transmission cable. Inset shows individual PowerBuoy™. A 10-Megawatt OPT power station would occupy only approximately 4 acres of ocean space 


 

Wave Power in Australia

 

Oceanlinx (formerly Energetech)has produced a prototype model of a new type of oscillating water column. The system uses a parabolic wall, which focuses wave energy into the column. Initial testing has been encouraging and Oceanlinx received a $750 000 grant through the Australian Greenhouse Office's  to construct a 300 kW wave power generator on the breakwater at Port Kembla (see Figure 13). The ocean trial of the Oceanlinx wave energy device took place at Port Kembla on October 26 2005. A proportion of the power generated was used to produce desalinated water on-board the device. The measured power indicates the device performs better than previously predicted from wave tank, wind tunnel, and computational fluid dynamic (CFD) testing. In two metre waves with periods of seven seconds, the results from the trial indicate the device will produce 321 kW, compared with previous predictions of 268 kW. Based on the recent test results, a full scale project should power up to 1500 homes, or produce three million litres of water per day per production unit. 2005). In October 2008, Oceanlinx recieved $AUD16 million from an investor syndicate comprising the New Energy Fund, Espírito Santo Ventures and Emerald Technology Ventures to further develop their technology and to progress key projects (Oceanlinx, 2008).

 

Figure 13 The Oceanlinx wave powered generator off the coast at Port Kembla NSW Australia
.

CETO

CETO is a wave power generation system that uses arrays of submerged diaphragm units that pressurise seawater to operate land based generation equipment that desalinates water and produces electricity. CETO’s developers,  are testing a wave energy prototype that is housed in a 20 metre steel hull and is deployed in around 7 metres of water near the shore at Fremantle (see Figure 14). The prototype has a tower visible about 5 metres above sea level for access and communications during the testing and trial phase. Subsequent commercial units will not have the tower. CETO is anchored permanently on the sea floor, as opposed to floating, or semi submersible devices. This protects against storms and other ocean forces.

 

 

Figure 14 The CETO 1 wave energy prototype in Fremantle, being transported into position

Dennis Kelly, managing director of Seapower Pacific, said CETO was a breakthrough in renewable energy technology at the right time, given the worldwide demand to produce power and water from clean sources. "Unlike other wave energy technologies that require undersea grids and a costly marine qualified plant, CETO requires only a small diameter pipe to carry high pressure sea water ashore at 7000 kpa (1000 psi) to either a turbine to produce electricity, or to a reverse osmosis filter to produce fresh water," Kelly said. "The prototype is expected to generate up to 100 kW of electricity, enough for 100 homes. In desalination mode the prototype is expected to produce about 300,000 litres of fresh water per day"  2005). CETO is also not exposed to possible damage from storms or shipping interference because of its location on the seabed. The complete CETO 2 prototype unit was deployed in January 2008 and is the only fully-submerged wave energy system to produce high-pressure seawater pumped ashore to spin hydro-electric turbines onshore (CETO, 2008).

 

Further Information

RISE Information Portal - Information regarding renewable energy resources, technologies, applications, systems designs and case studies.

Wikipedia - Wave Power

 

References

Archimedes Wave Swing, 2004. Homepage  (Accessed 26 November 2008).

AW-Energy, 2005. Homepage  (Accessed 26 November 2008).

CETO, 2008. Major milestones (Accessed 26 November 2008).

Energetech, 2005. Homepage (Accessed 27 February 2007).

Energyme.com. Webpage  (Accessed 26 November 2008).

Green Energy Works, 2006. Wave Power (Accessed 26 November 2008).

JAMSTEC, 2005. The Mighty Whale (Accessed 26 November 2008).

Oceanlinx, 2008. Homepage  (Accessed 26 November 2008).

Ocean Power Delivery, 2005. Homepage  (Accessed 26 November 2008).

Ocean Power Technologies, 2005. Homepage  (Accessed 26 November 2008).

Pelamis Wave Power Ltd, 2008. Homepage  (Accessed 26 November 2008).

The UN Atlas of the Oceans, 2006. Homepage  (Accessed 26 November 2008).

Wavegen, 2006. Islay  (Accessed 26 November 2008).

Wave Dragon, 2005. Website – principals  (Accessed 26 November 2008).

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