wave power

Scottish sea monsters ride the crest of a wave

1 May 2007

Although wave energy technology is still in its infancy, with most projects in the prototype stage, Europe is leading the way in terms of R & D and political awareness. Scotland shows significant potential, assisted by the recent government announcement of £13m of awards under the Marine Energy Fund, the largest beneficiary of which is to be Ocean Power Delivery, developer of the Pelamis wave energy device.

The areas of the world where wave climates are most ‘energetic’ are the western coast of Europe and southern regions of South America. Scotland and Ireland, with their long coast lines and westerly prevailing winds, show the most potential in Europe with wave power levels of up to 75 kW/m, compared to 25 kW/m on southern Europe’s atlantic coastline and 4-11 kW/m in the Mediterranean basin. The economically recoverable wave resource for the UK alone has been estimated at 87 TWh per year, with 78 TWh/y from Scottish waters. The UK Department of Trade and Industry (DTI) has forecast that wave power could be generating 35 TWh per year by 2025, approximately 20% of the UK’s electricity demand.

The UK government has in recent years shown substantial support for wave renewables projects, notably through the DTI’s £50m marine renewables deployment fund and the renewables obligation (RO) scheme whose current target is 6.7% rising to 15.4% by 2015/16. The government-funded Carbon Trust has also sought to support marine energy companies through its 2005 marine energy challenge programme and technology acceleration project established at the end of last year. Given that at present the average generating cost from marine technologies is around 0.08r/kWh, compared to the EU’s average electricity cost of 0.04r, and around 0.06r for wind, further development is required before the technology is fully competitive even with other renewables.

In Scotland, however, the governing executive has gone further with the RO, setting its target at 40% by 2020, by which point, it forecasts, the marine renewables sector alone could provide some 7000 jobs. To accelerate the push towards this target, the executive recently announced funding of £13m to be awarded to companies developing marine energy projects in Scottish waters.

The funding is to be split among nine bidders whose technology is to undergo testing at the European Marine Energy Centre facilities in Orkney, off the north coast of Scotland. As part of the executive’s allocation, £2.5m was ring-fenced for EMEC – to develop and maintain its wave test centre at Billia Croo on the Orkney mainland and tidal testing facilities on the nearby northern island of Eday. The Billia Croo site consists of four test berths linked via a substation on the mainland to the 11 kV distribution network and the EMEC communications centre in Stromness.

Of the three principal types of marine energy technology, offshore systems are seen to offer the most potential. UK offshore resources are estimated at 50 TWh per year, with near-shore at 7.8 TWh per year and tidal stream at 18 TWh per year. Offshore projects are also favoured for their lower noise and visual impact, in addition to reduced adverse effects on biological habitats, compared to other renewable and wave technologies.

Scottish Power subsidiary CRE Energy’s offshore project has received the largest proportion of the executive’s announced funding, some £4.1m, an allocation that will help pay for the company’s £10m offshore wave project, which aims at deploying a 3 MW wave farm off Orkney. The unit is expected to begin operation in 2008, employing four 750 kW Pelamis wave energy converters (WECs) provided by Edinburgh-based Ocean Power Delivery. At present the farm is the largest offshore project of its type in a mature state of development in the UK.

Pelamis design

OPD first successfully linked a pre-production prototype of its Pelamis device to the grid during tests in August 2004 at EMEC. Following this successful trial, cost reduction and performance enhancement investigations were carried out, culminating, in January, in the test of a full-scale prototype in the North Sea. These trials included testing the device’s mooring attachment system, in addition to survivability tests in waves over 5 m high. The prototype has now been re-installed at EMEC to undergo further tests on various new modifications including a remotely operated mooring attachment system, measures to improve the hydraulic power generation system and an improved control system to enhance power capture.

The Pelamis WEC is a 150 m long, 3.5 m wide, 700 tonne semi-submerged, articulated structure composed of four cylindrical sections linked by three two-degrees-of-freedom hinged joints. The tubular segments were provided by renewables specialist Camcal. The eponymously named Pelamis (Pelamis platurus is a sea snake found in shallow waters in the tropics) is best moored at depths of around 50-70 m, typically 5-10 km from the shoreline, where the high energy levels found in heavy swell waves can be accessed. The wave-induced motion of the structure’s joints is resisted by hydraulic rams that pump high-pressure oil through hydraulic motors, via smoothing accumulators, which then drive generators.

Three of the WEC’s four sections are designed as power modules, consisting of two 125kW generators. Provided by ABB, these generators were selected by OPD for their modified sealing system designed specifically for marine applications, and their ingress protection (IP) rating of 67. The IP rating is a standard used to define an item of electrical equipment’s level of protection against dust and humidity – a rating of 67 corresponds to dust tightness, combined with protection against immersion in water but not continuous immersion. The generator was further adapted for installation in the slimline Pelamis frame by a reduction in its frame size from 3.15 to 2.80 m, resulting in a weight reduction from 860 to 725 kg.

The generators are linked by a common 415V, 3-phase bus. With six generators installed, each Pelamis is able to continue operating in the event of a partial failure. A single transformer is used to step up the voltage to an appropriate level for transmission to the shore. The high voltage power is fed through a single umbilical cable to a junction on the sea bed, and then to the shore via a conventional sub-sea cable. Several devices can be connected together and linked to the shore through a single seabed cable. The complete machine is held in position using a mooring system consisting of floats and weights to prevent the mooring cables becoming taut. The Pelamis is moored so as to ensure that the 5 m drooped conical head positions itself facing directly into the oncoming incident waves.

OPD’s research and development on its Pelamis WEC began 9 years ago. Ongoing testing began in 2001 on a one-seventh scale pre-production prototype in the Firth of Forth near Edinburgh. The prototype had the same technical configuration as that intended for the full-scale model and therefore acted as a representative intermediate scale demonstration model. These tests continued until 2002, when survivability and control tests began on 1:7, 1:20, 1:33, and 1:50 scale prototypes at the wide testing tank at the Ecole Centrale de Nantes in France. In continuing development on its Pelamis designs, OPD is seeking to reduce generating costs, with the aim of making the technology competitive with wind and other renewable technologies by 2010.


There are numerous challenges that must be faced in the development of wave power projects. The irregular nature of wave amplitudes means that obtaining maximum efficiency over a range of excitation frequencies can be problematic. The Pelamis mechanism deals with this problem by storing energy in hydraulic accumulators which provide a smooth flow of fluid to the variable displacement drive motor, resulting in a steady generator output, thereby reducing grid connection problems. In addition, marine hazards and extreme weather conditions must be accounted for. Pelamis has the ability to load shed even if its rated power wave amplitude is exceeded. Conversely, in less energetic waves the device can improve its power capture capability by controlling the restraint applied to the machine’s joints – carried out by hydraulic rams – allowing the resonant response to be turned up to maximise capture efficiency, or turned down to limit loads and motions. The final installation of the device is simplified because it only involves connection to the mooring lines, sea bed cable and flexible umbilical, which are placed during the site’s preparation.

Portuguese Pelamis

A wave farm project for which OPD has already provided its Pelamis WECs is that led by Grupo Enersis of Portugal. Construction of the 2.25 MW array started in January 2006, followed in March by the shipment of three Pelamis devices from the Camcal factory on the Isle of Lewis to the site at Póvoa de Varzim, Aguçadoura, off Portugal’s north-west coast. Completion of the project’s first stage is expected in the coming months with the installation of the three WECs, given stable sea conditions. Enersis has already agreed the wavefarm’s grid connection rights.

The installation procedure, which will employ barge systems, was tested successfully last year at EMEC. The farm’s substation and submarine cable have both been tested successfully by the Portuguese electromechanics group, Efacec. The Pelamis design has also been independently verified by the offshore consultancy group of WS Atkins in accordance with offshore codes and standards set by the independent environmental risk group Det Norske Veritas.

A further investment of h70m is planned for the project’s second phase, which is expected to be completed by 2009. This would involve the installation of a further 38 Pelamis machines, for which the letter of intent has already been signed, bringing the farm’s total capacity to 24 MW covering an area of almost 1km2.

The political climate in Portugal is helping to hasten the deployment of renewables sourced generation with the government aiming at 39% of generation by 2010, 80% of which, it hopes, will be provided by hydro sources. The government is also offering a feed-in tariff of h0.24/kWh for the first 20 MW of power generated – which may be extended to 50 MW – and tax credits for generation from renewables.

The development of the Enersis project on this particular stretch of coastline was considered favourable because of the area’s moderate climate and deep water levels close to the shoreline, which ensures that the Pelamis can be moored at around 1 km, so avoiding problems for navigation and fishing that come into play at distances beyond 5 km. The area’s well-developed infrastructure of ports and grid connections was also an advantage.

Cornish project

A third project involving the Pelamis machine is that under development by E.On and Ocean Prospect. An agreement was signed last September to develop a wave farm of seven Pelamis devices 20 km off the north coast of Cornwall in the south west of England. The further progress of the project, which is named Westwave, is dependent upon the South West of England regional development agency’s Wave Hub project, to which it will be connected. The wave hub concept consists of an electrical grid connection point 12-15 km offshore into which up to four different wave energy projects could be connected, which it is hoped will greatly simplify and shorten the permitting process for developers of other projects in the area. The Wave Hub received research and development funding of £4.5m from the DTI, with the remainder of the its funding, some £15 million, coming from the RDA itself. A decision from the DTI is expected in May regarding planning permission. It is anticipated that if permission is granted for the Wave Hub then the installation of Westwave would begin during 2008.

Ocean Power Technologies plans to connect its PowerBuoy device (see panel) to Wave Hub if its tests at the Orkney site prove successful, and the Fred Olsen group its F03 device. The F03 consists of a semi-submersible floating platform with 21 point absorbers mounted in vertical hydraulic cylinders, the hydraulic pressure thus created being used to drive generators.

Location of EMEC facilities on the Orkney Islands 1. Tidal test centre 2. Wave test centre (Billia Croo) 3. Data centre Ocean Power Delivery’s Pelamis wave energy absorber Artist’s impression of a multiple Pelamis array Internal view of the Pelamis power conversion module How the Pelamis machine absorbs wave energy in two planes How the Pelamis machine absorbs wave energy in two planes

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