In 2006, the world’s first commercial wave farm will be operational in Portugal. Using tried and tested Pelamis technology, this 2.25 MW plant will sow the seed of true commercial development of wave power technology, and could make Portugal a world-leader in wave energy.

Ocean Power Delivery Ltd (OPD) recently announced the signing of an order with a Portuguese consortium to build the initial phase of what will be the world’s first commercial wave farm to generate electricity from ocean waves. The consortium is led by Enersis SGPS, SA. Enersis is part of the Semapa Corporation and is one of Portugal’s leading renewable energy companies with 100 MW of mini hydro, 500 MW of wind farms under operation/construction and a further 500 MW under development.


Figure 1. The Pelamis is a semi-submerged, articulated structure composed of cylindrical sections linked by hinged joints
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The initial phase will consist of three Pelamis P-750 machines located 5 km off Portugal’s northern coast, near to Póvoa de Varzim. The €8 million ($9.7 million) project will have an installed capacity of 2.25 MW, and is expected to displace over 6000 t per year of carbon dioxide emissions from conventional generating plant. A letter of intent has also been issued for a further 20 MW of Pelamis machines before the end of 2006 subject to satisfactory performance of the initial phase. The building of the machines is underway and the first phase is due to be completed in early 2006.

Pelamis machines

The Pelamis is a semi-submerged, articulated structure composed of cylindrical sections linked by hinged joints. The wave-induced motion of these joints is resisted by hydraulic rams, which pump high pressure oil through hydraulic motors via smoothing accumulators. The hydraulic motors drive electrical generators to produce electricity. Power from all the joints is fed down a single umbilical cable to a junction on the sea bed. Several devices can be connected together and linked to shore through a single seabed cable.

The design of Pelamis incorporates a novel (patented) joint configuration. This is used to induce a tuneable, cross-coupled resonant response, which greatly increases power capture in small seas. Control of the restraint applied to the joints allows this resonant response to be ‘turned-up’ in small seas where capture efficiency must be maximized or ‘turned-down’ to limit loads in survival conditions. All power conversion equipment is contained within a relatively small (wind turbine nacelle sized) unit called a Power Conversion Module. The machine consists of three Power Conversion Modules, each rated at 250 kW, giving a maximum power output of 750 kW. Power from each module is fed to a transformer contained within the nose section where the voltage is stepped up to minimize transmission losses to shore. The machine is held in position by a compliant mooring system which maintains enough restraint to keep the Pelamis on station but allows the machine to swing head-on to oncoming waves. Reference is achieved by spanning successive wave crests. The Pelamis is designed to be moored in waters approximately 40-100 m in depth (typically 5-10 km from the shore) where the high energy levels found in deep swell waves can be accessed.

Development and testing

The Pelamis P-750 WEC is the result of six years extensive design and development. OPD’s philosophy throughout has been to incrementally model, test and verify each element of the technology and ensure that this is fully understood before proceeding to the next stage. Three key elements that preceded the build of OPD’s first full-scale pre-production prototype (launched in February 2004) were:

  • A ‘full-systems’ 1/7th scale prototype tested at sea (2001)
  • A full-scale power conversion module tested on land (2003)
  • An 18 month design verification to the same factors of safety as an offshore oil and gas platform with WS Atkins.

This is the first time such a design verification has ever been undertaken for a wave energy converter. As specific design codes for wave energy converters do not yet exist, both American Petroleum Institute (API) and Det Norske Veritas (DNV) design codes for offshore structures were used wherever possible. Key factors examined included the extreme loads and fatigue loads on the structure over the initial 15 year design life.


Figure 2. The project will have an installed capacity of 2.25 MW. It is expected to displace over 6000 t per year of CO2 emissions from conventional power plant
Click here to enlarge image

In terms of survivability the machine has been verified to meet the 100-year design wave, which in the case of the European Marine Energy Centre in Orkney is a 28.5 m high wave. OPD self-imposed the need for this verification believing it would be essential to gain confidence from commercial customers, insurers and financiers, and that it would be a prudent step to take prior to full-scale deployment. A key outcome of this process was to enable insurance to be provided for the prototype system at a lower cost than would otherwise have been the case. Insurance is a significant part of the operating costs, even for more mature renewable technologies such as wind turbines due to their relatively high capital costs compared to fossil fuel plant, and it is essential for insurers to gain experience and confidence.

Prototype testing

Following three sets of sea-trials, OPD reached a significant milestone in August 2004 – installation of the Pelamis prototype at the European Marine Energy Centre, Orkney and the first electricity supplied into the UK grid from offshore wave energy. This machine was designed to be a pre-production prototype rather than just a technology demonstrator.

Testing has fully validated the Pelamis concept:

  • The prototype captured and generated electricity from waves in accordance with projections
  • The machine could be safely handled, towed, installed, connected to the grid, operated and removed
  • No fundamental engineering problems were identified
  • The control, alarm and data capture systems all performed as expected, both for general machine operation and for fault identification and analysis
  • All systems have proven to be reliable, although there have been a number of minor problems, particularly with the joint module hydraulic systems. All problems to date have been resolved without major works
  • Maintenance and rectification works have been manageable using modest facilities and equipment.

As would be expected for a prototype system a variety of issues have arisen that have needed addressing. Nevertheless no fundamental issues have been identified – the issues encountered to date have primarily been build or Quality Assurance related. The machines for the Portuguese project will be similar to the prototype but will incorporate a number of modest design improvements, allowing more flexibility in build and increasing power conversion efficiency in small waves. Testing will continue in parallel with build of the machines for Portugal.

Portuguese drivers

Portugal has a good wave resource, with typical energies of 30-40 kW/m and a mean available power of 13.7 GW (120 TWh/year). In comparison, current Portuguese electricity demand is 39.1 TWh/year. The country has experienced very rapid growth in electricity demand over the past ten years, yet still has one of the lowest electricity consumption per capita in the EU. This growth has resulted in CO2 emissions being more than 40 per cent over 1990 levels in 2002, according to United Nations data. To limit emissions, Portugal has set a target for renewable generation of 39 per cent by 2010, requiring an additional 4000-5000 MW of new renewable generating capacity, including 50 MW of wave energy.

A key constraint on renewable project development in Portugal is the availability of grid capacity, which is allocated by tender on a periodic basis by the Direcção Geral de Geologia e Energia for each renewable resource. Most wind projects are built on inland ridges, where wind speeds are high but grid capacity is limited due to the low population density. By contrast, Portugal’s wave resource is close to population centres and existing grid infrastructure, making its development subject to fewer restrictions.

From the Portuguese perspective the key drivers to enable an emerging wave energy market to develop are:

  • The strategic recognition of the importance of Portugal’s wave energy resource which could meet a large proportion of Portugal’s electricity demand and improve balance of payments, reducing dependence on imported energy sources.
  • The potential to contribute towards meeting international emissions reduction targets.
  • The potential to derive economic benefits in manufacturing and employment by being one of the first countries to establish a significant market in wave energy.

Economics

OPD’s Portuguese project benefits from a special feed-in tariff established by the Portuguese government to support wave energy. While this feed-in tariff is higher than that provided for hydro and wind projects (~€70 and €90/MWh respectively) it is lower than that provided for solar photovoltaics (€420/MWh). It is also limited to a specific MW capacity such that the impact on the overall consumer price of electricity is minimal.

While the wave tariff is relatively high compared to conventional generation, it recognises the fact that early small-scale projects suffer from ‘diseconomies of scale’ – for example the fixed costs required for the sub-sea export cable, substation and project development work are proportionally very high for a small scale project. It also recognises the fact that all technologies have required some form of support to make the transition from first prototypes to mainstream electricity production. For example, gas turbines were only available for use in mainstream electricity generation following a long period of investment, development and production in the military, aerospace and marine sectors, while nuclear fission has benefited from state support of research and development.

Another example is the wind industry, which has seen generating costs fall by 80 per cent over the past 20 years, making wind cost competitive with conventional generation on some of the best sites.

The positive message for wave energy is that its opening costs are around half of the wind industry’s some 20 years ago. Indeed EPRI concluded that wave energy is expected to be a lower cost option than wind energy for the same market deployment. This is not surprising when one considers that wave energy is one of the most concentrated forms of renewable energy. The difficulty in harnessing this resource to date has been less about power production per se but more about how to deal with the statistically infrequent but significant forces that can occur in storm waves without incurring excessive costs in steel and concrete structures. Pelamis offers a unique and elegant solution to this problem by being a survivable concept where energy adsorption is limited in extreme waves and avoids the need for structures to resist these forces.

In Portugal, OPD is working closely with Enersis and other Portuguese institutions including the Agência de Inovação, Agência Portuguesa para o Investimento, Portuguese Wave Energy Centre, Instituto Hidrografico, and INETI. Having had 17 years of experience developing, constructing and operating mini hydro schemes, and nine years with wind farms, Enersis now believes the time is right for wave energy to become the new Portuguese indigenous renewable resource and has backed the Pelamis system as the way to deliver this.