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STRUCTURE
 
II. Lisez le texte et dites si les affirmations suivantes sont vraies ou or fausses. Indiquez le numéro de paragraphe dans lequel vous avez trouvé la réponse
1- Le principe de fonctionnement de certaines centrales d’énergie produite par les vagues repose sur le déplacement d’une colonne d’air qui actionne une turbine.
2- A l’heure actuelle, la production totale d’énergie produite par les vagues ne représente que 2 terawatts.
3- L’électricité produite par les vagues a un coût de revient deux fois moindre que l’électricité provenant de sources d’énergie non renouvelables.
4- Le centre EMC a été financé par une organisation européenne.
5- Le nouveau site expérimental va vendre l’énergie produite au réseau électrique national.
6- La construction Osprey vendait l’équivalent de 2 megawatts au réseau électrique national.
7- Le premier prototype de centrale d’énergie produite par les vagues s’appelait the Edinburgh Duck.
8- Des mesures gouvernementales ont été adoptées en Grande-Bretagne pour soutenir le développement de l’énergie marémotrice.
9- En théorie, les centrales d’énergie produite par les vagues devraient pouvoir fournir 10% de la consommation totale d’énergie.
10- Contrairement aux autres centrales, Limpet est construite sur une côte.
11- Pelamis sera une centrale flottante placée en haute mer.
12- Elle pourra produire davantage d’électricité que Limpet.
 

Power from the waves

In a unique test site off the Scottish coast, rival wave machines will compete head to head as they feed electricity to the grid. Michelle Knott reports on big plans to lift marine energy off the drawing board.

THE rough, tumbling waters off Billia Croo, near Stromness in the Orkney Islands, are a sailor's nightmare. This coast faces the full fury of the north Atlantic, and any structure an engineer is rash enough to build here is guaranteed a heavy pounding. Which is why a team of engineers has chosen this spot for a unique experiment. When it opens next month, Billia Croo will become home to the world's first purpose-built test site for machines designed to extract power from the waves and turn it into electricity.

With funding of £5.65 million from the UK government, the European Marine Energy Centre (EMEC) aims to solve many of the problems that have dogged the construction of prototype wave-power generators in the past. With storm-proof moorings and armoured cables, installing and testing new devices should be simple and cheap. EMEC can also accommodate up to four machines at a time, so designers can directly compare devices under identical conditions, giving them a chance to spot the smallest design tweaks that provide the most benefit for energy generation. The site even has a built-in connection to the electricity grid, which allows the experimental generators to begin earning money for their creators the moment testing begins. "If you're in the early stages of a project, getting connected can be a major part of the cost," says David Langston, development manager at Wavegen, based in Inverness, which is expected to be one of the lab's first customers. "EMEC should definitely be a real help."

Nearly 30 years after British engineer Stephen Salter launched the grandaddy of wave-power devices, the Edinburgh Duck, wave energy is still lagging behind wind and tidal power as a practical source of renewable energy. Part of the problem is that testing wave power machines has proved an expensive and risky business. The powerful ocean swell needed to create useful amounts of power can all too easily wreak havoc on the machines. Wavegen's ambitious 2-megawatt Osprey device was destroyed off the Scottish coast by bad weather in 1995 before it could even be completed. Storms have also set back the European Commission's experimental wave energy plant in the Azores by 2 years. Only Limpet, Wavegen's 500-kilowatt prototype power station on the island of Islay, off Scotland's west coast, is successfully supplying the grid. It is the UK's only wave-power generating station and even this is a shoreline installation. Wave-power engineers reckon that generators floating offshore stand the best chance of commercial success as they will stay clear of the pounding waves in the surf zone, and should generate more energy per wave than the equivalent shoreline installation.

Encouraged by the facilities at EMEC and aided by government funding, Wavegen is trying again. This time it plans to develop a floating offshore device which will be tested next year. The idea is to use the same basic principle as the Limpet, which relies on wave motion to move a column of air through a vertical cylinder and uses the airflow this creates to drive a turbine.

However , EMEC's very first customer will be the Pelamis, or sea snake, developed by Edinburgh-based Ocean Power Devices (OPD). The company plans to use EMEC to launch a 750-kilowatt prototype next month. This will be the first full-scale snake to put to sea, but the developers are confident that it will be up to the job. "It is designed for a 15 to 20-year lifespan," says Max Carcass, business development director for OPD, and to withstand a monstrous swell up to 28 metres high, an event predicted to happen no more than once a century on average.

So what will these developers get from the new lab? Each test device will connect to its own dedicated "socket", anchored into a concrete block on the seabed. Each socket is linked to dry land by a cable that combines an electrical core capable of transmitting 2.3 megawatts and two fibre-optic links, one to transmit data from the device back to the control room onshore, the other to send out instructions to control it.

It's not just the surface of the sea off Billia Croo that is bursting with energy. Currents near the seabed are strong enough to keep the rocky bottom scoured clean, and if left unprotected, the cables would be damaged as they rub against rocks. To help protect them they are armoured and kept in place under heavy, shaped concrete mats which hold them firm even in the surf zone.

From the beach, the power cables are linked via a pair of substations to the national electricity grid, while the communication fibres go to a data collection centre in Stromness about 3.5 kilometres away. There each berth has a separate control suite, which the companies hiring space at EMEC can connect to remotely, from their own offices, via the internet. For use of one of the berths, developers are likely to be charged an annual fee plus a slice of any revenue they make from selling electricity they generate. Though charges have not yet been finalised, John Griffiths, EMEC's chief technical consultant, says tariffs will be designed to encourage "efficient and reliable generation", which he says will "substantially offset the costs of testing if the device performs well".

By cutting the cost of testing wave energy machines in a realistic environment, EMEC should help developers turn their good ideas into commercial reality. Yet EMEC alone is unlikely to be able to drag marine energy off the drawing board and onto the commercial energy market. Electricity generated by wave power costs at least twice as much as electricity generated by conventional, non-renewable sources. At present there are only a handful of wave-power devices in service around the globe, producing no more than a few tens of megawatts of energy in total. Compare this with the full potential for wave power, which the World Energy Council estimates to total about 2 terawatts, or about 10 per cent of global electricity consumption, and it becomes clear just how far the technologies have still to go.

Nevertheless, schemes are being launched at various sites around the world. For example, a long-awaited wave-energy scheme at Port Kembla, about 50 kilometres south of Sydney in Australia, is due to be completed this year. And in the US, a planned prototype installation will enable the Makah people of Washington state to sell energy from wave power directly to the local power company.

In the UK, a recent White Paper committed the government to backing wave power. Although funding remains tight, EMEC is Just the latest in a number of British developments that are aiming to make wave power a reality. The New and Renewable Energy Centre (NAREC) at Blyth in Northumberland opened last year to provide wave tank facilities for testing scale models before developers put to sea. Meanwhile, Seapower, the marine energy industry's trade association, is working with the British Standards Institution to put together performance standards by which investors and insurers will be able to judge a particular scheme. The problem for funders is that there is no benchmark for wave power devices. So the aim is to provide standards that will allow the non-specialist to compare the efficiency of different designs during tests at EMEC, for example, as well as offering a consistent view on the amount of energy that can be generated at a particular site.

"We've got to convince politicians and investors who don't believe in numbers or graphs," says Salter. And the way to do this, he insists, is to test wave-power machines in their working environment. Paper designs are not going to pull in the investors. "They want to see something churning about in the water," says Salter. EMEC aims to give them just that.

Michelle Knott is a technology journalist based in Cumbria