March 14, 2015 | MELBOURNE | From the print edition
NO LAND stands between Antarctica and Australia’s west coast—just a vast ocean, rippled and rocked by the Roaring Forties. For centuries these westerlies, which blow between latitudes 40° S and 50° S, powered ships sailing from Europe to Asia. These days, they are also creating waves in the world of renewable energy. At the end of February, a demonstration project designed to use the ocean swell they produce went live. As a result Australia’s largest naval base now gets part of both its electricity and its fresh water courtesy of the ’Forties.
Carnegie Wave Energy, in Perth, has been working since 1999 on what it calls CETO technology. Ceto was the ancient Greek goddess of sea monsters, and Carnegie’s particular monsters are buoys that resemble giant macaroons. They float a metre or two below the ocean’s surface, bobbing up and down in the swell and generating electricity as they do so. The current version, CETO 5, has a capacity of 240kW per buoy. Three of the beasts are now tethered to the sea bed 3km from HMAS Stirling, on Garden Island. They also help to run a desalination plant on the base, for fresh water is a valuable commodity in Western Australia’s arid climate.
The buoys themselves are 11 metres across, made of steel and filled with a mixture of seawater and foam to give them a density slightly below that of water, so that they float. Being submarine means that, unlike previous attempts to extract power from waves, they are not subject to storms and the constant battering that life at the interface between sea and air brings. As Michael Ottaviano, Carnegie’s boss, observes, savvy swimmers in Australia know to dive under—not through—an approaching wave, to avoid getting smashed. The same applies to buoys.
Even below the surface, though, the swell is enough to generate power. Each buoy’s rising and falling drives, as the diagram shows, a pump attached to the seabed at the bottom of that buoy’s tether. This pump pushes water through a pipe to a power station on Garden Island. There, the water’s pressure spins turbines that turn a generator. This arrangement produces about 5% of the base’s electricity.
The pressure can be put to a second use, too—to run a process called reverse osmosis, which removes salt from water. Osmosis happens when a solution of salt and a body of pure water are kept apart by a membrane permeable to water molecules, but not to the ions of which salt is composed. The resulting “osmotic” pressure on the water propels it into the brine through the membrane. Apply sufficient physical pressure to the brine, though, to overcome the osmotic pressure, and H2O will go the other way—which is a neat trick if you want to desalinate seawater.
Creating the necessary pressure requires a lot of energy. Reverse-osmosis desalination plants thus tend to guzzle diesel or electricity. But CETO 5 dispenses with all that. It delivers water at a high enough pressure for reverse osmosis to happen automatically. As a result, about a third of Stirling’s freshwater comes from desalination driven by wave power.
Carnegie aspires to bigger and better buoys it hopes will generate a megawatt each when launched in 2017. These versions, CETO 6, will be 20 metres across and will produce electricity inside themselves instead of at an onshore power plant. That means no pipe is needed; a submarine power cable will do instead.
This is just as well, because the idea is to site CETO 6 further offshore, where the swell is greater but the difficulties of installing a pipe are too. Nor is there any need, in the case of CETO 6, for machinery on the seabed to regulate the flow of water into the pipe. That reduces both capital and maintenance costs. And CETO 6 units are designed to be self-contained, so they can be towed back to land for upkeep.
The lack of a water pipe means, of course, that CETO 6 will not be able to run a desalination plant directly. But in most of the world, where rain is more plentiful than it is in Western Australia, reverse osmosis is a niche application. And, if desalination really is desired, part of the electricity produced can be diverted to that use.
The extra swell and lower running costs mean that CETO 6’s electricity will be cheaper than CETO 5’s. Mr Ottaviano reckons that if CETO 5 were deployed en masse, in “wave farms” with a capacity of 25MW, it could produce electricity at a cost of 30-40 US cents a kW-hour, which is competitive with diesel. At a similarly large scale, CETO 6’s electricity would, Mr Ottaviano says, cost about 20 cents a kW-hour. Ultimately, he thinks, economies of scale could bring that down to 12-15 cents a kW-hour for a 100MW wave farm. That is about the same as the cost of grid electricity in this part of Western Australia, allowing for the subsidy now given to renewable energy sources.
Carnegie also has its sights on markets farther afield. Military bases around the world need secure supplies of energy and water. And wave energy is attractive to island countries like the Maldives that must, at the moment, import fossil fuel at some expense. Whether submarine wave power of this sort will ever become truly mainstream is moot. But Carnegie is showing that, in appropriate circumstances, it could indeed have the wind behind it.