Renewable Energy: the Tide Is Turning

Renewable energy: The tide is turning

. 08 October 2008 . From New Scientist Print Edition. . Jason Palmer

Tapping the tides Advertisement

WELCOME to the Bay of Fundy in eastern Canada, home to the highest tides in the world. Here, 100 billion tonnes of Atlantic seawater flow in and out of the 270-kilometre-long bay every day. The sea level at Fundy rises by an average of 11 metres, reaching a maximum of 17 metres at the narrowest point, twice a day without fail, thanks to the moon's gravitational pull. Could this tidal movement be used to generate power?

The unwavering predictability and scale of the tides in some parts of the world make them an attractive renewable energy source. The World Energy Council estimates that Fundy's tides alone could generate 17,000 gigawatt-hours (GWh) of energy per year. Some estimates put the energy in the world's tides at as much as 1 million GWh per year, or about 5 per cent of the electricity generated worldwide, though only a fraction of this is likely to be exploited due to practical constraints.

With so much to gain, it is little wonder that in recent years interest in tidal power has risen and investment has flowed in. Yet it is an enterprise fraught with engineering and environmental challenges. In addition, tidal energy is unevenly distributed across regions and countries, with local geography determining whether it will be economically viable in any given area. Still, there are plenty of researchers, small companies and would-be entrepreneurs vying to harvest the ocean's bounty of "blue energy".

One method for harvesting tidal energy is the use of barrages - dams across a bay or river mouth that are opened as the tide comes in, allowing the bay to fill with water. At high tide the barrages are closed and later the water empties out through hydroelectric turbines, generating electricity. The roughly 12- hour tidal cycle ensures that this process occurs without fail twice a day.

The poster child for tidal energy is in Brittany, France, where a tidal barrage sits astride the Rance estuary. In operation since 1966, the La Rance plant provides 70 megawatts (MW) of power on average and has long since paid for itself. According to French electricity company EDF, which runs the plant as well as other power stations, La Rance's tidal energy costs ¬0.20 per kilowatt-hour, which is less than the company's average cost.

Similar barrages are in operation in Canada, Russia and China, albeit on a much smaller scale. Meanwhile, South Korea is building the world's largest tidal power plant at Sihwa Lake, 25 kilometres south-west of Seoul. Scheduled for completion next year, the plant will have a capacity of 254 MW, enough to power the nearby city of Ansan.

La Rance and Sihwa Lake will look like puddles if two ambitious projects proposed by Russia are realised. Engineers are planning to build a 15-GW barrage at Mezenski Bay on the White Sea and an 8-GW plant in Tugurski Bay in the far east of the country, to help power nearby industry.

The barrage approach has many detractors, however. Environmentalists claim that the water trapped in the bay or estuary by the barrage floods tidal plains and mudflats, displacing wildlife such as the shorebirds which rely on them for food. Barrages could also have more far-reaching effects. Computer models of the Bay of Fundy have shown that a large barrage would affect the tides as far away as Boston, 500 kilometres to the south.

However, studies at La Rance reveal that the area now has greater biodiversity than before the barrage was installed. It also draws a quarter of a million tourists every year, helping to boost the local economy.

Barrages draw the most fire, though, because they are huge infrastructure projects that require many years of work and enormous investment before they produce any juice. For example, the Severn estuary in the UK enjoys the world's third-highest tidal range with an average of 7 metres, rising to over 14 metres, which could provide up to 17,000 GWh of electrical energy per year. Proposed Severn barrage schemes have met with resistance, though, because they would cost upwards of £15 billion and require decades to complete.

There is a potentially cheaper way to exploit tidal energy that is also kinder to the marine environment: use currents to drive turbines. In essence it is the same idea as a wind farm, but under water. This approach has already been shown to work, having provided energy to the northern Norwegian town of Hammerfest since 2003. In the US, Verdant Power has provided energy from a small tidal-current project in the East River, New York, to a supermarket and parking garage since 2006.

Recent months have seen a spurt of activity on a grander scale. In August, UK company Marine Current Technology used tidal currents in Strangford Lough, Northern Ireland, to provide power to the national grid. The firm hopes the scheme will be at its full 1.2-MW capacity by next month. In March, Lunar Energy, another UK company, signed a £500 million deal to build a 300-turbine farm off the South Korean coast - the largest of its kind yet proposed. In New Zealand and India, small pockets of interest are slowly developing into prototypes and tentative energy-production deals.

The idea is not limited to tidal estuaries. Currents in large rivers and the open ocean could also do the job. Frederick Driscoll's team at Florida Atlantic University, Dania Beach, is spearheading an effort to capture energy from the Gulf Stream as the tide passes through a narrow channel off the Florida coast. The team is using a prototype 20-kilowatt turbine.

Estimates for the eventual cost of electricity from these tidal current schemes vary widely, primarily because the technology is unproven on a large scale. A report commissioned by the Carbon Trust, an organisation set up to help UK businesses develop low-carbon technologies, puts the cost of energy generated by tidal currents at between 12 and 15 pence per kilowatt-hour, making it four times as expensive as large wind farms.

However, Roger Bedard of California's Electric Power Research Institute believes that the cost will fall steeply as more turbines are installed. The tidal current industry shares so much technical know-how with the wind industry, the innovation costs should also fall. So can tidal energy provide us with boundless clean energy? Perhaps. It could meet a substantial chunk of a country's electricity demands but, as it depends on geography, only for a lucky few.

Read all the articles in our special issue on renewable energy

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Catching the waves

There's energy aplenty in the oceans' waves - taming them is the tricky part

Moored 5 kilometres off the coast of northern Portugal is the world's first wave farm. Built by UK company Pelamis Wave Power (PWP), the farm comprises three enormous floating cylinders connected by articulated joints. As the articulated structures bend with the waves, they drive hydraulic pistons which in turn operate turbines capable of generating up to 2.25 megawatts (MW) of power.

Worldwide, waves could provide anything from 1000 to 10,000 gigawatts of power, according to the World Energy Council. This means that waves could provide far more energy than the tides. Harvesting that energy is the tough part.

There are widely varying designs among the 60 or so proposed solutions to the wave power challenge, says Stephen Wyatt, an expert in marine energy at the UK's Carbon Trust. "What we haven't seen yet are any clear winners." Unlike with wind and tidal power, there has been no convergence of technology and it doesn't look like happening any time soon. Many seemingly good ideas haven't gone much further than the bar napkins on which they were conceived.

Some trends are emerging, however. Articulated structures like the PWP machines are one option. Another approach uses an oscillating water column, in which a cylinder tethered to the seabed is fitted with a piston connected to a buoy floating at the surface. As the buoy rises and falls with the waves, it moves the piston up and down, which drives water through turbines, generating power.

Vancouver-based company Finavera Renewables plans to use this approach to build a demonstration wave machine in Humboldt county, California. The company has signed a deal with California utilities giant Pacific Gas and Electric to provide a 2-MW wave plant that will plug into the grid in 2012.

Other approaches simply collect water from waves crashing over them. As the water flows back towards the sea it is channelled through turbines that generate power. A Danish company called Wave Dragon has successfully trialled prototype devices.

The trouble with waves is that their height and frequency vary hugely depending on the weather and geography. A technology designed to work best for metre-high waves arriving every few seconds will be inefficient for smaller, more frequent ones. What's more, wave farms have to withstand storms and freak, giant waves, and this makes them expensive to build. Like tidal power, location is paramount.

Jason Palmer