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Technology Corridor

Going With the Flow

June 15, 2003
By Jennifer Alvey

Tidal energy technology improves, but is it enough?

Could ocean energy be the next big thing in renewables?

The San Francisco Board of Supervisors in early May approved a pilot project, estimated at $2 million, to test technology that produces electricity from the tides in the San Francisco Bay.

The city hopes to produce 1 MW with the project and add it to the San Francisco grid by Jan. 1, 2006.

The idea of using the ocean to produce power is hardly new-it goes back as far as 1200 A.D., when farmers in Great Britain and France trapped sea water in ponds to power mills as the tide dropped. But in the modern age of pulling electricity from the grid, ocean energy has not been economic.

Improvement in technology and rising natural gas costs may change the economics of ocean power during the next decade, though, making ocean energy a viable-not to mention pollution-free-option for some coastal communities.

Joseph Neil, CEO of HydroVenturi, certainly hopes so. His company has been one of the driving forces behind the San Francisco project, and it is a leading contender to land the pilot there.

While tidal energy projects are new to the United States, Europe has been pursuing them for decades. In 1966, France commissioned a 240-MW tidal plant in Brittany on the River Rance. Although the plant still operates, its owner, Electricité de France, has not expressed interest in building more tidal projects.

More recently, the Philippines has embarked on a $2.8 billion tidal energy project, the Dalupiri Ocean Power Plant. Phase one of the project is expected to produce in the neighborhood of 100 MW. Project developer Blue Energy, of Canada, projects initial construction costs to be comparable to those of conventional power projects, possibly less as the scale of the project increases.

Advantage: Predictability

One advantage tidal energy has over both wind and solar is predictability. High and low tides are known months, years, even decades in advance. The twice-daily high and low tides produce four peak wave velocity events every day. Of course, the downside is that these peak wave periods don't necessarily coincide with peak electricity demand periods. But since the peak tidal electricity times are known in advance, adding tidal energy to the mix would allow generators to plan better than they can with solar and wind.

There are three main types of technologies that convert ocean energy into electricity:

• Tidal energy makes use of rising tides or currents through a relatively narrow channel to drive a turbine; in some installations, water is trapped behind a gate and released to spin the turbine at low tide.
• Wave energy harnesses the movement of surface waves to pump a working fluid through a turbine.
• Ocean thermal energy conversion uses the temperature gradient between varying layers of the ocean to create steam, which then drives turbines.

The San Francisco project will use the tidal power of its bay, which has several potential sites, including the bottleneck at the point where the Golden Gate Bridge crosses.

Disadvantage: Cost

The utility industry has been highly skeptical of tidal energy, and rightly so, says HydroVenturi's Neil. Tidal turbines, which look like underwater windmills, do work, he says, but typically are very expensive to maintain, due to the harshness of the marine environment. Also, unlike water that powers hydroelectric dams by being forced through essentially one opening, tides flow in different directions at different times of the day and year. Tidal turbines consequently cannot use the same paradigm as hydroelectric dams, according to Neil.

Tidal turbines do possess one key advantage over tidal fences, the other main tidal energy technology: Tidal turbines affect the environment much less.

While tidal energy installations, turbine or fence, certainly pollute far less than even the cleanest gas-fired plant, tidal plants do raise environmental concerns. Early tidal fences, in particular, were wide and low structures placed across a channel of fast-flowing water like an estuary. But blocking so much of the channel often destroyed feeding grounds for wading birds and blocked migration paths of fish, among other deleterious effects.

Best of Both Worlds?

HydroVenturi claims that its tidal fence design is cost-effective and does not harm marine life. Neil says that the company's fences contain no moving parts under water; the fence is constructed solely of steel and concrete, not unlike bridges and oil platforms. With the turbines sited on shore and driven by air compression (see Figures 1 and 2), repairs are at least as easy to effect as in a typical fossil fuel plant, so they are not particularly costly. Indeed, Neil predicts that his company's tidal power plants can go 50 to 70 years without much needed in the way of maintenance.

Engineering teams have been working on making tidal fences more environmentally friendly. Neil says that the tidal fences used by HydroVenturi are screened by a mesh grill designed to ensure that whatever marine life can get through the grill can also get through the device safely. As water accelerates through the venturi (essentially, a funnel), water pressure does not increase, he says. To fish, the water's acceleration would seem akin to being in a fast-moving stream-only a few knots of speed, according to Neil, rather than "some other-worldly" high rate of speed.

But even with these improvements, HydroVenturi's initial capital expenditure cost still outpaces gas-fired generation. The average cost of a turnkey, gas-fired plant can run as high as $1.1 million per megawatt installed, exclusive of fuel costs. Cap-ex for tidal energy plants runs from $1 million to $1.5 million per megawatt installed, exclusive of fuel costs.

Yet simply comparing the cap-ex of the two plant types is deceptive, according to Neil

One of the most obvious differences between the two types of plants is operations costs. Tidal plants have no fuel costs, as opposed to gas-fired generators. Also, Neil says, the maintenance costs for the kinds of power installations HyrdoVenturi plans to build are low.

HydroVenturi already operates a proof-of-concept facility in Grimsby, England, on the Humber River near the east coast of Britain. The 50-kW generation target for the facility has been exceeded, Neil says, and the cost of power production there has been economically where the company expected it to be. Otherwise, he says, HydroVenturi would not have pushed the San Francisco project.

The California coast offers some of the best potential tidal energy resources in the country, as do coastal areas of Massachusetts and New York. If HydroVenturi can live up to its claims, tidal energy could be the next big thing in those energy-starved, environmentally sensitive areas.

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Jennifer Alvey is associate editor at Public Utilities Fortnightly.

 

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