Many forms of renewable power generation, such as wind and solar, suffer from differing power output depending on how intense the natural energy source is at any particular point in time. One neat exception to this is a tidal barrage with multiple lagoons. By managing the water level in each, it is possible to smooth out power generation between tides, as well as make output constant between days with bigger tides and those with smaller tides. It is also possible to use such systems to store excess energy from other renewable generation sites (such as winds farms running at full power during times of low demand) and to release energy at times of maximum demand, or when output from other renewable options is flagging.
With two lagoons and pumps for both, there are a huge number of options. You can maintain one pool at a ‘high’ level, and the other at a ‘low’ level, topping up the former using natural high tides or pumping and drawing down the latter in the same ways. When the tide is high, you can generate power by letting water flow into the low pool from the sea, or by letting water flow into the low pool from the high pool. When the tide is low, you can generate power by letting water flow from the high pool out to sea, or from the high pool into the low pool. Whenever you are producing power, you can use it for any mixture of supplying the grid, pumping up the high pool, and pumping down the low pool.
The combination of pumping with tidal lagoons is even better than conventional pumped storage. This is because you can actually produce more energy letting the previously pumped water flow than it took to do the pumping. Wikipedia explains:
If water is raised 2 ft (61 cm) by pumping on a high tide of 10 ft (3 m), this will have been raised by 12 ft (3.7 m) at low tide. The cost of a 2 ft rise is returned by the benefits of a 12 ft rise. This is since the correlation between the potential energy is not a linear relationship, rather, is related by the square of the tidal height variation.
David MacKay’s book also has a detailed section on tidal pumping and two-lagoon arrangements.
Of course, tidal power is not without environmental consequences. It will certainly alter the marine ecosystems that exist wherever facilities are built, and may create consequences in river systems located behind the barrage. That being said, the many advantages of tidal power as an energy generation and energy storage option mean that it probably has an important role to play in building a sustainable global society.
1-MW tidal turbine to be submerged this fall in Bay of Fundy
Nova Scotia Power has partnered up with Dublin, Ireland-based OpenHydro Group to install a 1-megawatt tidal turbine to the seabed in the Bay of Fundy. It’s OpenHydro’s first installation of its 1-MW machine and is expected to be fully operational later this fall. Over two years the two companies will collect operational data, including impacts on environment, robustness of equipment, and power generation. The sub-sea base was manufactured by a local company in Dartmouth, Nova Scotia.
Ingenious it may be, but commercial viability is a long way off. Tidal-power partisans praise its reliability and easiness on the eye—in contrast with the giant wind turbines near some New England tourist havens—and note that over half of America’s electricity is used in states that border on the ocean. But there are limitations. Most of America’s tidal-energy capacity is in Alaska, too far from big population centres. Industry analysts reckon that, at maximum capacity, tidal power could generate 13 gigawatts nationwide, small beer compared with the 35 gigawatts of wind generation that already exists. Still, areas like Maine could benefit if the costs go down. A recent study shows that Maine could generate 250 megawatts from the tide, 100 of that in the Eastport area alone.
Perhaps the biggest benefit could be to Eastport’s economy. In a county where unemployment reaches 13% in some months, young people are moving away, replaced by seasonal part-timers. “If I wasn’t doing this I wouldn’t have a local job,” says Ryan Beaumont, an ORPC employee who used to work in the sardine industry. The town is striving to make the venture succeed: offering cheap office space and allowing use of its idle port and tugboats. This month Eastport received a $1.4m federal grant to build a manufacturing plant for the ocean-energy industry.
Shooting the Moon
A rush to harness Britain’s tides, with just one problem
COMPARED with solar and wind energy, which are booming, tidal power is an also-ran in the clean-energy stakes. But if you did want to build a tidal power station, there are few better sites than the estuary of the River Severn, in Britain. Its tidal range, the difference in depth between high and low tides, of around 15 metres is among the largest in the world.
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As he describes in a paper just published in the Proceedings of the Royal Society, though, an engineer called Rod Rainey thinks he has a way around both problems. He proposes to replace the conventional turbines of previous planned schemes with a much older technology. Specifically, he plans to span the estuary with a line of breast-shot water wheels. This is a design that dates back to the early days of the Industrial Revolution. Examples can be found fixed to the sides of picturesque old watermills.
But there would be nothing old-fashioned about Mr Rainey’s wheels. Thirty metres high and 60 wide, they would be made, in shipyards, from ordinary steel. Two hundred and fifty of them, along with the necessary supporting structures, would be floated into place and secured to the seabed, creating a line 15km long. Together, they could supply power at an average rate of 4GW. That is about as much as two biggish nuclear power stations would manage. Substituting one of the wheels with a set of locks would provide a shipping channel about twice the width of that through the Isthmus of Panama, permitting upstream ports such as Avonmouth and Cardiff to continue operating.
On paper, at least, Mr Rainey’s scheme looks attractive. Some of its advantages are environmental. The “breast” in a breast-shot water wheel is a structure on the riverbed (or, in this case, the seabed) that forms a near-watertight seal with the vanes on the bottom of the wheel. This means that if a motor is used to reverse the direction in which the wheel is turning, it will act as a pump instead of a generator. By pumping at the right points in the tidal cycle, such a system could minimise the impact on water levels behind it, helping preserve wetlands and other desirable habitats. Conventional turbines turn quickly, mincing any fish that come too close. Mr Rainey’s water wheels, by contrast, would revolve at a comparatively stately three metres a second. This is slow enough, he reckons, to permit fish to swim through easily.
Tidal Power is the Next Big Green Wave | The Walrus