Renewable and Predictable

The energy of the tide is a result of the Earth’s rotation combined with the gravitational pull of the moon and sun. Together, they move the water in the world’s oceans back and forth.

By capturing the kinetic energy in this ebb and flow, the movement of water can be converted into a source of electricity. And that electricity is renewable: the forces of the sun, moon, and earth are not depleted by harnessing the flow of water.

Tidal power is also predictable. While wind and solar are also excellent sources of renewable electricity, they provide intermittent supply. By contrast, we can predict the movement of the tides today, tomorrow…and for the next 500 years to come.

Understanding Tides

Tides and tidal currents are caused by gravitational forces of the Sun and Moon acting upon the oceans of Earth as it spins. Because of its proximity to the Earth, the Moon exerts roughly twice the tide-raising force of the Sun. So the Moon plays a dominant role in controlling our tides and dictating their cycles each day.

Our tides repeat themselves once every 12 hours and 25 minutes – or twice a lunar day – which is the time it takes for Earth to rotate once relative to the Moon.

Tides produced by the Moon:

  • M2 (semidiurnal lunar) 1/2 lunar day = 12h 25min
  • O1 (diurnal lunar) 1 lunar day = 24h 50min
Tides produced by the Sun:

  • S2 (semidiurnal solar) 1/2 solar day = 12h 00min
  • K1 (diurnal solar) 1 solar day = 24h 00min

Perigean-spring vs. Apogean-neap tides

The greatest tidal range and fastest water speeds occur during “perigean spring tides.” During spring tides, the Sun and Moon’s gravitational pulls are aligned so they pull together (at the new and full Moon, when the Sun, Moon and Earth form a line). During perigean tides the Moon, in its elliptical orbit, is nearest Earth (“perigee”) and the lunar tide is greatest.

“Apogean neap tides” yield the lowest tidal range and slowest currents (the Moon is in first or third quarter, so the Sun and Moon are separated by 90° and their gravitational forces partially cancel each other, and “apogee” is when the Moon in its elliptical orbit is farthest from Earth resulting in the smallest lunar tide).

Fundy tides are unusual in that they respond as strongly to the perigee-apogee influence as they do to the spring-neap influence. There is roughly a 7.4-day interval between spring and neap tides, a 13.8-day interval between perigean and apogean tides, and the extra-large, perigean-spring tides recur about every 206 days.

Fundy Tides

The primary cause of the immense tides of The Bay of Fundy is a resonance of the Bay of Fundy/Gulf of Maine system. The system is effectively bounded at its outer end by the edge of the continental shelf with its approximately 40:1 increase in depth. The system has a natural period of approximately 13 hours, a Q-value (efficiency) of about 5, and is driven near resonance, not directly by the Moon, but by the dominant semidiurnal tides of the Atlantic Ocean.

The gentle Atlantic tidal pulse pushes the waters of the Bay of Fundy/Gulf of Maine basin at nearly the optimum frequency to cause a large vertical range of the tide in the Bay of Fundy, particularly at its eastern end in Minas Basin.

The Bay of Fundy

On a flood tide, 160 billion tonnes of seawater flows into the Bay of Fundy — more than four times the estimated combined flow of all the world’s freshwater rivers during the same 6-hour interval.

The vertical tidal range can be over 16 metres — giving the Bay of Fundy the highest tides in the world. The horizontal range can be as much as 5 kilometres, exposing vast areas of ocean floor.

The tidal currents in the Bay of Fundy are fast, exceeding 10 knots (5 m/s, or 18 km/hr) at peak surface speed.

Early research from the California-based Electric Power Research Institute (EPRI) identified the Bay of Fundy as potentially the best site in North America for tidal power generation, with a world-class resource close to an existing electricity grid. In the Minas Passage alone, EPRI estimated a nearly 300 megawatt potential (equal to enough power for about 100,000 homes).

More recent research by Dr. Richard Karsten at Acadia University suggests there is more than 7,000 megawatts of potential in the Minas Passage, 2,500 megawatts of which can be extracted without significant effect on peak tide height. Models indicate upwards of 50,000 megawatts of energy exists in the entire Bay of Fundy.

The Guinness Book of World Records states the world’s highest average tides are in the Bay of Fundy, where the mean spring range in the Minas Basin is 14.5 metres (47.6 feet). The highest tide on record in the Bay was 21.6 metres (70.9 feet) in 1869.