Tidal power

About tidal power
Tides can provide large quantities of predictable energy. In suitable locations, energy can be harvested from tidal currents. Also ordinary ocean currents can in some locations reach high enough speeds to be useful for energy production. In total, it is estimated that about 1.100TWh can be harvested from tidal and ocean currents every year.

Stable resource
Tides are the result of the gravitational pull from the Moon and the Sun on the Earth’s land masses and water masses. The force of gravity weakens with distance, so the Moon pulls hardest on the ocean facing the Moon, somewhat less on the Earth itself, and weakest on the ocean facing away from the Moon. Thus two high tides are formed, one on each side of the planet, and the areas in between experiences low tides. Because the Moon orbits the Earth every 24h and 50 minutes, the high (or low)tides are separated by 12 hours and 25 minutes.

The water in the oceans redistribute itself between high and low tides, but have to go around land masses and across both deep and shallow areas. In some parts of the world large ocean volumes, geometry and bathymetry combine to provide the conditions for huge height differences between high and low tides. In other areas, the difference between high and low tides are almost imperceptible.  In Norway highest tidal ranges can be found in the north of the country. Here, several suitable locations for tidal turbines exists, e.g. in narrow straights and mouths of fjords. The currents will turn around every 6 hours, so the production cycle of the power plant can be predicted with high accuracy.

Known technology
Tidal power plants are usually based on two relatively well-known principles:

  • Damming water e.g. in fjords
  • Tidal turbines installed in the current

By building a dam across e.g. the mouth of a fjord, the tides will produce a difference in water height on each side of the dam. The water can be led trough turbines to produce power much in the same way as in river dams, though with different kinds of turbines due to the low water column height.

Tidal turbines are very similar to windmills, but submerged. One or more “propellers” turn a generator. The turbines can be located in straights and other areas with strong currents, and can either be bottom-mounted or floating.

Norwegian developers
In Norway several different companies work on tidal power generation. Four known installations are either operational or in development:

  • Hammerfest Strøm HS300 at Kvalsund, Finnmark. Bottom-mounted turbine, 300kW generator capacity.
  • Hydra Tidal Morild II at Gimsøystraumen, Lofoten. Floating turbine, 1,5MW generator capacity.
  • Moonfish Power. Concept for a floating turbine, 880kW generator capacity.
  • Aqua Energy Solutions. Concept and prototype based on “sails” on a guide wire, 120kW generator capacity.

The three turbine based concepts all use known principles from windmills on land. The main challenge for the developers is to get the plants robust enough, and at the same time economical, both in terms of installation and operations/maintenance. The size of each turbine is usually a compromise: Longer blades produce more power, but will also be subjected to greater loads from turbulence, cavitation and collisions with floating objects in the water. Usually production capacity is increased trough installing more turbines at a location.

The last concept, from Aqua Energy Solutions, is very different from the others. It is based on sails attached to an “endless” wire between two pulleys. The tidal current generates hydrodynamic lift on the sails, which pulls the wire along. A generator is attached to the pulleys. The generation capacity can be increased by either increasing the length of the string (and the number of sails) or by installing several plants in series.