How do tidal turbines work?
Tidal turbines are underwater, or partially underwater, electrical generators that work much like wind turbines except underwater. While in principle the same, the higher density of water, and differing pressure and temperature conditions at depth in a water column does mean that tidal turbines tend to smaller and more rugged in design.
While this might sound very cutting edge, using tidal power to do work is actually nothing new. Tidal mills, for example, were a very common practice in many parts of the world since the middle ages.
One of the main benefits of tidal turbines is the fact that their energy source, the tide, is a consistent and reliable source of kinetic energy. This makes the technology a very attractive proposition as it doesn't require backups from more traditional power generation when the wind doesn't blow or the sun doesn't shine.
Tidal turbines are generally installed on the seabed at locations with high tidal current velocities, or strong continuous ocean currents, where they extract energy from the flowing water.
Water currents and tides are driven by a combination of factors, including temperature gradients and salinity variations within the water, global wind systems, and gravitational forces from the Earth's Moon and, to a lesser extent, the Sun.
Variation of the moon's gravitational field over the volume of the earth causes a "tidal bulge" - displacement of small amounts of water in the oceans over a very large area. The ocean bulges reflect from shorelines, setting up currents, resonant motion, and standing waves, and driving coastal tides. Coastal topography can also intensify the fluctuations in coastal water height fluctuations.
But, it is a little more complicated than that, as you can imagine. It is not only the water column that distorts in response to gravitational forces but also the Earth too (ever so slightly). In fact, this is why the Earth is actually more ellipsoid than spherical in shape -- though ever so slightly. In fact, the Earth is slightly "podgier" at the equator by around 21,300+ meters (70,000 feet) than compared to the poles (on average).
Not only that, but Earth's "internal" gravitational forces are also not equal across its surface and are constantly changing (though very slowly).
It is a combination of these factors, that primarily causes the two high tides and two low tides which anyone who lives by a coastline are more than familiar with. All this water moving up and down, twice a day, has an immense amount of energy that has the potential for human exploitation.
And that is where technologies like tidal turbines "earn their crust". By effectively tapping into an endless supply of free energy, tidal turbines are pegged as being an important part of the energy mix of many nations around the world -- especially those with large tidal ranges, like the UK.
To tap into this energy source, tidal generators work much like wind turbines, except that movements in the water column turn the blades, and subsequently turn a dynamo to generate electricity.
Tidal turbine rotors tend to be smaller than wind turbines (though not always) as water is much denser than air. This has the added benefit that the turbines can be placed closer together while still generating equivalent amounts of electricity to their "supra- marine" cousins.
This generated electricity is then transferred to the electrical grid via undersea cables that travel from the turbines to the land.
What is the largest tidal turbine system in the world?
At present, one of the largest tidal turbine projects is located off the coast of Invergordon in Scotland. This generator is a true monster, with 18-meter (59-foot) blades, weighing in at over 140 tonnes (286,000 pounds), and stands an eyewatering 22.5 meters (74-feet) tall.
This enormous tidal turbine is able, in ideal conditions, to generate enough electricity to power around 1,000 homes. That is impressive, to say the least.
While this all sounds great, tidal turbines do come with some limitations. For example, they need to be sited near to the point where the electricity they generated will be consumed. For this reason, among others, tidal turbine "farm" locations can be limited, but are ideal if the conditions are right -- like in Scotland and other locations around the United Kingdom.
In the US, most tidal turbine projects are located along the coast of Alaska, where population centers are still relatively small.
However, in contrast to wind power, tidal power is very predictable. For this reason, it is easy to decide where to put tidal turbines and determine how much energy they will generate.
The blades of tidal turbines also need to be completely submerged to operate which means they are "out of sight, out of mind" and cannot spoil the view like large wind farms. Win, win.
How much do tidal turbines cost?
The cost of any large infrastructure project, like tidal turbines, varies widely, depending on conditions at the intended site, as you can imagine. In general, tidal turbines are inherently more expensive to invest in than their above-water alternatives.
Some estimates for tidal energy in places like Canada estimate costs of around $0.66 Canadian per kWh to install. This compares to around $0.2-0.3 per kWh for offshore, above-water turbines.
In the rest of the world, the costs come in at around this level too.
- Sihwa Tidal power station in South Korea - The largest, and most expensive tidal installation to date, it has an installed capacity of 245MW and cost around $560 million to build between 2003 and 2011. Technically a tidal barrage system rather than standalone tidal turbines, it comprises a series of submerging bulb turbines to generate electricity.
- La Rance tidal power station in France - This is one of the oldest in the world and was built in 1966. It has a capacity of 240 MW and cost around $918 million in 2019 inflation-adjusted dollars. This is also another tidal barrage-type installation.
What is the Atlantis project?
SIMEC Atlantic Energy ("Atlantis") is working on a number of projects involving specially-designed tidal turbines developed in collaboration with Asturfeito SAU and GE. Atlantis is, in their own words, "a global sustainable energy company, aiming to become the leading independent sustainable power generator in the UK."
To date, they have successfully developed sustainable energy projects with a total capacity of 1,000MW around the world. These projects have included the world’s largest flagship free-stream tidal power project, MeyGen. The company also provides supplies and maintenance services for their turbines.
Their tidal turbines tend to be fixed to the seabed via a gravity base or drilled pylons and are connected to the electrical grid using an armored power export cable. This cable is normally controlled using a standard SCADA system.
Atlantis' turbines are pretty sophisticated and have active pitch and yaw, and can be installed with relative ease once the foundations are complete on the seabed. According to Atlantis, this can typically be achieved within 45 minutes to install the nacelle unit.
Units can also be retrieved in an equally rapid time. Installation and decommissioning also do not involve the use of divers, making it safer and cheaper.
Each of their turbines has a typical operational life of 25 years with 5-yearly maintenance cycles.
Unlike wind turbines, Atlantis' tidal turbines keep their power conditioning equipment, like inverters, converters, frequency controls, etc, onshore in a dedicated substation building. This enables quick and easy access for maintenance and fault finding.