Tidal Energy

If you’re looking for clean, free energy… a song comes to mind.

Tide after tide.
If you flow I will catch – I’ll be waiting.
Tide after tide.

With no particular link to Cyndi Lauper, waves just want to have fun so let’s allow them to do so while catching their drift as a potential energy source using tidal turbines.

Introduction:

Wave energy is a form of hydropower used to convert energy obtained from tides into mechanical and/or electrical power. Wave energy is produced when electricity generators are placed on the surface of the ocean. The energy provided is most often used in desalination plants, power plants and water pumps. Energy output is determined by wave height, wave speed, wavelength, and water density.

sea wave during storm
Figure 1 Ocean Waves

How are Tides Generated:

Tidal forces are periodic variations in gravitational attraction exerted by celestial bodies. It is these forces that are responsible for the currents in the world’s oceans. A local, strong attraction on a part of the ocean allied with moving celestial bodies and the rotation of the Earth leads this bulging part of water to meet the adjacent shallower waters of the shoreline which creates the tides.

Looking back at the equation for gravity it can be concluded that the celestial bodies that most influence tides are the Moon due to its proximity and the Sun due to its size. It is then easy to understand that due to the non-circular, eccentric and inclined orbit of the Moon around us as well as of the Earth around the Sun then there are times during the year (1 revolution of the Earth around the sun) as well as for every revolution of the Moon around the Earth (once every 27 days) that the tides are non-uniform and therefore change every day. In addition to the aforementioned influences, tides also depend on the local geography (both in terms of sea floor and coastlines). Since the Moon does not plan on stopping its orbit anytime soon, and neither does the Earth around the Sun or around its axis, this means that tidal energy is indeed a renewable energy.

Also, as it does not create any byproducts (water is being pushed on a blade) it is also considered a clean one. A fun fact on tides and their effect on reducing the angular momentum of the Earth; During the last 620 million years the duration of a day based on the planet’s rotation has increased to the 24 hours we know today from 21.9 hours (corresponding to a loss of 17% of its rotational energy) due to water pumping losses at the planet level through natural restrictions along coastlines and therefore viscous dissipation at the sea bed along with turbulence. The effects of tidal power stations are however negligible with regards to Earth’s loss of angular momentum. Additionally, as it is known to surfers around the world the shape of bays/estuaries can also magnify (or reduce) the intensity of the tides.

History:

Humans have been using the tide to convert kinetic energy into mechanical work for a very long time. Evidence of Tide Mills have been found in Europe as early as the Middle Ages and as well as on the East Coast of North America.  Tide Mills work when incoming water was contained in storage ponds and as the tide went out it turned waterwheels that converted the kinetic energy into mechanical work to mill grain. In the 19th century, technology allowed turning this work into electricity to allow for the flexibility of use and distribution of power that we know today.

The first large-scale power plant using tidal energy (barrage technology) started operations in 1966 in Rance, France, with a total power of 240 MW achieved through 24 turbines. This was the biggest until 2011 when South Korea took the lead to produce 254 MW using only 10 turbines. The Rance plant continues to supply about 0.12% of France’s power demand (~130,000 households) and represents a power density of 2.6 W/m2 and a cost of electricity of EUR0.12/kWh vs. the national average of EUR 0.15/kWh and vs. EU average of EUR 0.20/kWh. The Rance barrage spans 750 m (about half a mile) with a tidal basin of 22.5 km2 (or 9 sq. miles). This site was deemed favorable to tidal power due to its wide average between high and low tide levels of 8 meters (26.2 ft) – the highest in France – with a maximum of 13.5 m (44.3 ft) during the perigean spring tide (when the moon’s location is the closest to the Earth and it coincides with a spring tide, about 3-4 times yearly). With a plant costing about EUR 94.5M, it has paid for itself in less than 20 years.

Figure 11 La Rance barrage
Figure 2 La Rance barrage – https://www.power-technology.com/features/featuretidal-giants-the-worlds-five-biggest-tidal-power-plants-4211218/

In comparison, the South Korean power station utilizes a 12.5 km long seawall and a 30 km2 basin for a total cost of $355.1 M to produce over 550 GWh yearly. These two plants are among the eight operational power stations operating as of August 2011. Other locations of interest based on tide data include China, England, Canada and Russia.

Energy Conversion Principles:

There are multiple methods to convert tidal energy into electrical energy.

Tidal Barrage

The goal is similar in concept to that of dams used along rivers worldwide; a high potential amount of water is stopped from flowing to a low potential water space and is released through hydraulic turbines. In the case of rivers these always flow in one direction (toward the sea/ocean) and regular opening of the valves allows controlling the amount of water on either sides of the dam. However, since potential tidal energy is dealing with shifting tides, the flow goes both ways; when the tide is rising the valves are open which lets the water rise on both sides of the barrage. (“Barrage” is the French word for “Dam”.) When the water reaches its highest point, the valves shut off resulting in two spaces; a trapped, constant depth, high potential water storage called a tidal basin (more French!) and a retiring water going with the decreasing tide. Letting the water flow from the storage back to the ocean allows spinning the turbines, therefore generating power.

Sihwa Lake tidal power station
Figure 3 Sihwa Lake tidal power station – https://www.power-technology.com/features/featuretidal-giants-the-worlds-five-biggest-tidal-power-plants-4211218/

Tidal Stream Generator

These are also called TEC (Tidal Energy Converters) and are “standalone” machines operating like wind turbines but in the water of rivers or estuaries. They first appeared during the oil crisis in the 1970’s and remain the cheapest and least ecologically damaging form of tidal power generation. Given that the power generated by a turbine is proportional to the density of the fluid that flows through it, water is more attractive than using air even at low velocities, because it is about 800 times denser than air.

The power also varies with the cube of the flow velocity which means the hydraulic turbine needs about one-tenth of the flow speed a wind turbine needs to generate the same amount of energy. Practically speaking, only areas where the tide velocity is above 1 m/s are attractive with 2-3 m/s flows resulting in about 4 times more energy compared to a similarly sized wind turbine. As a reference, the tides move at 4 m/s (13 ft/s) in the strait of Strangford Lough in Northern Ireland. It is to be noted that they are most effective in shallow waters, and just like wind turbines, their arrangement is complex due to the fluid-structure interaction from an upstream turbine disrupting the wave that the downstream device is trying to harness. For example, a 1MW prototype is in the works for a project in Scotland where the rotor diameter is only 18 meters. Different devices can be used to generated mechanical work. Among the most common ones we can find:

Bottom-mounted axial turbines vs. cable tethered turbine
Figure 4 Bottom-mounted axial turbines vs. cable tethered turbine- https://en.wikipedia.org/wiki/Tidal_stream_generator#Vertical_and_horizontal_axis_crossflow_turbines
  1. Axial turbines – Similar in concept to windmills, can be mounted on the seabed (like wind turbines) or tethered using cables
  2. Crossflow turbines – Turbine that can be used either vertically- or horizontally-deployed featuring helical designs
  3. Flow augmented turbines – Enclosed turbines (instead of non-ducted ones) helping with upstream flow accelerating and therefore additional power generation
  4. Oscillating devices – Devices using no rotating equipment but instead make sure of an oscillating airfoil that can extract power in one or both flow directions
  5. Venturi effect devices – Devices making use of pressure difference due to Venturi effect to generate power using an auxiliary system
  6. Tidal kite turbines – Underwater kite system that simply moves with the tide to produce mechanical work

 

Dynamic Tidal Power

Concept patented in 1997 that remains untried yet promising. The idea is to separate the waters perpendicular to the coast in such a way that a difference in sea level (head) is created on either side of a dam-like structure (~ 30 km long) that would be fitted with bi-directional turbines to convert the energy.

Pros

Below are listed some of the advantages of tidal energy over other methods:

  1. Tides are more predictable than wind and sun
  2. Cheaper electricity production cost compared to even nuclear power generation – 1.8 cents/kWh vs. 2.5 cents/kWh, mostly due to its very low running cost.
  3. Clean and renewable energy
  4. Tidal fences (in contrast to tidal barrages) do not interrupt fish migration or alter hydrology
  5. Bigger power density compared to wind turbines but with similar disadvantages
  6. Effective technology even at low speeds
  7. Long lasting equipment – the French barrage has been operating successfully for over 50 years.

 

Tidal turbine
Figure 5 Tidal turbine – https://www.power-technology.com/features/featuretidal-giants-the-worlds-five-biggest-tidal-power-plants-4211218/

Cons

Just like any other technology, this one also has disadvantages. These include:

  1. Historically relatively high cost and limited availability of sites with sufficiently high tidal ranges and/or flow velocities
  2. Some biological impact both in terms of ecosystem migration and death related to impact from the turbine blades.
  3. Noise generation for wildlife that tend to abandon such places (more impact than offshore wind turbines since tidal generators are in the water where sound travels faster and further compared to in the air). The acoustics of such devices also may confuse species like dolphins who use echolocation to navigate.
  4. Difficulty to maintain equipment due to inaccessibility leads to corroded parts in contact with water
  5. Barrages are much more expensive than tidal stream generators due to the added construction, materials, etc.

 

Sound Waves
Figure 6 Sound waves in air vs. water – https://dosits.org/science/sounds-in-the-sea/how-does-sound-in-air-differ-from-sound-in-water/

As we can see, tides are the source of a cyclic, continuously renewed energy that is free of any in-use pollution while presenting a similar environmental disadvantage and technology to wind turbines above ground. Just like for wind energy, the location of tidal turbines needs to be thoroughly researched to ensure that sufficient currents are present all year round in order to produce the target goal of electricity production coming from nuclear plants.

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