Like HEP (hydroelectric power), tidal power is a renewable energy source, converting water movement into electricity. But unlike HEP, tidal power stations are few and far between, despite their longevity and reliability. Why? Well, tidal power plants require specific conditions, their construction can cause considerable environmental damage – and they're not cheap to build either! Keep reading to discover why this energy source has yet to reach its full potential.
Tidal Power: Definition
Before tidal power is explained, let's recap tides themselves.
Tides are the regular rise and fall of sea level caused by the gravitational pulls of the Moon and the Sun.
These gravitational pulls cause 'bulges' in the ocean, forming high tides. These are alternated with low tides as the Earth rotates. It takes 12 hours and 25 minutes for an area to go from one high tide to another high tide (vice versa for low tide). The tides are also impacted by the monthly lunar cycle. Twice a month, during new or full moons, the largest spring tides occur. During half moon periods, the smallest neap tides occur.
A neap tide is when the difference between high and low tides is at its lowest whereas spring tide is when the difference between high and low tides is the largest.
Fig. 1 - Tide diagram
So, What is Tidal Power?
Now we've recapped tides; we can begin to explore tidal power.
Tidal power is a renewable source of energy produced by surging ocean waters during the rise and fall of tides.
Tidal Power Generation
Energy cannot be created or destroyed – the amount of energy in the universe will always remain constant. However, energy can be transferred from one form or object to another.
When a kettle is boiled, electrical energy is transferred into thermal energy.
Tidal energy is a form of kinetic energy – i.e., the energy of movement. Tidal power is generated by the force of the moving tides pushing against a turbine.
A turbine is a series of blades mounted on a rotor shaft.
The rotor shaft of a turbine is attached to a generator, which converts the kinetic energy of the rotor into electrical energy. And thus, tidal power!
Tidal Power Turbines
Tidal turbines are installed on the sea floor. Water is approximately 800 times more dense than air, so tidal turbines need to be as sturdy as possible. The extra weight makes tidal turbines expensive to build compared to other forms of renewable energy generation.
The average wind turbine costs up to $4 million to build – but a tidal power plant can a shocking $15 million!
However, the extra building costs soon pay for themselves. Tidal turbines can capture more energy using the same size blades as a wind turbine.
Tidal Power Plants
There are three types of power plant used to generate electricity from the tides:
- Barrages
- Lagoon construction
- In-stream turbines.
Tidal Barrages
A tidal barrage is a large dam-like structure, constructed across tidal rivers, bays, and estuaries. Turbines are attached to the underside of the dam.
The barrage gates are opened as the tides begin to rise, then closed once high tide has been reached, creating a pool. The trapped water is gradually released, spinning the turbines as it travels back out to sea. Engineers control the release to ensure a steady rate of energy generation.
Lagoon Construction
Tidal lagoons are bodies of water partly enclosed by a natural or man-made barrier. Natural tidal lagoons are often found at estuaries, where they receive a constant supply of freshwater.
A lagoon power plant would function much like a tidal barrage; trapping water and controlling its release through turbines.
In-stream Turbines
Tidal turbines are placed directly into a tidal stream.
A tidal stream is a fast-flowing current created by tides.
The fastest tidal streams often occur in straits or inlets; land masses funnel the water.
In-stream tidal turbines are most effective when placed in a shallow tidal stream. This maximises energy production, and causes minimal disturbance to ships and marine life.
Advantages of Tidal Power Services
Like many other forms of renewable energy, tidal power is carbon-free, producing no greenhouse gases or other pollutants. But tidal power services have a few other traits that make them stand out from the crowd.
Reliability
Tides go in and out like clockwork every day, regardless of the season or weather. This makes tidal power a predictable and reliable source of renewable energy.
Energy Efficient
Tidal power has a higher energy density than most other forms of renewable energy.
Energy density is the amount of energy stored in a system.
Furthermore, tidal turbines operate at 80% efficiency – higher than solar panels or wind turbines.
Longevity
Tidal power plants can last a long time. The average lifespan of a tidal barrage is 100 years – four times longer than a wind or solar farm!
Even nuclear power plants are short-lived compared to tidal plants; reaching retirement after 35 years of operation.
Limitations of Tidal Power Services
Producing renewable energy using the power of the tides is a newly emerging field, but one with great potential.
However, there are very few commercial-sized tidal power plants worldwide. Why is this?
Tidal Ranges
Not all coastlines are suitable locations for tidal power plants. This is because the tidal range varies around the world.
Tidal range is the vertical difference between a high and a low tide in a given area.
Using tidal power as a form of renewable energy is most suitable in an area with large tidal ranges. These areas are typically coastal locations. Land formations and topology can also affect tidal range – land masses that funnel water, restricting its ability to dissipate, typically produce large tidal ranges.
The Severn Estuary has the second-largest tidal range in the world, averaging 15 metres between high and low tide. This extreme range is formed by the English and Welsh landmasses funnelling the water.
In the early 21st century, the government considered building a tidal barrage on the Severn Estuary to utilise this huge tidal range. It would have had the potential to generate up to 5% of the UK's electricity. However, the project was abandoned.
For a tidal power plant to be economically viable, the coastline requires a tidal range of at least ten feet. Some parts of the world experience small tidal ranges of only three feet, such as the Gulf of Mexico, the Caribbean, and the Mediterranean. In these places, the construction of tidal power plants would not be worthwhile.
Impacts on the Environment
Tidal power projects also raise ecological concerns, which vary depending on the type of power plant.
Tidal Barrages
Tidal barrages significantly affect the land in the tidal range. The mean sea level within the barrage increases, whilst salinity inside the lagoon decreases. These changing conditions affect the plants and animals that can inhabit the area. Fish are other marine animals are often blocked in the lagoon, getting caught in the blades. The feeding grounds of birds are often flooded, so they migrate elsewhere.
Lagoon Construction
This type of power plant has minimal environmental impact. Typically constructed with natural materials, these power plants are submerged at high tides. This allows fish to swim around the structure, but prevents larger animals such as sharks from penetrating the lagoon. A high fish population would support high bird populations.
In-stream Turbines
Installing in-stream turbines can disrupt the tide. The environmental impact depends on the size of both and turbine and the tidal stream. The blades of these turbines turn slowly, to minimise marine life getting caught in the system.
Fig. 3 - In stream turbine
Tidal power converts kinetic energy of the tides into electricity using underwater turbines. It's a renewable, predictable and efficient source of energy, but there are few working plants due to tidal range requirements and ecological impacts.
Tidal Power - Key takeaways
- Tidal power is a renewable source of energy produced by surging ocean waters during the rise and fall of tides.
- Energy is generated by the force of the tides pushing against turbines, which convert the kinetic energy of the water into electricity.
- There are three types of tidal power plants: tidal barrages, tidal lagoons, and in-stream turbines.
- Tidal power is a reliable and predictable source of renewable energy. Power plants are also highly efficient and long-lived.
- However, tidal power plants are only suitable in parts of the world with a tidal range of at least ten feet. Furthermore, their construction can cause significant ecological damage.
1. David Thomas, Introducing Oceanography, 2012
2. Department for Business, Enterprise and Regulatory Reform, Severn tidal power feasibility study: BERR appoints contractors, 2008
3. Kim Rutledge, Tidal Energy, National Geographic, 2022
4. Kyle Browning, Commercial Wind Turbine Cost ($1 Million Cost Breakdown), Climatebiz, 2022
5. N-Sci Technologies, Tidal Energy and How it Works, 2019
6. PMI, Making tidal energy more affordable, 2016
7. Rebecca Foster, There is no time to ignore the clockwork reliability of tidal power - Rebecca Foster, The Scotsman, 2022
8. Richard Howell Fleming, Atlantic Ocean, Britannica, 2021
9. Ste Wright, Where are the world’s largest tidal ranges?, Tides Today, 2022
10. Tethys Engineering, La Rance Tidal Barrage, 2019
11. U.S. Energy Information Administration, Electricity explained, 2021
12. U.S. Energy Information Administration, Tidal power, 2021
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