How does tidal energy work?

What is tidal energy?

Tidal energy is a type of renewable power generated by capturing the natural movement of the ocean's tides. As the sea rises and falls throughout the day, this movement creates kinetic energy that's converted into electricity using specially designed turbines.

While this sounds like a new concept, tidal power has been around for decades. The world's first major tidal power station, La Rance Tidal Power Plant in Brittany, France, was built in the 1960s and still operates today, producing 500 to 600 million kilowatt-hours (kWh) of electricity every year.

More recently, South Korea opened the Sihwa Lake Tidal Power Station — currently the largest tidal energy facility in the world, capable of producing 254MW (megawatts) of electricity throughout its lifespan. To put this into perspective in terms of household appliances, a typical dishwasher uses 1.8 kWh per cycle, meaning a single tidal power station could be used to power 141,000 dishwashers at the same time! These projects have paved the way for tidal energy developments in coastal regions across the globe, including right here in the UK.

How does tidal energy work?

Tidal energy produces electricity by utilising the movement of the sea, which is controlled by the gravitational pull of both the moon and the sun. As the tides rise and fall twice each day, they create powerful water currents that can be captured and converted into electricity.

Specially designed turbines are installed on the sea floor in areas with strong tidal flows. As water moves past, the turbine blades rotate, driving a generator that turns this kinetic energy into electrical power. This electricity then travels via underwater cables to the shore, where it's fed into the power grid and distributed to homes and businesses across the country.

Types of tidal energy

There are actually several different methods for capturing tidal energy. The three main types are tidal turbines, barrages, and lagoons.

Let's take a look at how each works:

Tidal turbines

Tidal turbines are placed on the seabed in areas with strong tidal currents. They work by harnessing the natural ebb and flow of the tide as water rushes past their blades, spinning them to generate electricity.

These turbines are most effective in narrow stretches of water where the sea is funnelled between different areas of land. When large volumes of water are pushed through these tight gaps, they create powerful currents with high kinetic energy — perfect for spinning turbines and generating more electricity. The Pentland Firth, a stretch of water between mainland Scotland and the island of Orkney, is home to several tidal turbines that take advantage of these conditions.

Tidal barrages

A tidal barrage is essentially a dam built across a river estuary, using the movement of the tides to produce electricity. During high tide, water flows through a set of sluice gates into a reservoir behind the barrage. As the tide falls, these gates close, trapping the water inside and creating a difference in height between the sea level and the water held in the reservoir. When the stored water is released, it rushes through a series of turbines at high speed, spinning generators to create electrical power.

Tidal lagoons

Unlike barrages that block existing estuaries, tidal lagoons are artificial enclosures built out into the sea or along the coastline. They use concrete walls to create a new, contained area of water rather than damming a natural waterway. As the tide rises, sluice gates open to let seawater flow into the lagoon. Then, when the tide falls, the trapped water is released back into the sea through underwater turbines, spinning them to generate electricity.

Advantages of tidal energy

Tidal energy has many benefits, making it an attractive option for generating renewable power. Some of the main advantages are:

• Highly reliable and predictable — Unlike wind turbines or solar panels, which depend on weather conditions and aren't always worth the investment, tidal power is incredibly consistent. There are two high tides and two low tides every single day, regardless of the weather, season, or time of year. This makes it easy to forecast exactly how much energy will be generated, meaning better planning and grid management.

• Renewable and clean — Tidal energy produces no greenhouse gases or pollutants, making it an environmentally-friendly alternative to the traditional burning of fossil fuels. As the tides are powered by gravity, they'll continue indefinitely — giving us a seemingly endless source of clean power.

• High energy density — Tidal energy is more efficient than other renewable sources. Water is much denser than air, which means tidal turbines can generate more power from slower-moving currents than wind turbines can from faster-moving air. In fact, tidal turbines can be up to 80% efficient, outperforming solar panels and wind generators.

• Low maintenance costs — Once tidal energy systems are up and running, they're fairly cheap to maintain.

• Coastal protection — Tidal barrages can provide an added benefit by reducing the damage caused to the coastline by high tidal surges, offering some protection to local communities whose homes may otherwise be at risk of flooding or erosion.

Disadvantages of tidal energy

While tidal power has plenty of advantages, it's also important to acknowledge some of the drawbacks that come with this technology:

• High upfront costs — Building tidal power plants, whether they're turbines, barrages, or lagoons, takes significant investment. The construction process is complex and expensive, costing on average anywhere between $3–15 million (£2–10 million), which can make developing new projects difficult.

• Limited suitable locations — Tidal energy is most effective in areas with strong tidal currents. This limits where tidal power plants can be built, meaning not every coastal region can take advantage of this technology.

• Environmental impact — Tidal power can unfortunately have an impact on marine wildlife. Barrages, in particular, can disrupt the migration patterns of fish and other sea creatures, while the build-up of silt behind these barriers can alter local ecosystems.

• Limited generation time — Tidal turbines only produce electricity when the tide moves in or out, typically around 10 hours daily. During slack tide (when the water is still), no power is generated, which means this method can't provide constant, round-the-clock electricity.

• Water quality concerns — In enclosed systems like barrages and lagoons, the water can't be replenished as easily. This can lead to sediment build-up and reduced water quality along the coastline.

Is tidal energy renewable?

One of the biggest benefits of tidal energy is the fact that it's completely renewable, meaning it plays a crucial role in our efforts to reduce reliance on fossil fuels as we work towards Net Zero. Net Zero is a strategy that aims to balance the amount of greenhouse gases we produce with the amount we remove from the atmosphere.

The more electricity generated through tidal turbines, the less we need to rely on fossil fuels, which means fewer greenhouse gas emissions contributing to global warming. Similarly to how solar panels work by generating electricity using the sun's rays, tidal power is renewable because it uses the kinetic energy generated by the ocean. This natural process is predictable and infinite, meaning we can continue to generate power in this way forever.

What is tidal energy used for?

Tidal energy is mostly used to generate electricity that powers homes, businesses, and industries nationwide.

In regions with strong tidal currents, this energy can be fed directly into the local power grid. For more remote coastal communities where the traditional energy infrastructure might be limited or even unavailable, tidal energy offers a reliable off-grid solution that can keep the lights on without needing a connection to the national grid.

Tidal energy can also work alongside other renewable methods, such as solar panels or wind turbines, to create hybrid power systems. This combination helps provide a more consistent power supply, as different energy sources can complement each other when one isn't generating at full capacity.

How much electricity can tidal power create?

Tidal power can generate impressive amounts of electricity. A power station with 61 turbines installed on the seabed can produce up to 400MW of energy in total, which works out at roughly 6.5MW for each turbine — enough to power thousands of homes from a single unit.

Let's look at what that means for powering homes: tidal turbines typically generate electricity for around 10 hours per day when the tide is actively moving. If a 6.5MW turbine operates at full capacity during those 10 hours, it would generate approximately 23,725 MWh of electricity every year.

Since a typical UK household uses around 2,700 kWh (2.7 MWh) on a yearly basis, a single turbine could power roughly 8,800 homes annually. When you think about each tidal power plant having dozens of these turbines, the potential of this energy source becomes clearer.

Of course, the actual output depends on factors like tidal strength, turbine efficiency, and location, but these figures suggest just how powerful tidal energy can be as part of our wider renewable energy system.

How much does tidal energy cost?

Setting up tidal turbines or building lagoons can be expensive. However, the large volume of power they generate once they're up and running helps offset those initial costs.

The good news is that the cost of tidal energy has been dropping over time. Since 2018, the cost of generating power from tidal streams has fallen by 40%, meaning it's becoming a more competitive renewable energy source. As the technology develops and more power plants become active, these costs are expected to continue falling.

As it stands, the average cost of generating tidal stream energy is around £150 per megawatt-hour per 100MW of turbine capacity installed. As more turbines are added and the industry grows, this cost is expected to fall — dropping to approximately £90 per megawatt-hour at 1GW (gigawatt) of turbine capacity, and potentially as low as £80 per megawatt-hour at 2GW.

While this might sound confusing, this means that the more we invest in tidal energy infrastructure, the cheaper it becomes to produce.

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