# What Is Tidal Energy?

> Tidal energy is electricity generated from the rise and fall and the flow of the sea. Here is how it works, why it is so predictable, the main technologies, and why the UK is one of the best places on Earth for it.

*Section: Environment — By Elena Marsh (Environment & Climate Correspondent) — Published August 13, 2023 — 6 min read*

Canonical URL: https://dailyjunction.org/environment/what-is-tidal-energy
Tags: tidal energy, renewable energy, marine energy, clean energy, tidal power

## Key takeaways

- Tidal energy is power generated from the movement of the sea caused by tides, driven by the gravitational pull of the Moon and Sun.
- Its biggest advantage is predictability: tides can be forecast years ahead, unlike sunshine or wind.
- The two main approaches are tidal stream, using underwater turbines, and tidal range, using barrages or lagoons to trap water.
- The UK has some of the strongest tides in the world and is a global leader in developing tidal stream technology.
- It remains a small part of the energy mix because of high costs, limited suitable sites and environmental concerns.

The sea is never still. Twice a day, vast volumes of water rise and fall along the world's coastlines, dragged by the gravity of the Moon and the Sun. That endless motion carries an enormous amount of energy, and tidal power is the attempt to capture it. Among renewables it has a rare and valuable quality: it is utterly predictable. We know precisely when the tide will turn next week, next month and next year. Here is what tidal energy is, how it is harnessed, and why the UK is one of the best-placed countries on Earth to use it.

## What it is

**Tidal energy is electricity generated from the movement of seawater caused by tides.** As the tide comes in and goes out, and as water flows through channels and estuaries, that motion can be used to drive turbines, producing power in much the same way that moving air drives a wind turbine.

Tides themselves are created by the **gravitational pull of the Moon and, to a lesser extent, the Sun** on the oceans, combined with the Earth's rotation. Because those astronomical forces are constant and well understood, the tides follow a reliable cycle. That makes tidal energy a renewable source in the same family as solar, wind and [geothermal energy](/environment/what-is-geothermal-energy), but with a distinctive twist that sets it apart from all of them.

## Why predictability matters

The single biggest selling point of tidal energy is that it can be **forecast with great accuracy far into the future**.

Solar and wind are *intermittent*: clouds and calm days make them variable and only roughly predictable a short time ahead. Tides are different. They are governed by the orbits of the Moon and Sun, so tide tables can be calculated years in advance with confidence. The energy still comes and goes through the day as the tide ebbs and flows, but unlike the weather, exactly *when* it will do so is known.

> Tidal energy is variable but not unpredictable. You cannot generate at every hour, but you know precisely which hours those will be, which makes the output far easier to plan around.

This predictability is genuinely useful for managing an [electricity grid](/science/what-is-electricity), where matching supply to demand is the central challenge. Knowing exactly when tidal power will arrive allows operators to plan other sources around it. Water is also far denser than air, so a tidal current carries much more energy than wind of the same speed, meaning compact turbines can produce substantial power.

## The main technologies

There are two fundamentally different ways to capture tidal energy, and they work on different principles.

### Tidal stream

**Tidal stream** systems use underwater turbines placed in fast-flowing tidal currents, effectively an underwater wind farm. As the tide pushes water past the blades, they turn and drive a generator. The turbines can capture flow in both directions, generating as the tide comes in and again as it goes out.

This is the faster-growing approach, partly because it can be installed without damming an estuary and so has a smaller impact on the surrounding environment. Arrays of tidal stream turbines are being deployed in places with strong, reliable currents.

### Tidal range

**Tidal range** systems exploit the difference in water height between high and low tide. The classic method is a **tidal barrage**, a dam-like structure across an estuary with gates and turbines. Water is held back as the tide rises, then released through the turbines as it falls (and sometimes captured on the incoming tide too), generating power from the height difference.

A variation is the **tidal lagoon**, an enclosed area of coastline built for the same purpose without blocking an entire estuary. Tidal range projects can generate large amounts of power but require major construction and tend to have a bigger effect on local habitats.

| Approach | How it works | Main trade-off |
| --- | --- | --- |
| Tidal stream | Underwater turbines in tidal currents | Lower impact, but needs strong flows |
| Tidal barrage | Dam across an estuary holds and releases water | High output, but disrupts the estuary |
| Tidal lagoon | Enclosed coastal basin holds and releases water | Flexible siting, but costly to build |

## The challenges

If tidal energy is so reliable, why does it supply only a tiny fraction of the world's electricity? Several obstacles stand in the way.

- **High cost.** Building and maintaining structures in the harsh marine environment is expensive. Saltwater corrodes, storms batter, and access for repairs is difficult, so tidal projects have high upfront and running costs compared with mature technologies.
- **Limited sites.** Only coastlines with a large tidal range or strong currents are suitable, and not all of those are near where power is needed. The number of genuinely good locations worldwide is modest.
- **Environmental impact.** Large barrages can alter the flow of an estuary, affecting fish, birds and the wider [ecosystem](/environment/what-is-an-ecosystem) that depends on the natural cycle of the tides. Underwater turbines raise questions about marine life too, though their impact is generally smaller.
- **Early stage.** Tidal stream in particular is still developing, so it has not yet reached the economies of scale that have driven down the cost of wind and solar.

These factors explain why tidal remains a small, specialised part of the renewable picture, even as its reliability makes it attractive in principle.

## Why the UK matters

Few countries are better suited to tidal energy than the United Kingdom. Surrounded by sea and shaped by some of the **largest tidal ranges in the world**, Britain has an exceptional natural resource.

The Severn Estuary, between England and Wales, has one of the highest tidal ranges anywhere and has long been studied for large barrage and lagoon schemes, though none has yet been built at scale, partly because of cost and environmental concerns. Meanwhile, the strong currents around Scotland and its islands have made the waters off the north of Britain a global proving ground for **tidal stream** technology, with operating arrays feeding power to the grid.

This combination of strong tides and engineering expertise has put the UK at the forefront of tidal development internationally. Whether it becomes a major contributor depends on bringing costs down, but the resource itself is among the best on the planet.

## The bottom line

Tidal energy generates electricity from the rise, fall and flow of the sea, driven by the gravity of the Moon and Sun. Its standout quality is predictability: unlike sunshine or wind, the tides can be forecast years ahead, making the power easy to plan around, while the density of water packs a lot of energy into compact turbines. The two main routes are tidal stream, using underwater turbines, and tidal range, using barrages or lagoons. High costs, a limited number of suitable sites and environmental concerns have kept it small so far. But for a country like the UK, blessed with some of the world's strongest tides, it remains a genuinely promising piece of the low-carbon puzzle.

## Frequently asked questions

### What is tidal energy in simple terms?

It is electricity made from the movement of the sea as tides rise and fall and water flows in and out. That movement is used to turn turbines, much as wind turns a wind turbine. Because tides are driven by gravity, the energy is highly predictable.

### Is tidal energy renewable?

Yes. It draws on the tides, which are produced by the gravitational pull of the Moon and Sun and will continue indefinitely. The resource is not depleted by use, so tidal energy is a renewable source like solar, wind and geothermal.

### What is the difference between tidal stream and tidal range?

Tidal stream technology places turbines in fast-flowing tidal currents and works like an underwater wind farm. Tidal range technology uses a barrage or lagoon to hold back water at high tide, then releases it through turbines, exploiting the difference in water height.

### Why isn't tidal energy more widely used?

It is expensive to build, only a limited number of coastlines have strong enough tides, and large barrages can disrupt marine habitats. These factors have kept it a small, though growing, part of the renewable mix despite its reliability.

## Sources

- [International Energy Agency](https://www.iea.org/)
- [U.S. Energy Information Administration](https://www.eia.gov/)
- [UK Met Office](https://www.metoffice.gov.uk/)

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