How does tidal power work and what are its advantages?

A common concern about green energy is whether we can rely on it. However, not all renewables are made the same.

What is tidal power?

Tidal power is produced by harnessing the energy from the moon and the sun’s influence on sea levels, known as tides.

It’s a fully renewable resource, which can be accurately predicted decades in advance.

Unlike wind or solar, it operates every day of the year. The only time it doesn't produce energy is when the tide turns, so there are short gaps in energy output every six hours or so.

There are two different forms of tidal power: tidal range and tidal stream.

Tidal range power

Tidal range power generates electricity based on the difference (known as ‘the head’) in water level either side of a marine wall.

As the tide comes in, water is stored behind the marine wall. As the tide ebbs, the stored water remains at its stored level, creating the difference in water levels.

The greater the tidal range, the greater the potential head, determining how much energy can be generated.

The wall therefore acts as a dam, with the water travelling through turbines similar to the ones used for hydroelectric power. Power can be generated both as the tide ebbs and floods.

The length of marine walls can vary depending on location and whether a barrage (connecting opposite sides of an estuary) or a lagoon (a water storage basin projecting out from the same shoreline) is being considered.

A barrage has the advantage of having a shorter marine wall (so costs less) but it blocks the upstream estuary to fish and shipping. As such, more of the local habitat is affected, compared with a lagoon.

Tidal range projects tend to be closer to where the demand is, which means less energy is lost while in transit, and shorter transmission lines are needed. They can also help with stabilising the grid as the turbines can respond very quickly to any fluctuations in grid frequency.

Tidal range projects around the world

The UK’s west coast has some of the highest tidal ranges in the world.

The highest in the UK is over 14m in the Severn Estuary, but tidal ranges of over 10m also exist in North Wales and north-west England.

There are current proposals for tidal range power projects in the Mersey Estuary in north-west England and the Severn Estuary in south-west England.

At present, there are two large tidal range power projects operating in the world – a 240MW barrage in La Rance Estuary in Brittany, France and a 250 MW plant at Sihwa Lake in the Gyeonggi Province of South Korea.

Both are expected to run for 120 years or more. Although expensive to build, their long-term cost is low.

For example, when the Rance tidal power station opened in 1966, its electricity was the most expensive to produce in France. It took about 20 years to pay for itself, and now, its electricity is cheaper than that provided by solar and nuclear.

Tidal stream power

Tidal stream power generates electricity in the same way as a wind turbine, but it’s underwater.

Instead of using wind, it uses the ocean currents from the change in tides, resulting in less environmental impact compared with tidal range.

But this comes at the expense of a shorter operating life (around 25 years).

The turbines are smaller in diameter than wind turbines, but still require deep water, making them harder to maintain.

They are, at this stage in their development, significantly more expensive. The most recent renewables auction revealed offshore wind costs of less than £59/MWh, while tidal stream came in around £170/MWh. This difference should reduce over time.

Tidal stream power in the UK

Tidal stream energy could meet over 10% of the UK’s current electricity needs, research from Plymouth University suggests.

18MW of tidal stream capacity has been developed so far, primarily in the Orkneys (for example, the Meygen project) and more is under development, including at Morlais in Anglesey.

As tidal stream technology is also not as mature as tidal range, the UK also has a number of test sites for turbine development.

What are the barriers to tidal power?

There have been many attempts to realise tidal range power in the UK dating back to the 1920s.

They have not succeeded, mainly due to short-term changes in policy such as electricity privatisation in the 1980s, the dash for gas in the 1990s and the credit crisis of the late 2000s.

Other barriers include the high upfront costs, lack of investment pipeline and ecological concerns. For tidal stream, which didn’t start until the early 2000s, barriers include uncertainty over the technology and cost.

How can engineers support its take-up?

With the increased urgency of dealing with climate change, there’s a need for the UK to substantially increase its low carbon electricity generation. This has led to a renewed interest in using this indigenous, reliable, natural energy resource.

Engineers can support its take-up by demystifying what tidal power is and the role it can play in reducing carbon emissions.

In particular, they should highlight tidal power’s ability to create jobs and contribute to lower energy costs in the long term.

Both technologies have received support from the UK government over the past two decades but there’s still much to do.