A rough guide to tidal lagoons

A “giant man-made lagoon” in Swansea bay could be used to generate electricity for 120,000 homes a year, if the government says yes to a new planning application. But what exactly is a tidal lagoon – and what does the proposal mean for the UK’s electricity supplies?

Tidal Lagoon Power submitted plans for £850m tidal lagoon in Swansea Bay today. If it’s built, the structure will be the world’s largest power-generating lagoon.

The company also has bigger ambitions. It says tidal lagoons could supply eight per cent of the country’s electricity within just ten years. So far, reactions to the proposal appear pretty positive – but supporting the young technology could be expensive.

What is a tidal lagoon? 

A tidal lagoon is a man-made enclosure created in a tidal area. It looks like a harbor or marina,  and acts to separate a body of water away from the natural ebb and flow of the tides.

The movement of the tide in or out means that a difference in water levels builds up in the lagoon, compared to the water around it, in much the same way as a man-made lock on a river does. Once the difference is big enough, sluice gates are opened – allowing water to rush through the gaps, turning big turbines installed underwater. The rotating turbines generate electricity.

Here’s what the process looks like, in picture form:

Source: Tidal Electric Ltd

Under its new proposal, Tidal Lagoon Power proposes to construct a lagoon in Swansea Bay. This will involve building six miles of sea wall, according to the BBC. The wall would look like this:

Source: Tidal lagoon Swansea Bay website 

The following map shows where Swansea Bay is, in relation to the Bristol channel:

Reliable, predictable energy

A tidal lagoon generates electricity twice in one tide – once when the tide is coming in, and once when it’s going out. There are four tidal flows a day at Swansea bay, twice coming in and twice going out. This means the Swansea lagoon will be generating electricity for 14 hours out of 24, according to the company.

The tide flows in and out in a predictable pattern, so the energy it generates is reliable. That means it doesn’t need to be ‘ backed up‘ by gas or coal power stations held on standby – which is good news its ability to reduce greenhouse gas emissions.

Better than the barrage? 

This isn’t the first time a proposal has been made to generate electricity from Swansea’s tides. The Severn Estuary has the second highest tidal range in the world, so it would be surprising if a government keen to bump up its power production from renewables ignored it entirely.

An argument raged until just a few months ago over another proposal, known as the Severn Barrage. Under this idea, an artificial barrier stretching from shore to shore across the Severn Estuary could have been used to impound a large area of water much further up the estuary:

Simplified map of the proposed Severn Barrage, which would have been situated much further up the Severn estuary. Created by Friends of the Earth (pdf).

There were several proposals for barrages in different places, but the basic idea was again that they would generate electricity as the tide drove the water through turbines embedded in the structure.

However, the barrage was opposed by some green groups on the grounds that it would be environmentally damaging. They were particularly worried that it would negatively impact on important bird populations, by flooding intertidal mudflats.

It was also extremely expensive, with an estimated price tag of £25 billion. The case for the barrage was ” unproven“, according to a report from the Energy and Climate Change Committee last summer – ultimately sounding a death knell for the project.

Getting an industry going 

Some green groups see lagoons as less environmentally damaging, because they are much smaller and have a far less significant impact on the tidal flow of the estuary.

The view isn’t universal, however. The RSPB offered a cautious welcome to the lagoon proposal today, stating that it’s glad to see the back of the barrage, but warned “we will need to look at the details”.

The company hopes that the lagoon at Swansea Bay is the first of many. It says it could construct another five lagoons at sites including Colwyn Bay and Liverpool Bay. In total the six tidal lagoons could be generating 8% of the country’s total electricity, according to the firm behind the schemes.

Paying for it 

The prospect of a reliable, predictable renewable energy source, reducing greenhouse gas emissions from the country’s power system, supported by environmentalists and led by a British-owned company, may be attractive to many. But there is one potential stumbling block, and it’s a predictable one. The proposal is expensive.

In order to be financially viable, the Swansea lagoon proposal would require a subsidy of £180 per megawatt-hour (MWh) – that’s higher than the current level of support for offshore wind. Overall, the company says the lagoon at Swansea Bay will cost £750 to £850 million to construct; and all five projects would require £12 billion.

Tidal Lagoon Power argues that this subsidy can reduce rapidly over the next few years, as the technology develops and new projects are brought online. In the short-term, however, it’s a lot of money.

We’ll see how the government reacts over the next few months. The planning application is now due to be reviewed by the government’s Planning Inspectorate, before going through a period of public examination. Ultimately, the secretary of state for energy and climate change – currently Lib Dem Ed Davey – will make the decision.