Recently at work we’ve been doing presentations on areas of the energy industry that piques our interest. It’s been a fun way of rekindling the “extra-curricular” activities we’re all missing while adjusting to working from home.
We hear a lot of talk about the rising contribution of solar, wind, even nuclear energy to the UK mix- to name just a few energy sources. When my turn came around I selected a renewable source that has some wonderfully rich history, but that has fallen out of fashion in recent times: tidal energy.
While tidal energy does not usually fill out headlines in the UK, the little discussion that does come about is still rather disproportionate.
Over the course of 2018, tidal energy provided less than 0.003% of the UK’s total electricity mix. The total installed capacity at the moment is 6.3 MW, which is comparable to just a single wind turbine (Hornsea One, the UK’s newest wind farm, has 174). Quite appropriately, in most reports of the UK energy mix tidal doesn’t even appear on its own; it simply falls into the category of “Other”. So you might ask - why is it worth talking about at all?
For one thing, the UK’s capacity for tidal power is among the highest in the world. By size alone, the UK is a relatively small country: the UK represents less than 3 % of Europe’s total landmass, or 7 % of Europe’s costal perimeter. In contrast, it’s estimated that if all the tidal potential in Europe was realised, the UK alone would account for over half of the total power generated.
Why then does the UK have the potential to generate so much tidal power? And why doesn’t it use it?
To answer the first question we must look to the source of tidal energy: the Moon. Simply put, as the Moon races overhead (or more accurately, as the Earth rotates below it), the oceans of the Earth are pulled and thrown about due to the gravity of the Moon. This creates rising and falling sea levels that oscillate on a daily basis, though there are also patterns that cycle monthly or even yearly (Spring tides, Neap tides, etc.)
Looking at this in more detail, there’s a couple of parts to how this works.
While most of the things we interact with on a daily basis are pretty well firmed down; buildings have foundations, trees have roots, the seas and oceans are much more fluid, and therefore experience differential effects from the Moon’s gravity.
Pictured above is a simplistic model of the Earth-Moon system, with the Earth’s oceans represented as the blue band around it. Gravity works stronger the closer you get to an attractor, so it’s easy to imagine that the part of the ocean that is closest to the Moon is in effect pulled away from the Earth’s surface - the resulting bulge creates a high sea level in the location directly under the Moon. As the Earth rotates this bulge moves around the surface, creating a high tide that appears in any one location once every 24 hours.
This might cause some alarm bells to ring, as most places on the planet experience not one, but two high tides every day.
This brings us to the second part of the puzzle. The oceans are not the only thing affected by the Moon’s gravitational pull - the Earth is itself, too. Since the Earth feels the Moon’s gravity more strongly than the oceans at the back side of the Earth, compared to them the Earth is also pulled closer to the Moon.
This results another bulge of high sea level at the backside of the Earth - the other side of the Earth from the first bulge. As before, as the Earth rotates the second bulge also moves around the surface. Therefore at any point on the Earth’s surface you’ll encounter two high tides (caused by the bulges) every 24 hours, and two low tides in the gaps in between. If you were to graph the sea level over the course of a day you’d get something that looks like the following graph:
A word of warning; this is a very simplistic explanation of how tides come about. In reality, there are many more things that affect tidal ranges at any place on the coast of the world’s continents; such as the geography of bay a piece of coast sits in, the size and shape of the oceans around it and the weather it’s experiencing. Even the position of the Sun with respect to the Earth and Moon plays a part.
As such it would suffice to say the map of the world’s tides is pretty complex. However, as shown in the graph below, the United Kingdom is blessed by some of the world’s largest tidal ranges.
The greatest ranges are found in the mouth of the River Severn. The funnel shape of the Bristol Channel and the open ocean behind it contribute to a tidal range that regular approaches 15 metres.
This extreme range manifests itself in some extraordinary phenomena. On such example is the Severn Bore: a couple times a year the planets align and a large tidal wave gets propelled up the River Severn. The wave has so much energy that on a good year it’s possible to surf the wave for miles upriver.
Over the years there have been some bright sparks that have recognised that even on an average day, the volume of water flowing in and out the mouth of the Severn has a lot of potential energy. The most effective way to harness this is to build a barrage, focussing bulk of the tidal flow through a small stream in which turbines can be placed to generate electricity. In fact, there have been plans for such a barrage dating back to the 1800s.
This isn’t just a pie-in-the-sky industrial revolution pipe dream, and the plans have been under serious study since the 1980s. To summarise nearly half a century of back and forth, three main sites have been examined over the years.
1. “English Stones” Barrage length: 4 miles Peak energy generation: 1,000 MW 2. “Brean Down” Barrage length: 10 miles Peak energy generation: 8,000 MW 3. “Lynmouth” Barrage length: 18 miles Peak energy generation: 15,000 MW
The latter of these iterations has an almost unfathomably large capacity. The largest single power plant in the UK is the coal plant Drax, which has a maximum capacity of 4,000 MW. To put these both in perspective, the peak UK energy demand the day of writing was approximately 30,000 MW. Therefore, a tidal barrage stretching from Lynmouth in North Devon to Porthcawl in South Wales could generate about half of the peak energy demand for the entire UK.
The advantages of such a project are not only in capacity. The electricity generated by tidal power is clean, renewable and secure. Tidal barrages have low maintenance costs, and very long project lives (some estimates exceed 300 years). An additional benefit of having a physical connection between South Wales and the South West could bring fast transport links to some rather remote parts of the country.
So again the question is; if the UK has such a large potential for tidal energy, and the electricity is creates is cheap, clean, and secure, why hasn’t such a barrage been built? The simple answer is that the energy is simply not cheap enough. When compared to a project similar in scope and location - the Hinkley Point C nuclear reactor - many of the figures fall in favour of the barrage. However the most important figure of all, cost per unit of electricity produced, the nuclear option pulls ahead.
Severn Barrage (Brean Down version) Capacity 8 MW Total capital cost: ~£25 billion Cost per MWh: £150 - £350 Project life: up to 300 years
Hinkley Point C Nuclear Reactor Capacity: 3.2 MW Total capital cost: ~£25 billion Cost per MWh: £92.50 Project life: ~60 years
In addition, concerns over silt build up and disruption of the local ecology in the Bristol Channel has prevented the project from gaining wider public support. Perhaps more importantly, tidal power is still a rather immature technology, so even though it has been discussed for nearly a century and a half, perhaps such a large-scale venture is still ahead of its time.
How wise was the UK government’s 2016 decision to select the Hinkley Point C Nuclear Reactor over the Severn Barrage remains to be seen, however it has been considered by many as the final nail in the coffin for this awe-inspiring project.
Is this the end of tidal power in the UK entirely? Not quite. The UK is still home to two of the world’s nine active tidal generators. Despite the Severn Barrage plans being placed on the back burner, the UK is still seen as a pioneer in tidal power technology. The world’s first commercial-scale tidal stream generator (a slightly different technology to a tidal barrage, essentially an underwater wind turbine) was installed at Strangford Narrows, Northern Ireland in 2008, however it has since been decommissioned in 2016. This marks an end to over a millennia of tidal power in this site; remains of a tidal mill have been found nearby dating back to the year 787.
As such, the future of tidal power in the UK appears to be focussed on remote and community-driven initiatives rather than large-scale nationally-backed projects. But with the looming threat of climate change growing ever-nearer, serious action must be taken to meet the ambitious goals of net-zero economies. With globalised society as we know fracturing in front of our eyes, the need for energy security is greater than ever. Is now the time to resurrect the Severn Barrage? As Chaucer put it, time and tide wait for no man.