Rise and fall

Why haven’t we harnessed the power of the tides? (9 minute read)

Years ago, when I was living in Christchurch and struggling with the isolation of moving to a city where I knew almost nobody, I discovered the remarkable beach at Birdling’s Flat. I remember it being a grey, desolate day, and the beach was a grey, desolate place. The steeply sloping bank of gravel stretched for miles until it disappeared in the white haze of mist whipped up from the waves by the ceaseless wind. There was barely a tree in sight, nothing to shelter the string of tatty baches built from asbestos panels and corrugated iron.

I stood on the beach, feeling the freezing southerly wind on my face and listening to the gravel tumble in the foaming water. I could sense what I was later to learn about the beach – it was not a place where you would want to get in the water. The waves are unpredictable and there’s a deadly undertow. Even on the finest day, people don’t swim there, although it’s a good place to fish and fossick for agate and other pretty stones.

But even as I sensed the danger and desolation of the place, I felt it soothe me. Partly it was the wind, but more than anything it was the movement of the sea. There’s something so comforting about the constant motion, the wash of the waves and the rise and fall of the tide.

I’ve been thinking about Birdling’s Flat because it’s a place which, for me, epitomises the power of the sea. Although we often give it little thought, there is a lot of energy in the movement of the waves and tides. My interest in the sea’s energy was captured by a recent mention of generating electricity from the tides on the BBC World Service. Right now, New Zealand is uncomfortably aware of the unpredictable nature of generating electricity from the wind and rain. Could we use the constant and entirely predictable source of renewable energy which surrounds us?

To my surprise, I found that people had been doing this for far longer than I imagined. Before we used the movement of water to generate electricity, we used it to move stones which could grind grain to flour. I’d only ever heard of this being done on rivers and streams, but the tides have been used for this purpose for around 1500 years. In fact, tide mills were once quite common on the coasts of Europe, North America and China. Where I’ve been able to find detailed descriptions, it appears that tide mills didn’t strictly use the movement of the tide. Instead, they captured water in a large pond when the tide came in, then directed the outflowing water through a sluice to drive a water wheel.

Before we can harness the tide, we need to understand it, and that’s more difficult than it first appears. We’ve all been told that the tides are caused by the gravitational pull of the moon, but that explanation leaves a lot of unanswered questions. I spent several hours confusing myself in trying to understand precisely how tides work, and I’m still not convinced I understand correctly. But one crucial point I learned is that it isn’t just the moon affecting the tides – it’s also the sun and the landforms on Earth.

The sun’s influence causes the variations we call spring tides and neap tides. These are influenced by the position of the sun in relation to the moon. When the sun and the moon are lined up and their gravity is acting together, we get a larger tide known as a spring tide. When they are acting in opposition, we get a smaller tide, known as a neap tide. There are videos explaining the moon and tide on the page linked here, because sometimes words can be hard to follow.

The shape of the land also influences tidal movement. Just as water flowing down a river varies in speed depending on the gradient and the shape of the riverbed, tidal flows of water in the sea are influenced by the shape of the land. Narrow channels, such as Cook Strait between New Zealand’s North and South Islands, funnel large volumes of water through a small space. As a result, the tidal flows around Cook Strait are complex and treacherous. as well as profoundly counter-intuitive. High tide in Wellington, for example, is several hours out of phase with high tide on the other side of the strait, somewhere like Picton in the Marlborough Sounds. In fact, high tide in Wellington almost coincides with low tide on the other side of the strait.

I don’t have any pictures of Birdlings Flat I can find, so here is Baring Head on Wellington’s South Coast, looking across Cook Strait to the Marlborough Sounds (image credit: Melanie Newfield)

It also turns out that just because we can see what is happening on the coast, it doesn’t mean we know precisely what is happening under the water. A study around ten years ago, for example, showed that at the same time and same point on the map, water at different depths in Cook Strait can be flowing in opposite directions. Does this matter? Depending on the approach we take to generating energy from the tide, yes, it could.

The first efforts to generate electricity from the tide used a structure which is like a combination of an ancient tide mill with a hydro-electric dam. Known as a tidal barrage, it’s a dam-like structure built across an estuary. The water flow is controlled by sluice gates and directed through turbines. A tidal barrage is an established technology – the Rance Power Station in France has been operating since 1966 and produces slightly less than the wind farm at Makara, near Wellington. Unlike a wind farm, though, its energy production is continuous.

While there are a number of tidal barrages around the world, most of them are smaller than Rance. As far as I can tell, there are relatively few locations with enough tidal flow to make a tidal barrage economically feasible. They also have a number of negative effects on the estuaries where they are installed, affecting marine life, impeding boat traffic and making the water murkier. But they are still likely to have a place – South Korea constructed one which is larger than Rance around ten years ago, and Britain is planning an even larger one on the Mersey.

Tidal barrages aren’t the only option, though. Another approach is known as tidal stream, which is placing turbines in the sea in areas with a high flow – more like a wind farm than a hydro dam. A single tidal turbine can generate much more energy than a wind turbine of equivalent size, but they are also much more difficult and expensive to build.

Turbines, whether for wind farms, hydro stations or tidal generation are built from steel. Putting aside the challenge we have in dealing with the carbon emissions from steel production (I wrote about this issue a couple of years back), there’s a specific problem when we put steel into the sea – it corrodes much faster than in fresh water. This is not an insurmountable problem, after all, we’ve been putting steel structures such as ships and oil platforms into the sea for many years. There’s considerable research on coatings we can put on the steel to protect it. Nonetheless, it adds to the cost.

Another process which happens under the sea is a challenge too. Marine algae and animals like to live on surfaces, so anything under the water soon gets covered with growth. It’s not just a bit of slime either – these communities can be staggeringly large and complex. We term this biofouling when it happens on something that we have put into the water, although that’s a rather biased name, since it’s a perfectly natural process and only a problem if it interferes with our activities. It could certainly interfere with tidal energy generation. A turbine covered with barnacles isn’t going to produce electricity efficiently, and so the turbines are likely to require specialised coatings to prevent growth and a considerable amount of maintenance. (I could easily write a whole article on biofouling, because it intersects with a number of issues I’m interested in, including invasive species and pollutants, not to mention the fact that biofouling on ships increases fuel use and therefore emissions, but I’ll save that for another time.)

The biggest challenge, though, appears to be the difficulty of installing and maintaining turbines in areas with powerful currents. As a result, generating electricity from tidal streams is expensive – more expensive than nuclear energy, which currently is the most expensive way of generating electricity. However, since 2018, the cost of tidal stream electricity has dropped by more than 40%, and further cost reductions are predicted. Currently, the world’s largest tidal stream station is in the Orkney Islands off Scotland, and it’s still relatively small, although a much larger station is planned. While wind and solar will remain much cheaper for the foreseeable future, the predictability of the tide means that it’s likely to make a valuable contribution to energy production.

Tidal stream stations are not without environmental impacts, of course – no means of generating electricity is. It’s hard to predict exactly what impacts they will have, although changes to the movement of sediment and avoidance by marine species are both likely. Evidence from the existing small stations suggests that the impacts are relatively localised to the area around the stations, and will be much less than producing energy from fossil fuels. Nonetheless, we still have to make hard choices if we want to use tidal energy.

How far has New Zealand gone down the path of developing tidal power? We’ve certainly looked at it – plans and proposals date back at least as far as 2005. One promising location is Tory Channel in the Marlborough Sounds, which would take advantage of those powerful Cook Strait currents but avoid the worst of the weather and rough seas in the strait itself. Another location is in the Kaipara Harbour, north-west of Auckland. There was a serious proposal to install turbines which would have provided enough energy to power 250,000 homes – resource consent was granted in 2011, but the project was scrapped in 2013 as were all other plans and proposals, around half a dozen of them. As far as I can tell, this has largely been down to cost and market factors. I’m not the person to analyse these issues, but the article I’ve linked to here gives a good analysis. However, it’s worth noting that in New Zealand, as well as overseas, limited, inconsistent or poorly-directed government support has been part of the problem.

Right now, New Zealand is facing the consequences of relying on the weather, and the market, for electricity. Despite what some politicians seem to think, fossil fuels aren’t the answer. We can’t keep delaying action on climate change and assuming that other countries will solve the problem. Part of the solution may be in shifting away from relying entirely on centralised supply. The Australian government has supported rooftop solar, while the New Zealand government has not. But we also have to look again at geothermal and tidal energy generation, two predictable, reliable sources of renewable energy which are available to us if we are prepared to make the investment.