Q&A: Tidal Power

Question: You listed all the forms of renewable energy in one of your recent posts. You said that to harness the power of tides you need to place turbines underwater. Seems to me that an electrical generator immersed in sea water wouldn’t last very long. Wouldn’t it be better to build a gate system like they did for Venice, and then catch the high tide behind it? That way you could use the same type of turbines they use for hydro-power in dams. And from experience, we know that type of turbine can last for decades! — MW, St. John, N.S.

Answer: First, MW is referring to this post: https://sky-lights.org/2023/07/31/hygroelectricity/. You are correct that running an electrical turbine under (salty) seawater is a lot more demanding than pouring fresh water from a dam through a turbine. The design life of those turbines is around 25 years, but there are many in the field that have run 50 years or more and are still working just fine. Turbines immersed in seawater will definitely require more frequent maintenance, but modern stainless steel and aluminum alloys, and carbon fiber composite components, are remarkably durable in a saltwater environment. The larger issue is clogging by debris and aquatic flora and fauna.

As you can see in the animation, turbines placed in the path of the tide’s ebb and flow will generate electricity during both phases of the tide. They can be positioned above normal sea level as shown (to make for easier servicing during low tide), or completely immersed even at low tide to capture even more of the available energy. The animation shows the former, as it’s easier to see the mechanics of what happens. In practice, the turbines aren’t shaped like “water wheels” … they look more like wind turbines. But again, I chose the “water wheel” shape for simplicity.

The scheme you suggest (known formally as a tidal barrage) is actually in use in a few locations but most tidal power facilities use the tidal stream method. It’s ideal for locations where the flow speed of the tide is increased by having to flow through narrow inlets or straits. And they can be surrounded by a mesh cage to limit damage to marine creatures.

The barrage method you suggest has its advantages in certain ideal situations, where the topography of the coast cooperates, but in general they are far more disruptive to the marine ecosystem. They also need to accommodate shipping, and the installation of locks really complicates the engineering. Fossil fuel power plants have an average output of around 500 MW, and the largest wind and solar farms in the US have outputs of 1550 MW and 579 MW, so to offset a fossil fuel plant itdal sources need outputs of a similar magnitude.

For a tidal power plant, that requires a lot of surface area. The largest in the world is the Sihwa Lake Tidal Power Station in South Korea. It has a power output of 254 MW but requires a surface area of around 50 km2 (20 mi2), and experiences an average tidal range of 5.6 m (18 ft). And it is of the barrage type you inquired about. It used an existing seawall constructed for flood mitigation that already had a lock.

Whatever area your containment structure, the greater the tidal range the more power will be available. And that’s true for tidal stream operations also, as greater ranges result in greater flow speeds. The greatest tidal range in the world is found in the Bay of Fundy, Nova Scotia, where it can reach 16 m (53 ft), with flow speeds of 5–6 m/s (16–20 ft/s). If this could somehow be harnessed, each tide would generate 2500 MW of power — enough to power all of Nova Scotia.

This map shows where the Bay of Fundy is located, as well as its tide-enhancing geography. Click to enlarge:

The Bay of Fundy is 75 km (45 mi) wide at its month, so constructing a barrage would be cost prohibitive. There’s a lot of boat traffic in the Bay, and it’s home to a unique ecosystem that supports a large fishing and tourism industry, so that also argues against a barrage. But the province is planning a large scale tidal stream project that, as I explained earlier, will cause less disruption than a barrage. You can read more about it here.

So your barrage suggestion works well in some places, but harvesting tidal energy requires solutions tailored to the environment, tidal range, and flows speeds. Currently tidal power provides a mere 1.5% of the world’s electricity, but that is poised to scale up significantly. Tidal power, unlike solar or wind, is delivered like clockwork twice every day regardless of weather.

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