Along every seacoast, tides roll in and out in a very predictable rhythm. The moon and sun both exert a gravitational pull on seawater. The water’s movement creates tremendous natural kinetic energy. Tidal power is a renewable energy resource.

It is more predictable than either wind or solar, but it is still intermittent and non-dispatchable. That is, it can’t be turned on and off like a fossil fuel or nuclear plant.

Think of tidal energy as a subset of hydroelectric energy. That is, it comes from the movement of water. When a dam produces electricity, the energy of falling water drives a turbine. With tidal energy, the water moves back and forth to and from shore. That the water doesn’t always flow in the same direction is but one difference.

Although tidal power generation has great potential, its technology is the least developed of any renewable. This article will look at some of the reasons.

Some characteristics of tidal energy

Tidal energy emits no greenhouse gases. It shares that benefit with wind and solar power. Water is about 800 times denser than air. So a tidal flow of only several miles per hour can generate as much power as a wind turbine in an unimaginable wind speed of 300 miles per hour. Tidal turbines can therefore also be much smaller and closer together than wind turbines.

For the same reason, tidal power requires much less space. The largest onshore wind farm in the US, Roscoe Wind Farm in Texas, occupies 100,000 acres. The largest solar farm in the US, Solar Star in California, occupies 3,200 acres.

Neither one of those farms is the largest in the world. By contrast, the largest tidal power plant in the world, Sihwa Lake Tidal Power Station in South Korea, was simply added to an existing seawall.

The World Energy Council estimates that it may be possible to install as much as 1,000 gigawatts of tidal power by mid-century. That’s about half of what the world now gets from coal.

The International Renewable Energy Agency estimates that tidal power costs between 20-27¢ per kilowatt-hour. That’s by far more expensive than any other existing technology, but prices will drop quickly once the technology matures.

Tidal power technology is about where wind power was in the 1970s and ’80s. Unlike wind energy’s three-blade rotors, the tidal energy industry hasn’t settled on a standard design. Besides rotors like fan blades, some companies use Archimedian screws or the kinds of helical blades familiar in reel lawnmowers. The turbines can operate near the seafloor, near the surface, or somewhere in the middle.

Companies try a variety of approaches. Some fail spectacularly. Some run out of money before they can refine their ideas. Eventually, some tidal designs will succeed.

Tidal energy pros and cons

Advantages

Because tides flow continuously and predictably, engineers can more easily design tidal energy systems. And because water is so much denser than air, tidal turbines can use much smaller blades and operate at much lower speeds.

Although not many tidal energy generators exist, it is not a new technology. LaRanch started operating in France in 1966 and is still going. Tidal energy equipment lasts longer than solar or wind farms. It even lasts longer than nuclear power plants. Compare an expected lifespan of about 25 years for wind and solar installations, 60 years for nuclear, and 100 years for tidal energy.

It’s too early, though, to get excited about the prospect of abundant tidal power generation.

Disadvantages

For one thing, it comes with a much higher initial construction cost. The constantly moving tides make installation very expensive. Although long-term operation does not have a high cost, the up-front costs mean that return on investment will take a long time.

Ongoing research costs also make manufacturing expensive. Many tidal power generators resemble dams. Science doesn’t yet know how they influence currents and waves. Or, for that matter, their impact on marine life.

And that impact goes beyond having a large submerged structure with rotating blades. Tidal generators change the patterns of sedimentation, nutrients, and dissolved oxygen around them. Also, they emit electromagnetic field (EMF) radiation. Some marine life makes its own magnetic fields to evade predators, among other uses.

These environmental impacts may be harmless or extremely disruptive. No one knows. Therefore, companies operating tidal energy systems must hire experts in geography, marine biology, and ecology to identify and mitigate any bad effects.

It requires environmental monitoring around the clock to do so.  No one can know the impact of any technology until it has been in place a long time.

And while tidal power equipment is designed to operate in harsh conditions, available sensors are not.

Tides rise and fall influenced by the gravitational pull of the sun and the moon. Astronomers understand that action fairly well. No one yet really understands what happens in the water. Nearly all coasts experience two high tides a day, but some places have only one and others have four.

Necessary research to discover the technology’s environmental impact will be slow and expensive. Zero impact is too much to expect. It’s first necessary to discover and measure the impact and then decide what’s acceptable.

Tidal energy generation in Nova Scotia

The Bay of Fundy sits between the Canadian provinces of Nova Scotia and New Brunswick. The difference between high tide and low tide there can be as much as 54 feet, the largest tidal range in the world. Each tide has four times the estimated flow of all the world’s rivers combined.

One common and ancient way to harness tidal power uses dams to trap water in mill ponds at high tide. At low tide, the water is released down sluiceways to operate waterwheels. Such tidal mills were common along the Atlantic coast from colonial times through the 19th century.

Inventors have tried to harness the Bay of Fundy to make electricity for more than a century. Early attempts developed the tidal mill technology on a large scale. Among other problems, they threatened to ruin the bay for commercial fishing.

More recently, an attempt to lower a turbine the size of a six-story building into Minas Passage failed. The designer had underestimated the power of the current, which destroyed the structure in just a few days. Shortly after another attempt, the company lost its financing.

Most tidal power designs involve anchoring equipment to the ocean floor. Others take a different approach: small floating platforms.

A floating device costs less to build and install. It’s easier and safer to maintain, and if something goes wrong, it’s easier to remove it. Their turbines swing up independently for maintenance, so it’s not necessary to shut everything down at once. Unusually high currents and large floating debris cannot harm the rigs. They just kick the turbines out of the water.

Sustainable Marine Energy, an example of floating tidal power generators

Sustainable Marine Energy (SME) had built it business plan around servicing islands that rely on diesel generators, such as Brier Island in the Bay of Fundy.

SME’s power generator looks like a boat with three hulls and four rotors. The entire floating structure can pivot. Therefore, the craft can generate electricity both on the ebb and flow. It survived Hurricane  Dorian in September 2019, which, when it reached Canada, had 96-mph winds.

When it becomes operational in summer 2020, it will be the only floating tidal power generator in North America.

By working in the Bay of Fundy, SME has chosen the most treacherous tidal conditions in the world to develop its technology. If successful, it will be able to build other platforms to match conditions anywhere else.

Testing indicates that fish and marine mammals can avoid the blades even in a very swift current. But like every other tidal energy company, SME must spend a lot of money on research. Absence of evidence of a problem is not the same as absence of a problem.

SME actually started developing its idea in Scotland, but then the British government cut subsidies for tidal energy. It chose to move its experimental equipment to Brier Island, in part to get government support. Nova Scotia now gets about half of its power from coal. It is interested in both wind power and tidal power to meet Canada’s goal of a 30% reduction of greenhouse gas emissions by 2030.

Sooner or later, tidal energy will become a mainstream power source. It might happen only after solar and wind farms occupy all the easy and inexpensive locations. Two hundred years ago, coal started to make water wheels obsolete. Wouldn’t it be ironic if it’s water wheels that finally make coal obsolete?

Sources:

The push for tidal power faces its biggest challenge yet  / Elizabeth Royte, Smithsonian. April 2020
Riding the renewable wave: tidal energy advantages and disadvantages / Talal Husseini, Power Technology. October 26, 2018
Tidal energy — all renewables are not created equal / James Conca, Forbes. July 27, 2017

Photo credits:
High tide at Laguna Royale. Some rights reserved by DefMo.
Wave energy concepts. Public domain from Wikimedia Commons
SeaGen tidal energy generator.© Copyright Rossographer and licensed for reuse under this Creative Commons License
High tide at Cullercoats. Some rights reserved by Akuppa John Wigham
Cape d’Or lighthouse. Some rights reserved by Dennis Jarvis
Floating tidal energy platform. Public domain from Wikimedia Commons