Wind turbines and solar panels will undoubtedly play the central role. But both depend on the weather: wind turbines stand still on calm days, while solar panels generate less in overcast conditions and nothing at night. That variability is driving interest in more predictable sources of clean power.

One promising option lies beneath the ocean’s surface.

Tidal stream turbines work much like wind turbines, only under the sea. As tides flow in and out, predictably, twice a day in places like the United Kingdom, the moving water spins turbine blades to generate electricity. This power is then transmitted to shore via cables laid along the seabed. Unlike wind and solar, tides are governed by the gravitational pull of the moon and sun, making them highly predictable years in advance.

Governments are beginning to take notice. The UK and France are investing in tidal stream energy and plan to install at least 400 megawatts (MW) of capacity over the next decade; enough to power a city like Leeds or Amsterdam.

Other countries, including Canada, the United States, China and Japan, are also exploring the technology, albeit with much smaller scale projects.

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Despite this growing interest, a basic question remains: how much electricity can tidal currents actually produce, and where is it located?

I’ve teamed up with experts from around the world to help answer those questions. In our new research, we identified more than 400 potential tidal energy sites across 19 countries in Europe, the Americas, Asia and Australasia. These are locations where water flows fast enough, and at suitable depths, for turbines to operate.

Scientists typically describe tidal energy in three stages. The theoretical resource is the total energy in tidal currents. The technical resource is the portion that current turbine technology could realistically capture for electricity production. Finally, the practical resource accounts for constraints such as shipping routes, fishing activity and marine conservation areas. In practice, only a small fraction – around 1 percent to 20 percent – of the theoretical energy can actually be used to generate electricity.

Even so, the potential is significant. Across 90 of the most-studied sites, we estimate that tidal turbines could generate around 110 terawatt-hours of electricity each year – roughly equivalent to the annual electricity demand of Portugal.

Some countries stand out. The US, UK, New Zealand, Canada, China and Indonesia have the largest overall tidal energy resources. In places such as the UK, Indonesia and New Zealand, tidal energy could supply at least 10 percent of current national electricity demand. In larger countries like the US and China, the resource is still substantial but represents a smaller share of total electricity demand.

A striking finding is how concentrated this energy is, as more than half of the global tidal resource we identified is located in just six sites. These include the Pentland Firth between mainland Scotland and the Orkney Islands, the Alderney Race between the Channel Islands and France and the Minas Passage in a part of Canada known for the world’s highest tides. Two major sites in Alaska (Chatham Strait and Cook Inlet) are also among the most energetic in the world.

However, many of these locations are far from major population centers. Remoteness presents a significant challenge, as building the infrastructure needed to transmit electricity over long distances can be costly and complex. This is one of several factors that may reduce the amount of energy that can be practically harnessed.

Tidal power tends to be strongest in narrow areas between large islands or landmasses, where lots of water is “pushed through” the gap. But finding the most promising hot spots also depends on factors like seabed geography or localized ocean currents, which need specialized and detailed research.

This means there are gaps in the data. For many promising sites, particularly in countries such as Norway, South Korea and the Philippines, detailed measurements of tidal currents are still lacking. For this reason, global estimates of tidal energy potential could increase significantly as more data becomes available.

Our findings broadly support projections from the European Commission, which anticipates up to 8 gigawatts (GW) of tidal stream capacity in Europe. However, global projections of more than 100 GW – roughly the electricity demand of the entire UK – remain uncertain without better data and more comprehensive site assessments.

With enough investment and data, plus careful site selection, tidal stream energy offers something rare in renewables: power you can predict years in advance. In an electricity system increasingly affected by the weather, that reliability could make it a disproportionately valuable part of the world’s transition to clean energy.

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The writer is a senior research associate at the Department of Engineering Science, University of Oxford. This article is republished under a Creative Commons license. The views expressed are personal.

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