Tidal power is a promising renewable energy source, but production costs, a limited number of suitable locations, and technological challenges hinder its expansion.

Tidal power leverages the rise and fall of oceanic tides to capture potential or kinetic energy and convert it into other energy forms, often electricity. There are two methods of harnessing tidal power. One method resembles a hydroelectric dam, called tidal barrages, and another relies on underwater turbines that have blades that rotate as water flows by, powering a generator in the process.

Tidal turbines may be installed in water sources ranging from areas with strong ocean currents to tidal streams and estuaries. They may be installed on their own, but larger energy projects commonly install connected rows of turbines, called an array. Tidal barrage systems operate similarly to hydroelectric dams using dam-like structures and submerged gates to control water levels throughout the day and to direct water through turbines.

Michael Howland, Assistant Professor of Civil and Environmental Engineering at MIT, studies the physics of Earth’s atmosphere and renewable energy generation systems. He says that tidal power has one advantage over other renewable energy resources like wind and solar: predictability.

“Variations in wind patterns, weather, and turbulence make it ​​inherently challenging to predict [wind farms’ electricity generation] across different time scales,” he says. Tidal patterns, however, are well-known and well-understood, reducing the need for backup energy sources. “That's a clear incentive for using tidal power,” he adds.

Another benefit of tidal power is its relatively high power output. Because water is roughly 830 times denser than air, tidal or ocean currents can generate more energy per unit area than winds.

Despite these advantages and the skyrocketing demand for clean, renewable energy, tidal power hasn't taken off in the same way that solar and wind energy have. There are only a handful of commercially-operating tidal power plants worldwide, the largest of which is the Sihwa Lake Tidal Power Station in South Korea. The United States doesn't have any large-scale tidal power plants, although a few small-scale projects exist.

“The fundamental question is one of economics,” says Brian Polagye, Associate Professor of Mechanical Engineering and Director of the Pacific Marine Energy Center at the University of Washington. Because of the early stage of the technology, tidal power is an expensive source of energy: according to a 2019 study, commercial-scale tidal energy is estimated to cost $130-$280 per megawatt-hour (1), compared to $20 per megawatt-hour for wind (2). High upfront costs of building plants, expenses associated with maintaining machinery that can survive corrosive seawater, and the pricey engineering work that goes into them make up a significant portion of that cost discrepancy. Polagye adds that the supply chain for tidal power also isn't yet capable of providing necessary components and technologies at scale to make this energy source and, as of now, "everything’s pretty custom.” In fact, the market discrepancy between tidal and other, more mature, renewable energy systems is actually growing because the cost of generation from wind and solar generation continues to drop.

Beyond the economic difficulties, the tidal power industry also must overcome technical challenges such as the lack of an established and routine production market and legislative red tape. (One tidal power project set up in New York's East River required 23 different permits from 14 agencies to operate, as reported in Grist.)

Then there's the environmental impact, the extent of which isn't fully understood yet. Depending on the size and design of the system, tidal barrages can kill off flora and fauna, change salinity and sediment levels, and disrupt coastal ecology (3,4). Researchers like Polagye are trying to figure out how to harness tidal currents with much less environmental impact. “We're learning from our mistakes and trying to identify problems and stress mitigation proactively before they occur,” he says.

Tidal power is thriving in some countries. In Scotland, a 600-ton turbine anchored right off of the Orkney Islands is already generating power. The turbine, named the O2, is projected to meet the energy demands of 2,000 homes for the next 15 years. Recently, the UK also introduced a new set of incentives that specifically support tidal energy. The tide may also turn in the US: last year, the Department of Energy announced a $27 million investment in research and development around tidal and wave energy technology.

Howland believes that tidal power will be a piece of the renewable energy pie and used in tandem with other forms, but it’s not yet clear how large that piece will be.

“Oftentimes, discussions frame renewables as being against one another,” says Howland. “If our goal as a society is to decarbonize our electricity generation, we can't just pick one. We have to do all of them at the same time.”

Thank you to Norman Macdonald of Ross Shire for the question. You can submit your own question to Ask MIT Climate here.

Published April 12, 2022.

FOOTNOTES

1 U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy. "Powering the Blue Economy: Exploring Opportunities for Marine Renewable Energy in Maritime Markets." April 2019.

2 U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy. "Land-Based Wind Market Report: 2021 Edition."

3 Rtimi, Rajae, Aldo Sottolichio and Pablo Tassi. "Tidal Patterns and Sediment Dynamics in a Hypertidal Estuary Influenced by a Tidal Power Station." Journal of Coastal Research, 95(sp1), May 2020. doi:10.2112/SI95-293.1

4 Clark, Nigel A. "Tidal Barrages and Birds." Ibis International Journal of Avian Science, vol. 148, issue S1, March 2006. doi:10.1111/j.1474-919X.2006.00519.x