Tidal Power: Renewable Ocean Energy

Tidal power is a type of renewable energy which can be harvested from the ebb and flow of tides. We outline the main types of tidal technologies which are being developed to generate electricity, including barrages, lagoons and tidal streams. Read about the challenges of tidal energy, including high costs and environmental impacts, as well as its future prospects.
Seagen Tidal Stream Generator
SeaGen Turbine: decommissioned due to challenges of operating in extremely aggressive tides. Photo: ©Siemens

Sustainable Marine Energy

In theory, the ocean has an abundant amount of renewable energy which can be harnessed to generate green electricity. The energy is continuous and easily sustainable. So the use of marine renewables – like tidal and wave power – will certainly help to limit climate change around the world. But stiff challenges remain. Wave power is still in its infancy, and while tidal power technology has been around for decades, adoption has yet to occur on any significant scale.

Nonetheless, as our climate crisis worsens, expect to see significant growth in marine energy investments. After all, the ocean remains a huge and untapped source of sustainable energy. If we are to stabilize global warming we simply can’t afford to ignore the ocean’s waves and tides.

How Do Oceans Influence Climate Change
Effects of Global Warming on the Oceans

What Is Tidal Energy?

Tidal energy is a form of hydropower. It is power produced by the surge of ocean water during the rise and fall of tides. This ebb and flow movement of water is used to turn a tidal generator and produce electricity. The energy from tides comes mostly from the Moon’s gravity and a little from the Sun’s gravity. The moon’s force is much greater because it is 389 times closer to planet Earth than the sun.

Relationship between tides and currents. Ebb and Flow Animation.
This shows the relationship between tides and currents. As the tide rises, water moves toward the shore. This is called a flood current. As the tide recedes, the water moves away from the shore. This is called an ebb current. The movement of water toward and away from the shore is illustrated by the movement of the green seaweed. Tidal energy is generated by harnessing the power of this movement. Animation: NOAA

Usually, there are two high tides and two low tides each day, which makes tidal power very predictable. In comparison, solar power and wind power are less predictable because sometimes the sun doesn’t shine and the wind doesn’t blow.

Tidal plants can only be installed along coastlines where the difference in water levels must be at least 5 meters (16 ft) between tides to create electricity.

In some parts of the world, the speed and difference in heights is greater because of the local geography and the tilt of the planet. By placing devices that harness some of the tidal energy in these narrow channels, we can generate electricity and power communities in the process.

A Renewable Source Of Energy to Replace Fossil Fuels

Tidal energy is a renewable and sustainable source of energy, which means it is replenished by nature so that it can be used over and over again. The ocean covers 71 percent of Planet Earth, and if we could harvest its energy, we could possibly supply enough green electricity to meet four times the global electricity demand of today. 1

The point is, as global warming intensifies, we need to find a way to replace dirty fossil fuels like coal, oil and petroleum and natural gas. After all, it’s the greenhouse gas emissions from these high-carbon fuels that drives global warming. Among the benefits of renewable energy is the fact it produces very low greenhouse gas emissions, and very little air pollution.

With enough investment, tidal energy can become one more way of helping us to decarbonize our energy system. It is renewable, predictable and, if combined with storage, highly flexible.

Other renewables in the energy mix include: solar energy, wind power, underground geothermal energy, organic biomass and hydropower (hydroelectricity). Hydrogen power is the latest of the new clean energies. If its initial problems can be ironed out, it could provide an inexhaustible supply of storable energy.

How Much Electricity Is Generated by Tidal Power?

Very little. Currently total global marine energy generation capacity is only about 500 megawatts (MW), most of which comes from tidal power (495 MW), while wave power accounts for a mere 2 MW. 2

Marine Energy Electricity Generation

Targets for Marine Energy Electricity Generation for 2025 and 2030.
Ocean power electricity generation (tidal and wave power). Where we are now, and where we need to be by 2025 and 2030. Source: IEA Sustainable Development Scenario.

In 2019, electricity generation from marine technologies increased by an estimated 13 percent. While this rate of increase is significantly above that of the previous three years, generation is way behind the necessary target. To be on track with the Sustainable Development Scenario (SDS), the annual growth rate should be 23 percent between now and 2030. 3 Could it be that our climate plan can’t cope and is no longer fit for purpose?

In any event, even though there have been recent improvements tidal power design and turbine technology (such as cross flow turbines and new axial turbines) – deployment of new tidal installations is still falling short.

The United States for example, does not have any commercially operating tidal energy power plants, although several projects are in various stages of development.

Advanced marine projects ranging from 10 kW to 1 MW have been deployed in some countries, mostly in the U.K, Canada, Australia and China. However, these demonstration and small commercial projects are expensive and have not yet achieved the economies of scale necessary to trigger significant cost reductions and commercial take-up.

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List of Tidal Power Stations

Only a handful of commercial tidal power stations are in operation around the world (although more are in the pipeline). Those currently in place include the following:

NameCapacity
Megawatts
LocationDeployment Date
Sihwa Lake Tidal Power Station254 MWSouth Korea2011
Rance Tidal Power Station240 MWFrance1966
Annapolis Royal Generating Station20 MWCanada1984
MeyGen6 MWUnited Kingdom2017
Jiangxia Tidal Power Station3.2 MWChina1980
Kislaya Guba Tidal Power Station1.7 MWRussia1968
Uldolmok Tidal Power Station1.5 MWSouth Korea2009
Eastern Scheldt Barrier Tidal Power Plant1.25 MWThe Netherlands2015
Bluemull Sound Tidal Stream Array0.3 MWUnited Kingdom2016
Source: EIA

Types of Tidal Energy Generators

There are currently three different ways to harness tidal energy: (1) tidal barrages (2) tidal streams and (3) tidal lagoons. We also discuss more recent concepts including tidal fences and dynamic tidal power (DTP).

Tidal Barrages

Largest Tidal Power barrage dam in the world, Sihwa Lake, South Korea.
Tidal Barrage Power Plant Sihwa Lake, South Korea. Inaugurated in 2011, with an electricity generation capacity of 254 MW, it became the largest tidal power plant in the world. It has a 12.5km boardwalk. The station’s mean operating tidal range is 5.6 m (18 ft), with a spring tidal range of 7.8 m (26 ft). Photo: CC BY-SA 3.0

Tidal barrages are a type of tidal range technology. Tidal range technology utilizes the range difference between the tide height inside and outside the impounded area.

A large dam called a barrage is constructed across a tidal river, bay or estuary. Turbines inside the barrage harness the power of the moving tides, much in the same was as a hydroelectric dam harnesses the power of a flowing river. The barrage gates open as the tide rises. Then at high tide, the gates close causes the water to pool in a lagoon. Water can then be released at a pace which can be controlled by engineers to drive a turbine and generate electricity.

While electricity generation from tidal barrages employs the same principles as hydroelectric generation, the main difference is that tidal currents flow in both directions. The turbines and sluice gates in a tidal barrage can be designed to generate power one way, i.e. ebb-only, or flood-only, or bi-directional.

Tidal range technologies have a long history, especially when compared with less mature technologies like wave energy and tidal streams. The extraction of energy from tides has occurred for hundreds of years. There is even evidence of a tide mill in Strangford Lough in Northern Ireland, dating back to the early 6th century where an 8 meter wide dam enclosed a 6,500 sq m area of sea water. 4

The technology that is required to build a modern barrage has been around for over 80 years, but there are still several challenges to overcome, the most pressing being the high construction costs and the environmental impact.

Tidal power produces no greenhouse gases when in use. In fact, it can save up to 1 million tons of carbon dioxide per TWh. 5 However, this does not take into account the significant greenhouse gas emissions expended in the construction phase – including the use of materials like cement required to build the barrage – for more, read our article: Cement Emissions Bad For Climate Change.

While barrage schemes do create calmer waters and provide protection against rising sea levels, they fundamentally alter the area’s marine ecosystem. Ecologists point to the impacts of building a dam across an estuary, which can change the flow of the tidal current, affecting marine life. Fish, particularly migratory species, can become blocked in or out of the tidal lagoon.

Turbines move quickly in barrages, and marine life can become trapped in the blades. For example, at the tidal power plant at the Rance River estuary in France, a flatfish plaice is now extinct in the area and the marine food web has been permanently changed.

Tidal Streams

Tidal stream generators essentially look and work like underwater wind turbines. They harness the kinetic energy of moving water to turn turbines and generate electricity. These sorts of generators have a low visual effect because for the most part they are submerged under water.

The higher density of water means that the blades can be significantly smaller and turn more slowly than wind turbines yet they still deliver a potentially comparable amount of power.

As opposed to using the rising and falling movement of the tides, tidal stream generators take advantage of the fast-moving sea currents (tidal streams), which flow when tides are moving in and out. These tidal streams cause the turbines to rotate, turning the generators to generate electricity.

They have the advantage of being cheaper than tidal barrages – but are still costly. By their very nature tidal stream devices are placed in some of the most hostile ocean conditions, adding hugely to costs and the problem of shelf-life. SeaGen was the world’s first industrial-scale turbine, located at Strangford Lough, in Northern Ireland. Installed in 2008, it has since been decommissioned because of rough seas.

On the other hand, tidal stream generators have a smaller environmental impact than barrages because the turbines turn more slowly and hence pose less of a threat to sea life. On the downside, they generally can’t produce as much power as barrage dams.

There are currently no commercially viable tidal stream generators operating anywhere in the word. 6 A project by MeyGen in Scotland is currently the largest planned tidal stream project in the world and is the only commercial multi-turbine plant to have commenced construction. 7 The plan is to develop a capacity of 398 MW on a site just 2 kms from Scotland’s north-east tip. Only time will tell if the project reaches its maximum potential.

Tidal stream technology, is still in the relatively early stages of development. And as such the design space for devices is crowded. The most promising designs currently are the horizontal axis turbine (HAT), vertical axis turbine and oscillating hydrofoil.

Tidal Lagoons

A tidal lagoon is a power station that generates electricity from the natural rise and fall of the tides. It works in a similar way to tidal barrages by capturing a large volume of water behind a man-made structure which is then released to drive turbines and generate electricity. Unlike a barrage, which spans an entire river estuary in a straight line, a tidal lagoon encloses an area in a u-shape.

To date there are no tidal lagoons anywhere in the world. Developers in the UK were planning a £1.3bn plant in Swansea, Wales, said to be able to produce 320 MW (nameplate capacity), and power the equivalent of around 155,000 homes. In July 2020 the British government withdrew support and planning permission expired.

Tidal Fences

A tidal fence is a sort of cross between a tidal barrage and tidal stream system. Tidal fences are composed of individual vertical-axis turbines mounted together to form a fence like structure.

A tidal fence is also known as a caisson. These submerged fences form a sort of steel dam (barrage) across an inlet or estuary. (Usually a tidal fence uses vertical axis cross flow turbines, similar to Savonius Rotor or Darrieus Helicoil wind turbines.) Tidal currents flow past the turbine blades, causing them to rotate, which in turn powers generators to create free electricity. But unlike tidal barrages, a fence does not block the flow of water, making them cheaper to install and more marine-life friendly.

Dynamic Tidal Power

This is a new form of tidal range technology, which is still in the theory stage and has yet to be actually tested. Dynamic tidal power (DTP) is a way of capturing tidal energy by building a water head using a dam perpendicular to the coast. With a barrage, the dam is placed across the mouth of a river or a bay to enclose a tidal basin.

DTP may be a viable alternative to barrage development which has been relatively slow to develop due to the strict requirements relating to natural tidal conditions and concern over the environmental impact caused by the enclosure of a basin. 8 9

What Are The Advantages of Tidal Power?

Despite the slow start, there has been some progress in the tidal power sector. Several milestones were achieved in the 2009-2019 decade. These range from the commissioning of the world’s largest grid-connected tidal barrage in Sihwa Lake, South Korea, as well as numerous tidal stream devices deployed in open-sea waters for testing. Within this past decade, the cumulative energy produced from wave and tidal stream energy alone has increased from less than 5 GWh in 2009 to about 45 GWh in 2019. 10.

Greenhouse Effect Explained
Greenhouse Gas Statistics

1. Tidal Power Life Cycle Emissions

One study examined the comparative sustainability of 4 renewable electricity technologies and found that tidal power was the most environmentally friendly in terms of greenhouse gas emissions – namely carbon dioxide (CO2). Using a life cycle analysis, CO2 emissions were calculated from construction to decommissioning, including maintenance (periodic component replacement plus machinery use).

The comparative results showed that tidal power generation was associated with 1.8 g of CO2/kWh, wind with 3.0 g of CO2/kWh, hydroelectric with 4.6 g of CO2/kWh and geothermal energy with 5.6 g of CO2/kWh. 11

2. Predictable Energy

Tides are interesting because unlike other renewables such as wind and solar power, they are predictable. The gravitational forces of the moon and sun are not going to stop anytime soon.

Furthermore, tidal power is more efficient at lower speeds than it’s nearest competitor, wind power. Water is 830 times denser than air, so tidal turbines can generate electricity at speeds/velocities as low as 1m/s. In contrast, most wind turbines begin generating electricity at 3m/s-4m/s, or 7mph-9mph. 12

3. Tidal Energy Doesn’t Take Up Much Space

The largest tidal project in the world is the Sihwa Lake Tidal Power Station in South Korea, with an installed capacity of 254MW. The project, established in 2011 was easily added to an existing 12.5km-long seawall which was built in 1994 to protect the coast against flooding and to support agricultural irrigation. Compare this to some of the largest wind farms, such as the Roscoe wind farm in Texas, which takes up a sprawling 400 square km of farmland – bigger than five Manhattan islands. 13 Changing land-use is a factor in climate change, and needs to be considered.

4. No Visual Pollution

Unlike wind farms and solar farms, tidal turbines can be completely submerged and leave no visual impact on the landscape.

5. Abundant Source of Energy

The potential global market of tidal power is between 150-800 TWh (terawatt hours) per year, or up to EUR 40bn ($50bn) per year. According to the EU, under favorable regulatory and economic conditions, ocean energy technologies (tidal and wave) could meet 10 percent of the EU’s power demand by 2050. 14

Did You Know?

Bathymetry is the study of ocean and lake floors. It is the underwater equivalent of hypsometry (measurement of land elevation) or topography (study of land surfaces). The name bathmetry comes from Greek bathus, deep, and metron, meaning measure.

La Rance Tidal Power Station, France
La Rance tidal power station in France was built in the 1960s. For many years it was the largest of its kind, with an output of 240 megawatts. Photo: Public Domain

What Are The Disadvantages of Tidal Power?

1. High Costs

One of the biggest drawbacks to tidal power is the high upfront costs which makes it unattractive to investors. The cost of constructing a tidal power generation plant varies depending on what type of technology is used. For example, tidal energy turbines need to be much sturdier than wind turbines, because of the high density of water. Meantime, the proposed 320 MW Swansea Bay Tidal Lagoon project in Wales, was priced at £1.3bn ($1.67bn). In comparison, The Tengger Desert Solar Park cost around $530m for a total installed capacity of 850 MW.

2. Limitations of Site Requirements

While there is an estimated 3000GW of gross tidal power in the oceans, less than 3 percent is located in locations suitable for power generation. 15 This is mainly because tidal stream energy extraction is extremely site specific. Bathymetric charts of the ocean floor must show favorable conditions, spring peak currents need to be just right, and proximity to the coast to minimize electricity transmission costs is essential. In the United States, there are also legal concerns about who owns underwater land.

3. Storage Issues

Powerful tides only happen normally 10 hours out of each day, this means the tidal energy storage capacity must be developed.

4. Environmental Concerns

A report commissioned by the National Oceanic and Atmospheric Association (NOAA) identified several environmental effects. 16 It mentioned:

  • Alteration of currents and waves.
  • Alteration of habitats for benthic ecosystems (seabed). 17
  • Stressors from noise due to device operation or installation or both.
  • Emission of electromagnetic fields – associated with the generator and power electronics on a device or power cable.
  • Toxicity of paints, lubricants and antifouling coatings.
  • Interference with marine animal movements and migrations.
  • Death of marine animals by rotor blades or other moving parts.

The Future Prospects

Outside of a handful of successes, most tidal energy undertakings are still in the demonstration phase. For this reason, it can be said that the industry as a whole has yet to prove that it has a role in climate change mitigation strategies. However, in the next decade, with the right political partnership and investor support, we are likely to witness tidal energy becoming a fully fledged sustainable energy source.

Sustainable Energy of the Future

The latest hope for low carbon energy is Nuclear Fusion, a costly quest for limitless ‘always-on’ power that would revolutionize our energy system.

Further Reading

References

  1. IRENA and “Ocean Energy Europe Partner to Drive Ocean Energy Industry”. December 2020 []
  2. “Marine Energy Marine energy (or ocean energy) encompasses wave, tidal stream, tidal range, ocean thermal, ocean current, run-of-river, and salinity, etc., through which energy can be harnessed from oceans.” Energy for Sustainable Development, 2020 []
  3. IEA Tracking Power: 2020 Report. []
  4. “Harnessing the Tides, the Early Medieval Tide Mills at Nendrum Monastery, Strangford Lough”. Archaeological Monographs No 7, TSO, 2007.[]
  5. “A Review of Tidal Power Resource Evaluation for Tidal Barrage.” Abbas Ghasemi et al, 2019.[]
  6. “Tidal power renewable energy.” Climate Technology Centre & Network.[]
  7. “Largest Planned Tidal Stream Project.” Hydro International August 2020. []
  8. “Analysis of characteristics of Dynamic Tidal Power on the west coast of Korea.” Young HyunPark. 2017. []
  9. For more on DTP technology read : “Numerical study of hydrodynamic mechanism of dynamic tidal power.” Peng Dai et al. Water Science and Engineering, 2018. []
  10. Annual Report An Overview of Ocean Energy Activities in 2019. IEA.[]
  11. “Comparison of Life Cycle Carbon Dioxide Emissions and Embodied Energy in Four Renewable Electricity Generation Technologies in New Zealand.” Bridget M Rule eat al. 2009. []
  12. “Riding the renewable wave: tidal energy advantages and disadvantages.” Power Technology 2020. []
  13. “As climate threat looms, Texas Republicans have a solution: giant wind farm everywhere.” USA Today 2019. []
  14. “Tidal flows generate huge potential for clean electricity”. EU Commission 2020. []
  15. The World Offshore Renewable Energy Report 2004-2008. []
  16. “Environmental Effects of Tidal Energy Development” Report. NOAA 2010. []
  17. The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, stream or lake. []
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