EARTHDAY Explainers – VII

Tidal Power: Making Waves in the Energy Game

By Allen Huang
As the world races to combat climate change and reduce our dependence on greenhouse gas emitting fossil fuels, a centuries-old energy source is making waves—literally. The rhythmic rise and fall of ocean tides, driven by the gravitational forces of the Moon and Sun along with the Earth’s rotation, is emerging as an increasingly reliable source of renewable energy.

Tidal energy shares similarities with hydroelectric power, particularly in its use of natural water movement to generate electricity. However, it generally requires a smaller geographic footprint as it does not require large dams or barriers or reservoirs, and, depending on the technology used, can have a lower impact on local ecosystems.
These advantages make tidal energy a compelling solution for coastal communities, offering a reliable and consistent source of renewable power that can help reduce carbon emissions. Unlike wind and solar, which are intermittent, tidal energy is predictable, driven by the natural gravitational forces of the moon and sun, ensuring a steady energy supply. Moreover, tidal power systems can be designed to have minimal impact on marine ecosystems, with some technologies allowing for integration into existing coastal infrastructure.

By tapping into the vast energy potential of the oceans, tidal energy can power local grids, support energy independence, and help reduce reliance on fossil fuels, all while preserving the health of marine environments. While the modern world is waking up to the power of today’s energy – other ancient civilizations were all too aware of its power.

A Little Bit of Wave History

The origins of tidal energy date back over two thousand years, with the use of tide mills occurring in the Roman Empire around the 1st century BCE.

These early systems harnessed the natural ebb and flow of tidal waters to power grain mills. A typical tide mill featured a waterwheel or a set of paddles positioned in a tidal channel or basin. As the tide came in, water would fill a turn the wheel. The movement of the wheel was then transferred via a system of gears to rotate the millstones, grinding grain into flour. During the falling tide, water was released through sluices, and the process would reset as the tide came back in.

In many tide mills, the waterwheel operated both as a power source for grinding and a mechanism for controlling the flow of water, making them highly efficient in areas with strong tidal currents. By the 7th and 8th centuries CE, tide mills were common across Europe, especially in tidal regions like Britain, France, and the Low Countries. These systems were not only used for milling grain but also for other tasks, such as pumping water and sawing timber.

Some continued to operate well into the 19th century, long after steam power had begun to replace traditional water mills. But it is the ingenuity of these early tide mills which laid the groundwork for the advanced tidal power systems we are seeing gradually develop across coastal locations today.

What’s the Big Win With Tidal?

The biggest benefit of tidal power comes from the high level of power conversion rates. Similar to hydroelectricity, approximately 80% of the power the turbines collect from tidal energy becomes usable electricity.

However, tidal energy systems can only generate electricity during tidal flow periods, typically 4-6 hours per tidal cycle. This creates intermittency, as tidal energy production pauses between cycles. While tidal is predictable, it offers less consistent energy generation compared to wind and solar, which can operate for more extended periods.

Predictability and Reliability

Plus tidal energy is one of the most predictable forms of renewable energy because tides follow a regular, cyclical pattern driven by the gravitational pull of the moon and the sun. This means that tidal energy generation can be forecast years in advance, making it more reliable than other renewable sources like wind and solar, which are dependent on weather conditions.

Tidal energy is not weather-dependent. Tidal energy production happens every day, without interruption, as long as there is a tidal cycle. This provides a consistent and stable supply of energy for coastal communities, and even potentially for the wider national grid.
Because tidal energy is predictable and reliable, it can act as a complement to other intermittent renewable sources like wind and solar power. For example, when solar or wind energy output is low due to weather conditions, tidal energy could fill the gap, providing more stable and continuous energy. Hybrid systems that combine tidal energy with wind and solar could be highly effective in achieving grid stability and reducing the reliance on fossil fuels.

How Does Modern Tidal Power Work?

Primarily speaking, tidal energy systems primarily fall into two categories: tidal range power, by which tidal barrages are the most common form, and tidal stream power. While tidal ranges generate electricity in a fashion similar to traditional hydroelectric power, with water falling from an elevated place onto turbines to generate electricity, tidal stream power actually works more like wind energy, with bladed turbines rotating from the force of the ocean currents to power generators. All of these systems are each suited to distinct conditions and operational requirements.

Tidal Range Energy In More Detail

Tidal range refers to the vertical difference in height between high tide and low tide. It’s a measure of how much sea level rises and falls due to the gravitational pull of the moon and sun, along with the Earth’s rotation. The bigger the range, the more potential energy can be generated. In places where the tide has a large range—sometimes as much as 15 meters or more—there is more energy available to capture.
Tidal range systems work best in areas with strong tidal movements and wide, shallow coastal regions. These include places like the Bay of Fundy in Canada, where the tidal range can reach up to 16 meters, or the Severn Estuary in the UK, where the difference can be more than 13 meters. Other countries with favorable conditions include South Korea, France, and parts of China, which have natural harbors, estuaries, or coastal regions that experience high tidal fluctuations.
In case you want more detail, there’s a formula for calculating the amount of energy that can be produced from tidal range systems: 𝑃 = 𝜌 ⋅ 𝑔 ⋅ 𝐴 ⋅ ℎ²

Where:
  • 𝜌 is water density
  • 𝑔 is gravitational acceleration
  • 𝐴 is the impounded area (the area where water is collected)
  • ℎ is the height difference between high and low tide.

    The larger the tidal range and the area where water can be captured, the more energy can be generated

    • Tidal Stream Energy In More Detail

    Tidal stream, on the other hand, refers to the horizontal movement of water caused by tidal currents, which occur when the tide goes in and out. These currents are created as the ocean water moves back and forth, pulled by the gravitational forces of the moon and sun. The result is fast-moving streams of water, much like underwater rivers, that can flow through narrow channels, along coastlines, or between islands.

    Some of the best locations for harnessing tidal stream energy include the Pentland Firth in Scotland, where tidal currents reach speeds of up to 10 knots, or the Mersey Estuary in England. The Strait of Messina between Italy and Sicily also has strong tidal flows, as does the Cook Strait in New Zealand. These areas have strong, consistent tidal currents that can generate enough energy to power local communities and industries.
    The energy from these fast-moving tidal streams can be captured using special machines called tidal stream generators, which work much like underwater wind turbines. As the water flows past these turbines, it causes the blades to turn, and that turning motion is converted into electricity. This clean, renewable energy can be used to power homes, businesses, and even entire cities along coastlines.

    In contrast to tidal range systems, which rely on the rise and fall of water levels, tidal stream energy uses the movement of the water itself, making it a more flexible option for areas with strong, consistent currents. And since tidal stream generators can be installed on the seafloor, they have less impact on the surrounding environment compared to large tidal dams.

    Tidal Barrages In More Detail

    Tidal barrages, which are estuarine barrages, akin to massive dams. Built across estuaries (a partially enclosed body of water that connects rivers or streams to the open sea) with high tidal ranges, they trap water during high tide and release it through turbines at low tide, generating electricity.

    The power generated is proportional to both the area of the impounded water and the square of the height difference between the water levels inside and outside the basin, making tidal range energy most effective in locations with large natural tidal ranges and suitable geography for creating large impoundments.

    Unlike river-based hydroelectric facilities, where water can flow only by one direction, tidal barrages can be either one-directional or two-directional, also known as “double generation.” The traditional unidirectional generation produces electricity during one phase of the tidal cycle, which is typically when water flows out of the reservoir through turbines during low tide.
    However, two-directional generation harnesses power during both tidal phases: as water flows into the reservoir during high tide and as it flows out during low tide. Whether or not a tidal barrage is one or two-directional usually depends on the tidal range itself: the larger the range, the more likely it would be a one-directional facility.

    To sum up how the tidal energy methods differ:

  • Tidal range is the difference in water levels between high and low tide, which can be used to generate energy.
  • Tidal stream, uses moving water to spin turbines and generate power.
  • Tidal barrage is a specific system that captures tidal range energy by building a dam to trap water and release it through turbines.

    • What Other Emerging Tidal Technologies Exist?

    In addition to the relatively mature tidal barrages and tidal streams, there are other new techniques being developed to harness wave power: tidal lagoons and dynamic tidal power. Both of which are still mostly theoretical concepts that have not really come to fruition.

    Tidal lagoons, which is also a form of tidal range power, do not require the existence of estuaries to be built. As the name suggests (lagoons are semi-enclosed bodies of water slightly separated from the ocean), tidal lagoons are built as standalone structures that can be placed in areas with high tidal ranges to form artificial lakes.
    These lagoons use walls to trap water during high tide and release it through turbines during low tide, generating electricity. Tidal lagoons can also function in a two-directional manner, generating power as water flows both into and out of the lagoon, without being disruptive to the local environment as much as the tidal barrage.

    Dynamic tidal power, or DTP, on the other hand, is a much bolder attempt to collect electricity from tidal power. It involves constructing long, dam-like structures—potentially up to 60 kilometers—in open coastal waters at an angle to the shoreline. These structures, equipped with turbines, exploit the phase difference between tidal currents on either side of the dam to generate electricity. As tidal currents interact with the dam, a pressure differential is created, driving water through the turbines and producing power.

    Neither Tidal Lagoons or Dynamic Tidal Power Are Widespread – Why?

    Neither tidal lagoons nor dynamic tidal power have been realized on a large scale due to the significant technological, financial, and environmental challenges both of them come with. Firstly tidal lagoons require substantial upfront investment and face uncertainties around their long-term economic viability. For instance, the proposed Swansea Bay Tidal Lagoon in the UK has been indefinitely stalled due to the high costs as well as concerns about its cost-effectiveness compared to other renewable technologies.
    Dynamic tidal power faces even greater hurdles, and the concept so far has not progressed beyond theoretical models. The immense scale of construction and lack of proven prototypes make it a high-risk venture, at a time when other more feasible alternatives already exist.

    Challenges of Tidal Power: Cost

    The biggest hurdle for wider tidal power adoption is indeed cost. Initial capital investment for building tidal power facilities can be very high compared to other renewable energy sources like wind or solar. This is because tidal power requires expensive infrastructure, including dams, turbines, and complex engineering solutions that need to withstand the harsh conditions of the ocean.
    For example, the Sihwa Lake Tidal Power Station in South Korea, with an installed capacity of 254 megawatts, is one of the largest tidal power stations in the world. It was built at a cost of $298 million in 2011. Plus, the cost per megawatt of capacity for tidal power is higher compared to other renewable energy sources like wind or solar.

    Environmental Concerns

    While any damage tidal may do pales in comparison to the harms of fossil fuels there are some environmental impacts associated with tidal barrages in particular. Researchers suggest that tidal barrages could destroy benthic habitats (the physical environment on the bottom of a body of water where organisms live and grow), causing potential disruption to underwater ecosystem patterns. But unlike land-based solar farms or wind turbines, tidal energy installations often have a smaller geographic footprint overall.

    Then other concern is sedimentation and tidal patterns can be altered by the tidal barrages which also threaten migratory fish routes, and given tidal barrages strongly resemble the structures used to make impoundment dams, the challenges they pose for the environment do bear an eerie resemblance.
    For tidal stream facilities, despite the relatively low impact to the environment, the harsh marine conditions themselves pose their own set of challenges toward the long-term operability of these projects. Tidal energy systems are constantly exposed to powerful ocean currents, wave action, and saltwater corrosion, all of which can degrade materials and components over time.

    Having said all of this, like other forms of renewable energy, tidal power produces zero emissions while generating electricity. Unlike fossil fuels, tidal energy doesn’t release carbon dioxide (CO2) or other harmful pollutants into the atmosphere, making it an important tool in the fight against climate change.

    Will Tidal Energy Have A Future?

    The global potential of tidal power in theory is enormous. One estimate in 2023 suggests that tidal power globally could produce 1,200 terawatt-hours of electricity per year, which is 14% of the global renewable energy production level in that year. However, that number is under threat, ironically, from rising sea levels, as shifting geographies could render formerly ideal locations useless.

    Tidal energy offers many unique advantages: it’s predictable, reliable, and on the whole it is environmentally friendly. It’s a great match for coastal communities and has the potential to complement other renewable energy sources like solar and wind. While challenges like high initial costs and some environmental concerns do exist, tidal power is a promising renewable energy solution, especially as technology advances and costs decrease.

    Tidal power offers a predictable, sustainable wave of opportunity—meaning our oceans might be the key to a cleaner, more reliable energy future.