Tidal Energy: All You Need to Know
The pursuit of non-polluting renewable energy sources has led to increasing interest in the energy potential of our seas and oceans.
There are a variety of different technologies (thermal, wave, offshore wind, ocean current energy) have been developed to harness the energy in large water bodies.
However, tidal energy is one of the most promising sources that have yet to be tapped on a large scale.
The seas can provide a vast amount of energy in the form of waves, currents, and heat.
But the energy in water bodies is not easy to capture and convert into electricity. Therefore, tidal energy has not been widely adopted as a primary source of renewable energy.
What is Tidal Energy?
A tide refers to the regular rising and falling of seawater associated with the gravitational attraction of the moon and sun.
Tidal energy is a form of renewable energy that captures the kinetic energy of moving tides to generate electricity.
It takes advantage of the tremendous power of the ocean's tides using a variety of different technologies including tidal turbines, barrages, and in-stream generators.
Although tidal energy production is still in its infancy it has the potential to provide a consistent and reliable source of electricity without significant environmental impacts.
But today, tidal energy lags behind wind and solar in terms of installed capacity. There are a number of tidal power stations in operation around the world. But they still account for a very small percentage of global renewable energy production.
The Sihwa Lake Tidal Power Station in South Korea is the world’s largest tidal power station. It is located on the west coast of the country and has a capacity of 254 MW. The United States has no tidal power stations currently in operation, but there are a few in the development stages.
How does Tidal Energy work?
Tidal currents are created by the gravitational pull of the moon and sun on the earth. The tidal bulge (a mound of water that forms as a result of the tidal current) moves back and forth as the tide rises and falls.
The strength of a tidal current is determined by the tidal range (the difference between the high and low tide), the width of the channel, and the friction along the sides of the channel.
The tidal range and width of the channel are determined by the moon's and sun's distance from the earth, while the friction is determined by the shape of the coastline and the ocean bottom.
The amount of energy in a tidal current is proportional to the square of the tidal range. This means that a doubling of the tidal range (from, say, six feet to twelve feet) would quadruple the amount of energy in the tidal current.
Tidal energy technologies utilize either the kinetic energy of moving water or the potential energy of water at different elevations.
Kinetic tidal energy systems
Kinetic tidal energy systems extract power from the movement of the tides. This type of system typically consists of turbines that are mounted in or near the water body that is being harnessed. The tidal current spins the turbines, which then generate electricity.
The main challenge with kinetic tidal systems is extracting energy from a moving body of water without causing significant environmental damage. There is a lot of potential energy in tidal currents, but it is not easy to capture and convert into electricity.
In-stream generators are a type of tidal energy system that extracts power from the difference in water levels between two points in a water body.
This technology is still in development but has the potential to be more environmentally friendly than other types of tidal systems.
Potential tidal energy systems
Potential tidal energy systems use the difference in height between high and low tides to generate electricity.
These systems typically consist of a dam-like structure that is built across the entrance of a bay or estuary. As the tide rises and falls, the water level changes, and this difference in height is used to turn a turbine and generate electricity.
The main challenge with potential tidal systems is that they can be affected by weather conditions, and they can also have a significant environmental impact. Barrages are the most common type of potential tidal system.
So far, we've discussed two types of tidal energy systems: kinetic and potential. However, these two categories encompass a wide range of different technologies. Now let's take a closer look at some of the most common types of tidal systems.
Tidal Barrage
A tidal barrage is a structure that prevents water from flowing between two bodies of water. It is constructed across a narrow section of a river, estuary, or bay with a tidal range of more than 5 meters.
Tidal barrages operate on the same principles as hydroelectric dams, with the exception that tidal currents flow in both directions.
The main advantage of a tidal barrage is that it is very efficient at converting tidal energy into electricity. The downside is that they can be expensive to build, and they can have a significant environmental impact.
A tidal barrage will usually have turbines, waterwheels, sluice gates, embankments, and ship locks as part of the design.
A tidal barrage may be unidirectional or bidirectional and incorporate bulb turbines, straflo or rim turbines, and tubular turbines.
There are a number of tidal barrages in operation around the world, including the La Rance Tidal Power Station in France, the Sihwa Lake Tidal Power Station in South Korea, and the Rance Tidal Power Station in the United Kingdom.
There are two types of tidal barrages: single-basin and double-basin.
Single Basin Tidal Barrages
Single-basin systems consist of a single basin and typically necessitate the construction of a barrage across a bay or estuary. In a single basin, there are three different ways to generate power.
Ebb generation
Ebb generation is a process that uses the falling tide to generate electricity. As the tide falls, it flows through the barrage and into the basin.
This causes the water level in the basin to rise, and as the tide continues to fall, it flows out of the basin and back into the river or estuary. This process is repeated over and over again to generate electricity.
Flood generation
During the flood tide, the sluice gates and turbines are maintained in a closed position until there is a significant hydrostatic head across the barrage.
When a sufficient hydrostatic head is reached, the turbine gates are opened allowing the water to flow into the basin.
Because of the effects on shipping and the environment, flood production is a less desirable technique of generating electricity. The average reduction in sea level within the basin causes these consequences on shipping and the ecosystem.
Two-way generation
This type of operation generates electricity by utilizing both the flood and ebb stages of the tide. The sluice gates and turbines are maintained closed until the flood cycle is nearing its end.
The water is then allowed to flow through the turbines, generating energy. The sluice gates are opened when the minimum hydrostatic head for generating energy is attained.
At high tide, the sluice gates are closed, trapping water behind the barrier until an adequate hydrostatic head is restored.
The water is then allowed to run through the turbines in the ebb mode to generate electricity. Two-way generation offers the advantage of reducing non-generation time and lowering generator costs due to lower peak power.
Double Basin Tidal Barrages
Double-basin systems have two basins. The main basin is essentially the same as that of a single-basin ebb generation system.
A double-basin system differs from a single-basin system in that a share of the electricity generated during the ebb phase is utilized to pump water into the second basin, allowing for some storage.
Hence, this system may adapt the delivery of electricity to fit consumer demands.
The ability to distribute electricity during times of high electricity demand is the primary advantage of double-basin systems over single-basin systems. However, due to the inefficiency of low-head turbines, double basin systems are improbable to become viable.
Due to the additional length of the barrage, the high construction costs of double-basin systems may also limit the development of this system.
What are the challenges of building more tidal barrages?
The current difficulties limiting the expansion of tidal barrage systems are substantial construction costs and environmental effects, with no major technical issues that need to be resolved.
To withstand the massive loads produced by dammed water, the building of a tidal barrage necessitates a massive amount of materials.
The ensuing high building costs are regarded as one of the most significant challenges when determining whether a site is economically viable for tidal energy production.
Because of advancements in turbine design, a routine repair can now be performed with greater ease; thus, maintenance is no longer considered a development issue. The decision to use tidal energy technology must be taken with the understanding that immediate changes to the surrounding ecosystem will occur.
The most significant downside of tidal barrages is their environmental impact. The construction of a dam across an estuary or bay may alter the flow of tidal currents, influencing marine life within the estuary.
The influence of a tidal barrage varies depending on location; nevertheless, there are few projects to compare. Water quality within the basin may also be impacted, such as sediment transfer, causing variations in water turbidity. The presence of a barrage will have an impact on maritime trade.
This maritime traffic problem is easier to handle for an ebb generating system, which keeps the basin at a considerably higher water level than a flood generation system.
Changes in sediment transportation are not entirely bad, and as a result, marine life may thrive in places where it would not ordinarily be found.
Tidal Turbine
A turbine is a device that extracts energy from a moving fluid (liquid or gas). Tidal turbines are placed in the path of the tidal current and use the kinetic energy of the moving water to turn a turbine.
The most common type of tidal turbine is the horizontal-axis tidal turbine. However, there are also vertical-axis tidal turbines. Horizontal-axis tidal turbines are more common because they are easier to manufacture and install.
Horizontal-axis tidal turbines
Horizontal-axis tidal turbines are placed in the path of the tidal current and use the kinetic energy of the moving water to turn a turbine. They are the most common type of tidal turbine that is currently being used.
Horizontal-axis tidal turbines have the advantage of being easier to manufacture and install than vertical-axis tidal turbines.
They also have the advantage of being able to use a greater range of water velocities. However, they are less efficient than vertical-axis tidal turbines.
Vertical-axis tidal turbines
Vertical-axis tidal turbines are placed perpendicular to the direction of the tidal current and use the vertical motion of water to turn a turbine.
They have a smaller rotor that spins around a vertical axis. The blades of the rotor are designed to capture as much energy as possible from the moving tidal current.
Vertical-axis tidal turbines have the advantage of being able to extract energy from smaller tidal velocities, but they are more susceptible to damage from large floating objects, such as boats.
Can tidal barrages and turbines be used in conjunction?
Tidal barrages and turbines can be used in conjunction with one another to create a more efficient tidal power system.
The use of tidal barrages and turbines together can create a more efficient tidal power system.
Tidal barrages can be used to store energy from the tidal current, and turbines can be used to extract energy from the stored water. This arrangement would allow for more consistent delivery of electricity to consumers.
Tidal turbines can also be used to supplement the power generated by a tidal barrage. If the tidal barrage is not able to produce enough power to meet the demand, the turbines can be used to make up the difference.
This arrangement would allow for more consistent delivery of electricity to consumers and would help to reduce the amount of power that is needed from other sources, such as fossil fuels.
Tidal Stream Generators
Tidal stream generators resemble and function similarly to underwater wind turbines.
Tidal stream generators, as opposed to harnessing the rising and falling movement of the tides, take advantage of the rapidly moving sea currents (tidal streams) that flow when the tides are in and out.
The tidal streams drive the turbines to revolve, which in turn turns the generators, generating power.
Tidal stream generators have the benefit of being significantly less expensive to create and having less of an environmental impact than a tidal barrage.
Because the turbines turn slowly, they have no effect on sea life. This is in contrast to tidal barrages, which can impede fish migration up rivers from the sea.
One of the primary benefits of this technology is the ability to integrate tidal stream generators into existing structures such as scaffolds and docks. Even more importantly, this lowers costs and has a lower environmental impact.
Tidal stream generators cannot produce as much power as tidal barrage systems. They are susceptible to saltwater corrosion and marine fouling, which can reduce the lifespan of the generators. However, material science is making rapid advancements to combat these issues.
Tidal Mills
A tide mill is basically a water mill that gets its power from rising and falling tides. It is typically constructed on a river estuary, where it is protected from waves but near enough to the sea to harness tides.
Rising tides flow into a millpond through a gate, and when the tide begins to recede, the gate automatically closes. This traps the water in the millpond and forces it to turn a water wheel, which is connected to a generator to produce electricity.
The main disadvantage of a tide mill is that it can only produce power when the tide is flowing in and out. This means that the mill cannot produce power 24 hours a day, as is possible with other forms of tidal power generation.
The tidal mill has been used for centuries to grind flour, saw wood, or power a mill.
Tidal mills were made out of a dam with sluices, a holding basin, and a float or a water wheel that converted the energy of flowing water into mechanical power to power flour mills sawmills, and even breweries, and were used to pump sewage as late as 1880.
Some of the earliest tide mills in Europe were located in the Rance estuary in France. This is the first tidal power plant in the world, and it's located on the Rance river.
It started its operations in 1966. Compared to earlier tidal mills, this one generates electricity during both high and low tides.
Tidal fences
A Tidal Fence is a tidal stream device that directly harnesses fast-moving undersea ocean currents for energy generation.
In many aspects, a tidal fence installation is a hybrid of a tidal barrage and a tidal turbine stream system. Tidal fences are placed in shallow water, where the tidal current is strongest.
The tidal fence consists of a number of vertical pipes that are installed in the seabed. The tops of the pipes are open to the air, while the bottom is closed off.
When the tide rises and water flows into the pipe, it creates a pressure difference that causes the air to flow in and out of the pipe, turning a turbine connected to a generator.
The main advantage of a tidal fence is that it can be placed in much shallower water than a tidal turbine stream system, making it easier and cheaper to install. In addition, the tidal fence does not require the construction of a dam, which could have an adverse environmental impact.
The main disadvantage of the tidal fence is that it is less efficient than a tidal turbine stream system. The pipes are also susceptible to marine fouling, which can reduce the lifespan of the generators.
What is the Tidal Range?
The tidal range is the difference in height between high tide and low tide. When the tide is at its highest, it is called a high tide.
When the tide is at its lowest, it is called a low tide. The difference in height between high tide and low tide is called the tidal range.
The higher the tidal range, the more potential energy there is to be captured. The tidal range can vary depending on the location and the time of year. For example, the tidal range in the Bay of Fundy is the highest in the world, with a range of up to 12 meters (38 feet).
Is tidal energy a renewable source of energy?
Yes, tidal energy is a renewable source of energy. The tides are generated by the gravitational force of the moon and sun on the earth, so they will always be available.
Tidal energy can also be used to generate electricity without damaging the environment, so it is a sustainable source of energy.
How much tidal energy is available?
Because many tidal power installations are not financially viable, tidal energy has not been widely adopted as a renewable energy source.
However, because water currents contain a vast amount of kinetic energy, tidal energy has the potential to become a major source of renewable energy. The total energy available from tidal sources is estimated to be about 3,000 gigawatts (GW; billion watts).
However, estimates of how much of that energy is present for power generation by tidal barrages vary from 120 to 400 GW depending on the location and the conversion technology.
What are the benefits of tidal energy?
Predictable energy output
Tidal energy is a predictable source of energy. The amount of energy available from tidal sources is determined by the size of the tidal range and the width of the channel.
This means that you can predict how much energy will be available from a tidal power plant months or years in advance.
No fuel costs
Tidal energy does not require any fuel to generate electricity. This makes it a cost-effective source of energy that only requires a one-time investment.
Reduced greenhouse gas emissions
Tidal energy does not produce any greenhouse gas emissions, so it is a clean and sustainable source of energy. That said, some environmental impacts may occur during the construction of a tidal power plant.
Minimal land use
Tidal energy installations require very little land to operate. This is because the turbines are placed in the ocean, where there is plenty of space. In contrast, solar and wind energy installations require large amounts of land to generate a significant amount of energy.
Inexpensive to maintain
Tidal energy is a reliable and low-maintenance source of energy. Once a tidal power plant is installed, there is very little ongoing maintenance required. The turbines and generators are designed to last for many years without requiring any major repairs.
High energy density
The kinetic energy in water is much higher than the kinetic energy in the air. This means that you can generate more electricity from a tidal turbine than you can from a wind turbine of the same size.
Vertical-axis and offshore turbines are more economical
Tidal turbines can be either vertical-axis or horizontal-axis. Vertical-axis turbines are more efficient and economical than horizontal-axis turbines, so they are the preferred type of turbine for tidal power plants. Offshore turbines are also more economical than turbines located on land.
Reduces the damage of high tidal surges
Barrages mitigate the effects of high tidal surges on the land. They also help to protect coastal communities and infrastructure from the damaging effects of high tides.
Tidal energy is one of the most promising existing renewable energy sources. Unlike wind, solar, and thermal energy, tidal energy has a long-term perspective and can be projected more precisely.
What are the drawbacks of tidal energy?
Early-stage development costs
Costs for ocean-energy devices remain exceedingly high since there are no benefits to mass production.
Each device must be thoroughly conceived, built, and tested in laboratories before being transformed into prototypes to be tested in actual ocean waters.
To survive, the long process of trial and error requires finance. Governments are now providing a large portion of funding since private enterprises do not see a high return on their investment.
A plethora of unproven prototypes
Wave and tidal motion can be captured using a slew of new and innovative technologies. Energy from waves can be used to move devices vertically or horizontally and generate motion that can be used to generate power.
Because waves are influenced by surface winds, meteorological conditions, and local topography, it is difficult to accurately measure wave motion.
A device that works well in one place may not work at all in another, making it challenging to come up with a universal design.
The turbines can disrupt marine life
Introducing any large mechanical device into an active ocean ecosystem is bound to be a problem.
Aquatic organisms can be harmed or killed by spinning blades, as well. A coastal barrage for tidal energy might have a devastating effect on the entire estuary environment.
Mechanical equipment can also leak lubricants and generate noises that are harmful to fish and other aquatic animals.
Despite their best efforts, engineers are unable to predict what would happen after their prototypes are submerged. As a result, it is impossible for them to fix these issues before they become much more problematic.
Unpredictable weather
While waves crash on the coast indefinitely, the wind's influence on the surface water changes their frequency and amplitude.
Rough terrain often benefits these devices because it generates more motion and thus more energy. But power users expect a steady supply of energy, not one that fluctuates with each low-pressure system. So, in some cases, battery storage is required.
Hurricanes, tsunamis, typhoons, and other natural disasters can quickly destroy equipment, while daily sea pounding wears it out over months and years.
Because these devices will be installed in high-energy areas traditionally avoided by subsea cable and construction operations, engineers must create extremely durable machinery.
Corrosion and bio-fouling
Metal alloys are frequently required for devices that convert waves into energy. Saltwater is also extremely corrosive to steel and other metals. To avoid corrosion, special care must be taken during design, construction, and installation.
Many metallic alloys are corrosion resistant, but their use in large-scale energy systems is prohibitively expensive.
Plus, organisms will cling to anything we dump into the water. Small animals and plants can attach themselves to underwater gadgets, causing costly breakdowns and maintenance.
Formation of silt behind the barrage
When a coastal barrage is used to create a tidal energy reservoir, it can also trap sediment behind it. Over time, this accumulating sediment can reduce the efficiency of the barrage and even block the passage of water.
The deposition of silt can also lead to a build-up of nutrients that can promote the growth of algae, which can in turn lead to a decrease in water quality.
Few suitable barrage sites
There aren't many places in the world where there is both a large tidal range and a lack of environmental sensitivities. Building a tidal energy barrage can have a lot of unintended consequences.
The best locations for tidal energy barrages are also the most environmentally sensitive, making it difficult to find an acceptable site. Even if a good location is found, it may be in a heavily populated area, which could lead to protests and public outcry.
Produces energy only 10 hours a day
Tides are only strong for 10 hours each day, implying the tidal energy plant cannot run at full capacity for the most part.
The output from tidal turbines varies with the tide, so they can't always be relied on to generate power when it's needed. This inconsistency makes it difficult to use tidal energy as a baseload power source.
Requires high-priced technology
The technology needed to harness tidal energy is expensive and not yet commercially available. Until the cost of this technology comes down, it will be difficult to justify its use.
The cost of tidal energy technology is still prohibitively high for widespread use. Until it becomes more affordable, this form of energy will remain out of reach for most people.
Conclusion
There are a lot of potential benefits to tidal energy, but there are also some drawbacks that need to be addressed before they can be relied on.
Some of the issues include unpredictable weather, corrosion and bio-fouling, the formation of silt behind the barrage, and few suitable barrage sites. Additionally, tidal energy production is only possible for a limited number of hours each day.
Despite these drawbacks, tidal energy is a promising form of renewable energy that should be further investigated.
