Tidal energy or tidal power is a renewable source of energy generated by the flow of ocean waters during the rise and fall of the tides. This rise and fall allows to convert the kinetic energy of water into usable power such as electricity. This mainly works by the water movements produced by the gravitational force between the Earth, the Moon and the Sun. This can be considered as a pure astronomical phenomenon that can be observed since the Earth rotates and possesses oceans. The basic principle of producing electrical energy involves a generator that converts this energy into electricity by seizing the kinetic energy from the sea current or tides [1–5].
Tidal energy does not cause emission of the greenhouse gases which are responsible for global warming or fog formation unlike the generation of electricity using fossil fuels. Therefore it is considered as a renewable source of electricity. Currently, three methods are available to produce tidal energy, such as: tidal streams, tidal barrages, and tidal lagoons. In this paper, all of them are discussed and a basic comparison in terms of cost, efficiency and availability is made.
Wind turbines draw energy from air currents and tidal stream generators draw the energy the same way using water currents. However, the power generated using an individual tidal turbine can be greater than that of a wind energy turbine .
A tidal barrage works by using the height difference ∆H of the high and low tides and extracts the potential energy. Potential energy is seized from the tides by tactical placement of particular dams, which allow to produce electricity by using tidal barrage .
A newer version of the tidal energy harnessing design option is to build round spongy walls surrounded by turbines that can seize the potential energy from the high and low tides. These artificially created reservoirs can be considered similar to those of tidal barrages without having any natural ecosystem .
A form of low-head hydroelectricity is the tidal power which uses low-head hydroelectric generating equipment. Since the technology is well developed, the main barrier is the construction cost. At least a construction period of e.g. 10 years and an enormous capital cost is required for a tidal energy project .
However, the tidal stream generator is using renewable energy, the tidal energy. Even though tidal energy is renewable, it can also can bring some adverse effects to the environment. No fuel is needed to operate the tidal stream generators. Thus, it can help save the fossil fuel, available naturally. Therefore, greenhouse gases, which can cause global warming, will not be produced. Besides that, after the tidal stream generators are built, their power will be free. Furthermore, with tidal stream generators, a large amount of electricity can be produced in countries with appropriate coast conditions.
Tidal stream generators can also produce power with greater efficiency. The tidal power can convert approximately 80% of kinetic energy into electricity, while the efficiency of energy generation by coal and oil is less than 50% . Tidal stream generators not only provide the electrical energy, but also helps to preserve the environment. However, the construction cost may increase due to the device damage by the strong ocean storms and saltwater corrosion. Furthermore, shifting tides, resulting from the changes in tidal movement, could significantly decrease the effectiveness of tidal stream generators .
2. Tidal energy
As mentioned in the previous section, tidal movement can be used to generate electrical energy. The Earth, the Moon, and the Sun all generate the gravitational and centrifugal force creating the tide. The rise and fall of the surface of the ocean are generated by the gravitational force between the Sun and Moon, while the revolution of the Earth and the Moon creates the centrifugal force. The Moon has 2.2 times larger gravitational force than the Sun, as it is much closer to the Earth .
The tidal phenomenon happens twice every day, and lasts approximately 50 minutes, and 28 seconds. The gravitational pull of the Moon can cause the water to bulge towards it. This bulge of water is greater on the side of the Earth nearest the Moon. In the same way, the rotation of the Earth-Moon system, producing the centrifugal force, causes an another water bulge on the other side of the Earth, which is the furthest away from the Moon. When a landmass lines up with this Earth-Moon system, the water around the landmass is at the high tide, also known as spring tide, and when the landmass is at 90° to the Earth-Moon system, the water around it is at low tide, also known as a neap tide. Therefore, during one period of rotation, each landmass is exposed to two high tides and two low tides. Since the Moon rotates around the Earth, the timing of these tides at any point on the Earth will vary, occurring approximately 50 min later each day .
2.1. Tidal streams
A tidal stream generator is a machine that uses the undersea current to generate electricity. In the tidal generators, the kinetic energy of the water is used to power the turbines. A tidal stream generator utilizes the concept of a wind turbine. Since the density of water is higher, the power generated by the tides can be greater than that of wind turbines . These tidal stream generators are most often built by the sea, as the sea has a higher current flow, so as to generate the greatest amount of electrical energy. Tidal stream generators require ocean currents to turn the turbine, generating electricity. The forces acting upon the mean flow create the ocean current and direct the movement of seawater . Breaking waves, the wind, the Coriolis Effect, cabling, temperature and salinity all affect the mean flow. The gravitational force of the Sun and the Moon creates the tides. The current’s strength and direction can be influenced by: the depth contours, the interaction with other currents, and shoreline configuration. The bottom currents scour and sort the sediments, thus affecting the kind of bottom developing in the area, either soft or hard, fine-grained or coarse. Bottom material defines the organic environment that may develop in an area [6, 7].
Placing the turbines at sea is a complicated procedure, as the tidal generators are large and may disturb the tide that it is meant to exploit. Turbines of the tidal streams generator are arrayed underwater in rows, just like some wind farms. Furthermore, the most suitable current speed range for the tidal stream generators is between 3.6 and 4.9 knots (4 and 5.5 miles per hour (mph)) . If a tide generates a flow speed between 4–5.5 mph, a tidal stream turbine can generate more power than a wind turbine. The ideal depth of water for location of tidal turbine farms is between 20 to 30 meters .
However, tidal stream generators are not that popular compared to wind turbines. Despite that, in December 2008, a tidal stream generator was built in Ireland, to generate about 12 MW .
Axial turbines consist of a set of rotors. It has a fixed pitch blade of high efficiency. Tidal exchange is needed for the axial turbines to rotate [9, 10]. The AR-1000 is the first axial turbine which generates 1 MW power output with a 2.65 m/s rotation speed. This was developed by Atlantis Resources Corporation. Furthermore, in Marine Current Turbine in Strangford Lough located in Northern Ireland, a prototype SeaGen was installed in April 2008 . SeaGen generates 1.2 MW power output. SeaGen consists of two axial flow rotors. The generator is propelled by one rotor. Since the rotor pitch can be adjusted within the range of 180 degrees, the turbines are able to effectively generate electrical energy . In addition to that, a company called Tenax Energy of Australia has been granted permission to build 450 turbines around the Australian city of Darwin. The axial turbine of a 15 m rotor diameter with a gravity support is proposed. This type of turbine can operate in deep water, below the shipping channels. An estimated number of about 300 to 400 houses could be powered by each of such turbines [11, 12]. A UK-based company called Tidal stream has started building an axial turbine (Triton 3) in the Thames . The full scale Triton 3 turbine has a power capacity of 3 MW while Triton 6 of 10 MW.
Vertical and horizontal axis crossflow turbines
This type of turbine can be installed vertically or horizontally. This designe allows to generate electical energy using any deep water current, both river and tidal. This design has been used in South Korea to build a cross-flow turbine, generating 1 MW. Moreover, it is being expanded to generate 90 MW.
Flow augmented turbines
A duct and shroud (stator ring) are used to measure flow augmentation in flow augmented turbine. The shroud increases the power output of flow augmented turbines. An Australian company launched a rollout of the shrouded turbine. The company has calculated that two small flow augmented turbines would generate 3.5 MW .
2.2. Tidal barrage
The tidal barrage is part of renewable energy solution that consists of building tidal locks, dams, long wall, or sluice gate. It manages the potential energy of the ocean by capturing and storing it. The dam is constructed across the basin or tidal inlet, creating a single enclosed tidal reservoir. The lowest part of the barrage dam is placed at the seafloor and the tidal barrage is built at the highest level, so that the seawater can reach the highest peak of the annual tide. Seawater flows through an underwater tunnel built in the tidal barrage, using the sluice gates at the entrance and exit points. The tidal turbine generator is placed within this tunnel to harness the kinetic energy of the tidal waves by spinning thanks to the the seawater flow. The basic principle of the tidal barrage is similar to hydroelectric generation, except that the water can flow in two directions in a tidal barrage, contrary to a hydroelectric plant, in which the water flows in one direction only. In the presence of high tides, the seawater can flow into the tidal reservoir and fill it up. During the outgoing ebbing tides, the water can flow in the opposite direction emptying the reservoir .
A tidal barrage is attractive due to a very long service life and a cost-effective electricity production. However, the capital cost of building a new barrage is very high and can have a big impact on wildlife.
The tidal barrage utilizes the nature of tide which rises and falls due to the gravitational pull between the Earth, the Sun and the Moon. These phenomena cause the seawater to move towards the Sun and Moon. This movement of seawater increases the sea level. However, on the open-ocean, the increase in the sea level is marginal due to the fact that the large surface area has a deeper depth for it to flow into. The rise in the sea level is significant only near the coastline. This is due to the upward sloping gradient of the sea bed. This funnel helps to direct the water into the lagoons, river inlets, and estuaries. A tidal barrage generation utilizes the tidal range, the vertical difference between the low and high tides of seawater level. The height difference between the high and low tide produces the potential energy of the tides. The difference in water level at either side of the dam is exploited for electricity generation. There are three types of tidal barrage operations: the Flood generation, the Ebb generation and finally the two-way generation [14–22].
Tidal barrage Ebb generation
Tidal barrage ebb generation uses falling tides, commonly referred to as the ebb tide to generate electricity. The sluice opens during low tide so that the tidal basin fills up at the rate determined by the upcoming tide. The sluices are closed as the water level within the basin reaches the threshold point at high tide. This traps the seawater within the reservoir. As the seawater level outside the basin drops during the outgoing tide, there emerges a difference between the higher level of water trapped in the basin and the actual ocean level, called the head height [15, 32–38].
The difference in the head height during the start of the ebb tide and the falling tide is sufficient to initiate the electrical energy generation. Sluice gates connected to the turbines within the tunnel are opened to allow water to flow [3, 39–46]. This allows to exploit the potential energy stored within the water. The turbines generate electricity until the head height difference gets low. After that, the turbines are disconnected and the sluice is closed.
Figure 1 shows the tidal barrage Ebb generation [3, 26].
Fig. 1. Tidal barrage Ebb generation [3, 26]
As the rising tides move closer towards the land, the high tide starts to enter the reservoir of the barrage. Once the high tide level reaches its peak, the sluice gates are closed completely. As the water level goes down outside the barrage, sluice gates are opened to release the stored water. While the water is flowing out through the tunnel, the turbine rotates and produces electrical energy [14, 23–25]. The flood tidal barrage is a one-way method of tidal energy generation. It is restricted to 6 hours per tidal cycle due to the fact that the reservoir has to fill up.
The turbines used are low-speed turbines, as the water flow through the tunnel to the tidal basin is a slow process. This slow filing cycle is not harmful for the ecosystem as sea life can enter the enclosed basin without being exposed to fast-rotating turbines. As the tidal basin is filled with water at high tide, the sluice gates are opened which allows all the stored water to return to the ocean.
This type of tidal energy production is less efficient, compared to ebb generation, because the total amount of kinetic energy within the lower half of the basin is smaller than the total kinetic energy present in the higher half of the basin due to the effect of gravity.
Two-way tidal barrage generation
Two-way tidal barrage utilizes the phenomena of both the rising and falling tide to generate electricity. It requires better utilization of the sluice gates for more accurate control. If the tide ebbs, the seawater flows in or out via the same gate system. The turbine rotates in both directions, according to the direction of the water flow .
The main drawback of this method is that it is less effective in generating electricity. Moreover, the design and automation of bi-directional energy production are more expensive and less efficient.
2.3. Tidal lagoons
Tidal lagoon refers to a man-made structure that is connected to the ocean by one or more inlets, closing off a tidal sea area and generating electricity from hydro turbines. Furthermore, tidal lagoons can have different water depths and such lagoons can be found on all continents. Hydro turbines are placed inside concrete turbine housings and permanently submerged .
When the breakwater rises, a difference in water level is generated. This is referred to as the “head” height. When the water level reaches the desired head height, sluice gates are opened and water flows into the lagoons . The water inside the lagoon flows through turbines, generating electricity. Moreover, a tidal lagoon generates electrical energy twice, as the tide comes in and goes out. Furthermore, lagoons are expected to be active for an average of 14 hours a day .
The tide flow has a predictable pattern, therefore the energy generated from tidal lagoons is reliable. Hence, tidal lagoons do not need gas or coal power stations to be used as backup stations. This directly helps to reduce carbon dioxide emissions .
Since the lagoons can be built with natural materials, the environmental impact of tidal lagoons is low. They can be used as low breakwaters at low tide and can be submerged at high tide. This allows smaller fish to swim without any difficulty. Since lagoons are made from natural materials, they do not increase carbon dioxide emission. Moreover, tidal lagoons pose a much lower danger than nuclear power plants.
There are two design options for the lagoon wall. The first uses sandy material, which is used in many marine construction sites . The second material is a quarry run, similar to coastal reinforcements and harbour walls. For both designs, large rocks will be positioned on top of the lagoon wall to protect it from degradation.
Unfortunately, the efficiency of a tidal lagoon is rather low. Furthermore, constructing a lagoon is expensive and many conditions have to be taken into consideration before their construction. For example, biodiversity, constructing area, etc. Because of that, currently there are no functioning power plants on tidal lagoons .
3. Simulation investigations
There is an ever increasing number of scientific publications analysing the tidal stream generators using various methods of modelling and simulation. In this paper, SolidWorks (2015) and Computational Fluid Dynamics (CFD) software have been used for simulation of a tidal stream generator and tidal barrage, to provide a detailed analysis.
3.1. Tidal Stream Generator
The main part of the tidal stream generator is the turbine. To understand the effect of blade rotation, turbine modelling method called Immersed Body Force (IBF) has been introduced . The Fb Function is used to represent the force generated by the turbine. This force creates flows with different momentums and rotational speeds. This force Fb can be represented as the sum of two forces FD and FL.
As shown in Fig. 2 (a) and (b), the body force is considered as uniform throughout the volume of the blades . The force generated by the top blade is considered as a drag force. Furthermore, the other two blades consist of both drag and lift forces. An additional force has been used to create a circulation flow .
Fig. 2. (a) Force direction of the turbine blade; (b) Force direction with an additional force 
The power output is equal to
Pext – power output from the turbine [W], Cp – Power coefficient, ρ – density [kg/m3], A – the designed turbine area [m2], v – stream velocity [m/s].
Table 1 shows the specifications of the turbine and the generator used in the computation. To identify the velocity diversity the CFD software has been used. The velocity increases dramatically as the fluid passes through the turbine, as shown in Fig. 2. The reason to have a higher velocity values near the blades, is the drag and lift forces acting on the fluid flow . In Tab. 2, the output power corresponding to the stream velocity was presented.
Tab. 1. Turbine and generator specifications
Tab. 2. Output power vs stream velocity
From Fig. 3, it can be seen that the maximum power is generated at maximum velocity. Therefore, the stream velocity is nearly directly proportional to the output power which is supported by other research [1, 2, 5,12, 13, 15].
Fig. 3. The power output of the turbine
3.2. Tidal Barrage
To evaluate the power generated from the tidal barrage, some important parameters should be considered. The model includes a gain factor to indicate the effect of the structure. It is related to the velocity of the flow rate as well. The output power P of the tidal barrage is related to the rotational velocity of the rotor ω multiplied by the total torque M produced on all the blades by the water flow.
where b is the distance from the center of rotation to the point where the force F is being applied .
Furthermore, the rotational velocity ω is equal to the relative velocity of the flow Vrel over the distance (radius r) between the tip of the blade and the centre of the blade
The output power is
To find the torque generated, Morrison’s formula should be used :
where: Cd – Drag coefficient, A – Blade’s surface, Vrel – Relative velocity of the flow in respect to the blade.
Maximum power production is achieved when the rotational speed is 30% . The reason is that if the rotor velocity is equal to the flow speed, then no force would be generated because Vrel becomes zero. Therefore, the power generation equation can be rearranged as
Height of the structure h = 12 m
Width of base structure L = 55 m
Length of the blades l = 10 m
Height of the blades Hw = 3 m
Rotor diameter ø = 7 m
Depth of the rotor in the base = 1 m
Water depth = 50 m
To understand the spread velocity near the tidal barrage CFD software has been used.
Equations (3), (4) and (5) are valid under steady condition, but in a real scenario, the blades would not rotate under a steady condition. Therefore, to observe the velocities, the tidal barrage should be simulated without blades and after that, virtual blades should be considered for calculation part . Table 3 and Fig. 4 present the relation between the output power and the stream velocity.
Fig. 4. Power output vs stream velocity
Tab. 3. The output power in relation to the stream velocity
In the simulation studies, the power output is determined from the speed of the liquid. The simulation was done on the flow of the tidal stream which can be altered as the tidal stream can accelerate, decelerate, and reverse over varying depth. Data is sampled starting from a velocity of 1.8 m/s to up until 3.0 m/s. The simulated tidal stream generator has steady growth on the graph presenting the relationship between the power output and the tidal stream velocity. The power curve shows that the velocity is directly proportional to the output power. The maximum power of 2096 kW is generated when the velocity is at its peak which is 3.0 m/s. The initial growth rate on the graph is almost linear, but the increase is quite significant as the velocity increases. The graph documented an increase of 168.37 kW for a velocity change from 1.8 m/s to 2.0 m/s. However, the increase in power is quite significant for a higher velocity. The velocity increase from 2.8 m/s to 3.0 m/s increases the power output by 392 kW. This leads to a conclusion that the power will increase significantly as the value of the stream velocity keeps increasing.
For the tidal barrage analysis, the generator is composed of a cross-flow turbine with a horizontal axis. It is supported by a base structure fixed at the bottom. Data is sampled at instant velocities of: of 4 m/s, 2.12 m/s, 2.5 m/s, 3 m/s and 3.5 m/s. The output power increases exponentially from 2.21 m/s to its peak at 3.5 m/s. The collected data varies according to the tidal cycle, changing twice a day. The current generated increases as the water level rises, and becomes null at still water level, which is the peak water level. Then, it reverses direction as the water level drops till the still water reaches the lower water level. The tidal stream cycle continues at a certain interval. This interval represents the two peak values of tides of different intensity, occurring every 24 hours.
Tidal energy is a clean and renewable source of energy. It is also more predictable in comparison with other energy sources. Using tidal energy, possesses a great potential of producing a huge amount of electrical power.
The development of tidal barrage is constrained by its enormous construction cost. However, in the near future, tidal barrage systems may prove to play a key part in worldwide electricity generation, since there is a possibility of a price increase of fossil fuels.
Several potential locations for tidal barrage sites have already been identified, with a potential for producing electrical energy on a large scale. However, so far only four such sites have been constructed, out of many potential tidal barrage locations. There are several sustainability issues with the construction of tidal barrages, such as the harm towards marine life and effects on water quality. Still, the tidal barrage system is considered an established and reliable solution, requiring no breakthrough technology.
On the other hand, a lesser impact on the environment has been observed when using tidal currents in comparison with tidal barrages. However, the full extent of the environmental impact has yet to be discovered. Since tidal current devices are still in the early stage of development, further research and technological advancement are necessary. Among the various technological developments, electricity transmission, maintenance, installation and loading conditions are important to focus on. In order to produce a large amount of electricity using tidal currents, these issues need to be seriously focused on.