Introduction to Tidal Power Generation

Tidal power or tidal energy is a form of hydropower that converts the energy obtained from tides into useful forms of power, mainly electricity. Tides are caused by the combined gravitational forces of Sun and Moon on the waters of the revolving Earth. When the gravitational forces due to the Sun and the Moon add together, tides of maximum range, called spring tides, are obtained. On the other hand, when the two forces oppose each other, tides of minimum range, called neap tides, are obtained. Tides repeat themselves once every 12 hours and 25 minutes. In one year there are approximately 705 full tidal cycles.

Energy can be harnessed from the tides in two ways: using the change in height of the tides (potential); and using the flow of the water (kinetic). Tidal power is very sensitive to speed. The power output varies as the cube of the speed.  In other words, if the water flows twice as fast, it makes eight times the power. Also, tidal turbines do not have to spin as fast as windmills to generate power, because water is 832 times denser than air. As the kinetic energy content of a tidal stream flows per unit time, which is the same as the hydro power ( P ), the available energy can be calculated in terms of velocity ( V ), swept cross-sectional area ( A ) perpendicular to the stream flow direction, and the density of the water ( ρ ), which for sea water is approximately 1025 kg/m³. Providing the velocity is uniform across the cross-sectional area, at any instant in the tidal cycle the amount of energy available will be: P = ½ ρ A.V³.

Tides are more predictable than wind energy and solar power. When built, we will be able to tell the National Grid on any given day, any given hour, when we will be generating and how much we can generate. That is something neither wind nor solar can do. But, among sources of renewable energy, tidal power has traditionally suffered from relatively high cost and limited availability of sites with sufficiently high tidal ranges or flow velocities, thus constricting its total availability. Tidal power technology is constantly evolving. However, the most common technology today can be classified into four main categories:

 

1. Tidal stream generators:

Tidal stream generators make use of the kinetic energy of moving water to power turbines, in a similar way as windmills use moving air. It can utilizes both ebb and flood tides for power generation. This method is gaining in popularity because it’s removable, it can be scaled up gradually (from one device, to an array, to a larger farm), and has lower potential costs and ecological impact (compared to barrages) but installation and maintenance are challenging.

 

2. Tidal barrage:

Tidal barrages are very similar to the Dams in hydroelectric plants, except that they are much larger as they are built across the full width of a tidal estuary or bay. The tidal range (difference between high and low tide) needs to be in excess of 5 meters (some articles suggest 7 meters) for the barrage to be workable. Sluice gates are opened to allow the basin to fill during the incoming flood tide. The gates are then closed and the water is held until ebb tide creates a suitable head (i.e., the drop in elevation from the water source to the power generator). Water is then released through turbines and produces electrical energy just like a hydroelectric power plant. This continues until the water level behind the barrage reaches the minimum operating point and the sluice gates are closed. As the tide again begins to rise, the sluice gates are again opened to enable the repetition of the cycle. Tidal barrages have very high infrastructure costs and are very damaging on the local environment (marine life).

 

3. Tidal lagoons:

Tidal Lagoons are similar to barrages but have a much lower cost and impact on the environment. They are self contained structures cut off from the rest of the sea. It works in pretty much the same way as a tidal barrage as when the tide rises the lagoon fills and when it falls the water is then released through the turbines. They can be configured to generate continuously, which is not the case with barrages.

 

4. Dynamic tidal power:

Dynamic tidal power is still theoretical and has not been tried, but requires the building of dams that are tens of kilometers long to regulate water flow.

 

Components of Tidal Power Plant

The components of a tidal power station are as follow:

A. Barrage: A barrage is a dam-like structure, usually made out of concrete, used to capture the energy from masses of water moving in and out of a bay or river due to tidal forces. The barrage houses both the turbine and sluice gates.

 

B. Turbines: A tidal turbine, similar to a wind turbine, converts the horizontal movement (kinetic energy) of the water during the incoming and outgoing tide to electrical energy. There are two basic types of turbines: the horizontal axis turbine, and the vertical axis (cross flow) turbine. There are a large number of different designs undergoing development and testing. They are located in the passageways that the water flows through when gates of barrage are opened.

 

C. Sluices: Sluice gates are the ones responsible for the flow of water through the barrage.

 

D. Basin: The area behind the barrage where water is stored.

 

E. Power Cables: Generated electrical energy is brought to shore via a standard submarine electrical cable, which is normally installed in a trench in the seafloor and under the beach at the shore. The cable runs to an interconnection substation. From here, the electricity is transferred to consumers through the transmission grid.

 

F. Onshore Facility: The submarine electrical cable would be connected to a local power distribution grid or a long-distance power transmission grid. The control and monitoring of devices and transformers would be carried out remotely using fiber-optic cables or other communication devices.

Tidal barrage power generation schemes

1. Single-Basin Tidal Barrage Schemes:

These schemes require a single barrage across the tidal basin. There are three different methods of generating electricity with a single basin. All of the options involve a combination of sluices which, when open, can allow water to flow relatively freely through the barrage, and gated turbines, the gates of which can be opened to allow water to flow through the turbines to generate electricity.

 

A. Ebb Generation Mode: During the high tide, incoming water is allowed to flow freely through sluices in the barrage. At the end of high tide, the sluices are closed and water retained behind the barrage. When the water outside the barrage has fallen to sufficient head i.e., sufficient height between the basin's water level and the open water level, the basin water is allowed to flow out though low head turbines and to generate electricity. The system can be considered as a series of phases. Typically the water will only be allowed to flow through the turbines once the head is approximately half the tidal range. This method will generate electricity for, at most, 40% of the tidal range.

 

B. Flood Generation Mode: The sluices and turbine gates are kept closed during the high tide to allow the water level to build up outside the barrage. As with ebb generation, once a sufficient head has been established the turbine gates are opened and water can flow into the basin, generating electricity. This approach is generally viewed as less favourable than the ebb method, as keeping a tidal basin at low tide for extended periods could have detrimental effects on the environment and on shipping. In addition, the energy produced would be less, as the surface area of a basin would be larger at high tide than at low tide, which would result in rapid reductions in the head during the early stages in the generating cycle.

 

C. Two-Way Generation: It is possible, in principle, to generate electricity during both ebb and flood currents. In addition, there would be additional expenses associated in having a requirement for either two-way turbines or a double set to handle the two-way flow. Advantages include, however, a reduced period with no generation and the peak power would be lower, allowing a reduction in the cost of the generators.

 

2. Double-Basin Tidal Barrage Schemes:

All single-basin systems suffer from the disadvantage that they only deliver energy during part of the tidal cycle and cannot adjust their delivery period to match the requirements of consumers. Double-basin systems have been proposed to allow an element of storage and to give time control over power output levels. The main basin would behave essentially like an ebb generation single-basin system. A proportion of the electricity generated during the ebb phase would be used to pump water to and from the second basin to ensure that there would always be a generation capability.

It is anticipated that multiple-basin systems are unlikely to become popular, as the efficiency of low-head turbines is likely to be too low to enable effective economic storage of energy. The overall efficiency of such low-head storage, in terms of energy out and energy in, is unlikely to exceed 30%. It is more likely that conventional pumped storage systems will be utilized. The overall efficiency of these systems can exceed 70% which is likely to prove more financially attractive.

 

Cooperating double basin system: This scheme consists of two basins, at different elevation connected through turbine. The sluices in the high and low level basin communicate with sea water directly as shown in Figure. The high level basin sluices are called the inlet sluices and the low level as outlet sluices. The basic operation of the scheme is as follows.

Suppose the upper basin is filled with water. The water is allowed to flow to the lower basin through the turbine. Therefore, the level in the upper basin falls and that in the lower basin rises. At an instant when the rising level in the basin is equal to the level of the falling tide, the outlet gates are opened. When the tide reaches its lower most level, the outlet gates are closed. After a while the tide rises. When its level becomes equal to the low level of the upper basin, the inlet gates are opened. As a result, the level of the upper basin starts rising. At the same time, the turbines are fed from the upper basin transferring water to the lower basin, thus raising level of water there. When the tide reaches its peak value, the inlet gates are called again. Thus the cycle is repeated.

 

Advantages of Tidal Power Generation

There are many advantages of generating power from the tide; some of them are listed below:

1. Tidal power is a sustainable energy resource.

2. It reduces fossil fuels dependence.

3. It has very less visual impact.

4. The artificial lakes created for tidal power can be used for leisure activities such as boating, creating a tourist attraction and also results in new areas of sheltered water, attractive for fish, sea birds, seals and seaweed.

5. Tidal energy is available worldwide on a large scale from deep ocean waters.

6. Tidally driven coastal currents provide an energy density four times greater than air

7. A feature which gives them an advantage over both wind and solar systems is that the tidal currents are both predictable and reliable.

8. Seawater is 832 times as dense as air; therefore the kinetic energy available is much greater than air.

 

Disadvantages of Tidal Power Generation

Unfortunately, there are also disadvantages and limitations to generating tidal power. Some of these are:

1. Utilization of tidal energy on small scale is not economical.

2. Tidal fences could present some difficulty to migrating fish.

3. Tidal power plants can be developed only if natural sites are available.

4. Tidal power systems do not generate electricity at a steady rate and thus not necessarily at times of peak demand.

5. As the sites are available on the bays which are always far away from load centers, the power generated has to be transmitted to long distances. This increases the transmission cost and transmission losses.