Tidal Energy: An Overview

There are more than 77 seas in the world with more than one-lakh square kilometre area. Nearly 1,335,000,000 cubic kilometres of the Earth’s volume is occupied by the seas. The tides that occur on the surface of the sea has a great potential to generate a lot of energy. Tidal energy is touted to be a potential source of energy soon. In this article, I would be explaining tidal energy in detail.

Tides As A Source Of Energy

What Are Tides And How Are They Caused?

Tides are the periodic rise and fall in sea levels. They are caused by the combined effects of the gravitational forces exerted by the Moon and the Sun, and the rotation of the Earth. The gravitational attraction of the moon causes the seas to bulge in the direction of the moon. Another bulge occurs on the opposite side because the Earth is also being pulled toward the moon. Two tides occur in a day because the Earth is rotating about its axis for 24 hours daily. Issac Newton was the first person who explained the occurrence of tides scientifically.

Occurrence Of High And Low Tides

Spring tides are strong tides that occur when the Sun, Moon and Earth are in a line. One must remember that spring tides are not associated with the spring season. The gravitational force of the moon and the sun contribute to spring tides. The spring tides occur only during the full moon and new moon. The Proxigean Spring Tide is a rare case where the tide is extremely high. This occurs when the moon is unusually close to the Earth and in its new moon phase. The proxigean spring tide occurs at most once every 18 months. Neap tides are weak tides that occur when the gravitational forces of the Moon and the Sun are perpendicular to one another. These tides only occur during the quarter moon phases.

History Of Tidal Energy

The history of tidal energy dates back to 900AD. Tidal Power Plants were initially made using naturally occurring tidal basins. To harness the tidal energy, a barrage was built across the opening of the basin. This allowed one side of the basin to fill when the tide was rising and when the tide dropped the water was released through a waterwheel. Back then there was no need for electricity. This innovation was not used to generate electricity but used for grinding grains. These were called tide mills.

Tide Mills abandoned village near Newhaven, East Sussex, England

The 17th century witnessed the migration of the Europeans to North America and other countries. These migrants took the idea of tide mills to America. The first tide mill was established by the European settlers in America at Maine. Following which tide mills were established all over the United States of America. By the end of the 19th century, the idea of using the power of tides to generate electricity became a serious discussion.

Tidal Power Written By A.M.A.Struben

In the year 1921, A. M. A Struben wrote a book named Tidal Power. This book neatly explained various methods involved in capturing the energy from the tides. Dexter Cooper, an engineer came with the idea of harnessing the power of tides. This marked the beginning of converting the tidal energy to generate power. In the year 1924, the US Federal Power Commission carried out a study on large scale power generation using tidal energy in regions enclosing the Bay of Fundy and Passamaquoddy Bay. Unfortunately, the study could not produce desirable results and the Great Depression stopped the funding for this project. In the year 1956, another set of studies were commissioned by Nova Scotia Light and Power of Halifax. The study was aimed to develop tidal power generation plants on the Nova Scotia side of the Bay of Fundy. The cost was yet another major concern for the study but it was noted that tidal energy can generate millions of horsepower.

La Rance Tidal Barrage in France

The 1960s saw major progress in the tidal energy domain. In the year 1965, the La Rance Tidal Barrage was built in France. It is the first commercial-scale and modern tidal power plant. It is located in the Rance Estuary near St. Malo, France. A total of 24 turbo generators are used in the La Rance Tidal Barrage. This tidal plant has been working till producing clean energy. In 1982, the second commercial-scale tidal barrage was built in Nova Scotia. This tidal power plant uses the STRAFLO Turbine, an invention by Escher-Wyss of Switzerland. Currently, the world’s largest tidal energy power plant is the Sihwa Lake Tidal Power Station located in South Korea. It has a capacity of generating 254 MW of tidal power.

Types Of Tidal Power Plants Based On The Number Of Basins

Types Of Tidal Power Plants Based On The Number Of Basins

The tidal power plants are classified into two categories based on the number of basins used for power generation. They are further subdivided based on the number of cycles required to generate power. They are as follows:

1. Single Basin Systems
2. Double Basin Systems

Single Basin Systems

Single Basin Type Of Tidal Power Plants can generate power intermittently. It is the simplest way of generating power from tides. The basin is separated from the sea by a garage or a dyke. The sluice or the gateway is opened during a high tide to fill the entire basin. The turbine generator units are installed within the ducts inside the barrage to generate electricity. Single Basin Systems are further classified into three more types. They are as follows:

1. Single Basin One Way Cycle: It is the simplest form of a tidal power plant. In this type, the basin is allowed to get filled flood tide and during the ebb tide. The water flows from the basin to the sea passing through a turbine and generator to produce power. The power is available only for a short duration of time for the ebb tide.
2. Single Basin Two Way Cycle: In this type of tidal power plant, power is generated both during the flood tide as well as the ebb tide. The power generated is intermittent like the Single Basin One Way Cycle but the generation period is increased as compared to that of the Single Basin One Way Cycle. One of the major constraints involved in this type of tidal power plant is the complicated arrangement of the turbines. The same turbine is used as a prime mover for the ebb and tide flows pass through the turbine in opposite directions. Variable pitch turbine and dual rotation generator are used in the Single Basin Two Way Cycle system.
3. Single Basin Two Way Cycle With Pump Storage: This type of system is used to generate power during both the ebb and flow tides. Machines that have the ability to generate power and pump the water in either direction are used. A part of the energy produced is used to introduce a difference in the water levels between the basin and sea at any time of the tide. This is achieved by pumping water into the basin up or down. The Single Basin Two Way Cycle With Pump Storage System has a longer period of power production as compared to that of the Single Basin Two Way Cycle and Single Basin One Way Cycle.

Double Basin Systems

In this type of arrangement, the turbine is set up between two basins. One basin is intermittently filled by the flood tide while the other basin is drained by the ebb tide. This type of system has a small power generating capacity but continuous power is available with this type of system. One of the major drawbacks of this system is that 50% of the potential energy is lost in introducing the variation in the water levels of the two basins. The various types of double basin systems are:

1. Simple Double Basin Type
2. Double Basin With Pumping

Various Technologies Used To Generate Power From The Tides

Tides are indeed a potential source of energy. This energy needs to tapped to generate electricity. There are six different methods of generating electricity from the tides. They are as follows:

1. Tidal Barrages
2. Tidal Steam Generator
3. Dynamic Tidal Power
4. Tidal Lagoon
5. Tidal Fences
6. Tidal Turbines

Tidal Barrages

Tidal Barrages

A tidal barrage is a dam-like structure with turbines installed at the bottom of the reservoir. Instead of storing the water like a conventional dam, a tidal barrage allows the water to enter into the basin during the high tide and releases the water during the low tide. This is done by measuring the tidal flow and controlling the sluice gates at key times of the tidal cycle. Tidal barrages are one of the oldest methods of generating power from the tides. The tide mills worked very similarly to that of the tidal barrages.

Tidal Steam Generators

Tidal Steam Generator

A tidal steam generator is used to convert the kinetic energy of the moving water to run a turbine to produce electricity. They are very much similar to the operation of wind turbines. The power available from the tides is higher than that of the wind because water has a higher density as compared to air. They are the most environmentally friendly method to generate electricity from tides. They are generally submerged partially or completely in the water. The tidal steam generator works on the Venturi effect.

Dynamic Tidal Power

Dynamic Tidal Power

Dynamic tidal power is the technology that uses the difference between the potential energy and kinetic energy of tides to generate electricity. Long dams are built from the coasts straight into the seas and oceans. During the movement of tides, water on one side of the dams is at a higher level than the other side. These dams have turbines installed to produce electricity. These dams are equipped with bidirectional turbines which can flip 180 degrees after each tide to generate power when the tide comes in and leaves the reservoir.

Tidal Lagoon

Power Generation Cycle In A Tidal Lagoon

A tidal lagoon is a man-made enclosure created in a tidal area. It acts as a harbour and also acts as a barrier to separate a water body from the natural ebb and flow of the tides. The movement of tides in and out creates a difference in the water levels in the lagoon as compared to the surrounding water bodies. Once the difference in water levels is large enough to generate electricity, the sluice gates are opened and the water rushes into the enclosure. This is used to turn the big turbines to produce electricity.

Tidal Fence

Tidal Fences

The tidal fence is another form of tidal steam technology that utilizes fast-moving ocean currents to generate electricity. They are individually positioned around the ocean floor. They are composed of individual vertical-axis turbines. These vertical-axis turbines are mounted together within a single fence-like structure. These tidal fences are used to harness the power of the underwater tides to generate electricity.

Tidal Turbines

Tidal Turbines

Tidal turbines are very much similar to that of wind turbines. They are always placed on the seafloor. These tidal turbines are placed in regions where the tidal floor is very strong. The density of water is 800 times higher than that of air. These tidal turbines are sturdier and heavier than wind turbines. These tidal turbines are designed to withstand dynamic loads from the tides. These tidal turbines are more expensive than wind turbines. With the same blade size, a tidal turbine can harness more energy than a wind turbine.

Advantages Of Tidal Energy

Nearly 70% of the earth’s crust is covered by the seas. The seas have immense energy that can be harnessed to produce electricity. The advantages of tidal energy are as follows:

1. Environment Friendly: Tidal energy is a clean source of energy. It does not generate greenhouse gases into the environment. Tidal power doesn’t generate gaseous by-products on the other hand conventional fossil fuels generate a lot of harmful gases. Tidal energy generation doesn’t produce any undesirable noise as it works underwater.
2. Predictability And Reliability Of The Tides: The tides are far more predictable as compared to that of the wind. The predictability of tides makes it easier for choosing the right site for establishing the tidal power plant. Generally, two high tides and one low tide occurs in a day.
3. Renewable: The tides are an inexhaustible source of energy like wind, solar, hydropower and other non-conventional sources of energy. Tides occur daily due to the combined effects of the gravitational forces exerted by the Moon and the Sun, and the rotation of the Earth.
4. Tidal Energy Is Effective At Low Speeds: It's clear that water is 800 times denser than air. Tidal energy can generate electricity at a speed as low as 1m/s whereas wind turbines can generate electricity at speed of 3m/s. Tidal turbines have an 80% conversion efficiency while wind turbines have a maximum of 59% conversion efficiency.
5. Cost-effective: Tidal power plants have a longer life span as compared to other power plants using non-conventional sources of energy. A tidal powerplant is designed to last for at least 75 to 100 years. Despite the high initial upfront cost, good returns can be obtained in the long run. The La Rance Tidal Barrage was built in the year 1965 in France and it has been working flawlessly until now.
6. Economic Benefits Of Tidal Power Plants: Tidal power plants can reduce a nation’s dependence on imported energy supplies. It also plays a vital role in increasing the nation’s energy security. Tidal power plants can generate a lot of employment opportunities for skilled and unskilled labourers.

Challenges Involved In Harnessing The Power Of The Tides

Despite the environmental and economic advantages of tidal energy, there exist some challenges in harnessing the total potential of the tides to generate electricity. The challenges are as follows:

1. Tidal Energy Is Not A Constant Source Of Energy: Despite the predictable nature of tides, tidal energy is not a constant source of energy because the tides occur due to the combined effects of the gravitational forces exerted by the Moon and the Sun, and the rotation of the Earth. A tidal power plant on average can produce power for almost 10 hours daily. It can only generate power intermittently.
2. High Upfront Cost: Building a tidal powerplant would cost approximately 3 Million USD to 15 Million USD. Tidal Energy remains an unexplored domain in many developing countries. This is indeed a huge amount of money to invest despite the long term generation cost is quite low. For example, in December 2014, the Indian Institute of Technology, Chennai in association with CRISIL Risk and Infrastructure Solutions Limited conducted a study on India’s potential to harness Tidal Power. The study stated that tidal power potential is estimated at around 12,455 MW. The potential areas with low/medium tidal wave strength were found in the Gulf of Khambat, Gulf of Kutch & southern regions in Gujarat, Palk Bay- Mannar Channel in Tamil Nadu, and Hoogly river, South Haldia & Sunderbans in West Bengal. Tidal energy is still in the Research & Development (R&D) phase. It has not been implemented on a commercial scale in India. The earlier efforts for harnessing tidal power were not successful because of the high capital cost ranging from Rs. 30 crores to Rs. 60 crores per MW.
3. Environmental Impact: Despite the zero-carbon dioxide emission by the tidal power plant, there are a few negative impacts on the environment due to the tidal power plants. Firstly, changing the tidal flows by damming a bay or estuary could, however, result in negative impacts on aquatic and shoreline ecosystems, as well as navigation and recreation. Secondly, electromagnetic radiations emitted from the tidal power plant can harm marine life. Finally, tidal turbines have killed many fishes. Tidal barrages prevent the migration of fishes and cause flooding of surrounding areas that forever changes the landscape.
4. Unavailability Of Suitable Locations: There are more than 100 seas all across the globe. One must remember, that all the seas across the globe are not suitable for generating electricity from tides. The difference in water levels between the high tide and low tide must be above 5 meters to generate electricity from the tides and this cannot be achieved at all the seas.
5. Novel Technology: Despite this technology of harnessing the energy from the tides is centuries old, there’s a need for immense research and development in this domain.

Conclusion

High Potential Tidal Energy Resources In The World

Tidal Energy is truly a sustainable and renewable source of energy. It can be considered as a potential source of energy in the future. It has been estimated that there is nearly 1 terawatt of exploitable power is stored in the world’s oceans which is enough to power 10 billion 100-watt lightbulbs at once. It does not generate any carbon dioxide or any other harmful gases during its operation. The upfront initial cost to set up a tidal power plant is extremely high despite the long term generation cost is quite low. There is an immense need for Research and Development in this sector. Its quite clear that tidal energy is an intermittent source of energy as a tidal power plant can only generate electricity for a maximum of 10 hours only. The energy generated during those ten hours must be stored for future use. In the future, I would want to see a suitable infrastructure for storing the energy produced from the tides. To prevent the fishes from being killed due to the collision with the tidal turbines, the manufacturers of the tidal turbines can implement sensors that would detect the presence of the fishes and can activate the brakes to stop the rotation of the blades of the tidal when the fishes are very near to it.