Oceans cover more than 70% of Earth’s surface, making them the world’s largest solar collectors. The ocean can produce two types of energy: thermal energyfrom the sun’s heat, and mechanical energy from the tides and waves .These energies are non-polluting, reliable, and very predictable.
Potential of oceans as a renewable energy source:-

Tidal energy :-

• Tidal Energy, also known as Tidal Power is classified as an alternate energy or better known as the renewable source of energy. It is one of the forms of hydropower energy that exercises energy of the oceanic tides to generate electricity.
• As the celestial bodies- Earth, Sun, and the Moon change their positions throughout the year with these changes in position the gravitational pull exerted by these bodies result in water movements and thus the energy created can be harnessed to generate electricity.
• The energy obtained from the rise and fall of tides is called the tidal energy.
• Tidal barrages or dams are constructed across a narrow opening to the sea. Water rushes into the dam when the sea level rises. This moves the blades of the turbines which are attached at the opening of the dam. This results in the generation of electricity.

Some of the advantages of tidal energy are:

1. Inexhaustible source of energy
2. Highly predictable energy source
3. Environment friendly
4. Operational and maintenance costs are low
5. High energy density

Some of the disadvantages of tidal energy are:

1. High tidal power plant construction costs
2. Negative influence on marine life forms
3. Location limits
4. Variable intensity of sea waves.

Indian scenario:-

• As of March 2017, India announced of its 7500 Km long coastline, where the height of high tide was recorded over 5 mtrs higher than the low tide which can essentially capture the potential tidal power.
• The Ministry of New and Renewable Energy estimated that the country can produce 7000 MW of power in the Gulf of Khambhat in Gujarat, 1200 MW of power in the Gulf of Kutch in Gujarat and about 100 MW of power in the Gangetic delta of Sunderbans in West Bengal.

Ocean thermal energy:-

• The sun’s heat warms the surface water a lot more than the deep ocean water, and this temperature difference creates thermal energy.
• In OTEC, scientists use the temperature difference between the hot surface of the ocean and the cooler, deeper layers beneath to drive a heat engine except that no fuel is burned and a temperature gradientexists in the oceans naturally. The best place to find such a combination is in the tropics (between the latitudes of about 20°N and 20°S).

Disadvantages:-

1. The biggest problem with OTEC is that it’s relatively inefficient.
2. Significant amount of the electricity generated (typically about a third) has to be used for operating the system (pumping the water in and out).
3. It implies that OTEC plants have to be constructed on a relatively large scale, which makes them expensive investments.
4. Large-scale onshore OTEC plants could have a considerable environmental impact on shorelines, which are often home to fragile, already threatened ecosystems such as mangroves and coral reefs.

India:-

• India is geographically well-placed to generate ocean thermal energy, with around 2000 kms of coast length along the South Indian coast, where a temperature difference is available throughout the year. That means, about 1.5×106 square kilometres of tropical water in the Exclusive Economic Zone around India with a power density of 0.2 MW/km2.
• The total OTEC potential around India is estimated as 180,000 MW, considering 40% of gross power for parasitic losses.
• However, the cost estimates of ocean energy as against conventional energy is still being worked out, as the country is still in a nascent stage of development of the technology and start generation.
• OTEC is capital intensive and is economical only at very large scales. The complexities and challenges in a large offshore OTEC plant are many and hence a land based small rating one is being attempted at Kavaratti to power a low temperature thermal desalination plant,
• India’s maiden Ocean Thermal Energy Conversion (OTEC) project is coming up in Kavaratti, capital of the Lakshadweep archipelago, off the south-western coast after almost three and a half decades of initial plans.

Ocean wave energy:-

• Wave Energyalso known as Ocean Wave Energy, is another type of ocean based renewable energy source that uses the power of the waves to generate electricity. Unlike tidal energy which uses the ebb and flow of the tides, wave energy uses the vertical movement of the surface water that produce tidal waves.
• Wave power converts the periodic up-and-down movement of the oceans waves into electricity by placing equipment on the surface of the oceans that captures the energy produced by the wave movement and converts this mechanical energy into electrical power.
• Wave Energy” is an indirect form of wind energy that causes movement of the water on the surface of the oceans and by capturing this energy the motion of the waves is converted to mechanical energy and used to drive an electricity generator

Wave Energy Advantages

1. Wave energy is an abundant and renewable energy resource as the waves are generated by the wind.
2. Pollution free as wave energy generates little or no pollution to the environment compared to other green energies.
3. Reduces dependency on fossil fuels as wave energy consumes no fossil fuels during operation.
4. Wave energy is relatively consistent and predictable as waves can be accurately forecast several days in advance.
5. Wave energy devices are modular and easily sited with additional wave energy devices added as needed.
6. Dissipates the waves energy protecting the shoreline from coastal erosion.
7. Presents no barriers or difficulty to migrating fish and aquatic animals

Wave Energy Disadvantages

1. Visual impact of wave energy conversion devices on the shoreline and offshore floating buoys or platforms.
2. Wave energy conversion devices are location dependent requiring suitable sites were the waves are consistently strong.
3. Intermittent power generation as the waves come in intervals and does not generate power during calm periods.
4. Offshore wave energy devices can be a threat to navigation that cannot see or detect them by radar.
5. High power distribution costs to send the generated power from offshore devices to the land using long underwater cables.
6. They must be able to withstand forces of nature resulting in high capital, construction and maintenance costs.

Wave energy in India:-

• The total available potential of wave energy in India along the 6000 Km of India’s coast is estimated to be about 40,000 MW – these are preliminary estimates. This energy is however less intensive than what is available in more northern and southern latitudes.
• Scientists at the Indian Institute of Technology (IIT) Madras have now found a simple fix to increase the amount of power generated by oscillating water columns.
• Despite being an attractive power source, wave energy remains largely unexploited due to large costs required to build these structures.

Current Energy
Marine current is ocean water moving in one direction. This ocean current is known as the Gulf Stream. Tides also create currents that flow in two directions. Kinetic energy can be captured from the Gulf Stream and other tidal currents with submerged turbines that are very similar in appearance to miniature wind turbines. As with wind turbines, the constant movement of the marine current moves the rotor blades to generate electric power.

Conclusion:-

As Government of India steps up its effort to reach the objectives to contemplate its Renewable Energy and climate change objectives post 2022, it is opportune to explore all possible avenues to stimulate innovation, create economic growth and new jobs as well as to reduce our carbon footprint.
Given the long-term energy need through this abundant source, action needs to be taken now on RDD&D front in order to ensure that the ocean energy sector can play a meaningful part in achieving our objectives in coming decades.
MNRE looks over the horizon at a promising new technology and considers the various options available to support its development. Over 100 different ocean energy technologies are currently under development in more than 30 countries. Most types of technologies are currently at demonstration stage or the initial stage of commercialization.