New and Renewable Energy Minister R.K. Singh has approved a proposal to declare ocean energy as renewable energy.

The ministry of new and renewable energy (MNRE) has clarified to all stakeholders that energy produced using various forms of ocean energy - such as tidal, wave, ocean thermal energy conversion - shall be considered as renewable energy and eligible for meeting non-solar renewable purchase obligations (RPO).

This step has been taken by the MNRE after some renewable energy developers sought clarification in this regard.

Under the RPO, distribution companies (discoms) are required to have certain proportion of clean energy supplies. The proportion is fixed by state power regulators. The discoms can also buy renewable energy certificates in lieu of mandated clean energy supplies from the developers or renewable power generators.

Background

Oceans cover 70 per cent of the Earth’s surface and represent an enormous amount of energy in the form of wave, tidal, marine current and thermal gradient.

A variety of different technologies are currently under development throughout the world to harness this energy in all its forms.

The total identified potential of tidal energy is about 12,455 megawatts (MW), with potential locations identified at Khambhat and Kutch regions, and large backwaters, where barrage technology could be used.

The total theoretical potential of wave energy along India’s coast is estimated to be about 40,000 MW.

Ocean thermal energy conversion has a theoretical potential of 1,80,000 MW in India subject to suitable technological evolution.

Deployment is currently limited, but the sector has the potential to grow, fuelling economic growth, reducing carbon footprint and creating jobs not only along the coasts but also inland along its supply chains.

As India steps up its effort to contemplate its renewable energy and climate change objectives after 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.

India has a long coastline with the estuaries and gulfs. The MNRE looks over the horizon at development of new technology and considers various options available to support its deployment.

Tidal energy  

The tidal cycle occurs every 12 hours due to the gravitational force of the moon. The difference in water height from low tide and high tide is potential energy.

Similar to traditional hydropower generated from dams, tidal water can be captured in a barrage across an estuary during high tide and forced through a hydro-turbine during low tide.

The capital cost for tidal energy power plants is very high due to high civil construction and high power purchase tariff.

To capture sufficient power from the tidal energy potential, the height of high tide must be at least 5 m greater than low tide. The Gulf of Cambay and the Gulf of Kutch in Gujarat on the west coast have the locations in the country where potential exists.

Wave energy

Wave energy is generated by the movement of a device either floating on the surface of the ocean or moored to the ocean floor. Many different techniques for converting wave energy to electric power have been studied.

Wave conversion devices that float on the surface have joints hinged together that bend with the waves. This kinetic energy pumps fluid through turbines and creates electric power.

 Stationary wave energy conversion devices use pressure fluctuations produced in long tubes from the waves swelling up and down. This bobbing motion drives a turbine when critical pressure is reached.

Other stationary platforms capture water from waves on their platforms. This water is allowed to runoff through narrow pipes that flow through a typical hydraulic turbine.

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.  Similar to wind turbines, the movement of the marine current moves the rotor blades to generate electric power.

Ocean Thermal Energy Conversion

Ocean thermal energy conversion (OTEC) uses ocean temperature differences from the surface to depths lower than 1,000 m to extract energy.

A temperature difference of only 20 degrees Celsius can yield usable energy. Research focuses on two types of OTEC technologies to extract thermal energy and convert it to electric power: closed cycle and open cycle.

In the closed cycle method, a working fluid, such as ammonia, is pumped through a heat exchanger and vaporised. This vaporised steam runs a turbine. The cold water found at the depths of the ocean condenses the vapour back to a fluid where it returns to the heat exchanger.

In the open cycle system, the warm surface water is pressurised in a vacuum chamber and converted to steam to run the turbine. The steam is then condensed using cold ocean water from lower depths.