Authors: Prasad K. Bhaskaran
Energy Review, Vol 4. Issue 2. 2022
Increasing energy demands across many locations of the nation, pressure on developmental activities, and rapid deterioration of air quality index in urban locations have led to investigating various renewable sources of energy for sustainable use. The oceans comprise about 71% of the Earth’s surface and cover an area of about 361 million km2, and the various possible sources of marine energy can result from the tidal stream, tidal variations, waves, offshore wind, ocean thermal gradients, ocean currents, river runoff and salinity gradients. In the context of the North Indian Ocean region, preliminary investigation as well the identification of potential locations for harvesting ocean wave energy started in 1988, and the first pilot wave energy plant was set up in 1990. Though considerable research works were carried out on renewable marine energy extraction, limited studies are conducted so far on ocean wave energy and its potential that has wide implications for coastal and island locations in terms of energy demands. During the financial year 2019-2020, the renewable energy sector contributed only 21.2% against the total power consumption, and the contribution from marine energy was negligible. Estimates signify that the contribution from the ocean and tidal currents comprises about 5 TW of energy and about 1-10 TW of wave energy. It is interesting to note that ocean waves can provide about 20 times more available energy per square meter compared to wind and solar energy.
Ocean waves result from the manifestation of energy transfer at the air-sea interface. Swells are long period surface gravity waves generated by wind action at the air-sea interface in a conducive growing environment having sufficient fetch and duration. Physical mechanisms associated with swell wave propagation and its characteristics have immense practical applications ranging from off-shore activities, coastal protection, beach restoration, design of wave energy converters, ocean wave prediction, wind-wave climate studies, and many more. Propagation characteristics of long period swells is indeed challenging due to its complexity and spatial variability. It is interesting to note that energy propagation from swells can reach distant locations travelling thousands of km in the open ocean reaching various destinations along the coast. Energy loss in the course of swell propagation due to inner friction, and viscosity, effects are very small. For any given region the wind-wave climate is characterized by the combined effect resulting from mutual interaction between multiple wave systems. The propagation characteristics of swells and the associated physical mechanisms have emerged as a great interest for many researchers because of their huge potential in wave energy.
The Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC-AR5) highlights the importance of waves and their active feedback in improving the existing climate models. The swell wave system plays an important role in redistributing the wave energy during their trail of propagation to long distances thereby determining the wind-wave climate across the global oceans. A comprehensive understanding of the regional-wise wave characteristics has immense importance in context to climate change and estimation of wave energy potential. Remotely forced swells can modulate and modify the locally generated wind-waves of a given region. Hence, a proper understanding of the long-term variability of swell wave parameters is very essential for determining the potential of ocean wave energy.
A recent study conducted by our team at IIT Kharagpur has divided the entire Indian Ocean into six sub-domains classified into zones covering the Arabian Sea, Bay of Bengal, South China Sea, Tropical South Indian Ocean, Western extra-tropical South Indian Ocean and the Southern Ocean, and the Eastern extra-tropical South Indian Ocean and the Southern Ocean. A detailed study was carried out to investigate the monthly, seasonal, intra-seasonal, and inter-annual variability of wind waves in the Indian Ocean region. Further, the study had analysed the regional-wise characteristics, spatio-temporal distribution, and variability of high threshold significant wave heights in the region. A methodology was developed to identify the potential regions of swell wave variability, their trends, and maxima to understand the influence and role of various climate indices affecting the variability and trends of wind seas and swells. The study has led to identifying the variability of extreme swell wave climate, wave power density, and directionality for the Indian Ocean and coastal regions in a changing climate scenario. In addition, a methodology was developed to assess the changes in seasonal directionality of swell wave propagation and extreme swell energy flux for selected regions along the Indian coast.
Multi-platform calibrated satellite altimeter wave data was used to emphasize the climatological trend and patterns in the wave energy distribution. A comprehensive analysis was performed for the six sub-domains listed above providing details on the monthly and seasonal wave climatology, annual cycle, and pentad-scale trends. Interesting observations from the study signify that for the extra-tropical South Indian Ocean, the 99th, 95th, and 90th percentile values for waves are active for more than six months in a year highlighting the potential source and generation area of wave energy. Lower percentile significant wave heights are also significant exhibiting a pentad scale oscillation over this region and a decadal variability over the North Indian Ocean and tropical South Indian Ocean regions. Swells that propagate from the western sector of the extratropical South Indian Ocean are the primary source of swell wave power in the tropical and subtropical Indian Ocean, reaching different locations of peninsular India with an average travel time of 7-8 days. The study points out that there is no significant trend in the swell wave energy flux over the north Bay of Bengal; and in addition, a strong La Niña epoch can decrease the directional swell wave energy flux. Other climate indices such as Indian Ocean Dipole, and El Niño-Southern Oscillation have a significant correlation with a directional spread in the tropical Indian Ocean, especially in the nearshore regions. The study is expected to have wide potential in the planning of renewable energy resources from ocean waves in the deep and nearshore locations of the Indian coast. By pointing out the potential of ocean swells, this article thus places the importance of further research in this area.
(Dr. Prasad K. Bhaskaran is a professor at the Department of Ocean Engineering and Naval Architecture, Indian Institute of Technology Kharagpur, India.)■□■