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The demand for energy supply has allowed looking into options beyond fossil-based energy sources. If 71% of the earth's surface is the ocean, how can we harness energy from waves?
Table of Contents:
Background
Extracting energy by renewable means such as solar, hydropower, and the wind is quite popular in terms of implementation and installation. Even though the vast surface of the earth is covered by water, extracting energy from the ocean isn't popular and is still in the early developmental stages. Ocean provides multiple ways in which useful energy can be extracted, namely, waves, currents and tides. However, the random nature of waves and tides, extreme weather, seasonal variation in sea states, disturbance to flora and fauna and difficulty in transmission pose some of the difficulties in adapting to the ocean energy harnessing.
Waves
Ocean waves are defined as the periodic undulation or oscillations of the ocean surface caused
due to motion of the winds. Thus we derive the fact that waves result from the motion of the wind. In this blog, we concentrate only on wave energy devices.
Classification based on location
Wave energy converters can be classified into the following two categories based on the location of the converter.
Onshore devices: Wave energy converters that are installed near the coast are called onshore devices. The advantage of onshore devices is that they do not need long transmission cable lines. However, as the waves near the shore are of lesser amplitude, less energy can be trapped by such devices.
Offshore devices: Wave energy converters that are installed in the sea are classified as offshore devices. Offshore devices need long transmission cable lines to transfer the captured power back to the near coastal regions. Small scale offshore devices find their application in navigation buoys and warning signal installations near icebergs or such dangerous sea areas.
Classification based on energy tapping method
Based on the way energy is tapped from waves, wave energy converters can be classified into the following categories:
Overtopping: During the crest of a wave, the overtopping water is collected into a large storage reservoir. This water then passes through a turbine and returns to the sea. This process continues, and energy can be extracted by connecting the shaft of the turbine with an electric generator. An alternative to enhance the efficiency of the system could be the provision of a convergent duct before passage through the turbine.
Oscillating bodies: Under the action of waves, a floating body experiences oscillations. The floating body is further connected to mechanical or hydraulic driven electrical generators. The floating bodies are generally moored to the sea bed. To capture the stronger waves, these oscillating bodies are mostly located far from the shores.
Point absorbers with power take-off mechanisms: Point absorbers are devices which have smaller dimensions in relation to the incident wavelength. Energy from the point absorbers can be trapped by their heaving motion over the surface of the water. Else, the energy can also be extracted by submerging the point absorbers below the sea surface and capturing energy using pressure gradients. The point absorbers are generally coupled with suitable power take-off mechanisms to utilize the energy efficiently. The linear motion (heave) is converted to rotary motion using linear electric generators.
Oscillating water column (OWC): Oscillating water column is a turbine housing a cylindrical hollow column and lower end of which interacts with the waves while the other upper face is open to the atmosphere. The periodic crests and troughs of the oscillating waves at the lower end oscillate the trapped air above them. This reciprocating airflow causes the turbine to rotate; the rotation of the turbine is in a single direction even though the air is reciprocating in nature.
Stages of wave energy conversion
In brief, the wave energy conversion in most of the devices takes place in the following three stages:
Stage 1 - Hydrodynamic stage: Conversion of sea wave elevations into pressure gradients
Stage 2 - Mechanical stage: Conversion of pressure gradient into mechanical energy by mechanical means
Stage 3 - Electrical stage: Conversion of mechanical energy into electrical energy using an electric generator
Indian Wave Energy Scenario
India has immense potential for wave energy as it possesses a coastline of approximately
6000 km. Research on wave energy in India was initiated by the wave energy group at
Indian Institute of Technology Madras during the year 1983. It was concluded by the early reports oscillating water column is the best suitable method for wave energy extraction in India. This led to the construction of OWC, also termed as “India wave energy plant”, capable of producing 150 kW by the year 1990. The plant has been installed at Vinzhinjam, Kerala.
Summary
We discussed the various wave energy harvesting techniques
Alongside, we also discussed the stages in conversion from wave to wire
Lastly, in brief, we discussed the Indian wave energy scenario