The energy from ocean waves has huge potential. It alone could supply the world's electricity needs. The total power of waves available on the world's coastlines is estimated at 2 to 3 million megawatts. In some locations, the wave energy density can average 40 megawatts per kilometer of coastline. The real challenge is how to harness wave energy efficiently and with minimal environmental, social, and economic impacts.
Oceans and seas are the sources of wave energy. Waves are created by the interaction of winds with the surface of the ocean. It has both kinetic and potential energy. Wave energy has high concentration between the latitude of 40° and 60° in the southern and northern hemisphere. Coast lines of New Zealand and Japan, the west coast of Europe and the USA are suitable for wave energy generation.
TERMS AND DEFINITIONS
1. Ocean wave: The up and down and to and fro motion of water is called ocean wave. It is periodic and sinusoidal in nature.
2. Wave energy: It is defined as the rate at which energy is transferred across one meter line at right angle to direction of propagation of wave.
3. Crest: The top most point of the sinusoidal wave is called crest.
4. Trough: The bottom most point of the sinusoidal wave is called trough.
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| Sinusoidal Ocean Wave |
5. Wave height (H): It is the vertical distance between the crest and trough.
6. Amplitude (a): It is the maximum height of the wave from the baseline.
a = H/2.
7. Wave length (λ): It is the horizontal distance between the two successive crests or troughs.
8. Wave period (T): It is the time taken by the wave to cover a distance of one wavelength.
9. Frequency (f): It is the number of wavelengths passed through a given point per second. f = 1/T.
10. Wave velocity (c): It is defined as the horizontal distance covered by the wave per unit time. It is also called celebrity.
c =λ/T meter/sec.
11. Relationship between wavelength and time period: It is given by the following expression λ = 1.56 T² meter.
12. Relationship between wave height (H) and wind speed (U): The empirical formula showing the relation between the wave height and wind speed is given by the expression.
H = 0.085 U² meters
U is wind speed in knots (1 knot = 1.4 km/hour)
FACTORS AFFECTING WAVE ENERGY
Wave energy depends mainly on three factors:
1. Wind speed: Wave energy is proportional to wind speed. The higher the wind speed, the more is the wave energy.
2. Effective fetch value: It is the distance on the ocean surface over which the wind blows before reaching the reference point. The more is the fetch value, the more is the wave energy.
3. Depth of water: The more the depth of the ocean water column, the more is the wave energy.
WAVE ENERGY GENERATION
Wave energy generation is a developing technology. Although many wave energy devices have been invented, only a small number have been tested and evaluated and very few of these have been actually tested in ocean waves-testing is usually undertaken in a wave tank. Wave energy fluxes are smallest in summer and greatest in winter.
Wave has both potential energy and kinetic energy. Up and down motion of water in vertical direction is due to potential energy and propagation of waves in horizontal direction is due to kinetic energy.
WAVE ENERGY CONVERSION DEVICES
There are a number of types of wave conversion devices. The main types of these devices are given below.
Float Type Wave Energy Generator
In this type, a square float moves up and down with water. Four vertical ai files guide the float. A piston is attached with the float which compresses air in a stationary cylinder. The cylinder piston arrangement acts as a reciprocating air compressor. When the piston moves downwards it draws atmospheric air into the cylinder through an inlet check valve. When the piston moves upwards air is compressed in the cylinder which sends air through an outlet check valve to four underwater floating tanks via the four airfiles. These tanks serve two purposes, namely, buoyancy and air storage. The compressed air in the storage tank drives an air turbine which is coupled with an electrical generator. The generator produces electrical power which can be transmitted to the shore through cable.
Limitations of this type are:
1. It requires a linear array of a few km to produce power of 100 MW.
2. Sometimes water enters the turbine.
3. Problem of corrosion of metal parts due to saline water.
4. Power transmission to shore is costly.
5. Design to withstand storms.
Oscillating Water Column (OWC) Type Energy Generator
India has a oscillating water column type wave energy system 150 kW capacity installed near Thiruvananthapuram. Wave power available at the site is 13 kW/m. This system consists of a chamber of size 10 m × 10 m with a height of 15 m. The chamber has a side opening through which water comes into the chamber for collecting wave energy. An air turbine is installed in the chamber which operates similar to a wind turbine. This turbine has a dia of 2 m and is coupled to an induction generator.
Dolphin Type Wave Energy Generator
The main components of Dolphin type wave generators are described below:
1. The supporting structure: It is built in the sea bed to provide support to the equipment. The structure is mounted on a pile foundation. It is also called a dolphin.
2. Stationary generator: It is installed on the top of the supporting structure. It converts wave energy received through connecting shafts to electrical energy.
3. Floating generator: It collects wave energy from a float (buoy) through a gear arrangement and generates electrical power.
4. Gear arrangement: Gear arrangement is provided for both the generators. This arrangement converts the movement into continuous rotational motion and generates electricity at the generator output.
5. Connecting shaft: It is connected between the stationary generator and the floating generator. Connecting shaft provides the rolling motion about its own fulcrum and drives the stationary generator.
6. Float: It is connected to the other end of the connecting shaft. It has both rolling and oscillatory motion. It drives the floating generator and generates electric power.
High Level Reservoir Type
Sea water is fed to the high level reservoir on the shore by a pressure amplifier which is operated by wave energy. The potential energy of the water in the reservoir is used to run a hydraulic turbine which drives a generator and produces electric power.
Hydraulic Accumulator Type
This is similar to a high level reservoir type without any reservoir. It has mainly three components, namey, pressure amplifier; hydraulic accumulator and a hydraulic turbine. Wave enters the cylinder of the pressure amplifier from the bottom and drives the main piston. This amplifies the pressure of a closed loop fluid to about 5 bar. This high pressure fluid is sent to a hydraulic accumulator where it maintains constant pressure due to an air cushion. The fluid from the accumulator is passed through a hydraulic turbine like Pelton wheel or Francis turbine. The fluid drives the turbine which is coupled with the generator and produces electric power.
MERITS AND DEMERITS OF WAVE ENERGY
Merits
1. The power produced from wave energy is concentrated unlike solar and wind energy which are dilute in nature.
2. It is a renewable energy source and can produce a significant amount of energy.
3. It is free, no fuel is needed and no waste is produced.
4. It does not require a large land area.
5. Very little pollution.
6. Operation and maintenance is not expensive
Demerits
1. A Smaller number of sites for large wave energy generation is available where waves are consistently strong.
2. Wave energy conversion devices are complicated in construction.
3. These devices have to withstand the high force of storms.
4. Large capital cost is associated with wave energy generation.
5. It can disturb or disrupt life including changes in the distribution and types of marine life near the shore.
6. Poses a possible threat to navigation from collisions due to the low profile of the wave energy devices above the water, making them undetectable either by direct sighting or by radar.
7. The energy generation depends on the waves, so it gives variable energy supply.
8. Sea water is saline so it corrodes metallic parts.
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