Chapter 6 – Ocean Energy

Chapter 6 – Ocean Energy

All form of energy on earth is derived by solar energy directly or indirectly. About 75% area of globe is covered with water. There is large potential of energy production through ocean energy harvesting. Ocean energy can be harnessed by three different means; (i) Tidal energy (ii) Ocean wave energy system (iii) Ocean thermal energy conversion.

6.1 Tidal Energy

In each day two high and two low tides occur due to gravitational effect of moon and sun. The effect of moon is 2.6 times more than the sun. Lunar day is 50 minutes more than solar day (24 hour). Due to this variation time of tides vary on each day, but it can be predicted accurately. In one month, when sun, moon and earth are in one line maximum height of tide occurs. It is the day of new moon and full moon and known as spring tides. Similarly for two times in a month, Sun and moon are at right angles, the height of tide is minimum on these days and known as neap tides. The difference of height between high tide and low tide is called tidal range.

6.1.1 Characteristics of Tidal Power

·       It does not depend on whether and climate condition.

·       The time of tides rise and fall can be predicted accurately.

·       The electricity production is not continuous.

6.1.2 Electricity Production through Tidal Power

Barrage is used to collect water in one or more basin during high tides through sluice gate. At the time of low tide water is released back into ocean through turbines installed in barrage, which produce electricity. Hence electricity is produced twice in a day. If reversible turbines are used, electricity may be produced during filling of basin as well as during emptying. In this way electricity may be produced for four times during a day.

6.1.3 Types of Tidal Power Plants

            i. Single-basin single-effect plan

Electricity is produced during low tide using one basin.

           ii. Single-basin double effect plan

Electricity is produced during high tide as well as during low tide in one basin.

          iii. Double-basin with linked-basin operation

Two separate basins are linked with power plant and two sluice gates are connected to ocean.

          iv. Double-basin with paired basin operation

Two basins (high and low) have their own sluice gate and their own power plant. This is similar to two individual single-basin single-effect basin operations.

6.1.4 Important Components of Tidal Power Plant

·       Barrage

·       Sluice Gate

·       Turbines

6.2 Ocean Wave Energy System

Because of the wind large waves are produced in ocean water. The regular and large up and down of water waves can be converted into mechanical energy or electricity. The waves have more amplitude as one goes farther from the coast. India has 6000 km ocean coast, therefore it has large potential of ocean wave energy system. Indian coast has been divided in six regions for this purpose. The potential ranges within 5 to 16 kW/m  during north-east monsoon  and 17 to 47 kW/m  during south-waste monsoon.

6.2.1 Factors affecting wave energy

i. Speed of wind/amplitude of waves

ii. Effective fetch value/ the distance over which in the ocean uninterrupted wave is present.

iii. Depth of sea water/ larger the depth more will be the wave energy.

6.2.2 Energy available from wave energy

The following parameters are important for considering the wave energy.

H = Wave height. It is the distance between trough to the crest. (0.2 to 3 m)

a = Amplitude of the wave = H/2

l = wave length

T = Time period of wave (4 to 12 s)

f = Frequency

C = Wave velocity or celerity = l/T

T = 1/f

l = 1.56 T²

The energy available in random sea is

P = 0.96 H²T kW/m of wave crest

Wave energy system has both potential and kinetic energy.

6.2.3 Wave Energy Principle

The principles have been categorized in two varieties:

i.                 Oscillating water column

ii.               Wave activated point absorbers

6.2.3.1 Oscillating water column (OWC)

In this system an enclosed water column is formed in the ocean. As the crest and trough is produced in ocean wave, water column is increased or decreased inside the column. This results in decrease/increase of air column above the water column. Hence air is exhaled or inhaled inside the OWC. In the path of air flow air turbine is situated which produce electricity.

6.2.3.2 Wave activated point absorbers

In this system some part of device oscillates in comparison to other fixed part due to wave oscillation mechanism.

i.                 Buoy type machine

It is a hydraulic system to turn a motor generator. The buoy is a floating part of a system, which oscillates up and down with ocean wave. This motion is coupled with up-down motion of counterweight through pulley. The up-down motion of counterweight is attached to piston which can operate a generator.

ii.               Dolphin type wave energy generator

In this system a floating generator is attached with stationary generator via shaft and gear to produce electricity.

iii.              Oscillating ducks

In this case several duck shaped systems are installed in an array perpendicular to wave motion. Ocean waves are absorbed by these ducks and ducks starts to oscillate, which in turn attached to mechanical system to produce electricity.

6.3 Ocean Thermal Energy Conversion

Heat radiation from sun is absorbed by ocean water. Hence the surface layer of ocean water is at high temperature while deep water is at low temperature. This temperature difference may be of 20^°C and it may run the heat engine.

6.3.1 Closed Cycle OTEC System

·             In this system liquid ammonia is used as working substance. In evaporator due to presence of hot water ammonia changes into vapour and pressurized ammonia runs the turbine to produce electricity.

·             The rest of liquid which is not converted into vapour goes into separator.

·             The pressure of ammonia vapour decreases after passing through turbine. This vapour condenses into liquid in condenser in presence of cold water.

·             This liquid ammonia again comes into evaporator and cycle goes through.

6.3.2 Open Cycle OTEC System

·             In open system hot water from ocean goes into vacuum chamber, where water changes into steam.

·             This steam runs the turbine to produce electricity.

·             The steam condenses into fresh water due to cold water of ocean.

·             In open cycle system fresh water is produced as by-product.

6.3.3 Thermoelectric OTEC

In this device thermocouple is used to produce electricity as the junctions are kept at hot and cold water of ocean.

6.4 Osmotic Pressure

When two liquids of different concentration are mixed through, there is flow of solvent through membrane from higher concentration of solvent to low concentration of solvent, this process is known as osmosis. The membrane should be such that it can allow the passage of solvent molecules but not the solute molecules. The external pressure required   to stop the spontaneous flow through membrane is known as osmotic pressure.

The osmotic pressure can be utilized in producing electricity. If river is passed through a semi-permeable membrane at the place, where it meets the ocean, large amount of osmotic pressure can be harnessed. The ocean has higher concentration of salt in comparison to river of fresh water; hence on passing through semi-permeable membrane the process of osmosis takes place.

The method of harnessing osmotic pressure is related to Pressure Retarded Osmosis (PRO). In a chamber, ocean water is pumped. The fresh river water is passed into chamber through semi-permeable membrane. It increases the water level inside the chamber. The pressure produced by water will start the turbine to produce electricity.