The ocean and sea constitute about 70% of the earth 's surface. This absorbs solar energy due to which a temperature gradient of about 20°C exists between surface water and deep ocean water. This temperature gradient can be utilized in a heat engine to generate electrical power. The power obtained by this principle is called ocean thermal energy conversion (OTEC).
The first ocean thermal energy conversion (OTEC) plant was built by Georges Claude in 1930 in Matanzas Cuba. This system generated 20 kW of electrical power by using a low pressure turbine.
WORKING PRINCIPLE OF OTEC
The main objective of ocean thermal energy conversion (OTEC) is to convert the solar energy trapped by the ocean into electrical energy. The working principle of OTEC is based on the ocean's natural temperature gradient of about 20°C between the warm surface ocean water and cold deep ocean water at a depth of about 1000 m. According to Lambert's law of absorption of radiation.
Intensity I is given by the expression
I = Ioe-μh
where,
I = intensity of radiation at the surface.
h = depth of water
μ=absorption coefficient which depends upon the quality of water
In other words, the intensity of heat decreases with the increase in water depth. This temperature gradient is used to run a heat engine which drives an electric generator to produce electric power.
Due to low temperature gradients the overall efficiency of the OTEC system is 3 to 4%. The capital cost of OTEC is very high as the depth of about 1000 m is needed to obtain a temperature gradient of 20°C.
EFFICIENCY OF OTEC SYSTEM
The conversion efficiency of an OTEC system is given by the expression:
η = {(T₁-T₂)/T₁} x100
where,
T₁ = temperature of warm water in degree kelvin
T₂ = temperature of cold water in degree kelvin
The efficiency of a heat engine working between two temperature limits cannot be more that of Carnot cycle efficiency (n). Due to this the overall efficiency is obtained by multiplying a relative efficiency factor (E.F.) to conversion efficiency (η).
The overall efficiency of an OTEC system is given by
ηOTEC = (η) x E.F.
=(T₁-T₂)/T₁ ×E.F.×100
Relative efficiency factor EF is about 0.4 to 0.6.
CONVERSION TECHNOLOGY
The available thermal gradient between surface warm water and deep cold water can be used to heat up and vaporize a liquid called working fluid. The working fluid develops pressure when it is vaporized. When the vapor is passed through a turbine it expands and rotates the turbine. It is then condensed into liquid at the turbine outlet and the cycle is repeated. The working fluid used in case of open loop system is water and in case of closed loop system is ammonia.
The generally used working fluid ammonia has certain advantages such as easy availability, low cost and superior properties as transport medium. Ammonia as a working fluid has disadvantages of being toxic and inflammable.
Another option for working fluid is Chlorofluoro-carbon (CFC), which is not toxic or inflammable but they are harmful to the ozone layer of the atmosphere.
TYPES OF OTEC SYSTEM
There are mainly two types of ocean thermal energy conversion (OTEC) systems.
1. Open Cycle System
2. Closed Cycle System
Open Loop System or Claude Cycle System
In this system sea water is used as a working fluid. The warm water is evaporated in a chamber called an evaporator at a very low pressure. The vapor so formed is passed through a steam turbine where it expands and drives the turbine. The turbine is coupled with a generator which converts mechanical energy into electrical energy. Steam is generated at a very low pressure which requires a large volume of steam. So the diameter of the turbine is large and is of the order of 12 meter for 1 MW plant. The steam is condensed at the turbine outlet by the condenser. The water so condensed does not contain salts and can be used for drinking and irrigation purposes.
Advantages of Open Cycle System
1. The working fluid in an open cycle system is sea water which is not toxic or inflammable unlike ammonia which poses a problem when leaked.
2. It does not cause harm to protective ozone layer of the atmosphere unlike CFC.
3. Fresh water produced from condenser can be used for drinking and irrigation purposes.
4. Working fluid (i..e, water) is no threat to the environment.
Disadvantages of Open Cycle System
1. Huge amount of warm water is required for evaporation and huge amount of cold water is required for condensation.
2. The carbon dioxide dissolved in seawater is released into the atmosphere by the vacuum pump and it causes air pollution.
3. Volume of the working fluid is much larger than the closed cycle.
Closed Cycle OTEC System
It is also called the rankine cycle or Anderson cycle OTEC system. In this system, warm water from the ocean surface is pumped through an evaporator, where a low boiling point fluid like ammonia is evaporated. Then the vapor ammonia is passed through a turbine and drives it which runs a generator and produces electric power.
The working fluid is collected and condensed at the turbine outlet by the condenser with the help of cold deep water from the ocean. Liquid ammonia from the condenser is pumped back to the evaporator and the process is repeated. For all the time ammonia remains in a closed cycle and continuously circulated.
OTEC plant of 20 MW needs a large quantity of warm water for evaporator and huge amount of deep cold water for condensation.
Advantages of Closed Cycle System
1. Efficiency is higher in comparison to open cycle systems.
2. The size of the power plant decreases with increase of vapor pressure of working fluid. So overall cost is lower in comparison to open cycle systems.
3. The system is more compact in comparison to the open cycle.
Disadvantages of Closed Loop System
1. The working fluid ammonia is toxic and inflammable.
2. Working fluid like CFC may cause depletion of the protective ozone layer of the atmosphere.
MERITS AND DEMERITS AND LIMITATIONS OF OTEC SYSTEM
Merits
1. The OTEC system is a renewable energy source.
2. It is inexhaustible since it does not use any fuel.
3. Open cycle plants can also be used to produce fresh water for drinking purposes, which can solve the problem of scarcity of pure drinking water in coastal areas.
4. It has a great potential which has no emission of pollutant gasses.
5. The cold water (5°C) from the deep ocean can be used for air conditioning and refrigeration.
Demerits
1. The overall plant efficiency is very low and is of the order of 3%.
2. The capital cost of the plant is very high.
3. The working fluid used in closed cycle systems is ammonia, freon or propane which are very expensive.
4. Sea water is highly saline (contains salt) and thus it corrodes metallic parts.
5. Leakage of working fluid in closed cycle systems causes pollution and may disturb the habitat of the aquatic sea animals.
6. Huge amount of pipe work is required for the OTEC plant as the plant involves a depth of 1000 m between warm surface water and cold deep ocean water.
7. The OTEC plant should be strong and capable of withstanding ocean storms.
Limitations
1. Capacity of the turbogenerator is limited to 25 kW due to limitation. of a small temperature difference of 20°C between warm and cold waters.
2. Due to low pressure available, a large turbine is required.
3. Construction of floating power plants is difficult. 4. Since the OTEC power plants are away from the load centers and sometimes off-shore, the transmission cost is high.
5. Sea ecosystem gets disturbed due to construction of plants and pipelines.
POTENTIAL AND CHALLENGES OF OTEC SYSTEMS
Potential
Large amount of solar energy is stored in oceans and seas. On an average 60 million km² of tropical sea absorbs solar radiation equivalent to the heat content of 245 billion barrels of oil. If this energy could be tapped, it would provide huge energy potential. India has an overall potential of 50,000 MW of OTEC energy.
Currently there is a floating OTEC project named Sagar Shakti of 1 MW of installed capacity, 35 km off Tiruchendur coast in Tamil Nadu.
Challenges of OTEC Systems
1. A power plant based on OTEC poses an enormous challenge of building a 2500-3000 m long cold water pipe to transport the large volumes of deep sea water required from a depth of about 1000 m.
2. A 50 MW power plant will require a discharge of 150 cubic meter per second. That will require a pipeline of diameter of at least 8 m.
3. The cost of the pipeline is very high.
4. For off-share OTEC plants, there is a need for a submarine power cable.
5. Biofouling: Biofouling is the unwanted accumulation of algae, micro-organisms, marine animals and plants on the surface of pipes, evaporators and condensers. This reduces the efficiency of the OTEC.
CRITERIA FOR SITE SELECTION FOR OTEC PLANTS
For selecting the suitable site for installing on Ocean Thermal Energy Conversion (OTEC) plant, following factors may be taken into consideration.
1. The suitable site for the plant needs a difference temperature of about 20°C between the warm surface water and the cold deep ocean water. The higher the temperature difference, the more is the power output.
2. The site lies on the torrid and temperate zones between latitude 30° south and 30° North.
3. For a land based plant a site where slope falls off sharply from the land is ideal because the length of the cold water pipeline required is minimum.
4. In the floating type OTEC system the plant is placed on the ship which moves along the ship. The energy generated is consumed on the ship itself because it is very costly to transmit power from the sea to the seashore.
APPLICATIONS OF OTEC SYSTEMS
1. Power production by OTEC plants.
2. Fresh water production: In an open cycle OTEC plant, fresh water is produced.
3. Airconditioning and refrigeration: Cold water (5°C) being pumped to the surface can be used for air conditioning and refrigeration.
4. Hydrogen production: Hydrogen can be produced by electrolysis of water using power generated by OTEC plants.
CONCLUSION
It is a potential source of renewable energy which creates no emissions. It is also fuel free and has less environmental impact. In addition to power generation, it can supply pure water both for drinking and agriculture, and can provide refrigeration and air conditioning to the nearby coastal areas.
However, the OTEC system has high capital cost and lacks developed technology at present.
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