OTEC - Introduction, Availability, Working Principle, Types of OTEC Systems, Limitations and Advantages This is suitable for 8th-semester B.Tech students of AKTU, who have opted for Renewable Energy Resources (ROE086) as the open elective subject.

1. Ocean Thermal
Energy Conversion
(OTEC)
BY
KRISHNANAND (ASST. PROFESSOR)
DEPT. OF MECH. ENGINEERING
SR GROUP OF INSTITUTIONS
JHANSI

2. Content
 Introduction
 Availability of OTEC
 Working Principle of OTEC
 Performance Analysis of OTEC
 Types of OTEC Systems
 Limitations of OTEC
 Applications of OTEC
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3. Introduction
 Objective of OTEC:
 For conversion – Temperature Gradient is needed (of at least
20ºC).
 Found in Tropical Oceans
Solar Energy
Trapped by Ocean
Usable Energy
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4. Availability
 80% of Solar radiation incident on earth surface falls on the
surface of World’s ocean.
 Near equator, daily solar radiation flux - 17 to 20 MJ/sq. m
 Heat absorbed by water (good thermal storage capacity)
 Stored as sensible heat – heat the surface water up to 27 - 30ºC.
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5. Availability Contd…
 Temperature difference between surface and at depth of 1000 m.
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6. Availability Contd…
 Monthly Variation of Temperature of Tropical Ocean Water
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7. Working Principle
 Generation of electric power from OTEC is based on – utilization
of the natural temperature difference between warm sea water at
surface and cold sea water at some depth.
 Temperature of surface sea water – about 27ºC
 Temperature of water at depth of 1000 m – about 7 to 10ºC
 Two infinite heat reservoirs – Water at 27ºC
(at Surface)
Water at 7-10ºC
(at depth of 1000 m)
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8. Working Principle Contd…
 CARNOT CYCLE –
This temperature gradient utilized in prime mover (turbine)
Part of heat energy
Mechanical Energy
Electrical Energy
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9. Performance Analysis
Heat Source
(27ºC)
Heat Sink
(7-10ºC)
E Mechanical
Energy
Electrical
Energy
Work
Output
T1
T2
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10. Performance Analysis Contd…
 Carnot Cycle efficiency –
𝜂 = 1 −
𝑇2
𝑇1
 T1 = 27ºC or T1 = 27 + 273 = 300 K
 T2 = 7ºC or T2 = 05 + 273 = 280 K
 so Carnot efficiency,
𝜂 = 6.66%
 Actual efficiency of OTEC plant is less than the Carnot Cycle.
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11. Performance Analysis Contd…
 Actual efficiency of OTEC Power Plant is –
𝜂 0𝑇𝐸𝐶 = 𝐸𝐹 × 𝜂 𝑐
 𝜂 𝑐 = Carnot Cycle Efficiency
 EF = Relative Efficiency Factor (0.4 – 0.6)
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12. Performance Analysis Contd…
 Solar energy absorption by water takes place according to
Lambert’s law of absorption.
 Lambert’s Law of Absorption –
“Each layer of equal thickness absorbs the same fractions of light
that passes through it.”
 If I = Intensity of radiation
x = depth of water , Then:
ⅆ𝐼 𝑥
ⅆ𝑥
= 𝜇𝐼
or
𝐼 𝑥 = 𝐼0ⅇ−𝜇𝑥
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13. Performance Analysis Contd…
𝐼 𝑥 = 𝐼0ⅇ−𝜇𝑥
 𝐼0 = Intensity of radiation at the surface where x = 0
 𝐼 𝑥 = Intensity of radiation at a distance x below the surface
 𝜇 = Absorption coefficient
 The intensity falls exponentially with depth ‘x’ and depending
upon 𝜇, almost all the absorption occurs very close to surface of
deep water.
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14. Performance Analysis Contd…
 EXAMPLE – 1:
Determine the overall efficiency of an OTEC plant if the
temperature of warm water on the surface layer is 30ºC and
temperature of cold water in the depth of the tropical ocean is 8ºC.
It can be assumed that the efficiency factor (EF) of power plant is
0.5.
Solution :
T1 = 30 + 273 = 303 K, T2 = 8 + 273 = 281 K and EF = 0.5
𝜂 = (1 −
𝑇2
𝑇1
)X100
𝜂 = 7.26%
𝜂 0𝑇𝐸𝐶 = 𝐸𝐹 × 𝜂 𝑐
= 05X0.0726
= 0.0363
𝜂 0𝑇𝐸𝐶 = 3.63%
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15. Types of OTEC Systems
 Open Cycle System or Claude Cycle System
 Closed Cycle System or Anderson Cycle
 Power plant comprises of –
Evaporator
Condenser
Turbine
Pump
Working
Fluid
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16. Open Cycle System
 Simple flow diagram of Open Cycle System (why Open Cycle ?)
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17. Open Cycle System Contd…
 Detailed diagram of Open Cycle System
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18. Open Cycle System Contd…
 Working Fluid is - Sea Water
 No Heat Exchanger is needed
 Direct - contact between exhaust steam and cold spray water
Warm Surface
Water
Flash
Evaporated
(under High Vacuum)
Vapour
Derives
A Low Pressure
turbine
Exhaust steam
condensed
Using Cold sea
water
Derives
Generator
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19. Open Cycle System Contd…
 T-S Diagram of Open Cycle System
 1: Warm Surface water at 27ºC
 1-2: Hot water admitted to evaporator
 2-3: Production of steam in evaporator
 3-5: Expansion of steam
 5-7: Condensation (by direct contact)
 6: Sink temp. of deep cold water
 1 – 6: Open Cycle OTEC
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20. Closed Cycle System
 Simple flow diagram of Closed Cycle System (Why closed cycle ?)
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21. Closed Cycle System Contd…
 Detailed diagram of Closed Cycle System
Working
Fluid:
Ammonia
or
Freons
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22. Limitation of OTEC Power Plant
 Large size of steam turbine needed (due to low pressure).
 Very large vacuum pump needed (to maintain vacuum).
 Expensive working Fluid (Closed Cycle – Ammonia or Freons).
 High cost of plant.
 Capability of withstanding severe ocean storms
 Corrosion of metal parts due saline water.
 OTEC plant can alter and may damage the surrounding ecosystem.
 Construction of floating power plant is difficult.
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23. Application of OTEC Plant
 Electricity generation
 Can also produce desalinated water (irrigation, human consumption)
 Closed cycle OTEC plant can act as chemical treatment plant.
 OTEC Plant can be used to pump up deep sea water (air conditioning
and refrigeration)
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24. Content Sources
 Images - www.google.in
 Book – NON-Conventional Energy Resources by S.Hassan Saeed
and DK Sharma
 Data available on web.
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