Maarg Manthan · Topic 12.1

Ocean Thermal Energy Conversion (OTEC)

Environment & Ecology › Energy Resources · Topic 12.1

Ocean Thermal Energy Conversion (OTEC) generates electricity from the temperature difference between warm tropical surface sea water, at around 25 to 30 degrees Celsius, and cold deep water at 1,000 metres, at around 4 to 5 degrees. A temperature difference of at least about 20 degrees is needed, which restricts commercial OTEC to tropical regions. Three cycles exist: the closed cycle, in which warm water vaporises a low-boiling working fluid such as ammonia that drives a turbine and is condensed by cold water; the open cycle, in which warm sea water itself is flash-evaporated in a vacuum to make steam that drives a turbine and, after condensation, yields desalinated fresh water; and the hybrid cycle, which combines both. OTEC gives continuous base-load power, unlike wind and solar, and by-products such as fresh water, cooling and nutrient-rich cold water for aquaculture. In India the National Institute of Ocean Technology (NIOT), Chennai, has worked on OTEC and low temperature thermal desalination in Lakshadweep.

5 min read · UPSC Prelims · Environment & Ecology · Energy Resources

At a Glance

Needs About 20 C temperature gap
Cycles Closed, open, hybrid
Suitable zone Tropical seas
Power type Continuous base load
Indian body NIOT, Chennai
Exam link UPSC Prelims, Environment & Ecology: Energy Resources

Where Does It Fit?

This is the first topic of the macro on energy resources. It opens the account of ocean-based renewable energy, which continues with tidal and wave energy. Previous topic: Oxygen Stratification in Water Bodies. Next topic: Tidal Energy and Wave Energy.

How Does OTEC Work?

Cycle	Working
Closed cycle	Warm surface water heats a working fluid such as ammonia in an evaporator; the vapour turns a turbine; cold deep water condenses it for reuse
Open cycle	Warm sea water is flash-evaporated in a low-pressure chamber; the steam drives a turbine and is condensed by cold water into fresh water
Hybrid cycle	Combines closed cycle power with open cycle fresh water production
Temperature need	At least about 20 degrees C difference between surface and deep water
Plant location	On land, on the shore, or on floating platforms near deep water
Cold water pipe	A long pipe draws cold water from about 1,000 m depth
By-products	Desalinated water, air-conditioning, aquaculture and hydrogen or ammonia production

What Are the Advantages, Limits and Indian Efforts?

Advantages: renewable, continuous, no fuel and low carbon
Fresh water: open and hybrid cycles produce drinking water, valuable for islands
Limits: low efficiency of around 3 to 5 percent, high capital cost and large cold water pipes
Environmental concerns: effects of pumping deep water on marine life and release of dissolved gases
Site constraint: deep cold water must be close to shore
India potential: large potential around peninsular India and the islands, with estimates of tens of thousands of megawatts; check official figures
NIOT work: low temperature thermal desalination plants at Kavaratti, Minicoy and Agatti in Lakshadweep
Past trials: a floating 1 MW OTEC plant off Tamil Nadu was tried in the early 2000s but did not succeed

What Else Should You Remember?

Temperature difference: the difference between the surface and 1,000 m depth should be at least about 20 degrees
Tropical belt: between about 20 degrees north and south, warm surface water is available throughout the year
Working fluid: ammonia has a low boiling point and is used in closed-cycle plants
Evaporator and condenser: heat exchangers that transfer heat to and from the working fluid
Flash evaporation: sudden vaporisation of warm water in a vacuum chamber
Desalination: fresh water is the condensed steam of the open cycle
LTTD: low temperature thermal desalination uses warm surface and cold deep water to make fresh water without a turbine
Kavaratti plant: the first LTTD plant in India, set up in Lakshadweep in 2005 with capacity of about 100,000 litres a day
Island use: OTEC and LTTD help islands with limited fresh water
Hawaii: a pioneering OTEC site in the United States, with the Natural Energy Laboratory of Hawaii Authority
Georges Claude: French engineer who built an early OTEC plant in Cuba in 1930
Japan: has long run OTEC research and small plants
Efficiency: low because the temperature difference is small compared with fuel-burning plants
Capital cost: high due to large pipes, platforms and heat exchangers
Biofouling: growth of marine organisms in pipes and exchangers reduces performance
Corrosion: salt water attacks equipment
Cold nutrient-rich water: deep water can support fish and seaweed farming when discharged properly
Base load: OTEC runs day and night, helping grid stability
Ocean energy types: OTEC, tidal, wave and salinity gradient
Blue economy: India’s Deep Ocean Mission includes ocean energy and desalination
Link to previous: see Oxygen Stratification in Water Bodies
Link to next: see Tidal Energy and Wave Energy
Link to renewables: see Renewable Energy and Sustainable Development
Caution: OTEC is still at pilot stage, and plant sizes and potential figures differ across studies
Exam hint: remember 20 degree difference, three cycles and the NIOT link

Why Does It Matter for the Exam?

Gap needed: 20 C
Fluid: Ammonia
Agency: NIOT
Current affairs link: Deep Ocean Mission
Common errors: calling OTEC tidal energy or placing it in cold seas

As an analytical point, OTEC offers steady renewable power and fresh water for tropical islands, but its low efficiency and high cost keep it at pilot scale. Small island applications that couple power and desalination are the most realistic near-term use.

Why Is This Topic Important?

Source Sea heat gap
Type Base load
Limit Cost

What Came Next?

The next topic covers tidal energy and wave energy. Next topic: Tidal Energy and Wave Energy.

Key People and Terms

OTEC Ocean Thermal Energy Conversion
LTTD Low temperature thermal desalination
Thermocline Zone of falling temperature
Working fluid Heat-carrying liquid
Flash evaporation Quick vaporisation
NIOT National Institute of Ocean Technology

Exam Corner

Points to Remember

Needs about 20 C difference.
Three cycles: closed, open, hybrid.
Fluid: ammonia.
Gives fresh water.
NIOT is in Chennai.

Do Not Confuse With

OTEC and tidal energy Heat gap versus tidal range
Open and closed cycle Sea water steam versus working fluid
OTEC and LTTD Power versus water production

Memory Hook

Warm top, cold deep, twenty degrees apart, ammonia turns the wheel.

Mains Angle

Questions ask for principle and cycles. These points help.

Principle Heat gap
Value Base load
Limit Cost

A Question You May Face

An original practice question, not a past paper question.

“Explain the principle of Ocean Thermal Energy Conversion and discuss its prospects in India.”

How to Answer

Principle Heat gap.
Cycles Three.
India NIOT.
Limit Cost.

GS Relevance

Prelims: principle, cycles, Indian efforts. GS Paper 3: ocean energy and technology.

Frequently Asked Questions

What is Ocean Thermal Energy Conversion?

Ocean Thermal Energy Conversion is a technology that produces electricity from the difference in temperature between warm surface water and cold deep water in tropical oceans. A difference of at least about 20 degrees Celsius is needed. The heat difference is used to vaporise a fluid or sea water that turns a turbine, giving steady, renewable power.

What are the types of OTEC cycles?

There are three cycles. The closed cycle uses a low-boiling working fluid such as ammonia that is vaporised by warm water and condensed by cold water. The open cycle flash-evaporates warm sea water to make steam and produces fresh water. The hybrid cycle combines the two to produce both electricity and fresh water.

Why is OTEC suitable for tropical islands?

Tropical islands have warm surface water all year and deep cold water close to shore, which suit OTEC. They also often lack fresh water and rely on costly imported fuel, so a plant that gives power, drinking water and cooling is valuable. India's Lakshadweep islands use low temperature thermal desalination for this reason.

What are the limitations of OTEC?

OTEC has low conversion efficiency of a few percent, needs large heat exchangers and long cold water pipes, is expensive to build and maintain, suffers biofouling and corrosion, and can only be used where deep cold water is close to the shore. Most plants remain pilots, so it is not yet commercially widespread.

What has India done on OTEC?

India's National Institute of Ocean Technology in Chennai has studied OTEC and built low temperature thermal desalination plants in Lakshadweep, starting with Kavaratti in 2005. A floating 1 megawatt OTEC plant tried off Tamil Nadu in the early 2000s did not succeed, so India continues research and pilot work.

PYQ Practice — Statement Analysis

1 OTEC needs a temperature difference of about 20 degrees Celsius.

True

Between surface and deep water.

2 OTEC works best in polar seas.

False

It works best in tropical seas.

3 The open cycle produces fresh water.