What is OTEC (Ocean Thermal Energy Conversion)?

Principle of Ocean Thermal Energy Conversion

Ocean Thermal Energy Conversion (OTEC) technologies use the temperature difference between warm sea water at the surface of the ocean and cold sea water at 0.8 – 1.0 km depth from top of the sea level to produce electricity. It operates generally with temperature difference of 20 degrees centigrade (°C) or more.

The warm seawater is used to produce a vapour that acts as a working fluid to drive turbines.  The cold water is used to condense the vapour and ensure the vapour pressure difference drives the turbine.

Fluid in the OTEC

• In the open cycle OTEC, sea water is used as the working fluid where ammonia is used as fluid in the closed cycle OTEC. 
• When we use mixture of water and ammonia in the closed cycle OTEC, it is called as the Kalina Cycle. 
• The OTEC also consists of electricity cables to transfer electricity back to shore and water ducting system.

Why Ammonia?

• The use of ammonia as a working fluid reduces the size of the turbines and heat exchangers required.

Challenges of OTEC

• Size of the water ducting systems that need to be deployed in the large scale OTEC plants. A 100 MW OTEC plant requires cold water pipes of 10 meter diameter and length of 1000 m.
• Lacking of operating experience
• The technological challenge is that the small temperature difference requires very large volumes of water at minimum pressure losses. This requires large seawater pumps, large piping systems, and large cold water pipes operating almost continuously in a hostile and corrosive environment.
• A 100 MW OTEC plants would have several seawater pumps, each the same size as a locomotive engine. These pumps would guide 750 tonnes per second of seawater through the OTEC system (US. Department of Energy)
• Salt water and sedimentation is biggest challenge for ocean thermal energy, it could damage ocean bound machines.

Advantages of OTEC

• Able to provide electricity on a continuous  basis, while also providing cooling without electricity consumption
• The capacity factor of OTEC plants is around 90%-95%, one of the highest for all power generation technologies.
• As efficiency of the Carnot cycle is very low (maximum 7%), it does not impact on the feasibility of the OTEC because the fuel is absolutely free
• As the sea water is desalinated, pure drinking water is available for local communities.
• By using Low Temperature Thermal Desalination (LTTD) technologies, sea water can produce fresh water includes dehumidification.
• Bad weather and natural disasters interrupt electricity and water supplies but ocean thermal energy still potential to provide water as well as electricity.

Disadvantages of OTEC

• The energy losses due to pumping are around 20% - 30%.
• The open cycle OTEC system must be carefully sealed to prevent leakage in the atmosphere air and destroying vacuum.
• The floating platform installation has comparatively lower land use but it requires grid cables to be installed to land, higher construction and maintenance cost.