Renewable Energy: Water Power; Part 3 of 3

March 14th, 2011 by

This is the final segment on five types of hydrokinetic technologies, all mentioned in Sustainable Infrastructure: The Guide to Green Engineering and Design by S. Bry Sarté.  These systems are microhydroelectric, wave energy, vortex-induced vibration (VIV), tidal power and ocean thermal energy conversion (OTEC).  My final blog on the topic will discuss OTEC power: how it works, different types of systems, and its many different applications.

Ocean Thermal Energy Conversion (OTEC):

OTEC uses the differences in temperature between cold, deep ocean water and warm, shallow water to power a heat engine capable of producing electricity.  OTEC is most efficient when there is a large temperature difference (approximately 36 degrees Fahrenheit), thus the best potential sites are tropical areas.  OTEC systems are closed-cycle, open-cycle or a hybrid of the two.

Closed-cycle systems pump warm ocean water through a heat exchanger where it is used to heat a working fluid with a low boiling point (such as ammonia).  The working fluid is vaporized and the expanding vapor turns a turbo-generator.  Cold seawater is then pumped through a second heat exchanged to condense the vapor back into a liquid.

Diagram of Closed Cycle OTEC, Source: http://en.wikipedia.org/wiki/File:Otec_Closed_Diagram_in_English.JPG

Open-cycle systems use the warm seawater directly instead of a secondary fluid.  A low-pressure container causes the water to boil, and like in the closed system, the expanding vapor drives a turbine which, in turn, powers an electrical generator.  This is also a desalination process, as the steam is fresh water.  The steam is condensed back to liquid form by exposure to the cold ocean water.

Diagram of Open Cycle OTEC, Source: http://en.wikipedia.org/wiki/File:Otec_Open_Diagram_in_English.JPG

Hybrid systems evaporate warm seawater in a vacuum, similar to in the open-cycle systems.  However, this steam vaporizes a working fluid, similar to a closed system.  Again, the expanding vapor fuels a turbine to produce electricity and the cold ocean water is used to condense the vapor back into a liquid.

Sites can be located on or near land, on continental shelves or as floating facilities.  Each has its advantages and disadvantages mainly related to the difficulty and cost of installation and maintenance and the safety of the facility in turbulent conditions.

OTEC is primarily thought of as a source of renewable energy; however, it has several useful byproducts and uses.

OTEC Applications, Source: http://www.nrel.gov/otec/applications.html

First, as mentioned above, desalinated water is produced in open or hybrid cycles.  A 2MW plant could be capable of producing approximately 150,000 cubic feet of desalinated water per day!

Secondly, the cold seawater needed for an OTEC system can also be used in chilled water coils to provide air conditioning.  The InterContinental Bora Bora Resort and Thalasso Spa is the first hotel in the world to use seawater air conditioning for all of it’s cooling needs.

Deep ocean water is some of the most nutrient rich water in the world.  For this reason OTEC systems support mariculture, which is the farming of aquatic organisms.  Salmon, lobster, trout, oysters, clams, and spirulina are just some of the species that can be raised in pools supplied by OTEC pumped water.

The cold ocean water can also be used in land-based agriculture, by running the water through underground pipes and chilling the soil.  This allows plants to be grown in non-native climates.

OTEC is a promising technology; however, there are still economic, political and environmental concerns.  Hopefully, careful site selection can minimize negative impacts.

All of the water sources of renewable energy are capable of producing huge amounts of energy.  Hopefully in the near future, these technologies will be further optimized so that they can be implemented in more locations with lower costs and environmental impacts than fossil fuel electricity generation.  Water power might well become the “wave of the future”  for renewable energy.

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