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January 03, 2013

Study by Makai Ocean Engineering Reveals Biological Impact



Ocean thermal energy conversion has been a controversial project due to its potentially harmful impact on marine life. However, a new study carried out by Makai Ocean Engineering, and funded by the U.S. Department of Energy, revealed that the biological impact can be minimized.

The results of the study soften the regulatory hurdle the technology was facing, potentially clearing the way for pilot projects that will move OTEC developers toward building 100-MW deep ocean power plants.

The OTEC technology works by utilizing the higher surface temperature of the oceans to evaporate liquefied ammonia, which then helps drive turbine like steam. The technology capitalizes on the low boiling point of liquefied ammonia. The technology, if successfully implemented, will greatly offset the use of fossil fuels to generate clean electricity.

A typical 100-MW OTEC plant would consist of rigs out in the ocean that would pump the 25-degree-Celsius surface ocean water into an evaporator. The process would vaporize the liquefied ammonia, which will eventually turn the turbines. On the other hand, the engines also pump the cold, ocean water of approximately five degrees Celsius, through a condenser that liquefies back the ammonia vapors. This creates a continuous cycle in a closed and efficient system.

The potential biological impacts arise from the discharge of the cold water, obtained from around a kilometer below the ocean surface, into the 20-meter-to-40-meter, near- surface depth. In this threshold, the ecosystem-sustaining phytoplankton populations thrive. The cold water brought up into this threshold could cause a phytoplankton proliferation or an unwanted algal bloom.

However, the published report on Makai Ocean Engineering’s research, entitled "Modeling the Physical and Biochemical Influence of Ocean Thermal Energy Conversion Plant Discharges", reveals that if the cold water is discharged back at least around 70 meters below the surface, it would avoid the biological harm associated with the technology.

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Edited by Brooke Neuman


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