Editing Ocean thermal energy conversion (section)

== Environmental impact ==

Carbon dioxide dissolved in deep cold and high pressure layers is brought up to the surface and released as the water warms. {{citation needed|date=September 2012}}

Mixing of deep ocean water with shallower water brings up nutrients and makes them available to shallow water life. This may be an advantage for aquaculture of commercially important species, but may also unbalance the ecological system around the power plant.

OTEC plants use very large flows of warm surface seawater and cold deep seawater to generate constant renewable power. The deep seawater is oxygen deficient and generally 20–40 times more nutrient rich (in nitrate and nitrite) than shallow seawater. When these plumes are mixed, they are slightly denser than the ambient seawater.<ref name="bioplume">{{cite journal|last=Grandelli|first=Pat|title=Modeling the Physical and Biochemical Influence of Ocean Thermal Energy Conversion Plant Discharges into their Adjacent Waters|url=http://www.osti.gov/bridge/servlets/purl/1055480/1055480.pdf|website=US Department of Energy - Office of Scientific and Technical Information|access-date=27 March 2013|doi=10.2172/1055480|year=2012}}</ref> Though no large scale physical environmental testing of OTEC has been done, computer models have been developed to simulate the effect of OTEC plants.

=== Hydrodynamic modeling ===

In 2010, a computer model was developed to simulate the physical oceanographic effects of one or several 100 megawatt OTEC plant(s). The model suggests that OTEC plants can be configured such that the plant can conduct continuous operations, with resulting temperature and nutrient variations that are within naturally occurring levels. Studies to date suggest that by discharging the OTEC flows downwards at a depth below 70 meters, the dilution is adequate and nutrient enrichment is small enough so that 100-megawatt OTEC plants could be operated in a sustainable manner on a continuous basis.<ref name="hydroplume">{{Cite book |doi = 10.23919/OCEANS.2011.6107077|chapter = Physical and biological modeling of a 100 megawatt Ocean Thermal Energy Conversion discharge plume|title = Oceans'11 MTS/IEEE Kona|pages = 1–10|year = 2011|last1 = Rocheleau|first1 = Greg J.|last2 = Grandelli|first2 = Patrick|isbn = 978-1-4577-1427-6|s2cid = 22549789}}</ref>

=== Biological modeling ===

The nutrients from an OTEC discharge could potentially cause increased biological activity if they accumulate in large quantities in the [[photic zone]].<ref name="hydroplume" /> In 2011 a biological component was added to the hydrodynamic computer model to simulate the biological response to plumes from 100 megawatt OTEC plants. In all cases modeled (discharge at 70 meters depth or more), no unnatural variations occurs in the upper 40 meters of the ocean's surface.<ref name="bioplume" /> The picoplankton response in the 110 - 70 meter depth layer is approximately a 10–25% increase, which is well within naturally occurring variability. The nanoplankton response is negligible. The enhanced productivity of diatoms (microplankton) is small. The subtle phytoplankton increase of the baseline OTEC plant suggests that higher-order biochemical effects will be very small.<ref name="bioplume" />

=== Studies ===

A previous Final Environmental Impact Statement (EIS) for the United States' NOAA from 1981 is available,<ref>{{cite web|title=Final Environmental Impact Statement for Commercial Ocean Thermal Energy Conversion (OTEC) Licensing|url=http://hinmrec.hnei.hawaii.edu/wp-content/uploads/2010/01/OTEC-Programmatic-EIS-NOAA-1981.pdf|website=U.S. Dept of Commerce, National Oceanic and Atmospheric Administration|access-date=27 March 2013|archive-date=23 October 2020|archive-url=https://web.archive.org/web/20201023074930/http://hinmrec.hnei.hawaii.edu/wp-content/uploads/2010/01/OTEC-Programmatic-EIS-NOAA-1981.pdf|url-status=dead}}</ref> but needs to be brought up to current oceanographic and engineering standards. Studies have been done to propose the best environmental baseline monitoring practices, focusing on a set of ten chemical oceanographic parameters relevant to OTEC.<ref>{{cite web|author1=L. Vega|author2=C. Comfort|title=Environmental Assessment of Ocean Thermal Energy Conversion in Hawaii|url=http://hinmrec.hnei.hawaii.edu/wp-content/uploads/2010/01/Environmental-Assessment-of-OTEC-in-Hawaii1.pdf|website=Hawaii National Marine Renewable Energy Center|access-date=27 March 2013|archive-date=24 October 2011|archive-url=https://web.archive.org/web/20111024034258/http://hinmrec.hnei.hawaii.edu/wp-content/uploads/2010/01/Environmental-Assessment-of-OTEC-in-Hawaii1.pdf|url-status=dead}}</ref> Most recently, NOAA held an OTEC Workshop in 2010 and 2012 seeking to assess the physical, chemical, and biological impacts and risks, and identify information gaps or needs.<ref>{{cite web|title=Ocean Thermal Energy Conversion: Assessing Potential Physical, Chemical, and Biological Impacts and Risks |url=http://coastalmanagement.noaa.gov/otec/docs/otecjun10wkshp.pdf|website=National Oceanic and Atmospheric Administration, Office of Ocean and Coastal Resource Management |access-date=27 March 2013}}</ref><ref>{{cite web|title=Ocean Thermal Energy Conversion: Information Needs Assessment |url=http://coastalmanagement.noaa.gov/otec/docs/otecassessment.pdf|website=National Oceanic and Atmospheric Administration (NOAA) Office of Response and Restoration (ORR) and the Environmental Research Group at the University of New Hampshire (UNH)|access-date=27 March 2013}}</ref>

The [[Tethys (database)|Tethys database]] provides access to scientific literature and general information on the potential environmental effects of OTEC.<ref>{{cite web|title=Tethys |url=https://tethys.pnnl.gov/|website=Tethys |publisher=PNNL}}</ref>