Shifting to renewable energy is an important global challenge, and there are many technologies available to help reduce carbon dioxide emissions. Seawater air conditioning (SWAC) is a renewable ocean thermal energy technology that will soon be implemented in Honolulu, Hawaii, on the island of Oahu. The SWAC system will operate by using cool water from 500 m depth in a heat exchange system and then will release this nutrient-rich water back into the ocean at a shallower depth of 100–140 m. The introduction of a plume of warmed (but still relatively cool) deep seawater has unknown impacts on the tropical marine environment. Possible impacts include increases in primary production, changes in water chemistry and turbidity, and changes in the local food web. We used moored instruments and shipboard profiling to describe oceanographic parameters at the future SWAC effluent site. Parameters varied with the M2 internal tide, and denser water was correlated with higher nitrate, lower oxygen, and lower chlorophyll a (correlation coefficients 0.55, –0.58, and –0.75, respectively). The nitrate concentrations in the plume will be >30.0 µmol kg–1, while ambient concentrations range from <2.0–9.8 µmol kg–1. Irradiance levels at the effluent depth are sufficient to support net photosynthesis, and the effluent’s location in the pycnocline could lead to rapid horizontal advection of the plume and expansion of the spatial scale of impacts. These baseline data provide an understanding of pre-impact conditions at the future SWAC site and will enable a more accurate environmental assessment. A comprehensive and well-resolved environmental monitoring effort during SWAC operation will be necessary to quantify and understand these impacts.

Comfort, C.M., M.A. McManus, S.J. Clark, D.M. Karl, and C.E. Ostrander. 2015. Environmental properties of coastal waters in Mamala Bay, Oahu, Hawaii, at the future site of a seawater air conditioning outfall. Oceanography 28(2):230–239, https://doi.org/10.5670/oceanog.2015.46.

» Detailed Methodology (92 KB pdf)
Specifics of moored and profiling instruments and programming.

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