Oceanographic Parameters to Explore the Environmental Impacts of OTEC Installations

Before field trials of ocean thermal technology (OTEC) become operational, researchers need a solid understanding of the environmental impacts of these installations. Due to the large thermal gradient and irregular bathymetry off the coast of Hawaii, the archipelago has several potential OTEC sites in the works. A pilot plant is under construction south of Barber’s Point, Oahu, and a commercial plant may be constructed off of Kahe Point, Oahu. A Final Environmental Impact Statement was conducted in 1981 for the Barber’s Point site, but this report needs to be brought up to current oceanographic and engineering standards. Comfort and Vega (2011) suggest a protocol for environmental baseline monitoring, which focuses on ten chemical oceanographic parameters, and addresses existing gaps in knowledge of ecology and oceanography near the two OTEC sites. In the operation of an OTEC plant, seawater intake pipes draw warm water from a depth of 20 m and cold water from a depth of approximately 1000 m. The water masses are mixed and discharged at 60 m or deeper. An environmental impact analysis can help to determine the optimal mixed seawater discharge level. —Meredith Reisfield
Comfort CM, Vega L. 2011. Environmental Assessment of Ocean Thermal Energy Conversion in Hawaii. Hawaii National Marine Renewable Energy Center, Hawaii Natural Energy Institute, University of Hawaii at Manoa, Honolulu, HI, p 1-8.

During the operation of an OTEC plant, large water masses are redistributed. A 5 MW OTEC plant requires 25 m3/s of both cold and warm water flow. The daily flow tops 2 million cubic meters of water. The redistribution of large volumes of water could significantly impact an ecosystem, affecting primary production, nutrient concentrations, and densities of larval fish and other plankton. Comfort and Vega propose using existing data sets as baseline environmental information for OTEC. They recommend an additional year-long, directed, baseline monitoring program to address gaps in existing knowledge. Fortunately, as researchers at the Hawaii Natural Energy Institute acknowledge, significant data are already available to describe current circulation, and oceanographic parameters such as temperature, salinity, nutrients, and primary production off the coast of Oahu. The OTEC plume’s trajectory has also been modeled. OTEC operation raises several concerns about biological impacts. The redistribution of water on a large scale will affect the temperature stratification, salinity, oxygen and nutrient levels near the site. A primary concern in OTEC installations is the potential for upwelled nutrients to fertilize surface waters and prompt phytoplankton blooms. To avoid altering the primary productivity in surrounding waters, it is crucial that the plumes discharged from OTEC facilities settle at a sufficiently low depth so that the potential for functional biomass increase is reduced. Small organisms, including plankton and fish larva, can easily be trapped in the intake pipes with high mortality due to sudden and significant temperature and pressure changes. Many organisms migrate vertically throughout the water column on a daily basis, so understanding which organisms may be entrapped requires further knowledge of the ecosystem. Since floating objects in the ocean tend to accumulate large groups of fish and seabirds, larger organisms are likely to interact with the OTEC installations.  These organisms are less likely to be entrained in the system due to their larger size and swimming abilities which will allow them to easily manage the current flow. Vibration of the deep water pipe will create a signal that could be detected by marine mammals and fish, creating a risk of disruptions in marine mammal communication and navigation. The researchers propose additional monitoring of seasonal oceanographic parameters at relevant locations, further plankton sampling across multiple depths and time periods, and acoustical monitoring at the installation site to quantify baseline noise levels both before and after installation of the OTEC facility. Comfort and Vega note that given the wide availability of current data, gaps in knowledge could be quickly and efficiently addressed with one year of directed baseline monitoring. Studying the effects of an OTEC facility in operation with sufficient baseline data, rather than simply modeling these outcomes, could ensure that commercial OTEC plants have a minimal environmental impact. 

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