Environmental Engineering Reference
In-Depth Information
Effects on Ocean Ecosystems
Impacts of construction of an OTEC facility will depend on whether the project is
located onshore or offshore. An offshore facility would require the installation of
large, long water conduits on the seabed to access deep water. Alternatively, OTEC
projects located on offshore platforms would depend on subsea cables to transfer
electricity to shore. The installation and maintenance of pipelines and electrical
cables would disturb bottom habitats and generate EMFs. Structures could become
colonized with marine organisms and attract fish. Depending on the location of the
warm water intake and discharges, these fish might be more susceptible to entrain-
ment, impingement, or contact with the discharge plume.
The potential environmental effects of OTEC operation have been considered
by a number of authors (Abbasi and Abbasi, 2000; Harrison, 1987; Holdren et al.,
1980; Myers et al., 1986; Pelc and Fujita, 2002). Myers et al. (1986) provided the
most comprehensive assessment of the possible effects on the marine environment
resulting from operation of the types of OTEC facilities that were contemplated in
the early 1980s. Most of the likely effects were expected to be physical and chemi-
cal changes in the ocean surface waters arising from the transfer of large volumes
of cool, deep water. Abbasi and Abbasi (2000) suggested that OTEC plants will
displace about 4 m
3
/s of water per megawatt of electrify output from both the
surface layer and the deep ocean layer, and then discharge the water at some inter-
mediate depth. The warm water intake would be located at a depth of about 10 to
20 m, and the cold water intake might extend to a depth of 750 to 1000 m (Myers
et al., 1986). The large transfer of water may disturb the thermal structure of the
ocean near the plant, change salinity gradients, and change the amounts of dis-
solved gases, dissolved minerals, and turbidity. The transfer will result in an artifi-
cial upwelling of nutrient-rich deep water, which may increase marine productivity
in the area. The stimulation of marine productivity may be especially strong in
tropical waters, where nutrient levels are often low, and could have detrimental
effects on nearby sensitive habitats such as coral reefs. Moreover, carbon dioxide
will also be released when the deep water is warmed and subjected to lower pres-
sures at the surface. The possible amounts of carbon dioxide released have not
been rigorously quantified; some estimate that the quantities will be minute (Pelc
and Fujita, 2002), and others suggest that the contribution will be relatively large
(Holdren et al., 1980). The relatively high carbon dioxide and low dissolved oxygen
content of the deep water may alter pH and dissolved oxygen concentrations in a
surface mixing zone.
The large heat exchangers will have to be treated with biocides (e.g., chlorine or
hypochlorite) in order to prevent the growth of bacterial slimes and other biofouling
organisms; volumes of biocides would be proportional to the large volume of heating
and cooling water. Degradation of the heat exchanger materials will result in chronic
releases of metals (e.g., copper, nickel, aluminum). Accidental release of the working
fluid that is evaporated and condensed to drive the turbine could have toxic effects.
The potential for acute and chronic toxicity and bioaccumulation of metals from
deep ocean water will have to be considered (Fast et al., 1990).
