3.1.4. Environmental impacts
Though fairly benign in environmental impact compared to traditional power plants, OTEC poses some potential environmental threats, especially if implemented on a large scale. Data from existing electric generating stations on the coast provide insight into possible impacts of OTEC plants. These stations impact the surrounding marine environment mainly through heating the water, the release of toxic chemicals, impingement of organisms on intake screens, and entrainment of small organisms by intake pipes, all of which are concerns for OTEC. Large discharges of mixed warm and cold water would be released near the surface, creating a plume of sinking cool water. The continual use of warm surface water and cold deepwater may, over long periods of time, lead to slight warming at depth and cooling at the surface [6]. Thermal effects may be significant, as local temperature changes of only 3–4°C are known to cause high mortality among corals and fishes. Aside from mortality, other effects such as reduced hatching success of eggs and developmental inhibition of larvae, which lower reproductive success, may result from thermal changes [14]. Increased nutrient loading resulting from the discharge of upwelled water could also negatively impact naturally low-nutrient ecosystems typical of tropical seas.
Toxic chemicals, such as ammonia and chlorine, may enter the environment from an OTEC plant and kill local marine organisms. Ammonia in closed-cycle systems would be designed not to contact the environment, and a dangerous release would be expected to result only from serious malfunction such as a major breakdown, collision with a ship, a greater than 100-yr storm, terrorism, or major human error [6]. The impact of chlorine will likely be minimal, as it would be used at a concentration of approximately 0.02 ppm daily average, while the EPA standard for marine water requires levels lower than 0.1 ppm [6].
Impingement of large organisms and entrainment of small organisms has been responsible for the greatest mortality of marine organisms at coastal power plants thus far [14]. The magnitude of this problem depends on the location and size of the plant; however, if marine life is attracted to OTEC plants by the higher nutrient concentrations in the upwelled cold water, large numbers of organisms, including larvae or juveniles, could be killed by impingement or entrainment. For floating plants, victims of impingement would be mainly small fish, jellyfish, and pelagic invertebrates, while for land-based plants crustaceans would be the most affected [6].
Finally, a small amount of CO2 is released to the atmosphere by OTEC power generation. Bringing deepwater to the surface where pressure is lower allows some of the sequestered CO2 in this deepwater to outgas, especially as the water is warmed, reducing the solubility of CO2. However, this carbon emission is very minute compared to the emissions of fossil fuel plants.
OTEC could significantly improve quality of life in SIDS, where the current cost of power is at a premium and the benefits of desalinated water, mariculture and air conditioning would have a major impact. Further research into environmental impacts is necessary, but if the technology is shown to be benign, the development of OTEC for SIDS should be a priority. Plants in developed tropical sites that face high power prices should also be encouraged, if appropriate sites at which environmental damage will be negligible can be found. Because the governments of the SIDS that would benefit most from OTEC cannot afford such a high capital investment, governments of developed states should contribute to the research effort and investment for OTEC in developing countries. Appropriate measures should be taken to control environmental impacts including:
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Refraining from siting OTEC plants in sensitive areas including prime fishing grounds, spawning areas, and sensitive reef habitats.
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Making use of discharge for ancillary benefits, which prevents discharges from altering local water temperature significantly.
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Carefully regulating the use of toxins such as ammonia and chlorine, and avoiding coating the plants with toxic hull coatings used on ships in harbors which are known to pollute the waters.
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Relying mainly on relatively small plants. While there may be economic benefits to scaling up, large-scale plants are more likely to damage a local community through discharge or impingement/entrainment. Also, benefits from economies of scale are likely to dwindle at the 50 MW scale [15]. Similarly, if several small OTEC plants are used these plants must be suitably spaced to prevent altering local ecology too significantly at any one site [6].