Ocean Thermal Energy Conversion (OTEC) is a new technology that could play an important part in the future of renewable energy sources. It takes advantage of the natural temperature gradient, between warm water at the surface and cold water at depth, which exists all around the world. Governments have been interested in the potential of OTEC technology as a cost effective means of generating clean, renewable electricity.
One of the major problems that stands in the way of the success of OTEC, is the unknown ecological impact of pumping vast amounts of nutrient-rich water from the depths of the ocean up to the surface. It has been estimated that a small OTEC plant of just 5MW capacity would have a daily flow of more than two million cubic metres of water; what a 2011 environmental report described as “an unprecedented environmental modification that must be rigorously evaluated.”
The main concern is the potential for huge phytoplankton blooms as the nutrient-rich deep sea water mixes with the warm water near the surface.
The Department of Energy (DoE) recently published a report on a study by Makai Ocean Engineering which found that the effects of a 100MW OTEC plant in the waters off the coast of O’ahu, Hawaii, would not have any significant impact on the population and reproduction of phytoplankton.
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The report stated that Makai, a veteran of more than 30 years in the field of OTEC research, created situations to simulate the “biochemical effects of the nutrient-enhanced seawater plumes that are discharged by one or several 100 megawatt (MW) OTEC plants.”
Makai created a model which consisted of “four separate ducts discharging a total combined flow rate of 420 cubic meters/second of warm water and 320 m3/s of cold water in a mixed discharge.”
Makai explained in its results that, “as the nitrate is advected and dispersed downstream, only a fraction of the deep ocean nutrients (< 0.5 umol/kg perturbation) mix upward where they are utilized by the ambient phytoplankton population.”
“This nutrient perturbation causes a phytoplankton perturbation of approximately 1 mgC/m3 (~10% of average ambient concentrations) that covers an area 10×5 km in size at the 70 to 90m depth. Thus, the perturbations are well within the natural variability of the system, generally corresponding to a 10 to 15% increase above the average pico-phytoplankton biomass.”
By. Charles Kennedy of Oilprice.com