shear currents on the cold-water pipe. With regard to seasonal variations, estimates based on equation 1 and the annual cycle of temperature suggest that a 20 percent variation in power output can be expected with a 1,000-m pipe at Hawaii over the course of the year (Rand Corporation, 1980; Cohen, 1982). Even more dramatic changes result from the SST fluctuations due to El Niño or La Niña in the central tropical Pacific, where the committee estimates variations in power production as high as 50 percent. The assessment group largely fails to address the temporal variability issue. The GIS database would be of much more use if it included at least monthly resolution, which for the present 2-year run would at least allow evaluation of specific El Niño or La Niña conditions that are important for OTEC in the tropical Pacific. It would also be useful to have some measure of internal tidal displacements, if only for high-priority sites like Hawaii.
Given the substantial seawater requirements of OTEC plants, the number and spatial density of plants would be a major consideration when considering available power. Plants need to be scaled and designed to minimize their own back effects so they do not adversely affect the locally available temperature contrast. There will also be a maximum plant spatial density beyond which plant discharges would begin to interfere with one another. At regional and global scales there could be a variety of impacts on the ocean arising from widespread deployment of OTEC. Since OTEC is essentially a mixing process, promoting the flux of heat down the vertical temperature gradient, massive deployment of OTEC could actually enhance thermohaline circulation. The potential impacts of these effects, such as decreased tropical surface temperatures or increased primary productivity due to an influx of nutrients from deep cold water, would require careful modeling and would remain speculative until actual plant operations commenced.
Instead of looking at plant spacing issues or the size of individual plants, the OTEC assessment group focused on the supply of cold water as the resource limit. They used the flow speeds at the depth of the cold-water pipe in the HYCOM model to estimate possible plant densities. The size of the ultimate resource available with massive deployment of OTEC plants is a highly speculative question worthy of significant study on its own. The assessment group chose to adopt a figure from the literature (Nihous, 2007b) that was developed by assuming that the net cold water upwelling from all OTEC plants would not be too large a fraction of the net thermohaline overturning circulation. The volume of cold water required by the plant was met by a specified change in the deep layer thickness, which was adjusted to meet the Nihous global estimate of OTEC potential. However, this assumes that the cold-water supply is limited in the ocean, an idea that is not universally accepted. Most modern