Abstract
Variability of the water mass structure north of St. Croix in the Virgin Islands Basin was observed during a 2.5 month study of corrosion and biofouling on OTEC heat exchanger performance in the summer of 1977. Daily STD profiles and weekly hydrocasts were taken of the upper 1500m from a Tracor Marine barge moored 15 km north of St. Croix in 3600m water depth. The largest temporal fluctuation in water properties occurred ín the Tropical Surface Waters of the upper 100m due primariìy to advection of this spatially inhomogeneous water mass past the moor. Currents in the upper layer were also highly variable with speeds ranging from 0 to 50 cm/sec and numerous directíon reversals. Subsurface currents appeared to be more steady and toward the west at 10 to 15 cm/sec.
The waters used in the heat exchanger test were pumped continuously from the Tropical Surface Waters at a depth of 20m, which is within the surface mixed layer defined by temperature but at the base of the surface salinity mixed layer. Intake salinity variations of 1.7°/oo over a one-month period were coherent with similar changes in the upper 60m of TSW. Variation in water properties below the Tropical Surface Water was small. The mean and ranges of temperature and salinity at 1000m were only 5.4 +0.5°C and 35.0 +0.06°/oo, respectively. Temperature of the surface mixed waters was also quite steady with a total range of only 0.9°C from 27.8 to 28.7°C during the experiment. The thermal resource available for OTEC power plants defined as the vertical temperature difference AT between the surface mixed waters and subsurface water averaged 23°C at a depth of 1000m with a standard deviation of +0.2°C. The depth to reach a ∆T of 20°C varied from a minimum of 660m to a maximum of 740n. Historical data indicate that the maximum depth to reach a ∆T of 20°C would occur in the winter and would not exceed 956m. Thus, from thermal resource considerations, the waters north of St. Croix are considered an excellent location for an OTEC site.