Physical and numerical modeling of the external fluid mechanics of OTEC pilot plants

Abstract

This study examined the near field external fluid mechanics of symmetrical OTEC pilot plant designs (20-80 MWe) under realistic deep water conditions. The objective was to assess the environmental impact of different plant configurations and to determine if pilot plants can be expected to operate without degrading the thermal resource available for power production. Physical modeling studies were conducted to investigate the variation of near field plume dynamics and the sensitivity of recirculation to different pilot plant designs. Experiments were conducted in a thermally stratified 12m x 18m x 0.6m basin, at an undistorted length scale ratio of 1:300, which allowed the upper 170m of the ocean to be studied. Measurements included temperature, dye concentration and visual observation from photographs. Both mixed and non-mixed discharge concepts were investigated. Discharge port design included two, four or eight discrete circular ports, with significant variations in the MWe/port ratio, issuing either horizontally or vertically. A range of ambient uniform current speeds was investigated while an ambient density profile, representative of potential sites off of Hawaii and Puerto Rico, was chosen.

A previously calibrated integral jet model (Hirst, 1971a) was tested against experimental observation to develop a valid, predictive tool that would facilitate study of conditions that were not modeled with the present experimental set-up. The model was modified to more accurately represent the dynamics of the OTEC discharge in the near field. Major modifications included adjustment of the equations that characterized the starting length (length of ZOFE); introduction of jet deflection in the ZOFE; introduction of a lateral spreading formulation that allowed the "squeezing" effects of the ambient stratification to be simulated; and introduction of an aspect factor, which accounted for interaction of a number of closely spaced vertical jets issuing from a circular array. Overall agreement between prediction and observation was quite good. The potential environmental impact of the discharge plume from an OTEC plant over a broad range of realistic conditions was assessed through additional sensitivity simulations.

Results indicate that little recirculation occurs for the designs considered in this study. The recirculation that does occur appears to be the result of plume upwash in the lee of the plant and, possibly, internal wake effects on the plant bow. Environmental impact is argued to be proportional to the degree of perturbation caused by the OTEC discharge to the upper mixed layer. For the conditions considered in the sensitivity study the OTEC plume remained below the upper mixed layer except for the largest layer depths considered (H~ 100m). These larger depths are near the maximum values reported for either Hawaii or Puerto Rico and represent the only conditions where significant perturbations may be likely.