Research on the external fluid mechanics of ocean thermal energy conversion plants : report covering experiments in a current

Abstract

This report describes a set of experiments in a physical model study to explore plume transport and recirculation potential for a range of generic Ocean Thermal Energy Conversion (OTEC) plant designs and ambient conditions. Tests were conducted in a thermally-stratified 12 m x 18 m x 0.6 m basin, at an undistorted length scale ratio of 1:300, which allowed the upper 180 m of the ocean to be studied. Conditions which have been tested include a range of plant sizes (nominally 200 MWe - 600 MWe); a range of discharge configurations (mixed vs. non-mixed evaporator and condenser flows, multiple vs. radial slot discharge port(s), variation of discharge-intake separation and variation of discharge angle); and a range of ambient current speeds (0.15 - 1.0 m/s), and density profiles (surface mixed layers of 31 to 64 m). The tests described herein complement those reported previously (Adams et al., 1979) for a stagnant-ambient environment.

Measurements included temperature, dye concentration and visual observations from still and motion pictures. Results derived from these measurements are presented in tables and graphs in prototype dimensions for direct use by OTEC designers. Many of the results are also analyzed and presented in non-dimensional terms to extend their generality. No significant recirculation was observed for any tests with a discharge directed with a vertical (downward) component. For tests with a horizontal discharge, recirculation was observed to be a complex function of a number of parameters. For sufficiently shallow discharge submergence, low to moderate current speeds, and with plants employing a radial slot discharge, recirculation could result from dynamic pressures caused by the proximity of the free surface - despite the negative plume buoyancy. This mode was labelled "confinement-induced" recirculation and led to measurements of direct recirculation ranging from 25% to 40%.

For certain combinations of ambient current speed and generally positive plume buoyancy (resultIng from deeper discharge submergence), the plume was observed to billow upward resulting in "current-induced" recirculation. This was observed for both radial slot and multiple port discharge configurations although somewhat greater recirculation was observed with the former configuration. Measured recirculation for current-induced recirculation fell in the range 0 to 10% with a peak occurring at intermediate current speeds of about 0.5 m/s. Experiments with a mixed evaporator and condenser discharge showed less tendency for direct recirculation of either type than the separate (evaporator only) discharges, but the effects of recirculation, as measured by the drop in evaporator intake temDerature (below the ambient temperature at the level of the intake) were not very different. A simple mathematical model, based on the governing length scales, was successfully calibrated to the observed values of direct recirculation for the radial discharge case.

Various measures of plume transport were summarized to help designers predict the impact of OTEC operation on the environment and to establish guidelines for spacing of multiple plants. Minimum near field dilutions were observed in the range between 5 and 10 indicating that the peak concentration of any chemicals contained in the discharge would be between 10 and 20% of the discharge concentration. Near field horizontal and vertical dimensions of the plume wake were found to be correlated with a length scale derived from discharge kinematic momentum flux and ambient current speed. The rise and fall of the equilibrium plume elevation (above or below the discharge elevation) was found to be governed by a ratio of length scales based on the ambient density profile and the discharge kinematic momentum and buoyancy fluxes.