SACTI

SACTI (Seasonal and Annual Cooling Tower Impact) is the climatological version of the ANL/UI model. It contains an algorithm for the cooling tower plume shadow calculation. The shadow calculation is important because an increase of the shadow lasting on a certain region could have negative effects on farming, photovoltaic power plants and recreational areas (beaches, parks, etc).

SACTI contains a pre-processor that builds the model meteorological classes. Each hour of meteorology is attributed to a certain meteorological class. Classes are built basing on parameters like stability classes and ratio between plume exit velocity and wind speed. This choice assures that plumes of the same class have similar shape and, at the same time, it assures that the number of classes per wind direction is low enough (about 35 categories per each wind direction sector). The plume shadow is calculated using the wind direction and the sun height at a specific hour of the day. The model assumes that the plume is a cone frustum, where the minor basis has radius equal to the tower radius and the major basis has radius equal to the maximum radius of the visible plume. The ground all around the tower is separated in circular sectors with specific width and length. If the sector center is under the shadow, the model assumes that the all sector is under the shadow.

Compared to other models, SACTI model does not assume an opaque plume, but it assumes that a certain quantity of direct solar radiation could pass through the plume. SACTI determines also the fraction of direct solar radiation absorbed by the plume, which is a function of the plume thickness.

SACTI applications to simulate hypothetical NDCT (Natural Draft Cooling Tower) plants have shown a non-relevant reduction of the energy density at ground. Major effects are always observed close to the source, in general in regions located inside the plant. The model has shown shadow effects greater for MDCT (Mechanical Draft Cooling Tower) than for NDCT. This is due to the fact that MDCTs, even if smaller, produce a visible plume closer to the ground. The SACTI model has been validated using laboratory and field data.

SACTI allows to calculate the main aspects associated to the atmospheric impact of cooling towers:

  • drift dispersion and deposition
  • formation of the visible plume
  • shadowing induced by the visibile plume
  • reduction of the solar radiation below the visible plume
  • induced fog formation
  • induced ice formation

SACTI is composed by four modules which must be launched in sequence:

  • The first module (PREP) reads the hourly meteorological data, analyse them together with the release conditions, then selects a limited number of meteorological categories to simulate.
  • The second module (MULT) simulates the atmospheric dispersion of the vapour plume and of the drift drops.
  • The third module (TABLES) analyses the results of the second module together with the input information in order to determine the environmental impacts.
  • Finally the fourth module (PAGEPLOT) is used to prepare the plots of the results.

Additional information may be found in:

  • Carhart R.A. and Policastro A.J. (1991) A second-generation model for cooling tower plume rise and dispersion I. Single sources. Atmospheric Environment, Vol. 25A, N. 8, 1559-1576.
  • Carhart R.A., Policastro A.J. and Dunn W.E. (1992) An improved method for predicting seasonal and annual shadowing from cooling tower plumes. Atmospheric Environment, Vol. 26A, N. 15, 2845-2852.
  • Policastro A.J., Dunn W.E. and Carhart R.A. (1994) A model for seasonal and annual cooling tower impacts. Atmospheric Environment, Vol.28, N. 3, 379-395.