Modeling and Optimization of the Cooling Tower under Unsaturated Outlet Air Conditions

Abstract

This thesis presents a comprehensive methodology for engineers and plant managers to use for optimizing the combined chiller power and cooling tower fan power for cooling systems without the traditional assumption that the exiting air is saturated. The most widely accepted and utilized classical models are the Merkel model and the Poppe model. However, the former is somewhat inaccurate and the latter has significant computational burdens. Thus, this work uses the Braun model supplemented with simple field measurements to provide an accurate procedure for optimizing combined cooling tower and chiller performance that is suitable for field use.

The effectiveness cooling tower model, developed by Braun (1988), is utilized to describe the cooling tower performance with supplemented with field measurements. The chiller model presented in the EnergyPlus Engineering Reference book based on the condenser entering temperature is applied. The models are coupled to determine the optimal operation for the cooling system. To optimize the system, correlations between the power consumption of chillers and cooling tower fans and approach temperatures are investigated and regressed. The performance curves based on these correlations can help to achieve a “near-optimal” operation over a range of operating conditions. By doing so, an optimized approach temperature and temperature range is given and evaluated at over a range of annual operating condition. The impact of the wet-bulb temperature is also explored. The procedure developed predicts that about 15% of the measured energy consumption of the chillers and cooling tower fans in the Connally Building chiller plant at the Texas A&M University System would be saved if the optimal control in this study were applied to the 68 days across 12 months that were analyzed in this thesis.