Thermodynamic Modeling and Optimization of a Screw Compressor Chiller and Cooling Tower System

Abstract

This thesis presents a thermodynamic model for a screw chiller and cooling tower system for the purpose of developing an optimized control algorithm for the chiller plant. The thermodynamic chiller model is drawn from the thermodynamic models developed by Gordon and Ng (1996). However, the entropy production in the compressor is empirically related to the pressure difference measured across the compressor. The thermodynamic cooling tower model is the Baker & Shryock cooling tower model that is presented in ASHRAE Handbook - HVAC Systems and Equipment (1992). The models are coupled to form a chiller plant model which can be used to determine the optimal performance. Two correlations are then required to optimize the system: a wet-bulb/setpoint correlation and a fan speed/pump speed correlation. Using these correlations, a "quasi-optimal" operation can be achieved which will save 17% of the energy consumed by the chiller plant.