Steady-state simulation and optimization of an air cooled chiller

Abstract

In this paper, we propose a rigorous model for representing the operation of an air cooled chiller of a large office building and some simple correlations for predicting global heat exchange coefficient and pressure drop of the evaporator. The proposed models and a simplified physical one that assumes constant evaporator's heat exchange coefficient and pressure drop are used to simulate and optimize the operation of the chiller. The rigorous model shows that large variations of the order of 10% can be observed in the prediction of global heat exchange coefficient of the evaporator. We also present correlations for predicting specific enthalpies and entropies of superheated R-410, capable of representing Chemour's tabulated values within a maximum error of 2,5% for the entire expected operating region of chillers for air conditioning systems. All models are programmed as non-linear programming problems and solved by a sequential quadratic programming algorithm. Simulation and optimization results show that all physical models are easily and rapidly solved and that the more rigorous model should be preferred to optimize the operation of the chiller and to adequately predict limiting operating points and off-design situations. CPU time for all simulations was below 0,3 s using an Intel Centrino Duo type processor. Optimizing the operational point by means of the rigorous model might enable significantly reducing energy consumption depending on the disturbances affecting the operation of the chiller (e.g. of the order of 7%). We further show that the simple correlations are capable of predicting the operation of the chiller within acceptable deviations (predicting errors in compressor's power are up to 3%) and might be used to preliminarily test advanced control applications. It is also shown that evaporator's pressure drop should not be neglected as is a common practice, since prediction errors in superheating might be of the order of 3 °C. Finally, the rigorous model should be used to analyze limiting operating points and the effect of changing manipulated variables on important process constraints like superheating.

Keywords

• Central chiller system
• Optimization
• Rigorous model
• Buildings