Oil-free centrifugal chiller optimal operation

Abstract

Centrifugal chillers, using variable-speed turbo compressors with magnetic bearings, are becoming very com- mon in Heating, Ventilation and Air Conditioning (HVAC) systems. Such solution guarantees superior energy efficiency, mostly under part load conditions, compared with traditional equipments, and it provides additional advantages such as light weight and a compact package. On the other hand, turbo machinery adds its own complexity to the whole HVAC system and its efficient management is a non-trivial task. In this paper a hybrid optimisation technique is employed to determine optimal operation, under various working conditions, for air-condensed water centrifugal chillers. The proposed method provides opti- mal solutions using a combination of two algorithms: A random population-based optimiser, the Gravitational Search Algorithm (GSA), followed by the deterministic Levenberg-Marquardt (LM) algorithm. The hybrid method effectively overcomes the problem of high sensitivity to initial conditions of LM tech- nique and a shortcoming of GSA which reduces its searching efficiency when close to the optimum. The hybrid method has been tested in a Matlab ® -based simulation environment where the performance of an air-condensed centrifugal chiller is adequately evaluated. Simulation results guarantee high energy efficiency in a wide range of chiller working conditions. I. INTRODUCTION In HVAC chiller systems the application and integration of magnetic bearings, coupled with variable speed centrifugal compressor, yield significant advantages as compared to conventional compressor technology (such as reciprocating, scroll, or screw). This solution guarantees significant reduc- tions in energy consumption and environmental emissions associated with energy production. At full load, compressor efficiency is similar to that of the finest screw compressors, whereas at part load, the coupling with a variable frequency drive ensures the highest available efficiency. This is an intrinsic characteristic of centrifugal compressors and it can save up to 30% of energy costs as compared to other types of compressors. Moreover, infinitely variable unloading can be delivered from the full 100% capacity down to 20%, a value representing the typical load. The compressor integrated variable frequency drive provides unmatched flex- ibility for off-peak operation. Inlet guide vanes also provide stable operation at light loads or high head conditions, further improving machine unloading capability. This type of turbo machinery provides additional advantages such as light weight and a compact package, improved reliability and reduced maintenance through the elimination of wear, oil and oil management systems, low noise with virtually no