Optimal operational efficiency of chillers using oil-free centrifugal compressors.

In chiller design, oil-free variable-speed centrifugal compressors are becoming increasingly popular. However, the management of systems equipped with this kind of compressor is a non-trivial task. This work focuses on the efficient operation of variable-speed air-condensed chillers with variable-speed centrifugal compressors paired with (oil-free) magnetic bearings. Multiple operating conditions, at any moment in time, together with wide cooling ranges and potentially high energy efficiencies during off-peak demands create the need for an open-loop energy optimisation strategy via an efficiency-based fitness function. The physical variables of this function and their constraints are discussed including several variable dependencies. The problem of devising strategies for improving chillers' efficiency is here formulated as a model-based optimisation and it is solved by means of an ad hoc hybrid algorithm which combines a deterministic method and stochastic one. The results of simulations, which are based on two chiller layouts, show the potential of the proposed approach. [ABSTRACT FROM AUTHOR]