## 1 Introduction
### Conventional and Optimal Design of a Chiller Plant  
The size and configuration of a chiller plant are two of the most important influencing factors of energy performance, which determine whether the HVAC system could operate within its high efficient region or not [3]. The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) handbook [4] introduced a conventional chiller plant design method, which mainly focused on how to fulfill the peak cooling load at a rated operating point. In order to determine the size of a chiller plant, two typical methods for estimating the building cooling load are suggested as follows [5]:  
1. The cooling load of a typical design day is determined according to the statistic historical outdoor weather conditions and maximum boundaries.
2. Simulation software such as EnergyPlus [6] and TRNSYS [7] are employed to generate the annual cooling load distribution according to typical meteorological year (TMY) data, behaviors of occupants, schedules of plug-in appliances, lightings, etc.  
Based on the above-mentioned methods, the determination process of chiller plant cooling capacity could be considered as a deterministic model based on simulation [8]. In fact, there are many inevitable differences between the actual operating parameters (e.g., weather conditions, internal heat gains, and indoor occupants) and those used in the design calculation (usually referring as uncertainties), which will result in a large deviation between the actual cooling load and the design cooling load [9]. Generally, designers usually prefer to size the chiller plant with a larger cooling capacity than the maximum cooling load (i.e., adding a safety factor) to make sure that the chiller plant could supply sufficient cooling capacity in satisfying the cooling demand under a series of cooling load scenarios for safety consideration [10; 11]. This might easily result in oversizing problems, which reflect as that chiller plant frequently operates within its low efficient region, and thus a large amount of energy is wasted [12; 13; 14].  
Since full load conditions (i.e., peak load) occupy a very small proportion of the whole cooling season, some studies have proposed that more attention should be paid to part load conditions, which also requires the chiller plant to serve at its high efficient region [9]. A chiller plant consisting of multiple chillers is considered as an effective option to improve efficiency by combining the optimal sequence control during operation [15; 16; 17; 18]. When the actual cooling load is less than the peak cooling load, some chillers could be switched off and then the operating chillers can operate at a relatively high part load ratio (PLR) within their high efficient regions. The use of high efficiency chillers is considered as another effective way to ensure the operational performance of a chiller plant at high levels [19]. For instance, variable-speed chillers that have good performance characteristics even under very low part load conditions are recommended to be employed when the chiller plant operates at part load conditions frequently [20]. In addition, the optimization of chiller plants can also be achieved by integrating hybrid chillers with different types of compressors and/or driven by different energy sources, so that all operating chillers can operate within their optimal loading ranges [21].