A Case Study on Staging Mixed-Age Computer Room Air Conditioning Systems Considering Performance Degradation

Computer room air conditioning (CRAC) systems are commonly used in small to medium-sized data centers due to their straightforward installation and control. A key operational challenge throughout a CRAC system's life cycle is the degradation of its coefficient of performance (COP). Conventional data center energy models often assume new or homogeneous equipment operating conditions. However, in data centers with multi-unit CRAC configurations, factors such as differing equipment ages, varied maintenance schedules, and individual unit faults lead to heterogeneous performance degradation. In practice, this heterogeneous nature leads to a significant yet often overlooked opportunity for energy optimization. This paper presents a case study of a four-unit CRAC system exhibiting degradation to demonstrate an optimal staging control strategy that explicitly accounts for heterogeneous COP conditions. A dynamic testbed model was developed in Modelica, incorporating a rule-based state graph control that prioritizes the use of the most efficient CRAC units. The proposed COP degradation-aware control was benchmarked against a traditional uniform part load ratio (PLR) staging approach, showing a 4.74% reduction in total cooling energy consumption. These results highlight the importance of considering real-world degradation effects when designing advanced staging controls to enhance the energy efficiency of aging data center facilities.