Multivariate diagnosis analysis for chiller system for improving energy performance

The operating status of a large chiller system in buildings with central air-conditioning could vary at a given cooling demand and ambient condition. This study investigates the potential change of operating statuses and its influence on the system coefficient of performance (COP)-cooling energy output divided by the total electric power input. The system studied comprised four large and one small chillers. A set of primary chilled water pumps operated in different numbers to deliver the required flow rate of chilled water to the operating chillers. The actual flow rate to meet the building cooling demand was regulated by the secondary chilled water pumps. A set of evaporative cooling towers was installed to match the heat rejection required for the chillers. Comprehensive operating data were collected at 15-min intervals in 8 months. 18.46% of the total operating conditions were transitional statuses and unconventional operating statuses by switching on or off one extra cooling tower. This led to a 33.9-64.9% drop in individual system COPs. An examination of the variation of system load ratios and ambient wet bulb temperatures showed that the system COP was higher at the pair-up operation of system components even at higher wet bulb temperatures. A survival analysis indicated that the transitional statuses depended mainly on the temperature of chilled water returning to the evaporators and the temperatures of cooling water entering and leaving the condensers. Minimizing unnecessary transitional statuses and restoring the pair-up operation of system components could save electricity by 2.72%.