Analysis of a commercial ice-storage system: Design principles and measured performance

Abstract

This paper summarizes an analysis of the measured performance of an ice-storage system in a large office building. The partial storage system was installed at a cost below a conventional cooling system when the building was initially constructed. The design analysis concluded that by reducing on-peak electric demand through shifting cooling energy use to nighttime operation, the system would save about $38 000 per year in electricity costs. During 1987, savings were roughly 10% of this estimate. Reasons for the discrepancy include the inability of the system to deliver the design cooling load, and poor operating and maintenance practices. The assessment is based upon monitoring of the cooling load, compressor electricity use, chilled-water pump electricity use, and whole-building electricity use, each measured at 15-minute intervals. A primary objective of the monitoring project was to compare electricity costs of the actual system to what they would have been with a conventional cooling system, without storage. We estimated the hourly electricity use of the base case conventional system using the measured hourly cooling load data and a part-load efficiency curve for a 1617 kW t (460 ton) centrifugal chiller. Five months of cooling load data were available (February - June). The maximum electric demand shift was 150 kW e (6.1 W e/m 2). The average monthly peak shift was 95 kW e. Over these five months the system COP decreased from 2.82 to 1.74, while the estimated COP for the conventional system ranged from 2.84 to 2.75. Since the analysis period, the system has undergone major retrofitting to alleviate the occasional overheating and further reduce electricity costs.