Computer modeling of the energy performance of screw chillers

The energy crisis and mandatory energy conservation standards together with the advancement in computer technology led to rapid development of the energy calculation procedures for the prediction of the performance and energy requirements of buildings. DOE-2 is one of the major public domain programs, which is used throughout North America, however this program does not contain computer models for screw compressor chillers. The main objective of this study is to evaluate the energy performance of screw-compressor chillers, based on thermodynamic principles and parameter identification from the manufacturers' data. The thermodynamic model used in this study was based on the ASHRAE Toolkit-I for primary HVAC system energy calculations. Results showed that the screw compressor chillers perform almost similar to centrifugal chillers at off-design conditions under full load; however at part load regime, screw chillers perform completely different from centrifugal chillers. It was observed that unlike the centrifugal chillers, the screw chillers have the best performance at part load ratio about 0.7, and at low part load ratio the COP is significantly low. After the energy performance correlation for screw compressor chillers were derived, they were used in the DOE-2 program to compare the energy performance of an office building in Montreal with that of centrifugal chillers. Results showed that the electric demand of screw chiller is significantly higher. Meanwhile the monthly peak electric demand is almost constant throughout the year, while that of centrifugal chillers shows a significant variation from month to month. Beside the correlations for energy performance of screw compressor chiller based on the DOE-2 generic format, some other type of correlations were developed using the identified parameters, which can be used in the Fault Detection and Diagnostic (FDD) systems to check continuously the performance of chillers during the operation