Effect of Shorter Compressor On/Off Cycle Times on A/C System Performance

Abstract

Effects of compressor on/off cycle times shorter than typical a/c heat exchanger thermal time constant (~1 min) were examined as a method of cooling capacity regulation. Short compressor on/off cycle times enable the use of heat exchangers continuously throughout entire off-cycle, thus reducing on-cycle temperature lift and increasing COP compared to conventional cycling. Experiments were done for a typical residential a/c system for run time fractions from 0.3 to 1.0, and cycle periods from 10 to 80 seconds. The objective was to understand how pulsed refrigerant flow from the compressor affects performance of the other components. Refrigerant side heat transfer resistance, pressure drop, and magnitude and curvature of heat exchanger surface temperature fluctuations were identified as the most influential factors affecting system performance. Virtually identical effects were observed for wet and dry coil experiments. Results at a run time fraction of 0.56 were compared to continuous (simulating variable speed) operation of a smaller compressor providing the same cooling capacity. Cycling the compressor at cycle periods of 10-80 sec degraded the performance of other components enough to reduce COP by approximately 2.5-7.5%, respectively. The corresponding COP loss for conventional long-cycling was 11.5%, all on the condenser side, because evaporator surface temperature was held constant to ensure that sensible and latent capacities were comparable. Comparisons of actual variable speed and cycling strategies must also account for parasitic power such as inverter losses and differences in fan and blower power.