Experimental investigation on variable speed operation of air conditioning

Abstract

Paper presented at the 9th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Malta, 16-18 July, 2012.

Air conditioning installations are typically designed with enough cooling capacity to satisfy the desired minimum temperature under the heaviest load. Since heat loads tend to be less than the maximum designed loads, the system will most often be working under part-load rather than full-load conditions. Operating at these part-load conditions at fixed capacity will be more expensive than if the capacity were able to match the required load. Varying capacity air conditioning systems, which make use of inverter technology, have been developed in order to track the required cooling load more closely. These high quality air conditioners are advertised as consuming around 30 per cent less energy than conventional systems. This experimental investigation looked into the steady state performance and start-up power requirements of an inverter driven refrigeration unit versus fixed speed operation. A three phase (745 W motor) air conditioning laboratory setup was used in direct-on-line (fixed speed) mode and also with a variable frequency drive (inverter). The results obtained showed that the cooling coefficient of performance increased as the operating frequency was reduced. This means that an improved performance can be achieved at lower operating frequencies. A reduced start-up power was required for the inverter driven system, however the additional power requirements for the inverter resulted in the mains driven system being more efficient at full-load conditions. The improved coefficient of performance at lower frequencies resulted in an improved performance for the inverter driven system when compared to a conventional system at part-load conditions.