Improving the energy efficiency of a control cabinet air conditioner through the use of variable refrigerant flow capacity control

Abstract

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

Control cabinet air conditioners are used to regulate the temperature of an enclosure containing electrical equipment. Normally, these air conditioners are selected for worst-case conditions, and work on an on/off control. This paper describes the work carried out to analyse the performance of an air conditioner (AC) with variable refrigerant flow (VRF) for a masters degree dissertation [1]. The performance of a standard on/off air conditioner was first measured. The implementation of VRF in a control cabinet was carried out successfully by installing a variable-speed compressor within a standard AC unit. Experiments performed showed that the energy savings are 14% at full load and between 8 and 32% at part load. For most conditions, the enclosure temperature could be controlled to a stable value with a flat enclosure temperature profile. Maintaining a stable enclosure temperature reduces electronic component failure. A computer model was created using Microsoft Visual Basic for Applications, which could be used within Microsoft EXCEL. For a given set of ambient temperatures and enclosure loads, the model estimates the power consumption of a standard AC and a VRF AC and calculates the potential savings. When applied to various scenarios, savings of 18-25% were achieved. The system efficiency can be improved further by other changes to the AC design. A mathematical software model of the AC was built using Visual Basic Express 2005, to evaluate these potential improvements. It was shown that the COP could be improved by increasing the air-flows and by controlling evaporator superheat. By using an electronic expansion valve, the degree of superheat could be accurately controlled. Changes in refrigerant charge were found to have more effect at high ambient temperatures, with the cooling capacity being maximised with only small changes in power consumption.