Steady-State Performance Prediction for a Variable Speed Direct Expansion Air Conditioning System Using a White-Box Based Modeling Approach

Abstract

When using a certain type of Heating, Ventilation & Air Conditioning (HVAC) systems, it is primary to obtain their steady-state operating behaviors for achieving a better indoor thermal environment. This paper reports a development of a white-box-based dynamic model for a direct expansion (DX) air conditioning (A/C) system to predict its steady-state operating performance under variable speed operation. The established model consists of five sub-models, i.e., a compressor, an electronic expansion valve, an evaporator, a condenser and a conditioned space. Each sub-model was developed based on partial lumped parameter approach. Using the available data generated from an experimental DX A/C system, both transient and steady-state behaviors predictions agreed well with the experimental ones. With the help of the validated white-box model, the inherent steady-state operating performance expressed in terms of the relationship among total cooling capacity (TCC), equipment sensible heat ratio (E SHR) and coefficient of performance (COP) under various speed combinations of compressor and supply fan were further examined. The results show that a higher COP could be achieved when the DX A/C system was operated at a higher fan speed or a lower compressor speed for dealing with a larger required E SHR. This model could be helpful for A/C system design and controller development.