Howard Cheung

A method to calculate uncertainty of empirical compressor maps with the consideration of extrapolation effect and choice of training data

Although compressor maps are powerful tools to estimate compressor power consumption quickly, their ability to extrapolate outside their training data range is always questioned. In order to quantify the effect of extrapolation, a method to calculate the uncertainty of compressor map outputs is developed in the current article. The method considers four major components of uncertainties due to various sources such as measurement uncertainties of training data and equation of state. The change of the predicted map uncertainty with the degree of extrapolation and the map accuracy are shown for eight different compressor maps. The results indicate that the uncertainty from model random error increases significantly as the maps extrapolate though extrapolation does not necessarily imply inaccurate compressor map outputs. The results also show that the uncertainty due to training data is the most significant component of uncertainties when the maps are not extrapolated.

An empirical model for simulating the effects of refrigerant charge faults on air conditioner performance

This article introduces a simple empirical model that can be readily implemented in building simulation programs to model the effects of refrigerant charge on air conditioner performance. Under- or over-charged refrigerant faults are relatively common and impact performance of equipment. It is important to understand their overall impacts in the context of whole building energy consumption and comfort in order to prioritize their importance relative to other building issues. This article develops an empirical model using some existing data. The model applies corrections to the rated values of cooling capacity, energy input ratio, and sensible heat ratio of existing air-conditioning equipment modeling approaches. Model predictions are compared with experimental results obtained from three air conditioners that have fixed orifices as expansion devices. The accuracy of the model is compared with a more complex empirical model and is shown to provide better accuracy for predicting cooling capacity and sensible heat ratio.