Fu Wing Yu

Commercial buildings can take up one-third of the energy related carbon emissions. There is limited research on forecasting cooling demands to evaluate sustainable air conditioning systems under climate change. This paper develops a simplified cooling demand model based on the time series of climatic and architectural variables to analyze carbon reduction by a sustainable chiller system. EnergyPlus is used to simulate hourly cooling demands of a hypothesized high-rise office building in Hong Kong under a change of architectural parameters and future climate conditions. An hourly cooling demand model with R² above 0.9 is developed with inputs of the window-to-wall ratio, outdoor air enthalpy, global solar radiation, wind speed and their two steps ahead. The validated model is then used to analyze carbon reduction potentials by free cooling and a full variable speed chiller system. The low carbon technologies reduce carbon emissions by over 20% with but the reduction shrinks to 2.51–4.93% under future climate conditions. The novelty of this study is the simplified cooling demand model based on the time series of climatic and architectural variables. The significances of this study are to quantify carbon reduction by a sustainable chiller system under climate change and to appeal for more carbon reduction technologies for carbon neutrality.

Very few studies have considered modelling the statuses of loading and component combinations (LC) in chiller systems. This study uses a decision tree (DT) and a neural network (NN) to model accurately six output levels of LC in terms of 11 predictor variables. The NN model gave a higher correct prediction of 89.52% than 84.44% in the DT model. A variable dropout analysis from the DT and NN models generalizes the top five significant variables: the statuses of cooling towers and primary chilled water pumps, and the supply temperature, return temperature and flow rate of chilled water. The system coefficient of performance (SCOP) modelled by NN has an improved R² of 84.18% versus 74.62% when the significant variables are included as inputs. The SCOP maximization brings three optimal strategies—implementing chiller sequencing, operating components in pairs and optimizing chilled water supply temperature—which reduce CO₂ emissions by 99,815–346,987 kg.

Mist is increasingly applied to precool outdoor air in heat rejection. This study investigates how the coefficient of performance (COP) of an air-cooled chiller varies with a mist precooler at different levels of cooling effectiveness. A multivariate regression model was developed to simulate the operating variables of an air-cooled chiller with mist precooling. The model was validated with typical performance data of an air-cooled centrifugal chiller. The COP would increase by up to 30%, depending on the cooling effectiveness and the wet bulb depression-the difference between the dry bulb and wet bulb temperatures of outdoor air. At a large wet bulb depression, the percentage increase of COP tended to correlate linearly with the chiller capacity. Yet at a small wet bulb depression, the dynamic control of condensing temperature resulted in a non-linear relationship between the percentage change of COP and the cooling effectiveness. Further experimental work is required to optimize cooling effectiveness for the maximum COP.

This article examines the actual operating performance of centrifugal chillers with oil-free magnetic bearings (hereafter oil-free chillers) to analyze energy saving potentials for commercial buildings. A chiller system retrofit for a shopping arcade involved replacing three existing constant speed water-cooled centrifugal chillers by three oil-free centrifugal chillers of the same capacity. The coefficient of performance was calculated based on operating data logged at 1-h intervals for over 1 year before and after replacement. Oil-free chillers with variable speed control operated at a higher coefficient of performance under part load conditions, reducing the total electricity consumption of the shopping arcade by 9.6%. If the average technical efficiency in data envelopment analysis was improved to 1 from 0.6 in the existing case, the average system coefficient of performance could increase from 6.2 to 10.3. The improvement involved controlling the temperature of supply chilled water at a lower boundary curve and operating an equal percentage of capacity among chillers. It is economically viable to design an all-variable speed chiller system with oil-free chillers instead of the conventional constant speed system for a reference office building in a subtropical climate.