R.K.Y. Sit

Evaporative cooling technologies for air-cooled chillers for building energy performance improvement

Where water conservation is concerned, central air-conditioning systems in commercial buildings are installed with air-cooled chillers for comfort cooling. This study examines evaporative cooling technologies for air-cooled chillers to improve their energy performance. Two common devices for cooling air intake to condensers are evaporative coolers and mist generators. Depending on weather conditions and packing materials, evaporative coolers can enhance the refrigeration effect and chiller energy performance at various degrees. Yet, different configurations and designs for air-cooled condensers call for tailor-made design of the coolers. The coolers also impose additional flow resistance to condenser air and, in turn, additional fan power. Mist generated in front of the condensers brings no flow resistance to condenser air and allows the dry bulb temperature approach closely the corresponding wet bulb temperature. The cooling effectiveness can be maximized with lesser water consumption. The benefit of evaporative cooling for air-cooled chillers is more appealing in future climate scenarios in a subtropical climate, considering the higher difference between the dry bulb and wet bulb temperatures. Integration of control and optimization strategies with mist precooling has been discussed.

Analysis of centrifugal chillers with oil-free magnetic bearings for enhancing building energy performance

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.

Cooling effectiveness of mist precooler for improving energy performance of air-cooled chiller

Mist is increasingly applied to precool outdoor air in heat rejection. This study investigates how the coefficient of performance of an air-cooled chiller varies with a mist precooler at different levels of cooling effectiveness. A multi-variate 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 coefficient of performance 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 coefficient of performance 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 coefficient of performance and the cooling effectiveness. Further experimental work is required to optimize cooling effectiveness for the maximum coefficient of performance.

Carbon Emissions of Chiller Systems in Hong Kong Hotels under Climate Change

ABSTRACTBuilding energy simulation is a common technique to forecast future energy use and develop strategies for meeting carbon reduction targets. The purpose of this study is to analyze the trend of electricity use and carbon emissions of chiller systems—the most energy intensive type of system—using building energy simulation for hotels in subtropical zones under climate change. Based on a typical meteorological year weather file for subtropical Hong Kong, weather data were forecasted for climate change scenarios in 2020, 2050 and 2080. The building simulation program EnergyPlus was used to model a reference hotel with two typical chiller system designs. Simulation results show that a system capacity extension by up to 5% could be considered to satisfy the increasing cooling demand for a 15-year operating span. Various strategies have been discussed for chiller systems to reduce carbon emissions by the demand side. A rigorous carbon intensity target by power companies should be in place to reduce the i...

Critique of the Design and Operation of Sustainable Air-cooled Chillers in Commercial Buildings

ABSTRACTWhere water shortage is concerned, air-cooled chillers are typically installed to provide cooling energy in commercial buildings with significant electricity consumption. This study investigates performance data and technical specifications on a wide range of air-cooled chiller products. The coefficient of performance (COP) of the chillers at full load operation ranges from 2.4 to 3.7, with 2.9, 3.1 and 3.3 in the 25th, 50th and 75th percentiles, respectively. A survey on national standards and guidelines for the energy performance of air-cooled chillers shows that the minimum required COPs lie between 2.4-3.1 at full load operation. This suggests that over 50% of the existing products can meet the requirement and more stringent control should be in place to boost the use of high efficiency chillers. Control algorithms should be further exploited to judge if the chillers operate with desirable performance under part load conditions. Advanced controls are seldom applied to optimize their performanc...

Environmental benefits of sustainable chiller system under climate change

Chiller systems are commonly used to provide cooling energy in commercial buildings but with considerable electricity and water consumption. This paper discusses the environmental benefits of a sustainable chiller system with full variable speed control under climate change. Hourly weather data of climate change scenarios in 2020, 2050 and 2080 were predicted by using the program CCWorldWeatherGen, based on a typical meteorological year weather file for Hong Kong. The building simulation program EnergyPlus was used to model a full variable speed chiller system and a conventional system operating for a reference office building. Simulation results show that the environmental performance of the sustainable system is superior to the conventional ones with annual electricity saving of 25.7–31.5%, water saving of 1.8–2.2% and carbon emissions reduction of 25.6–30.3% under the three climate change scenarios. The sustainable system would bring an improvement in the annual average coefficient of performance even ...