Low-energy design for air-cooled chiller plants in air-conditioned buildings

Chillers are widely used for cooling buildings in the subtropical regions at the expense of considerable energy. This paper discusses how the number and size of air-cooled chillers in a chiller plant should be designed to improve their energy performance. Using an experimentally verified chiller model, four design options were studied for a chiller plant handling the cooling load profile of an office building. Using chillers of different sizes is desirable to increase the number of steps of total cooling capacity. This enables the chillers to operate frequently at or near full load to save chiller power. Pumping energy can also be saved because of the improved control of chilled water flow whereby the chilled water supplied by the staged chillers can match with that required by air side equipment for most of the operating time. It is estimated that the annual electricity consumption of chiller plants could drop by 9.4% with the use of unequally sized chillers. The findings of this research will offer guidance on how to select chillers of different sizes for a low-energy chiller plant.     

Life cycle analysis of enhanced condenser features for air-cooled chillers serving air-conditioned buildings

Air-cooled chillers are commonly used to provide cooling energy for air-conditioned buildings at the expense of considerable electricity. This paper examines the life cycle electricity cost of these chillers with the improved condenser features of condensing temperature control (CTC), evaporative pre-coolers (EC) and variable speed condenser fans (VSF). A validated model for an air-cooled screw chiller was used to ascertain how the individual and mixed features influence the annual electricity consumption of chillers in various operating conditions. It is estimated that the life cycle electricity cost savings range from HK$ 2,099,742 with EC to HK$ 6,399,564 with all the three features, with regard to a chiller plant serving an office building for 15 yr. The life cycle analysis reported here provides important insights into how to reap the benefits of energy efficient technologies for air-cooled chillers.     

Optimum load sharing strategy for multiple-chiller systems serving air-conditioned buildings

Many central cooling systems in air-conditioned buildings have multiple chillers to meet various cooling load requirements. This paper further develops optimum load sharing strategies for the chillers in order to maximize their aggregate coefficient of performance (COP). Based on the part load performance curves of air-cooled screw chillers, it is ascertained that for two equally sized chillers operating, one should carry a full load and the other should be partially loaded to meet the system load. When two chillers of different sizes are running, the larger chiller should be fully loaded and the smaller chiller should operate at part load in order that their combined capacity satisfies the system load. Such an uneven load sharing strategy for achieving maximum COP is independent of ambient conditions and the control of condensing temperature. The variable primary flow of chilled water should be applied to chillers in order to implement the strategy. The results of this paper are useful in developing low-energy chiller plants.     

Chiller sequencing control with enhanced robustness for energy efficient operation

This paper presents a strategy for improving the reliability and the energy efficiency of chiller sequencing control based on the total cooling load measurement of centralized multiple centrifugal chiller plants. The improvement is achieved as follows. Firstly, a fused measurement of building cooling load is used to replace the direct/indirect measurement. Secondly, the maximum cooling capacity of individual chillers is computed online using a simplified centrifugal chiller model. Thirdly, the online computed maximum cooling capacity is calibrated according to the quality of the fused measurement in order to deal with the possible misbehaviours in measurement instruments. A simplified model for computing the maximum cooling capacity is developed and validated using field data. The performance of the proposed chiller sequencing control strategy is tested and compared with a conventional chiller sequencing control algorithm. Test results are presented showing that the proposed strategy can effectively improve the reliability of chiller sequencing control and reduce the energy consumption of chiller plants.     

Energy performance of a cooling plant system using the inverter chiller for industrial building

The purpose of this paper is to clarify energy performance of the cooling plant system in the industrial building using actual measured operating data and numerical simulation analysis. One aspect of industrial buildings is that they have large energy consumption for manufacturing and air-conditioning compared with office and commercial buildings. Some examples of high-efficiency technologies installed in this particular cooling plant system are inverter chillers, integrated cooling towers and a free-cooling system. The inverter chiller which has been put on the market recently is state-of-the-art technology. The maximum COP of the inverter chiller reaches about 18 under certain conditions and integrated cooling towers make lower temperature cooling water as the whole capacity is large. Actual operating data indicates satisfied values for chiller and system COP during the running period and the simulation results show that the cooling plant system can cut down annual electric power consumption by about 48% compared with conventional cooling system.     

An alternative approach for the performance rating of air-cooled chillers used in air-conditioned buildings

Air-cooled chillers are widely used to provide cooling energy for air-conditioned buildings at the expense of considerable electricity. The (Air-Conditioning & Refrigeration Institute) ARI standard 550/590 sets out a rating condition to specify the coefficient of performance (COP) of the chillers under part-load conditions. This condition was found to be insufficient to deal with diverse operating conditions under the multiple chiller arrangement. This paper proposes an alternative approach to specifying more precisely the chiller COP under part-load conditions. It is desirable to establish a set of part-load performance curves showing how the chiller COP varies with the condensing temperature at various combinations of chiller loads and outdoor temperatures. The results of this paper will give engineers and researchers a better idea about how to specify the upper limit of condensing temperature for more energy efficient chillers and how chiller COP curves help compare air-cooled chillers for buildings in any climate zone and to estimate the annual electricity consumption of chillers satisfying any given building cooling load profile.     

Optimal design and operation of a thermal storage system for a chilled water plant serving pharmaceutical buildings

A group of buildings in the pharmaceutical industry located in Southern Germany is experiencing a trend of growing cooling loads to be met by the chilled water plant composed of 10 chillers of greatly varying cost effectiveness. With a capacity shortfall inevitable, the question arises whether to install an additional chiller or improve the utilization of the existing chillers, in particular those with low operating costs per unit cooling, through the addition of a chilled water thermal energy storage (TES) system. To provide decision support in this matter, an optimization environment was developed and validated that adopts mixed integer programming as the approach to optimizing the chiller dispatch for any load condition, while an overarching dynamic programming based optimization approach optimizes the charge/discharge strategy of the TES system. In this fashion, the chilled water plant optimization is decoupled but embedded in the TES control optimization. The approach was selected to allow for arbitrary constraints and optimization horizons, while ensuring a global optimum to the problem.     

Evaluation of alternative arrangements of a heat pump system for plume abatement in a large-scale chiller plant in a subtropical region

Heat pumps could be used to produce hot water for hybrid cooling towers for preventing the occurrence of plume in subtropical regions. The evaporative side of the heat pump system could be arranged either at the inlet side or at the outlet side of these cooling towers for cooling down the cooling water temperature. Alternatively, the evaporative side of the heat pump system could also be arranged at the evaporative side of chillers to reduce return chilled water temperature and therefore to reduce the cooling load of chillers. This study presents the evaluation of the impacts of these three arrangements of the heat pump system on the plume control performance and the energy performance in a large-scale chiller plant in Hong Kong. The performance prediction and evaluation of the chiller plant and the plume abatement system were conducted on a dynamic simulation platform. The results show that these three arrangements have almost the same plume control performance with sufficient plume control capability. The results also show that the arrangement of the evaporative side of the heat pump system for cooling down return chilled water temperature has much better performance that the other two arrangements for improving the overall energy efficiency.     

Part load performance of air-cooled centrifugal chillers with variable speed condenser fan control

Air-cooled centrifugal chillers are commonly used in commercial buildings but their performance analysis is lacking. This paper investigates the part load performance of the chillers via a thermodynamic model. The model was validated using a wide range of operating data from an existing chiller with specific settings of outdoor temperature and condensing pressure in controlling the condensing temperature. The validated model was developed specifically to ascertain the maximum coefficient of performance of chiller (COP) together with the strategy for optimizing the condensing temperature under various operating conditions. It is found that the highest COP occurs at a part load ratio (PLR) of 0.71–0.84, depending on the outdoor temperature and the control of condensing temperature, rather than at full load. Yet the chillers operating at such part load conditions will cause extra energy used for the early staging of chilled water pumps. To minimize the overall chiller plant energy consumption, it is still preferable to implement chiller sequencing based on the full load condition than on the aforementioned PLRs. The results of this paper present criteria for implementing low-energy strategies for operating air-cooled chillers satisfying a given building cooling load profile.     

A data fusion scheme for building automation systems of building central chilling plants

Accurate and reliable building load measurement is essential for robust chiller sequencing control, building air-conditioning system performance monitoring and optimization. This paper presents a scheme adopting the data fusion technique to improve the quality of building cooling load measurement of building automation systems. The strategy uses two types of measurement information on the cooling load, i.e., “direct measurement” of building cooling load, which is calculated directly using the differential water temperature and water flow rate measurements, and “indirect measurement” of building cooling load, which is calculated using a model using the instantaneous chiller electrical power input. Capitalizing their own advantages and disadvantages, a data fusion algorithm is developed to merge these two types of data to remove outliers and system errors as well as to reduce the impacts of measurement noises. Meanwhile, a method is implemented to provide quantitative evaluation of the degree of reliability of the merged measurement. Validation of the data fusion algorithm is conducted using field data collected from a chiller plant in a high-rising building in Hong Kong.     

Fusion of redundant measurements for enhancing the reliability of total cooling load based chiller sequencing control

Total cooling load based chiller sequence in multiple-chiller plants is essentially the best approach to stage a chiller on or off in order to satisfy thermal comfort requirement and achieve energy efficiency simultaneously. In practice, however, this approach cannot be reliably implemented. The reason is the measurement of the cooling load of multiple-chiller plants is not always consistent enough for staging chillers on or off appropriately. Measurement uncertainties, including noises, outliers and biases, have a significant influence on the performance of the sequencing operation. This paper develops a strategy of fusing available redundant measurements to reduce the measurement uncertainties. With a moving window, the proposed strategy can (i) remove measurement outliers according to a calibrated Moffat distance between redundant measurements; (ii) reduce the influence of measurement noises by merging redundant measurements; and (iii) calibrate the bias of the merged measurements. Simulation studies are represented to show the merits of the proposed strategy for improving the reliability of the total cooling load based chiller sequencing control.     

Energy signatures for assessing the energy performance of chillers

This paper investigates how energy signatures can be used as an alternative to an energy use intensity (describing the annual electricity consumption of chillers in kWh per unit floor area of a building in m²) to assess the energy performance of chillers with various design options and operating strategies. An energy signature is a best-fit straight line relating chiller power to a climatic index when chillers operate for a building cooling load profile. Sixteen combinations of four design options and four operating strategies for chillers serving a hypothetical hotel are studied by simulation. For each combination, an energy signature for the chillers is determined. The slope and intercept of the energy signature can be used to accurately predict the annual electricity consumption of the chillers and to evaluate the extent to which this consumption can drop when chiller efficiency is improved. It is desirable to develop reference energy signatures in relation to different characteristics of building cooling load as a yardstick for the minimum requirement of chiller performance. With this yardstick, the effectiveness of energy efficient measures in the operation of chillers could be identified.     

Strategy for designing more energy efficient chiller plants serving air-conditioned buildings

In cities located in the subtropical regions, air-cooled chillers are commonly used to cool commercial buildings almost year-round, which accounts for considerable electricity consumption in the long term. This paper explains how a chiller plant should be designed to enable the chillers to operate frequently with maximum performance. Four design options with respect to the number and size of chillers were studied for a chiller plant satisfying the year-round cooling demand of a hotel. For each design option, the annual electricity consumption of chillers and pumps was assessed using a sophisticated chiller model. The assessment showed that an electricity saving of 10.1% can be achieved by installing a chiller plant with six chillers of three different sizes instead of four equally sized chillers. The results of this paper will give engineers and researchers a better idea about how to select chillers of different sizes and how chiller part load performance curves can be used to evaluate improvements in the energy performance of a chiller plant with alternative designs.     

多台不同冷量冷水机组并联节能运行及控制

针对多台不同制冷量冷水机组并联运行的情况,分析了影响系统能耗的各因素,结合实例对比了不同运行方案的能耗,提出了制定节能运行方案的方法。对冷水机组群控的逻辑判据进行分析,从兼顾节能效果和设备使用寿命的角度,提出了将节能运行方案与负荷变化趋势预测相结合的群控策略。     

Design and performance of solar powered absorption cooling systems in office buildings

The paper contributes to the system design of solar thermal absorption chillers. A full simulation model was developed for absorption cooling systems, combined with a stratified storage tank, steady-state or dynamic collector model and hourly resolved building loads. The model was validated with experimental data from various solar cooling plants.As the absorption chillers can be operated at reduced generator temperatures under partial load conditions, the control strategy has a strong influence on the solar thermal system design and performance. It could be shown that buildings with the same maximum cooling load, but very different load time series, require collector areas varying by more than a factor 2 to achieve the same solar fraction. Depending on control strategy, recooling temperature levels, location and cooling load time series, between 1.7 and 3.6 m² vacuum tube collectors per kW cooling load are required to cover 80% of the cooling load.The cost analysis shows that Southern European locations with higher cooling energy demand lead to significantly lower costs. For long operation hours, cooling costs are around 200 € MWh⁻¹ and about 280 € MWh⁻¹ for buildings with lower internal gains and shorter cooling periods. For a Southern German climate, the costs are more than double.     

基于建筑全年动态冷负荷的冷水机组优化配置方案

提出一种在设计阶段对冷水机组方案进行优化配置的方法.首先,冷水机组的能耗计算简化为制冷机的实际制冷量和冷却水进口温度两个独立变量的函数.进一步,通过建筑动态负荷计算获得全年冷负荷频率特性以及相应的室外湿球温度分布,其中湿球温度决定了冷却水最低进口温度.最终,计算出各种冷水机组配置方案的全年以及不同冷负荷需求工况下的运行电耗,并得出最优化的节能方案.     

多台冷水机组的优化配置与节能运行研究

本文针对公共建筑中的空调能耗,尤其是冷水机组能耗过大的问题,建立了多台冷水机组配置模型及优化目标.以某办公建筑为例,分析了不同运行策略下多台冷水机组运行时的能耗,同时对不同策略和配置情况下的运行情况进行了经济性比较.分析可知,当选用多台机组时,平均分配负荷的运行策略比较节能,节能效果视组合方式的不同而不同,其中容量相同的组合方式采用该运行策略优势最为明显.     

深圳市6栋写字楼集中空调系统逐时冷负荷与能耗分析

以深圳市6栋高层写字楼集中空调系统全年运行数据为依据，分析了写字楼集中空调系统制冷主机逐时输出的冷负荷及年运行能耗，得到了各楼主机全年输出的逐时冷负荷的最大值，分析比较了各大楼的建筑面积、入住率、单位空调面积负荷、单位空调面积空调能耗，并比较了各楼的主机选型方案。     

香港环球贸易中心冷却水系统优化控制研究

提出了常规中央空调冷却水系统的优化控制策略.该控制策略是香港理工大学智能建筑研究所与香港新鸿基地产发展有限公司共同研究开发的中央空调系统系列智能优化控制策略之一.这一优化控制策略将应用在香港环球贸易中心.该方法可以自动设置系统冷却水的供水温度(即冷却塔的出水温度)以保证系统自适应于动态的工作条件并保持高效率.该方法在这一建筑及空调系统的虚拟环境下进行了测试和验证.     

上海某研发中心集中式空调冷源系统节能改造

针对该工程集中式空调系统由于机组容量配置过大而造成的冷源能量过剩问题进行了改造,使两幢楼原本各自独立的冷源系统在负荷需求量较小时合二为一,即用一幢楼的冷源设备来满足两幢楼的负荷需求,实现了较为节能的供冷方案。投资回报分析结果表明,该方案年平均节省电能8.1%,一年内即可收回投资。