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.     

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.     

Modelling of the coefficient of performance of an air-cooled screw chiller with variable speed condenser fans

Air-cooled chillers are generally recognized as energy intensive equipment in air-conditioned buildings in the subtropical climate. This paper considers how the use of variable speed condenser fans enables these chillers to operate more efficiently. The thermodynamic model of an air-cooled screw chiller was developed using the simulation program TRNSYS and validated using the field data and specifications of the chiller. The staging of condenser fans and the control of their speed in various operating conditions were described. A comparison was made on the coefficient of performance of the chiller in the steady state with various control strategies: head pressure control with constant or variable speed condenser fans; condensing temperature control (CTC) with constant or variable speed condenser fans. Potential improvements in the chiller COP due to the use of CTC with variable speed condenser fans were discussed. The findings of this paper are useful in developing more energy efficient air-cooled chillers.     

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.     

Air management modeling of condensing units in a confined space and its impact on the chiller system performance

The small air cooled chillers that serve an apartment building or residential villa often have the outdoor condensing units installed within a confined space. The installation distance between chiller and wall or between two chillers has significant impact on the chiller performance. In this study, three CFD (Computational Fluid Dynamics) approaches to condensing unit air management modeling are proposed and compared with each other first. The predicted air flow rates are compared to the test data as well. The comparison shows that the CFD approach with fan boundary definition is the most cost-effective, easy to be implemented, and accurate. Together with the chiller system modeling, a parametric study is further conducted to investigate the effect of the wall-chiller distance and the chiller-chiller distance on the chiller performance.     

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.     

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.     

冷水机组冷凝器水侧受阻故障模拟实验研究

采用理论分析和实验测试方法,对冷水机组冷凝器水侧受阻故障时空调系统冷水侧、制冷剂侧、冷却水侧的运行参数进行研究,确定对冷水机组冷凝器水侧受阻故障敏感的运行参数。冷却水出水温度、制冷剂冷凝温度、压缩机排气温度是对冷水机组冷凝器水侧受阻故障敏感的运行参数,可以作为检测识别该故障的主要依据。     

Economic benefits of optimal control for water-cooled chiller systems serving hotels in a subtropical climate

A water-cooled chiller system in an air-conditioned hotel can take up about one-quarter of the total electricity consumption and considerable amounts of water in the heat rejection process. This paper evaluates operating cost savings of a chiller system integrated with optimal control of cooling towers and condenser water pumps. A sophisticated chiller system model was used to ascertain how different control methods influence the annual electricity and water consumption of chillers operating for the cooling load profile of a reference hotel. It is estimated that applying load-based speed control to the cooling tower fans and condenser water pumps could reduce the annual system electricity use by 8.6% and operating cost by 9.9% relative to the equivalent system using constant speed fans and pumps with a fixed set point of 29.4 °C for cooling water temperature control. The ways to implement this advanced control for system optimization are discussed.     

Electricity end-use characteristics of air-cooled chillers in hotels in Hong Kong

This paper presents the operating efficiency of air-cooled chillers in three existing hotels and investigates the extent to which the annual electricity consumption can decrease by improving their efficiency. Chillers in these hotels tend to be improperly staged, causing their seasonal efficiency to rise by 0.05–0.12 kW/kW from a full load efficiency of 0.32 kW/kW. When chiller sequencing is restored, their seasonal efficiency could be enhanced to 0.34 kW/kW, which corresponds to an 8.8–22.7% drop in their annual electricity consumption. It is possible to further decrease the annual electricity consumption by 27.0–38.6% when the chillers operate under floating condensing temperature control instead of head pressure control. The implications of improved chiller efficiency for reducing the electricity demand of hotels are discussed.     

空调冷却水变流量控制方法研究

指出冷却水的出水温度和进出水温差的可变范围实际可以选取得较大,论述了冷凝温度控制法的特点,并将其与常规的定温差控制方式进行了比较,结论是当水泵相对于主机的电功率大于10%时,冷凝温度控制节能效果较好,控制上也简便易行.同时指出,虽然冷却水变流量对于冷水机组性能系数的影响是负面的和不可忽略的,但这一负面影响较之对水泵节能的正面影响要小.     

变频冷水机组部分负荷下冷却水定变流量性能研究

冷水机组大部分时间是在部分负荷下运行,而变频技术的发展使得变频压缩机和变频水泵越来越多地应用到制冷空调当中。本文针对制冷循环各部件建立了稳态模型,并在模型中引入RKS方程精确计算制冷剂的热物性。模拟结果表明:变频冷水机组在部分负荷下运行,定、变冷却水流量对机组蒸发温度影响不大,但冷凝温度有不同程度的降低。同时,考察定、变冷却水流量是否节能应着重考虑冷却水系统能耗在机组总能耗中所占的比例,冷却水系统能耗比例越大,变流量节能效果越明显。     

离心式冷水机组冷凝热回收的研究

传统空调机组的工作原理是将室内产生的热量通过热力循环转移到室外,这种方法虽然能满足室内的需求,但是造成了室外的热污染,同时在热力循环过程中产生的高位热能被白白浪费掉。复合冷凝技术通过改变传统的冷凝方式实现冷凝热回收,使空调系统在满足室内要求的同时又尽量利用高位冷凝热制取生活热水,同时在夏季可以停开热水锅炉,提高了能源利用效率的同时减少了能源消耗总量。本文从火用的角度对复合冷凝方式进行了研究,验证了复合冷凝可以极大地提高能源利用效率,并且在离心式冷水机组上进行了节能改造。机组在改造后稳定制取热水的同时没有影响到机组的制冷效果,机组运行稳定。     

Optimum placement of condensing units of split-type air-conditioners by numerical simulation

Split-type air-conditioners used in residential or office buildings often have the outdoor condensing units installed at the sidewalls or on the roofs. Installation distance from the supporting wall for the first group and the height of installation for the second group are two factors that affect the condenser efficiency. In this study, a CFD code is used to calculate the effect of distance from the supporting wall on the entrance air temperature and on the on-coil temperature of condenser installed between two walls. In the case of condenser installed on the roof, the effect of installation height of the condenser from the finished roof on on-coil temperature is investigated and the minimum recommended height of installation is determined.     

浅谈蒸发冷凝技术在地铁中的应用

通过蒸发冷却技术的应用分析,将三种蒸发冷凝冷却设备的优缺点作了比较,确定选用蒸发冷却冷凝冷水机组,并结合车站的布置形式,经分析比较得到最合理的车站通风空调系统设置方案,进而解决了地面冷却塔设置困难问题。     

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.     

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.     

Effect of condensing unit layout at building re-entrant on split-type air-conditioner performance

The use of split-type air-conditioners is popular in residential buildings of Hong Kong. In new apartment buildings, the building re-entrants are the most frequent choice of the project team for placing the outdoor condensing units. Nevertheless, the effect of heat dissipation from all condensing units at a re-entrant can be detrimental. Improper condenser layout incurs energy wastage, equipment de-rating and malfunction problems. Because of the continuing changes in outdoor air conditions, individual air-conditioner usage and loading, accurate prediction of the thermal impact at a re-entrant is virtually difficult. Based on computational fluid dynamics (CFD) analysis incorporating with an energy performance evaluation model, the effectiveness of various condenser layout schemes in building re-entrants under steady-state conditions was explored in this paper. It was found that there are limited hard-and-fast rules to give definite solutions. Provided that there is no physical access problem, an alignment of condensing units facing inward to induce a positive inflow of air into the re-entrant is a desirable solution.     

冷凝器在线清洗技术及其应用

分析了空调冷水机组制冷效率下降的原因,对提高冷机效率的传统方案和冷凝器在线清洗装置的应用效果进行了对比.冷凝器在线清洗装置利用循环水系统的自身压力,实现了冷机正常运行工况下冷凝器的在线清洗,具有方便管理、运行稳定、节能降耗、减少污染的优点.最后以莱工程为实例,计算分析了该装置的节能效果.