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.     

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.     

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.     

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.     

Advanced control of condensing temperature for enhancing the operating efficiency of air-cooled chillers

Air-cooled chillers are commonly used to provide cooling energy in commercial buildings in the subtropical climate. These chillers have long been considered inefficient because they operate under head pressure control where the condensing temperature is kept high in the refrigeration cycle. This paper considers a novel method to strategically lower the condensing temperature to enhance chiller efficiency at any outdoor temperature. An experiment is carried out on an air-cooled reciprocating chiller to confirm that an electronic expansion valve enables refrigerant to be adequately fed into the evaporator in all operating conditions, even when the condensing temperature falls to around 20 °C. By enhancing the heat rejection airflow of the condenser, both the condensing temperature and chiller power can decrease considerably with less fluctuation. According to the algorithm of staging condenser fans, this enhancement corresponds to resetting the set point of the condensing temperature based on any outdoor temperature. The potential and benefits of implementing this reset strategy are discussed.     

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.     

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.     

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.     

基于模型的离心式制冷机组系统优化控制策略研究

本文提出了基于模型的离心式制冷机组系统台数优化控制策略。该优化控制策略充分利用系统日常调试与机组自身提供的可靠数据,采用简化模型来评估单台机的最大制冷量、计算单台机的瞬时制冷量,并预测一定负荷下不同台数机组运行时的系统总电功率,寻求制冷机台数的优化组合,以使得制冷机组系统的性能系数最大化。     

离心式制冷机系统优化控制策略研究

提出了基于模型的离心式制冷机系统台数优化控制策略。该优化控制策略充分利用机组系统的日常调试与制冷机组自身提供的可靠数据，采用简化模型来评估单台机的最大制冷量、计算单台机的瞬时制冷量、并且预测在一定负荷下不同台数机组运行时系统的总电功率，从而寻求制冷机台数的优化组合使得制冷机组系统性能系数最大化。     

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.     

基于冷水量变化的冷水机组性能测试与故障诊断

研究了定流量/变温差和变流量/定温差条件下冷水量变化对机组性能的影响。结果表明:大型冷水机组蒸发器侧冷水大范围变流量不会影响系统的稳定性;在一定范围内冷水机组制冷量与冷水流量呈线性变化,系统的性能系数COP基本保持稳定,这为冷水泵的节能运行提供了良好的依据;蒸发器盘管中冷水流量较大时,允许的冷水流速变化范围可以适当增加;冷水量调节的极限速度为10.1%/min,否则会造成冷水机组运行不安全;蒸发器冷水流量变化引起的冷水机组性能变化可作为故障诊断的判断依据。     

Analysis of district cooling system with chilled water thermal storage in hot summer and cold winter area of China

A typical district cooling system (DCS) with a chilled water storage system is analyzed in hot summer and cold winter area in China. An analysis method concerning operation modes is proposed based on measured data, which is obtained by long term monitoring and on-site measurements of cooling season. The DCS operates at partial load for a large proportion of the cooling time; in particular, the partial-load rate (PLR) can be less than 25% for more than 50% of the total cooling season. In the night, PLR reaches 5% of the peak load. Thus, it is critical to achieve efficient operation under partial-load conditions of the DCS. Installation of chilled water thermal storage presents a solution to improve the working condition of the DCS and chillers. From the beginning to the end of the cooling season, the DCS operation can be summarized by typical operation modes according to cooling demand and chiller operation. For each mode, the base-load chiller operated at a high-load rate, with an average value of 0.88, and the coefficient of performance (COP) remained in a small range, between 4.2 and 5.2. The average energy efficiency ratio (EER) of the DCS for the cooling season was 3.65 and 3.81 for Years A and B, respectively. With respect to the economics, chillers used 90.2% of off-peak electricity, at only half the price of peak electricity.

     

Experimental investigation of adsorption chiller for Micro-scale BCHP system application

Building cooling, heating and power (BCHP), is an attracting cogeneration system for its economic and environmental friendly qualities. Therefore, it is meaningful to develop Micro-scale BCHP (MBCHP) system based on adsorption chiller and internal combustion gas engine for the use of residential and light commercial buildings. To guide the optimum matching and operation of adsorption chiller in MBCHP system correctly, this paper deals with the performance of adsorption chiller under varying heating conditions. Experimental results show that the value of COP is high in the operation mode of varying hot water inlet temperature with mass recovery in no heating pattern (VTNH). With the hot water inlet temperature of 65 °C, the value of COP is as high as 0.40 in VTNH mode. Under electricity output conditions from 6.0 to 8.3 kW in MBCHP system, VTNH mode is especially preferred when cooling demand is with priority.     

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。     

An integrated model for the design of air-cooled chiller plants for commercial buildings

Cooling load calculation is the first step in designing the air-conditioning system of a building. The calculated cooling capacity with appropriate buffer is then used to select the number and size of chillers in the system. N + 1 is a common formula used by designers to size the chiller plants in Hong Kong buildings, where N is the actual number of chillers required and 1 is a redundant chiller provided to ensure reliability. This paper reviews the problem of excess capacity and discusses the risk exposure of chiller systems without redundant chillers. The cooling load profiles of the chiller plants of four medium-sized commercial buildings in Hong Kong are reviewed. The risk exposure of chiller systems without redundant chillers can be minimized by applying risk-based preventive maintenance. The just-in-demand design reduces capital cost of the building and frees up funds for continuous energy measurement and improving the energy efficiency of chiller plant systems. This paper presents a model for designing chiller plants that improves the energy efficiency of the plant in a cost effective and thoughtful manner. It is designed with consideration of the life cycle of the plant and real-time continuous commissioning, monitoring, measurement, comparison and execution for better energy management.     

部分负荷下冷冻水变流量对水源热泵机组性能影响分析

建立水源热泵机组数学模型,进行部分负荷下冷冻水侧变流量对机组性能影响模拟。结果表明：变流量时流量变化范围并不与机组负荷变化成线性关系;无论定、变流量,部分负荷下冷凝温度下降,蒸发温度升高,机组COP随着负荷减小先增大后减小;在一定范围内冷冻水变流量对机组本身性能影响并不显著,却会使冷冻水泵功耗有较大幅度下降。     

上海某工厂空调系统热回收综合节能改造

冷水机组在制取冷量的同时会排放大量冷凝热,回收这部分冷凝热可使冷水机组的综合COP大幅提升,系统一次能源利用率大量提高,降低热水系统的运行成本。结合上海某工厂空调系统热回收综合节能改造的案例,分析采用冷凝热回收技术后的节能效果和经济效益,为同类工厂空调系统科学、经济、合理地设计改造和运行管理提供借鉴。     

基于遗传算法的冷水机组运行优化策略研究

本文通过采用回归方法获得冷水机组性能系数(COP)与部分负荷率(PLR)的关系,并建立各台冷水机组能耗方程,利用遗传算法求解其最小值,从而获得各冷水机组所承担的最优负荷比率,同时根据某一实际建筑计算了其全年的冷负荷率时间分布情况,在此基础对系统冷水机组全年的运行情况进行了优化,从而达到5.7%的节能效果。     

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.