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.     

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 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.     

酒店建筑空调和生活热水方案优化设计

本文利用负荷全年逐时计算数据,引入全寿命周期费效比方法,对北京一家酒店的冷源设计方案的初投资和运行费用比较分析,综合考虑初投资、控制策略、维保难度、运行费用等多方面因素,该项目最终推荐选用水冷冷水机组机＋电热水器的方案。     

Effects of utilizing seawater as a cooling source system in a commercial complex

This paper treats energy and cost performance of a cooling source system with indirect seawater utilization for air conditioning in a commercial complex. Seawater utilization has merits as a cooling source, because seawater temperature is lower than outdoor air in summer and it is cost effective because there is no water monetary cost. Actual operating data has been measured for about 2 years and the chiller and system co-efficient of performance (COP) have indicated about 4.77 and 2.93, respectively, even in summer season and the mean efficiency of the thermal storage system was about 89.9% taking into account heat loss of pumps. In addition, we have constructed simulation models for cooling tower systems, air cooling chiller systems and direct seawater utilization systems then compared them to this system. The electric power consumption of the indirect seawater utilization system was almost the same as the other systems except the air cooling chiller system, because using lower seawater temperature made the chiller efficiency higher. In conclusion, our results showed the indirect seawater utilization system was able to improve the system COP compared to air cooling chiller system, and cancel water consumption compared with the cooling tower system, and cut down an initial and maintenance costs compared with the direct seawater utilization system.     

空调冷却水系统最优节能控制策略

提出了一种空调冷却水系统最优节能控制策略。这种控制策略要求在系统调试过程中确定各种外界条件下最小能耗工况的水泵和风机的频率,在系统运行过程中,直接根据外界条件将水泵和风机的频率调到这个频率。介绍了在系统调试过程或运行过程中确定最小能耗工况下水泵和风机频率的方法。     

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.     

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.     

Optimal control of an HVAC system using cold storage and building thermal capacitance

This paper presents a comprehensive analysis on the optimal control protocol to minimize the daily operating cost of an air-conditioning system in a 33 600 m² office building. The system consists of two chillers, one designated for cold storage charging, the other for direct cooling, an air-handling unit, a cooling tower, and water pumps. This analysis determines the optimal protocol for indoor temperature and humidity control as well as operating point settings for the chiller control considering two thermal storage sources: (i) the thermal capacitance of the building, and (ii) a cold storage facility. The analysis is based on the thermodynamic modeling of the air-conditioning system including the thermal response of the building structure. A discussion of operating cost savings is presented for several outdoor conditions and electricity rates. Furthermore, the load shift potentials are discussed for each storage source. The results of this analysis indicate significant savings can be achieved by precooling the building during hours of low electricity rates. The resulting load shifting can be augmented by utilizing cold storage equipment. The results are discussed in detail for Seattle climatic conditions and electric rates. A regional comparison of the results for the four US locations, Seattle, WA; Phoenix, AZ; San Diego, CA; and the island of Hawaii, is given.     

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.     

Modeling and economic analysis of gas engine heat pumps for residential and commercial buildings in various climate regions of Iran

There is an increasing trend in using heat pumps in air conditioning (heating/cooling) systems of residential and commercial buildings. The required power to drive the compressor of vapor compression heat pump cycles may be provided by either an electrical motor or an internal combustion engine. In this paper thermal modeling and economic analysis of gas engine heat pumps (GEHPs) are presented based on energy and mass balance equations as well as the gas engine operating parameters (such as thermal efficiency, fuel consumption and fuel mass flow rate) and heat pump operating parameters (such as evaporator and condenser capacity and compressor input power). Based on the modeling results and with estimating GEHP fuel consumption, the economic analysis of using gas engine heat pumps (in comparison with the electrical heat pumps) at various climate regions of Iran, for both residential and commercial (office) buildings, and for both cooling and heating modes, was performed. Appropriate cost functions for predicting GEHP capital investment were proposed. Three approaches including equivalent uniform annual cost (EUAC), the annual cost of energy consumption, and payback period were applied in the economic analysis.     

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.     

Methodology for optimizing the operation of heating/cooling plants with multi-heat-source equipments

A methodology for optimizing the operation of heating/cooling plants with multi-heat-source equipments is proposed. The methodology decides the optimal combination of the running machines to minimize the energy consumption of a heating/cooling plant. The energy consumption, energy cost and carbon dioxide emission are simulated using a tool developed in the MATLAB^® Simulink^® environment, embedded with a module developed using MATLAB^® Stateflow^® that can automatically decide the on/off states of heat source machines corresponding to cooling and heating loads and an operational priority order of heat source machines. A case study is introduced to demonstrate the methodology, which is an actual heating/cooling plant located in Osaka, Japan. The plant consists of two absorption chiller/heaters, one centrifugal chiller, one ice chiller, and two air-source heat pumps. The proposed methodology is used to simulate the plant performance at six different operational priority orders of heat source machines. The best operational combination can reduce primary energy consumption by 19.7%, energy cost by 12.8%, and carbon dioxide emission by 29.6%, compared to present operation.     

过渡季节应用自然冷源的设计方法

在计算通风空调切换温度基础上,提出过渡季应用自然冷源的设计及节能评估方法,通过采用过渡季通风量控制策略减少风机运行能耗.上海地区某高大厂房采用此方案,与过渡季空调方案相比,负荷率0.8时,空调主机年节能率为29.3％,卸载控制策略时的风机能耗为风机满载运行时的41％～56％.提出以日期作为通风空调运行切换依据,并定量分析不保证率与节能率的关系.     

全年供冷的风冷式冷水机组问题分析与解决

风冷式冷水机组在环境温度较低时，可能出现冷量急剧减少等问题。本文对于产生问题的原因进行了分析，提出采用热力膨胀阀并联毛细管和控制冷凝风机开机数量的方法，来维持机组在大的环境温度变化范围内正常工作，给出了具体的控制策略。应用本文方法所设计的冷水机组，已在实际中得到应用，实现了全年正常供冷。     

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.     

冷水机利用变流量冷冻水的若干设计问题

目前变水量冷冻水系统的设计一般都采用一、二次双环路方式。但这种双环路配置方式会产生：使用两个水泵增加设备投资；冷冻水送水及回水的混合影响冷水机在部分负荷时的效率；实际的高负荷空调需求条件与最大设计工况条件不吻合时不能充分利用冷水机的全负荷容量等不良后果。设计单环路空调冷冻水系统可以避免上述问题。讨论了单环路变流量系统的长处以及可能产生的问题，并提供在整个运行范围内，能够实现安全、稳定和可靠的冷水机控制策略。     

Energy savings in a building using regenerative evaporative cooling

This paper explores the potential of reducing the annual energy consumption of a central air-conditioned building through advanced evaporative cooling systems. The building considered is a typical three floor library building of a University. The regenerative evaporative cooling technology is coupled with the liquid cooled water chiller system to accomplish the energy conservation objective. Comparisons of the regenerative evaporative cooling are made with simple evaporative cooling to bring out the importance such a system. The well-known building simulation software, TRNSYS is used to carry out the heat load calculations and the dynamic simulations of the building. Annual energy consumptions of different components of the air-conditioning system are estimated for the existing water chiller system as well as for both coupled evaporative cooling systems (simple and regenerative). The annual energy consumptions, the indoor temperature, the relative humidity and the thermal comfort index ‘PMV’ are compared for all the three different air-conditioning systems. The coupling of direct and regenerative evaporative cooling technologies with water chiller system has shown, respectively, 12.09% and 15.69% savings in annual energy consumption of the building, while maintaining PMV between ?1 and +1 for most of the hours in the year.     

高效全自动管道清洗系统在某集中冷站中的应用分析

广州地铁某集中冷站冷水机组采用珠江水作为冷却水源。在使用过程中,出现了机组传热温差急剧上升、经常故障报警和电能消耗异常等情况。为解决此问题,通过分析比较,安装了3套适合冷水机组冷凝器的全自动清洗系统CQM,实现了冷水机组高效、节能、环保运行。