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

Building and Environment 46 (2011) 196e209

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An integrated model for the design of air-cooled chiller plants for commercial buildings Raymond K.L. Chan, Eric W.M. Lee*, Richard K.K. Yuen Department of Building and Construction, City University of Hong Kong, Tat Chee Avenue, Kowloon Tong, Hong Kong

a r t i c l e i n f o

a b s t r a c t

Article history: Received 10 March 2010 Received in revised form 7 July 2010 Accepted 15 July 2010

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 þ 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. Ó 2010 Elsevier Ltd. All rights reserved.

Keywords: Air-cooled screw chillers Commercial buildings Chiller plant sizing Energy audit Redundancy Reliability

1. Introduction The first step in designing a heating, ventilating and airconditioning (HVAC) system of a building once the built environment and operational parameters have been taken into account is to calculate the cooling load. Electrical and mechanical (E&M) consulting engineers are responsible for calculating the cooling load capacity and specifying the equipment needed. In Asia, it is common for construction and design to occur at the same time. Compressed construction schedules mean that the design of cooling systems is usually quick and expedient, and often based on rules of thumb. Even with computer modeling, consulting engineers calculate the calculated cooling capacity with large safety margins for fear of liability should a shortage of cooling capacity occur due to changes in the practical operating situation. To ensure the reliability of cooling systems, redundant chillers are installed to safeguard the cooling capacity should one of the chillers fail at peak demand. Consequently, the chiller plant is frequently bigger than needed [1]. In fact, most the chiller plant is only operated at peak demand for a few months of the year. The provision of redundant chillers to increase the reliability of the

* Corresponding author. Fax: þ852 2788 7612. E-mail address: [email protected] (E.W.M. Lee). 0360-1323/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.buildenv.2010.07.013

chiller plant is comparatively expensive, and permits service and maintenance personnel to allow the chillers to break down. This means that only basic routine maintenance is carried out and limited resources are allocated to the maintenance and operation of the chiller plant. However, if redundant chillers were not supplied, then a stringent maintenance strategy would be necessary to maintain the reliability of the chiller plant. A more cost effective way than providing redundancy cooling capacity may be to upgrade the capacity of the independent refrigeration circuits of air-cooled chillers to an optimized level. The operation of chillers creates peak electricity demand and accounts for about a quarter of the total electricity consumption of commercial buildings in sub-tropical climates [2,3]. It is essential to understand the energy efficiency or coefficient of performance (COP), of these chillers to reduce the electricity demand. However, without the installation of basic measuring equipment and chiller plant control (CPC) systems, carrying out energy audits is difficult and can be inaccurate. Four medium-sized commercial buildings that use air-cooled screw chillers were studied to determine the extent of excess capacity in air coolant systems. Buildings with air-cooled screw chiller plants were chosen because of their prevalence in Hong Kong and the characteristics of the independent refrigeration circuits in air-cooled screw chillers. Although the Hong Kong government relaxed the restrictions on the use of fresh water for

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heat rejection systems in Hong Kong in 2000 [4], air-cooled chillers continue to be used in many buildings as they cannot be replaced without causing nuisance to neighboring buildings. The part load performance of such air-cooled chillers is not maximized because of the lack of proper specifications of the chiller COP at part load. Improved condenser features that are available for air-cooled chillers (including condensing temperature control, evaporative pre-coolers and variable speed condenser fans) are not always applied to improve the efficiency of chiller plant [5] because of a shortage of data for comparison and the lack of competency of operation personnel. However, such features provide opportunities to optimize the efficiency of chiller systems at part load when aircooled chillers are operating at 90% of their total operating hours. The saving in capital expenditure that this affords could be invested in the installation of basic measuring equipment, energy saving devices, and CPC for energy audits and the optimization of the chiller plant. The data collected with this equipment would also form a baseline for comparison with benchmarks in the market, and would allow the continuous monitoring of the energy use trend of air-conditioning systems. Basic measuring equipment of this kind and CPC for energy audits was never included in old buildings constructed at a time when there was less concern about energy use. In addition, with competent operation and maintenance professionals, the installation of measurement and monitoring systems will enable air-conditioning systems to be re-commissioned from time to time following changes in usage and application. The traditional way of designing, operating, and maintaining a chiller plant is based on the individual performance of the various pieces of equipment, and does not consider the life cycle performance of the entire system in terms of cost and energy savings. The objective of this paper is to review the cooling loads of the buildings against the cooling capacities of their chiller plant and develop an integrated model for the design of air-cooled screw chiller plants for small to medium-sized commercial buildings using sustainable criteria that meet operational and maintenance requirements while achieving an overall efficiency optimization at a reasonable cost. In Hong Kong, air-conditioning systems have been used to cool indoor environments since the 1950s. The COP of water cooled chillers ranging from 4.2 to 6.2 which is much higher than that of air-cooled chillers of 2.7e3.2, yet air-cooled chillers have been widely used in Hong Kong since the 1970s because of the shortage of fresh water supply for heat rejection. In the early 1990s, aircooled screw chillers became popular, gradually replacing aircooled reciprocating and centrifugal chillers due to their better reliability and more competitive price [6]. 1.1. Characteristics of air-cooled screw chillers In air-conditioning system, air-cooled screw chillers or other types of chiller plants are used to produce chilled water to air-side systems such as primary air fan coil systems, constant air volume systems and variable air volume systems in different zones of the building. An air-cooled screw chiller generally comprises a shell and tube evaporator, an air-cooled condenser with constant speed condenser fans and one or two refrigeration circuits in parallel depending on its size. Each refrigeration circuit includes one electronic expansion valve and one or two constant speed twinscrew compressors. These are operated under head pressure control whereby the heat rejection airflow of the condenser is regulated by staging several groups of condenser fans The number of condenser fan groups is kept at a minimum in most operating conditions to enable the condensing temperature to remain between 45  C and 50  C [7]. The cooling capacity can be controlled (continuously or in steps) by a modulating sliding valve in each

197

compressor. In sub-tropical climates, this is rated at an outdoor dry bulb temperature at 35  C for air entering the condenser with a zero fouling factor allowance, a water side fouling factor allowance of 0.000018 m2K/W for the evaporator, and a flow rate of 0.043 lit/s per kW for the evaporator’s water. The supply chilled water temperature has to be set at 7  C to meet the dehumidifying capacity of the air-side equipment. The returned chilled water temperature is floating at 12.5  C. The chiller will be loaded and unloaded when the returned chilled water temperature varies from the floating point. The COP is the cooling capacity in kW over the chiller power in kW, with higher values reflecting higher efficiency. The COP quoted by chiller manufacturers is always based on the rating condition of the Air-conditioning and Refrigeration Institute (ARI) Standard 590 [8]. A 15-year life cycle is the reference for air-cooled chillers in the guidance on the life cycle analysis of mechanical equipment in ASHRAE Standard 90.1 [9]. Medium-sized air-cooled screw chillers always have two independent refrigeration circuits with one or two compressors. Each independent refrigeration circuit can be operated independently even there is a failure in another refrigeration circuit. If one of the compressors fails in the same refrigeration circuit, then it is recommended that the whole circuit be taken out of service. The burnt compressor may transfer residue or acidic materials to the functioning compressor through the refrigeration circuit, causing it to fail. The cooling capacity of air-cooled screw chillers thus depends on the number of available refrigeration circuits, but not on the number of available compressors. Small to medium-sized commercial buildings usually use between 100 and 350 tons air-cooled screw chillers. Generally, the maximum cooling capacity of air-cooled screw chiller is 400 tons. The various models increase in steps of around 10e30 tons. This provides flexibility and the possibility of increasing capacity by installing the next model up, rather than increasing the number of chillers. The minimum unloading step of each compressor in an aircooled screw chiller can be set at around 15e30% of the full load by modulating the sliding valve. This allows the minimum unloading percentage of the chiller with twin compressors to be lower than 10% of the total capacity at steady operating conditions. The compressor power of an air-cooled water chiller can be saved by increasing the evaporating temperature or decreasing the condensing temperature. However, when the supply chilled water temperature is set at 7  C, the potential for reducing the compressor power by raising the evaporating temperature is rather limited, because the evaporating temperature varies within a narrow interval of between 3  C and 5  C over the entire range of chiller load conditions. It is possible to replace the head pressure control system with condensing temperature control to achieve a higher chiller COP, whereby for most of the operating time the condenser effectiveness can be maximized by staging all of the condenser fans to enable the condensing temperature to sit closely above any given outdoor temperature [10e14]. Unfortunately, Condensing Temperature Control (CTC) is still a dream solution under the current technology. Practically, manufacturers have difficulties in maintaining the pressure difference under CTC. It makes the CTC not commercially viable. The further development of magnetic bearing will be one of the possibilities to solve this problem. Under head pressure control, the chiller COP is difficult to determine accurately by a combination of outdoor temperatures and part load ratio (PLR), as it tends to fluctuate when the condensing temperature is passively controlled. When a condensing temperature control system is used, a set of regressive curves can be generated that show the relationship between the chiller COP, the outdoor temperature, and the PLR of the chillers,

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thus making it is possible to predict the chiller efficiency under various operating conditions. 1.2. Condensing temperature control to optimize the efficiency of air-cooled chillers When air-cooled water chillers in a chiller plant come into operation, the condensing temperature is generally monitored to ensure the safe operation of the compressors and condensers. The minimum pressure differential between evaporator and condenser shall be 60e80 psi to ensure proper lubrication to compressor. Experiments concluded that it is possible to continuously stage all condenser fans of an evaporative condenser to enable the condensing pressure to float at its lowest level to minimize the energy cost of the system. When the staging of condenser fans for air-cooled chillers is applied, the chiller COP will be improved. It is due to a situation where the heat rejection airflow is enhanced to decrease the condensing temperature. Under CTC, all condenser fans have to be staged in almost all operating conditions and this staging can prevent each condenser fan from cycling start and stop frequently. Condenser fans will sometimes have to be turned off step by step if the stage compressors have insufficient lubrication when over half of the compressors are idle and the outdoor temperatures is 20  C or below. To success implement CTC, chiller manufacturers have to replace thermostatic expansion valves with electronic expansion valves in air-cooled chillers and also maintain sufficient lubrication to compressor bearings. Under a finite range of heat rejection capacities, the condensing temperature varies within two boundaries. The lower boundary is the sum of the outdoor temperature and the log mean temperature difference (LMTD) on the condenser side, and the upper boundary is the sum of the temperature of the air leaving the condenser and the LMTD on the condenser side [11]. When the boundaries of the condensing temperature and a set of chiller efficiency curves are known, an upper limit of condensing temperature can be developed to specify the minimum COP at various combinations of outdoor temperatures and PLRs. For any given outdoor temperature, the upper limit of the condensing temperature can be adjusted based on the two boundaries. Under a condensing temperature control system, if the upper limit of the condensing temperature is set to be 15  C above the temperature of the air entering the condenser (Tcdae) (i.e. Tcdae þ 15  C), the chiller will definitely meet all of the requirements of the minimum chiller COP. There is a correlation between the increase in COP and the decrease in condensing temperature at different chiller PLRs [15]. 2. Cooling load calculation and chiller plant sizing The cooling load capacity of commercial buildings in Hong Kong is calculated by E&M consulting engineers at the design stage using various tools including hand calculation, computer software, and rules of thumb (e.g. 100 W/m2 or 0.028 TR/m2) [16]. Several cooling load calculation software packages are available on the market such as DOE-2E, HAP-E20II, TRACE 700, and Energy Plus. Trace 700 and E20II are commonly used by local consulting firms because of their simplicity. Advanced software such as BLAST and DOE-2 are more sophisticated, and hence less popular for use in commercial building projects. After preliminary calculations, E&M consulting engineers aim to achieve the peak cooling load while allowing some spare capacity as a safety margin. The revised cooling load capacity is checked using a rule of thumb. Chillers of equal size are typically designed for medium-sized chiller plants designed to handle a peak building cooling load of between 1000 kW and 7000 kW. Multiple (N) chillers of the same tonnage are selected because this makes water balancing easier and the parts can be

interchanged if failure occurs. Smaller chillers can be added if a night load is required after normal office hours. It is always the case that redundant chillers are provided to ensure reliability and thus N þ 1 chillers are installed. In their cooling load calculation, E&M consultants always mark up the calculated cooling load of buildings by 10e20% in determining the size of chiller plant at the design stage [17]. This means that the specified cooling capacity is always greater than the actual required cooling capacity. In this circumstance, even if no redundant chiller is provided; the specified cooling capacity of the chiller plant is sufficient for the actual cooling demand of the building. It is argued that if the quality and reliability of the chillers are good [18,19], then a redundant chiller is not necessary. If a redundant chiller is not provided, then the initial capital investment of the owner will be lower, and the property management companies that subsequently manage the building will be forced to engage in preventive maintenance of the chiller plant to increase its overall reliability. However, in practice concern about the reliability of the chillers means that redundancy is also provided for in the cooling capacity design. As the presence of the redundant chiller ensures that the peak cooling capacity of the building is achieved even if one chiller mal-functions, property management companies do not need to seriously consider the maintenance requirements of chiller plant. Most property management companies carry out only routine maintenance of chiller systems, sometimes employing small-scale services companies to complete the task. A rough estimation indicates that more than 50% of chiller service contracts in Hong Kong are carried out by the small-scale service companies. This is an interesting topic for further study, especially given the increasing consciousness of energy use. Optimizing the energy use of a building or chiller plant requires operation professionals with knowledge of energy management to monitor or upgrade the efficiency and effectiveness of the system using a building management system (BMS). Some high grade commercial office buildings in Hong Kong, especially those occupied by the investment banks or those that have telecommunication rooms and computer rooms that require air-conditioning with a high-degree of reliability [20], E&M consultants provide 100% additional standby chillers (2 N number of chillers) in the design to support the air-conditioning system, because the loss of data from a server or data centre that would be incurred by the failure of the air-conditioning system cannot be tolerated. The higher the grade of commercial building, the more redundant capacity is provided as a back up for the air-conditioning system. In reality, the chiller plant is only operated at full load for a few months during the summer. With a full understanding of the actual cooling load profile of the building and the characteristics of aircooled screw chillers, condition-based preventive maintenance can be applied to the chiller plant to improve reliability. This allows sufficient cooling capacity to be maintained in summer without the use of redundant chillers. Rather than installing a redundant chiller, resources can be directed to changing the chiller plant size or upgrading the chiller plant control to achieve better energy efficiency. The installation of basic equipment for conducting energy audits and energy saving devices for energy improvements can also be considered. Using an experimentally verified chiller model, it has been proven that different number and sizes of air-cooled chillers are desirable to increase the number of steps in the total cooling capacity. This enables chillers to frequently operate at or near full load to save chiller power and pumping energy [21]. Unfortunately, few E&M consultants are willing to try this type of approach in chiller plant sizing. One of the four commercial buildings studied here does use different numbers and sizes of chillers, and the excess

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capacity in the chiller plant of this building was less severe than that in the other three. However, this approach requires more accurate cooling load calculation at the design stage for new projects or comparison with the historical operating load profile for upgrading existing chiller plants. 3. Review of practical cooling load profiles and data analysis To understand the practical cooling load profile of air-cooled screw chiller plants in medium-sized commercial buildings in Hong Kong, data were collected from four commercial buildings (denoted Buildings A to D) that use air-cooled screw chillers. Tables 1 and 2 show the characteristics of the buildings and chiller plants. The daily operating log sheets of the chiller plants from July 2007 to December 2008 were provided by either the services company or property management company responsible for the buildings. The data were studied and transformed into cooling load profiles within the study period for further analysis. The period from July 2007 to December 2008 was selected because it included hot summers in 2007 and 2008, allowing critical periods in the cooling load profiles to be compared. 3.1. Operating log sheets of the chiller plants A review of the daily operating log sheets showed that they did not directly record the cooling capacity of the chillers at different intervals. However, they did provide the following useful information for the estimation of the cooling load of the chillers.  Running current of the compressors  Inlet and outlet pressure difference of the chilled water in the evaporator  Inlet and outlet temperature difference of the chilled water in the evaporator

3.2. Data transformation The percentage loading of chillers can be estimated from the running current of the compressors compared with that of the full load current. However, the running current of the compressors will vary at different ambient temperatures, and the percentage loading of the chillers will not exactly refer to the cooling capacity. Hence, these data can only be used as an estimation or reference. The best means of calculating the loading is to apply the pressure difference of the chilled water between the inlet and outlet of the evaporator to the pressure drop chart of the evaporator to obtain the

Table 1 Building characteristics. Attributes

Building A

Building B

Building C

Building D

Location Use

Central Office/ Clubhouse 1984 7400 25 Concrete composite West

Kwai Chung Office/Godown

Central Office

Sheung Wan Office

1982 12,894 15 Concrete composite South

1983 9250 25 Concrete composite South East

1975 6542 23 Concrete composite East

Upper roof floor 2005

Roof floor

Roof floor

2005

2005

Year of completion Gross area (m2) No. of floors Building structure Orientation of main façade Location of chiller plant Year of chiller plant upgrade

Roof floor 2004/2005

199

Table 2 Chiller plants characteristics. Building

Capacity and quantity

Type of operation (Manual/ BMS)

Type of maintenance (Routine/ Preventive)

Frequency of maintenance

Record of data logs of operating parameters

A

574 kW  2 705 kW  1 844 kW  1 1182 kW  4 1195 kW  4 265 kW  4

Manual

Routine

Monthly

Yes

BMS BMS Manual

Routine Routine Routine

Monthly Monthly Monthly

Yes Yes Yes

B C D

current flow rate of the evaporator. The pressure drop chart of the evaporator is usually provided by the chiller supplier in the installation, operation and maintenance manual (IOM) after the testing and commissioning of the chiller plant. The simultaneous cooling capacity produced by the chillers can also be calculated. If the water flow rate is provided at the header only, then the total water flow rate can be derived from the number of chillers operated to obtain the water flow rate in the individual evaporator or chiller. In buildings B and C, some of the values for the measured chilled water flow rate of the evaporator were greater than that of the design chilled water flow rate by more than 30%. It is because the operator used two pumps for one chiller to deliver more chilled water to certain areas that experienced a shortage of cooling capacity. This is an abnormal application as the excessive chilled water flow rate which can cause erosion and turbulent flow to the evaporator. Furthermore, the chiller supplier provides the minimum and maximum limits of the flow rate of each evaporator that ensures the heat exchange of the evaporator working within an acceptable range. Thus, even if a greater flow rate is delivered to the evaporator, the cooling capacity of the evaporator will not be increased in proportion to the flow rate. Generally, the overall cooling capacity of the chiller will only be increased by a maximum of 5% depending on the spare capacity of the compressor. Following the various procedures and adjustments, the hourly cooling load profiles of the buildings in the period of measurement (i.e. from 1 July 2007 to 31 Dec 2008) were obtained. Fig. 1 shows the hourly cooling load profiles of the buildings. For easy visualization, the weekly peak cooling load profiles of the buildings are presented in Fig. 2. Enlarged versions of the daily peak cooling loads of the buildings in the peak months and peak weeks are respectively shown in Figs. 3 and 4 for easy comparison.

3.3. Data analysis The chiller plant combinations of the four commercial buildings (Buildings A to D) that use air-cooled screw chillers were obtained. Details of the physical properties of the individual air-cooled screw chillers were also collected from the service companies or property management companies and are summarized in Table 3. The most important physical property of the individual aircooled screw chillers is the number of independent refrigeration circuits and the cooling capacity of each circuit. These data provide information on how much cooling capacity remains if one of the independent refrigeration circuits fails. The cooling capacity of the redundant chiller is first deducted from the total cooling capacity of the building. If the multiple chillers are not identical, then the smallest chiller or night load chiller is treated as the redundant chiller in the calculation. The actual available cooling capacities of the buildings are obtained which is the actual cooling capacity without redundancy as shown in Table 4. The first failure mode of a chiller is defined as the failure

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Fig. 1. Hourly cooling loads of Buildings A, B, C, and D from 1-Jul-07 to 31-Dec-08.

of the refrigeration circuit with the highest cooling capacity. The second failure mode is defined as the failure of the refrigeration circuits with the highest and the second highest cooling capacity. The third, fourth, fifth and sixth failure modes are similarly defined until the remaining cooling capacity is less than the cooling capacity at the peak demand. Based on the available cooling capacity at each step, for Building A, B and C, additional lines of actual cooling load (without redundancy), first and second failure modes overlap on Figs. 1e4 for easy comparison. Since the cooling load profile of Building D is well below its cooling capacities, only lines indicates the second, third and fourth failure modes are added Fig. 4. Fig. 1 is the hourly cooling load records of the buildings over the measuring time period (i.e. from 1 July 2007 to 31 Dec 2008). It

shows how frequent the available cooling capacities of the chiller plants would not meet with the cooling loads of the buildings. It indicates that only the cooling load of Building A exceeds its actual available cooling capacity in July 2007. Referring to Figs. 3 and 4, two peak cooling loads are recorded at 8ame10am 9 July 2007 and 8ame10am 19 July 2007. They exceed the actual available cooling capacity of Building A (i.e. 2123 kW) by just 4.0% and 1.1% respectively. It is also observed from Fig. 1 that the cooling loads of the other buildings occasionally exceed their cooling capacities of the first failure mode especially in summer. Details of the cooling load profiles in the peak weeks and peak months are illustrated in Figs. 3 and 4. Reader can easily spot the time and date of the peak cooling loads occurred.

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Fig. 2. Weakly peak cooling loads of Buildings A, B, C, and D from 1-Jul-07 to 31-Dec-08.

201

202

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Fig. 3. Daily cooling loads of Buildings A, B, C, and D in the peak months.

It is considered that the actual available cooling capacity is sufficient if the peak load profile is below the actual available cooling capacity of the building. On the contrary, if there is failure in the refrigeration circuits of the chillers, the reduced available cooling capacity may not be sufficient to cater the peak load. Chillers without

spare cooling capacity are therefore exposed to higher risk at higher failure modes. A summary of the number of hours of operations in different failure modes of the chiller plants of the buildings throughout the period of measurement is shown in Table 5. However, the failure of one of the independent refrigeration circuits

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Fig. 4. Daily cooling loads of Buildings A, B, C, and D in the peak weeks.

203

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Table 3 Combination and physical characteristics of the chiller plants. Building

A

B

C

D

Total available cooling capacity (kW) Total number of chillers Chiller combination (Quantity  kW) Number of refrigeration circuit per chiller Design cooling capacity of each circuit (kW) Number of compressor per circuit Cooling capacity of redundant chiller (kW) Actual cooling capacity (w/o redundancy) (kW)

2697 4 1  844 2 354/490 2/1 574 2123

4728 4 4  1182 2 591/591 2/2 1182 3546

4780 4 4  1195 2 597.5/597.5 2/2 1195 3585

1060 4 4  265 2 132.5/132.5 1/1 265 795

1  705 2 352.5/352.5 1/1

may not cause a shortage of cooling capacity to the building during a hot summer. It is because this surplus of cooling capacity was usually provided by the 10e20% mark up of the calculated cooling load at the design stage. Therefore, the four buildings should have sufficient cooling capacities during the hot summers.

4. Risk exposure Obviously, risk to chiller plant occurs when there is inadequate capacity to cool the building due to the failure of the chillers. The risk exposure is defined as the total number of hours over a year that the cooling load of a building is higher than the cooling capacity of its chiller plant. A summary of the risk exposure of the buildings is shown in Table 5. The table shows that any failure of any refrigeration circuit would have resulted in shortage of cooling capacity of Building A. However, the other buildings have zero risk exposure if they have no redundant chiller. The risk exposure of Building A in year 2007 is only 4 h. With the integrated design approach and the implementation of preventive maintenance which will be discussed in Section 5, the chiller plant should be able to cope with the 4 h’ risk exposure. Table 4 also shows that Building B, C and D have zero risk exposure not only without the redundant chillers but also with the failures of their largest independent refrigeration circuits. Building D even shows that its chiller plant can cope with the cooling load even with the failures of the largest 3 independent refrigeration circuits and with the absence of the redundant chiller during the peak summer period. When the cooling load of a building increases just marginally higher than the maximum cooling capacity of the chiller plant, the chillers may still be able to cope with the cooling load by the following extra cooling capacities. 1. The spare capacity of the compressor or chiller due to the overdesign of the HVAC system by the designer. 2. The spare capacity due to differences in the ambient and the outdoor temperatures which was over-estimated by the HVAC system designer. 3. The manufacturer’s design margin of the chillers.

Table 4 Risk exposure of the chiller plants.

2  574 2 287/287 1/1

Adding up all of these spare capacities may give a maximum extra 5% cooling capacity if the chillers are supplied by reliable chiller suppliers. This could be sufficient for the chillers to cope with the cooling load just beyond the design maximum cooling capacities of the chillers without the redundant chiller. Variable primary flow (VPF) can also be adopted to achieve the increased cooling capacity through the installation of a variable speed chilled water pump [22]. According to Figs. 1 and 2, in average, the buildings experienced half of their annual cooling loads between April and October. Therefore, it is suggested that the maintenance teams should focus on the preventive maintenance between November and March of each year. If the optimal inspection period does not fall within that period, the maintenance staff should adjust the schedule and carry out the inspection or necessary services before April of each year. Considering the mean time between failures (MTBF) of critical parts, some spare parts would have to be ordered outside of the replacement schedule. The application of this risk-based preventive maintenance strategy means that the risk exposure of chiller plants without redundant chillers in terms of system performance loss and environment or ecological loss can be kept to a minimum or even zero.

5. Integrated design New technologies and applications in the HVAC industry could improve the energy efficiency and operation of the chiller plant. However, E&M consultants rarely consider these at the design stage and even if they do their designs may be rejected by the developers

Table 5 Summary of chiller failures in Buildings A, B, C and D. Building Mode

A

B

C

Cooling capacity of failure mode (kW)

Building A

Building B

Building C

Building D

Actual cooling capacity e w/o redundancy First failure mode Second failure mode Third failure mode Fourth failure mode Fifth failure mode Sixth failure mode

2123

3546

3585

795

1633 1279 926.5 574 287 0

2955 2364 1773 1182 591 0

2987.5 2390 1792.5 1195 597.5 0

662.5 530 397.5 265 132.5 0

D

Building load (kW)

>2123 1633e2123 1279e1633 Building Load (kW) w/o Redundancy >3546 First failure 2955e3546 Second failure 2364e2955 Mode Building Load (kW) w/o Redundancy >3585 First failure 2987.5e3585 Second failure 2390e2987.5 Mode Building Load (kW) w/o Redundancy >795 First failure 662.5e795 Second failure 530e662.5 Third failure 397.5e530 Fourth failure 265e397.5 w/o Redundancy First failure Second failure Mode

Risk exposure

Allowable circuit failure

2007

2008

4h 248 h 710 h 2007

0h No circuits would 152 h be failed 1194 h 2008 One independent circuit would be 0h failed 3h 60 h 2008 One independent circuit would be 0h failed 3h 13 h 2008 Three independent circuits would be 0h failed 0h 0h 0h 129 h

0h 0h 0h 2007 0h 0h 70 h 2007 0h 0h 0h 0h 89 h

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at the final approval stage because of the budget constraints or the inherent complexity of such technology compared with conventional designs. Nevertheless, if these new technologies and applications could be included in the design, then the overall cost will be reduced and the system would be more effective.

205

BACnet at the system level of the BMS, and can be accepted by most control suppliers in the market without leading to monopoly [24]. The end user is even able to remotely control and monitor the chiller plant through the Internet. 5.2. Variable primary flow (VPF)

5.1. Chiller plant control (CPC) Central computerized building management systems (BMS) for managing and operating the various systems within a building and usually incorporates controls for an HVAC system, a security system, a fire system, a lighting system, a lift system and so forth [23]. However, the building control suppliers without the specific HVAC knowledge have great difficulty in providing an appropriate control system for a specific application and optimization of the chiller plant. Traditional control strategies for chillers are based only on the conventional sequencing control or the monitoring of certain operating parameters such as the leaving chilled water temperature or loading when starting and stopping the chillers. Also, the chiller plant control is considered as a subsystem of building management system provided by the BMS suppliers. Over the last decade, with the rapid development of the advanced chiller controllers and the advancement in hardware and software, the chiller plant control can now be carried out by standalone systems which are able to optimize the performance of the chiller plants based on the particular characteristics of the chillers. The system therefore runs with minimum required number of chillers with the optimum efficiency of the whole chiller plant. CPC systems can monitor various operating parameters of chillers, such as the leaving chilled water temperature, running amperage of the compressor, and lubricant temperature. They can identify the system or chillers operating at base, peak, and swing, and also allow soft-start application. CPC systems can provide reliable and flexible plant operation by switching to another standby chiller automatically to replace a problem chiller without loss of chilled water flow. They are also useful analytical tool for the operators and the facilities management personnel to identify the deteriorated component of the chiller plant through the provision of regular diagnostic reports. If repair works or service at the identified deteriorated component can be carried out prior to the system failure, the reliability of the chillers or chiller plant will be increased. In the past, proprietary control protocols as shown in Fig. 5 allowed control systems to lock in a single supplier. The more advanced CPC systems illustrated in Fig. 6 use the open communication protocol

To extend the cooling capacity of chillers, more chilled water can be delivered to the evaporator for heat rejection if the compressor has a sufficient margin to handle it. It is impossible in the conventional primary-secondary system because the constant flow production side of the chilled water loop is hydraulically decoupled from the variable flow distribution side. However, chiller manufacturers have now revealed that specific chillers can accommodate the condition with evaporator flow rates down to 0.46 m/s. It extends the chiller’s ability to operate effectively in low flow conditions. The number of successful cases of VPF installation is growing in recent years. The building professionals are more interested in the actual implementation of the VPF. The VPF design uses variable flow pumps in place of the constant flow primary pumps to circulate the chilled water throughout the chilled water loop. The pressure drops along the loop previously overcome by the primary pumps are now overcome by the distribution pumps in the VPF design. The VPF designs still include a bypass line but a modulating control valve, which is not necessary in decoupled system, is added. At low load condition, the modulating valve in the bypass delivers the water necessary to maintain the minimum evaporator flow limit of each operating chiller. In VPF designs, the pump control is independent to the chiller sequencing control. While, the pump controller adjusts the speed of the pump to maintain a predetermined differential pressure across two specific points in the system, the plant controller turns on and off the chillers to achieve the cooling capacity to meet the building cooling load. The VPF designs require fewer pumps and piping connections comparing to the conventional primaryesecondary-circuit systems. Thus, fewer electrical equipment are required and the plant has a smaller footprint. These factors reduce the initial cost of the chiller plant [22]. However, the chilled water pump must be upgraded to include a variable speed drive to cope with the CPC requirements. 5.3. Basic equipment for energy measurement In the past, efficient energy conversation and utilization technologies were expensive. The high cost of fossil fuel and the

Fig. 5. Traditionally, chiller plant control has been considered a subsystem of building management and has thus been provided by BMS suppliers.

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Fig. 6. Advanced Chiller Plant Control (CPC) system complemented with chiller supply.

environmental pollution concerns demand for the equipments of HVAC system with energy efficient design. However, without the effort from the management, the advancement in technology itself may not be sufficient to achieve the energy efficient objective. The commitment and support of an organization’s management team play an important role in the successful implementation and execution of the energy management plan. A monitoring system should be established to collect the energy related data of the building system and store as a database for future evaluation of the energy conservation opportunities. A detailed energy audit should then be performed. Once the energy conservation opportunities have been implemented, it is crucial to monitor and collect the energy related data to reveal the successfulness of the implementation [25,26]. Certain monitoring instruments (e.g. pressure and temperature sensors) for energy audit should be probed into some particular locations of the HVAC system for monitoring the cooling load of the building. However, such instrument may not sufficiently be provided in most of the existing old buildings including the 4 commercial buildings of this research since these old buildings which were not designed with the theme of energy conservation. Table 6 summarized the monitoring instrument in these four commercial buildings. With the measurements taken by the instruments listed in Table 6, the actual operating characteristics of the chiller plants can be compared with their original designs. Such comparison can identify the areas of inefficiency which can also be identified from the data logs of the flow rates and corresponding changes in temperature and pressure. The parameters for comparison include the chiller COP, pump efficiency, piping system frictional loss and control mechanisms. The local code of practice regarding the energy efficiency of airconditioning installations [27], guidelines for energy performance based on the building energy code [28], and the Executive Summary for Private Offices and Commercial Outlets [29] are all good references for determining the efficiency of chiller plants. The Electrical and Mechanical Services Department of the HKSAR government guidelines on energy audits [26] and the chapter on Energy Management in the Building Operations and Management of the ASHRAE Handbook for HVAC Applications [25] are also helpful for general reference. The measuring instruments required to monitor energy efficiency can generate real-time data for comparison and allow chiller plant operators to secure maximum efficiency operation by themselves. Although specialized audit teams or companies can provide

portable ultrasonic flow meters and power analyzers to determine chiller cooling capacity and electric power, this still requires the assistance and cooperation of operation and maintenance personnel to provide information on the building’s operational characteristics and the technical characteristics of the chiller plant. Most energy audits are top-down or corporate initiatives and are not well suited to existing facilities and staff competency. This creates an additional workload for the system operators and property management companies, which may means that they do not provide full support for energy audits. However, if this is made part of the responsibilities of the operators and the chiller plant is sufficiently well designed to provide the necessary operational data, then the energy audit will not be a one-off exercise, but rather a real-time continuous monitoring and improvement program. If measuring instruments are designed into the system and installed at the very beginning, then operators with adequate knowledge of energy management and building services engineering will be able to continuously verify that the air-conditioning system is Table 6 Provision of measuring instruments for energy audits of chiller plant. Item

Measured parameter

Instruments

A

B

C

D

Chiller

Chilled water supply temperature Chilled water return temperature Chilled water flow rate Power usage Internal operating parameter, if any Power usage Differential pressure Chilled water supply temperature Chilled water return temperature Chilled water flow rate Chilled water supply temperature Chilled water return temperature Chilled water flow rate Dry Bulb (DB) Temeprature/Relative Humidity (RH)

Temperature sensor Temperature sensor Flow meter

U

U

U

U

U

U

U

U

Power meter High level data, if any Power meter Pressure sensor Temperature sensor Temperature sensor Flow meter

U U

U

U

U

U

U

U

U

Chiller Internal Log Chilled Water Pump Main header CHW

Secondary CHW loop

Weather

Building

U

U U

Temperature sensor Temperature sensor Flow meter RH sensor/DB sensor

U

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207

the low load condition to maintain the minimum evaporator flow limit of each operating chiller. Overspent in supply and installation of a modulating control valve is reserved (US$6410/ system). g. Installation of a variable speed drive in the secondary pumps. Overspent in supply and installation of variable speed drives is reserved (US$6410/pump for cooling capacity > 705 kW and US $3846/pump for cooling capacity < 705 kW). h. Provision of basic measuring equipment for continuous measurement (e.g. flow rate of the chilled water and power consumptions of the equipments) is necessary to monitor the operating conditions of the chiller plant. Overspent in provision of basic metering and monitoring devices is reserved (US $38,462/system). i. The CPC system is able to identify any deteriorated component of the chiller plant. It can provide an early warning to the maintenance team to service the component prior to the system failure. Therefore, the implementation of the CPC system can effectively increase the reliability of the chiller plant. Overspent in provision and installation of CPC is reserved (US$64,103/system).

operating at maximum efficiency. To cope with the above requirements, property management companies should no longer employ non-skilled workers to operate chiller plants. 6. Financial projection This section studies the cost saving of the chiller plant by the implementation of the integrated design. If the integrated design model is applied to the four commercial buildings under study at the design stage, the differences from the conventional design approach are listed as follows: a. According to the analysis in Section 3, a chiller plant without redundant chiller can still provide sufficient cooling capacity to cope with the cooling load of the building. Therefore, no redundant chiller is provided to the chiller plant. Saving in supply and installation cost of redundant chiller and associated piping and accessories is gained (US$1282/Tons for cooling capacity < 705 kW and US$1026/Tons for cooling capacity > 705 kW). b. Without the provision of the redundant chiller, the risk of the building cooling load exceeding the available cooling capacity of this chiller plant becomes higher. It is advised to select the chillers by one or two commercial size greater than the required. It can provide spare capacity to the overall chiller plant. Overspent in larger size chiller is reserved (US$321/Extra Tons for cooling capacity > 574 kW and US$385/Extra Tons for cooling capacity < 574 kW)). c. For existing chiller plant, after the removal of the redundant chiller, the maintenance of the same size of pumps, valves and piping should be improved. d. Although redundant chiller is considered not necessary to be provided according to the analysis in Section 3, it is still recommend the provision of pumps, valves and piping for an extra chiller for possible future expansion as a sustainable design. This is not a mandatory requirement. Overspent in related piping and accessories is reserved (US$897/Tons for cooling capacity < 705 kW and US$705/Tons for cooling capacity > 705 kW) if this part is adopted. e. In order to maximize the energy saving of the proposed integrated design, it is recommended to replace all small, inefficient and low head primary pumps in the VPF system. Saving in supply of pumps is gained (US$2564/pump for cooling capacity > 705 kW and US$1282/pump for cooling capacity < 705 kW). f. A modulating control valve should be installed in the bypass line of the VPF system to provide the necessary water flow at

If the integrated design were adopted to estimate the cooling load of the four commercial buildings, fewer chillers or chillers with smaller capacities would be used compared with a conventional design. In terms of the installation cost, the installation cost saving in unprovision of redundant chillers and primary pumps can offset the expense of installing the VPF system, the variable speed drive in the secondary pumps, a CPC system, flow meters and power meters. In addition, fewer chillers or chillers with smaller capacities would require fewer electrical accessories to support and also requires a smaller area for the chiller plant. The installation cost saving by the integrated design for Buildings A, B, C and D are shown in Tables 7 and 8. This installation cost saving will provide incentive and initiative to developers and E&M consultants for adopting of the integrated design without violating the budget constraint. In fact, the integrated design will bring huge potential energy saving in operating the chiller plant. Regarding the saving in running cost throughout its operating life of 15 years, without the redundant chillers, it depends on the degree of improvement of energy efficiency upon applying the integrated design. The chiller efficiency is optimized and pump power is minimized by the integrated design. The overall saving can be estimated by measuring the improvement in efficiency and using equations (1) and (2):

Table 7 Integrated design and associated cost savings for Buildings A, B, C and D. Building

A

B

C

D

Total available cooling capacity at conventional design (kW) Chiller combination at conventional design (Qty.  kW)

2697 1  844 1  705 2  574 1  844 1  705 1  574 2123 2123 490 2613

4728 4  1182

4780 4  1195

1060 4  265

3  1182

3  1195

3  265

3546 2955 591 3546

3585 2987.5 597.5 3585

795 397.5 132.5 530

3  1182

3  1195

2  265

228.21

228.21

93.59

Chiller combination at conventional design e w/o Redundancy (Qty.  kW) Actual available cooling capacity e w/o Redundancy (kW) Practical peak cooling capacity Required (kW) Limited spare cooling capacity on largest refrigeration circuit (kW) Total required cooling capacity with limited spare capacity at integrated design (kW) Chiller combination at integrated design (kW) Estimated initial cost savings in chiller combination between conventional and integrated design (Thousand US$)

1  925 2  844 43.59

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Table 8 Break down of installation cost variances for Building A, B, C and D. Building

A

B

C

Saving in deletion of redundant chiller (Thousand US$) Saving in primary pump (Thousand US$) Overspent in upgrading of capacity (Thousand US$) Overspent in adding modulating valve (Thousand US$) Overspent in adding VFD to secondary pumps (Thousand US$) Overspent in provision of metering & monitoring devices (Thousand US$) Overspent in adding CPC (Thousand US$) Overall estimated savings in chiller combination between conventional and integrated design (Thousand US$)

208.97

346.15

346.15

96.15

7.69

10.26

10.26

5.13

(44.87)

0.00

0.00

96.15

(6.41)

(6.41)

(6.41)

(6.41)

(19.23)

(19.23)

(19.23)

(7.69)

(38.46)

(38.46)

(38.46)

(25.64)

(64.10)

(64.10)

(64.10)

(64.10)

228.21

228.21

43.59

D

93.59

Saving in Chiller ¼ Chiller CapacityðTonsÞ  15 Years  Operating HoursðHours=YearÞ  Improvement in EfficiencyðkW=TonsÞ  Tariff RateðUS$=kWhÞ (1)

at peak load in summer is reduced to a minimum. Details of riskbased preventive maintenance are not discussed or elaborated in this paper. However, the assumption that applying risk-based preventive maintenance increases the reliability of chiller plant is adopted as one of the sustainable criteria in the integrated design model. The selection of higher quality and more reliable chiller suppliers, the full utilization of the characteristics of air-cooled screw chillers, and the adoption of risk-based preventive maintenance make a redundant chiller unnecessary but ensure that the reliability of the chiller plant of a building can still be maintained. E&M consultants must design building services systems with adequate equipment for measurement and optimization. The saving on the initial investment of the chiller plant made when an integrated design is applied leaves resources free for the developer to consider the installation of equipment for energy audit and optimization. The air-conditioning system of a building should be tested and re-commissioned from time to time (at least every three to five years) to respond to the changing load demands and characteristics. Updating the operating profile will also make the optimization of the system will more effective. An integrated model for the design of a chiller plant using aircooled screw chillers maximizes the COP at part load and maintains the highest reliability of the chiller plant using risk-based preventive maintenance to generate real-time continuous energy optimization at the lower possible costs. The algorithm used in the model is shown in Fig. 7. The development of a dedicated energy audit team will aid the continuous monitoring and improvement of the energy efficiency

Saving in Pump ¼ 15 Years  Operating HoursðHours=YearÞ  Improvement in EfficiencyðkWÞ Accurately calculate the cooling load

 Tariff RateðUS$=kWhÞ (2)

Add a 10% or 20% buffer, which is around one or two sizes greater than the largest independent refrigeration circuit

7. Discussion and recommendations Redundancy can make a chiller plant system more reliable. However, it also causes resource wastage and ignorance of the importance of maintenance and efficient operation. If the cooling load calculation of a building is accurate and detailed at the design stage, then the designed cooling capacity can be minimized to suit the peak demand cooling capacity with limited spare capacity. The maximum tonnage of each air-cooled screw chiller is about 400 tons. For medium-sized commercial buildings, 200- to 350-ton air-cooled screw chillers are often used. If the risk exposure is considered to be high, then spare capacity or a 10e20% mark up of the calculated cooling capacity can be added to each designated chiller which should avoid the need for an additional or redundant chiller. Where there is uncertainty about the further expansion or the requirement for a greater cooling capacity, E&M consultants can design the system with a redundant chiller. Space will be reserved at the installation stage, but the extra chiller will not actually be installed until practical operating data support its addition. This gives the system sufficient flexibility for future expansion or efficient optimization after a few years of operation. Risk-based preventive maintenance is a combination of the riskbased maintenance strategy and condition-based preventive maintenance. A risk-based preventive maintenance strategy can be applied to a chiller plant to lower the risk exposure. When the optimal inspection interval is set and the mean time between the failure of components considered, the risk exposure of the system

Select N chillers with not more than one refrigeration circuit as spare cooling capacity

Design an E&M installation for N chillers and the installation of primary variable flow, measuring equipment, and chiller plant control system for energy audits

Carry out proper testing and commissioning of the system or chiller plant and continuously collect the operation data and characteristics for analysis

Study the feasibility of the application of improved condenser features to improve the part load efficiencies of the chillers

Produce part load efficiency curves and carry out real-time continuous monitoring of part load efficiencies increasing them to the maximum with improved condenser features Implement risk-based preventive maintenance (RBPM) strategy for the chiller plant

Implement energy management opportunities (EMO) at regular intervals according to cost and complexity

Re-commission the system or chiller plant for every three to five years and collect data for analysis and the identification of EMO

Fig. 7. An integrated model for the design of air-cooled screw chiller plant.

R.K.L. Chan et al. / Building and Environment 46 (2011) 196e209

of HVAC systems and chiller plant. The team will require competent operators with knowledge of energy management and the application of risk-based preventive maintenance strategy to maintain the reliability of the chiller plant. The operators must know all of the details of the operating parameters of the system, and be prepared to undertake an energy audit whole heartedly for job satisfaction and a sense of achievement. Compared with the topdown and ad hoc reactive energy audits by external auditors, auditing carried out by a dedicated in-house team should be a smooth and effective process. However, there are few professionals able to conduct energy audits and management in Hong Kong and the government should thus put more resources into training local Certified Energy Managers (CEM). This will require the involvement of local universities through the provision of postgraduate courses in energy management. The life cycle electricity cost is an important criterion for determining the economic benefits of measuring the energy efficiency of chiller plant through their service life. In Hong Kong, electricity bills are usually charged to tenants through the monthly management fee which means that tenants pay them. There is thus little initiative for owners to carry out preventive maintenance and the optimization of chiller plant. However, it is the social responsibility of an engineer to design a system and optimize its efficiency using a life cycle approach to save the earth’s resources for the next generation. When raw materials become less available and more expensive, the cost of investing in measuring equipment and CPC for energy audits and continuous monitoring will become less significant. However, it will be too late to wait for this economic offset to become apparent. The new integrated design concept of a chiller system without a redundant chiller and the application of risk-based preventive maintenance offer the means of saving resources now. 8. Conclusion Adopting a risk-based preventive maintenance strategy can help to strengthen the reliability of chiller plant, and renders the installation of redundant chillers unnecessary. If accurate and detailed cooling load calculations for a building are made at the design stage, then the specified cooling capacity can be minimized through the provision of a reasonable mark up of the calculated cooling capacity. Spare cooling capacity can then be added to independent refrigeration circuits, rather than installing an additional chiller in the system. The characteristics of the independent refrigeration circuits and the wide capacity range of air-cooled screw chillers allow the design of chiller plants with limited spare cooling capacity. This design approach will not only save building owners in terms of the initial investment in the chiller plant, but also increase the incentive for preventive maintenance to increase the reliability of air-conditioning systems. The saving in capital investment could be used to install measuring equipment and chiller plant control systems for better energy management. Such systems would allow the continuous monitoring and optimization of chillers at part load throughout their life cycle. Most owners pay for redundant chillers to ensure the reliability of the chiller system as a whole. However, that means that little attention need be paid to preventive maintenance and optimization. Consequently, owners or tenants pay more in the total life cycle cost of airconditioning systems. The integrated design of air-cooled screw chiller plants enables the provision of just-in-demand cooling capacity with limited spare capacity. With competent full-time energy management and maintenance personnel in place, it should be possible to closely and continuously monitor the operating characteristics of chiller plants to optimize their lifetime efficiency and reliability through the adoption of a risk-based preventive maintenance strategy.

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Acknowledgement The work described in this paper was fully supported by a grant from CityU (Project No. 7008028).

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