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Economic benefi ts of optimal control for water cooled chiller systems serving hotels in a subtropical climate F W Yu a K T Chanb aHong Kong Community College The Hong Kong Polytechnic University Hong Kong China bDepartment of Building Services Engineering The Hong Kong Polytechnic University Hong Kong China 1 Introduction Hotels play an important role in sustaining the economic growth of Hong Kong Yet they constitute one of the major energy end users in the commercial sector because of their round the clock operation A previous survey 1 indicated that the overall electricity consumption in the hotel segment increased from 443 to 921 GWh a 108 2 increase from 1988 to 2000 With the increasing supply of local hotels it was estimated that their electricity demand could grow by 36 8 GWh per annum over the period of 2001 2005 2 One possible way to moderate the increasing electricity demand is to improve the energy perfor mance of air conditioning systems which account for around half of the total electricity consumption According to studies on the electricity consumption of 16 local hotels 3 4 their energy use intensity EUI the annual electricityconsumption inkWh per unit fl oor area of a building in m2 varied widely from 198 to 926 kWh m2with an average of 406 kWh m2 For hotels with central air conditioning plants water cooled chiller systems with cooling towersarecommonlyusedtoprovide coolingenergyinthe formof chilled water to maintain the thermal conditions required for indoor areas The operation of chillers and cooling towers leads to the peak electricity demand and accounts for about half of electricity consumption for air conditioning Moreover cooling towers rely on evaporation of water in the heat rejection process leading to considerable water consumption when the chillers are operating Variable speed technology has long been considered a standard energy effi cient feature to enhance the energy performance of chillersystems Hartman 5 launchedtheequalmarginal performanceprinciple EMPP toassistinoptimizingthe performance of chiller systems in which variable speed drives VSDs are applied to all the chillers condenser water pumps and cooling tower fans Yet the all variable speed arrangement is seldom considered in system design and VSDs are applied solely to the secondary loop pumps and cooling tower fans in most systems Variable fl ow of chilled water is increasingly used to reduce energy use given that pump energy can be saved when the primary pumps deliver less fl ow to their dedicated chillers at part load conditions The successful application of variable primary fl ow depends on how the fl ow and chiller capacity can be adjusted to match changing load conditions 6 This application is subject to the design of the chilled water distribution circuit and the airside cooling coils are required to be furnished with two way control valves in order to allow the fl ow of chilled water to drop under the reduced load conditions For a constant air volume system requiring high latent cooling capacity the potential of reducing the chilled water fl ow rate under part load conditions is rather limited and so are the pump energy savings Gordon et al 7 and Hydeman et al 8 developed chiller models to study variations of chiller COP coeffi cient of perfor mance at different condenser water fl ow rates No analysis was Energy and Buildings 42 2010 203 209 A R T I C L EI N F O Article history Received 27 July 2009 Received in revised form 25 August 2009 Accepted 25 August 2009 Keywords Water cooled chiller Cooling tower Economic analysis Optimum control A B S T R A C T 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 evaluatesoperatingcostsavingsofachillersystemintegratedwithoptimalcontrolofcoolingtowersand condenser water pumps A sophisticated chiller system model was used to ascertain how different control methods infl uence the annual electricity and water consumption of chillers operating for the cooling load profi le 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 fi xed set point of 29 4 8C for cooling water temperature control The ways to implement this advanced control for system optimization are discussed 2009 Elsevier B V All rights reserved Corresponding author Tel 852 37460416 fax 852 23647375 E mail address ccyufw hkcc polyu edu hk F W Yu Contents lists available at ScienceDirect Energy and Buildings journal homepage 0378 7788 see front matter 2009 Elsevier B V All rights reserved doi 10 1016 j enbuild 2009 08 016 made on the trade off between the chiller power tower fan power and condenser water pump power under the reduced fl ow conditions Alsonocontrolregimewasgivenonhowthecondenser water fl ow should vary in response to the chiller load and wet bulb conditions Variable speed control is increasingly applied to cooling tower fans to reduce their cycling frequency and allow better heat rejection control with decreased fan power 9 The dynamic characteristics of cooling towers with the varying heat rejection airfl ow impose more complications in system optimiza tion Lu et al 10 presented a model based optimization strategy for the condenser water loop of a chiller system The strategy involved minimizing the total power of the chillers condenser water pumps and cooling tower fans taking into account the interaction between the varying air fl ow rate and condenser water fl ow rate for a given heat rejection rate The infl uence of the condenser water entering temperature on both the chiller power and cooling tower performance was analysed Yet they did not explain the control logic of the condenser water pump and cooling tower fan for system optimization Graves 11 reformulated the Gordon Ng model 7 in order to optimize a system designed with two chiller pump pairs and one cooling tower The chiller model was coupled with an Ntu effectiveness Ntu no of transfer units model 12 for evaluating the cooling tower performance Drawing on the chiller tower interaction the modifi ed model was used to analyse how the system COP varied with the changing condenser water fl ow Yet the tower model discounted the water loss due to evaporation and asingleNTUvaluewasassumedtorepresentthetower performance at different water and airfl ow rates Two correlations were identifi ed to facilitate near optimal system operation one was the linear relationship between the cooling water set point and wet bulb an analogy to the fi xed approach method another was the linear relationship betweenthe tower fan speed and pump speed Benton et al 13 developed a regression model to represent the improved cooling tower simulation algorithm CTSA The algorithm simply indicated the cooling tower approach cooling water leaving temperature subtracted from wet bulb tempera ture as a dependent variable of the wet bulb temperature range temperature difference of cooling water condenser water fl ow and fan power It remains to be ascertained how to evaluate an optimal set point for cooling water temperature or the optimal fan speed control in response to dynamic characteristics of cooling towers at part load operation The aforementioned studies have demonstrated important modelling techniques to analyse chiller system performance and some insights on optimizing the control of chiller and cooling tower systems with variable speed control for condenser water pumps and tower fans Yet none of the reported models are comprehensive enough to assess power relationships of chillers condenser water pumps and cooling towers together with water consumption in the heat rejection process with regard to various control methods of cooling towers and condenser water pumps Furthermore most of the studies focus only on electricity sayings without considering the likely trade off between water and electricity savings Morequantitativefi guresandeconomicanalysisareconsidered necessary in order to reap the potential benefi ts of applying optimization technologies to chiller systems The objective of this study is to investigate the economic benefi ts of a water cooled chiller system with the following energy effi cient technologies variable speed control of cooling tower fans a constant approach a fi xed and low cooling water leaving temperature variable condenser water fl ow These technologies are adaptable to most existing chiller systems with minor modifi cations This paper fi rst describes a hotel and its chiller system The method to simulate hourly building cooling loads is presented A sophisticated chiller system model was used to analyse howthe system COP varies with different technologies and to predict the annual electricity and water consumption of the chiller system An assessment was made on the water and electricity cost savings resulting from the individual and mixed uses of the technologies The signifi cance of this study rests on providing more quantitative analysis to promote water cooled chiller systems with optimal operating strategies in order to boost their environmental performance in terms of annual electricity and water consumption and at the same time to reduce their operating costs 2 Evaluation of hourly cooling loads of the hotel Table 1 summarizes the features of the hotel to be modelled The features were compiled into a building description fi le for the multi zone model in the simulation program TRNSYS 15 14 used Table 1 General information about the hotel and its air conditioning systems General Gross fl oor area GFA m2 52 020 Total air conditioned area m2 45 540 87 5 GFA U values of wall window roof W m28C 1 9 5 4 0 7 Shading coeffi cient of glass0 55 AreaGuestroomsShops and restaurants Area per fl oor m2 2 0102640 Air conditioned area per fl oor m2 2 0101560 Number of fl oors186 Cooling temperature set point 8C 2422 Relative humidity 5050 Ventilation rate L s person 7 55 Occupancy m2 person 185 Equipment power density W m2 1250 Lighting power density W m2 1835 Air conditioning system operating hours Monday Friday0100 24000800 2300 Saturday0100 24000800 2300 Sunday0100 24000800 2300 Air side system details Type of air handling unitsFan coil units FCUs CAV AHU and FCUs Chilled water fl ow control2 way valve2 way valve Chiller plant operating hours all days 0100 2400 F W Yu K T Chan Energy and Buildings 42 2010 203 209204 for building system energy analysis The hotel had average characteristics in terms of the number of guestrooms total fl oor area and EUI according to the energy end use surveys for the local hotel sector 2 4 Based on a typical example of hourly weather data under the subtropical climate of Hong Kong the annual cooling energy of the hotel was evaluated to be 17 692 414 kWh withatotalof8152coolinghourswhichaccountfor93 1 ofatotal of 8760 h year Fig 1 shows the frequency distribution of the hourly building load ratios a ratio to the peak cooling load of 4753 kW The building load ratios ranging from 0 5 to 1 accountfor 47 of the total operating hours This suggests that the chiller system needs to operate under part load conditions for most of the time Higher building load ratios are generally associated with higher outdoor temperatures though a certain range of building load ratios especially below 0 5 could be present at a large range of outdoor temperatures 3 Method of studying the chiller system 3 1 Arrangement of the chiller system To meet the peak cooling load of 4753 kW the total capacity of the chiller system was designed to be 4800 kW As Table 2 illustrates the system contains three identical chillers rated at a cooling capacity of 1600 kW each This multiple chiller arrange mentis commonlyusedto enhance fl exibility and back up capacity at part load operation The capacity modulation of each centrifugal chiller is done by varying the opening of the inlet guide vanes Each chiller is coupled with a constant speed chilled water pump to deliver the rated fl ow rate to the chiller evaporator There is a differential pressure by pass pipe in the single loop pumping system to balance the fl ow of chilled water supplied by the staged chillers against the demand of chilled water required by the airside system coils Chiller sequencing was implemented under which all the running chillers carried the same part load and no additional chillers in the chiller arrangement started to operate until each of the operating chillers was running at full load Following this strategy for staging chillers two and three chillers operated when the building cooling load exceeded 1600 and 3200 kW respec tively Each chiller operating was interlocked with one chilled water pump one condenser water pump and one cooling tower 3 2 Calculation of the annual electricity consumption of the chiller system A sophisticated chiller system model was developed using TRNSYS 15 in order to compute the annual electricity consumption of chillers pumps and cooling tower fans The model considered the real process phenomena including the capacity control of compressors and mechanistic relations between chiller compo nents In simulating the operation of a chiller of any given size the compressors and the condenser satisfi ed the mass balance of refrigerant and the energy balance at the evaporator The cooling towers were modelled using the Ntu effectiveness approach with which a change in the heat transfer effectiveness of the tower at various combinations of airfl ow and water fl ow is taken into account Analgorithmwasincludedinthecoolingtowermoduleto compute the operating variables of the cooling towers based on a given set point of cooling water leaving temperature Details about the development of the model and its validation were given elsewhere 15 The hourly chiller power was computed by using the chiller system model based on the hourly weather data and the load which each operating chiller carried The electricity consumption of the chilled water pumps varied step by step depending on the number of chillers operating The fan power of each cooling tower operating depends on whether it is on off or variable speed control is used and on the set point of the temperature of cooling water leaving the cooling tower The overall annual electricity con sumption of the chiller plant is the sum of all chiller power pump power and cooling tower fan power for all operating hours 3 3 Prediction of water consumption costs and electricity costs The make up water of a cooling tower serves to compensate for three parts of water loss evaporation drift and bleed off The cooling tower module within the system model considered the overall mass and energy balance of cooling water and air to estimate the evaporation loss The drift rate and bleed off rate were assumed to be 0 2 and 0 6 respectively of the cooling water circulation rate with regard to the use of the traditional chlorination water treatment technology The make up water cost of HK 4 58 m3and the sewagecharge of HK 1 2 m3frombleed off were used to calculate the overall water consumption and discharge cost for the operation of cooling towers based on the guidelines stated in the pilot scheme for the wider use of fresh water in evaporative cooling towers for energy effi cient air conditioning systems 16 The calculation of the electricity cost of the chiller plant was based on the maximum demand tariff of a Fig 1 Frequency distribution of the hourly data of building load ratios Table 2 Details of the chiller system Total cooling capacity kW Three identical sets of chillers pumps and cooling towers 4800 For each chiller Compressor typeCentrifugal Refrigerant typeR134a Nominal cooling capacity kW 1600 Nominal compressor power kW 280 7 COP at full load5 7 Design chilled water supply return temperature 8C 7 12 5 Design chilled water fl ow rate L s 72 Design condenser water entering leaving temperature 8C 33 38 Design condenser water fl ow rate L s 87 For each cooling tower Heat rejection capacity kW 2004 Design entering leaving temperature 8C 34 4 29 4 Water fl ow rate L s 87 Air volume fl ow rate m3 s 63 Fan motor power kW 22 8 Drift loss of nominal fl ow 0 2 Design water evaporation rate of nominal fl ow 0 53 Design wet bulb outdoor temperature 8C 28 Rated power of each chilled water pump kW 28 Rated power of each condenser water pump kW 21 6 F W Yu K T Chan Energy and Buildings 42 2010 203 209205 local power company and the overall electricity consumption calculated from the chiller system model The tariff structure involves different rates for the monthly demand charges and energy charges The demand charge is HK 42 1 kVA in the month for the fi rst 400 kVA and HK 41 1 kVA for the next additional kVA The energy charge is HK 1 023 unit kWh for the fi rst 200 units supplied per month per kVA of maximum demand and HK 0 962 per unit for each additional unit supplied 4 Detailed analysis of chiller system performance with various energy effi cient technologies Simulation was carried out on eight operating schemes given in Table 3 Schemes 1 4 represent a constant speed confi guration for both the cooling tower fan and condenser water pump Under this confi guration the tower fan is cycled on and off to deliver the heat rejection airfl ow required to meet a given temperature of cooling water leaving the cooling tower Tctwl for any heat rejection The condenser water pump is staged continuously to provide the chiller operating with the rated fl ow of condenser water for all loading conditions For Schemes 5 8 variable speed control is applied to both the tower fan and condenser water pump in order to improve their energy effi ciency at part load operation Under this control the fan speed is modulated in a range of 10 100 full speed