Analysis of Air-Side Economizers in Terms of Cooling-Energy Performance in a Data Center Considering Exhaust Air Recirculation

energies

Article Analysis of Air-Side Economizers in Terms of Cooling-Energy Performance in a Data Center Considering Exhaust Air Recirculation

Seonghyun Park 1 and Janghoo Seo 2,*

1 Department of Architecture, Graduated School, Kookmin University, 77 Jeongneung-ro, Seongbuk-gu, Seoul 02707, Korea; [email protected] 2 School of Architecture, Kookmin University, 77 Jeongneung-ro, Seongbuk-gu, Seoul 02707, Korea * Correspondence: [email protected]; Tel.: +82-2-910-4593

Received: 24 January 2018; Accepted: 13 February 2018; Published: 17 February 2018

Abstract: Demand is soaring for data centers with advanced data-processing capabilities. In data centers with high-temperature information technology (IT) equipment, enormous cooling systems are operated year-round. To date, studies have aimed to improve the cooling efficiency of server-room units, but cooling-energy-consumption analysis considering the recirculation of exhaust air (EA) has not been researched to a sufficient degree. This study analyzed the cooling-energy saving effects considering the EA-recirculation and supply-air (SA)-temperature conditions when direct and indirect air-side economizers were applied to a data center in Korea. Thirteen case studies were conducted. The results showed that when the EA-recirculation ratio in the direct air-side economizer was 15%, its annual cooling-energy consumption increased by approximately 6.1% compared to the case with no recirculation. The indirect air-side economizer also exhibited an approximately 9% increase in cooling-energy consumption. On the other hand, when the SA temperature changed to 22 ◦C, the annual cooling-energy consumption of the direct and indirect air-side economizers decreased by approximately 67% and 55%, respectively, compared to a central chilled-water system. This indicates the importance of developing measures to prevent EA recirculation and of securing a wind path for the improvement of air-conditioning efficiency in data centers at the design stage.

Keywords: data center; air-side economizer; energy simulation; cooling system; recirculation

1. Introduction Demand is soaring for data centers with advanced data-processing capabilities, owing to the rapid advancement of the information technology (IT) industry [1,2]. The global IT-related market size in 2016 was reported as $153 billion [3]. IT equipment accounts for 40% of total data-center-operating costs [4]. In data centers with high-temperature IT equipment, enormous cooling systems are operated year-round to minimize the economic losses caused by malfunctions and errors of IT equipment [5]. Cooling-energy consumption accounts for approximately 31% of the total energy consumption in data centers [6]. Therefore, studies have been actively conducted on cooling systems that can improve heat-source efficiency by introducing air-side economizers to reduce the cooling energy consumed by data centers [7–12]. The air-side-economizer system can significantly reduce the cooling energy consumed by data centers. Representative air-side economizers applicable to data centers are divided into direct and indirect types, and the appropriate type must be selected at the design stage by considering the salt or dust contamination caused by regional characteristics [13]. A direct air-side economizer must effectively discharge the heated air that passes through a server rack to the outside, while the indirect air-side economizer must control the air flow through the

Energies 2018, 11, 444; doi:10.3390/en11020444 www.mdpi.com/journal/energies Energies 2018, 11, 444 2 of 14 installation of a separate wind path to prevent the recirculation of heated air. To date, many studies have been conducted on the improvement in cooling efficiency in the server-room unit, but analysis of cooling-energy consumption by considering the recirculation of exhaust air (EA) according to the shape of the data-center building has not been adequately researched. EA recirculation mixes part of the EA with outdoor air (OA) and causes a rise in temperature, thereby lowering the cooling-load-reduction effect in air-side economizers. This means that it is necessary to consider the effects of recirculating air dischargedEnergiesEnergies to the2018 20182018, outside11,, 1111, x,, xFORx FORFOR PEER PEER when REVIEW REVIEW the changes in cooling-energy consumption with the2 applicationof22 ofof 14 1414 of an air-side economizer are analyzed. When cooling-energy consumption is calculated by considering AA direct direct air air‐side‐‐sideside economizer economizer must must effectively effectively discharge discharge the thethe heated heated air air that thatthat passes passes through throughthrough a a EA recirculation, a more accurate analysis of this quantity will be possible. In addition, in server serverserverserver rack rackrack to toto the thethe outside, outside, while while the thethe indirect indirectindirect air air‐side‐‐sideside economizer economizer must must control controlcontrol the thethe air air flow flowflow through throughthrough racks locatedthethethe installation installationinstallation in the server of of a aseparate separateseparate rooms wind wind of a path datapath to toto center, prevent prevent “hot”the thethe recirculation recirculationrecirculation and “cold” of of heated heated aisles air. air. are To To alternatelydate, date, many many arranged to efficientlystudiesstudiesstudies supply have have been been the conducted conducted coldconducted air on dischargedon the thethe improvement improvementimprovement from in inin the cooling coolingcooling computer-room efficiency efficiency in inin the thethe server air serverserver conditioning‐room‐‐roomroom unit, unit, but but (CRAC) or computer-roomanalysisanalysis airof of handlercooling coolingcooling‐energy‐‐energy (CRAH) consumption consumptionconsumption to the server by by considering consideringconsidering racks [14 ].the thethe In recirculation general,recirculationrecirculation large of of exhaust dataexhaust centers air air (EA) (EA)(EA) are operated accordingaccording to toto the thethe shape shapeshape of of the thethe data data‐center‐‐centercenter building building has has not not been been adequately adequately researched. researched.researched. EA EA using underfloor distribution systems. recirculationrecirculationrecirculation mixes mixes part part of of the thethe EA EA with with outdoor outdoor air air (OA) (OA)(OA) and and causes causescauses a arise riserise in inin temperature, temperature,temperature, thereby therebythereby Thislowering meansloweringlowering thatthe thethe cooling coolingcooling the cooling-energy‐load‐‐loadload‐reduction‐‐reductionreduction effect effect consumption in inin air air‐side‐‐sideside economizers. economizers. of the same This This heat-source means means that thatthat it it itis system isis necessary necessary can to toto be reduced by improvingconsiderconsiderconsider the the the coolingthe effects effects of efficiency, of recirculating recirculatingrecirculating thus air air discharged increasing discharged to toto the the thethe outside temperature outside when when the thethe of changes thechangeschanges supply in inin cooling coolingcooling air‐ (SA)energy‐‐energy delivered to the serverconsumption room.consumptionconsumption This with studywith the thethe calculated application application theof of an cooling-energyan air air‐side‐‐sideside economizer economizer consumption are are analyzed. analyzed. of When anWhen existing cooling coolingcooling‐ dataenergy‐‐energy center using consumptionconsumptionconsumption is isis calculated calculatedcalculated by by considering consideringconsidering EA EA recirculation, recirculation,recirculation, a amore more accurate accurate analysis analysis of of this thisthis quantity quantity dynamic-energy simulation and analyzed the cooling-energy saving effect considering the various willwill be be possible. possible. In InIn addition, addition, in inin server serverserver racks racksracks located locatedlocated in inin the thethe server serverserver rooms roomsrooms of of a adata data center, center,center, “hot” “hot” and and condition“cold” of“cold” EA aisles aisles recirculation are are alternately alternately and arranged arranged SA-temperature to toto efficiently efficiently supply supplysupply when the thethe direct cold coldcold air air and discharged discharged indirect from fromfrom air-side the thethe computer computercomputer economizers‐ ‐‐ are applied toroomroomroom a data air air conditioning center conditioningconditioning in Korea. (CRAC) (CRAC)(CRAC) or or computer computercomputer‐room‐‐roomroom air air handler handler (CRAH) (CRAH)(CRAH) to toto the thethe server serverserver racks racksracks [14]. [14].[14]. In InIn general,general, large largelarge data data centers centerscenters are are operated operated using using underfloor underfloor distribution distribution systems. systems.systems. 2. Cooling LoadThisThis means and means Energy that thatthat the thethe cooling Consumption coolingcooling‐energy‐‐energy consumption consumptionconsumption of a Conventional of of the thethe same samesame heat heat Data‐source‐‐sourcesource Center system systemsystem can cancan be be reduced reducedreduced byby improving improvingimproving the thethe cooling coolingcooling efficiency, efficiency, thus thusthus increasing increasingincreasing the thethe temperature temperaturetemperature of of the thethe supply supplysupply air air (SA) (SA)(SA) delivered delivered 2.1. Overviewtototo the thethe of server serverserver Data room. room. Centerroom. This This study studystudy calculated calculatedcalculated the thethe cooling coolingcooling‐energy‐‐energy consumption consumptionconsumption of of an an existing existing data data center centercenter usingusing dynamic dynamic‐energy‐‐energy simulation simulationsimulation and and analyzed analyzed the thethe cooling coolingcooling‐energy‐‐energy saving savingsaving effect effect considering consideringconsidering the thethe Tablevarious1various shows condition conditioncondition an overview of of EA EA recirculation recirculationrecirculation of a data and center.and SA SA‐temperature‐‐temperaturetemperature This major when when data direct direct center and and locatedindirect indirectindirect inair air‐side Seoul,‐‐sideside Korea, has four undergroundeconomizerseconomizers are are floorsapplied applied and to toto a adata 12 data ground center centercenter in inin floors;Korea. Korea. the IT server rooms are located from the second to the 10th floors.2.2. Cooling Cooling In Load addition, Load and and Energy Energy the data Consumption Consumption center has of of a a chillersConventional Conventional installed Data Data Center Center in the underground machine room and uses a central chilled-water system, which cools the IT equipment by producing chilled water and supplying2.1.2.1. Overview Overview it to of the ofof Data Data server Center Center rooms on each floor. The floor height is 4800 mm, and an underfloor distribution systemTableTable 1 1shows isshowsshows used an an overview foroverview air distribution.of of a adata data center. center.center. InThis This the major major air-distribution data data center centercenter located locatedlocated system, in inin Seoul, Seoul, the Korea, Korea, air cooledhas has by the fourfour underground floorsfloors and 12 ground floors;floors; thethe ITIT serverserver roomsrooms are locatedlocated fromfrom thethe secondsecond toto thethe CRAH coolingfour underground coil is supplied floors and to 12 the ground server floors; racks the through IT server therooms cold are aisleslocated of from the the access second floors to the and server 10th10th floors. floors.floors. In InIn addition, addition, the thethe data data center centercenter has has chillers chillerschillers installed installedinstalled in inin the thethe underground underground machine machine room roomroom and and rooms, andusesuses the a acentral heatedcentralcentral chilled chilledchilled air returns‐water‐‐water system, system,system, to CRAH which which throughcools coolscools the thethe IT the ITIT equipment equipment hot aisles. by by producing producing chilled chilledchilled water water and and supplyingsupplyingsupplying it it itto toto the thethe server serverserver rooms roomsrooms on on each each floor. floor.floor. The The floor floorfloor height height is isis 4800 4800 mm, mm, and and an an underfloor underfloor distributiondistribution system systemsystem is isis used used for forfor air Table air distribution. distribution. 1. Overview In InIn the thethe of air air data‐distribution‐‐distribution center. system, system,system, the thethe air air cooled cooledcooled by by the thethe CRAHCRAH cooling coolingcooling coil coilcoil is isis supplied suppliedsupplied to toto the thethe server serverserver racks racksracks through throughthrough the thethe cold coldcold aisles aisles of of the thethe access access floors floorsfloors and and serverserverserverClassification rooms, rooms,rooms, and and the thethe heated heated Parameter air air returns returnsreturns to toto CRAH CRAH through throughthrough the thethe Value hot hot aisles. aisles. Unit Location Seoul, Korea - TableTable 1. 1. 1.Overview Overview of of data data center. center.center. Latitude 37.523 N ClassificationClassification ParameterParameter ValueValue UnitUnit Longitude 126.871 E Datacenter LocationLocation Seoul,Seoul,Seoul, Korea Korea ‐ ‐ ‐ − Gross floorLatitudeLatitude area 37.523 64,70037.52337.523 NN m 2 DatacenterDatacenter CRACLongitude capacityLongitude 126.871126.871126.871 30 E E RT GrossGross floor floorfloor area areaarea 64,70064,70064,700 mm−2 −−22 FloorCRACCRAC height capacity capacitycapacity 30 48003030 RTRT mm CeilingFloorFloorFloor height height height 4800 120048004800 mmmm mm Server room CeilingCeiling height height 120012001200 mmmm ServerServer room roomroom Access floor height 600 mm AccessAccess floor floorfloor height height 600600600 mmmm ◦ SA conditionSASASA condition conditioncondition 13.5, 13.5,13.5, 50 5050 °C,°°C, % % C, %

PanoramaPanoramaPanorama of ofof data data center center centercenter CeilingCeilingCeiling grill grillgrill of of ofof hot hot hot aisles aisles aislesaisles side side side AirﬂowAirflowAirflow diagram diagram diagram in in inthe thethe server serverserver server room roomroom room

Energies 2018, 11, 444 3 of 14

2.2. BoundaryEnergies 2018 Condition, 11, x FOR PEER of Energy REVIEW Simulation 3 of 14

To2.2. calculate Boundary theCondition cooling-energy of Energy Simulation consumption of the existing central chilled-water system in the data center, the cooling load of the server rooms must first be calculated. Table2 shows the boundary To calculate the cooling‐energy consumption of the existing central chilled‐water system in the conditionsdata center, of the the energy cooling simulation. load of the server For therooms heat must generated first be calculated. by IT servers, Table 2 6480shows server the boundary racks were assumedconditions to be installed,of the energy including simulation. 4 kW-class For the server-rack heat generated 720 by units IT servers, per floor. 6480 The server converted racks were value of serverassumed heat was to 2880be installed, kW per including floor. The 4 kW cooling‐class loadserver generated‐rack 720 units by resident per floor. personnel The converted and lightingvalue of was considered.server heat For was load-pattern 2880 kW per changes floor. The according cooling toload server generated operations, by resident values personnel ranging and from lighting 0.7 to 1.0 werewas applied considered. for each For hour. load‐ TRNSYSpattern changes (17.2, according Thermal to Energy server Systemoperations, Specialists, values ranging Madison, from WI,0.7 to USA) was used1.0 were to quantitatively applied for each analyze hour. theTRNSYS annual (17.2, cooling-energy Thermal Energy consumption System Specialists, of the data Madison, center (8760WI, h). The meteorologicalUSA) was used datato quantitatively for Seoul provided analyze the by annual TRNSYS cooling were‐energy used asconsumption the OA conditions of the data [15 center]. (8760 h). The meteorological data for Seoul provided by TRNSYS were used as the OA conditions [15]. Table 2. Boundary condition for calculating cooling load.

Table 2. Boundary condition for calculating cooling load. Classification Description IT equipmentClassification heat load Converted value ofDescription server heat 2880 kW × 9 (floor) IT equipment heat load Converted value of server heat 2880 kW × 9 (floor) People load Personnel/daily schedule of 20 residing personnel People load Personnel/daily schedule of 20 residing personnel WeatherWeather data data Seoul, Korea (obtained from TRNSYS) LightingLighting load load 1616 Wm Wm−2−2 ×× 22502250 m−−2 2(floor(floor area) area) TimeTime 00–0800–08 h h 09–2009–20 h h 20–24 20–24 h h Server rackServer load rack ratio load ratio 0.70.7 1.01.0 0.8 0.8

Server operationServer operation

Applied for each hour (0.7–1.0) Applied for each hour (0.7–1.0)

2.3. Devices2.3. Devices Used Used in Central in Central Chilled-Water Chilled‐Water Systems Systems

2.3.1.2.3.1. Water-Cooled-Chiller Water‐Cooled‐Chiller Component Component The componentsThe components of theof the existing existing data-center data‐center coolingcooling system system and and their their specifications specifications are listed are listed in in Table3Table. The 3. coefficient The coefficient of performance of performance ( COP(COP),), whichwhich represents represents the the performance performance index index of a ofchiller, a chiller, varies depending on the target inflow temperatures of the chilled and cooling water. varies depending on the target inflow temperatures of the chilled and cooling water.

Table 3. Specifications of central chilled‐water system. Table 3. Speciﬁcations of central chilled-water system. Component Specifications Unit Power Consumption Chiller Q = 4567 kW, COP = 6 8 765 kW ComponentR Speciﬁcations Unit Power Consumption Cooling tower Forced draft .Cooling tower, QR = 6345 kW 8 120 kW Chiller −1 −1 −1 8 765 kW Chilled water pump m chw = 56,400QR kg=∙h 4567, Cp,chw kW, = COP 4.19 kJ =∙ 6kg ∙K 8 110 kW −1 . −1 −1 Cooling water pump m cw = 93,600 kg∙h , Cp,chw = 4.19 kJ∙kg ∙K 8 160 kW Cooling tower Forced draft Cooling tower, QR = 6345 kW 8 120 kW Outdoor air supply fan 26,450 CMH 648 11 kW . −1 −1 −1 Chilled water pump mNumberchw = 56,400 of rows kg· h= 5, ,CNumberp,chw = of 4.19 tubes kJ· kg= 60 ·K 8 110 kW Cooling coil 324 7.5 kW . −1 −1 −1 CoolingCRAH water pump Tubem cwouter= 93,600 diameter kg· h12 mm;,Cp,chw inner= 4.19diameter kJ·kg 11 mm·K 8 160 kW Capacity 22,800 CMH 324 7.5 kW Outdoor air supply fan 26,450 CMH 648 11 kW Number of rows = 5, Number of tubes = 60 The COP Coolingvalue of coil the chillers used in this study was 6 based on the set chilled324‐water temperature 7.5 kW CRAH Tube outer diameter 12 mm; inner diameter 11 mm of 7 °C and the Capacitycooling‐water‐inflow temperature 22,800 CMH of 30 °C [15]. The chiller 324 COP is calculated 7.5 kW using Equation (1) and capacity under current conditions is calculated using Equation (2):

COPCOPratedCOPratio (1)

Energies 2018, 11, 444 4 of 14

The COP value of the chillers used in this study was 6 based on the set chilled-water temperature of 7 ◦C and the cooling-water-inﬂow temperature of 30 ◦C[15]. The chiller COP is calculated using Equation (1) and capacity under current conditions is calculated using Equation (2):

COP = COPrated × COPratio (1)

Capacity = Capacityrated × Capacityratio (2) The chiller load is calculated using Equation (3).

. . Qload = mchw × Cp,chw × (Tchw,in − Tchw,set) (3)

In addition, the part load ratio (PLR), which is the ratio between the load capacity and design capacity of the chillers, was investigated and reﬂected in the simulation analysis. PLR is calculated by Equation (4), while the outlet temperature of the chilled-water stream is given as Equation (5).

. Q PLR = load (4) Capacity

. Qmet Tchw,out = Tchw,in − . (5) mchw × Cp,chw

2.3.2. Cooling-Tower Component The cooling tower is of a closed-circuit type. The cooling-water temperature difference (range) between the inlet and outlet of the cooling tower and the difference (approach) between the cooling-water-outlet and inlet-air wet-bulb temperatures was 5 ◦C. The fan power of the cooling tower was set to an operational rate of at least 80% to prevent freezing in winter. In addition, the heat source used as the cooling and chilled water had a speciﬁc heat of 4.19 kJ·kg−1·K−1. This cooling-tower model relied on the basic premise that the saturated-air temperature is the temperature at the air–water interface and that of the outlet ﬂuid. With this assumption, the air enthalpy can be calculated from Equation (6) [16].

. Q ( ) = ( ) + f hsat Tf ,out ha Ta,in y−1 (6) . . ma ma × 1 − exp −λdesign . ma,design

Here, y = 0.6 for most applications, and the designed outlet-fluid-temperature, air- and fluid-flow-rate, and inlet-air conditions are required by the cooling-tower model as parameters. We can use Equation (7) to calculate the design conditions (hair(Tair,out,design)) as an unknown in order to solve for λdesign [17]. " # hsat(Tf ,in,design) − hair(Ta,in,design) λdesign = ln (7) hsat(Tf ,out,design) − hair(Ta,out,design) The outlet-air enthalpy can be determined from the energy balance on the cooling tower and can be expressed as Equations (8) and (9).

. Qfluid,design = mfluid × Cp,fluid Tfluid,in,design − Tfluid,out,design (8)

 .  Qﬂuid,design hair Tair,out,design = hairTair,in,design + .  (9) mair Energies 2018, 11, 444 5 of 14

The power of the cooling-tower fan is calculated using Equation (10).

. j 2 3 k Pfan = Pfan,rated a0 + a1(γa) + a2(γa) + a3(γa) + ... (10)

2.3.3. Cooling-Coil Component The central chilled-water system reduces the load on the chillers by bypassing the chilled-water flow into CRAH depending on the server-room heat. In this study, a simplified cooling coil was fitted as our model. The total heat-transfer rate across a chilled-water coil cooling a moist air stream is the difference in enthalpy across either flow stream, given as Equation (11).

. . Qt = mw × Cp,w × (Tw,out − Tw,in) (11)

A third empirical relation for the total heat transfer is taken from the American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) and given as Equations (12)–(16) [18].

. Qt = Nrow × A f × BRCW × WSF × LMTD (12)

1 C C C C C = × 2 × 3 × 4 × 5 × 6 C1 2 3 2 4 (13) BRCW Va Vw Vw Va VwVa 2 2 2 3 2 3 WSF = k1 + k2∆T1 + k3∆T1∆T2 + k4∆T1 + k5∆T1∆T2 + k6∆T1 ∆T2 + k7∆T1∆T2 + k8∆T1 ∆T2 3 3 (14) +k9∆T1 ∆T2

∆T1 = Tdp,in − Tw,in (15)

∆T2 = Tdb,in − Tw,in (16) where BRCW is the heat transfer per unit face area per log-mean-temperature difference for an entering water temperature equal to the dew-point temperature of the entering air stream, WSF is a wetted surface factor, and LMTD is the log mean temperature difference between air and water-flow streams. In this study, BRCW and WSF were calculated using the empirical constant values of previous research [18]. However, operating conditions of cooling coil should be considered to use this equation because this model was developed to fit in the condition of specific range in terms of face air velocities, chilled water velocities, entering chilled water temperature, and dry bulb temperature of entering and leaving air. The velocities of the air and water streams are calculated using Equations (17) and (18).

. ma Va = (17) ρa × A f

. 4m = a Vw 2 (18) ρw × π × di × Ncoil

2.3.4. Fan and Pump Components All fans and pumps operate at constant speed with on/off signal control. Fans have a variable air-flow rate, and pumps have a constant water-flow rate. The fan and pump efficiencies were set to 65%. The power consumed by the fan and pump can be calculated as Equations (19) and (20), respectively [15]. j 2 3 ik . Pfan = Pmax c0 + c1β + c2β + c3β + ... + ci β , if m > 0 (19)

Ppump = Prated × ηmotor (20) Energies 2018, 11, 444 6 of 14

2.4. Cooling Load and Energy Consumption Figure1 shows the cooling-load-calculation results. The cooling load generated in the server rooms of the data center ranged from 18,000 to 24,000 kW, depending on the operation rate of the server, independent of the OA. This was because approximately 98% of the cooling load was generated by the server racks, meaning that enormous cooling energy was consumed even when the ambient temperature was low in winter. The percentage of power consumed by each component out of the total annual power consumption of the central chilled-water system is shown in Figure2. The chillers accounted for 47% of the total power consumption. Therefore, it is expected that the power consumption of the chillers will decrease just by improving the cooling efﬁciency of the server rooms Energiesbecause 20182018 of,, increasing1111,, xx FOR PEER the REVIEW set-chilled water and SA temperature. 66 ofof 1414

Figure 1. MonthlyMonthly coolingcooling load load of server server room. room.

Figure 2. PercentagePercentage of power consumption consumption of each each component. component.

Figure 3 shows the monthly cooling‐energy consumption of the central chilled‐water system. Figure 33 showsshows thethe monthlymonthly cooling-energycooling‐energy consumptionconsumption ofof thethe centralcentral chilled-waterchilled‐water system.system. The monthly power consumption of the components excluding the cooling tower and chillers was The monthly power consumption of the components excluding the cooling tower and chillers was constant. This isis because only thethe temperaturetemperature of thethe chilled water returningreturning toto thethe chillers throughthrough constant. This is because only the temperature of the chilled water returning to the chillers through thethe bypass control changes. The flowflow raterate of thethe pump or thethe air volume of CRAH are held constant, the bypass control changes. The ﬂow rate of the pump or the air volume of CRAH are held constant, regardlessregardless of thethe serverserver‐‐roomroom heat and OA conditions. InIn addition, thethe energy consumed by thethe regardless of the server-room heat and OA conditions. In addition, the energy consumed by the chillers chillers increasedincreased with thethe ambient temperature.temperature. This was because COP decreased because thethe increased with the ambient temperature. This was because COP decreased because the temperature temperaturetemperature of thethe cooling water flowingflowing intointo thethe chillers increasedincreased as thethe outdoor‐‐air wet‐‐bulb of the cooling water ﬂowing into the chillers increased as the outdoor-air wet-bulb temperature rose. temperaturetemperature rose.rose. This means thatthat thethe energy consumed toto produce thethe samesame amount of 5°C chilled This means that the energy consumed to produce the same amount of 5◦C chilled water increased. water increased.increased. The energy consumed by thethe cooling towertower also increasedincreased slightlyslightly inin thethe summersummer The energy consumed by the cooling tower also increased slightly in the summer when the outdoor-air when thethe outdoor‐‐air wet‐‐bulb temperaturetemperature was relativelyrelatively high. wet-bulb temperature was relatively high.

Figure 3. Monthly coolingcooling energyenergy consumptionconsumption of centralcentral chilledchilled‐‐water system.system.

Energies 2018, 11, x FOR PEER REVIEW 6 of 14

Figure 1. Monthly cooling load of server room.

Figure 2. Percentage of power consumption of each component.

Figure 3 shows the monthly cooling‐energy consumption of the central chilled‐water system. The monthly power consumption of the components excluding the cooling tower and chillers was constant. This is because only the temperature of the chilled water returning to the chillers through the bypass control changes. The flow rate of the pump or the air volume of CRAH are held constant, regardless of the server‐room heat and OA conditions. In addition, the energy consumed by the chillers increased with the ambient temperature. This was because COP decreased because the temperature of the cooling water flowing into the chillers increased as the outdoor‐air wet‐bulb temperature rose. This means that the energy consumed to produce the same amount of 5°C chilled water increased. The energy consumed by the cooling tower also increased slightly in the summer Energies 2018, 11, 444 7 of 14 when the outdoor‐air wet‐bulb temperature was relatively high.

Figure 3. Monthly cooling energy consumption of central chilled chilled-water‐water system.

3. Effect of Air-Side Economizers on Server Cooling in the Data Center

3.1. Case Study Table4 shows the 13 cases examined in this study. In Case 1, the central chilled-water system was applied to the target data center; this is our reference case, and the SA temperature was 13.5 ◦C. The air-side economizers were divided into direct and indirect types. The parameters affecting the cooling-energy performance of each air-side economizer were the set SA temperature and the EA recirculation ratio. The recirculation ratio was set to 5%, 10%, and 15% by taking advantage of the fact that the EA and OA grills of the data center are generally installed on the same side of the building.

Table 4. Condition of case.

Case Cooling System SA Set Temperature EA Recirculation Rate Case 1 Central chilled cooling system - Case 2 0% Case 3 13.5 ◦C 5% Case 4 10% With direct air-side economizer Case 5 15% Case 6 18 ◦C 0% Case 7 22 ◦C Case 8 0% Case 9 5% 13.5 ◦C Case 10 10% With indirect air-side economizer Case 11 15% Case 12 18 ◦C 0% Case 13 22 ◦C

Equation (21) is used to calculate TOA∗, which was inﬂuenced by the recirculation of exhaust air.

TOA∗ = TOA × (1 − rre,EA)+TEA × rre,EA (21)

Moreover, the SA temperatures were set, according to the improvement in cooling efﬁciency, as 18 ◦C and 22 ◦C. When EA recirculation was 15% under the climate conditions of Seoul, the direct-air-side-economizer-availability time decreased from 52% to 47% per year. On the other hand, when the SA temperature condition was changed to 22 ◦C, it signiﬁcantly increased from 52% to 78%, even for an EA recirculation of 15%. Energies 2018, 11, 444 8 of 14

In addition, when a direct air-side economizer is used, equipment malfunctions arising from dust from OA must be prevented. Standard 52 of ASHRAE uses the minimum-efficiency-reporting-value (MERV) index and recommends MERV 11 through MERV 13 to improve the indoor air quality of the server rooms when OA is introduced. It recommends the use of no less than MERV 8 filters [19]. Therefore, in this study, it was assumed that MERV-8 and MERV 11 filters were installed in the OA fans, and that energy simulation was performed by considering an increase in fan power due to pressure loss. Energies 2018, 11, x FOR PEER REVIEW 8 of 14 3.2. Direct Air-Side Economizer Figure4 shows a schematic of a cooling system using a direct air-side economizer. When the outdoor temperature is low, OA is directly supplied or mixed with the return air (RA) and supplied to the server rooms to produce a cooling effect. When the outdoor temperature is high, the existing centralEnergies chilled-water 2018, 11, x FOR PEER system REVIEW is operated. 8 of 14

Figure 4. Direct air‐side economizer schematic.

In this study, the OA availability time was calculated by assuming that OA was introduced at 25% mixing‐ratio intervals, according to the temperature or enthalpy section under the climatic conditions of Seoul (Figure 5). As a result, the OA‐availability times of the temperature and enthalpy control were 6269 and 5556 h, respectively. In addition, 100% OA was set to be supplied to the server rooms under the condition thatFigureFigure direct 4. 4. Direct introductionDirect air-side air‐side economizerof economizer OA was schematic. possible.schematic. Figure 6 shows the SA condition delivered to the server rooms via CRAH when OA was introducedInIn this this study,into study, each the the OAsection. OA availability availability In the time 75% time was mixing was calculated calculated‐ratio bysection, assuming by assuming the SA that conditionthat OA wasOA introducedwas became introduced a athigh 25% at‐ temperaturemixing-ratio25% mixing intervals,‐andratio lowintervals,‐ accordinghumidity according tostate the temperature byto OAthe temperatureand or RA enthalpy mixing. or sectionenthalpy Thus, under whensection the the climaticunder direct the conditions airclimatic‐side economizerofconditions Seoul (Figure isof operated,Seoul5). As (Figure a result, the control 5). the As OA-availability a is result, simple the when OA times ‐theavailability mixing of the temperature ratio times is ofcalculated the and temperature enthalpy by section, control and but enthalpy werethere are6269control areas and were where 5556 6269 h, the respectively. and SA condition5556 h, In respectively. addition, that passes 100% In through addition, OA was the set100% mixing to beOA chamber supplied was set tofails to be the to supplied server meet the rooms to ASHRAE the under server standardtherooms condition under [20]. that the directcondition introduction that direct of introduction OA was possible. of OA was possible. Figure 6 shows the SA condition delivered to the server rooms via CRAH when OA was introduced into each section. In the 75% mixing‐ratio section, the SA condition became a high‐ temperature and low‐humidity state by OA and RA mixing. Thus, when the direct air‐side economizer is operated, the control is simple when the mixing ratio is calculated by section, but there are areas where the SA condition that passes through the mixing chamber fails to meet the ASHRAE standard [20].

(A) (B)

Figure 5. 5. AnnualAnnual outdoor outdoor air air condition condition and and outdoor outdoor air air (OA) (OA) mixing mixing ratio. ratio. (A) ( ATemperature) Temperature control; control; (B) (EnthalpyB) Enthalpy control. control.

Figure6 shows the SA condition delivered to the server rooms via CRAH when OA was introduced into each section.(A) In the 75% mixing-ratio section, the( SAB) condition became a

Figure 5. Annual outdoor air condition and outdoor air (OA) mixing ratio. (A) Temperature control; (B) Enthalpy control.

Figure 6. SA condition according to the OA mixing ratio. (A) Temperature control; (B) Enthalpy control.

Energies 2018, 11, x FOR PEER REVIEW 8 of 14

Figure 4. Direct air‐side economizer schematic.

Energies 2018, 11, 444 9 of 14 high-temperature and low-humidity state by OA and RA mixing. Thus, when the direct air-side (A) (B) economizer is operated, the control is simple when the mixing ratio is calculated by section, but there are areasFigure where 5. Annual the SAoutdoor condition air condition that passes and outdoor through air the(OA) mixing mixing chamber ratio. (A) Temperature fails to meet control; the ASHRAE (B) standardEnthalpy [20]. control.

Figure 6. SA condition according to the OA mixing ratio. (A) Temperature control; (B) Enthalpy control. Figure 6. SA condition according to the OA mixing ratio. (A) Temperature control; (B) Enthalpy control.

3.3. Indirect Air-Side Economizer The system schematic and operation algorithm of the indirect air-side economizer is shown in Figures7 and8, respectively. RA was first subjected to sensible heat exchange by adding an indirect heat exchanger to the existing central chilled-water system and was then supplied to the server rooms through cooling coils. The operation plan of the indirect air-side economizer was set to control three stages: one in which RA* (the RA state when it passes through the primary indirect-heat exchanger) was at or below 13.5 ◦C, one in which RA* was over 13.5 ◦C but below 22.5 ◦C, and one in which OA was 22.5 ◦C or higher. When OA is 22.5 ◦C or higher, it is difficult to expect the cooling effect of the indirect air-side economizer; hence, the central chilled-water system, i.e., the existing cooling system, is operated. When RA* is greater than 13.5 ◦C but less than 22.5 ◦C, a parallel-operation mode is activated in which the indirect air-side economizer is responsible for the partial load and the central chilled-water system is used for the remaining cooling load. When RA* is less than 13.5 ◦C, the cooling load of the server rooms is handled only by the indirect air-side economizer. The heat exchanger component model described by Equations (22) and (23) is used [21]. . mOA TRA∗ = TRA + εHX . × (TOA − TRA) (22) mRA . . . HXflow ratio = (mOA + mRA)/2mRA (23) In this study, the heat exchanger effectiveness was set at 70% based on the Energy Performance Index of the Korean government [22]. Here, as the heat exchanger effectiveness increases, the temperature passing through the heat exchanger becomes closer to the outside air, and it is possible to reduce the coil load and save the cooling energy. In addition, when the indirect air-side economizer and central chilled-water system were operated in parallel, the parallel operation was stopped and only the central chilled-water system was operated if the cooling energy consumed was higher than that consumed by a single operation of the existing central chilled-water system. On the other hand, in the case of the indirect air-side economizer, MERV-8 filters were applied because OA was not directly introduced into the server rooms, and the power consumed by the OA fan was calculated by considering the corresponding pressure loss. Energies 2018, 11, x FOR PEER REVIEW 9 of 14

3.3. Indirect Air‐Side Economizer The system schematic and operation algorithm of the indirect air‐side economizer is shown in Figures 7 and 8, respectively. RA was first subjected to sensible heat exchange by adding an indirect heat exchanger to the existing central chilled‐water system and was then supplied to the server rooms through cooling coils. The operation plan of the indirect air‐side economizer was set to control three stages: one in which RA* (the RA state when it passes through the primary indirect‐heat exchanger) was at or below 13.5 °C, one in which RA* was over 13.5 °C but below 22.5 °C, and one in which OA was 22.5 °C or higher. When OA is 22.5 °C or higher, it is difficult to expect the cooling effect of the indirect air‐side economizer; hence, the central chilled‐water system, i.e., the existing cooling system, is operated. When RA* is greater than 13.5 °C but less than 22.5 °C, a parallel‐operation mode is activated in which the indirect air‐side economizer is responsible for the partial load and the central chilled‐water system is used for the remaining cooling load. When RA* is less than 13.5 °C, the cooling load of the server rooms is handled only by the indirect air‐side economizer. The heat exchanger component model described by Equations (22) and (23) is used [21].

m OA TRA*TRAεHX TOA TRA (22) m RA

HXflow ratio(m OAm RA)/2m RA (23) In this study, the heat exchanger effectiveness was set at 70% based on the Energy Performance Index of the Korean government [22]. Here, as the heat exchanger effectiveness increases, the Energiestemperature2018, 11, 444 passing through the heat exchanger becomes closer to the outside air, and it is possible10 of 14 to reduce the coil load and save the cooling energy.

Figure 7. Indirect air‐side economizer schematic. Energies 2018, 11, x FOR PEER REVIEWFigure 7. Indirect air-side economizer schematic. 10 of 14 In addition, when the indirect air‐side economizer and central chilled‐water system were operated in parallel, the parallel operation was stopped and only the central chilled‐water system was operated if the cooling energy consumed was higher than that consumed by a single operation of the existing central chilled‐water system. On the other hand, in the case of the indirect air‐side economizer, MERV‐8 filters were applied because OA was not directly introduced into the server rooms, and the power consumed by the OA fan was calculated by considering the corresponding pressure loss.

FigureFigure 8. 8.Indirect Indirect air-sideair‐side economizer economizer operation operation algorithm. algorithm.

3.4. Cooling Energy Consumed by Air‐Side Economizers 3.4. Cooling Energy Consumed by Air-Side Economizers A case study on the change in cooling‐energy consumption of the air‐side economizer according toA the case EA study‐recirculation on the change ratio and in cooling-energy SA‐temperature consumption change was conducted. of the air-side The results economizer are shown according in to the EA-recirculationTable 5 and Figure ratio 9. and When SA-temperature the EA‐recirculation change wasratio conducted. was 15%, Thethe resultsannual arecooling shown‐energy in Table 5 and Figureconsumption9. When was the approximately EA-recirculation 6.1% higher ratio than was that 15%, in the Case annual 2 (recirculation cooling-energy ratio 0%). consumption On the other was approximatelyhand, when 6.1% the SA higher temperature than that was in reduced Case 2 (recirculation to 22 °C when ratio the direct 0%). On air‐ theside other economizer hand, whenwas the SA temperatureoperated, the was annual reduced cooling to‐energy 22 ◦C whenconsumption the direct was air-side approximately economizer 67% lower was operated,than that of the the annual cooling-energycentral chilled consumption‐water system. was Therefore, approximately it is expected 67% that lower the improvement than that of in the cooling central efficiency chilled-water in the server rooms will significantly decrease the cooling‐energy consumption of the data center. system. Therefore, it is expected that the improvement in cooling efficiency in the server rooms will In the indirect air‐side economizer, at an outdoor temperature below 13.5 °C, an HX effectiveness significantly decrease the cooling-energy consumption of the data center. of 70%, and a recirculation ratio of 0%, the amount of time spent processing the◦ cooling load with onlyIn the using indirect OA was air-side 3529 economizer,h per year. This at anincreased outdoor to temperatureapproximately below 7000 h 13.5 per yearC, an when HX effectivenessparallel of 70%,operation and a with recirculation the existing ratio cooling of 0%, system the was amount included. of time In addition, spent processing the operation the of cooling the indirect load with onlyair using‐side OAeconomizer was 3529 was h perpossible year. except This increasedfor a certain to period approximately when the SA 7000 temperature h per year was when set parallelto operation22.5 °C. with Thus, the the existing annual cooling cooling‐energy system consumption was included. was reduced In addition, to approximately the operation 45% compared of the indirect air-sideto that economizer of the existing was central possible chilled except‐water for system. a certain period when the SA temperature was set to 22.5 ◦C. Thus,This result the annual is similar cooling-energy to the annual consumptioncooling‐energy wassaving reduced ratio found to approximately by Ham et al. 45%[21]. comparedThis to thatprevious of the study existing analyzed central the chilled-water cooling‐energy system. consumption for various indirect air‐side economizers. As a result of the case study, it was confirmed that although the EA‐recirculation ratio was adjusted This result is similar to the annual cooling-energy saving ratio found by Ham et al. [21]. to 15%, the increasing of cooling‐energy consumption was not higher than EA‐recirculation ratio. This previous study analyzed the cooling-energy consumption for various indirect air-side economizers. As a result of the case study, it wasTable confirmed 5. Results that of case although study. the EA-recirculation ratio was adjusted to 15%, the increasing of cooling-energy consumption was not higher than EA-recirculation ratio. Case 1 2 3 4 5 6 7 Existing Operation system Direct air‐side economizer system SA condition 13.5 °C 18 °C 22 °C HX effectiveness ‐ Mixing chamber (Perfect mixing) EA recirculation ratio ‐ 0% 5% 10% 15% 0% Available time of outdoor air (h) ‐ 4536 4413 4289 4169 5665 6838 Annual energy consumption (MWh) 69,121 38,815 39,538 40,374 41,164 31,206 22,642 Energy saving ratio ‐ 43.8% 42.8% 41.6% 40.4% 54.9% 67.2% Case 8 9 10 11 12 13 Operation system Indirect air‐side economizer SA condition 13.5 °C 18°C 22 °C HX effectiveness Heat exchanger effectiveness 70% EA recirculation ratio 0% 5% 10% 15% 0% Available time of outdoor air (h) 6988 6748 6594 6312 8206 8707 Annual energy consumption (MWh) 46,820 48,230 49,105 50,911 39,812 30,913 Energy saving ratio 32.3% 30.2% 29.0% 26.3% 44.1% 55.3%

Energies 2018, 11, 444 11 of 14

Table 5. Results of case study.

Case 1 2 3 4 5 6 7 Existing Operation system Direct air-side economizer system SA condition 13.5 ◦C 18 ◦C 22 ◦C HX effectiveness - Mixing chamber (Perfect mixing) EA recirculation ratio - 0% 5% 10% 15% 0% Available time of outdoor air (h) - 4536 4413 4289 4169 5665 6838 Annual energy consumption (MWh) 69,121 38,815 39,538 40,374 41,164 31,206 22,642 Energy saving ratio - 43.8% 42.8% 41.6% 40.4% 54.9% 67.2% Case 8 9 10 11 12 13 Operation system Indirect air-side economizer SA condition 13.5 ◦C 18◦C 22 ◦C HX effectiveness Heat exchanger effectiveness 70% EA recirculation ratio 0% 5% 10% 15% 0% Available time of outdoor air (h) 6988 6748 6594 6312 8206 8707 Annual energy consumption (MWh) 46,820 48,230 49,105 50,911 39,812 30,913 Energy saving ratio 32.3% 30.2% 29.0% 26.3% 44.1% 55.3% Energies 2018, 11, x FOR PEER REVIEW 11 of 14

FigureFigure 9. Annual 9. Annual cooling cooling energy energy consumption consumption of each of case. each case.

4. Conclusions 4. Conclusions The present study used dynamic energy simulation to analyze the cooling‐energy saving effect The presentconsidering study EA‐recirculation used dynamic and SA‐ energytemperature simulation conditions towhen analyze direct and the indirect cooling-energy air‐side saving economizers were applied to a data center in Korea. The conclusions of this study are as follows: effect considering EA-recirculation and SA-temperature conditions when direct and indirect air-side economizers wereWhen appliedair‐side economizers to a data center were used in Korea.to reduce The the conclusionscooling‐energy of consumption this study of are the as data follows: center, the SA temperature exceeded the range recommended by ASHRAE when the mixing • When air-sideratio was economizers calculated by dividing were used the OA to condition reduce theat regular cooling-energy intervals. Therefore, consumption the mixing of the data ratio of OA must be adjusted to meet the target SA condition according to the conditions of RA center, therecirculation SA temperature in the server exceeded rooms and the of OA. range recommended by ASHRAE when the mixing ratio was In calculated the direct air by‐side dividing economizer, the when OA the condition EA‐recirculation at regular ratio was intervals. 15%, the annual Therefore, cooling the‐ mixing ratio of OAenergy must consumption be adjusted increased to meet by approximately the target SA 6.1% condition compared accordingto Case 2. In tothe the indirect conditions air‐ of RA side economizer, the cooling‐energy consumption increased by approximately 9%. This is recirculation in the server rooms and of OA. because the use of OA was impossible owing to EA recirculation under climate conditions that • In the directwould otherwise air-side allow economizer, it. Therefore, when EA grills the must EA-recirculation be installed such that ratio the recirculation was 15%, rate the annual cooling-energyis lower consumption than 5%, or the increased envelope plan by approximatelymust be implemented 6.1% at compared the design stage to Case of the 2. Indata the indirect air-side economizer,center. the cooling-energy consumption increased by approximately 9%. This is  When the SA‐temperature condition was changed to 22 °C, the annual cooling‐energy because theconsumption use of OA of the was direct impossible and indirect owing air‐side toeconomizers EA recirculation was reduced under by approximately climate conditions 67% that would otherwiseand 55%, respectively, allow it. Therefore, compared to EA the grills case for must the central be installed chilled‐water such system. that the Therefore, recirculation it is rate is lower thanexpected 5%, or that the improving envelope the plan cooling must efficiency be implemented in server rooms at thewill design significantly stage reduce of the the data center. • When thecooling SA-temperature‐energy consumption condition of data centers. was changed to 22 ◦C, the annual cooling-energy  When the indirect air‐side‐economizer method is applied to data centers, server‐rack consumptionmalfunctions of the can direct be prevented and indirect because air-sidefine dust economizersand moisture cannot was be reduced directly introduced by approximately 67% andinto 55%, server respectively, rooms. In addition, compared it is to expected the case that for increment the central of SA chilled-water set temperature system. and HX Therefore, it is expectedefficiency that as improving well as EA the‐recirculation cooling efﬁciencyminimization in will server largely rooms reduce will the signiﬁcantly cooling‐energy reduce the cooling-energyconsumption consumption of data centers, of data which centers. accounts for enormous amounts of energy each year. Acknowledgments: This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (NRF‐2015R1C1A1A01052784).

Author Contributions: Seonghyun Park provided main idea of the simulations, analyzed the simulation results and wrote the paper. Janghoo Seo reviewed the paper. All authors have read and approved the final manuscript.

Conflicts of Interest: The authors declare no conflict of interest.

Energies 2018, 11, 444 12 of 14

• When the indirect air-side-economizer method is applied to data centers, server-rack malfunctions can be prevented because ﬁne dust and moisture cannot be directly introduced into server rooms. In addition, it is expected that increment of SA set temperature and HX efﬁciency as well as EA-recirculation minimization will largely reduce the cooling-energy consumption of data centers, which accounts for enormous amounts of energy each year.

Acknowledgments: This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (NRF-2015R1C1A1A01052784). Author Contributions: Seonghyun Park provided main idea of the simulations, analyzed the simulation results and wrote the paper. Janghoo Seo reviewed the paper. All authors have read and approved the final manuscript. Conflicts of Interest: The authors declare no conflict of interest.

Nomenclature

2 Af face area of a coil (m ) ai normalized power coefficients . . ci coefficient of a polynomial relating P/Pmax to m/mmax (dimensionless) BRCW base rating of a coil (W·m−2·K−1) C1~C6 empirical constant Capacity chiller capacity under current condition (W) Capacityrated chiller rated capacity (W) Capacityratio chiller capacity under current conditions divided by the rated capacity (dimensionless) CMH cubic meters per hour (m3·h−1) COP coefficient of performance (dimensionless) COPrated chiller rated coefficient of performance under current conditions (dimensionless) COPratio chiller COP under current conditions divided by the rated COP (dimensionless) −1 −1 Cp specific heat (kJ·kg ·K ) CRAC computer-room air conditioning CRAH computer-room air handler d diameter (mm) EA exhaust air Ec total energy consumption of a central chilled-water system (W) Ei total energy consumption of an indirect-air-side economizer system (W) h specific enthalpy (J·kg−1) h(T) heat-transfer coefficient between the fluid and air at a given temperature T (W·m−2·K−1) HX heat exchanger IT information technology k1~k9 empirical constant LMTD log-mean temperature difference between air and water flow (◦C) . m mass-flow rate (kg·h−1)

Ncoil number of parallel water-ﬂow circuits Nrow depth measured in number of rows of coil tubes OA outdoor air P power consumption (W) PLR chiller-part load ratio (dimensionless) . Q heat-transfer rate (W) . Qmet load met by chiller (W) RA return air rre ratio of recirculation SA supply air T temperature (◦C) V velocity (m·s−1) WSF wetted-surface factor (dimensionless) Energies 2018, 11, 444 13 of 14

Greek Symbols

β control function (0 ≤ β ≤ 1) γ ratio of flow rate to designed flow rate ε effectiveness η efficiency of pump motor λ thermal conductivity (W·m−1·K−1) ρ density (kg m−3) Subscripts a air chw chilled water cw cooling water db dry bulb temperature design design condition dp dew point temperature f fluid HX heat exchanger i inside of tube in inlet max maximum out outlet R rated sat saturated-air condition set set point t total w water

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