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Energy 36 (2011) 2802e2811

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Energy

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Heat pipe based cold energy storage systems for datacenter energyconservation

Randeep Singh*, Masataka Mochizuki, Koichi Mashiko, Thang NguyenThermal Technology Division, R&D Department, Fujikura Ltd., 1-5-1, Kiba, Koto-ku, Tokyo 135-8512, Japan

a r t i c l e i n f o

Article history:Received 6 May 2010Received in revised form15 February 2011Accepted 15 February 2011Available online 17 April 2011

Keywords:Data center coolingHeat pipeCold energy storageThermal managementEnergy conservationGreen data center

* Corresponding author. Tel.: þ81 3 5606 1174; faxE-mail address: randeep.singh@jp.fujikura.com (R

0360-5442/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.energy.2011.02.021

a b s t r a c t

In the present paper, design and economics of the novel type of thermal control system for datacenterusing heat pipe based cold energy storage has been proposed and discussed. Two types of cold energystorage system namely: ice storage system and cold water storage system are explained and sized fordatacenter with heat output capacity of 8800 kW. Basically, the cold energy storage will help to reducethe chiller running time that will save electricity related cost and decrease greenhouse gas emissionsresulting from the electricity generation from non-renewable sources. The proposed cold energy storagesystem can be retrofit or connected in the existing datacenter facilities without major design changes.Out of the two proposed systems, ice based cold energy storage system is mainly recommended fordatacenters which are located in very cold locations and therefore can offer long term seasonal storage ofcold energy within reasonable cost. One of the potential application domains for ice based cold energystorage system using heat pipes is the emergency backup system for datacenter. Water based cold energystorage system provides more compact size with short term storage (hours to days) and is potential fordatacenters located in areas with yearly average temperature below the permissible cooling watertemperature (w25 �C). The aforesaid cold energy storage systems were sized on the basis of metrologicalconditions in Poughkeepsie, New York. As an outcome of the thermal and cost analysis, water based coldenergy storage system with cooling capability to handle 60% of datacenter yearly heat load will providean optimum system size with minimum payback period of 3.5 years. Water based cold energy storagesystem using heat pipes can be essentially used as precooler for chiller. Preliminary results obtained fromthe experimental system to test the capability of heat pipe based cold energy storage system haveprovided satisfactory outcomes and validated the proposed system concept.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Datacenters are one of the fastest growing sectors in the market.It is expected that power consumed by the datacenter nearlydoubles every 5 years [1]. Electric energy consumption by the datacenter contributes to its major operational cost. As the powersupplied to the data center processing units is ultimately dissipatedas heat, therefore significant fraction of the electric power isrequired for cooling data center [2,3]. It is estimated that for everywatt of power consumed by the compute infrastructure, anotherone-third to half watt is needed to operate the cooling infrastruc-ture. A data center with thermal load output of 8800 kW canconsume more than $4 million a year just for cooling purpose. Inmost of the countries worldwide, major portion of the electricpower is generated from the non-renewable energy sourcesincluding coal, gas and nuclear which pollutes earth’s atmosphere

: þ81 3 5606 1514.. Singh).

All rights reserved.

by greenhouse gas emission. In this regard, energy conservationbased cooling systems for data center can provide two foldadvantage, firstly they can reduce the electricity consumption andthus operational cost for thermal management and secondly theycan minimise the carbon emission in the environment [4e6].

In the present paper, design, thermal analysis and economics ofan innovative heat pipe based cold energy storage system has beendiscussed in detail and compared with the existing chiller basedrefrigeration system. Results of the preliminary tests conducted onthe heat pipe based cold energy system are also presented tovalidate the proposed system concept.

2. Heat pipe based cold energy storage system

2.1. Principle

Theproposed systemutilizes thermal diode characteristics of thewickless heat pipe or thermosyphon to capture and store the cold

Cold ambient Hot ambientTa

Ts

Ta < Ts (Bottom Heat Mode) Heat pipe operational

TTT

Ta > Ts (Top Heat mode) Heat pipe non-operational)

Vapour flow

Ground level

Heat pipe

Fins

Ta

Wick

Cold energy storage

Liquid

Liquid flow

Ts

Ambient air temperature

Storage temperature

Natural convection

Fig. 1. Thermal diode character of the thermosiphon.

R. Singh et al. / Energy 36 (2011) 2802e2811 2803

energy from the ambient to the storage media. Fig. 1 depicts theworking principle of the thermosyphonwhich can extract heat fromthe high temperature storagemedia to low temperature ambient bymeans of continuous evaporation-condensation process.

In other words, the thermosyphon can only transfer heat whenoperating in the bottom heat mode (evaporator below condenser)which is possible when ambient temperature is lower than storagetemperature. For ambient temperature higher than storagetemperature i.e. top heat mode or evaporator above condenserconfiguration, there will not be any heat transfer from ambient tostorage media other than negligible heat conducted along thethermosiphon tube wall. Cold energy system based on this

Cold plate

Chiller

Indirect Contact Type Cooling

Tower

3-way valve

3-way valve

3-way valv

Tcold water storage

Cold room

D

Pump

Cold

Fig. 2. Datacenter facility with proposed heat

principle can be used as daily based (night to day) or seasonal based(winter to summer) storage. In addition to this, storage media forsuch system can store cold energy in sensible (single phase) as wellas latent (two-phase) form.

2.2. System description

Fig. 2 shows the schematic of the data center cooling systemutilizing the proposed heat pipe based cold energy storage. Theoverall system consists of the data center facility, heat exchanger,cold energy storage system, electrical chiller and a cooling tower. Itshould be noted, which will also be proved through later analysis,that due to seasonal variation of ambient temperature and size/economics of the storage system, the chiller will be required withthe proposed system. Here, the main incentive of the presentdesign is the electricity cost savings incurred from the downtime ofthe chiller equipment. The cold storage provides the chilled waterfor extracting heat from the rack chipsets via indirect contact highlyeffective plate type heat exchanger which also helps to avoidcontamination of the liquid cooled heat sink. In this case, thechiller-cooling tower system is connected to the cold storage andhelps to provide extra cold energy to the storage water, as required,depending on the sized capacity of the heat pipe system andambient temperature. As mentioned before, the cold storage can besimple water storage or ice storage depending on the geographicallocation, yearly weather conditions of the place and economics ofthe system. The three-way flow control system between plate heatexchanger and cold energy storage can be used to optimise coldfluid temperature such as to avoid condensation of moisture fromroom air and to avoid over discharging of cold energy storagethereby saving energy. Between chiller and cold storage, the threeway flow control valve can be used to improve COP of chiller and toavoid over cooling of the cold storage fluid. For very low ambienttemperatures, the cold storage fluid can be cooled by passingdirectly through cooling tower using three-way flow control on thehot and cold fluid lines.

Rack

Tfluid-mean

Tcpu

Tcold plate inside

TIM

Plate Heat Exchanger

e

3-way valve

Tplatehx cold fluid Tplatehx hot fluid

Hot room

atacenter

Energy Storage System

Tcold plate outside

pipe based cold energy storage system.

RTIM RWall Rconv Rplate Hx

Ccold energy

storage system

Rnat conv

R evap

wall

R evap

conv

R cond

conv

R cond

wall Ramb

Rchiller R cooling tower

TCPU

Tambient

Tambient

Tcp outer wall T cp inner wall T mean fluid

cold plate

T mean cold storage

T TS evap-outside

TTS evap-inside Tvapour

T TS cond-inside

TTS cond-outside

Tcooling water

QCPU

Qchiller-out

Qthermosyphon-outCold plate Plate Hx

Chiller (active cooling system)

Heat pipe (passive cooling system)

Fig. 3. Thermal resistance and capacitance diagram for the datacenter thermal management system based on the heat pipe cold energy storage system.

R. Singh et al. / Energy 36 (2011) 2802e28112804

2.3. System thermal analysis

The thermal resistanceecapacitance diagram for the completecooling system as presented in Fig. 3 consists of different heat flowresistance elements and a central cold energy capacitance element.Here, the cooling of the cold storage that acquires the heat dissi-pated by the chipsets central processing unit (CPU) is provided bythe chiller as active cooling system and thermosiphonmodules aspassive cooling system.

a b

Fig. 4. (a) Thermal resistance diagram showing different heat flow resistances from CPU to cand (c) Thermal resistance diagram for the thermosyphon.

2.3.1. Thermal resistancesThe thermal resistance elements, depicted in Figure 3, consist of

heat flow resistance from data center CPU to cold storage via plateheat exchanger, ambient to cold storage via thermosyphon andambient to cold storage via chiller. In the proposed design, the firsttwo resistance elements are useful to analyse the thermal charac-teristics of heat pipe system and are further discussed in detail.

CPU to Cold Storage: Heat flow from the CPU to the cold energystorage tank has to encounter different heat conduction andconvection thermal resistances as detailed in Fig. 4(a). Water is

c

old water storage, (b) Thermosyphon schematic showing temperature points of interest

R. Singh et al. / Energy 36 (2011) 2802e2811 2805

used as the heat transfer as well as heat storage fluid. The coppercold plate which is comprised of liquid cooled microchannel heatsink is attached to the active thermal footprint of the CPU usingthermal interface material (TIM). Heat dissipated by CPU to the coldplate via TIM is transported by force convection of water to the hotside of indirect contact plate heat exchanger fromwhere the heat istransferred to the cold water flowing through the cold side ofexchanger and ultimately to the cold energy storage. Fig. 4(a) alsolists the important thermal parameters to determine the individualresistances. Heat conduction resistance, Rcond, across a plate withthickness t, heat flow area Ac and thermal conductivity k is givenby:

Rcond ¼ tkA

(1)

Convective resistance, Rconv, between fluid and heat transfersurface with contact area A and heat transfer coefficient h is givenby:

Rconv ¼ 1hA

(2)

On the basis of given assumptions, the temperature differencebetween the cold storage and CPU isw40 �C. It should be noted thathigher storage temperature is an advantage for high heat transferby the thermosyphon as larger temperature difference betweenstorage and ambient will be available throughout the year. Never-theless, the CPU temperature sets the upper limit on the storage

Fig. 5. (a) Hourly temperature data for Poughkeepsie, New York for year 2008 (b) Wind spwarmest month average temperature for different locations.

water temperature. The maximum permissible temperature for theCPU is in the range of 100 � 5 �C with the nominal designtemperature within 80 � 5 �C. However, keeping the safety marginand owing to the fact that the performance of the CPU decreaseswith the increase in temperature, in this case the set CPUtemperature is considered to be 65 �C. Working through thethermal network and observing the abovementioned factors, forcold water type storage the water temperature of around 25 �C canbe regarded as optimum.

Thermosyphon: Heat extracted by the thermosyphon from thecold storage is mainly dependent on the thermosiphon geometry,cold energy storage temperature, ambient temperature and windspeed. Fig. 4(b and c) shows the thermal resistance componentsand the relevant thermal parameters for the thermosyphon heattransfer prediction. The thermosyphon tube is made from stainlesssteel and condenser fins from Aluminum. R134a was used as theworking fluid due to its superior heat transfer performance at lowertemperatures, in this case ambient temperature, unlike waterwhich has high merit number at higher heat pipe operatingtemperatures. The thermosyphon evaporator and condensersections of 3 m and 2 m length respectively are optimized to givehigh heat transfer rate for the given storage water temperature of25 �C. Due to the higher thermal resistance from storage fluid toevaporator wall (natural convection heat transfer) and low heattransfer area (no fins on evaporator section), evaporator lengthgreater than 3 m can improve thermosyphon heat transferperformance but it is mainly limited by the height and constructionconstraints for the cold energy storage.

eed readings for Poughkeepsie, New York for year 2008 and (c) Freezing index versus

R. Singh et al. / Energy 36 (2011) 2802e28112806

2.3.2. Cold storage capacitanceThe volume of the cold storage and the number of the heat pipe

modules are the important design parameters for the cold energystorage which are mainly dependent on the expected downtime forthe chiller, type of storage (cold water or ice), expected systempayback time, heat pipe geometry and metrological conditions ofthe place. Effect of these factors on the system sizing will be dis-cussed in the subsequent sections.

In the present investigation, cold energy storage size and designoptimization is conducted on the basis of the metrological condi-tions for Poughkeepsie, New York. Fig. 5(a) and (b) presents thehourly ambient temperature data in �C and wind speed in m/s forPoughkeepsie for year 2008 respectively which is used to deter-mine the yearly heat transfer rate of the thermosiphon. The loca-tion has yearly average temperature of 10 �C and yearly averagewind speed of 1.68 m/s Fig. 5(c) plots the freezing index versuswarmest month average temperature for different locationsthroughout the world. Freezing index (FI) is defined as the numberof below 0 �C days per year. For example, if a place has an averagetemperature of �4 �C for 100 days in a year then the freezing indexof the place is 400 �C days (e.g. Poughkeepsie). Freezing index isuseful to estimate the ice forming potential of the heat pipe for thegiven climatic conditions.

Depending on the type of storage (ice or cold water) and theyearly temperature/wind data of the place, the number of heat pipemodules and storage volume required the handle predeterminedpercentage of chiller load can be calculated. In themodeling, simplepayback time period is used as the deciding parameter to optimizethe cold energy storage size.

2.4. System design and sizing

The cold energy storage system was sized for 8800 kW or2500 USRT (1 USRTw 3.5 kW) data center and as per Poughkeepsie(POU), New York weather conditions. In the following sections, thetwo types of systems i.e. ice storage and cold water storage aredesigned and discussed in detail.

2.4.1. Ice storage based cold energy storageBased on the freezing index of 400 �C days for Poughkeepsie, the

ice formation capacity of a single thermosyphon module can becalculated from heat pipe heat transfer rate when the ambienttemperature is below 0 �C. On an average, as per thermosyphongeometry, a single module will be able to transfer w252 W of heatthat will account for w7 m3 of ice each year. This is calculated onthe basis of freezing index of 400 �C days which is equivalent to

0

20

40

60

80

100

120

140

0 20 40 60 80 100 120

Chiller Cooling Load Handled by Ice Storage, %

Nu

mb

er o

f Ice S

to

rag

e M

od

ules, x 10³ Freezing Index: 400 (~Poughskeepsie)

For example, to handle 20% of the chiller load for whole year ~ 25,435 thermosyphon modules

are required

Fig. 6. Heat pipe requirements for ice based storage to handle different percentage ofdatacenter load.

winter season of 100 days with mean temperature of �4 �C. Givensome tolerance for the yearly variability in the winter temperature,storage size of 10 m3 per heat pipe module is considered in thepresent design. It should be noted that there will be decrease in theice generation rate with the growth in the low conductive ice layeraround the thermosyphon evaporator outer diameter which is notconsidered in the above mentioned values. In order to overcomethis problem, an ice removal system based on the skin effect usinghigh frequency alternating current is under investigationwhich canremove the ice layer from evaporator on the periodic basis. In otherwords, the above values give an estimation of the maximum heattransfer and thus maximum ice forming potential ofthermosyphon.

In discharging mode, the water flowing through the cold sideof the plate heat exchanger can be pumped in thin layer over theformed ice to discharge the cold energy storage. Fig. 6 plots thenumber of heat pipe modules required to handle predeterminedpercentage of the datacenter heat load. As evident from the graph,the number of modules required is very high (e.g. 25,435 heatpipes to handle 20% of yearly datacenter heat load) owing to thelow heat transfer rate of thermosyphon because of low availabletemperature difference between zero degree water and winterambient. Also, ice formation is possible only in winter seasonwhich will require very large system size to form and store enoughice for year long operation. Such a system can be recommendablefor low capacity datacenters, for winter time thermal control ofdata center and for location with very high freezing index (i.e. verylow or below zero temperatures throughout the year). The presentdatacenter heat capacity is very high (w8.8 MW) which makes theice storage system economically non-viable for year long thermalcontrol of the data center. One of the domains identified for the icestorage system is emergency support system for the datacenterwhich is required as backup and for relatively shorter support time(w5e6 h maximum).

2.4.2. Water based cold energy storageThe disadvantages of the ice storage system can be addressed by

considering the water based storage that can accumulate coldenergy in the form of sensible cooling of water rather than latentenergy based cold storage in the form of ice. As discussed in theprevious section, the cold water temperature of 25 �C can beconsidered as optimum from CPU operatibility and longevity pointof view. Cold water storage will positively provide larger temper-ature difference (and thus high heat transfer) between cold water(w25 �C) and ambient air for most of the year (except peak summerseason when ambient temperature is >25 �C) unlike ice storage

0123456789

10

0 20 40 60 80 100

Chiller Load, %

Nu

mb

er o

f H

ea

t P

ip

e M

od

ule

s (x

10

³)

For example, to handle 20% of the chiller load for whole year ~ 1,836 thermosyphon modules are

required

Fig. 7. Heat pipe requirements for cold water based storage to handle differentpercentage of datacenter load.

Fig. 8. (a) Yearly variation in the CPU and cold water storage temperatures with and without chiller for the case when heat pipe based cold water storage system is sized to handle100% datacentre heat load (b) Percentage of datacenter heat load handled by heat pipe based cold water storage and chiller throughout the year for the case when heat pipe coldenergy storage system is sized to handle 100% datacenter heat load.

R. Singh et al. / Energy 36 (2011) 2802e2811 2807

which is operational only during below zero ambient temperature(i.e. peak winter season). This will help to reduce the heat pipemodules and therefore cold energy storage size. In order to designthe cold water system size, yearly average ambient temperature of10 �C for Poughkeepsie is considered. From the results of analysis,as shown in Fig. 7, it can be seen thatmodules requirements for coldwater storage is much less than ice storage system (e.g.w1836 heatpipes to handle 20% of yearly data center heat load). An importantpoint to note here is that the number of modules is decided on thebasis of the energy balance using average ambient temperature of10 �C and hence there can be wider variation in the heat transfer indifferent seasons throughout the year. For example, according tothe hourly temperature data for a single thermosiphon, the yearlyaverage heat transfer rate is w944 W with minimum of w0 Wduring peak summer and maximum of w2583 W during peakwinter season. This gives a clear indication that the chiller will bestill needed in the overall thermal control system even if the heatpipe storage is design to handle 100% datacenter load due tounavailability of positive temperature gradient to dump heat fromwater storage to ambient in the peak summer season.

Fig. 8(a) presents the variation in the cold water storage and CPUtemperature through the year for cold energy system design tohandle 100% of yearly datacenter output heat load. It is evidentfrom the dotted lines, which presents the system operationwithoutchiller, that cold water and CPU temperature increases beyond25 �C and 65 �C limits respectively during the summer seasonwhenthe ambient temperature approaches storage temperature of 25 �C.The solid lines present the controlled cold storage temperature at�25 �C by operating the chiller. In Fig. 8(b), the blue line depicts thepercentage of datacenter load handled by heat pipe storage andpink line presents the cooling provided by chiller on daily basis. Theheat pipe system is able to cool the datacenter for most of thewinter and intermittently in autumn and spring season. However,chiller is required to run at nearly 100% capacity for most of thetime during peak summer months. It is also noted from the graphthat heat pipe system designed to handle 100% data center load isoversized for winter season and therefore provides excessivecooling capacity. This is not an economically viable design. Asa result, the size of the cold water storage is further optimized onthe basis of the economical analysis to avoid system oversizing. Itshould be noted for the cold water storage system only energyanalysis is performed. For size optimization, space considerationsimilar to ice storage is assumed (w10 m3 per heat pipe module).

2.5. Economical analysis: system size optimization

The heat pipe based cold water storage system size is optimizedon the basis of the simple payback timewhich is defined as the ratioof the total system capital cost to the annual savings from theproposed system. Fig. 9(a) plots different cost figures associatedwith the datacenter thermal control assembly with respect to theheat pipe system size express as percentage of yearly datacenterload (or percentage of chiller downsizing). Heat pipe modules cost(@ 157 $/pc) and land plus storage construction cost (@ 300 $/m2)presents linear rising trend with the downsizing of the chillerequipment. Here, the storage space requirement is estimated on thebasis of 10 m3 capacity per heat pipe module which accounts for3.3 m2 (w1.8 m � 1.8 m) area for 3 m deep storage. Such an extentof gap around uniformly spaced thermosiphon condensers will alsoprovide superior heat transfer by free convection and radiationmodes from condenser fins to the ambient.

Chiller capital cost is constant for different heat pipe systemsizes as it is designed for 8800 kW (@ 571 $/kW) so that it can run atfull load during peak summer season as required. The chilleroperational or electricity cost per year (@ 0.3 $/kWhr) presentsa linear drop till 60% heat pipe system size followed by less steepslope. In this case, the annual electricity cost is determined from thecumulative datacenter load that is expected to be handled by thechiller in a year, as per hourly ambient temperature variation andgiven heat pipe system capacity.

Till 60% heat pipe system size, the linear decreasing trendjustifies the use of the heat pipe based cold water storage systemfor winter and autumn/spring season where the designed systemcan handle most of the datacenter load. Beyond 60%, the electricitycost does not drop significantly due to high ambient temperaturesin summer seasonwhich demands the chiller operationmost of thetime.

Fig. 9(b) plots the savings from the electricity cost (difference ofthe electricity cost without heat pipe system and with heat pipesystem), system total capital cost (sum of cost related to heat pipemodules, chiller and land/construction) and simple payback timeperiod. It is evident from the payback line that heat pipe systemdesigned to handle 60% of the yearly datacenter heat load willprovide least payback time (w3.5 years) and thus optimum systemsize. This represents both performance and cost optimized designfor the heat pipe based cold water storage system. The proposedheat pipe system size of 60% will provide a green datacenter design

Fig. 9. (a) Cost of different components for the thermal management of the datacenter versus heat pipe based cold energy storage size expressed in terms of percentage ofdatacentre yearly heat load. (b) Total cost of datacenter thermal management system, savings from cold energy storage and payback time versus different sizes of heat pipe coldenergy system expressed in terms of percentage of datacenter yearly heat load.

R. Singh et al. / Energy 36 (2011) 2802e28112808

approach with total of 10.4 kilotons of carbon dioxide reduction peryear. The principle of the heat pipe based cold water system can beimplemented in the form of precooler for the datacenter chillerwhere it can decrease the temperature of the coolant by certaindegree, depending on its size, therefore reducing the electricdemand for the chiller.

This water based cold energy will operate as follow:

Winter season: only heat pipe system operation.Autumn and spring season: heat pipe system opera-

tion þ chiller operation (if cold water temperature > 25 �C).Summer season: chiller operation continuously (switches off,

if cold water temperature< 25 �C)þ heat pipe system operation(if ambient temperature < 25 �C).

3. Experimental study: proof of concept

The novel concept of heat pipe based cold energy storage systemhas been experimentally tested at Fujikura facility located in Aomoriin Japan. Fig.10(a andb) shows thedetails of theheat pipe based coldstorage module and the experimental test facility at Fujikura. Theheat pipe module was made of stainless steel with aluminium finsand R134a as the working fluid. In this case, the heat pipe outerdiameter was 50.8 mm with 2.6 m evaporator length and 0.75 mcondenser length. The condenser consisted of 76 fins with 200 mmdiameter and 1mm thickness. Figure 10 (a) also presents location ofthermocouples installed in the cold storage tank (T1 toT4), ambientair (T5), adiabatic section (T6) and condenser section (T7). In order tominimize heat leaks from the ambient, the evaporator section of the

Fig. 10. (a) Heat pipe based cold energy storage module (b) Experimental test facility at Fujikura, Japan used to test the cold energy storage system.

R. Singh et al. / Energy 36 (2011) 2802e2811 2809

Fig. 11.

R. Singh et al. / Energy 36 (2011) 2802e28112810

thermosyphon along with the storage tank was installed under-ground and the outer surface of the tank was insulated using fibreglass insulation. This helps to minimise heat gain from the groundand ambient during the summer season.

Fig. 12

Figure 11 (a) presents the temperature trend at different loca-tions in the cold storage facility for the test period of 25 days duringwhich the ambient temperaturewas below zero degree Celsius. It isevident from the water temperature (thermocouples: T1 to T4) that

.

R. Singh et al. / Energy 36 (2011) 2802e2811 2811

storage temperature remain around 0 �C which could have resultedin the generation of ice fromwater. At the end of the test period, thestorage tank top cover was opened and it was visually confirmed, asshown in Figure 11(b), that heat pipe was able to capture coldenergy from sub-zero ambient and convert storage water into ice.It should be noted that ice formation mainly takes places aroundheat pipe evaporator portion that further validate the use of heatpipe for cold energy storage. It was estimated using assumptionsstated in section 2 that approximately 113 kg of ice was producedduring the test period.

Lab scale tests were also conducted under controlled conditionsto study the ice formation profile around the heat pipe. Figure 12 (a)shows the experimental test sample that consisted of stainless steelheat pipe with its bottom half installed inside the fibre-glassinsulated water container. The test sample was placed inside thecold chamber maintained at sub-zero temperature to transfer coldenergy from chilled chamber air to water inside cold storage usingheat pipe. Figure 12 (b) shows the generated ice profile around theheat pipe evaporator that is tapering downwards with larger icethickness at the evaporator top portion than bottom. The shape ofthe generated ice around heat pipe evaporator can be explained bythe anomalous variation in the density of water during coolingprocess. As the water temperature reduces from initial conditionstill 4�C, the density of the liquid, like most other fluids, increases.Below 4�C, reduction in the water temperature results in thedecrease of liquid density. Therefore, as the water in the coldstorage tank achieves temperature below 4�C, temperaturegradient is established along the evaporator length with lowtemperature (i.e. low density) liquid on the top and high temper-ature (i.e. high density) liquid at the bottom. As a result,commencement of ice formation as well as its generation rate ishigher at the top that produces a downward tapering profile of icearound heat pipe evaporator.

4. Conclusions

In summary, the present paper has proposed the novel conceptfor the thermal management of the data center on the basis of the

heat pipe based ice storage or cold water storage system. Thesetwo types of storage approaches can help to minimize the thermalload on the chiller units and thus save electricity and associatedcost. Ice based cold energy storage system is useful for low tomedium capacity datacenter located at very cold regions withaverage yearly ambient temperature below zero most of the timeround the year. This kind of system can provide long term storage.Such a system can be used as emergency cooling backup fordatacenters. Cold water storage system can provide short term andcompact cold energy storage system for small as well as large sizedatacenter and is more recommendable option for locations withyearly ambient temperature below the allowable cooling watertemperature. Such a system can be used as precooler for data-center chiller thereby reducing its electricity demand. Looking atthe massive electricity usage by today’s datacenters, the proposedheat pipe based cold energy storage system for thermal manage-ment of datacenter can help to address the energy crisis presentlyfaced by the world.

References

[1] US Environmental Protection Agency (2011, March 25). Energy Star [Online].Available: http://www.energystar.gov/index.cfm?c=prod_development.server_efficiency.

[2] Brill KG. 2005 e 2010 Heat density trends in data processing, computersystems, and telecommunications equipment: perspectives, implications andthe current reality in many data centers. The Uptime Institute; 2005.

[3] Schmidt RR, Crus EE, Lyengar MK. Challenges of data center thermalmanagement. IBM Journal of Research and Development July September,2005;49(4/5).

[4] Patel CD, Sharma R, Bash CE, Beitelmal A. Thermal Considerations in CoolingLarge Scale High Compute Density Data Centers, Proceedings of 2002 InterSociety Conference on Thermal Phenomena, IEEE.

[5] Moore J, Sharma R, Shih R, Chase J, Patel C and Ranganathan P. Going beyondCPUs: The potential for temperature-aware data centers. In Proceedings of thefirst workshop on temperature-aware computer systems, 2004.

[6] Schmidt R, Iyengar M, Steffes J, Lund V. Co-generation- Grid Independent Powerand Cooling for a Data Center, Proceedings of the ASME 2009 InterPACKConference, IPACK2009, July 19e23, 2009, San Francisco, USA.