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Heat pipe based cold energy storage systems for datacenter energy conservation
Energy 36 (2011) 2802e2811
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Energy journal homepage: www.elsevier.com/locate/energy
Heat pipe based cold energy storage systems for datacenter energy conservation Randeep Singh*, Masataka Mochizuki, Koichi Mashiko, Thang Nguyen Thermal 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
a b s t r a c t
Article history: Received 6 May 2010 Received in revised form 15 February 2011 Accepted 15 February 2011 Available online 17 April 2011
In the present paper, design and economics of the novel type of thermal control system for datacenter using heat pipe based cold energy storage has been proposed and discussed. Two types of cold energy storage system namely: ice storage system and cold water storage system are explained and sized for datacenter with heat output capacity of 8800 kW. Basically, the cold energy storage will help to reduce the chiller running time that will save electricity related cost and decrease greenhouse gas emissions resulting from the electricity generation from non-renewable sources. The proposed cold energy storage system 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 for datacenters which are located in very cold locations and therefore can offer long term seasonal storage of cold energy within reasonable cost. One of the potential application domains for ice based cold energy storage system using heat pipes is the emergency backup system for datacenter. Water based cold energy storage system provides more compact size with short term storage (hours to days) and is potential for datacenters located in areas with yearly average temperature below the permissible cooling water temperature (w25 C). The aforesaid cold energy storage systems were sized on the basis of metrological conditions in Poughkeepsie, New York. As an outcome of the thermal and cost analysis, water based cold energy storage system with cooling capability to handle 60% of datacenter yearly heat load will provide an optimum system size with minimum payback period of 3.5 years. Water based cold energy storage system using heat pipes can be essentially used as precooler for chiller. Preliminary results obtained from the experimental system to test the capability of heat pipe based cold energy storage system have provided satisfactory outcomes and validated the proposed system concept. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Data center cooling Heat pipe Cold energy storage Thermal management Energy conservation Green data center
1. Introduction Datacenters are one of the fastest growing sectors in the market. It is expected that power consumed by the datacenter nearly doubles every 5 years [1]. Electric energy consumption by the data center contributes to its major operational cost. As the power supplied to the data center processing units is ultimately dissipated as heat, therefore significant fraction of the electric power is required for cooling data center [2,3]. It is estimated that for every watt of power consumed by the compute infrastructure, another one-third to half watt is needed to operate the cooling infrastructure. A data center with thermal load output of 8800 kW can consume more than $4 million a year just for cooling purpose. In most of the countries worldwide, major portion of the electric power is generated from the non-renewable energy sources including coal, gas and nuclear which pollutes earth’s atmosphere * Corresponding author. Tel.: þ81 3 5606 1174; fax: þ81 3 5606 1514. E-mail address: [email protected] (R. Singh). 0360-5442/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2011.02.021
by greenhouse gas emission. In this regard, energy conservation based cooling systems for data center can provide two fold advantage, firstly they can reduce the electricity consumption and thus operational cost for thermal management and secondly they can minimise the carbon emission in the environment [4e6]. In the present paper, design, thermal analysis and economics of an innovative heat pipe based cold energy storage system has been discussed in detail and compared with the existing chiller based refrigeration system. Results of the preliminary tests conducted on the heat pipe based cold energy system are also presented to validate the proposed system concept.
2. Heat pipe based cold energy storage system 2.1. Principle The proposed system utilizes thermal diode characteristics of the wickless heat pipe or thermosyphon to capture and store the cold
R. Singh et al. / Energy 36 (2011) 2802e2811
Natural convection
principle can be used as daily based (night to day) or seasonal based (winter to summer) storage. In addition to this, storage media for such system can store cold energy in sensible (single phase) as well as latent (two-phase) form.
Ambient air temperature Ta
Cold ambient
Hot ambient
Ta
2.2. System description
T
Fins
Vapour flow
Liquid flow
Ground level
Storage temperature
Heat pipe Ts
Ts Wick
Liquid
Cold energy storage
Ta > Ts (Top Heat mode) Heat pipe non-operational)
Ta < Ts (Bottom Heat Mode) Heat pipe operational
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Fig. 1. Thermal diode character of the thermosiphon.
energy from the ambient to the storage media. Fig. 1 depicts the working principle of the thermosyphon which can extract heat from the high temperature storage media to low temperature ambient by means of continuous evaporation-condensation process. In other words, the thermosyphon can only transfer heat when operating in the bottom heat mode (evaporator below condenser) which is possible when ambient temperature is lower than storage temperature. For ambient temperature higher than storage temperature i.e. top heat mode or evaporator above condenser configuration, there will not be any heat transfer from ambient to storage media other than negligible heat conducted along the thermosiphon tube wall. Cold energy system based on this
Fig. 2 shows the schematic of the data center cooling system utilizing the proposed heat pipe based cold energy storage. The overall system consists of the data center facility, heat exchanger, cold energy storage system, electrical chiller and a cooling tower. It should 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 with the proposed system. Here, the main incentive of the present design is the electricity cost savings incurred from the downtime of the chiller equipment. The cold storage provides the chilled water for extracting heat from the rack chipsets via indirect contact highly effective plate type heat exchanger which also helps to avoid contamination of the liquid cooled heat sink. In this case, the chiller-cooling tower system is connected to the cold storage and helps to provide extra cold energy to the storage water, as required, depending on the sized capacity of the heat pipe system and ambient temperature. As mentioned before, the cold storage can be simple water storage or ice storage depending on the geographical location, yearly weather conditions of the place and economics of the system. The three-way flow control system between plate heat exchanger and cold energy storage can be used to optimise cold fluid temperature such as to avoid condensation of moisture from room air and to avoid over discharging of cold energy storage thereby saving energy. Between chiller and cold storage, the three way flow control valve can be used to improve COP of chiller and to avoid over cooling of the cold storage fluid. For very low ambient temperatures, the cold storage fluid can be cooled by passing directly through cooling tower using three-way flow control on the hot and cold fluid lines.
Datacenter Rack
Tcpu Tcold plate outside TIM Tcold plate inside
Cold plate
Tfluid-mean Indirect Contact Type Cooling Tower Pump
3-way valve 3-way valve
Tplatehx cold fluid
Tplatehx hot fluid
3-way valve
Chiller 3-way valve
Plate Heat Exchanger
Tcold water storage
Cold room Hot room Cold Energy Storage System Fig. 2. Datacenter facility with proposed heat pipe based cold energy storage system.
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T TS evap-outside T mean cold storage Cold plate
TCPU QCPU
RTIM
RWall
Rnat conv
Plate Hx Rconv
Rplate Hx
T cp inner wall
T mean fluid cold plate
wall
TTS evap-inside
Ccold energy Tcp outer wall
R evap
Rchiller
Heat pipe (passive cooling system)
T TS cond-inside R evap R cond conv
conv
Tvapour
R cond Ramb
wall
TTS cond-outside
Qthermosyphon-out
Tambient
R cooling tower
storage system
Qchiller-out
Tcooling water
Tambient
Chiller (active cooling system)
Fig. 3. Thermal resistance and capacitance diagram for the datacenter thermal management system based on the heat pipe cold energy storage system.
2.3. System thermal analysis The thermal resistanceecapacitance diagram for the complete cooling system as presented in Fig. 3 consists of different heat flow resistance elements and a central cold energy capacitance element. Here, the cooling of the cold storage that acquires the heat dissipated by the chipsets central processing unit (CPU) is provided by the chiller as active cooling system and thermosiphonmodules as passive cooling system.
a
b
2.3.1. Thermal resistances The thermal resistance elements, depicted in Figure 3, consist of heat flow resistance from data center CPU to cold storage via plate heat exchanger, ambient to cold storage via thermosyphon and ambient to cold storage via chiller. In the proposed design, the first two resistance elements are useful to analyse the thermal characteristics of heat pipe system and are further discussed in detail. CPU to Cold Storage: Heat flow from the CPU to the cold energy storage tank has to encounter different heat conduction and convection thermal resistances as detailed in Fig. 4(a). Water is
c
Fig. 4. (a) Thermal resistance diagram showing different heat flow resistances from CPU to cold water storage, (b) Thermosyphon schematic showing temperature points of interest and (c) Thermal resistance diagram for the thermosyphon.
R. Singh et al. / Energy 36 (2011) 2802e2811
used as the heat transfer as well as heat storage fluid. The copper cold plate which is comprised of liquid cooled microchannel heat sink is attached to the active thermal footprint of the CPU using thermal interface material (TIM). Heat dissipated by CPU to the cold plate via TIM is transported by force convection of water to the hot side of indirect contact plate heat exchanger from where the heat is transferred to the cold water flowing through the cold side of exchanger and ultimately to the cold energy storage. Fig. 4(a) also lists the important thermal parameters to determine the individual resistances. Heat conduction resistance, Rcond, across a plate with thickness t, heat flow area Ac and thermal conductivity k is given by:
Rcond ¼
t kA
(1)
Convective resistance, Rconv, between fluid and heat transfer surface with contact area A and heat transfer coefficient h is given by:
Rconv ¼
1 hA
(2)
On the basis of given assumptions, the temperature difference between the cold storage and CPU is w40 C. It should be noted that higher storage temperature is an advantage for high heat transfer by the thermosyphon as larger temperature difference between storage and ambient will be available throughout the year. Nevertheless, the CPU temperature sets the upper limit on the storage
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water temperature. The maximum permissible temperature for the CPU is in the range of 100 5 C with the nominal design temperature within 80 5 C. However, keeping the safety margin and owing to the fact that the performance of the CPU decreases with the increase in temperature, in this case the set CPU temperature is considered to be 65 C. Working through the thermal network and observing the abovementioned factors, for cold water type storage the water temperature of around 25 C can be regarded as optimum. Thermosyphon: Heat extracted by the thermosyphon from the cold storage is mainly dependent on the thermosiphon geometry, cold energy storage temperature, ambient temperature and wind speed. Fig. 4(b and c) shows the thermal resistance components and the relevant thermal parameters for the thermosyphon heat transfer prediction. The thermosyphon tube is made from stainless steel and condenser fins from Aluminum. R134a was used as the working fluid due to its superior heat transfer performance at lower temperatures, in this case ambient temperature, unlike water which has high merit number at higher heat pipe operating temperatures. The thermosyphon evaporator and condenser sections of 3 m and 2 m length respectively are optimized to give high heat transfer rate for the given storage water temperature of 25 C. Due to the higher thermal resistance from storage fluid to evaporator wall (natural convection heat transfer) and low heat transfer area (no fins on evaporator section), evaporator length greater than 3 m can improve thermosyphon heat transfer performance but it is mainly limited by the height and construction constraints for the cold energy storage.
Fig. 5. (a) Hourly temperature data for Poughkeepsie, New York for year 2008 (b) Wind speed readings for Poughkeepsie, New York for year 2008 and (c) Freezing index versus warmest month average temperature for different locations.
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2.3.2. Cold storage capacitance The volume of the cold storage and the number of the heat pipe modules are the important design parameters for the cold energy storage which are mainly dependent on the expected downtime for the chiller, type of storage (cold water or ice), expected system payback time, heat pipe geometry and metrological conditions of the place. Effect of these factors on the system sizing will be discussed in the subsequent sections. In the present investigation, cold energy storage size and design optimization is conducted on the basis of the metrological conditions for Poughkeepsie, New York. Fig. 5(a) and (b) presents the hourly ambient temperature data in C and wind speed in m/s for Poughkeepsie for year 2008 respectively which is used to determine the yearly heat transfer rate of the thermosiphon. The location has yearly average temperature of 10 C and yearly average wind speed of 1.68 m/s Fig. 5(c) plots the freezing index versus warmest month average temperature for different locations throughout the world. Freezing index (FI) is defined as the number of below 0 C days per year. For example, if a place has an average temperature of 4 C for 100 days in a year then the freezing index of the place is 400 C days (e.g. Poughkeepsie). Freezing index is useful to estimate the ice forming potential of the heat pipe for the given climatic conditions. Depending on the type of storage (ice or cold water) and the yearly temperature/wind data of the place, the number of heat pipe modules and storage volume required the handle predetermined percentage of chiller load can be calculated. In the modeling, simple payback time period is used as the deciding parameter to optimize the cold energy storage size. 2.4. System design and sizing The cold energy storage system was sized for 8800 kW or 2500 USRT (1 USRT w 3.5 kW) data center and as per Poughkeepsie (POU), New York weather conditions. In the following sections, the two types of systems i.e. ice storage and cold water storage are designed and discussed in detail. 2.4.1. Ice storage based cold energy storage Based on the freezing index of 400 C days for Poughkeepsie, the ice formation capacity of a single thermosyphon module can be calculated from heat pipe heat transfer rate when the ambient temperature is below 0 C. On an average, as per thermosyphon geometry, a single module will be able to transfer w252 W of heat that will account for w7 m3 of ice each year. This is calculated on the basis of freezing index of 400 C days which is equivalent to
Freezing Index: 400 (~Poughskeepsie) 120
100
80
60
For example, to handle 20% of the chiller load for whole year ~ 25,435 thermosyphon modules are required
40
20
0 0
20
40
60
80
100
2.4.2. Water based cold energy storage The disadvantages of the ice storage system can be addressed by considering the water based storage that can accumulate cold energy in the form of sensible cooling of water rather than latent energy based cold storage in the form of ice. As discussed in the previous section, the cold water temperature of 25 C can be considered as optimum from CPU operatibility and longevity point of view. Cold water storage will positively provide larger temperature difference (and thus high heat transfer) between cold water (w25 C) and ambient air for most of the year (except peak summer season when ambient temperature is >25 C) unlike ice storage
Number of Heat Pipe Modules (x10³)
Number of Ice Storage Modules, x 10³
140
winter season of 100 days with mean temperature of 4 C. Given some tolerance for the yearly variability in the winter temperature, storage size of 10 m3 per heat pipe module is considered in the present design. It should be noted that there will be decrease in the ice generation rate with the growth in the low conductive ice layer around the thermosyphon evaporator outer diameter which is not considered in the above mentioned values. In order to overcome this problem, an ice removal system based on the skin effect using high frequency alternating current is under investigation which can remove the ice layer from evaporator on the periodic basis. In other words, the above values give an estimation of the maximum heat transfer and thus maximum ice forming potential of thermosyphon. In discharging mode, the water flowing through the cold side of the plate heat exchanger can be pumped in thin layer over the formed ice to discharge the cold energy storage. Fig. 6 plots the number of heat pipe modules required to handle predetermined percentage of the datacenter heat load. As evident from the graph, the number of modules required is very high (e.g. 25,435 heat pipes to handle 20% of yearly datacenter heat load) owing to the low heat transfer rate of thermosyphon because of low available temperature difference between zero degree water and winter ambient. Also, ice formation is possible only in winter season which will require very large system size to form and store enough ice for year long operation. Such a system can be recommendable for low capacity datacenters, for winter time thermal control of data center and for location with very high freezing index (i.e. very low or below zero temperatures throughout the year). The present datacenter heat capacity is very high (w8.8 MW) which makes the ice storage system economically non-viable for year long thermal control of the data center. One of the domains identified for the ice storage system is emergency support system for the datacenter which is required as backup and for relatively shorter support time (w5e6 h maximum).
120
Chiller Cooling Load Handled by Ice Storage, %
Fig. 6. Heat pipe requirements for ice based storage to handle different percentage of datacenter load.
10 9
For example, to handle 20% of the chiller load for whole year ~ 1,836 thermosyphon modules are required
8 7 6 5 4 3 2 1 0 0
20
40
60
80
100
Chiller Load, % Fig. 7. Heat pipe requirements for cold water based storage to handle different percentage of datacenter load.
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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 handle 100% 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 cold energy storage system is sized to handle 100% datacenter heat load.
which is operational only during below zero ambient temperature (i.e. peak winter season). This will help to reduce the heat pipe modules and therefore cold energy storage size. In order to design the cold water system size, yearly average ambient temperature of 10 C for Poughkeepsie is considered. From the results of analysis, as shown in Fig. 7, it can be seen that modules requirements for cold water storage is much less than ice storage system (e.g. w1836 heat pipes to handle 20% of yearly data center heat load). An important point to note here is that the number of modules is decided on the basis of the energy balance using average ambient temperature of 10 C and hence there can be wider variation in the heat transfer in different seasons throughout the year. For example, according to the hourly temperature data for a single thermosiphon, the yearly average heat transfer rate is w944 W with minimum of w0 W during peak summer and maximum of w2583 W during peak winter season. This gives a clear indication that the chiller will be still needed in the overall thermal control system even if the heat pipe storage is design to handle 100% datacenter load due to unavailability of positive temperature gradient to dump heat from water storage to ambient in the peak summer season. Fig. 8(a) presents the variation in the cold water storage and CPU temperature through the year for cold energy system design to handle 100% of yearly datacenter output heat load. It is evident from the dotted lines, which presents the system operation without chiller, that cold water and CPU temperature increases beyond 25 C and 65 C limits respectively during the summer season when the 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 the percentage of datacenter load handled by heat pipe storage and pink line presents the cooling provided by chiller on daily basis. The heat pipe system is able to cool the datacenter for most of the winter and intermittently in autumn and spring season. However, chiller is required to run at nearly 100% capacity for most of the time during peak summer months. It is also noted from the graph that heat pipe system designed to handle 100% data center load is oversized for winter season and therefore provides excessive cooling capacity. This is not an economically viable design. As a result, the size of the cold water storage is further optimized on the basis of the economical analysis to avoid system oversizing. It should be noted for the cold water storage system only energy analysis is performed. For size optimization, space consideration similar 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 optimized on the basis of the simple payback time which is defined as the ratio of the total system capital cost to the annual savings from the proposed system. Fig. 9(a) plots different cost figures associated with the datacenter thermal control assembly with respect to the heat pipe system size express as percentage of yearly datacenter load (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 chiller equipment. Here, the storage space requirement is estimated on the basis of 10 m3 capacity per heat pipe module which accounts for 3.3 m2 (w1.8 m 1.8 m) area for 3 m deep storage. Such an extent of gap around uniformly spaced thermosiphon condensers will also provide superior heat transfer by free convection and radiation modes from condenser fins to the ambient. Chiller capital cost is constant for different heat pipe system sizes as it is designed for 8800 kW (@ 571 $/kW) so that it can run at full load during peak summer season as required. The chiller operational or electricity cost per year (@ 0.3 $/kWhr) presents a linear drop till 60% heat pipe system size followed by less steep slope. In this case, the annual electricity cost is determined from the cumulative datacenter load that is expected to be handled by the chiller in a year, as per hourly ambient temperature variation and given heat pipe system capacity. Till 60% heat pipe system size, the linear decreasing trend justifies the use of the heat pipe based cold water storage system for winter and autumn/spring season where the designed system can handle most of the datacenter load. Beyond 60%, the electricity cost does not drop significantly due to high ambient temperatures in summer season which demands the chiller operation most of the time. Fig. 9(b) plots the savings from the electricity cost (difference of the electricity cost without heat pipe system and with heat pipe system), system total capital cost (sum of cost related to heat pipe modules, chiller and land/construction) and simple payback time period. It is evident from the payback line that heat pipe system designed to handle 60% of the yearly datacenter heat load will provide least payback time (w3.5 years) and thus optimum system size. This represents both performance and cost optimized design for the heat pipe based cold water storage system. The proposed heat pipe system size of 60% will provide a green datacenter design
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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 of datacentre 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 cold energy system expressed in terms of percentage of datacenter yearly heat load.
approach with total of 10.4 kilotons of carbon dioxide reduction per year. The principle of the heat pipe based cold water system can be implemented in the form of precooler for the datacenter chiller where it can decrease the temperature of the coolant by certain degree, depending on its size, therefore reducing the electric demand 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 operation 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 system has been experimentally tested at Fujikura facility located in Aomori in Japan. Fig.10(a and b) shows the details of the heat pipe based cold storage module and the experimental test facility at Fujikura. The heat pipe module was made of stainless steel with aluminium fins and R134a as the working fluid. In this case, the heat pipe outer diameter was 50.8 mm with 2.6 m evaporator length and 0.75 m condenser length. The condenser consisted of 76 fins with 200 mm diameter and 1 mm thickness. Figure 10 (a) also presents location of thermocouples installed in the cold storage tank (T1 to T4), ambient air (T5), adiabatic section (T6) and condenser section (T7). In order to minimize heat leaks from the ambient, the evaporator section of the
R. Singh et al. / Energy 36 (2011) 2802e2811
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.
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Fig. 11.
thermosyphon along with the storage tank was installed underground and the outer surface of the tank was insulated using fibre glass insulation. This helps to minimise heat gain from the ground and ambient during the summer season.
Fig. 12.
Figure 11 (a) presents the temperature trend at different locations in the cold storage facility for the test period of 25 days during which the ambient temperature was below zero degree Celsius. It is evident from the water temperature (thermocouples: T1 to T4) that
R. Singh et al. / Energy 36 (2011) 2802e2811
storage temperature remain around 0 C which could have resulted in the generation of ice from water. At the end of the test period, the storage tank top cover was opened and it was visually confirmed, as shown in Figure 11(b), that heat pipe was able to capture cold energy from sub-zero ambient and convert storage water into ice. It should be noted that ice formation mainly takes places around heat pipe evaporator portion that further validate the use of heat pipe for cold energy storage. It was estimated using assumptions stated in section 2 that approximately 113 kg of ice was produced during the test period. Lab scale tests were also conducted under controlled conditions to study the ice formation profile around the heat pipe. Figure 12 (a) shows the experimental test sample that consisted of stainless steel heat pipe with its bottom half installed inside the fibre-glass insulated water container. The test sample was placed inside the cold chamber maintained at sub-zero temperature to transfer cold energy from chilled chamber air to water inside cold storage using heat pipe. Figure 12 (b) shows the generated ice profile around the heat pipe evaporator that is tapering downwards with larger ice thickness at the evaporator top portion than bottom. The shape of the generated ice around heat pipe evaporator can be explained by the anomalous variation in the density of water during cooling process. As the water temperature reduces from initial conditions till 4 C, the density of the liquid, like most other fluids, increases. Below 4 C, reduction in the water temperature results in the decrease of liquid density. Therefore, as the water in the cold storage tank achieves temperature below 4 C, temperature gradient is established along the evaporator length with low temperature (i.e. low density) liquid on the top and high temperature (i.e. high density) liquid at the bottom. As a result, commencement of ice formation as well as its generation rate is higher at the top that produces a downward tapering profile of ice around heat pipe evaporator. 4. Conclusions In summary, the present paper has proposed the novel concept for the thermal management of the data center on the basis of the
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heat pipe based ice storage or cold water storage system. These two types of storage approaches can help to minimize the thermal load on the chiller units and thus save electricity and associated cost. Ice based cold energy storage system is useful for low to medium capacity datacenter located at very cold regions with average yearly ambient temperature below zero most of the time round the year. This kind of system can provide long term storage. Such a system can be used as emergency cooling backup for datacenters. Cold water storage system can provide short term and compact cold energy storage system for small as well as large size datacenter and is more recommendable option for locations with yearly ambient temperature below the allowable cooling water temperature. Such a system can be used as precooler for datacenter chiller thereby reducing its electricity demand. Looking at the massive electricity usage by today’s datacenters, the proposed heat pipe based cold energy storage system for thermal management of datacenter can help to address the energy crisis presently faced 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, computer systems, and telecommunications equipment: perspectives, implications and the current reality in many data centers. The Uptime Institute; 2005. [3] Schmidt RR, Crus EE, Lyengar MK. Challenges of data center thermal management. IBM Journal of Research and Development July September, 2005;49(4/5). [4] Patel CD, Sharma R, Bash CE, Beitelmal A. Thermal Considerations in Cooling Large Scale High Compute Density Data Centers, Proceedings of 2002 Inter Society Conference on Thermal Phenomena, IEEE. [5] Moore J, Sharma R, Shih R, Chase J, Patel C and Ranganathan P. Going beyond CPUs: The potential for temperature-aware data centers. In Proceedings of the first workshop on temperature-aware computer systems, 2004. [6] Schmidt R, Iyengar M, Steffes J, Lund V. Co-generation- Grid Independent Power and Cooling for a Data Center, Proceedings of the ASME 2009 InterPACK Conference, IPACK2009, July 19e23, 2009, San Francisco, USA.
Contents lists available at ScienceDirect
Energy journal homepage: www.elsevier.com/locate/energy
Heat pipe based cold energy storage systems for datacenter energy conservation Randeep Singh*, Masataka Mochizuki, Koichi Mashiko, Thang Nguyen Thermal 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
a b s t r a c t
Article history: Received 6 May 2010 Received in revised form 15 February 2011 Accepted 15 February 2011 Available online 17 April 2011
In the present paper, design and economics of the novel type of thermal control system for datacenter using heat pipe based cold energy storage has been proposed and discussed. Two types of cold energy storage system namely: ice storage system and cold water storage system are explained and sized for datacenter with heat output capacity of 8800 kW. Basically, the cold energy storage will help to reduce the chiller running time that will save electricity related cost and decrease greenhouse gas emissions resulting from the electricity generation from non-renewable sources. The proposed cold energy storage system 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 for datacenters which are located in very cold locations and therefore can offer long term seasonal storage of cold energy within reasonable cost. One of the potential application domains for ice based cold energy storage system using heat pipes is the emergency backup system for datacenter. Water based cold energy storage system provides more compact size with short term storage (hours to days) and is potential for datacenters located in areas with yearly average temperature below the permissible cooling water temperature (w25 C). The aforesaid cold energy storage systems were sized on the basis of metrological conditions in Poughkeepsie, New York. As an outcome of the thermal and cost analysis, water based cold energy storage system with cooling capability to handle 60% of datacenter yearly heat load will provide an optimum system size with minimum payback period of 3.5 years. Water based cold energy storage system using heat pipes can be essentially used as precooler for chiller. Preliminary results obtained from the experimental system to test the capability of heat pipe based cold energy storage system have provided satisfactory outcomes and validated the proposed system concept. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Data center cooling Heat pipe Cold energy storage Thermal management Energy conservation Green data center
1. Introduction Datacenters are one of the fastest growing sectors in the market. It is expected that power consumed by the datacenter nearly doubles every 5 years [1]. Electric energy consumption by the data center contributes to its major operational cost. As the power supplied to the data center processing units is ultimately dissipated as heat, therefore significant fraction of the electric power is required for cooling data center [2,3]. It is estimated that for every watt of power consumed by the compute infrastructure, another one-third to half watt is needed to operate the cooling infrastructure. A data center with thermal load output of 8800 kW can consume more than $4 million a year just for cooling purpose. In most of the countries worldwide, major portion of the electric power is generated from the non-renewable energy sources including coal, gas and nuclear which pollutes earth’s atmosphere * Corresponding author. Tel.: þ81 3 5606 1174; fax: þ81 3 5606 1514. E-mail address: [email protected] (R. Singh). 0360-5442/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2011.02.021
by greenhouse gas emission. In this regard, energy conservation based cooling systems for data center can provide two fold advantage, firstly they can reduce the electricity consumption and thus operational cost for thermal management and secondly they can minimise the carbon emission in the environment [4e6]. In the present paper, design, thermal analysis and economics of an innovative heat pipe based cold energy storage system has been discussed in detail and compared with the existing chiller based refrigeration system. Results of the preliminary tests conducted on the heat pipe based cold energy system are also presented to validate the proposed system concept.
2. Heat pipe based cold energy storage system 2.1. Principle The proposed system utilizes thermal diode characteristics of the wickless heat pipe or thermosyphon to capture and store the cold
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Natural convection
principle can be used as daily based (night to day) or seasonal based (winter to summer) storage. In addition to this, storage media for such system can store cold energy in sensible (single phase) as well as latent (two-phase) form.
Ambient air temperature Ta
Cold ambient
Hot ambient
Ta
2.2. System description
T
Fins
Vapour flow
Liquid flow
Ground level
Storage temperature
Heat pipe Ts
Ts Wick
Liquid
Cold energy storage
Ta > Ts (Top Heat mode) Heat pipe non-operational)
Ta < Ts (Bottom Heat Mode) Heat pipe operational
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Fig. 1. Thermal diode character of the thermosiphon.
energy from the ambient to the storage media. Fig. 1 depicts the working principle of the thermosyphon which can extract heat from the high temperature storage media to low temperature ambient by means of continuous evaporation-condensation process. In other words, the thermosyphon can only transfer heat when operating in the bottom heat mode (evaporator below condenser) which is possible when ambient temperature is lower than storage temperature. For ambient temperature higher than storage temperature i.e. top heat mode or evaporator above condenser configuration, there will not be any heat transfer from ambient to storage media other than negligible heat conducted along the thermosiphon tube wall. Cold energy system based on this
Fig. 2 shows the schematic of the data center cooling system utilizing the proposed heat pipe based cold energy storage. The overall system consists of the data center facility, heat exchanger, cold energy storage system, electrical chiller and a cooling tower. It should 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 with the proposed system. Here, the main incentive of the present design is the electricity cost savings incurred from the downtime of the chiller equipment. The cold storage provides the chilled water for extracting heat from the rack chipsets via indirect contact highly effective plate type heat exchanger which also helps to avoid contamination of the liquid cooled heat sink. In this case, the chiller-cooling tower system is connected to the cold storage and helps to provide extra cold energy to the storage water, as required, depending on the sized capacity of the heat pipe system and ambient temperature. As mentioned before, the cold storage can be simple water storage or ice storage depending on the geographical location, yearly weather conditions of the place and economics of the system. The three-way flow control system between plate heat exchanger and cold energy storage can be used to optimise cold fluid temperature such as to avoid condensation of moisture from room air and to avoid over discharging of cold energy storage thereby saving energy. Between chiller and cold storage, the three way flow control valve can be used to improve COP of chiller and to avoid over cooling of the cold storage fluid. For very low ambient temperatures, the cold storage fluid can be cooled by passing directly through cooling tower using three-way flow control on the hot and cold fluid lines.
Datacenter Rack
Tcpu Tcold plate outside TIM Tcold plate inside
Cold plate
Tfluid-mean Indirect Contact Type Cooling Tower Pump
3-way valve 3-way valve
Tplatehx cold fluid
Tplatehx hot fluid
3-way valve
Chiller 3-way valve
Plate Heat Exchanger
Tcold water storage
Cold room Hot room Cold Energy Storage System Fig. 2. Datacenter facility with proposed heat pipe based cold energy storage system.
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T TS evap-outside T mean cold storage Cold plate
TCPU QCPU
RTIM
RWall
Rnat conv
Plate Hx Rconv
Rplate Hx
T cp inner wall
T mean fluid cold plate
wall
TTS evap-inside
Ccold energy Tcp outer wall
R evap
Rchiller
Heat pipe (passive cooling system)
T TS cond-inside R evap R cond conv
conv
Tvapour
R cond Ramb
wall
TTS cond-outside
Qthermosyphon-out
Tambient
R cooling tower
storage system
Qchiller-out
Tcooling water
Tambient
Chiller (active cooling system)
Fig. 3. Thermal resistance and capacitance diagram for the datacenter thermal management system based on the heat pipe cold energy storage system.
2.3. System thermal analysis The thermal resistanceecapacitance diagram for the complete cooling system as presented in Fig. 3 consists of different heat flow resistance elements and a central cold energy capacitance element. Here, the cooling of the cold storage that acquires the heat dissipated by the chipsets central processing unit (CPU) is provided by the chiller as active cooling system and thermosiphonmodules as passive cooling system.
a
b
2.3.1. Thermal resistances The thermal resistance elements, depicted in Figure 3, consist of heat flow resistance from data center CPU to cold storage via plate heat exchanger, ambient to cold storage via thermosyphon and ambient to cold storage via chiller. In the proposed design, the first two resistance elements are useful to analyse the thermal characteristics of heat pipe system and are further discussed in detail. CPU to Cold Storage: Heat flow from the CPU to the cold energy storage tank has to encounter different heat conduction and convection thermal resistances as detailed in Fig. 4(a). Water is
c
Fig. 4. (a) Thermal resistance diagram showing different heat flow resistances from CPU to cold water storage, (b) Thermosyphon schematic showing temperature points of interest and (c) Thermal resistance diagram for the thermosyphon.
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used as the heat transfer as well as heat storage fluid. The copper cold plate which is comprised of liquid cooled microchannel heat sink is attached to the active thermal footprint of the CPU using thermal interface material (TIM). Heat dissipated by CPU to the cold plate via TIM is transported by force convection of water to the hot side of indirect contact plate heat exchanger from where the heat is transferred to the cold water flowing through the cold side of exchanger and ultimately to the cold energy storage. Fig. 4(a) also lists the important thermal parameters to determine the individual resistances. Heat conduction resistance, Rcond, across a plate with thickness t, heat flow area Ac and thermal conductivity k is given by:
Rcond ¼
t kA
(1)
Convective resistance, Rconv, between fluid and heat transfer surface with contact area A and heat transfer coefficient h is given by:
Rconv ¼
1 hA
(2)
On the basis of given assumptions, the temperature difference between the cold storage and CPU is w40 C. It should be noted that higher storage temperature is an advantage for high heat transfer by the thermosyphon as larger temperature difference between storage and ambient will be available throughout the year. Nevertheless, the CPU temperature sets the upper limit on the storage
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water temperature. The maximum permissible temperature for the CPU is in the range of 100 5 C with the nominal design temperature within 80 5 C. However, keeping the safety margin and owing to the fact that the performance of the CPU decreases with the increase in temperature, in this case the set CPU temperature is considered to be 65 C. Working through the thermal network and observing the abovementioned factors, for cold water type storage the water temperature of around 25 C can be regarded as optimum. Thermosyphon: Heat extracted by the thermosyphon from the cold storage is mainly dependent on the thermosiphon geometry, cold energy storage temperature, ambient temperature and wind speed. Fig. 4(b and c) shows the thermal resistance components and the relevant thermal parameters for the thermosyphon heat transfer prediction. The thermosyphon tube is made from stainless steel and condenser fins from Aluminum. R134a was used as the working fluid due to its superior heat transfer performance at lower temperatures, in this case ambient temperature, unlike water which has high merit number at higher heat pipe operating temperatures. The thermosyphon evaporator and condenser sections of 3 m and 2 m length respectively are optimized to give high heat transfer rate for the given storage water temperature of 25 C. Due to the higher thermal resistance from storage fluid to evaporator wall (natural convection heat transfer) and low heat transfer area (no fins on evaporator section), evaporator length greater than 3 m can improve thermosyphon heat transfer performance but it is mainly limited by the height and construction constraints for the cold energy storage.
Fig. 5. (a) Hourly temperature data for Poughkeepsie, New York for year 2008 (b) Wind speed readings for Poughkeepsie, New York for year 2008 and (c) Freezing index versus warmest month average temperature for different locations.
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2.3.2. Cold storage capacitance The volume of the cold storage and the number of the heat pipe modules are the important design parameters for the cold energy storage which are mainly dependent on the expected downtime for the chiller, type of storage (cold water or ice), expected system payback time, heat pipe geometry and metrological conditions of the place. Effect of these factors on the system sizing will be discussed in the subsequent sections. In the present investigation, cold energy storage size and design optimization is conducted on the basis of the metrological conditions for Poughkeepsie, New York. Fig. 5(a) and (b) presents the hourly ambient temperature data in C and wind speed in m/s for Poughkeepsie for year 2008 respectively which is used to determine the yearly heat transfer rate of the thermosiphon. The location has yearly average temperature of 10 C and yearly average wind speed of 1.68 m/s Fig. 5(c) plots the freezing index versus warmest month average temperature for different locations throughout the world. Freezing index (FI) is defined as the number of below 0 C days per year. For example, if a place has an average temperature of 4 C for 100 days in a year then the freezing index of the place is 400 C days (e.g. Poughkeepsie). Freezing index is useful to estimate the ice forming potential of the heat pipe for the given climatic conditions. Depending on the type of storage (ice or cold water) and the yearly temperature/wind data of the place, the number of heat pipe modules and storage volume required the handle predetermined percentage of chiller load can be calculated. In the modeling, simple payback time period is used as the deciding parameter to optimize the cold energy storage size. 2.4. System design and sizing The cold energy storage system was sized for 8800 kW or 2500 USRT (1 USRT w 3.5 kW) data center and as per Poughkeepsie (POU), New York weather conditions. In the following sections, the two types of systems i.e. ice storage and cold water storage are designed and discussed in detail. 2.4.1. Ice storage based cold energy storage Based on the freezing index of 400 C days for Poughkeepsie, the ice formation capacity of a single thermosyphon module can be calculated from heat pipe heat transfer rate when the ambient temperature is below 0 C. On an average, as per thermosyphon geometry, a single module will be able to transfer w252 W of heat that will account for w7 m3 of ice each year. This is calculated on the basis of freezing index of 400 C days which is equivalent to
Freezing Index: 400 (~Poughskeepsie) 120
100
80
60
For example, to handle 20% of the chiller load for whole year ~ 25,435 thermosyphon modules are required
40
20
0 0
20
40
60
80
100
2.4.2. Water based cold energy storage The disadvantages of the ice storage system can be addressed by considering the water based storage that can accumulate cold energy in the form of sensible cooling of water rather than latent energy based cold storage in the form of ice. As discussed in the previous section, the cold water temperature of 25 C can be considered as optimum from CPU operatibility and longevity point of view. Cold water storage will positively provide larger temperature difference (and thus high heat transfer) between cold water (w25 C) and ambient air for most of the year (except peak summer season when ambient temperature is >25 C) unlike ice storage
Number of Heat Pipe Modules (x10³)
Number of Ice Storage Modules, x 10³
140
winter season of 100 days with mean temperature of 4 C. Given some tolerance for the yearly variability in the winter temperature, storage size of 10 m3 per heat pipe module is considered in the present design. It should be noted that there will be decrease in the ice generation rate with the growth in the low conductive ice layer around the thermosyphon evaporator outer diameter which is not considered in the above mentioned values. In order to overcome this problem, an ice removal system based on the skin effect using high frequency alternating current is under investigation which can remove the ice layer from evaporator on the periodic basis. In other words, the above values give an estimation of the maximum heat transfer and thus maximum ice forming potential of thermosyphon. In discharging mode, the water flowing through the cold side of the plate heat exchanger can be pumped in thin layer over the formed ice to discharge the cold energy storage. Fig. 6 plots the number of heat pipe modules required to handle predetermined percentage of the datacenter heat load. As evident from the graph, the number of modules required is very high (e.g. 25,435 heat pipes to handle 20% of yearly datacenter heat load) owing to the low heat transfer rate of thermosyphon because of low available temperature difference between zero degree water and winter ambient. Also, ice formation is possible only in winter season which will require very large system size to form and store enough ice for year long operation. Such a system can be recommendable for low capacity datacenters, for winter time thermal control of data center and for location with very high freezing index (i.e. very low or below zero temperatures throughout the year). The present datacenter heat capacity is very high (w8.8 MW) which makes the ice storage system economically non-viable for year long thermal control of the data center. One of the domains identified for the ice storage system is emergency support system for the datacenter which is required as backup and for relatively shorter support time (w5e6 h maximum).
120
Chiller Cooling Load Handled by Ice Storage, %
Fig. 6. Heat pipe requirements for ice based storage to handle different percentage of datacenter load.
10 9
For example, to handle 20% of the chiller load for whole year ~ 1,836 thermosyphon modules are required
8 7 6 5 4 3 2 1 0 0
20
40
60
80
100
Chiller Load, % Fig. 7. Heat pipe requirements for cold water based storage to handle different percentage of datacenter load.
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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 handle 100% 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 cold energy storage system is sized to handle 100% datacenter heat load.
which is operational only during below zero ambient temperature (i.e. peak winter season). This will help to reduce the heat pipe modules and therefore cold energy storage size. In order to design the cold water system size, yearly average ambient temperature of 10 C for Poughkeepsie is considered. From the results of analysis, as shown in Fig. 7, it can be seen that modules requirements for cold water storage is much less than ice storage system (e.g. w1836 heat pipes to handle 20% of yearly data center heat load). An important point to note here is that the number of modules is decided on the basis of the energy balance using average ambient temperature of 10 C and hence there can be wider variation in the heat transfer in different seasons throughout the year. For example, according to the hourly temperature data for a single thermosiphon, the yearly average heat transfer rate is w944 W with minimum of w0 W during peak summer and maximum of w2583 W during peak winter season. This gives a clear indication that the chiller will be still needed in the overall thermal control system even if the heat pipe storage is design to handle 100% datacenter load due to unavailability of positive temperature gradient to dump heat from water storage to ambient in the peak summer season. Fig. 8(a) presents the variation in the cold water storage and CPU temperature through the year for cold energy system design to handle 100% of yearly datacenter output heat load. It is evident from the dotted lines, which presents the system operation without chiller, that cold water and CPU temperature increases beyond 25 C and 65 C limits respectively during the summer season when the 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 the percentage of datacenter load handled by heat pipe storage and pink line presents the cooling provided by chiller on daily basis. The heat pipe system is able to cool the datacenter for most of the winter and intermittently in autumn and spring season. However, chiller is required to run at nearly 100% capacity for most of the time during peak summer months. It is also noted from the graph that heat pipe system designed to handle 100% data center load is oversized for winter season and therefore provides excessive cooling capacity. This is not an economically viable design. As a result, the size of the cold water storage is further optimized on the basis of the economical analysis to avoid system oversizing. It should be noted for the cold water storage system only energy analysis is performed. For size optimization, space consideration similar 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 optimized on the basis of the simple payback time which is defined as the ratio of the total system capital cost to the annual savings from the proposed system. Fig. 9(a) plots different cost figures associated with the datacenter thermal control assembly with respect to the heat pipe system size express as percentage of yearly datacenter load (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 chiller equipment. Here, the storage space requirement is estimated on the basis of 10 m3 capacity per heat pipe module which accounts for 3.3 m2 (w1.8 m 1.8 m) area for 3 m deep storage. Such an extent of gap around uniformly spaced thermosiphon condensers will also provide superior heat transfer by free convection and radiation modes from condenser fins to the ambient. Chiller capital cost is constant for different heat pipe system sizes as it is designed for 8800 kW (@ 571 $/kW) so that it can run at full load during peak summer season as required. The chiller operational or electricity cost per year (@ 0.3 $/kWhr) presents a linear drop till 60% heat pipe system size followed by less steep slope. In this case, the annual electricity cost is determined from the cumulative datacenter load that is expected to be handled by the chiller in a year, as per hourly ambient temperature variation and given heat pipe system capacity. Till 60% heat pipe system size, the linear decreasing trend justifies the use of the heat pipe based cold water storage system for winter and autumn/spring season where the designed system can handle most of the datacenter load. Beyond 60%, the electricity cost does not drop significantly due to high ambient temperatures in summer season which demands the chiller operation most of the time. Fig. 9(b) plots the savings from the electricity cost (difference of the electricity cost without heat pipe system and with heat pipe system), system total capital cost (sum of cost related to heat pipe modules, chiller and land/construction) and simple payback time period. It is evident from the payback line that heat pipe system designed to handle 60% of the yearly datacenter heat load will provide least payback time (w3.5 years) and thus optimum system size. This represents both performance and cost optimized design for the heat pipe based cold water storage system. The proposed heat pipe system size of 60% will provide a green datacenter design
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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 of datacentre 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 cold energy system expressed in terms of percentage of datacenter yearly heat load.
approach with total of 10.4 kilotons of carbon dioxide reduction per year. The principle of the heat pipe based cold water system can be implemented in the form of precooler for the datacenter chiller where it can decrease the temperature of the coolant by certain degree, depending on its size, therefore reducing the electric demand 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 operation 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 system has been experimentally tested at Fujikura facility located in Aomori in Japan. Fig.10(a and b) shows the details of the heat pipe based cold storage module and the experimental test facility at Fujikura. The heat pipe module was made of stainless steel with aluminium fins and R134a as the working fluid. In this case, the heat pipe outer diameter was 50.8 mm with 2.6 m evaporator length and 0.75 m condenser length. The condenser consisted of 76 fins with 200 mm diameter and 1 mm thickness. Figure 10 (a) also presents location of thermocouples installed in the cold storage tank (T1 to T4), ambient air (T5), adiabatic section (T6) and condenser section (T7). In order to minimize heat leaks from the ambient, the evaporator section of the
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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.
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Fig. 11.
thermosyphon along with the storage tank was installed underground and the outer surface of the tank was insulated using fibre glass insulation. This helps to minimise heat gain from the ground and ambient during the summer season.
Fig. 12.
Figure 11 (a) presents the temperature trend at different locations in the cold storage facility for the test period of 25 days during which the ambient temperature was below zero degree Celsius. It is evident from the water temperature (thermocouples: T1 to T4) that
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storage temperature remain around 0 C which could have resulted in the generation of ice from water. At the end of the test period, the storage tank top cover was opened and it was visually confirmed, as shown in Figure 11(b), that heat pipe was able to capture cold energy from sub-zero ambient and convert storage water into ice. It should be noted that ice formation mainly takes places around heat pipe evaporator portion that further validate the use of heat pipe for cold energy storage. It was estimated using assumptions stated in section 2 that approximately 113 kg of ice was produced during the test period. Lab scale tests were also conducted under controlled conditions to study the ice formation profile around the heat pipe. Figure 12 (a) shows the experimental test sample that consisted of stainless steel heat pipe with its bottom half installed inside the fibre-glass insulated water container. The test sample was placed inside the cold chamber maintained at sub-zero temperature to transfer cold energy from chilled chamber air to water inside cold storage using heat pipe. Figure 12 (b) shows the generated ice profile around the heat pipe evaporator that is tapering downwards with larger ice thickness at the evaporator top portion than bottom. The shape of the generated ice around heat pipe evaporator can be explained by the anomalous variation in the density of water during cooling process. As the water temperature reduces from initial conditions till 4 C, the density of the liquid, like most other fluids, increases. Below 4 C, reduction in the water temperature results in the decrease of liquid density. Therefore, as the water in the cold storage tank achieves temperature below 4 C, temperature gradient is established along the evaporator length with low temperature (i.e. low density) liquid on the top and high temperature (i.e. high density) liquid at the bottom. As a result, commencement of ice formation as well as its generation rate is higher at the top that produces a downward tapering profile of ice around heat pipe evaporator. 4. Conclusions In summary, the present paper has proposed the novel concept for the thermal management of the data center on the basis of the
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heat pipe based ice storage or cold water storage system. These two types of storage approaches can help to minimize the thermal load on the chiller units and thus save electricity and associated cost. Ice based cold energy storage system is useful for low to medium capacity datacenter located at very cold regions with average yearly ambient temperature below zero most of the time round the year. This kind of system can provide long term storage. Such a system can be used as emergency cooling backup for datacenters. Cold water storage system can provide short term and compact cold energy storage system for small as well as large size datacenter and is more recommendable option for locations with yearly ambient temperature below the allowable cooling water temperature. Such a system can be used as precooler for datacenter chiller thereby reducing its electricity demand. Looking at the massive electricity usage by today’s datacenters, the proposed heat pipe based cold energy storage system for thermal management of datacenter can help to address the energy crisis presently faced by the world.
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