3 termal energy storage

State-of-the-art on thermal energy storage technologies in data center
Lijun Liu
a
, Quan Zhang
a,
⇑
, Zhiqiang (John) Zhai
b,
⇑
, Chang Yue
a
, Xiaowei Ma
a
a
College of Civil Engineering, Hunan University, Changsha, Hunan 410082, China
b
Department of Civil, Environmental and Architectural Engineering, University of Colorado at Boulder, USA
article info
Article history:
Received 4 December 2019
Revised 23 May 2020
Accepted 24 July 2020
Available online 29 July 2020
Keywords:
Thermal energy storage
Data center
Energy saving
Emergency cooling
Cooling system design
abstract
Data center consumes a great amount of energy and accounts for an increasing proportion of global
energy demand. Low efficiency of cooling systems leads to a cooling cost at about 40% of the total energy
consumption of a data center. Due to specific operation conditions, high security and high cooling load is
required in data center. To achieve energy saving, cost saving and high security, novel cooling systems
integrated with thermal energy storage (TES) technologies have been proposed. This paper presents an
extensive overview of the research advances and the applications of TES technologies in data centers.
Operating conditions, energy mismatch and requirement of high security in data center were overviewed.
Principles and characteristics of TES technologies were discussed. Applications of passive TES coupled air
flow and applications of active TES integrated cooling system are summarizes, and the design and perfor-
mance of these TES integrated thermal systems are analyzed, with a focus on energy saving, cost savings
and high security.
Ó2020 Elsevier B.V. All rights reserved.
Contents
1. Introduction . . . ........................................................................................................ 2
2. Overview of data center. . . . . . . . . . . . . ..................................................................................... 2
2.1. Operating conditions. . . . ........................................................................................... 2
2.2. Requirement of high security and high cooling load . . . . . . . . . . . . . ........................................................ 3
2.3. Energy mismatches . . . . . ........................................................................................... 3
3. Principles and characteristics of TES . . . ..................................................................................... 4
3.1. Principles of TES technologies . . . . . . . . . . . . ........................................................................... 4
3.2. Characteristic of passive TES . . . . . . . . . . . . . ........................................................................... 4
3.2.1. TES materials . . . . . . . . . . . . . ................................................................................ 4
3.2.2. Configurations and heat transfer enhancement of TES . . . . . . . . . . . . . . . . . . .......................................... 5
3.3. Characteristic of active TES. . . . . . . . . . . . . . . ........................................................................... 5
3.3.1. TES materials . . . . . . . . . . . . . ................................................................................ 5
3.3.2. Configurations and heat transfer enhancement of TES . . . . . . . . . . . . . . . . . . .......................................... 5
4. Applications of passive TES coupled air flow . . . . . . . . . . . . . . . .................................................................. 6
4.1. TES embedded in enclosure . . . . . . . . . . . . . . ........................................................................... 7
4.2. TES based electronics cooling . . . . . . . . . . . . . ........................................................................... 8
5. Applications of active TES integrated cooling system . . . . . . . . ................................................................. 10
5.1. TES integrated traditional refrigeration system. . . . . . . . . . . . . . . . . . ....................................................... 10
5.1.1. TES based air-cooled system . ............................................................................... 10
5.1.2. TES based water-cooled system. . . . . . . . . . . . . . . . . . ............................................................ 12
5.2. TES integrated absorption refrigeration system . . . . . . . . . . . . . . . . . ....................................................... 13
5.2.1. Cold energy storage based absorption refrigeration system . . . . . . . . . . . . . . ......................................... 13
5.2.2. Heat storage based absorption cooling system. . . . . . ............................................................ 14
https://doi.org/10.1016/j.enbuild.2020.110345
0378-7788/Ó2020 Elsevier B.V. All rights reserved.
⇑
Corresponding authors.
E-mail addresses: [email protected] (Q. Zhang), [email protected]
(Zhiqiang (John) Zhai).
Energy & Buildings 226 (2020) 110345
Contents lists available at ScienceDirect
Energy & Buildings
journal homepage: www.elsevier.com/locate/enb

5.3. TES integrated free cooling system . . . . . ............................................................................. 14
5.3.1. TES combined heat pipe system . . . . . . . . . . . .................................................................. 15
5.3.2. TES coupling evaporative cooling system . . . . .................................................................. 15
6. Operation strategies based on TES to reduce operation cost. . . . . . . . . . . . . . . . .................................................... 17
7. Conclusions and future work. . . .......................................................................................... 18
Declaration of Competing Interest . . . . . . . . . . . . . . . . . ....................................................................... 18
Acknowledgements . . . . . . . . . . .......................................................................................... 18
References . .......................................................................................................... 18
1. Introduction
Data centers, which house computing servers, network equip-
ment, cooling devices, power supplying sets, and other related
equipment, experience fast growth as an integral part of informa-
tion and communication technology. Due to the massive computa-
tion and data interactions, data centers consume explosive amount
of energy. The energy consumption of data centers is approxi-
mately 1.1%-1.5% of the total global electricity consumption in
2011 and it will continue to increase with the rate that is doubling
every two years until 2020 [1–3]. Among this huge energy con-
sumption, cooling devices, as one of the main infrastructures pro-
viding proper operating conditions for servers, account for about
30–40% of total consumption, taking up the second largest propor-
tion, while IT equipment of servers, I/O devices and storages utilize
most of the energy [4–5]. Energy saving and energy efficiency
enhancement in cooling system of data center is urgent, and kinds
of technologies have been applied to achieve it, including free cool-
ing, air distribution optimization, variable frequency technology
and energy storage technology [6–9]. Among them, thermal energy
storage is one of the most promising technologies to enhance the
efficiency of energy sources (and increase the energy efficiency of
cooling system), which overcomes many mismatch between
energy supply and demand in terms of time, temperature or site.
Advantages of TES integrated energy systems include enhance-
ment of overall efficiency and reliability, better economic feasibil-
ity, less operating costs and less environmental pollution [9]. TES
technologies have been utilized in many occasions for years, and
various TES units and systems have been proposed and studied
extensively [10–12]. Many researchers studied performance of dif-
ferent thermal energy storage materials and different thermal
energy storage configures, which are the important impacts of
thermal energy storage technologies [13–14]. Besides thermal
energy storage materials and configures, applications of TES inte-
grated thermal management system (including cooling system
and air flow) in data center, shown its own characteristics as well
as inherent challenges, which are the focus of this review.
In the current, TES technologies of data center have been paid
more and more attention and are evolving rapidly. The purpose
of this paper is to provide the fundamental knowledge and a
review of existing literatures of TES in data center. Section 1 briefly
introduced energy consumption in data center and TES technolo-
gies. In section 2, operating conditions, energy mismatches, and
security requirement in data center are overviewed. Section 3 dis-
cusses principles and the characteristics of TES technologies in data
center with a focus on TES materials and TES configurations. Appli-
cations of passive TES coupled air flow, and applications of active
TES integrated cooling systems, are analyzed in section 4 and sec-
tion 5, respectively. Operation strategies are analyzed in section 6.
The last section summarizes the main findings and development
directions. The outlook of the components is shown in Fig. 1. This
article is expected to be helpful to understand the state-of-the-art
of TES in data center, and to improve the reliability and energy effi-
ciency of data center through the TES integration.
2. Overview of data center
2.1. Operating conditions
Cooling devices are applied to control temperature, humidity
and particle concentration to create suitable environments for IT
servers, which is determinant for the reliability and efficiency of
data center operations. In order to ensure a reliable and efficient
operation of computing servers, America Society of Heating, Refrig-
erating and Air-conditioning Engineers (ASHRAE) developed ther-
mal guidelines for data processing environment. Classes A1-A4
were proposed to define suitable temperatures and humidity set-
ting ranges for data centers at different cooling levels (as shown
in Fig. 2), and an indoor environment with temperature ranging
from 18 to 27 °C and moisture ranging from 9°C-15 °C DP (Dew
Point) and 60% RH(relative humidity) was suggested to be optimal
[15]. Cooling systems should provide proper temperature and
humidity for the computer room to maintain the safe and reliable
operation of the systems.
Nomenclature/Abbreviations
TES Thermal energy storage
LTES Latent thermal energy storage
STES Sensible thermal energy storage
DP Dew Point
RH Relative humidity
CPU Central processing unit
CRAC Computer room air conditioner
UPS Uninterrupted power supply
EES Electricity energy storage
PCM Phase change material
ATES Aquifer thermal energy storage
HX Heat exchanger
TBS Telecommunications base station
AHU Air handle unit
ACU Air conditioner unit
PCB Phase change board
UFAD Under-floor air distribution system
EDL Enthalpy difference laboratory
COP Coefficient of performance
PUE Power usage effectiveness
HP Heat pipe
TPCT Two-phase closed thermosyphon
TCO Total cost of ownership
2L. Liu et al. / Energy & Buildings 226 (2020) 110345

2.2. Requirement of high security and high cooling load
Because data centers are more sensitive to temperature
changes, the reliability requirement of cooling system is more rig-
orous than those in civil buildings. For this reason, almost all data
centers are equipped with emergency cooling systems, which can
turn on TES units to discharge cold energy under power failures.
Emergency cooling is one of the main applications of TES for data
center, which can often ensure the system security for at least
15 min [16].
At the same time, uniqueness of cooling systems attributes to
the unusual cooling load conditions in data centers. (1) Cooling
systems in data center run continuously for 24 h per day and
365 days per year, while cooling systems in civil buildings (e.g.,
office buildings) only operate at a certain time period (e.g., office
hours). (2) For human settlement environment, temperature is lim-
ited to 26 °C for comfort, but this is not necessary for IT servers in
data centers. The maximum permissible temperature for central
processing unit (CPU) is 100 °C, and the cooling system is usually
designed to keep CPU temperature below 85 °C[17–18]. For exam-
ple, water of 15–25 °C is enough in liquid cooling, while 7 °C water
is needed for civil buildings [19]. (3) Heat density of a data center
may reach 500–3000 W/m
2
[20–21], while the heat density of a
typical civil building is about 90–400 W/m
2
.
Requirement of high security and high cooling load in data cen-
ters leads to the development of data centers cooling system as a
separate field. TES integrated with cooling systems in data center
is usually applied to realize multi-targets including lower cost
and higher operational security.
2.3. Energy mismatches
Energy supply–demand mismatches exist in energy consump-
tion process. Thermal energy storage technology adapts to the vari-
ations in outdoor temperature and user cooling requirement (i.e.,
supply–demand mismatches). During the operation of data cen-
ters, five supply–demand mismatches commonly occur, including:
(a) Lower efficiency of traditional refrigeration systems in day-
time compared with its relatively higher efficiency at night-
time. Due to lower ambient temperature at nighttime,
traditional refrigeration systems including air-source chiller,
water-source chiller and direct-expansion air conditioner,
turns to be more efficient.
(b) Free cooling being used only at night or cold season. Free
cooling can solely be used when outdoor temperature is
lower than indoor temperature. It happens to both direct
and indirect fresh air side free applications. Generally, free
cooling only works when the temperature difference
between indoor and outdoor reaches 5–8 °C[22–26]. Free
cooling systems may also be found in water side economizer
applications through chiller operational adjustments and
related water system valve controls. Free cooling systems
should be improved and optimized according to actual
weather parameters and server working load variations.
(c) Peak-valley time of use tariff. Due to inactive nocturnal
activities, electricity price is much lower at night than that
during the daytime in many countries [27–28].
(d) Higher heat load of data center in daytime [29–31]. To pre-
vent servers from failure under higher heat load conditions,
cooling infrastructure of data center should have the auxil-
iary capacity and system to handle peak cooling loads. Even
though most data centers only reach peak utilization for a
small fraction of a load cycle, it will cost much more to meet
the requirements of suitable environments under the higher
heat load condition.
(e) Emergency cooling. The availability of data centers plays a
very vital role on the emergency occasions. ‘‘24 by Forever”
expectation was proposed early in 2005 on data center secu-
rity [32]. Challenges during power outages are much more
Fig. 1. Main components of the sections.
Fig. 2. Recommended and allowable operating conditions for four ASHRAE data
center classes [15].
L. Liu et al. / Energy & Buildings 226 (2020) 110345 3

significant than those under normal operations, because
power failure may impose more serious damages to data
centers, such as sudden shutdown of servers and fundamen-
tal destruction of IT equipment. Therefore, emergency cool-
ing is one of the cornerstones to maintain the reliability of
data centers [33–34].
To mitigate these ‘‘mismatches” challenges above, energy stor-
age technologies become inevitable and powerful, which can not
only improve energy utilization efficiency but also balance the dis-
crepancy between supply and demand of energy [35]. Electricity
energy storage and thermal energy storage (TES) are commonly
utilized. Electrical energy storage is an effective way to do
building-grid interaction just as uninterrupted power supply has
been utilized in data center for years, but batteries have drawbacks
like short lifespan, environmental pollution and so on. TES is
another way to achieve the same effect in the long run, being clean
and friendly to the environment.
3. Principles and characteristics of TES
3.1. Principles of TES technologies
In general, TES is divided into physical storage and chemical
storage [36]. Currently, various thermochemical energy storage
materials are at development stage and such a system is not yet
commercially available. What widely used in data centers is phys-
ical energy storage. Physical energy storage is further divided into
sensible thermal energy storage (STES) and latent thermal energy
storage (LTES). The commercial viability of LTES is limited by mate-
rial characteristics and its initial cost, as opposed to STES that is
mostly employed in data center. STES stores thermal energy
through its temperature changes, among which water is commonly
used as the storage material. LTES accumulates thermal energy
almost isothermally, by using phase change material (PCM) such
as ice [37].
STES saves thermal energy by variation of material tempera-
ture, and the storage materials undergo no phase change in the
storage process. The amount of thermal energy stored is propor-
tional to the temperature difference and specific heat capacity dur-
ing charging and discharging processes. It can be defined as,
Q¼Z
T
f
T
i
mC
p
dt ¼mC
ap
T
f
T
i
 ð1Þ
where Qis cold energy quantity, kJ/s; T
i
denotes initial temper-
ature, K; T
f
represents final temperature, K; mrefer to flow rate of
fluid, kg/s; C
p
is specific heat capacity, kJ/(kg*K); C
ap
denotes aver-
age specific heat capacity, kJ/(kg*K).
Most materials used in the TES application are inexpensive with
good thermal properties [38]. Concrete, metal, water and air are sen-
sible thermal storage materials usually seen. Water is generally used
as cold energy storage material in data centers, because of its low
price, high specific heat capacity and no pollution or corrosion [39].
LTES stores thermal energy when the storage materials undergo
a phase change process from one physical state to another. The
thermal energy storage capacity includes the portion that varies
with the temperature difference and the portion caused by phase
change of storage materials. The energy formula is given as
follows:
Q¼Z
T
r
T
i
mC
p1
dt þma
r
D H
r
þZ
T
f
T
r
mC
p2
dt ð2Þ
where Qis cold energy quantity, kJ/s; T
i
denotes initial temper-
ature, K; T
r
represents phase transition temperature, K; T
f
repre-
sents final temperature, K; mrefers to flow rate of fluid, kg/s; C
p1
is specific heat capacity from T
i
to T
r
, kJ/(kg*K); C
p2
is specific heat
capacity from T
r
to T
f
, kJ/(kg*K); a
r
denotes solidified fraction, %;
D H
r
represents latent heat, kJ/kg.
Phase change materials (PCM) are applied in LTES. PCM are a
group of materials that have an intrinsic capability of absorbing
and releasing heat during phase transition cycles [40]. PCM can
be classified as inorganic PCM including salts, salt hydrates and
alloys, and organic PCM, including paraffin, alcohols, fatty acids,
esters and others. Paraffin, which is mostly applied in data centers,
can be divided into crude paraffin and commercial paraffin. Crude
paraffin has good thermal properties (including transition temper-
ature and latent heat) but its price is high, while commercial paraf-
fin is a good substitute with less inferior thermal parameters, much
lower price and more convenient availability. The exothermic and
endothermic phase transitions of PCM are utilized effectively by
incorporating PCM into TES systems so that thermal loads are
met by controlling the systemic operating parameters. Researchers
are focusing extensively on improving the thermophysical proper-
ties of PCM towards commercialization [41–42]. Heating/cooling
strategies and performance enhancement techniques are practiced
to achieve wide and efficient utilization of PCM in TES systems
[43–44].
Both sensible and latent heat thermal energy storage is utilized
in data center, and could be viewed as substitutes for each other in
some cases. For convenient narration, TES are divided into passive
TES technologies and active TES technologies in this paper. Passive
TES is generally used inside the data center, corresponding to the
cooling terminals and coupled with air flow, while active TES is
integrated with cooling system and generally placed outdoors.
3.2. Characteristic of passive TES
Application of passive TES technologies includes TES embedded
in enclosure [45–47], TES based electronics cooling [48] and ther-
mal mass in data center [49–50]. TES embedded in enclosure and
TES based electronics cooling, often taking PCM as energy storage
materials, are placed dispersedly on the inner surface of enclosure,
and any other locations inside data center considering air flow
arrangement. TES is overcooled by the CRAC system to a low tem-
perature, and release cold energy when the temperature of the
data center increases.
Various thermal masses exist in data center, such as the cold air,
the recirculation air handler coils, supply air ducts, and the raised
metal floor. When the thermal masses in data center are over-
cooled by CRAC system to a lower temperature, they gain the
capacity to absorb heat later as thermal reserve space, to avoid
overheat [49].
From this perspective, thermal masses in data center are viewed
as passive TES, which take indoor air and other substances as sen-
sible energy storage materials. However, due to low specific heat
capacity and limited temperature difference between overcooled
temperature and overheat temperature, the cold energy stored in
these thermal mass is very little.
Energy capacity and heat transfer rate are the most important
parameters we concerned about, thus, materials and configura-
tions of passive TES are overviewed next.
3.2.1. TES materials
Thermal mass in data center requires no special design, and
relies on operation conditions and operation strategies which will
be analyzed in section 6. Different latent heat TES were placed dis-
persedly on the inner surface of enclosure, and any other locations
inside data center to store cold energy. Due to limitation of opera-
tion temperature of the data center, the transition temperature (i.e.
working temperature of TES) were strictly chosen. For passive TES
4L. Liu et al. / Energy & Buildings 226 (2020) 110345

applied in data centers, storage materials, working temperature
and the design storage period are summarized in Table 1 in detail.
Because of the uneven temperature distribution, working temper-
ature ranges from 20 °Cto62°C. As can be seen, passive TES cur-
rently applied in data center are latent heat TES except thermal
mass. The storage period is relatively short, and most passive TES
provides cold energy for a few hours or minutes.
3.2.2. Configurations and heat transfer enhancement of TES
When passive TES stores and releases thermal energy directly
through air flow or the volume of the TES is quite small, passive
TES is just encapsulated by metal or plastic, such as phase change
board [52–54,58], PCM block [62–63], to fix it in specific location.
Elaborate structure or configuration is not required. This is mainly
because in the heat transfer process, the thermal resistance of the
air is much greater, and the thermal resistance of the TES has little
influence on the heat transfer performance. What we are more
concerned about is the energy storage capacity, which is much
more relevant to storage materials. For TES based electronics cool-
ing, when heat transfer enhancement is needed [66–72], the struc-
ture design of TES are configurations of TES and more the layouts of
cooling system, which is to be analyzed in section 4.
3.3. Characteristic of active TES
Active TES technologies are applied through TES heat exchang-
ers which are often integrated with cooling system [73–74]. TES
heat exchanger could be sensible TES units and latent TES units,
including water tank [75–77], micro-encapsulated TES[78–80],
plate-type heat exchanger [81] and tube-in-tank TES [82]. Besides
layouts of cooling system which will be analyzed in section 5,
materials, configurations and heat transfer enhancement of the
active TES are the basis of the combined system.
3.3.1. TES materials
Water is a favorable material for active TES. Surface water and
aquifer water can be applied in data center cooling system [83].
Surface water, including rivers water or shallow-well water, which
is influenced by the climate very much, needs water tank to con-
tain, while aquifer water is underground with relatively stable
temperature and natural soil layers are regarded as the container
[84]. When water used, working temperature is most important
design parameter for active TES, The normal temperature differ-
ence between supplying and returning water is 5 °C[75–76]. For
LTES, storage material type, especially transition temperature (i.e.
working temperature), is one of the most important design param-
eters. For active TES applied in data centers, storage materials,
working temperature and the design storage period are summa-
rized in Table 2 in detail. As can be seen, water is commonly used,
and the working temperature ranges from 0 to 40 °C. Working tem-
perature of LTES ranges from 13 °Cto30°C, which is relatively
lower than that of passive TES as shown in Table 1.
3.3.2. Configurations and heat transfer enhancement of TES
When using water tank, water stratifies naturally because of
increasing density at lower temperature: the hot water flows
directly to the top, the cold water remains at the bottom, and the
intermediate region is the thermocline. This thermal stratification
is affected by several factors, such as tank size and shape, location
and geometry of inlets and outlets, fluid temperatures and flow
rates during charging and discharging process. The mixing effect
caused by the temperature difference could degrade the heat
source quality and have side effects on the system storage effi-
ciency. The horizontally partitioned water tank was proved to effi-
ciently achieve a good thermal stratification performance. The
thermocline moves from the bottom of the tank to the top of the
tank during the transient charging/discharging process and
behaves as a dynamic natural barrier which maintains the warm
water region separated from the cold region, therefore, the ther-
mocline layer should be designed as narrow as possible to achieve
a larger hot water volume in the tank, which indicates less mixing
between the hot and cold water [114–115]. The optimal economic
efficiency of water storage could be achieved when the mass of the
TES reached 2000 m
3
[75–76].
Table 1
Storage materials and working temperature of passive TES.
Author Type of PCM working
temperature
Storage
period
Marongiu[51] Glauber’s salts or
Nonadecane
31/32 Hours
Sun[52–54] Octadecane paraffin 27.4 Hours
Nörtershäuser
[55]
PCM 25/35 Hours
Zhang[56–57] PCM – Hours
Studniorz[58] PCM 20–30 Hours
Onishi[59] PCM – For
emergency
Zhang and Bai
[60–61]
PCM – Hours
Skach[62–63] Commercial paraffin 39 Hours
Weng[64] Tricosane, palmitic acid, or
lauric acid
42–4840–
4658–62
Minutes
Behi[65] RT42(PCM form Rubitherm
Company)
38–43 Minutes
Jaworski[66] Lauric acid 41.5 Minutes
[67–72] Paraffin wax 56–58 Minutes
Table 2
Storage materials and working temperature of active TES.
Author storage material Working
temperature
Storage
period
Saeed [81] hexadecane 15.5–18.3 Hours
Fang [82] Mixture of tetradecane and
hexadecane
15.01 For
emergency
Chang [85–86] mixture of kwai acid and
lauric acid
19.67 hours
Cui [87] Ternary fatty acid/expanded
graphite composite
15–25 Hours
Sun[88–89] Organic composite 18–20 Hours
Xiao[90] Ground source or PCM – Seasons or
hours
Drenkelfort[83] Aquifer 10 Seasons
You and zhang
[91–92]
PCM – Hours
Garday[93]Oro
[94–95]
Water 5.5 For
emergency;
hours
Oro[96] Hydrated salts 13 Hours
Xie[97] Hydrate – Hours
Chen[98] Water 25 Hours
Hu and Wang
[99–100]
Water – Hours
Hammann
[101]Wang
[102]
Water, PCM – Hours
Muren[103] Water – Hours
Haywood[104–
105]
PCM – Hours
Wang[106] Dodecyl stearic acid 20.2 Hours
Singh[17,107–
108]
Water, ice 0 Hours,
seasons
Wei[109] Water 38–40 Hours
Sundaram[110] Hydrate salt 28–30 Hours
Nall[111] Water – Hours
Luttrell [112–
113]
PCM slurry 21–27 Hours
L. Liu et al. / Energy & Buildings 226 (2020) 110345 5