3 termal energy storage

Energy & Buildings 226 (2020) 110345
Contents lists available at ScienceDirect
Energy & Buildings
journal homepage: www.elsevier.com/locate/enb
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
b
College of Civil Engineering, Hunan University, Changsha, Hunan 410082, China
Department of Civil, Environmental and Architectural Engineering, University of Colorado at Boulder, USA
a r t i c l e
i n f o
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
a b s t r a c t
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 performance of these TES integrated thermal systems are analyzed, with a focus on energy saving, cost savings
and high security.
Ó 2020 Elsevier B.V. All rights reserved.
Contents
1.
2.
3.
4.
5.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 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
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
Applications of passive TES coupled air flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1.
TES embedded in enclosure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.2.
TES based electronics cooling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
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
⇑ Corresponding authors.
E-mail addresses: [email protected] (Q. Zhang), [email protected]
(Zhiqiang (John) Zhai).
https://doi.org/10.1016/j.enbuild.2020.110345
0378-7788/Ó 2020 Elsevier B.V. All rights reserved.
2
L. Liu et al. / Energy & Buildings 226 (2020) 110345
Nomenclature/Abbreviations
TES
LTES
STES
DP
RH
CPU
CRAC
UPS
EES
PCM
ATES
HX
TBS
Thermal energy storage
Latent thermal energy storage
Sensible thermal energy storage
Dew Point
Relative humidity
Central processing unit
Computer room air conditioner
Uninterrupted power supply
Electricity energy storage
Phase change material
Aquifer thermal energy storage
Heat exchanger
Telecommunications base station
AHU
ACU
PCB
UFAD
EDL
COP
PUE
HP
TPCT
TCO
Air handle unit
Air conditioner unit
Phase change board
Under-floor air distribution system
Enthalpy difference laboratory
Coefficient of performance
Power usage effectiveness
Heat pipe
Two-phase closed thermosyphon
Total cost of ownership
5.3.
6.
7.
TES integrated free cooling system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3.1.
TES combined heat pipe system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3.2.
TES coupling evaporative cooling system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Operation strategies based on TES to reduce operation cost. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Conclusions and future work. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Declaration of Competing Interest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction
Data centers, which house computing servers, network equipment, cooling devices, power supplying sets, and other related
equipment, experience fast growth as an integral part of information and communication technology. Due to the massive computation and data interactions, data centers consume explosive amount
of energy. The energy consumption of data centers is approximately 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 consumption, cooling devices, as one of the main infrastructures providing proper operating conditions for servers, account for about
30–40% of total consumption, taking up the second largest proportion, 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 cooling, 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 enhancement of overall efficiency and reliability, better economic feasibility, 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 different 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 integrated 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.
14
15
15
17
18
18
18
18
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 technologies. In section 2, operating conditions, energy mismatches, and
security requirement in data center are overviewed. Section 3 discusses principles and the characteristics of TES technologies in data
center with a focus on TES materials and TES configurations. Applications of passive TES coupled air flow, and applications of active
TES integrated cooling systems, are analyzed in section 4 and section 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 efficiency 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, Refrigerating and Air-conditioning Engineers (ASHRAE) developed thermal guidelines for data processing environment. Classes A1-A4
were proposed to define suitable temperatures and humidity setting 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.
L. Liu et al. / Energy & Buildings 226 (2020) 110345
3
Fig. 1. Main components of the sections.
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 consumption process. Thermal energy storage technology adapts to the variations in outdoor temperature and user cooling requirement (i.e.,
supply–demand mismatches). During the operation of data centers, five supply–demand mismatches commonly occur, including:
Fig. 2. Recommended and allowable operating conditions for four ASHRAE data
center classes [15].
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 rigorous 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 limited 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 example, 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/m2 [20–21], while the heat density of a
typical civil building is about 90–400 W/m2.
Requirement of high security and high cooling load in data centers leads to the development of data centers cooling system as a
(a) Lower efficiency of traditional refrigeration systems in daytime compared with its relatively higher efficiency at nighttime. 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 prevent servers from failure under higher heat load conditions,
cooling infrastructure of data center should have the auxiliary 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 security [32]. Challenges during power outages are much more
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L. Liu et al. / Energy & Buildings 226 (2020) 110345
significant than those under normal operations, because
power failure may impose more serious damages to data
centers, such as sudden shutdown of servers and fundamental destruction of IT equipment. Therefore, emergency cooling is one of the cornerstones to maintain the reliability of
data centers [33–34].
To mitigate these ‘‘mismatches” challenges above, energy storage technologies become inevitable and powerful, which can not
only improve energy utilization efficiency but also balance the discrepancy 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 physical 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 material 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 temperature, and the storage materials undergo no phase change in the
storage process. The amount of thermal energy stored is proportional to the temperature difference and specific heat capacity during charging and discharging processes. It can be defined as,
Z
Q¼
Tf
mC p dt ¼ mC ap T f T i
ð1Þ
Ti
where Q is cold energy quantity, kJ/s; T i denotes initial temperature, K; T f represents final temperature, K; m refer to flow rate of
fluid, kg/s; C p is specific heat capacity, kJ/(kg*K); C ap denotes average 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 sensible 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:
Z
Q¼
Tr
Ti
Z
mC p1 dt þ mar DHr þ
Tf
mC p2 dt
ð2Þ
Tr
where Q is cold energy quantity, kJ/s; T i denotes initial temperature, K; T r represents phase transition temperature, K; T f represents final temperature, K; m refers 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); ar denotes solidified fraction, %;
DHr 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 temperature and latent heat) but its price is high, while commercial paraffin 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 properties 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 thermal 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 temperature, 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 overcooled 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 sensible 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 configurations 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 dispersedly on the inner surface of enclosure, and any other locations
inside data center to store cold energy. Due to limitation of operation temperature of the data center, the transition temperature (i.e.
working temperature of TES) were strictly chosen. For passive TES
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L. 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 temperature ranges from 20 °C to 62 °C. As can be seen, passive TES currently 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 cooling, when heat transfer enhancement is needed [66–72], the structure 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 exchangers 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 contain, 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 difference between supplying and returning water is 5 °C [75–76]. For
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
Octadecane paraffin
PCM
31/32
Hours
27.4
25/35
Hours
Hours
PCM
PCM
PCM
–
20–30
–
PCM
–
Hours
Hours
For
emergency
Hours
Commercial paraffin
Tricosane, palmitic acid, or
lauric acid
RT42(PCM form Rubitherm
Company)
Lauric acid
Paraffin wax
39
42–4840–
4658–62
38–43
Minutes
41.5
56–58
Minutes
Minutes
Sun[52–54]
Nörtershäuser
[55]
Zhang[56–57]
Studniorz[58]
Onishi[59]
Zhang and Bai
[60–61]
Skach[62–63]
Weng[64]
Behi[65]
Jaworski[66]
[67–72]
Hours
Minutes
LTES, storage material type, especially transition temperature (i.e.
working temperature), is one of the most important design parameters. For active TES applied in data centers, storage materials,
working temperature and the design storage period are summarized in Table 2 in detail. As can be seen, water is commonly used,
and the working temperature ranges from 0 to 40 °C. Working temperature of LTES ranges from 13 °C to 30 °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 efficiency. The horizontally partitioned water tank was proved to efficiently 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 thermocline 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 m3 [75–76].
Table 2
Storage materials and working temperature of active TES.
Author
storage material
Working
temperature
Storage
period
Saeed [81]
Fang [82]
hexadecane
Mixture of tetradecane and
hexadecane
mixture of kwai acid and
lauric acid
Ternary fatty acid/expanded
graphite composite
Organic composite
Ground source or PCM
15.5–18.3
15.01
19.67
Hours
For
emergency
hours
15–25
Hours
18–20
–
Drenkelfort[83]
You and zhang
[91–92]
Garday[93]Oro
[94–95]
Aquifer
PCM
10
–
Hours
Seasons or
hours
Seasons
Hours
Water
5.5
Oro[96]
Xie[97]
Chen[98]
Hu and Wang
[99–100]
Hammann
[101]Wang
[102]
Muren[103]
Haywood[104–
105]
Wang[106]
Singh[17,107–
108]
Wei[109]
Sundaram[110]
Nall[111]
Luttrell [112–
113]
Hydrated salts
Hydrate
Water
Water
13
–
25
–
For
emergency;
hours
Hours
Hours
Hours
Hours
Water, PCM
–
Hours
Water
PCM
–
–
Hours
Hours
Dodecyl stearic acid
Water, ice
20.2
0
Water
Hydrate salt
Water
PCM slurry
38–40
28–30
–
21–27
Hours
Hours,
seasons
Hours
Hours
Hours
Hours
Chang [85–86]
Cui [87]
Sun[88–89]
Xiao[90]
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L. Liu et al. / Energy & Buildings 226 (2020) 110345
Isaac et al [116] studied several TES systems applied in a water
cooled data center and a 1-D plug-flow model was developed to
simulate the temperature distribution of a water tank. The mixing
layer thickness was proved to account for the development of the
thermocline, which was also controlled by the mixing ratio at the
inlet section. The model was validated by experimental data, and
was proved to be fast and efficient to help design and optimize a
water TES. Huang et al. [117] simulated the charging and discharging processes of natural stratified water tank with large/small temperature differences. The thermocline in the case of large
temperature difference between inlet and outlet was more stable
and had less fluctuation. More obvious temperature stratification
and higher efficiency were found in the case of large temperature
difference. When the cooling load of data center was very small,
the chiller was stopped and the cooling load was undertaken by
the TES to avoid partial load operation of the chiller and improve
its operating efficiency. When the data center operated at extreme
high working load, the TES was employed to utilize the cold energy
as a supplement cooling source.
Aquifer is a kind of geological formation containing the groundwater, and it stores thermal energy seasonally. Aquifer thermal
energy storage (ATES) consists of two wells. Cold water is extracted
from one well, and is recharged back to the other one after heat
exchange. ATES has lower investment and operation cost than classical water tank. However, the allowable temperature change, and
effects on environment need to be considered at the design and
operation stages. ATES is complex and limited in applications due
to the unpredictable hydrogeology [83].
Saeed et al. [81] presented an experimental investigation of a
TES vessel for load-shifting purposes. The novel TES was a platetype heat exchanger (HX) unit with water as the working fluid
and PCM as the energy storage medium. Schematic images for
the experimental HX and HX plate are shown in Fig. 3. The thermal
characteristics of the heat exchanger such as heat transfer coefficient, effectiveness, efficiency, water exit temperature, thermal
storage rate, total energy storage capacity and storage time were
experimentally evaluated as a function of various inlet conditions
including temperature and flow rate. Compared to conventional
storage systems, the compact parallel plate design showed an
enhanced performance with the effectiveness up to 83.1% when
Hexadecane was used. The proposed TES not only had substantial
thermal storage benefits, but also provided cost savings in infrastructure, equipment, and maintenance/operations compared to
conventional systems.
Fang et al. [82] numerically studied the basic unit of a tube-intank TES. Schematic design of the TES system can be seen in Fig. 4.
The thermal conductivity of the PCM was considered as constant to
assess the thermal energy storage potential of the TES. On one
hand, when the equivalent thermal conductivity of PCM increased
from 0.2 W/(mK) to 1 W/(mK), the outlet temperature reached
stability more quickly during the discharging process. On the other
hand, the outlet temperature profile hardly changed when the
equivalent thermal conductivity increased from 1 W/(mK) to
5 W/(mK). The capacity effectiveness was proved to be enhanced
Fig. 4. Schematic design of of the tube-in-tank TES: (a)complete system; (b) basic
unit with a single pipe with PCM [82].
compared with a same water tank. The characteristic curve of
TES was less influenced by the inlet temperature under the discharging condition. It was feasible for emergency cooling from
the perspective of cooling capacity.
Chang [85] developed a novel ternary two-way TES, as shown in
Fig. 5. Heat transfer model was established on the basis of the coupling of peak-valley electricity price, outdoor natural cold source
and phase change energy storage technology. In winter, the ternary
two-way TES was applied to directly replace the chiller. In summer, the TES was charged by chiller at night, when the outdoor
temperature and the electricity price were relatively low, and the
stored cold energy was then utilized as supplement during daytime. By integrating free cooling and mechanical refrigeration,
the novel TES not only saved energy, but also reduced the chiller
capacity. A mixture of kwai acid and lauric acid with the ratio of
7:3 was selected as PCM, which was a substantial part for the
TES design. Feng et al. [86] further studied the ternary two-way
phase TES. Working conditions of the TES were divided into five
detailed modes (charging by fresh air, charging by chiller, charging
by fresh air and chiller, discharging through return air, and cooling
through chiller), and the performance of each mode including temperature distribution, liquid phase distribution and the patterns of
charging and discharging, was analyzed, respectively.
4. Applications of passive TES coupled air flow
In order to quickly acquire the latest developments in this field,
an overview of innovative and representative passive TES systems
is summarized in Table 3. TES coupled air arrangement systems are
briefly described and the effects of TES are concluded in this table.
Fig. 3. Schematic image for the (a) experimental storage heat exchanger (HX) unit; (b) schematic picture for HX plate [81].
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L. Liu et al. / Energy & Buildings 226 (2020) 110345
Fig. 5. The ternary two-way phase change energy storage model: (a) schematic diagram of shell and tube HX; (b) schematic diagram of plate HX [85–86].
Table 3
Innovative and representative passive TES applied in data centers.
Type
TES in
Author
enclosure
Maintaining
the peak
temperature
of the server
below 50 °C
effectively
Sun[52–54]
Nörtershäuser
[55]
Non-
contact
TES
Achieving
energy saving
through peak
shift and
temperature
difference
between day
and night
Studniorz[58]
Onishi[59]
Zhang[60–61]
Skach
[62–
63]
Contact
TES
TES was
adjacent to
CPU
Weng[64]
Behi[65]
Jaworski[66]
[67–72]
System description
Effect
Marongiu[51]
PCM was encapsulated in bundles of aluminum tubes,
with one end of tubes outside and the other one inside of
the PCM container. The bundles of tube were installed at
the sides or bottom of the sealed enclosure.
PCM was encapsulated in aluminum board as PCM board. The molding PCM
board was mounted in the inner face of TBS exterior enclosure.
PCM was mixed with concrete
Achieving power saving rate of 18%-38% with 10 mm
thickness and 1 m2 PCM board
Obtaining the maximum load of the cabinet 1600 W,
1200 W and 1050 W for three different enclosures,
respectively
TES was integrated with the rectifying device, which was
mounted in the underfloor plenum, i.e. the upwind of the
server.
Zhang[56–57]
PCM was encapsulated in slabs and mounted at the downwind of the air
conditioner unit (ACU), i.e. the upwind of the server, and was applied in offgrid TBS.
TES was mounted at the downwind of the air handle unit
TES was arranged according to the pathway of the air flow, air supply section,
air returning section or any other section along the air flow way
Reducing peak cooling load by up to 12% or increasing the number of servers
by up to 14.6%
The adiabatic section of a heat pipe was covered with TES, with the evaporator
contacting with the heat spreader, and the condenser cooled by fan
The adiabatic section of a heat pipe was covered with TES, with the evaporator
contacting with the heat spreader, and the condenser cooled by cold water
Aluminum pipes filled with PCM was mounted on the base of heat sink, with
the fan blowing cooling air through pipes
Aluminum block was hollowly manufactured with passive pin–fin. PCM was
filled in the space between pin–fin
4.1. TES embedded in enclosure
Besides heat released by servers, heat transmitted through
external enclosure is also one main component of cooling load.
Thermal insulation material was used to reduce the cooling load
caused by external heat [118]. Due to the high heat density of
Reducing fuel consumption
Alleviating the need for UPS
Saving space, special auxiliary equipment and control
appliance
Saving 46% of the fan power consumption
Absorbing 11.7% of the heat dissipation of the power
supplier by the TES
Stabilizing the temperature of the electronics device and
protecting the electronics from rapid increase in heat
transfer rates
Achieving higher efficiency
the data center, it is necessary to cool the indoor environment
and server devices 24 h/365d all year around, therefore, the thermal insulation for isolating heat exchange between indoor and outdoor is not always an appropriate choice for the cooling demand.
TES is a more advanced technology applied in building enclosure
to fully utilize natural cooling source. Through the solidification
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L. Liu et al. / Energy & Buildings 226 (2020) 110345
and melting process of PCM, it transmits cold energy between the
outdoor environment and data center to reduce the energy consumption [119].
Dating back to 1997, Marongiu et al. [51] designed TES with
PCM encapsulated in bundles of tubes, and one end was placed
inside and the other outside. PCM melting and solidification process was numerically simulated, while corresponding experiment
was not conducted yet. Four cases with different kinds of PCM
and different pitches were studied, and the results indicated that
one of the four cases worked relatively well in maintaining target
temperature. This design can be considered as the prototype of
the application of PCM in enclosure. Only a limited portion of the
entire enclosure can be designed as the PCM heat exchanger. With
the development of manufacturing technologies, the fusion technology of PCM with enclosure material (e.g. concrete) and the
packaging technique of PCM become more practical. The applications of TES in enclosure were more flexible, and as required, the
whole building including walls, floors and roofs can be wrapped
with phase change board [45,47,120].
Sun et al. [52–54] designed and implemented TES in the enclosure of telecommunications base stations (TBSs). Based on the
phase change heat transfer process, the energy saving potential
of phase change board (PCB) in 5 different climatic regions in China
was simulated. The factors influencing energy saving efficiency and
economic benefit were discussed, including PCM thickness, PCM
transition temperature, ambient temperature, and diurnal temperature variation. Nörtershäuser et al. [55] discussed the main components of space cooling load of data centers, including cabinet
heat dissipation, external temperature and solar radiation heat.
Numerical models of different enclosure structures were established, including typical concrete wall, typical wall with internal
thermal insulation, typical wall with external thermal insulation,
and typical wall with external insulation and PCM. In a telecom
shelter without air conditioner, PCM helped to keep the indoor
temperature under 45 °C for a few days; While in case of a coldless
air conditioning system, the improvements with PCM was small.
Besides TES embedded in walls, TES can also be arranged in cabinets, or at the outside of cabinet or idle positions in computer
rooms, which are also kings of TES embedded in enclosure. According to the flow direction of cold air in computer rooms, TES could
be located at the upstream, downstream or other nearby locations
of the heat source. Airflow distribution optimization is one of the
important measures to save energy in data center [121]. When
TES is added in computer room, it would exert an influence on airflow distribution, and energy efficiency of TES would be affected by
the airflow in turn.
Zhang et al. [56–57] proposed an air distributor based on TES
and integrated the air distributor into the under-floor plenum of
under-floor air distribution system (UFAD). The sketch can be seen
in Fig. 6. For air source UFAD, air supplied from AHU passes
through air duct into the under-floor plenum. After exchanging
heat with PCM, air was supplied into sever room. Air of lower temperature at night and air of higher temperature in daytime was
supplied from AHU, after heat exchanger with PCM, air of constant
temperature was transported to the server room. For water source
UFAD, TES was charged by cold water in nighttime. Cold energy
was stored in TES at nighttime and released through the distributor
during the daytime. The design achieved operational cost saving
through peak shift and the temperature difference between day
and night.
Studniorz et al. [58] analyzed LTES applied in an existing offgrid TBS, based on a dynamic simulation model. The TES
charged/discharged directly by airflow can be seen in Fig. 8. The
PCM was macro-encapsulated in slabs and the transition temperature of the PCM was in the range of 20–30 °C. PCM was charged by
cold air from ACU and discharged to cool room air and supply it to
the computer room when ACU didn’t work. For cooling system of
constant supplying air temperature, the warm air in Fig. 7 (a)
was the same to the cold air in Fig. 7 (b). TES was demonstrated
to reduce the primary energy consumption in this off-grid TBS.
Onishi et al. [59] proposed TES for emergency cooling in computer rooms. The TES was solidified by the cold air supplied to
the computer room and stayed solid. When power failure occurred,
the fan driven by uninterrupted power supply (UPS) operated to
ventilate the computer room. The PCM was dissolved and the cold
energy was released within several minutes, until an emergency
power supply started up. This cooling system alleviated the need
for UPS, and turned out to be compact, efficient, and economical.
Zhang [60] attempted to install PCM according to the pathway
of air flow and store cold energy in the same pathway (shown in
Fig. 8). TES was dispersed on the floor and wall or the surface of
servers. Compared with existing energy storage technology, this
kind of TES did not need to occupy the internal or the external
space of computer rooms, and did not need any special TES auxiliary equipment or control appliance for cooling system. Bai et al.
[61] came up with a similar design, except that the TES was much
closer to servers.
4.2. TES based electronics cooling
TES-based electronics cooling has been paid great attention in
recent years as an emerging passive method [122–125]. When
the temperature of the heat spreader reaches the transition temperature of the specific TES, TES absorbs the latent heat without
raising temperature to maintain the stable temperatures of electronic devices.
Skach et al. [62–63] utilized TES for thermal time shifting when
the TES was arranged close to the processor inside the server. Three
homogeneous data center configurations, each of which was provisioned with a different type of servers, were evaluated using real
workload traces from Google. Simulation results showed that peak
cooling load was reduced up to 12 percent by TES, or in other notation, the number of servers was increased up to 14.6 percent without increasing the scale of the refrigerator. This design was able to
store cold energy to shape the thermal behavior of data center, and
released the stored energy only when it was beneficial.
Weng et al. [64] experimentally investigated the thermal performance of a heat pipe with TES for electronic cooling (shown in
Fig. 9). The adiabatic section of heat pipe was covered by TES,
which stored and released thermal energy depending upon the
heating power of evaporator and fan speed of condenser. Thermal
energy was stored in the TES When the heating power was too high
enough to be handled by the fan of condenser, and was released to
heat sink with low heating power at evaporator. Experimental
investigations were conducted to obtain the temperature distributions of the charging/discharging process. Different thermal energy
storage materials, volume of filling PCM, fan speed, and heating
power were investigated in the cooling module. The cooling module with tricosane as thermal energy storage materials saved 46%
of the fan power consumption compared with the traditional heat
pipe.
Behi et al. [65] introduced another kind of PCM-assisted heat pipe
module to improve the cooling process and avoid overheating inside
electronic devices. In this study, the condenser of the heat pipe was
cooled by a cold water loop, and the adiabatic section was seen as the
secondary condenser. Under the different intensity of power supplies, the thermal performance of the module was investigated
experimentally and numerically. The results revealed that 11.7% of
the heat dissipation was absorbed by the TES and the 75% of the heat
dissipation was transferred through the condenser. TES released
cold energy at transition temperature, to prevent the temperature
of electronic components from increasing dramatically, thus it elim-
L. Liu et al. / Energy & Buildings 226 (2020) 110345
9
Fig. 6. UFAD system combined with TES based on PCM: (a) air source; (b) water loop [56–57].
Fig. 7. Airflow direction of solidifying and melting process: (a) solidifying; (b)
melting [58].
inated thermal damage in electronic components. Although cooling
of water loop is much more efficient than air circulation one, the
structural design and water fluid allocation are much more complex
than that of air cooled condenser side.
Jaworski [66] proposed a new design of heat sink with PCM for
cooling of microprocessors. The schematic design of a simple PCMbased heat sink is depicted in Fig. 10. Aluminum pipe filled with
PCM was installed on the base of heat sink, and air flow blew over
the pipes vertically. Thermal characteristics and performance were
studied numerically. Results showed that a small amount of PCM
in heat sink significantly increased its capability to stabilize the
temperature of the microprocessor, indicating high potential of
PCM-based heat sink for electronics cooling.
During the application of PCM in cooling of electronic components, the poor thermal conductivity of PCM hinders its development. In addition to heat pipe and aluminum tube (or fins)
mentioned above, there are other ways to increase heat transfer
coefficient, including adding nano particles and metal foams
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L. Liu et al. / Energy & Buildings 226 (2020) 110345
Fig. 11. Photogragh of pin–fin heat sink: (a) with pin–fin; (b) without fin-pin
[67–70].
5. Applications of active TES integrated cooling system
Fig. 8. Typical layout of TES along airflow pathway [60].
According to cooling system of different types with which
active TES integrated, there are three application scenarios to be
analyzed in detail, TES integrated traditional refrigeration system,
TES integrated absorption refrigeration system enclosure, and
TES integrated free cooling system. In order to quickly acquire
the latest developments in this field, an overview of innovative
and representative active TES systems is summarized in Table 4.
TES based cooling system are briefly described and the effects of
TES are concluded in this table.
5.1. TES integrated traditional refrigeration system
Fig. 9. The reality of phase change cooling moudle [64].
[67–70]. Fig. 11(a) shows the diagram of pin–fin heat sink, where
the space between pin fins is filled with PCM. When the padding
is selected to be nano particles or metal foams, pin fins disappear,
as shown in Fig. 11 (b).
There are lots of researches on the performance of PCM-based
thermal management systems for electronic devices along with
thermal properties of PCM [71]. Parameters investigated ranged
from fins thickness, fins number, PCM types, PCM volume fraction,
to geometrical parameters and non-dimensional numbers [72].
Nevertheless, more extensive studies are conducted to reveal the
energy storage mechanism of PCM. In addition, the application of
PCM in the field of cooling electronics should be explored further
coupling with CPU workload.
Traditional cooling system refers to the cooling system driven
by compressor, which is divided into air-cooled system and
water-cooled system according to the heat transfer types of the
condenser. In water-cooled cooling system, cold water can be
obtained through cooling tower or directly from nature source.
TES integrated with traditional refrigeration system could take
advantages of discontinuous cold source to achieve higher efficiency and security, with mechanical refrigeration system in the
spare state. For an entire cooling system, it is probable that more
than one kind of cold source is applied to cool different parts
(TES and condenser). In this paper, an air-cooled (or watercooled) system refers to the mechanical refrigeration system (i.e.
the basic standby cooling system) in which condenser is cooled
by air (or water), while TES is charged by air, water, underground
cold source or any other available cold source.
5.1.1. TES based air-cooled system
Sun et al. [88] proposed a technology that combined PCM with a
natural cold source to reduce the space cooling energy of TBSs. The
schematic diagram of the TES inside a TBS is shown in Fig. 12. PCM
was utilized to store cold energy from outdoor air at night, and
release the stored energy for indoor cooling during daytime. A
Fig. 10. Schematic design of PCM-based heat sink: (a) graphic model; (b) plan sketch [66].
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Table 4
Innovative and representative active TES applied in data centers.
Type
Author
System description
Energy/cost saving
TES in traditional
refrigeration
system
Sun[88–89]
TES was charged through circulated water cooled by outdoor cold air, and
discharged through circulated water by indoor air as well.
The cooling system mainly consisted of dry cooler, ground source HX, standby
conditioner and TES, in which the ground source HX was also TES for seasonal
thermal energy storage.
Aquifer thermal energy storage was combined with air-cooled conditioner, to
provide chilled water together with air-cooled conditioner.
Cooling tower or dry cooler was utilized to charge TES.
TES was charged by cold water from chiller.
The average annual energy efficiency ratio of this
unit was 14.04 W/W
Utilizing ground source and cold ambient air
sufficiently
Xiao[90]
Drenkelfort
[83]
You[91–92]
Garday[93]
Oro[94–95]
TES in absorption
refrigeration
system
Oro[96]
TES could be charged by ambient cold air or cold water from chiller.
Xie[97]
The cooling system of the data center was combined with TES. It stored heat
emitted from the server and enhanced by heat pump system, which was also
utilized as the heat source for the hot-water supplying system.
The condenser and absorber was cooled by cooling tower, the generator
provided 55 °C water to spray and cool the computing CPU, and the evaporator
provided cold water to cool the storage room through TES.
Solar energy or exhaust gas and jacket cooling water of the combustion engine
generator was utilized by absorption refrigerator, and the cold energy
provided by absorption refrigerator was stored in TES.
TES was charged by absorption refrigerator, cooling tower or chiller.
Chen[98]
Hu[99]
Wang[100]
Hammann
[101]Wang
[102]
Muren[103]
TES in free
cooling
system
Haywood
[104–105]
Wang[106]
Singh
[31,107–
108]
Wei[109]
Sundaram
[110]
Nall[111]
Luttrell
[112–113]
The ambient cold energy in night was stored in TES to cool the condenser and
absorber of the absorption refrigerator, and the generator absorbed heat from
solar energy.
Solar energy and high quality heat of the data center was stored in TES to drive
the absorption refrigerator, which provided cold energy to the data center.
TES was integrated with the condenser to be cooled by the ambient air and
provide cold energy to condenser.
The evaporator of the heat pipe was mounted in the cold water tank and the
condenser was exposed to ambient air. TES was charged by low outdoor air or
cold water from chiller. The data center was cooled by cold water from the
TES.
The condenser of the heat pipe was mounted in the cold water tank, and the
evaporator was exposed to the air flow of the TBS. The TES was charged by the
outer surface of the water tank. The TBS was cooled by evaporator of heat pipe
through indoor circulated air.
Sealed steel container was filled with PCM encapsulated balls immersed in
water. The evaporator of the heat pipe was immersed in water, and the
condenser was exposed to air. The TES charged by ambient air through heat
pipe and was discharged through the surface of the steel container
Cooling tower was applied to charge the TES and TES provided cold energy to
the data center adapted to relatively high temperature.
Air indoor was cooled by cold water through indirect evaporation and direct
evaporation. PCM slurry was pumped to the downwind of the cooled air for
the charged/discharged process.
theoretical model was developed to assess the viability of this
technology. The results were validated with experimental data by
placing the proposed TES inside of an enthalpy difference laboratory (EDL). Simulations were carried out using weather data of five
cities located in five different climatic regions in southwest and
eastern China. The results indicated that the average annual
adjusted energy efficiency ratio of this unit was 14.04 W/W. In
addition, a mathematical model was developed to simulate the
operation of the proposed system. Energy savings ratio was used
as the criterion to evaluate energy savings in the five specified
cities [89]. Based on the TES unit integrated refrigeration system,
Chen et al. [126] further proposed six operating modes to reduce
the cooling energy. Free cooling using outdoor air provided significant energy saving for space conditioning. Refrigerating capacity,
cooling capacity, cold storage rate, cold supply, cold storage, and
coefficient of performance (COP) were experimentally calculated
and analyzed. Results indicated that the proposed refrigeration
system performed the requested tasks steadily. Further, Liu and
Cheng et al. studied other aspects of the novel design including
Reducing the overall energy demand of cooling
system
Sufficiently utilizing cold ambient air
Providing cold energy for 15 min and reducing the
scale of UPS; Reducing the annual electrical cost by
3%
Energy consumption of the cooling system was
decreased by 15%–22%
The COP of heat pump was raised at low ambient
temperature. The energy consumption of cooling
system was reduced.
The energy saving efficiency, exergy efficiency and
operation cost saving ratio could be up to 51%,17%,
and 71%
Sufficiently utilizing solar energy or exhaust gas
and jacket cooling water, and saving electrical
energy
TES stored cold water produced by free cooling,
absorption refrigerator or the chiller
Above 15% COP enhancement was achieved
PUE fell less than one
The PUE was decreased to 1.51. The energy saving
rate reached 28%.
Minimizing the chiller cooling capacity, and saving
electricity and cost
The temperature of the TBS was controlled with
the fluctuation range of 2 °C
Absorbing heat dissipated during the hottest part
of the day, it is viable and reliable for TBS installed
in remote regions
Eliminating the chiller
Expanding the range of operation temperature up
to 6 °C, and reducing the water consumption
economic effectiveness [127] and numerically simulation model
[128–129].
Xiao et al. [90] proposed another kind of air-cooled system, in
which underground source water was applied to charge TES
(shown in Fig. 13). Underground source water and soil could also
be considered as thermal energy storage materials, which were
seasonal storage sources of cold energy. In hot summer, ground
source had much lower temperature than ground surface and environment. Through this design, underground cold source was
extracted to TES and supplied to data center for cooling. In winter,
data center was cooled by the air-cooled system integrated with
TES, when outdoor temperature was low enough to achieve high
efficiency. If natural cold energy was not sufficient for data center,
the standby refrigerator would be switch on.
To further study, Drenkelfort et al. [83] integrated aquifer thermal energy storage (ATES) in data center to cut down cooling load
demand of the cooling system (shown in Fig. 14). Aquifer water
with seasonally stable temperature was utilized in the cooling
system and no water container was needed. Case studies with
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Fig. 12. Schematic diagram of the TES inside a TBS [88–89,126].
actual temperature profile of underground should be verified. In
addition, legal aspects are suggested to be deeply researched, since
benefits and requirements for operation of ATES are ambiguous.
Studies mentioned above mainly focus on utilizing of diurnal/
seasonal/ cold energy and low electricity price to achieve energy
saving and cost saving. You et al. [91] designed a kind of aircooled system to provide cold energy for emergency cooling to
data centers by the application of TES. Zhang et al. [92] realized
emergency through control strategies and TES technology.
Fig. 13. Schematic diagram of TES system using ground Source [90].
mid-size data centers for three different German locations were
conducted to determine the energetic and economic savings of
the ATES operation compared to a standard cooling system design.
Using site specific data (weather and hydrogeological data), the
implement of ATES reduced the energy demand of cooling system
drastically, mainly depending on underground structure. However,
only simulation study was conducted using specific data, while
5.1.2. TES based water-cooled system
The typical application of TES in water-cooling system was proposed by Garday et al. [93] in the white paper of 2007, as shown in
Fig. 15. This design was to meet emergency cooling requirements
of data center during power failure. Cold-water tank stored cold
energy required by data center for 15 min. During this period,
the IT equipment was supported by UPS after power failure, while
the diesel generator had not been started successfully. Under normal operating conditions, data center was supported by watercooling chiller, and a small portion of the chilled water was
pumped into cold water tank as an alternative. Supplying cold
energy without interval after power failure is the basic function
of emergency cooling. To meet this challenge, Wang et al. [130]
Fig. 14. Schematic diagram of the ATES cooling system [83].
L. Liu et al. / Energy & Buildings 226 (2020) 110345
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Fig. 16. Schematic diagram of water-cooled system combining hot-water supply
system [97].
Fig. 15. The typical application of TES in water-cooling system [93].
presented another method of switching between conventional
refrigeration and emergency cold sources. Oro et al. [94–95] simulated the efficiency performance of water-cooled system integrated
with TES, which was charged by redundant chiller. A dynamic
energy model was presented to study effects of different strategies
on operational conditions and power consumption. It was proved
to be feasible to store cold energy under the condition of offpeak electricity price, and the annual electrical cost was reduced
by 3%.
Mitchell [78] proposed another TES applied in data center. The
cooling system was just a typical refrigeration system mentioned
in Ref. [93]. The distinctiveness was the energy storage materials
applied, which were ice encapsulated in balls. The balls floated in
water tanks filled with glycol solution. Ice balls were frozen by glycol solution during nighttime when electricity price was lower, and
thawed during daytime. Cold glycol provided cool air to the data
center, which could reduce the demand for operation of chillers
during daytime. The unique TES was utilized in a data center in
Phoenix where climate was hot and dry, and the temperature
was up to 109°F, and an economizer could not work economically.
Power energy and water supply was saved. However, giant container was needed to hold ice balls in, and a large space was
needed.
Ling et al. [31] proposed a water-cooled cooling system. In this
novel cooling system, lake water was applied as natural cold
source when water temperature was lower than 12 °C. When
water temperature increased, water-cooled chiller would work
together with lake water. When water temperature increased
higher than 18 °C, the chiller worked independently. As can be seen
from the diagram, cold water was supplied to data center through
a cold storage tank. The TES could provide cold energy in case of
power failure to promise higher security and annul average power
usage effectiveness (PUE) of data center was below 1.2.
Oro et al. [96] further proposed a water-cooled system integrated with free cooling and TES. In this design, TES was charged
by the cold water from chiller as well as outdoor air. System performance was simulated under different European climates.
Depending on climates of different locations, energy consumption
of the cooling system for a 1250 kW data center was decreased by
15%–22%. The results revealed that this cooling system was economic in locations such as Amsterdam, Barcelona, Frankfurt, and
London, but was not profitable for Sweden where electricity price
was low.
Xie et al. [97] proposed a system combined cooling system of
data center and hot-water supplying system by the implementing
of TES (shown in Fig. 16). In winter when outdoor temperature was
extremely low, there was demand of hot water, and the heat dissipation of data center was used as the heat source for water-source
heat pump of the hot-water supplying system. As the temperature
of the heat accumulated in hydrate energy storage was much
higher than ambient temperature, the coefficient of performance
of the water-source HP raised substantially. At the same time,
energy consumption of the cooling system was reduced, since cooling tower and chiller were turned off.
5.2. TES integrated absorption refrigeration system
Absorption cooling system consists of condenser, evaporator,
absorber, circulation pump, throttle valve, other parts and generator which can absorb heat to boil the refrigerant [131]. As lots of
heat is vented from data center, absorption cooling system is an
efficient way to cool data center and absorb the heat at the same
time. For data center, TES used in absorption refrigeration can be
divided into heat storage and cold thermal energy storage according to the different charging and discharging temperatures, where
a high temperature TES is used to heat the generator and a low
temperature TES is applied to accumulate the redundant cold
energy.
5.2.1. Cold energy storage based absorption refrigeration system
Chen’s study proposed a two-stage LiBr/H2O absorption chiller
with spray cooling system for data centers, which was proved to
be efficient [132]. However, the additional chillers for treating
recycle water led to unnecessary energy consumption, thus further
research was aimed at improving the energy management of data
center through increasing cooling efficiency, recovering waste heat
and shifting peak load with TES [98]. The configuration of the
hybrid cooling system is shown in Fig. 17. In this study, the cooling
load was divided into computing racks heating load and storage
racks heating load. Meanwhile, these two different parts were
cooled by cooling water from the generator and the evaporator
respectively. The cooling load of computer room changed with
time, resulting in the COP change of absorption cooling system.
Due to an essentially unstable cooling load of storage racks, the
disparity of supply and demand was balanced through TES and a
complementary air conditioner. The thermodynamic and economic
models based on energy and exergy balance were developed. Comparison study of the proposed system and original cooling system
in Dawning 5000A supercomputer was performed. On the one
hand, a refined approach to energy utilization was beneficial to
energy conservation. On the other hand, to simplify the simulation
process, a lot of assumptions were made to establish the mathematical model, such as steady-state flow conditions, no leakage
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L. Liu et al. / Energy & Buildings 226 (2020) 110345
Fig. 17. Configuration of hybrid cooling system for data center [98].
and pressure drop, and equilibrium state of refrigerant in generator
and absorber, which all required experimental verification further.
Data centers are normally cooled by cold air or cold water
directly rather than spraying cooling, and in circumstance of sufficient solar energy, solar absorption cycle is also an optional renewable cooling method. Hu et al. [99] proposed a solar absorption
refrigerator within TES, which utilized solar energy and stored cold
energy in cold water tank during the period of abundant solar
energy, and released cold energy in TES to data center in the case
of poor solar illumination conditions. For off-grid system, Wang
et al. [100] utilized an internal combustion engine generator to
provide power to industrial park, while the exhaust gas and the
jacket cooling water of the combustion engine generator were supplied to the generator of absorption cycle. The schematic diagram
of the diesel generator driving absorption refrigerator is shown
in Fig. 18. The cold energy produced by absorption refrigerator
was supplied to data center, and the surplus was stored in TES
and released when needed.
Hamann et al. [101] proposed a data center cooling system
within TES utilizing free cooling and/or solar cooling. When outside temperature below a certain value, the data center took
advantage of free cooling and the chiller plant was bypassed. In
addition, the heat generated by the solar collector system drove
the absorption refrigerator to provide cooling water for the internal
water loop. Free cooling and absorption refrigerator shared a coldwater tank. Through control strategy, TES stored the cold water
produced by free cooling or by the low power-consumption
absorption refrigerator driven by solar energy. Wang et al [102]
Fig. 18. Schematic diagram of diesel generator driving absorption refrigerator
within TES [100].
further specified the arrangement of solar absorption refrigerator
and utilized more efficient PCM as energy storage material.
Muren et al. [103] investigated a computational model of an
absorption system to explore the advantages of nighttime cooling
and TES to enhance the performance of solar absorption refrigeration for peak cooling in data centers. As shown in Fig. 19, TES was
applied to store cold energy to cool the condenser and the absorber
rather than to cool data center directly. COP enhancement of 15%
was achieved with simple cold storage strategies. When optimally
designed, this type of system achieved higher energy efficiency,
and offered environmental and economic advantages that made
it an attractive initial step in incorporating solar powered absorption cooling into green data center designs. Since providing cold
source for condenser and absorber, TES in this design was utilized
to store the lower-grade energy, thus the volume needed would be
much larger, which significantly increased the investment in system construction.
5.2.2. Heat storage based absorption cooling system
Haywood et al.[104] proposed an absorption cooling system
making use of the waste heat generated by data center room’s
equipment as well as the sustainable power (i.e. solar energy).
The overall system level diagram for the work and heat flow paths
is shown in Fig. 20. By the use of TES, the challenge of providing
both enough cold energy to data center and enough exergy to drive
the cooling process, regardless of the thermal output of data center
equipment, was addressed. Haywood et al. [105] further studied
the purpose of capturing a high fraction of dissipated thermal
power by using water as heat transfer fluid. By effectively capturing at least 85% of the heat dissipated from the CPUs on each blade
server and efficiently transporting that thermal energy to drive
absorption chiller, it was possible to achieve high efficient PUE.
With external heat supplementation, such as solar thermal, the
value of PUE can fall to less than one.
Due to exploitation of the instability of solar energy and other
heat energy (i.e. heat dissipation in data centers), TES is generally
added in an absorption cycle to accumulate heat energy. By the
application of TES, the running time of absorption cycle was prolonged [133–134]. Youshida et al. [135] proposed a novel system
incorporating cold supply to data center and heat supply to other
facility. In this system, the heat from data center was stored in
TES, and supplied to absorber together with post-heating source
to enhance the efficiency of cooling system.
5.3. TES integrated free cooling system
Free cooling system runs spontaneously due to the temperature
difference between the indoor and the outdoor, and its efficiency is
rather high. Obviously, it is highly dependent on ambient temper-
Fig. 19. Representation of absorption cooling system with TES [103].
L. Liu et al. / Energy & Buildings 226 (2020) 110345
Fig. 20. Overall system level diagram showing the work and heat flow paths [104].
ature. In order to maximize the energy utilization efficiency of data
center, various technical means such as TES system are used to
make full use of the indoor and outdoor temperature difference
(i.e., outdoor low ambient temperature). Strictly speaking, free
cooling is not completely energy-free, but it consumes energy
much less than mechanical refrigeration. Meanwhile, free cooling
is too dependent on the ambient temperature to provide enough
cooling capacity continuously, and thus mechanical refrigeration
is used as backup system in almost all the cooling system in data
center. TES in free cooling system is mainly divided into TES combined heat pipe system, and TES coupling evaporative cooling
system.
5.3.1. TES combined heat pipe system
Heat pipe, also known as thermal diode, is on the basis of the
principle of unidirectional heat conduction. Driven by capillary
force, gravity force or working medium pump, heat pipe consumes
little or no energy and is a very energy-saving heat transfer
method. Meanwhile, since heat transfer in heat pipe is based on
phase-changing heat transfer mechanism, heat transfer efficiency
is ten times that of common metals [136]. Integral heat pipes
and separated heat pipes both have been applied in data centers
[137–138].
Shi et al. [139] proposed a heat pipe/direct expansion combined
refrigeration system, where separated heat pipe was adopted.
Through combination of separated heat pipe and direct expansion
refrigeration, the system had two operating modes including heat
pipe and mechanical refrigeration modes. When outdoor temperature was low, the heat pipe mode started to run. When outdoor
temperature was high, the refrigeration mode was switch on. In
order to extend the effective cooling time of heat pipe mode and
make full use of outdoor cold sources, cold thermal energy storage
was added in this scheme [106], and the principle of the system
can be seen in Fig. 21. When the heat pipe was insufficient to
undertake all cooling load, the combined operation of heat pipe
and cold storage was adopted. The condenser was a ternary heat
exchanger, while the specific configuration of condenser was not
introduced in detail. In addition, the switching values of multiple
operating modes were not specified.
Compared with integral heat pipe, the structure of the heat
exchanger of separated heat pipe is relatively complicated, and
more so when combined with TES. Ternary heat exchangers are
designed to realize the TES’s charging/discharging process, thus
multi-mode applications of separated heat pipe with TES and the
refrigerator are rather complex. By contrast, the application of integral heat pipe in data center is paid relatively more attentions.
Singh proposed an integral heat pipe based on TES in data center,
mainly taking advantage of the thermal diode character of the heat
15
pipe [17,107–108]. As shown in Fig. 22, liquid cooling was used in
data center servers, and the cooling system outside the racks consisted of heat exchanger, cold energy storage system, electrical
chiller and a cooling tower. Multiple operating modes were
achieved. The TES was charged through the chiller as well as the
cooling tower when ambient temperature was low, even more,
when ambient temperature was low enough, TES was charged by
the integral heat pipe without any power consumption. Thermal
performance and economy efficiency were analyzed, and the system performance was optimized.
Wei et al. [109] studied a passive heat transfer system of heat
pipe with cold energy storage. Heat in the indoor space was
exported from the cold water tank by using heat pipe bundles,
and then the heat was released to the environment through natural
convection of the tank wall. As the cold source of the heat pipe, the
cold water tank was used to balance the temperature difference
between day and night and flat the temperature crest of the heat
pipe condenser in conventional scheme. The simulation results
showed that the water temperature was kept lower than the ambient air due to the release heat at night, but the effect of peak load
clipping was not obvious.
Sundaram et al. [110] designed a new passive cooling system
incorporating PCM and two-phase closed thermosyphon (TPCT)
heat exchangers to carry out thermal management for TBS. The
PCM and the evaporation section was placed indoor, and the condensation section was placed outside. The newly developed thermal system absorbed the equipment dissipated heat during the
hottest part of the day, stored it as latent heat and then released
it to the ambient through thermosyphon at night. Experimental
results indicated that a typical shelter fitted with an airconditioning system consumed 22,776 kWh, while the proposed
cooling system saved approximately 14 tons of carbon footprints
per year. The cooling system was viable and reliable for shelters
installed in desert and tropical regions, and for remote areas where
there was no power grid and the maintenance was minimized.
However, according to the results, the capacity of PCM may be
not sufficient in some months of a year, thus further optimal
design is needed to fully meet the cooling load requirement of electronic equipment.
5.3.2. TES coupling evaporative cooling system
In general, free cooling method is classified as airside free cooling, waterside free cooling, and heat pipe free cooling. Due to the
low thermal density and poor thermal conductivity of air, it is
almost impossible to use airside free cooling to store heat directly
or indirectly. Therefore, the water side free cooling technology
combined with TES is adopted in the data center.
Nall [111] proposed a compressor-less cooling system. The cold
water from the cooling tower firstly went into a water TES and
then was supply to air handle unit (AHU), the warm water then
returned to the cooling tower. For this design, the temperature difference during charging and discharging was enlarged. Considering
the wet bulb temperature and the atmospheric pressure associated
with elevation, effects of thermal storage materials and external
climate on the system was studied. Besides the prevailing metric
PUE, the total cost of ownership (TCO) was considered to be more
important. According to qualitative analysis, the expense added by
TES and the oversized cooling tower, was nearly offset the avoided
cost of the water chiller and the separate chilled water system.
On account of an indirect/direct evaporative cooler, two cooling
system designs were compared [112]: indirect/direct evaporative
cooler augmented with a direct expansion cooler and the same
indirect/direct evaporative cooler augmented with a thermal
energy storage system. The cooler designs are shown in Fig. 23.
The capital cost of the TES system was expected to be higher than
the direct expansion system, while utilization time and the operat-
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L. Liu et al. / Energy & Buildings 226 (2020) 110345
Fig. 21. Schematic diagram of HP-TES combined refrigeration system [106].
Fig. 22. Data center facility with heat pipe based cold energy storage system [17].
Fig. 23. Two indirect/direct evaporative cooler design: (a) Direct expansion augmented cooler; (b) TES augmented cooler [112].
L. Liu et al. / Energy & Buildings 226 (2020) 110345
ing cost of the TES system were much lower. Electrical power and
water of the TES system were saved according to quantitative analysis. For the sake of comparison, the evaluation of investment cost,
operation cost and water saving should be integrated into a unified
index, such as simple economic payback period, so as to make
more comprehensive analysis of the advantages and disadvantages
of different systems.
Luttrell et al. [113] further analyzed indirect/direct evaporative
cooler with TES to reduce the scale of the cooling system and
extend the upper temperature limit. Highly porous materials,
which allowed air to pass through with little pressure loss, were
applied as the evaporative media. Water was cooled outside by
the cooling tower and contained in the water sump. The cooled
water was then supplied to the indirect cooling heat exchanger
(HX) and the direct cooling HX inside. The indoor air passed
through the two HXs one by one to be cooled. Then, the cooled
air was propelled by a fan to the data center. Three idealized integration concepts were considered, seen in Fig. 24, which all
required moderate quantities of PCM slurry to achieve improvement. An increase of the upper temperature limit approached
6 °C at moderate humidity levels. Two cases offered potential to
substantially reduce water consumption for a wide range of diurnal conditions. Moderate quantities of PCM slurry was employed
to minimize water use.
TES implemented in this study was water-based slurry of
micro-encapsulated PCM. Small capsule size eliminated the effect
of low thermal conductivity of phase change materials, which
was one of the major engineering challenges. Some scholars have
started to study the application of micro-encapsulated PCM in data
centers [79–80], although it is still in the exploratory stage and has
great application potential in the future.
6. Operation strategies based on TES to reduce operation cost
On the basis of design and optimization of TES integrated systems, operation strategies were proposed. In the premise that
structure and thermal design of TES units and TES integrated cooling systems and air flow arrangement were matched well, only the
storage capacity and energy loss rate were considered for the TES
in the operation strategies. And most of the studies were focusing
on operation cost reduction by optimized algorithm.
Google and Apple applied the idea of TES for computer room air
conditioner (CRAC) to reduce the operation cost as well as uninterrupted power supply (UPS) energy storage [140–141].By shifting
(part of) the cooling load of data center from day to night hours,
thereby taking advantage of the lower ambient air temperature
and utilizing the off-peak tariff for electricity, , TES was proved to
17
reduce the energy consumption by 20% and the energy cost by
35% [142].
Wang et al. [50] focused on cutting down the electricity bill by
optimizing the control strategies of the thermal management system with utilization of TES. A smart cooling strategy was proposed,
which checked low price in the hour-head power market to overcool thermal mass in data center and absorb heat when power
price increased, which was also integrated with thermal storage
tank to store energy at night. The trace-driven simulation showed
that the cooling strategy achieved the desired cooling performance
with 16.8% less electricity bill than traditional cooling control. They
further optimized the control strategies by integrating TES (including thermal mass and thermal storage tank) and UPS [143]. A
dynamic control algorithm based on Lyapumov drift and Lyapunov
optimization was design to exploit energy storage, and a smart
cooling framework that dynamically coordinated different kinds
of storage techniques was developed, as shown in Fig. 25.
Song et al. [144] proposed a combined cooling heating and
power (CCHP) system to cool data center, which supplied cold
energy by electric chillers in off-peak hours and by absorption chillers in peak hours. Two operation strategies, following thermal
load (FTL) and following electric load (FEL), were simulated by
TRNSYS 17.0 , and the results was compared. The overall system
efficiency of FEL system with cool thermal storage is larger than
that of FTL. Compared with FTL, the primary energy consumption,
CO2 equivalent emissions and operation costs of FEL were reduced.
For the proposed CCHP system, with the introduction of cold
energy storage, the FEL strategies was better than the FTL
strategies.
Zhou et al, [145] further investigated the comprehensive operation cost reduction for data center using energy storage, considering electricity cost as well as cost of energy storage devices. Two
forms energy storage, thermal energy storage with electricity from
smart grid and battery storage with electricity from wind energy
and smart grid, were proposed. Based on emulated workload,
real-time wind speed, and real-time electricity price, timeaveraged electricity cost was optimized and normalized, and then
compared with electricity supply without energy storage. The
results showed that storage capacity and the location of data center affected the cost of storage devices and the energy supply, and
energy storage didn’t always turn to reduce comprehensive operation cost of data center.
Guo et al. [146–147] explore the problem of joint geographical
load balancing, delay-tolerant workload scheduling, and thermal
storage management for green energy integration in geographically passive data center. An online control algorithm, Stochastic
Cost Minimization Algorithm (SCMA), with provable performance
guarantee based on the Lyapunov optimization framework, was
Fig. 24. Three idealized integration concepts: (a) TES integration with the water sump; (b) TES integration with HX before media; (c) TES integration HX after media[113].
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L. Liu et al. / Energy & Buildings 226 (2020) 110345
cooling systems at different levels with TES integration need be
further developed before they can be widely deployed for public
market. This review summarizes the present progresses of TES
applications in data center, which enlightens the trends and directions for future TES development for data center applications.
Declaration of Competing Interest
Fig. 25. control flow of smart cooling framework in off-peak hours.
proposed to formulated the problem. The effectiveness SCMA was
demonstrated through both analytical analysis and numerical evaluations, was proved to provide an explicit trade-off between cost
saving and workload delay.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared
to influence the work reported in this paper.
Acknowledgements
This work was supported by the International Science and Technology Cooperation Project of China (2017YFE0105800), and
National Natural Science Found of China (No.51878254).
7. Conclusions and future work
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