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Free cooling technologies for data centers: energy saving mechanism and applications

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Abstract

Air conditioning and cooling systems account for about 40 % of total electricity usage in data centers. Free cooling is a novel and promising technology that can decrease the load ratio of electrical chiller and save cooling energy consumption accordingly, through making full use of natural free cooling source. In this paper, four typical free cooling systems are analyzed and compared, to show their mechanisms, main features, energy saving effects and applicable situations respectively. (1) Direct fresh air cooling is of the highest free cooling potential. However, it is hard to meet indoor air quality demand due to the indoor-outdoor air mixing. (2) Rotating wheel heat exchanger can be used for indirect free cooling, since indoor and outdoor air flow in different paths for heat exchange. While its power usage effectiveness (PUE) increases inevitably under the same climatic conditions. (3) Heat pipe can be integrated with rack back plate to enhance heat transfer with free cooling sources. Its cooling efficiency can increase by 3-5 times compared to traditional heat exchangers. (4) In water-based free cooling system, a heat exchanger is installed in parallel with electrical chiller and the system can work under three modes according to different outdoor temperature. Increasing the load ratio of free cooling can decrease PUE and save electricity usage. In practical applications, the cooling system design for data centers depends on various factors, such as indoor air quality requirement, local climatic conditions, energy saving demands, room space, capital investment and operation costs.
ScienceDirect
Available online at www.sciencedirect.com
Available online at www.sciencedirect.com
ScienceDirect
Energy Procedia 00 (2017) 000–000
www.elsevier.com/locate/procedia
1876-6102 © 2017The Authors. Published by Elsevier Ltd.
Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling.
The 15th International Symposium on District Heating and Cooling
Assessing the feasibility of using the heat demand-outdoor
temperature function for a long-term district heat demand forecast
I. Andrića,b,c*, A. Pinaa, P. Ferrãoa, J. Fournierb., B. Lacarrièrec, O. Le Correc
aIN+ Center for Innovation, Technology and Policy Research -Instituto Superior Técnico,Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal
bVeolia Recherche & Innovation,291 Avenue Dreyfous Daniel, 78520 Limay, France
cDépartement Systèmes Énergétiques et Environnement -IMT Atlantique, 4 rue Alfred Kastler, 44300 Nantes, France
Abstract
District heating networks are commonly addressed in the literature as one of the most effective solutions for decreasing the
greenhouse gas emissions from the building sector. These systems require high investments which are returned through the heat
sales. Due to the changed climate conditions and building renovation policies, heat demand in the future could decrease,
prolonging the investment return period.
The main scope of this paper is to assess the feasibility of using the heat demand outdoor temperature function for heat demand
forecast. The district of Alvalade, located in Lisbon (Portugal), was used as a case study. The district is consisted of 665
buildings that vary in both construction period and typology. Three weather scenarios (low, medium, high) and three district
renovation scenarios were developed (shallow, intermediate, deep). To estimate the error, obtained heat demand values were
compared with results from a dynamic heat demand model, previously developed and validated by the authors.
The results showed that when only weather change is considered, the margin of error could be acceptable for some applications
(the error in annual demand was lower than 20% for all weather scenarios considered). However, after introducing renovation
scenarios, the error value increased up to 59.5% (depending on the weather and renovation scenarios combination considered).
The value of slope coefficient increased on average within the range of 3.8% up to 8% per decade, that corresponds to the
decrease in the number of heating hours of 22-139h during the heating season (depending on the combination of weather and
renovation scenarios considered). On the other hand, function intercept increased for 7.8-12.7% per decade (depending on the
coupled scenarios). The values suggested could be used to modify the function parameters for the scenarios considered, and
improve the accuracy of heat demand estimations.
© 2017 The Authors. Published by Elsevier Ltd.
Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and
Cooling.
Keywords: Heat demand; Forecast; Climate change
Energy Procedia 143 (2017) 410–415
1876-6102 © 2017 The Authors. Published by Elsevier Ltd.
Peer-review under responsibility of the scientific committee of the World Engineers Summit – Applied Energy Symposium & Forum: Low
Carbon Cities & Urban Energy Joint Conference.
10.1016/j.egypro.2017.12.703
10.1016/j.egypro.2017.12.703 1876-6102
Available online at www.sciencedirect.com
ScienceDirect
Energy Procedia 00 (2017) 000000
www.elsevier.com/locate/procedia
1876-6102 © 2017 The Authors. Published by Elsevier Ltd.
Peer-review under responsibility of the scientific committee of the World Engineers Summit Applied Energy Symposium &
Forum: Low Carbon Cities & Urban Energy Joint Conference.
World Engineers Summit Applied Energy Symposium & Forum: Low Carbon Cities & Urban
Energy Joint Conference, WES-CUE 2017, 1921 July 2017, Singapore
Free cooling technologies for data centers: energy saving
mechanism and applications
Yin Zhanga,b, Zhiyuan Weib, Mingshan Zhangc*
aSchool of Architecture and Environment, Sichuan University, Chengdu 610065, China
bDepartment of Building Science, Tsinghua University, Beijing 100084, China
cResearch Institute of Nationalities, Southwest Minzu University, Chengdu 610041, China
Abstract
Air conditioning and cooling systems account for about 40 % of total electricity usage in data centers. Free cooling is a novel and
promising technology that can decrease the load ratio of electrical chiller and save cooling energy consumption accordingly,
through making full use of natural free cooling source. In this paper, four typical free cooling systems are analyzed and compared,
to show their mechanisms, main features, energy saving effects and applicable situations respectively. (1) Direct fresh air cooling
is of the highest free cooling potential. However, it is hard to meet indoor air quality demand due to the indoor-outdoor air
mixing. (2) Rotating wheel heat exchanger can be used for indirect free cooling, since indoor and outdoor air flow in different
paths for heat exchange. While its power usage effectiveness (PUE) increases inevitably under the same climatic conditions. (3)
Heat pipe can be integrated with rack back plate to enhance heat transfer with free cooling sources. Its cooling efficiency can
increase by 3-5 times compared to traditional heat exchangers. (4) In water-based free cooling system, a heat exchanger is
installed in parallel with electrical chiller and the system can work under three modes according to different outdoor temperature.
Increasing the load ratio of free cooling can decrease PUE and save electricity usage. In practical applications, the cooling system
design for data centers depends on various factors, such as indoor air quality requirement, local climatic conditions, energy
saving demands, room space, capital investment and operation costs.
© 2017 The Authors. Published by Elsevier Ltd.
Peer-review under responsibility of the scientific committee of the World Engineers Summit Applied Energy Symposium &
Forum: Low Carbon Cities & Urban Energy Joint Conference.
Keywords: Data center; Refrigeration; Air conditioning; Free cooling; Energy efficiency
* Corresponding author. Tel.: +86-28-85522790; fax: +86-28-85524605.
E-mail address: swunzms@126.com
2 Author name / Energy Procedia 00 (2017) 000000
1. Introduction
During recent years, information communication and internet technologies are undergoing a dramatically fast
development [1]. Thus the increasing demand for data processing caused a rapid growth in the data centers
containing IT equipment, rack servers and related devices. It is reported that the total energy consumption in data
centers doubled from 2005 to 2010 [2]. The IT devices are often of huge heat emissions and show high demands for
temperature and humidity control in data centers, so that the cooling and air conditioning system is indispensable.
According to a recent survey, the refrigeration and air conditioning system accounts for about 40 % of total
electricity usage for data centers [3]. Therefore, in order to reduce the energy consumption of air conditioning is of
great importance in energy efficiency for the whole data center.
There are many effective ways to save cooling energy usage in data centers, such as indoor air distribution
optimization, heat transfer enhancement for rack servers, thermal performance improvement of chillers and so on [4].
Zimmermann [5] applied the hot water system to refrigeration system in IT rooms and established the energy model.
Ebrahimi [6] introduced different cooling systems according to working conditions and found that energy efficiency
could be increased substantially through low grade energy recovery. Marcinichen [7] proposed the two phase
cooling technologies and put forward the method to evaluate heat recovery ratio. In addition, free cooling is a novel
and promising technology that can decrease the load ratio of electrical chiller and save cooling energy consumption
accordingly, through making full use of natural free cooling source [8]. Because of the high efficiency and low
emissions, free cooling technologies utilized in data centers causes more and more attentions during recent years.
In this paper, four typical free cooling technologies and corresponding air conditioning systems are analyzed and
compared, to show their mechanisms, main features, energy saving effects and applicable situations respectively.
This work is of significance in guiding the design of refrigeration system with free cooling technologies for practical
data centers.
2. Air Conditioning Systems in Data Center
IT devices always show high necessities in working conditions, especially the indoor temperature (22±2 oC) and
humidity (50±5 %) control. Therefore, the air conditioning system is of great significance in space cooling for such
data centers, considering the huge and consecutive heat emissions [9]. Fig. 1 shows the typical refrigeration system
for data centers. The electrical chiller is used to produce low temperature water in its evaporator and then the chilled
water is delivered to the terminal air handling units to take away the emission heat from racks. On the other hand,
the condensation heat of the chiller is exhausted to the ambient through the cooling tower.
Fig. 1. Typical refrigeration system in data centers.
Such refrigeration system constitutes the dominant part of energy consumption in data centers, except for the IT
devices themselves. Power usage effectiveness (PUE) is often used to evaluate the energy consumption level for
data centers [4].
total IT AC else
IT IT
PU PU PU PU
PUE PU PU

 
(1)
Condenser
Chiller
Indoor handling unit
Cooling tower
Rack
Yin Zhang et al. / Energy Procedia 143 (2017) 410–415 411
Available online at www.sciencedirect.com
ScienceDirect
Energy Procedia 00 (2017) 000000
www.elsevier.com/locate/procedia
1876-6102 © 2017 The Authors. Published by Elsevier Ltd.
Peer-review under responsibility of the scientific committee of the World Engineers Summit Applied Energy Symposium &
Forum: Low Carbon Cities & Urban Energy Joint Conference.
World Engineers Summit Applied Energy Symposium & Forum: Low Carbon Cities & Urban
Energy Joint Conference, WES-CUE 2017, 1921 July 2017, Singapore
Free cooling technologies for data centers: energy saving
mechanism and applications
Yin Zhanga,b, Zhiyuan Weib, Mingshan Zhangc*
aSchool of Architecture and Environment, Sichuan University, Chengdu 610065, China
bDepartment of Building Science, Tsinghua University, Beijing 100084, China
cResearch Institute of Nationalities, Southwest Minzu University, Chengdu 610041, China
Abstract
Air conditioning and cooling systems account for about 40 % of total electricity usage in data centers. Free cooling is a novel and
promising technology that can decrease the load ratio of electrical chiller and save cooling energy consumption accordingly,
through making full use of natural free cooling source. In this paper, four typical free cooling systems are analyzed and compared,
to show their mechanisms, main features, energy saving effects and applicable situations respectively. (1) Direct fresh air cooling
is of the highest free cooling potential. However, it is hard to meet indoor air quality demand due to the indoor-outdoor air
mixing. (2) Rotating wheel heat exchanger can be used for indirect free cooling, since indoor and outdoor air flow in different
paths for heat exchange. While its power usage effectiveness (PUE) increases inevitably under the same climatic conditions. (3)
Heat pipe can be integrated with rack back plate to enhance heat transfer with free cooling sources. Its cooling efficiency can
increase by 3-5 times compared to traditional heat exchangers. (4) In water-based free cooling system, a heat exchanger is
installed in parallel with electrical chiller and the system can work under three modes according to different outdoor temperature.
Increasing the load ratio of free cooling can decrease PUE and save electricity usage. In practical applications, the cooling system
design for data centers depends on various factors, such as indoor air quality requirement, local climatic conditions, energy
saving demands, room space, capital investment and operation costs.
© 2017 The Authors. Published by Elsevier Ltd.
Peer-review under responsibility of the scientific committee of the World Engineers Summit Applied Energy Symposium &
Forum: Low Carbon Cities & Urban Energy Joint Conference.
Keywords: Data center; Refrigeration; Air conditioning; Free cooling; Energy efficiency
* Corresponding author. Tel.: +86-28-85522790; fax: +86-28-85524605.
E-mail address: swunzms@126.com
2 Author name / Energy Procedia 00 (2017) 000000
1. Introduction
During recent years, information communication and internet technologies are undergoing a dramatically fast
development [1]. Thus the increasing demand for data processing caused a rapid growth in the data centers
containing IT equipment, rack servers and related devices. It is reported that the total energy consumption in data
centers doubled from 2005 to 2010 [2]. The IT devices are often of huge heat emissions and show high demands for
temperature and humidity control in data centers, so that the cooling and air conditioning system is indispensable.
According to a recent survey, the refrigeration and air conditioning system accounts for about 40 % of total
electricity usage for data centers [3]. Therefore, in order to reduce the energy consumption of air conditioning is of
great importance in energy efficiency for the whole data center.
There are many effective ways to save cooling energy usage in data centers, such as indoor air distribution
optimization, heat transfer enhancement for rack servers, thermal performance improvement of chillers and so on [4].
Zimmermann [5] applied the hot water system to refrigeration system in IT rooms and established the energy model.
Ebrahimi [6] introduced different cooling systems according to working conditions and found that energy efficiency
could be increased substantially through low grade energy recovery. Marcinichen [7] proposed the two phase
cooling technologies and put forward the method to evaluate heat recovery ratio. In addition, free cooling is a novel
and promising technology that can decrease the load ratio of electrical chiller and save cooling energy consumption
accordingly, through making full use of natural free cooling source [8]. Because of the high efficiency and low
emissions, free cooling technologies utilized in data centers causes more and more attentions during recent years.
In this paper, four typical free cooling technologies and corresponding air conditioning systems are analyzed and
compared, to show their mechanisms, main features, energy saving effects and applicable situations respectively.
This work is of significance in guiding the design of refrigeration system with free cooling technologies for practical
data centers.
2. Air Conditioning Systems in Data Center
IT devices always show high necessities in working conditions, especially the indoor temperature (22±2 oC) and
humidity (50±5 %) control. Therefore, the air conditioning system is of great significance in space cooling for such
data centers, considering the huge and consecutive heat emissions [9]. Fig. 1 shows the typical refrigeration system
for data centers. The electrical chiller is used to produce low temperature water in its evaporator and then the chilled
water is delivered to the terminal air handling units to take away the emission heat from racks. On the other hand,
the condensation heat of the chiller is exhausted to the ambient through the cooling tower.
Fig. 1. Typical refrigeration system in data centers.
Such refrigeration system constitutes the dominant part of energy consumption in data centers, except for the IT
devices themselves. Power usage effectiveness (PUE) is often used to evaluate the energy consumption level for
data centers [4].
total IT AC else
IT IT
PU PU PU PU
PUE PU PU

 
(1)
Evaporator
Condenser
Chiller
Indoor handling unit
Cooling tower
Rack
412 Yin Zhang et al. / Energy Procedia 143 (2017) 410–415
Author name / Energy Procedia 00 (2017) 000000 3
In Eq. (1), PUtotal and PUIT represent the power usage amounts of the overall data center and the IT equipment
respectively. Therefore, a lower PUE value means that the data center is more energy saving. Hence, reducing
energy consumption of air conditioning system (PUAC) proves to be an effective way to decrease PUE and save
power usage for the whole data center. Therein, free cooling technology is one of the most promising approaches.
3. Typical Free Cooling Technologies
3.1. Direct fresh air cooling
As Fig. 2 shows, when the outdoor temperature is low, the fresh air can be brought in directly through ventilation
system for free cooling. Such a system can make most use of the ambient free cooling potentials without any extra
refrigeration equipment. Nonetheless, due to the indoor-outdoor air mixing, it is hard to meet the indoor air quality
(IAQ) requirement for direct fresh air cooling system. Thus such a ventilation system often work with other fresh air
handling equipment, including dehumidification device, filters and air cleaners, to remove moistures, dusts and other
pollutants, which will inevitably increase the primary and operation costs. As a result, Although of high energy
efficiency and low PUE value, the direct fresh air cooling systems are merely applied in developed countries, such
as Microsoft data center in Dublin and Hewlett-Packard data center in London, where direct fresh air cooling
systems are installed with advanced indirect evaporation cooling equipment for air cleaning (PUE=1.08).
Fig. 2. Direct fresh air cooling system.
Fig. 3. Free cooling system based on rotating wheel heat exchanger.
3.2. Rotating wheel heat exchanger
To solve the indoor air quality problem of the aforementioned system, heat exchangers can be added between the
indoor and outdoor airs. Therein, rotating wheels are widely used. Fig. 3 gives the schematic diagram of rotating
wheel heat exchangers where the wheel keeps rotating at a speed of 10-12 r/min and the airs flows into different
paths to avoid mixing [10]. Then the indoor air flows back to the data center for space cooling after heat exchange
Fresh air handling unit
Data center
Racks
Exhaust fan
Condenser
Wheel
Air cleaner
Filter
Outdoor
Indoor
Filter
Evaporater
Supply fan
4 Author name / Energy Procedia 00 (2017) 000000
whereas the heated outdoor air is exhausted. Compared to the direct fresh air cooling, such a system based on the
rotating wheel heat exchanger is in fact an indirect free cooling system, which can significantly improve the indoor
environment. However, due to the extra medium heat exchange process, the free cooling effect of such an indirect
free cooling system decreases inevitably under the same climatic conditions, increasing PUE value as a result. So in
order to guarantee a reliable refrigeration system, electrical cooling equipment is often integrated with the rotating
wheel heat exchanger. For instance, in data center of China Mobile Company at Harbin, only if outdoor temperature
is lower than 23 oC, does the rotating wheel system work. Otherwise, the data center is refrigerated via electrical
cooling. In addition, such a rotating wheel heat exchange system usually occupies a large space because of the
relatively low air-air heat transfer efficiency, which also limits its applications.
3.3. Heat pipe back rack
Heat pipe is a kind of heat exchanger, where phase change materials are used to facilitate the high-efficient heat
transfer. Integrating the heat pipe heat exchangers with racks is favorable for heat transfer enhancement between the
high temperature racks and the free cooling sources. As Fig. 4 shows, multiple heat pipes are packed into a board
installed at the rack back. Compared to traditional heat exchangers, the cooling efficiency of such heat pipe rack
back can be increased by 3-5 times, which can decrease the PUE value dramatically. For example, the annual
electricity usage amount of IT devices in China Mobile data center in Jiangsu province is 6.83×105 kWh. After heat
pipe back racks taking place of traditional ones, the annual electricity consumption decreases considerably from
1.37×106 kWh to 1.07×106 kWh, making PUE decrease from 2.00 to 1.49 as a result.
Fig. 4. Heat pipe back rack. Fig. 5. Water-based free cooling system .
3.4. Water free cooling system
As Fig. 5 shows, the main difference between water-based free cooling system and traditional air conditioning
system is that a heat exchanger is installed in parallel with electrical chiller to make full use of free cooling
capacities from cooling tower. In other words, according to climatic conditions (especially the wet bulb temperature),
the whole system can work under three different modes: (1) when the outdoor temperature is low (winter), the
cooling water can be used to produce chilled water directly through the heat exchanger and the chiller can be turned
off, so that the system works under free cooling mode; (2) when the outdoor temperature is high (summer), the
chiller is activated instead while the cooling tower is only used to handle the condensation heat, so that the system
works under electrical cooling mode; (3) when the outdoor temperature is moderate (spring and fall), the chiller
and heat exchanger work together in parallel, so the system works under free cooling + electrical cooling mode.
Therefore, the working conditions of the water-based free cooling system are greatly impacted by the ambient
temperature variation. Taking the HUAWEI dater center at Langfang (a northern city in China) as an example (Fig.
Rack
Rack
Heat pipe back
Chiller
Pump
Pump
Terminal
devices
Cooling tower
Heat
exchanger
Yin Zhang et al. / Energy Procedia 143 (2017) 410–415 413
Author name / Energy Procedia 00 (2017) 000000 3
In Eq. (1), PUtotal and PUIT represent the power usage amounts of the overall data center and the IT equipment
respectively. Therefore, a lower PUE value means that the data center is more energy saving. Hence, reducing
energy consumption of air conditioning system (PUAC) proves to be an effective way to decrease PUE and save
power usage for the whole data center. Therein, free cooling technology is one of the most promising approaches.
3. Typical Free Cooling Technologies
3.1. Direct fresh air cooling
As Fig. 2 shows, when the outdoor temperature is low, the fresh air can be brought in directly through ventilation
system for free cooling. Such a system can make most use of the ambient free cooling potentials without any extra
refrigeration equipment. Nonetheless, due to the indoor-outdoor air mixing, it is hard to meet the indoor air quality
(IAQ) requirement for direct fresh air cooling system. Thus such a ventilation system often work with other fresh air
handling equipment, including dehumidification device, filters and air cleaners, to remove moistures, dusts and other
pollutants, which will inevitably increase the primary and operation costs. As a result, Although of high energy
efficiency and low PUE value, the direct fresh air cooling systems are merely applied in developed countries, such
as Microsoft data center in Dublin and Hewlett-Packard data center in London, where direct fresh air cooling
systems are installed with advanced indirect evaporation cooling equipment for air cleaning (PUE=1.08).
Fig. 2. Direct fresh air cooling system.
Fig. 3. Free cooling system based on rotating wheel heat exchanger.
3.2. Rotating wheel heat exchanger
To solve the indoor air quality problem of the aforementioned system, heat exchangers can be added between the
indoor and outdoor airs. Therein, rotating wheels are widely used. Fig. 3 gives the schematic diagram of rotating
wheel heat exchangers where the wheel keeps rotating at a speed of 10-12 r/min and the airs flows into different
paths to avoid mixing [10]. Then the indoor air flows back to the data center for space cooling after heat exchange
Fresh air handling unit
Data center
Racks
Exhaust fan
Condenser
Wheel
Air cleaner
Filter
Outdoor
Indoor
Filter
Evaporater
Supply fan
4 Author name / Energy Procedia 00 (2017) 000000
whereas the heated outdoor air is exhausted. Compared to the direct fresh air cooling, such a system based on the
rotating wheel heat exchanger is in fact an indirect free cooling system, which can significantly improve the indoor
environment. However, due to the extra medium heat exchange process, the free cooling effect of such an indirect
free cooling system decreases inevitably under the same climatic conditions, increasing PUE value as a result. So in
order to guarantee a reliable refrigeration system, electrical cooling equipment is often integrated with the rotating
wheel heat exchanger. For instance, in data center of China Mobile Company at Harbin, only if outdoor temperature
is lower than 23 oC, does the rotating wheel system work. Otherwise, the data center is refrigerated via electrical
cooling. In addition, such a rotating wheel heat exchange system usually occupies a large space because of the
relatively low air-air heat transfer efficiency, which also limits its applications.
3.3. Heat pipe back rack
Heat pipe is a kind of heat exchanger, where phase change materials are used to facilitate the high-efficient heat
transfer. Integrating the heat pipe heat exchangers with racks is favorable for heat transfer enhancement between the
high temperature racks and the free cooling sources. As Fig. 4 shows, multiple heat pipes are packed into a board
installed at the rack back. Compared to traditional heat exchangers, the cooling efficiency of such heat pipe rack
back can be increased by 3-5 times, which can decrease the PUE value dramatically. For example, the annual
electricity usage amount of IT devices in China Mobile data center in Jiangsu province is 6.83×105 kWh. After heat
pipe back racks taking place of traditional ones, the annual electricity consumption decreases considerably from
1.37×106 kWh to 1.07×106 kWh, making PUE decrease from 2.00 to 1.49 as a result.
Fig. 4. Heat pipe back rack. Fig. 5. Water-based free cooling system .
3.4. Water free cooling system
As Fig. 5 shows, the main difference between water-based free cooling system and traditional air conditioning
system is that a heat exchanger is installed in parallel with electrical chiller to make full use of free cooling
capacities from cooling tower. In other words, according to climatic conditions (especially the wet bulb temperature),
the whole system can work under three different modes: (1) when the outdoor temperature is low (winter), the
cooling water can be used to produce chilled water directly through the heat exchanger and the chiller can be turned
off, so that the system works under free cooling mode; (2) when the outdoor temperature is high (summer), the
chiller is activated instead while the cooling tower is only used to handle the condensation heat, so that the system
works under electrical cooling mode; (3) when the outdoor temperature is moderate (spring and fall), the chiller
and heat exchanger work together in parallel, so the system works under free cooling + electrical cooling mode.
Therefore, the working conditions of the water-based free cooling system are greatly impacted by the ambient
temperature variation. Taking the HUAWEI dater center at Langfang (a northern city in China) as an example (Fig.
Rack
Rack
Heat pipe back
Chiller
Pump
Pump
Terminal
devices Cooling tower
Heat
exchanger
414 Yin Zhang et al. / Energy Procedia 143 (2017) 410–415
Author name / Energy Procedia 00 (2017) 000000 5
6), electrical cooling, free cooling, electrical cooling + free cooling account for 51%, 32%, 17% of time respectively
in one year, resulting in that the annual average PUE value arrives at 1.35. Based on the previous analysis, the
energy saving effect of such systems mainly derives from the free cooling. Hence, increasing the load ratio of free
cooling can decrease PUE and save electricity usage.
Fig. 6. Free cooling system based on rotating wheel heat exchanger.
Table 1. Comparison between typical free cooling systems in data centers.
Direct fresh
air cooling
Rotating wheel
heat exchanger
Heat pipe
back rack
Water-based
free cooling
Air mixing
Yes No No No
Air quality resuirement
High Medium Low Low
Indoor humidity influence
High Medium Low Low
Occupied room space
Small Large Large Large
Electricity usage units
Fan Fan Fan Pump
Power consumption
Low High Low High
Primary investment
800 RMB/kW 1000 RMB/kW 1380RMB/kW 1250 RMB/kW
Maintenance
Filter, Fan Fan None Heat exchanger
Key technologies
Air cleaning;
Humidification
Humidity control;
Heat exchanger;
Heat pipe;
Anti-freezing
Temperature control
Anti-freezing
Applicable situations Low IAQ demand
High IAQ demand;
Enough space; Tout≤15
o
C
High IAQ demand;
Limited space;
Large cooling load;
Tout Tchilled water +1
o
C
4. Conclusions
Decreasing the electricity usage of refrigeration system through free cooling technology is greatly important for
energy saving in data centers. In this paper, four typical free cooling systems are introduced and analyzed, including
direct fresh air cooling, rotating wheel heat exchanger, heat pipe back rack and water-based free cooling system.
Furthermore, the comparison among these free cooling systems is conducted to show their mechanisms, main
features, energy saving effects and applicable situations respectively (Table 1). In practical applications, the cooling
system design for data centers depends on various factors, such as indoor air quality requirement, local climatic
conditions, energy saving demands, room space, capital investment and operation costs. This work is of significance
in guiding the design of refrigeration systems with free cooling technologies for practical data centers.
Acknowledgements
This research is financed by China Scholarship Council (201706245001), Sichuan Science and Technology
Program (17YYJC0994) and Sichuan University Post Doctor Research Program (2017SCU12020).
-10
0
10
20
30
01000 2000 3000 4000 5000 6000 7000 8000
Wet bulb temperature/
oC
Time/h
Free cooling
Electrical cooling+Free cooling
Electrical cooling
6 Author name / Energy Procedia 00 (2017) 000000
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Yin Zhang et al. / Energy Procedia 143 (2017) 410–415 415
Author name / Energy Procedia 00 (2017) 000000 5
6), electrical cooling, free cooling, electrical cooling + free cooling account for 51%, 32%, 17% of time respectively
in one year, resulting in that the annual average PUE value arrives at 1.35. Based on the previous analysis, the
energy saving effect of such systems mainly derives from the free cooling. Hence, increasing the load ratio of free
cooling can decrease PUE and save electricity usage.
Fig. 6. Free cooling system based on rotating wheel heat exchanger.
Table 1. Comparison between typical free cooling systems in data centers.
Direct fresh
air cooling
Rotating wheel
heat exchanger
Heat pipe
back rack
Water-based
free cooling
Air mixing
Yes
No
No
No
Air quality resuirement
High
Medium
Low
Low
Indoor humidity influence
High
Medium
Low
Low
Occupied room space
Small
Large
Large
Large
Electricity usage units
Fan
Fan
Fan
Pump
Power consumption
Low
High
Low
High
Primary investment
800 RMB/kW
1000 RMB/kW
1380RMB/kW
1250 RMB/kW
Maintenance
Filter, Fan
Fan
None
Heat exchanger
Key technologies
Air cleaning;
Humidification
Humidity control;
Heat exchanger;
Heat pipe;
Anti-freezing
Temperature control
Anti-freezing
Applicable situations
Low IAQ demand
High IAQ demand;
Enough space; Tout≤15 oC
High IAQ demand;
Limited space;
Large cooling load;
Tout Tchilled water +1 oC
4. Conclusions
Decreasing the electricity usage of refrigeration system through free cooling technology is greatly important for
energy saving in data centers. In this paper, four typical free cooling systems are introduced and analyzed, including
direct fresh air cooling, rotating wheel heat exchanger, heat pipe back rack and water-based free cooling system.
Furthermore, the comparison among these free cooling systems is conducted to show their mechanisms, main
features, energy saving effects and applicable situations respectively (Table 1). In practical applications, the cooling
system design for data centers depends on various factors, such as indoor air quality requirement, local climatic
conditions, energy saving demands, room space, capital investment and operation costs. This work is of significance
in guiding the design of refrigeration systems with free cooling technologies for practical data centers.
Acknowledgements
This research is financed by China Scholarship Council (201706245001), Sichuan Science and Technology
Program (17YYJC0994) and Sichuan University Post Doctor Research Program (2017SCU12020).
-10
0
10
20
30
01000 2000 3000 4000 5000 6000 7000 8000
Wet bulb temperature/oC
Time/h
Free cooling
Electrical cooling+Free cooling
Electrical cooling
6 Author name / Energy Procedia 00 (2017) 000000
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