Thermal Management and Energy Consumption in Air, Liquid, and Free Cooling Systems for Data Centers: A Review

Overview

With the continued growth of high heat-dissipation data centres, thermal management and reducing cooling energy have become major trends, but existing studies looked at individual factors without systematic summary. The review compiles the key factors, related research and optimisation methods for each cooling system: in air cooling, thermal management relates mainly to uniform distribution of hot and cold air, and adjusting perforated tile porosity can reduce energy consumption; in liquid and free cooling, climate conditions, structural design, coolant type and flow rate are the key factors.

The free-cooling families are direct and indirect airside economisers (including rotary-wheel and plate heat exchangers), water-side systems using seawater or lake water, and cooling-tower free cooling. Liquid cooling covers cold plates, heat pipes and immersion. In a Seoul reference-model comparison, direct air cooling saved about 68% of energy, indirect air cooling 50% and a water-side cooling cycle 15%. A combined direct-air plus evaporative-cooling plus waste-heat-reuse system in Tokyo gave 20.8% annual energy savings, and every 2 °C drop in indoor temperature reduced savings by 2.8-8.5%.

Direct airside economisers in cooler climates cut energy use by 15-22% (Oro et al.); very dry or very humid sites suit them less because of humidification or dehumidification energy, and Beijing and Harbin are judged unsuitable because of haze and dust. Comparing nine air-side heat exchanger types, energy-saving devices cut total cooling energy by 47.5% to 62%, and an efficient indirect airside economiser saved 63.6%. An indirect evaporative cooling unit in Shenzhen achieved a PUE of 1.249 at 5.3 kW per cabinet. A lake-water/cooling-tower free cooling plant reached average cooling-equipment EER of 16.8-49.3 and an average PUE of 1.11-1.15 under four load factors. Optimising chilled-water flow rate and CRAC inlet temperature in a Tianjin data centre gave 21.3% energy savings and a 4.43% PUE reduction.

Limits: as a review it compares studies from different climates and conditions, so figures are not strictly like for like. The authors highlight the free-cooling switchover point, climate (humidity, haze/dust) and coolant flow as the dominant design variables.

Key points

1. In a Seoul reference-model comparison, direct air cooling saved about 68% of energy, indirect air cooling 50% and a water-side cooling cycle 15%.
2. A combined direct-air plus evaporative-cooling plus waste-heat-reuse system in Tokyo gave 20.8% annual energy savings; every 2 °C drop in indoor temperature reduced savings by 2.8-8.5%.
3. Direct airside economisers in cooler climates cut energy use by 15-22%; very dry or very humid sites suit them less, and Beijing and Harbin are judged unsuitable because of haze and dust.
4. Comparing nine air-side heat exchanger types, energy-saving devices cut total cooling energy by 47.5% to 62%; an efficient indirect airside economiser saved 63.6%.
5. An indirect evaporative cooling unit in Shenzhen achieved a PUE of 1.249 at 5.3 kW per cabinet.
6. A lake-water/cooling-tower free cooling plant reached average cooling-equipment EER of 16.8-49.3 and an average PUE of 1.11-1.15 under four load factors.
7. Optimising chilled-water flow rate and CRAC inlet temperature in a Tianjin data centre gave 21.3% energy savings and a 4.43% PUE reduction.

Using it in design

• In preliminary cooling selection, make climate (humidity, haze/dust) and the free-cooling switchover point decisive inputs; direct airside economisers may be unsuitable in haze- and dust-prone regions.
• In air cooling, tune perforated tile porosity and raised-floor height as airflow variables; porosity adjustment can reduce energy use.
• Use published PUE ranges by system type (for example lake-water free cooling 1.11-1.15, indirect evaporative 1.249) as preliminary targets and verify them against the project climate.

Who should read it

Cooling designers and energy-efficiency specialists choosing among air, free and liquid cooling options.

Document contents

1. 1. Introduction
2. 2. Air Cooling for Data Centers (2.1 Thermal Management Performance: uniformity of temperature distribution, cooling efficiency; 2.2 Energy Saving and Consumption Reduction of System)
3. 3. Free Cooling System (3.1 Air-Side Economizers: direct, indirect; 3.2 Water-Side Economizers: direct water-cooled system, air-cooled system, cooling tower system)
4. 4. Liquid Cooling Systems (4.1 Indirect Liquid Cooling: single-phase, two-phase, heat pipe; 4.2 Direct Liquid Cooling: liquid immersion, spray cooling, jet impingement)
5. 5. Conclusions and Further Recommendations

Related standards

• ASHRAE TC9.9 Thermal Guidelines

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Publication details

Authors

Sijun Xu; Hua Zhang; Zilong Wang

Publisher

Energies (MDPI AG)

Year

2023

Volume

16

Issue

3

Pages

1279

Document language

English

Source type

Scientific

Access

Open licence — downloadable

DOI

10.3390/en16031279

Licence

CC BY 4.0 — share with attribution

Licence evidence

www.mdpi.com

Citation