Overview of data center cooling technologies

Demand for digital services is growing rapidly. According to IEA ⁽¹⁾, since 2010, the number of internet users worldwide has more than doubled to reach 4.9 billion in 2021, while internet traffic has expanded 20-fold over the same period. The growing demand for streaming content, evolving digital technologies, emerging blockchain IT infrastructure, and accelerated growth of Internet of Things (IoT) as a result of expanding machine-to-machine (M2M) technology explain this sharp rise in traffic. 

 

However, since 2010, despite this strong growth in demand, data centre energy use (excluding energy used for cryptocurrency mining) has grown only moderately, in part due to improvements in the efficiency of IT equipment, their cooling technologies, and a shift away from small, inefficient enterprise data centres towards more efficient cloud and hyperscale data centres ⁽¹⁾.

 

In terms of GHG emissions, the data centres and data transmission networks that underpin digitalisation accounted for around 300 Mt CO₂-eq in 2020, equivalent to 0.9% of energy-related GHG emissions (or 0.6% of total GHG emissions). Since 2010, like energy consumption, emissions have grown only modestly despite the rapidly growing demand for digital services. However, to get on track with the Net Zero Scenario, emissions need to be halve by 2030 ⁽¹⁾.  Since, according to the IIR ⁽²⁾, between 30% and 55% of data centre electricity consumption is used for cooling IT equipment, further improvements in the energy efficiency of cooling systems are a key issue. 

 

. In a review paper ⁽³⁾ presented at the 2022 IIR-co-sponsored Purdue International Refrigeration and Air Conditioning Conference, American researchers provide an overview and perspectives of cooling technologies in data centres. Generally, cooling strategies in these facilities may be divided into two categories: air-cooled systems and liquid-cooled systems, both of which can be implemented at different levels (from chip to room level).

 

Overhead/underfloor air delivery, hot/cold aisle layout, and hot/cold aisle containment are the primary strategies used to optimise air cooled system performance. The raised floor architecture has been widely adopted in data centres, but has substantial air flow leakage (about 25–50%). It has been found that the optimal ventilation system is a hard floor design with overhead cold air delivery and hot air return duct instead of room-based supply and return. Cold-aisle containment can better reduce the maximum inlet temperature of the racks and suppress temperature rise during cooling system failure, while hot-aisle containment can provide lower average inlet temperature of the racks with smaller standard deviation and is less affected by air tightness around the servers. As rack power density rises above 10 kW/rack and heat flux beyond 100 kW/cm², conventional air-cooled systems are not a viable solution for thermal management.  

 

Liquid cooling methods such as spray cooling, impingement jet, immersion cooling, liquid-cooled micro-channels, and heat pipes are among the emerging technologies to overcome the capacity limitations of air-cooled systems. Pertaining to immersion cooling, transitioning into sub-cooled two-phase flow boiling, enhancing heat transfer by adding micro-structures or irregularities to create more nucleation sites and higher heat transfer surface area, and utilis ing nanofluids are prominent enhancement strategies gaining attention among scholars. Submerging a power electronics module in a fluid can lead to a thermal resistance 25% of that of an air-cooled system, or 30-50% of that of a liquid-cooled system such as microchannel or spray cooling. Depending on the existing cooling system, overall heat load, and hot spots, the heat pipe system can serve the data centre as a stand-alone unit or in conjunction with an air-cooled system, a so-called hybrid system. Compared to typical air-cooled systems, the hybrid system can lower the annual cooling load factor and energy consumption by 37-58% and 20-70%, respectively. 

 

 

Sources

(1) IEA, Data Centres and Data Transmission Networks, Tracking report - September 2022, https://www.iea.org/reports/data-centres-and-data-transmission-networks  

(2) IIR, The Role of Refrigeration in the Global Economy (2019), 38th Note on Refrigeration Technologies, https://iifiir.org/en/fridoc/the-role-of-refrigeration-in-the-global-economy-2019-142028  

(3) Isazadeh A. et al, Cooling Technologies in Datacom Facilities: An Overview and Perspectives, https://docs.lib.purdue.edu/cgi/viewcontent.cgi?article=3491&context=iracc