Energy saving initiatives for cooling in data centres
In January 2021, Europe’s leading cloud and data centre operators signed the Climate Neutral Data Centre Pact, a self-regulatory initiative to become climate neutral by 2030. According to recent studies, free cooling methods may be effective in achieving significant energy savings goals.
Data centres are large capacity facilities whose primary function is to house information technology (IT) equipment (i.e. computer servers) and offer data services. [1, 2] Data centres are currently responsible for about 2% of global electricity consumption.  Depending on the scenario, it has been estimated that data centres will use around 3–13% of global electricity in 2030, compared to 1% in 2010.  Between 30% and 55% of this consumption is allocated to cooling IT equipment. 
Energy savings objectives
In January 2021, 25 companies and 17 associations signed the Climate Neutral Data Centre Pact, which sets the goal of achieving climate neutrality by 2030. Major European cloud and data centre operators, including AWS, Google and Equinix, as well as smaller national providers, have signed this agreement. The pact commits the signatories to increase and measure their efficiency, to use only renewable energy by 2030, to address water efficiency, take part in circular economy to repair and recycle servers, and reuse waste heat. The group also promises to use the Power Usage Effectiveness (PUE) standard, while addressing the need for a new metric to replace the PUE. 
The PUE is defined as the ratio of the amount of energy used by an entire data centre facility to the energy used solely for IT equipment. A PUE value of 1 represents the hypothetical situation where the energy consumption of all non-IT equipment (e.g. lighting, power supply, communication and cooling systems) would be zero. An average PUE is typically below 2.0, while data centres with high energy performance can achieve lower values, close to 1.2. 
As part of the Climate Neutral Data Centre Pact, all new data centres in cool climates will have to achieve an annual PUE target of 1.3 (operating at full capacity) by the beginning of 2025. Facilities in warm climates will only have to achieve a PUE of 1.4, since they need to use energy to cool their IT servers. Existing data centres with worse PUEs will have until 2030 to meet these targets. 
For more details on the actions to make data centres climate neutral by 2030, please visit: https://www.climateneutraldatacentre.net/self-regulatory-initiative/
Free cooling methods 
Reducing the energy consumption of the cooling system is key to achieve a low PUE value. For instance, thermal management can be improved by the appropriate use of locally available free cooling sources.
Free cooling is defined as the use of a natural cold source (air or water) to cool an indoor environment. When the cold source temperature is below the indoor operating temperature, the data centre can be cooled without a compressor-based refrigeration cycle. This mode of operation is commonly known as economizer cycle. Airside economizers utilize natural cold air to cool the data centre environment, while waterside economizers utilize natural cold water.
Air-side economizers cool the data centre indoor environment using the outdoor air directly or indirectly. With direct air-side economizers, filtered outdoor air is drawn into the indoor environment using fans, without any air conditioning or humidity control. Direct air-side economizers have the advantage of being less complex because these systems do not require pumps, cooling towers or heat exchangers.
Indirect air-side economizers use an intermediate air-air heat exchanger, between the recirculated indoor airflow and outdoor airflow. The additional heat transfer step reduces the effectiveness of the whole system. Nevertheless, in regions with constant high humidity levels, indirect air-side economizers can provide more free cooling hours than direct air-side economizers.
Overall, the performance of direct type economizers depends only on the regional climate and indoor operating conditions, while the performance of the indirect type also depends on the heat exchanger.
Water-side economizers can be classified as direct water-cooled system, air-cooled system and cooling tower system.
In direct water-cooled systems, natural cold water is used to cool the data centre indoor air without any further heat transfer steps.
Air-cooled systems use heat exchangers to cool down the circulating water. Despite lower effectiveness (i.e. fewer free cooling hours), it is simpler than direct water-cooled systems and limits the environmental impact caused by natural water consumption.
Cooling tower systems use an evaporative tower to reduce the circulating water temperature. According to the authors, cooling tower economizers are the most widely used in the waterside category because they overcome most of the problems encountered by other types of systems. The water consumption rate of a cooling tower is only about 5% of that of a direct water system. Furthermore, the amount of heated water discharged is very small, thus reducing the environmental harm.
Impact of free cooling methods on energy consumption
The Green Grid 2012 had estimated that the use of free cooling could result in savings of around 20% in cost, energy consumption and carbon emissions caused by cooling when compared to data centres that do not use free cooling. 
For example, it has been reported that one of Facebook data centres uses a direct air-side economizer system, achieving a PUE value as low as 1.09. One of Google data centres uses a direct water-side economizer system, achieving a PUE value lower than 1.20. Previous studies have shown that an indirect air-side economizer system with a high-efficiency heat exchanger can achieve an energy saving of 63.6%. A study in China found an energy saving rate of about 19.2% when using an indirect water-side economizer system with an open cooling tower. 
A recent study published in the International Journal of Refrigeration has proposed a methodology for assessing data centre free cooling operation in Brazil. The authors used the heat exchanger approach point applied to criteria from ASHRAE thermal guidelines with local weather data to estimate how many hours a data centre can be cooled without a compressor.  They found that cities such as Curitiba, São Paulo, Porto Alegre and Brasília can potentially operate in both air-side and water-side economizer modes for more than 3000 h per year, even considering conservative data centre thermal operating limits.
For more details on their assessment and the methodology used, please visit: https://iifiir.org/en/fridoc/free-cooling-potential-for-brazilian-data-centers-based-on-approach-143201
Limitations of free cooling methods
Although free cooling technologies offer significant energy savings, there are some drawbacks. Direct air-side economizers can affect the environmental parameters of data centres. Disadvantages include dust contamination and reduced reliability of the data centre due the absence of humidity or temperature control.  Direct water-side economizers are limited by cold water availability, meaning that the data centre must be located close to water sources. Indirect water-side economizers may face the problem of freezing cooling water. Therefore, it is necessary to implement cooling technologies that can make efficient use of natural cold sources without being limited by location and climate. 
 Amado, E. A., P. S. Schneider, and C. S. Bresolin. Free cooling potential for Brazilian data centers based on approach point methodology. International Journal of Refrigeration 122: 171-180. https://doi.org/10.1016/j.ijrefrig.2020.11.010
 Han, Z., Sun, X., Wei, H., Ji, Q., & Xue, D. (2020). Energy saving analysis of evaporative cooling composite air conditioning system for data centers. Applied Thermal Engineering, 116506. https://doi.org/10.1016/j.applthermaleng.2020.116506
 The Role of Refrigeration in the Global Economy (2019), 38th Note on Refrigeration Technologies. http://dx.doi.org/10.18462/iif.NItec38.06.2019
 Andrae, A.S.G.; Edler, T. On Global Electricity Usage of Communication Technology: Trends to 2030. Challenges 2015, 6, 117-157. https://doi.org/10.3390/challe6010117