Two-phase thermosiphon cooling in data centers: a comprehensive review of refrigerants for practical selection

Abstract

Data centers (DCs) play a crucial role in supporting artificial intelligence (AI), online platforms and cloud services. Currently, there are approximately 5 billion users worldwide, and this number is expected to grow by 5% annually. This surge will result in a substantial increase in energy consumption and will push us to reconsider appropriate energy management strategies. Two-Phase Thermosiphon (TPT) systems provide effective cooling for DCs through the natural circulation driven by gravity and the phase change of refrigerants. This process facilitates efficient heat transfer without the need for moving components, resulting in improved reliability and energy efficiency, which makes them well-suited for medium to high-density power applications. This paper presents a theoretical and practical in-depth review of common refrigerants used in TPT systems and evaluated them in terms of thermal capacity, operating conditions, geometrical design limitations, environmental impact, health and safety considerations, and international regulations. To synthesize the evaluation of the theoretical comparison findings, a decision matrix was developed to assess the overall suitability of selected refrigerants across all criteria. Water ranked highest, achieving an outstanding score of 89 out of 90, followed by methanol, ammonia, and ethanol with 58.5, 56.5, and 56, respectively. Other natural refrigerants such as pentane and 2-methylbutane demonstrated moderate suitability. In contrast, several alternatives received significantly lower scores due to safety concerns or unfavorable environmental characteristics. Based on in-practice comparisons, water exhibits higher thermal conductance, greater heat capacity, and enhanced thermal stability than many refrigerants commonly employed in TPT systems across a range of operating conditions.