Hybrid Cooling: The Present (and Future) of Data Center Cooling

As data centers grow in size and importance, the need for efficient and reliable cooling solutions becomes more critical. Cooling systems are essential for maintaining optimal temperatures and ensuring the performance and longevity of data center equipment. Traditionally, air cooling has been the primary method for regulating temperatures. However, the increasing density of high-performance computing (HPC) workloads and the rise of AI have driven the need for more sophisticated and tailored cooling strategies.  

This is where hybrid cooling, combining air and liquid cooling, comes into play. While liquid cooling will be nearly ubiquitous, it will coexist with air cooling as part of a hybrid solution whereby air and liquid techniques are harnessed together and optimized depending on the application, heat generated, costs, and facility capabilities. The hybrid cooling solution has emerged as the present of data center cooling. It will continue long into the future, offering a practical solution for both traditional and next-generation technologies.  

 What is Liquid Cooling? 

 Liquid cooling involves using a fluid—usually water or a specially formulated coolant—to absorb and remove heat from data center equipment. This method has long been utilized in other sectors, such as automotive and industrial applications, and has only recently gained traction in data centers. A recent Uptime Institute survey found that 16% of data center managers believe liquid cooling will become the primary cooling method within 1-3 years, and 41% think it will take 4-6 years. 

 Liquid cooling systems can be divided into two primary categories: 

1.  Direct-to-Chip Cooling: In this approach, coolants circulate through tubes that are in direct contact with the processor or other heat-generating components, effectively dissipating heat. Direct-to-Chip cooling provides efficient cooling at the source of heat generation, making it suitable for high-density applications. 
2. Immersion Cooling: In immersion cooling, servers or components are entirely submerged in dielectric (non-conductive) fluid. The heat generated by the components is transferred to the fluid, which a heat exchanger then cools. Immersion cooling can achieve significant cooling efficiency and is often used for specialized, high-performance environments. 

 Benefits of Liquid Cooling 

Liquid cooling offers several advantages over traditional air cooling, making it an attractive option for modern data centers. Here are some of the key benefits: 

1.  Increased Efficiency: Liquid cooling can be up to 3600 times more efficient than air cooling on per volume basis, thanks to liquids' higher thermal conductivity. This improved efficiency enables data centers to achieve lower temperatures with less energy, reducing the overall power usage effectiveness (PUE). 
2. Reduced Energy Consumption: Liquid cooling systems can maintain optimal temperatures with less reliance on mechanical cooling (e.g., air conditioners or fans), reducing the energy consumption associated with cooling infrastructure. This not only lowers operational costs but also contributes to sustainability goals. 

1. Enhanced Performance for High-Density Applications: With the rise of AI, machine learning, and other high-computing-power applications, data centers are handling more computationally intense workloads. Liquid cooling is better suited for these scenarios because it can effectively dissipate heat from high-density racks where air cooling would struggle to maintain the necessary temperatures. 

1. Space Savings: Because liquid cooling systems can more efficiently manage higher heat densities, data centers can achieve greater computing power in a smaller footprint. This translates to significant space savings, especially in edge data centers or facilities with limited room for expansion. 

What do we mean when we say hybrid cooling? Hybrid liquid cooling combines two cooling methods: liquid cooling (often Direct-to-Chip Liquid Cooling, or DCLC) and traditional air cooling. The goal is to optimize the strengths of both technologies for maximum performance, energy efficiency, and scalability.  

This approach allows data center operators to leverage the benefits of liquid cooling for high-density workloads while still using air cooling for less demanding equipment. Hybrid cooling can take different forms depending on the specific configuration of the data center, such as: 

1. Partial Liquid Cooling: In this setup, liquid cooling is used for the most heat-intensive components (e.g., high-performance servers), while air cooling is employed for the rest of the equipment. This targeted approach reduces the overall cooling load and improves energy efficiency. 

1. Integrated Hybrid Solutions: Hybrid cooling is a data center cooling system that uses both air and liquid cooling. Direct-to-chip liquid cooling (DCLC) removes heat from large electronic components with mounted cold plates. However, smaller board-level components – like resistors, capacitors, and solder traces – still generate heat and require air cooling. 

The mix of air and liquid cooling depends on which server components are connected to the liquid cooling loop. Air cooling can come from computer room air handlers (CRAHs), computer room air conditioners (CRACs), row-based cooling systems, or rear door heat exchangers. Liquid cooling is typically managed by a coolant distribution unit (CDU), which transfers heat to a chiller, dry cooler, water tower, or a combination of these systems. 

By combining both methods, hybrid cooling ensures efficient temperature control in high-performance data centers.  

Is Hybrid Cooling Just a Bridge, or Is it the Future? 

A debate in the data center industry is whether hybrid cooling is simply a transitional step towards fully liquid-cooled facilities or if it represents the future of data center cooling. While there is a growing consensus that liquid cooling will become nearly everywhere, there are several reasons why hybrid cooling is likely to be a long-term solution rather than just a temporary fix: 

Not all data center workloads have the same cooling requirements. Some servers may be used for storage or less computationally intensive tasks that do not generate significant amounts of heat. In these cases, air cooling is more than adequate. Meanwhile, AI workloads and HPC applications produce more heat than air cooling can efficiently handle, making liquid cooling necessary. Hybrid cooling allows data centers to efficiently manage both high-density and lower-density workloads under the same roof.  

Data centers often experience shifts in the types of workloads they support, whether due to changing customer needs or technological advancements. With a hybrid cooling system, data center operators can more easily adapt to these changes without completely overhauling their cooling infrastructure. For instance, as the proportion of high-density workloads grows, the data center can gradually shift from air cooling to more liquid cooling, ensuring the facility remains efficient and current. 

Retrofitting an existing data center to support 100% liquid cooling can be a significant investment, especially for smaller facilities. Hybrid cooling offers a cost-effective pathway to modernize cooling infrastructure. By implementing liquid cooling for the most heat-intensive racks and using air cooling for the remainder, data centers can optimize their budgets while still achieving substantial improvements in cooling efficiency. 

Having both air and liquid cooling systems in place provides an added layer of redundancy. If one system fails, the other can help maintain acceptable temperatures, reducing the risk of downtime. This redundancy is especially valuable in mission-critical environments where cooling system failures can have significant consequences.  

Technological Limitations and Air Cooling’s Persistent Role 

While liquid cooling can offer advantages, it’s important to note that air cooling will not disappear entirely from data centers. There will always be components or scenarios where air cooling is sufficient and even more practical than liquid alternatives. For example, backup systems, power distribution units, and lower-density servers may continue to rely on air cooling. Additionally, air cooling will likely remain a part of hybrid systems as a supplementary cooling method for liquid-cooled data centers. 

Why Hybrid Cooling Is the Way Forward 

As data centers evolve, cooling strategies must also advance to meet growing demands and environmental considerations. Hybrid cooling is not just a bridge between traditional air cooling and the future of liquid cooling; it is the future. The combination of liquid and air cooling allows for a balanced approach that can meet the diverse needs of modern data centers while providing flexibility for future growth. 

The hybrid approach enables data centers to cater to both high-density and low-density applications without requiring an all-in-one solution. As liquid cooling technologies mature and become more cost-effective, their adoption will increase. However, air cooling will still play a role in hybrid systems, providing backup, handling less demanding workloads, and supporting peripheral equipment. 

By adopting a hybrid cooling strategy, data centers can achieve optimal efficiency, reduce energy consumption, and adapt to changing requirements without incurring excessive costs. While fully liquid-cooled data centers may become more common, hybrid cooling solutions will remain the standard because they offer the best combination of performance, flexibility, and resilience. 

Wrap Up 

As powerful AI hardware is deployed at scale, more data center workloads will require liquid cooling to manage racks of high-density servers. However, traditional cloud and enterprise equipment isn’t going anywhere, and air cooling will remain essential for many years to come.  

The future of data center cooling is not a one-size-fits-all solution. Instead, air cooling and liquid cooling experts agree that both technologies will coexist for many years to come. Future data centers will transition from 100% air cooling to a hybrid of air and liquid cooling, driven by energy consumption concerns and sustainability goals.  

Hybrid cooling is already demonstrating its value as a scalable, efficient, and flexible solution that meets the diverse cooling needs of today's data centers while providing a pathway for future innovations. As data centers continue to evolve, the hybrid model will lead the way, delivering the cooling capabilities needed for tomorrow's high-performance computing challenges. 

By embracing hybrid cooling, data centers can ensure they are ready for the demands of AI, HPC, and other emerging technologies, all while maintaining energy efficiency and operational resilience. Hybrid cooling isn't just a stopgap; it’s the way forward.