Demand for high-performance computing and artificial intelligence (AI) is making data centres hotter. So how can we keep them cool? Liquid cooling.
Running an AI chatbot like ChatGPT or Copilot requires modern accelerators like graphics processing units or neural processing units. These are the chips that handle the advanced processing that makes AI possible. Performing trillions of operations per second generates heat—a lot of it. And these AI apps have millions of users.
Advanced, modern processing requires advanced, modern cooling.
Data centres already use a huge amount of energy. The National Energy System Operator (NESO) estimates that data centres accounted for 7 percent of commercial electricity demand in the UK in 2023. AI and other compute-intensive uses will only increase that.
" }A CDU distributes coolant to a closed-loop system. This schematic is simplified to convey the facility water/technical water system interface.
These advances in technology have increased the power density of data centres. Higher power density means each piece of information technology equipment (ITE) consumes more energy and produces more heat per square foot. In the 2010s, cloud computing rack densities climbed to 8 to 20 kilowatts (kW). Today, it’s common to have ITE loads over 100 kW per cabinet. Soon, they are expected to surpass 500 kW.
So, where are we today? And why are we transitioning to liquid cooling?
Conventional air cooling has met its limits
Without cooling, data centre servers would overheat and fail. So, these mission critical facilities need systems to remove heat from their servers, storage, and networking equipment.
Air cooling—where cool air is circulated through racks of equipment, dissipating heat—has been the standard approach to cooling ITE for decades. With high-density data centres, air cooling is no longer the best option. We are trending toward rack densities that can’t be cooled with air alone. We must find a way to move more heat efficiently.
What’s the advantage of liquid cooling in data centres?
Liquids can absorb more heat than air by volume, and they’re more thermally conductive.
With liquids, you get more value for your investment from a heat-transfer perspective. The specific heat-transfer ability of a liquid is far superior to that of air; you can move much more heat with a liquid than you can with the same volume of air. Because liquids transfer heat so well, liquid cooling can handle data centre cabinets with loads of 100 kW or more.
As we’ve said, heat load densities are increasing. That makes liquid cooling in data centres a popular solution. McKinsey projects the global data centre cooling market will hit $40 billion by 2030. And it expects that liquid cooling will account for at least $15 billion of that.
Liquid cooling in data centres comes in various formats
There are diverse options for liquid cooling. A rear door heat exchanger cools the air leaving the server rack but never touches the ITE directly. Chilled water systems run cold water into the environment, pull the heat out, and send it to the building water system.
Direct-to-chip (D2C) systems circulate coolant through a cold plate in contact with server electronics, transferring heat away in the process. Immersion cooling might be the most sci-fi looking approach. Entire servers and data racks are immersed in nonconductive fluid.
What are the components in a system for liquid cooling in data centres?
D2C cooling is still emerging. But it is gaining traction. This is especially true in new hyperscale and AI-focused data centres. Today, most liquid-cooled systems use a coolant distribution unit (CDU) for D2C cooling. A CDU distributes coolants (commonly 25 percent propylene glycol) to a closed-loop system, which circulates through cold plates mounted on ITE. The CDU keeps the temperature of the coolant supply steady.
The technical water system is the coolant loop that flows between the CDU and the ITE. It transfers heat gained from the ITE to the facility water loop via the CDU heat exchanger. A liquid-to-liquid heat exchanger in the CDU transfers ITE waste heat from the technical water system to the facility water system, which removes heat through the building’s mechanical systems.
The CDU keeps its cooling fluids separate from the building system. This is critical for ensuring coolant quality for ITE. A liquid system should maintain the manufacturer’s guidelines on coolant quality—things such as pH, hardness, conductivity, and dissolved solids—to help reduce equipment failure and improve performance.
While AI compute is the main driver for its adoption, liquid cooling in data centres has other benefits
Liquid cooling in data centres is more efficient than air cooling. It’s true, we can improve the effectiveness of air-cooling systems with aisle containment, cabinet chimneys, and in-row cooling units. But pushing all this air means data centre cooling systems consume roughly 30 to 40 percent of total data centre power. Liquid coolants are more efficient than air in these high-temperature racks.
Higher ITE loads demand a higher flow rate (for air or liquid) to cool them. But the liquid cooling system’s pumps use less energy than air fans to transfer the same amount of heat. When we look at the power usage effectiveness for data centres with liquid-cooled ITE, they spend less of their total energy on cooling and more on running IT equipment.
Higher temps mean higher efficiency. Because of liquid cooling’s heat-transfer performance, the coolant need not be “cold.” These systems can use warmer coolants, even up to 37°C. A dedicated liquid-cooling system designed for higher-temperature operations can achieve optimum efficiency. And use less water.
Liquid cooling in data centres also takes up less space in the data hall than air-cooling systems. This allows more space for the racks and higher-density computing.
And BREEAM reports that data centres that implement energy-efficient cooling systems, such as hybrid air/liquid solutions, can achieve lower operational costs.
" }A new hybrid cooling system at the NTT Data Center Campus in Garland, Texas, combines direct chip cooling with air-assisted cooling.
Hybrid cooling is an option, especially for retrofits
One option is to combine liquid cooling and air cooling in data centres. It’s a hybrid approach.
Air-cooled data centres—with direct evaporative air-handling units or air‑cooled computer room air-conditioning (CRACs)—can add liquid cooling using CDUs with liquid-to-air heat exchangers. This is a common approach to retrofitting older facilities, which might otherwise become obsolete. A facility water system makes it easier to cool the ITE with liquid, but it’s not required.
Often, CDU-based liquid cooling in data centres can’t capture all the heat by itself. So, some lower-capacity air cooling is still needed.
Can liquid-cooled data centres achieve leading sustainability standards?
While liquid cooling is becoming more common, sustainability-assessment methodologies are still evolving to reflect these new approaches. This presents challenges when demonstrating compliance with environmental assessment frameworks such as BREEAM.
Innovative cooling strategies often fall outside traditional modelling assumptions. Robust energy modelling, clear baseline comparisons, and well-documented evidence are therefore essential to demonstrate performance.
On hyperscale data centre projects in the US, our team developed an energy modelling methodology that enabled liquid-cooled facilities to document an improved energy performance for LEED certification by demonstrating equivalent or improved performance against conventional air-cooled designs. The principles developed through this work are applicable when assessing innovative cooling technologies against UK sustainability frameworks.
Here are four steps we’re taking to make LEED certification possible for data centres cooling their ITE with liquid systems.
" }Step 1: Our team worked on two hyperscale data centre projects for the same confidential client. Our models showed that liquid cooling offered:
- Server fan energy reductions of 70 to 80 percent
- Lower pump energy consumption
- Better heat rejection efficiency
- Reduced cooling plant needs
The results were impressive. The model showed gains far beyond LEED minimums for energy savings and power usage effectiveness.
And our models showed possible savings of 8 to 12 percent on IT equipment energy use thanks to better thermal management. (Project results are based on modelling and project-specific conditions; they may not be representative of all projects.)
Step 2: Liquid-cooled systems are new and not widely understood. So, we provided LEED reviewers with the information to help them. It included:
- Side-by-side comparisons to air cooling
- A guide to energy modelling for both systems
- Data showing energy use by component
Step 3: We used the LEED framework to measure and prove that a liquid-cooled system saved water versus a baseline air-cooled system. Our calculations show that the closed-loop system could save about 92,000 gallons of water per megawatt per year, when compared to evaporative systems.
Step 4: Next, we showed that liquid cooling could meet the International Energy Conservation Code requirements. We compared the total annual energy cost for the liquid-cooling design against a baseline building design. We demonstrated, through comparative analysis, that the liquid cooling design was capable of using less energy than code minimum air-cooled systems.
Whether working with BREEAM in the UK or LEED internationally, the key challenge is providing robust technical evidence that shows compliance where industry guidance has yet to fully evolve.
Education helps meet the challenges of liquid cooling deployment with hyperscalers
Our global team regularly works with hyperscalers on cooling systems for data centres. We have found that educating operations personnel on the fundamentals of liquid cooling, like those above, goes a long way in solving some of the challenges to deployment.
It helps to be on the same page. Material compatibility between the coolant, liquid-cooled ITE, and distribution system (piping, CDU, and accessories) can also pose design challenges. But despite these challenges, liquid cooling is here. And we need it.
" }Hybrid cooling systems, like this one at the NTT Data Center Campus in Garland, Texas, combine direct chip cooling with air-assisted cooling, which enhances efficiency at the chip level.
What are some ways data centre facilities can prepare to deploy liquid cooling?
There are several steps to get the liquid-cooling process started. They include a site audit, technical specifications, a review of space needs, and a discussion of sustainability.
Audit: Before you commit to converting your data centre to hybrid cooling, specialists will need to conduct a site audit. This will likely include:
- A computational fluid dynamics study of airflow in the facility
- An analysis of existing piping and modelling for the liquid system’s fluid network, CDUs, and other essentials
- An evaluation of ageing systems that may be reaching the end of their useful life
Specifications: Consider the technical requirements for the ITE you are planning to liquid cool. This includes fluid requirements, temperatures, and flows.
Space: Understand the space requirements for liquid cooling. The liquid cooling system will need space to add additional piping for the fluid network, CDUs, and treatment to support technical water and monitoring.
Sustainability: If you are seeking BREEAM certification on a liquid-cooled data centre, you should consider selecting equipment with modularity built in. This will help your system meet energy requirements.
Liquid cooling in data centres is essential
As demand for AI, machine learning, and advanced processing continues to grow, so does the heat in our data racks. Liquid cooling technology isn’t just critical to tomorrow’s data infrastructure.
It’s indispensable.
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