The Untapped Potential of Heat Reuse in Data Center Cooling Design

In the ever-expanding landscape of data centers, sustainability is no longer a mere buzzword. It’s a pressing necessity. As the demand for artificial intelligence (AI) and emerging technologies continues to skyrocket, power requirements are expanding exponentially. With this growth, hyperscalers and colocation companies are under increasing pressure to build new data centers in a more environmentally friendly way.

Cooling systems use 25-40% of the energy in data centers. Thus, cooling system design plays a pivotal role in the overall efficiency of the data center. Optimizing a cooling system can save power, water, and money for the operator. In recent years, outside factors have also helped move forward sustainable design. With the global increase in ambient temperatures, extreme weather events, including droughts, as well as the increase in chip densities and government regulations requiring more sustainable data center design, operators are now forced to rethink traditional designs.

Why Air-Cooled Chillers Have Been So Popular

In recent years, air-cooled chillers have been the primary choice for data center operators, and for good reason. Air-cooled chillers offer a host of benefits that make them an attractive choice for data center operators. Firstly, they have no evaporation, blowdown, or drift. They operate in a closed loop that requires no water consumption. This makes air-cooled chillers ideal for locations with water shortages or where water is expensive. Additionally, they require less maintenance compared to water-cooled chillers, which often necessitate cooling tower maintenance and water treatment programs.

The resilience of air-cooled chillers in extreme weather conditions is another advantage. With data centers increasingly being built in warm climates and remote locations, the challenges associated with operating cooling towers in freezing temperatures can be mitigated by adopting air-cooled chillers. Furthermore, air-cooled chillers are typically "packaged systems" which can include integrated options such as free cooling and are designed and configured at the factory, facilitating simplified delivery and installation.

Why would I rethink my air-cooled design?

Air-cooled chillers seem too good to be true and many ask why they would bother with another design. Anyone anticipating a significant load may want to consider a water-cooled design. A large quantity of air-cooled units on a roof can create microclimates causing overheating of chillers or components within the chiller. These designs can require load shedding, increased capacity sizing, and/or misted adiabatic cooling to combat these microclimates, requiring water use anyway. For larger installations, another design option can be a closed-loop water-cooled chiller system. By replacing cooling towers with fluid coolers in closed-loop configurations, water can be reused within the loop, eliminating the need for external water sources. Water-cooled chillers are significantly larger, up to and greater than 15mW per chiller. They are incredibly efficient and while they require inside installation, a fraction of the quantity is required.

Another significant benefit to water-cooled chillers is the greater potential for heat reuse.

What’s so hot about data center heat reuse?

Data center heat reuse is another innovative approach gaining traction worldwide, including requirements by legislation in some EU countries. By capturing and redirecting high-grade heat from servers to different heating networks. The feasibility of heat reuse depends highly on design and planning. In many cases, external organizations must be involved from the design onset. While proximity to urban areas is required for district heating, advancements in high-density chips and the required liquid cooling have increased output heat presenting opportunities for more data centers to implement this sustainable practice.

What can I do with this extra data center heat?

Data centers can contribute to efficient heating for industrial sites, indoor agriculture, fish farming, community gardens and pools, and urban infrastructure. There have even been applications to heat nearby parking lots for snow removal.

Liquid Cooling & Heat Reuse

The data center industry has been talking about liquid cooling for 15+ years stating, “it’s coming”. Well, it’s finally here. Not all chips require liquid cooling and many operators have begun implementing hybrid designs, using both air and liquid cooling. However, liquid cooling is required around 80kW/rack and many chip manufacturers are touting densities as high as 300kW/rack making liquid cooling the standard for their designs.

In addition to the standard benefits of liquid cooling, such as footprint optimization and high efficiency, the hotter water temperatures enable more options for heat reuse. One of the main deterrents from utilizing waste heat for district heating is the high temperature water required. With the use of heat pumps, district heating can be achieved. However, with liquid cooling and the very high temperatures, a heat pump is needed for a much smaller boost saving energy and cost.

It's Time To Think Outside the Chiller

Sustainability in data center cooling design is no longer optional but necessary in the face of increasing environmental and legislative pressures. Heat reuse, often underestimated, can play a crucial role in achieving sustainability goals. There are many different methods to optimize design efficiency, but some are not found within the operator’s cooling system or even within their data center. Forward and outward-thinking planning will need to be done moving forward to best capture efficiencies for our facilities in alignment with our communities.