Direct Liquid Cooling (DLC)

Air as the fundamental cooling medium is still the most common means to take heat out of a data center. Yet at some point in the not-distant future, heat fluxes for the most powerful processors will be too high to manage with direct air cooling. When it comes to heat transfer, liquids are fundamentally more efficient than air—because they are denser, have higher specific heat capacities, and lower thermal resistance.

Increasingly massive data center power requirements and rising density mean more heat transfer rate per unit area (heat flux), which pushes the limits of air-based cooling. We are already seeing that heat fluxes for the most powerful processors are too high to manage with air cooling. Air is not nearly as effective a heat transfer medium as liquid, and at some point is unable to efficiently remove all the heat generated by high-power chips.

Supporting the levels of heat flux that will come with 100+ kW rack densities will require direct liquid cooling to the rack with a direct tie to a chilled water loop via cooling distribution units (CDUs). When it comes to heat transfer, water is fundamentally more efficient than air—because it is denser, has a higher specific heat capacity, and a lower thermal resistance.

Direct liquid cooling is also much more sustainable. Here’s how:

• More heat transfer per unit transport energy—Because water is denser, has a higher specific heat capacity, and a lower thermal resistance compared to air, heat can be removed with dramatically less volumetric flow of water versus air. This allows for more heat transfer to be accomplished for the same amount of mechanical transport energy.
• Less MEP infrastructure and smaller buildings—Liquid cooling requires less piping infrastructure compared to the ductwork needed for air cooling. This, combined with the increased density enabled by liquid cooling, allows for mechanical, electrical and plumbing (MEP) infrastructure to be reduced in size, leading to smaller overall building footprints.
• Increased economization—The higher facility water temperatures achievable with liquid cooling increase the number of economization hours, reducing data center chiller Expanding the facility water system temperature delta can also improve efficiency of existing chillers and fluid coolers.
• CPU efficiency—The lower thermal resistance of water compared to air allows warmer entering water temperatures while still maintaining optimal CPU core temperatures for peak performance. Keeping CPUs below their thermal limit results in more useful work from the processors.
• Waste heat use—The higher return water temperatures from liquid cooling present opportunities for productive use of waste heat, such as for district heating, snow melting, or greenhouse applications, where location and transport energy factors align. This provides avenues to offset Scope 2 and potentially Scope 3 emissions.
• Better operating environment—Beyond sustainability benefits, liquid cooling produces a more human-friendly data hall environment, with reduced ambient noise and temperatures. This results in happier staff, enables greater diversity of talent, and improves productivity.

Stream has developed a configurable and innovative proprietary cooling direct liquid cooling (DLC) design—Stream’s Server Thermal Unit (STU). It supports both air cooling and liquid to the rack, with lower cost and less supply chain risk than the vendors who are providing their tenants off-the-shelf DLC solutions.

Explore more in our whitepaper Liquid Cooling is More Sustainable.