Cooling Data Centers – The End Game by Herb Zien
An Analogy – Lighting
Incandescent lighting was the standard for 100 years. This technology is so inefficient that scientists spent decades searching for an alternative, and since the 1980s most engineers assumed that Light Emitting Diodes would be the end game. However, LED technology was not ready for commercialization because bulbs could not meet market requirements in terms of color and cost. An entire industry, Compact Fluorescent Lighting, sprang up to transition from legacy to LED. Patents were filed, factories built, and distribution channels created, but all of this was temporary. When was the last time you bought a CFL bulb?
Legacy Technology for Cooling Data Centers
Immersing electronics in air is the legacy technology for data center cooling. Like incandescent bulbs, this is extraordinarily inefficient because air does not conduct heat. Massive fan power is required to overcome the natural inclination of air to keep heat moving from where it is generated to where it needs to be rejected. In addition, air conditioning, either direct expansion or evaporative cooling, usually is required to promote heat transfer. As a result, half the energy consumed by most data centers is wasted cooling the electronics.
Transitional Liquid Cooling Technologies
Transition liquid cooling technologies sprung up to partially address air-cooling deficiencies, Cold Plates and In-Row Cooling.
Cold Plates, originally developed to cool high-power processors in computer gaming products, are being modified for data center applications. Standard heat spreaders mounted on processors are replaced with heat exchangers that transfer heat from the processor to a fluid, usually water, which is pumped to a second exchanger that rejects the heat to air. Cold Plates are often referred to as “direct contact” systems, but there is no direct contact; only the hottest electronic components are covered by Cold Plates and there is a heat transfer barrier between the processor and transfer fluid. Furthermore, only about half the heat generated in a server is removed by the fluid; the rest is blown into the room by fans.
In-Row Cooling technology makes the room smaller. In legacy data centers a huge volume of air is cooled to remove heat generated in racks that occupy only about half of the floor space. In-Row Cooling, essentially mini-HVAC systems comprising fans and water-to-air heat exchangers, are installed in the racks to reduce the volume of air that must be cooled. This technology removes heat but is expensive and introduces water to the data center.
Total Immersion, where all heat generating components are immersed in a heat-conductive but electrically non-conductive medium, is a more energy-efficient solution than Cold Plates or In-Row Cooling systems, but most approaches are not ready for prime time. There are two forms of Total Immersion cooling, Two-Phase and Single-Phase.
Two-Phase Immersion systems are Tank-Based and electronic components are submerged in a refrigerant bath. Boiling occurs on the surface of heat generating components and vapor passively rises to the top of the enclosure, where it condenses on water-cooled coils and falls back into the tank. This technology has major drawbacks which include:
- The dielectric refrigerant is very expensive
- The refrigerant is highly volatile, and evaporates when the lid is opened for maintenance
- Cavitation associated with boiling can erode electronic boards
- Some refrigerants are toxic
- Water is required to condense the refrigerant vapor
Single-Phase Immersion systems can be either Tank-Based or Rack-Based.
Tank-Based systems resemble a rack tipped over on its back, with modified servers inserted vertically into slots in the tank. Drawbacks include:
- Scalability because the tank takes a lot of floor space
- Weight because there is a large volume of fluid
- Messiness because access to electronic components requires aprons and gloves
- Some systems use a mineral oil dielectric, which often has impurities
- All cooling is by bulk flow, which reduces efficiency
- Water is required to cool the dielectric
Rack-Based Single-Phase Total Immersion – The Final Answer
Rack-Based Single-Phase Total Immersion is the ultimate solution for cooling data centers. This technology, perfected by LiquidCool Solutions (LCS), overcomes negative perceptions the market may have had relating to immersion cooling. www.liquidcoolsolutions.com
Scalable in data center applications LCS-cooled hardware is Rack-Based and:
- Cost effective
- Energy efficient
- Neat and easy to maintain – https://www.youtube.com/watch?v=nQeygsow5DA&t=8s
- No water is required
For lighting CFLs were a transition technology from incandescent bulbs to LEDs. For data centers Cold Plates and In-Row Cooling systems are transition technologies. They are better than air but:
- Not as efficient as total immersion
- Fans are still required
- Heat removal hardware can get in the way of maintenance
- These systems can be expensive
Rack-Based Single-Phase Immersion Technology is the End Game for Cooling Electronics.