An Analogy – Lighting
Incandescent lighting was the standard for 100 years. Scientists spent decades searching for an alternative to this incredibly inefficient technology, 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 for 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 to serve an ephemeral market. When was the last time anyone bought a CFL bulb?
Legacy Technology for Cooling Data Centers
Immersing electronics in a bath of 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 is rejected. Air conditioning, either direct expansion or evaporative, usually is required to facilitate heat transfer. As a result, half the energy consumed by most data centers is wasted cooling the electronics. In addition, air promotes oxidation and corrosion, shortening the life of electronic components.
As with CFLs for lighting, transition technologies arose to address some of the deficiencies associated with using air to cool electronic equipment deployed in data centers.
Cold Plates, originally developed to cool high-power gaming computers, are being modified for data center applications. Standard air-cooled heat spreaders mounted on processors are replaced with chip-mounted heat exchangers that transfer energy from the processor to a fluid, usually water, which is pumped to a second exchanger that rejects the heat to air. Cold Plates are sometimes 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 thermal barrier between the processor and transfer fluid. About half the heat generated in a server is removed by the fluid; the rest is blown into the hot aisle 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 heat exchangers, are installed in rack rows to reduce the volume of air that must be cooled. This technology removes heat but is expensive and usually introduces water into the white space.
Total Immersion – The End Game
Total Immersion, where all heat generating components are immersed in a heat-conductive but electrically non-conductive fluid is the end game for cooling electronics. There has been resistance to widespread adoption because most configurations have not been ready for prime time in terms of scalability and maintenance, but negative perceptions seem to be changing with the introduction of technologies that address those concerns.
There are two forms of Total Immersion cooling, Two-Phase and Single-Phase.
Two-Phasesystems are Tank-Based. Electronic components are submerged in a refrigerant bath. Boiling occurs on the surface of heat generating components,and the vapor 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 including:
- The dielectric refrigerant is very expensive
- The fluidis volatile and evaporates when the tank cover is opened for maintenance
- Some refrigerants are toxic
- Cavitation associated with boiling can erode electronic boards and components
- Water is required to condense the refrigerant vapor
Single-Phase 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.Drawbacks include:
- Scalabilitybecause tanks take a lot of floor space
- Weight because there is a large volume of fluid
- Messiness when accessingthe electronic componentsfor maintenance
- All cooling is by bulk flow, which reduces efficiency
Rack-Based Single-Phase Total Immersion
Rack-Based Single-Phase Total Immersion is the gold standard for cooling data centers. This technologyovercomes negative perceptions the market may have had relating to immersion cooling, and offers benefits that air and other liquid cooling systems cannot match.
With Rack-Based Single-Phase Immersion technology:
- Data Centers cost less to build and operate
- No air conditioningis required
- No wateris consumed or introduced to the white space
- Maintenance is quick and neat
- Electronics are isolated from ambient air
- The power needed to cool racks is reduced up to 98%
- Waste heat from servers can be recovered and reused
- Data centers run silently
- Dielectric heat transfer fluid never needs replacement
- Causes of server failures are reduced
- Server chasses can be reused to reduce E-waste
Why Would Anyone Do Anything Else?