Dell technical expert: Comparison of five server thermal management technologies, single-phase DLC is more effective

  Recently, at a technical lecture organized by DCD, Dr. Tim Shedd, a Dell technology expert, revealed the performance comparison of five server thermal management technologies in a presentation titled "Performance comparison of five server thermal management technologies." The leading data center cooling technologies studied in the research include air cooling, single-phase immersion, two-phase immersion, two-phase direct liquid cooling, and single-phase direct liquid cooling (DLC, cold plate).

  Dell's research indicates that, compared to the other four data center cooling methods, single-phase direct liquid cooling (DLC) exhibits the highest thermal efficiency, providing a potential pathway to better sustainability and increased efficiency.

  The report notes that by 2025, CPU or GPU chip power is expected to reach up to 500W, with artificial intelligence and machine learning pushing GPU power to 700W and an anticipated future increase to 1000W.

More importantly, as power increases, there is a demand for lower chip packaging temperatures and smaller temperature differentials to ensure normal chip operation. Therefore, the challenges for thermal management systems intensify.

  The report uses typical data center server configuration data to construct a simplified thermal model, illustrating the applicability of these five thermal management technologies when processor power rises from 250W to 500W.

250W Processor

  In the past few years, when processor TDP was around 250W, all five thermal management technologies could efficiently cool typical data center racks, such as those deploying 32 dual-socket 250W rack-mounted servers. For a 2U rack-mounted server, the temperature differential between chip packaging and the air passing through the server was approximately 26°C. Therefore, with only 25°C of cool air, the chip temperature could be maintained at around 51°C, which is quite reasonable.

  At this point, the efficiency of single-server air cooling is comparable to single-phase immersion cooling.

In two-phase immersion cooling, the temperature differential between chip packaging and the cooling liquid is around 20°C, while DLC technology has an even lower differential. At typical flow rates of 1 lpm (1 liter per minute) or 2 lpm (2 liters per minute), the temperature differential between the DLC cold plate base and chip packaging remains within a range of 10°C.

350W Processor

  Currently, with individual processor power increased to 350W to 400W, the temperature differential required to dissipate chip heat to facility cooling water continues to rise.

For a cabinet cooling deployment with 32 dual-socket 350W rack-mounted servers, the temperature differential between air cooling (1U) and chip packaging exceeds 50°C. This means that cooling the server with 25°C cool air would result in a processor temperature of around 75°C, close to the processor's operating temperature limit.

  At this point, the effectiveness of single-phase immersion cooling is comparable to air cooling (1U), while air cooling (2U) can maintain a temperature differential between air and chip around 38°C.

  Additionally, the temperature differential between two-phase immersion cooling liquid and chip packaging is about 25°C, while single-phase DLC and two-phase DLC remain highly efficient. The temperature differential between two-phase DLC and chip is around 15°C, and at a flow rate of 1 lpm, the temperature differential for single-phase DLC is about 11°C.

It is evident that with processor power increased to 350W-400W, air cooling is approaching practical limits, requiring colder air and exacerbating cooling energy consumption.

500W

  In the next two to three years, processor TDP is expected to generally increase to 500W, posing significant challenges for air cooling. Innovative radiator design methods or reliance on larger sizes to allow more air to enter and cool the processor will be necessary.

  At this point, air cooling (1U), single-phase immersion cooling, and the temperature differential between chip packaging exceed 60°C. Two-phase immersion cooling remains effective, but the differential will rise to about 34°C. The temperature differentials between two-phase DLC and single-phase DLC (1 lpm) are similar, around 25°C, while single-phase DLC (2 lpm) has a smaller differential, around 17°C.

  It is worth noting that high-temperature water cooling in the range of 48°C to 50°C may present some real opportunities for heat energy reuse at this stage.

Summary

Air Cooling:

Increasing processor TDP poses growing challenges for air cooling.

Advances in radiators and fans may break through limits.

Typically encounters limitations on the impact of processor heat on other components.

DLC Cooling:

Single-phase cooling far exceeds 500W TDP.

Two-phase DLC can cool high TDP, although there are steam flow resistance issues that must be addressed.

Advances in system design or fluid technology may improve two-phase DLC.

Immersion Cooling:

Growing challenges with high TDP.

Advances in radiators and fluid technology may break through limits.

Two-phase is limited by fluid boiling point and condenser performance.

 

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