Two-Phase Mini-Thermosyphon For Cooling Of Datacenters: Experiments, Modeling And Simulations

Nowadays, datacenters heat density dissipation follows an exponential increasing trend that is reaching the heat removal limits of the traditional air-cooling technology. Two-phase cooling implemented within a gravity-driven system represents a scalable and viable long-term solution for datacenter cooling in order to increase the heat density dissipation with larger energy efficiency and lower acoustic noise. The present article builds upon the 4-part set of papers presented at 'THERM 2016 for a 15-cm height thermosyphon to cool a contemporary datacenter cabinet, providing new test data over a wider range of heat fluxes and new validations of the thermal hydrodynamics of our thermosyphon simulation code. The thermosyphon consists of a microchannel evaporator connected via a riser and a downcomer to a liquid-cooled condenser for the cooling of a pseudo-chip to emulate an actual server. Test results demonstrated good thermal performance coupled with uniform flow distribution for the new larger range of operating test conditions. At the maximum imposed heat load of 158 W (corresponding to a heat flux of 70 W cm(-2)) with a water inlet coolant at 20 degrees C, water mass flow rate of 12 kg h(-1) and thermosyphon filling ratio of 78%, the pseudo mean chip temperature was found to be 58 degrees C and is well below the normal thermal limits in datacenter cooling. Finally, the in-house LTCM's thermosyphon simulation code was validated against an expanded experimental database of about 262 data points, demonstrating very good agreement; in fact, the pseudo mean chip temperature was predicted with an error band of about 1 K.

Design Of Passive Two-Phase Thermosyphons For Server Cooling

Raffaele Luca Amalfi, Filippo Cataldo, Jackson Braz Marcinichen, John Richard Thome

The main objective of this paper is to utilize an improved version of the simulator presented at InterPACK 2017 to design a thermosyphon system for energy-efficient heat removal from 2-U servers used in high-power datacenters. Currently, between 25% and 45% of the total energy consumption of a datacenter (this number does not include the energy required to drive the fans at the server-level) is dedicated to cooling, and with a predicted annual growth rate of about 15% (or higher) coupled with the plan of building numerous new datacenters to handle the "big data" storage and processing demands of emerging 5G networks, artificial intelligence, electrical vehicles, etc., the development of novel, high efficiency cooling technologies becomes extremely important for curbing the use of energy in datacenters. Notably, going from air cooling to two-phase cooling, not only enables the possibility to handle the ever higher heat fluxes and heat loads of new servers, but it also provides an energyefficient solution to be implemented for all servers of a datacenter to reduce the total energy consumption of the entire cooling system. In that light, a pseudo -chip with a footprint area of 4 x 4 cm2 and a maximum power dissipation of 300 W (corresponding heat flux of about 19 W/cm2), will be assumed as a target design for our novel thermosyphon-based cooling system. The simulator will be first validated against an independent database and then used to find the optimal design of the chip's thermosyphon. The results demonstrate the capability of this simulator to model all of the thermosyphon's components (evaporator, condenser, riser and downcomer) together with overall thermal performance and creation of operational maps. Additionally, the simulator is used here to design two types of passive two-phase systems, an air- and a liquid-cooled thermosyphon, which will be compared in terms of thermal -hydraulic performance. Finally, the simulator will be used to perform a sensitivity analysis on the secondary coolant side conditions (inlet temperature and mass flow rate) to evaluate their effect on the system performance.

AMER SOC MECHANICAL ENGINEERS2020

State of the Art of High Heat Flux Cooling Technologies

Bruno Agostini, Bruno Michel, Jung Eung Park, John Richard Thome, Leszek Wojtan

The purpose of this literature review is to compare different cooling technologies currently in development in research laboratories that are competing to solve the challenge of cooling the next generation of high heat flux computer chips. Today, most development efforts are focused on three technologies: liquid cooling in copper or silicon micro-geometry heat dissipation elements, impingement of liquid jets directly on the silicon surface of the chip, and two-phase flow boiling in copper heat dissipation elements or plates with numerous microchannels. The principal challenge is to dissipate the high heat fluxes (current objective is 300 W/cm2) while maintaining the chip temperature below the targeted temperature of 85°C, while of second importance is how to predict the heat transfer coefficients and pressure drops of the cooling process. In this study, the state of the art of these three technologies from recent experimental articles (since 2003) is analyzed and a comparison of the respective merits and drawbacks of each technology is presented. The conclusion is that two-phase flow boiling in microchannels is the most promising approach; impingement cooling also has good prospects but single-phase liquid cooling is probably only a short-term solution. As an example of the state of the first technology, the Heat and Mass Transfer Laboratory at Ecole Polytechnique Fédérale de Lausanne has already achieved 200 W/cm2 of cooling in a first prototype, with a low pumping power, good temperature uniformity, and at the required maximal operating temperature.

2007

Efficiency Improvements of a Thermal Power Plant by Making Use of the Waste Heat of Large Datacenters using Two-Phase On-Chip Cooling

Nicolas Lamaison, Jackson Braz Marcinichen, John Richard Thome

Cooling of datacenters is estimated to have an annual electricity cost of 1.4 billion dollars in the United States and 3.6 billion dollars worldwide. Currently, refrigerated air is the most widely used means of cooling datacenter’s servers. Modern datacenters require a heat dissipation rate in the order of 5 to 15 MW and current air cooling technology represents around 45% of the total energy consumed. Based on the above issues, thermal designers of datacenters and server manufacturers now seem to agree that there is an immediate need to improve the server cooling process. On-chip cooling research is being developed in this context to propose a new, more efficient cooling technology. This also allows the recoveryof the heat rejected by the servers in a proper way, making it possible to reuse elsewhere, in another application. The present investigation develops a case study considering two different cooling systems applied on a datacenter and exploring the application of energy recovered in the condenser on a feedwater heater of a coal power plant. The effects of the evaporating and condensing temperatures on the cooling cycle performance and the potential to recover energy, and consequently the effect on the power plant efficiency, are evaluated. The analyses consider the main objective function to be the minimization of energy consumption, the corresponding CO2 footprint and operating costs. From the datacenter’s point of view, when compared with traditional air-cooling systems, energy consumption, without considering energy recovery, can be reduced by as much as 45% when using a liquid pumping cycle and 35% when using a vapour compression cycle. From the power plant point of view, the results showed that, when the pressure of the feedwater heater is optimized, an increase of up to 6.5% of the overall power plant efficiency can be obtained when using a vapour compression cycle to cool the datacenter. Considering the vapour compression cycle and a datacenter of 100000 blades, overall savings (considering the power plant and the datacenter as a whole system) of 2170 tons of CO2 and $0.34 million per MW of electricity production were obtained. Additional investigation was developed considering the effects of partial operation of the datacenter and/or the power utility on the parameters mentioned beforehand. For such an investigation the start-up was the ideal match between a datacenter of 100000 blades and a power plant, both with 100% of operational uptime. It has been shown that some cases could lead to impossible thermodynamic operations, meaning that special attention must be given when the design of such integrated utilities (datacenter and power plant) is made.