Green Cooling of High Performance Micro Processors: Parametric Study between Flow Boiling and Water Cooling

Due to the increase in energy prices and spiralling consumption, there is a need to greatly reduce the cost of electricity within data centers, where it makes up 50% of the total cost of the IT infrastructure. A technological solution to this is using on-chip cooling with a single-phase or evaporating liquid to replace energy intensive air-cooling. The energy carried away by the liquid or vapour can also potentially be used in district heating, as an example. Thus, the important issue here is “what is the most energy efficient heat removal process?” As an answer, this paper presents a direct comparison of single-phase water, a 50% water ethylene glycol mixture and several two-phase refrigerants, including the new fourth generation refrigerants HFO1234yf and HFO1234ze. Two-phase cooling using HFC134a had an average junction temperature 9 to 15˚C lower than for single-phase cooling, while the required pumping power for the CPU cooling element for single-phase cooling was on the order of 20-130 times higher to achieve the same junction temperature uniformity. Hot-spot simulations also showed that two-phase refrigerant cooling was able to adjust to local hot-spots because of flow boiling's dependency on the local heat flux, with junction temperatures being 20 to 30˚C lower when compared to water and the 50% water-ethylene glycol mixture, respectively. An exergy analysis was developed considering a cooling cycle composed by a pump, a condenser and a multi-microchannel cooler. The focus was to show the exergetic efficiency of each component and of the entire cycle when the subject energy recovery is considered. Water and HFC134a were the working fluids evaluated in such analysis. The overall exergetic efficiency was higher when using HFC134a (about 2%) and the exergy destroyed, i.e. irreversibilities, showed that the cooling cycle proposed still have a huge potential to increase the thermodynamic performance.

Experimental Evaluation and Control of Two-Phase Multi-Microchannel Evaporator Cooling Systems for High Efficiency Data Center

Duan Wu

Thermal designers of data centers and server manufacturers are showing a greater concern regarding the cooling of the new generation data centers, which consume considerably more electricity and dissipate much more waste heat, a situation that is creating a re- thinking about the most effective cooling systems for the future beyond conventional air cooling of the chips/servers. Potentially, a significantly better solution is to make use of on- chip two-phase cooling, which, besides improving the cooling performance at the chip level while also consuming less energy to drive the cooling process, also adds the capability to reuse the waste heat in a convenient manner, since higher evaporating and condensing temperatures of the two-phase cooling system (from 60-95 °C) are possible with such a new “green” cooling technology. In the present thesis, three such two-phase cooling cycles using micro-evaporation technology were experimentally evaluated with specific attention being paid to (i) controllability of the two-phase cooling system, (ii) energy consumption and (iii) overall exergetic efficiency, with the emphasis on (i). The controllers were evaluated by tracking and disturbance rejection tests, which were shown to be efficient and effective. The average temperatures of the chips were maintained below the upper limit of 85 °C of computer CPU’s for all tests evaluated in steady state and transient conditions. In general, simple SISO and SIMO strategies were sufficient to attain the requirements of control. Regarding energy and exergy analyses, the experimental results showed that all these systems can be thermodynamically improved since only about 6% of the exergy supplied is in fact recovered in the condenser in the present setup. Additionally, a series of tests covering a wide range of operating conditions under steady state regime were done. The main idea was to generate a “map of performance” of the different cooling systems in terms of energy consumption, energy recovery at the condenser and heat exchanger performance. A total of 120 tests were done which considered all combinations of the variables involved. Finally, empirical and semi-empirical correlations for overall thermal conductance and performance of all components and piping of all these systems were developed based on the experimental results obtained, which can be used for simulations and validations of potential codes developed to design and/or evaluate performance of cooling systems. An overall energy balance analysis for each system using the correlations developed showed that 99.17% of the experimental data were bounded within ± 10%.

EPFL2012

Dynamic flow control and performance comparison of different concepts of two-phase on-chip cooling cycles

Jackson Braz Marcinichen, Bruno Michel, John Richard Thome, Duan Wu

A hybrid on-chip two-phase cooling cycle specifically designed to cool server boards with chips of high performance computers was experimentally evaluated considering steady-state and transient operation of two parallel pseudo-chips and auxiliary electronics mimicking a real server board. Control strategies were developed and evaluated by reference tracking and disturbance rejection tests considering several setpoints of controlled variables. The hybrid cycle, operating with a common refrigerant R134a as the working fluid, was energetically and exergetically compared with two other cooling cycles experimentally evaluated in a previous study, one driven by an oil-free gear pump and another by an oil-free mini-compressor. The results showed that, for a specific steady state condition and heat load, respectively 28.9%, 51.9% and 62.5% of the energy out of the pump, compressor and hybrid cycles were associated with heat losses. The differences observed between the three cycles were justified firstly due to the concept of the cycles, i.e. cycles with the compressor showed as expected lower thermal performance than that with pump since its appeal is for energy recovery (benefitting from a higher condensing temperature) and secondly due to the irreversibilities observed in drivers, condenser and piping (thermal insulation). In summary, the three cycles proved to be efficient, simple and reliable concepts to cool server boards (CPUs, DIMMs etc.), showing high thermal performance and potential for heat recovery when compared with traditional air-cooling systems in current use in data centers. It can also be said that the pump cycle showed the best results in terms of energy and exergy, with the cooling and heat recovery performances reaching a maximum of about 5 and 1.8 times higher than the other cycles (worth noting that the focus in the present study was two-phase flow control and proof-of-concept of different cooling loops, meaning that no "optimal" system design was attempted and the differences above can be reduced). (C) 2013 Elsevier Ltd. All rights reserved.

Elsevier2014

A Review of On-Chip Micro-Evaporation: Experimental Evaluation of Liquid Pumping and Vapor Compression Driven Cooling Systems and Control

Jackson Braz Marcinichen, Jonathan Olivier, John Richard Thome