Werner Escher

A benchmark study on the thermal conductivity of nanofluids

This article reports on the International Nanofluid Property Benchmark Exercise, or INPBE, in which the thermal conductivity of identical samples of colloidally stable dispersions of nanoparticles or “nanofluids,” was measured by over 30 organizations worldwide, using a variety of experimental approaches, including the transient hot wire method, steady-state methods, and optical methods. The nanofluids tested in the exercise were comprised of aqueous and nonaqueous basefluids, metal and metal oxide particles, near-spherical and elongated particles, at low and high particle concentrations. The data analysis reveals that the data from most organizations lie within a relatively narrow band (±10% or less) about the sample average with only few outliers. The thermal conductivity of the nanofluids was found to increase with particle concentration and aspect ratio, as expected from classical theory. There are (small) systematic differences in the absolute values of the nanofluid thermal conductivity among the various experimental approaches; however, such differences tend to disappear when the data are normalized to the measured thermal conductivity of the basefluid. The effective medium theory developed for dispersed particles by Maxwell in 1881 and recently generalized by Nan et al. [J. Appl. Phys. 81, 6692 (1997)], was found to be in good agreement with the experimental data, suggesting that no anomalous enhancement of thermal conductivity was achieved in the nanofluids tested in this exercise.

On the Thermal Conductivity of Gold Nanoparticle Colloids

Nanofluids (colloidal suspensions of nanoparticles) have been reported to display significantly enhanced thermal conductivities relative to those of conventional heat transfer fluids, also at low concentrations well below 1% per volume (Putnam, S. A., et at. J. Appl. Phys. 2006, 99, 084308; Liu, M.-S. L., et al. Int. J. Heat Mass Transfer. 2006, 49; Patel, H. E., et al. Appl. Phys. Lett. 2003, 83, 2931-2933). The purpose of this paper is to evaluate the effect of the particle size, concentration, stabilization method and particle clustering on the thermal conductivity of gold nanofluids. We synthesized spherical gold nanoparticles of different size (from 2 to 45 nm) and prepared stable gold colloids in the range of volume fraction of 0.00025-1%. The colloids were inspected by UV-visible spectroscopy, transmission electron microscope (TEM) and dynamic light scattering (DLS). The thermal conductivity has been measured by the transient hot-wire method (THW) and the steady state parallel plate method (GAP method). Despite a significant search in parameter space no significant anomalous enhancement of thermal conductivity was observed. The highest enhancement in thermal conductivity is 1.4% for 40 nm sized gold particles stabilized by EGMUDE (triethyleneglycolmono-11-mercaptoundecylether) and suspended in water with a particle-concentration of 0.11 vol%.

On the Cooling of Electronics With Nanofluids

Nanofluids have been proposed to improve the performance of microchannel heat sinks. In this paper, we present a systematic characterization of aqueous silica nanoparticle suspensions with concentrations up to 31 vol %. We determined the particle morphology by transmission electron microscope imaging and its dispersion status by dynamic light scattering measurements. The thermophysical properties of the fluids, namely, their specific heat, density, thermal conductivity, and dynamic viscosity were experimentally measured. We fabricated microchannel heat sinks with three different channel widths and characterized their thermal performance as a function of volumetric flow rate for silica nanofluids at concentrations by volume of 0%, 5%, 16%, and 31%. The Nusselt number was extracted from the experimental results and compared with the theoretical predictions considering the change of fluids bulk properties. We demonstrated a deviation of less than 10% between the experiments and the predictions. Hence, standard correlations can be used to estimate the convective heat transfer of nanofiuids. In addition, we applied a one-dimensional model of the heat sink, validated by the experiments. We predicted the potential of nanofluids to increase the performance of microchannel heat sinks. To this end, we varied the individual thermophysical properties of the coolant and studied their impact on the heat sink performance. We demonstrated that the relative thermal conductivity enhancement must be larger than the relative viscosity increase in order to gain a sizeable performance benefit. Furthermore, we showed that it would be preferable to increase the volumetric heat capacity of the fluid instead of increasing its thermal conductivity.

Toward five-dimensional scaling: how density improves efficiency in future computers

We address integration density in future computers based on packaging and architectural concepts of the human brain: a dense 3-D architecture for interconnects, fluid cooling, and power delivery of energetic chemical compounds transported in the same fluid with little power needed for pumping. Several efforts have demonstrated that by vertical integration, memory proximity and bandwidth are improved using efficient communication with low-complexity 2-D arrays. However, power delivery and cooling do not allow integration of multiple layers with dense logic elements. Interlayer cooled 3-D chip stacks solve the cooling bottlenecks, thereby allowing stacking of several such stacks, but are still limited by power delivery and communication. Electrochemical power delivery eliminates the electrical power supply network, freeing valuable space for communication, and allows scaling of chip stacks to larger systems beyond exascale device count and performance. We find that historical efficiency trends are related to density and that current transistors are small enough for zetascale systems once communication and supply networks are simultaneously optimized. We infer that biological efficiencies for information processing can be reached by 2060 with ultracompact space-filled systems that make use of brain-inspired packaging and allometric scaling laws.

Direct Waste Heat Utilization From Liquid-Cooled Supercomputers

Chip microscale liquid-cooling reduces conductive and convective resistance thereby improving the efficiency of datacenters by allowing coolant temperatures above the free cooling limit in all climates. This eliminates the need for chillers and allows the thermal energy to be re-used in cold climates. Replacing the combustion processes for secondary users with recycled heat from the datacenter effectively eliminates carbon dioxide emission during the winter season and reduces operating cost throughout the year. The energy balance of emission-reduced datacenters is compared with a classical air cooled datacenter, a datacenter with free cooling in a cold climate zone, and a datacenter with chiller-mediated energy re-use. Hot water cooled datacenters reduce the effective energy cost by almost a factor of two compared to a current datacenter and reduce the carbon footprint by an even larger factor. Our energy re-use concept has been demonstrated in terms of cost and energy savings in a 60°C liquid cooled supercomputer. Additional alternative energy re-use schemes in hot climates for desalination and adsorption cooling allow close to full use of datacenter heat in all climates and all seasons. Output temperatures for these applications compared to space heating need to be 10–20°C higher which becomes possible through hotspot adapted cooling that eliminates mixing of fluids with different temperatures. In addition, interlayer cooled chip stacks allow double sided hotspot optimized cooling even closer to the heat source with low flow rates and low pumping power. This improves the large efficiency gain that becomes possible through 3D chip stacking.Copyright

Advanced liquid cooling for concentrated photovoltaic electro-thermal co-generation

We demonstrate an advanced packaging approach with an embedded silicon micro-channel water cooler where the photovoltaic cell is electrically connected by a metallization on the silicon substrate. The backside of the silicon substrate contains the micro-machined fluidic channels thereby minimizing the thermal resistance compared to a state — of — the — art package. This leads to a reduced temperature drop between the photovoltaic cell and the coolant, allowing an increase in the temperature of recovered heat. A low-pressure drop split-flow fluid manifold is implemented to distribute the coolant from one single input to the micro-channel array and back from two outlet ports. A thermal resistance of 0.12 cm2K/W was demonstrated, which allows for the removal of 100W/cm2 heat (>1000 suns) at a ΔT of 12K. Direct chip attached silicon coolers enable higher overall concentration factor thereby reducing photovoltaic cell cost. An additional benefit of silicon is its inertness against corrosion and the matching thermal expansion coefficient which allows building of systems with a very long lifetime. The split flow configuration reduces pumping power to about 5% of the system photovoltaic output. More complex manifold micro-channel systems are proposed to minimize the pumping power to a level below 1% and to cool arrays of cells on a single large substrate.

Lid-Integral Cold-Plate Topology: Integration, Performance, and Reliability

We demonstrate the lid-integral silicon cold-plate topology as a way to bring liquid cooling closer to the heat source integrated circuit (IC). It allows us to eliminate one thermal interface material (TIM2), to establish and improve TIM1 during packaging, to use wafer-level processes, and to ease integration in first-level packaging. We describe the integration and analyze the reliability aspects of this package using modeling and test vehicles. To compare the impact of geometry, materials, and mechanical coupling on warpage, strains, and stresses, we simulate finite element models of five different topologies on an organic land-grid array (LGA) carrier. We measure the thermal performance in terms of thermal resistance from cold-plate base to inlet liquid and obtain 15 mm2 K/W at 30 kPa pressure drop across the package. We build two different topologies using silicon cold-plates and injection-molded lids. Gasket-attached cold-plates pass an 800 kPa pressure test, and direct-attached cold-plates fracture in the cold-plate. The results advise to use a compliant layer between cold-plate and manifold lid and promise a uniformly thick TIM1 layer in the Si–Si matched topology. The work shows the feasibility of composite lids with integrated silicon cold-plates in high heat flux applications.

Ultra‐High‐Concentration Photovoltaic‐Thermal Systems Based on Microfluidic Chip‐Coolers

The electrical efficiency of a photovoltaic‐thermal system for coolant inlet temperatures ranging from 25 °C to 75 °C and concentrations from 500 to 1500 suns was investigated experimentally and theoretically. In this system absorbed radiation and thermal losses from the electric circuit are collected in a thermal circuit. This allows one to directly drive a thermal desalination process thereby contributing to an improved system efficiency. A triple‐junction solar cell was tested in two different configurations. At 1500 suns the electric efficiency of a silicon microchannel cooler package exceeded the efficiency of a reference package with a copper cooler by 2% and it remained fully functional up to concentrations of 4930 suns. We present a general model for concentrated photovoltaic‐thermal systems in which the standard efficiency modeling approaches for triple‐junction cells are extended by temperature and concentration dependencies. The currents were modeled both following the Shockley‐Queisser and a “...

Lid-Integral Cold Plate Topology Integration, Performance, and Reliability

We demonstrate the Lid-Integral Silicon Coldplate topology as a way to bring liquid cooling closer to the heat source IC. It allows to eliminate one thermal interface material (TIM2), to establish and improve TIM1 during packaging, to use wafer-level processes, and to ease integration in 1st level packaging. We describe the integration, and analyze reliability aspects of this package using modeling and test vehicle builts. To compare the impact of geometry, materials and mechanical coupling on warpage, strains and stresses, we simulate finite element models of five different topologies on an organic LGA carrier. We measure the thermal performance in terms of thermal resistance from coldplate base to inlet liquid and obtain 15mm2K/W at 30 kPa pressure drop across the package. We build two different topologies using silicon coldplates and injection molded lids. Gasket-attached coldplates pass an 800 kPa pressure test, direct-attached coldplates fracture in the coldplate. The results advise to use a compliant layer between coldplate and the manifold lid and promise a uniformly thick TIM1 layer in the Si-Si matched topology. The work shows the feasibility of composite lids with integrated silicon coldplates in high heat flux applications.Copyright

Zero-emission datacenters and 3D chip stacking

Summary form only given. Cloud computing has today become a widespread practice for the provisioning of IT services. Cloud infrastructures provide the means to lease computational resources on demand, typically on a pay per use or subscription model and without the need for significant capital investment into hardware. With enterprises seeking to migrate their services to the cloud to save on deployment costs, cater for rapid growth or generally relieve themselves from the responsibility of maintaining their own computing infrastructures, a diverse range of services is required to help fulfil business processes. In this talk, we discuss some of the challenges involved in deploying and managing an ecosystem of loosely coupled cloud services that may be accessed through and integrate with a wide range of devices and third party applications. In particular, we focus on how projects such as OpenStack are accelerating the evolution towards a federated cloud service ecosystem. We also examine how the portfolio of existing and emerging standards such as OAuth and the Simple Cloud Identity Management framework can be exploited to seamlessly incorporate cloud services into business processes and solve the problem of identity and access management when dealing with applications exploiting services across organisational boundaries.