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Highlights • Vertical channels enhance mass transfer and sorption rate in SAPO-34 coatings. • Optimal structuring is predicted by a characteristic transport length model. • Power-energy product per cycle is enhanced by 100% through structuring. • Structured coatings outperform other SAPO-34 configurations at short cycle times. Abstract The rate of water adsorption in SAPO-34 zeolite coatings for adsorption heat pumps has been shown to be limited by mass transfer. In the present contribution, uniformly spaced longitudinal channels of width 75 µm were introduced into SAPO-34 coatings and their effect on the heat and mass transport limitations during water sorption was explored. Different channel spacings at constant adsorbent mass per unit area were tested by means of temperature-swing adsorption measurements and thermal impedance analysis (TIA). The optimal ratio between the characteristic transport length (CTL) for heat transfer and the CTL for mass transfer was 6, which is in agreement with the TIA predictions and was validated experimentally. Geometries with CTL ratios greater or less than 6 exhibited higher thermal impedance and lower rates of water sorption. At the optimal CTL ratio, the water sorption rate was enhanced 2x at the same adsorbent mass per unit area when compared to unstructured SAPO-34 coatings. Compared to other adsorbent structures reported in literature, the structured coatings exhibit the highest power density and energy density at short cycle times. These findings may be used to improve efficiency and/or power in temperature-swing processes in which mass transfer in adsorbent coatings is rate-limiting. It is recommended to develop methods to structure coatings at the scales relevant for commercial adsorption heat pump modules, and further investigate the rate-limiting transport mechanisms in large adsorption heat exchanger modules. [ABSTRACT FROM AUTHOR]

Highlights • Rate of water sorption in SAPO-34 coatings is limited by mass transport. • Higher vapor pressure accelerates adsorption and slightly decelerates desorption. • Ragone plots provide the pareto-optimal cycle time for power and efficiency. • Mass transport must be improved to increase power & efficiency in coated adsorbers. Abstract Adsorption heat pumps have become an increasingly viable technology to use waste heat or renewable thermal energy for heating and cooling. The power density of this emerging technology is limited by the rate of heat and/or mass transfer in the adsorption heat exchanger (AdHEX) which drives investment costs. This work presents an experimental analysis of the mass and heat transfer during water sorption on SAPO-34 coatings to determine the limiting transport mechanism in state-of-the-art AdHEX. Isochoric temperature swings were carried out and evaluated using a recently introduced method to determine the relative importance of heat and mass transport impedances. Coatings with thicknesses between 60 and 460 µm were investigated and in all cases the sorption dynamics were limited by mass transport. Ragone plots were used to characterize the power and energy trade-off during thermal cycling of SAPO-34 in water vapor to identify the pareto-optimal cycle time for a specific coating thickness. With the knowledge of the rate-limiting mechanism, the overall transport rates of adsorbent coatings can systematically be improved to enhance transport rates in next-generation AdHEX. [ABSTRACT FROM AUTHOR]

The PLEIADES software environment is devoted to the thermomechanical simulation of nuclear fuel elements behaviour under irradiation. This platform is co-developed in the framework of a research cooperative program between Électricité de France ( EDF ), AREVA and the French Atomic Energy Commission ( CEA ). As many thermomechanical solvers are used within the platform, one of the PLEAIADES’s main challenge is to propose a unified software environment for capitalisation of material knowledge coming from research and development programs on various nuclear systems. This paper introduces a tool called mfront which is basically a code generator based on C++ (Stroustrup and Eberhardt, 2004). Domain specific languages are provided which were designed to simplify the implementations of new material properties, mechanical behaviours and simple material models. mfront was recently released under the GPL open-source licence and is available on its web site: http://tfel.sourceforge.net/ . The authors hope that it will prove useful for researchers and engineers, in particular in the field of solid mechanics. mfront interfaces generate code specific to each solver and language considered. In this paper, after a general overview of mfront functionalities, a particular focus is made on mechanical behaviours which are by essence more complex and may have significant impact on the numerical performances of mechanical simulations. mfront users can describe all kinds of mechanical phenomena, such as viscoplasticity, plasticity and damage, for various types of mechanical behaviour (small strain or finite strain behaviour, cohesive zone models). Performance benchmarks, performed using the Code_Aster finite element solver, show that the code generated using mfront is in most cases on par or better than the behaviour implementations written in fortran natively available in this solver. The material knowledge management strategy that was set up within the PLEIADES platform is briefly discussed. A material database named sirius proposes a rigorous material verification workflow. We illustrate the use of mfront through two case of studies: a simple FFC single crystal viscoplastic behaviour and the implementation of a recent behaviour for the fuel material which describes various phenomena: fuel cracking, plasticity and viscoplasticity. [ABSTRACT FROM AUTHOR]

A model reproducing strain softening behavior in ceramic materials is proposed. This model is base on a critical treatment of previous mechanical experimental results, mainly on uranium dioxide. The main hypothesis is that the strain softening phenomenon is related to an aging process, where some point defects move towards the dislocations and modify their velocity. This is different from most of models used up to now, as they were based on the hypothesis that only the initial lack of dislocations was responsible of the strain softening behavior. A model is first developed in a simple 1D framework. Evolution of the mechanical behavior with strain rate and temperature is well reproduced by this model. Then, the 1D model is extended to a 3D mechanical model, and mechanical compressive tests on UO 2 pellets are simulated. The 3D model well reproduces the observed asymmetrical shape of the compressed pellet if one considers that the material is not initially perfectly homogeneous, which highlights the importance of accounting for spatial heterogeneity of materials in models. [ABSTRACT FROM AUTHOR]

• Sole focus during the last decade on deep learning accuracy (Red AI), ignored economic, or environmental cost. • Universal metric that balances accuracy, complexity, and carbon footprint helps to select better models, and frameworks. • Recognition and training efficiency RE , TE , compare deep learning models and platforms in a universal fashion. • Sustainability is assessed with deep learning lifecycle efficiency and life cycle recognition efficiency DLLCE , RE LC. • Efficiency comparison among models with 4 – 30 – 1000 classes, on cloud and edge CPU, TPU, GPU, and NCS2. The single recent focus on deep learning accuracy ignores economic, and environmental cost. Progress towards Green AI is hindered by lack of universal metrics that equally reward accuracy and cost and can help to improve all deep learning algorithms and platforms. We define recognition and training efficiency as new universal metrics to assess deep learning sustainability and compare them to similar, less universal metrics. They are based on energy consumption measurements, on deep learning inference, on recognition gradients, and on number of classes and thus universally balance accuracy, complexity and energy consumption. Well-designed edge accelerators improve recognition and training efficiencies compared to cloud CPUs and GPUs due to reduced communication overhead. Cradle to grave sustainability of edge intelligence models and platforms is assessed with novel deep learning lifecycle efficiency and life cycle recognition efficiency metrics that include the number of times models are used. Artificial and natural intelligence efficiencies are compared leading to insights on deep learning scalability. [ABSTRACT FROM AUTHOR]

Developing strategies to reduce mass and heat transport limitations is one of the most important challenges in adsorption heat exchanger technology. Due to the strong coupling of mass and heat transport in these systems, it is difficult to determine the individual transport limitations quantitatively. In order to find an optimal design where heat and mass transport are balanced, a quantitative method that enables a direct comparison of these two transport phenomena is needed. In the present work, a novel experimental approach to discriminate between mass and heat transport is proposed based on the measurement of adsorbent temperature, heat exchanger surface temperature and vapor pressure. The methodology is applied to micro/mesoporous silica spheres arranged in a monolayer or bilayer loose grain configuration or in a monolayer configuration adhesively bonded to the substrate. While the monolayer configuration exhibits balanced heat and mass transport, we find that the bilayer and the thermally enhanced configurations are limited by heat and mass transport, respectively. The application of the proposed methodology to compare heat and mass transport limitations in other industrially-relevant adsorbent materials should greatly aid the design of more efficient adsorption heat exchangers for a wide range of applications. [ABSTRACT FROM AUTHOR]

The mechanical behaviour of UO 2 single crystal is under debate due to the unexpected multi-slip observations in the experiments that involve dislocations in 1 2 〈 110 〉 { 100 } slip systems but also in 1 2 〈 110 〉 { 110 } and 1 2 〈 110 〉 { 111 }. We propose a multi-scale model based on a composite slip in which, under the effect of cross-slip, part of the dislocation density in primary slip systems can be transferred in secondary systems with a lower propensity to glide but a more favourable orientation regarding the shear stress. This approach allows to describe the anisotropic mechanical response of UO 2 single crystal with an accuracy never reached up to now. After identifying the relevant slip systems depending on the orientation using a Schmid approach, dislocation dynamics simulations are used to assert if the cross-slip induces a composite slip and to quantify its effect on the flow stress which appears constrained by the activity of 1 2 < 110 > { 111 } systems. In agreement with this result, the composite slip is adapted to couple the activity of slip systems with common Burger vectors in a crystal plasticity framework for a closer comparison to the experiment. This multi-scale approach significantly improves our current knowledge on the links between dislocation microstructures and mechanical properties in UO 2. Composite slip mechanism appears as a candidate to explain unexpected plastic behaviours as often observed in complex materials with multiple slip modes underling that slip activation may be more complex than in usual constitutive laws. [Display omitted] [ABSTRACT FROM AUTHOR]

This work is devoted to the modeling of the mechanical behavior of coated or uncoated silicon nanoparticles, which constitute the anode active material in some lithium ion batteries, during their first lithiation at room temperature. The lithiation process induces a large volume expansion of the particles and a high level of stresses, which can lead to the fragmentation of the particles. Several approaches are proposed in order to estimate the volume expansion and the stress levels experienced by the particles. An original semi-analytical small strain model is first presented, which adapts the solution proposed by Seck et al. (2018) of the elastic-viscoplastic composite sphere problem subjected to radial loading, to the lithiated particle problem, in particular by considering the variation of phases properties during lithiation. The lithium concentration in the silicon particle is given by a sigmoid function (called logistic function) in order to mimic the reaction front between the phases. The implementation of the approach using the Hencky strain tensor Miehe et al. (2002) is proposed to take into account the large strains experienced by the particles. A complete description of the formulation is provided and the advantages are discussed. The importance of the large strains model is established by comparing it with the small strain one concerning the predictions of the pushing-out effect and the size effect of particles on their internal stresses during lithiation. Comparisons between our simulations and experimental data from Tardif et al. (2017) measuring the operando strain experienced by the pristine silicon gives the yield stress of the lithiated silicon. In addition, carbon-coated silicon nanoparticles are finally studied. We develop original closed-form expressions to predict the maximal stresses experienced by the coating at the end of the lithiation. Those expressions are used to re-estimate the fracture stress of pyrolitic carbon, considering a critical review of both pyrolitic carbon and lithiated silicon elastic properties. Finally, the mechanical effect of the coating on silicon during lithiation is studied. • The stress and strain evolutions in coated/uncoated silicon nanoparticles during their first lithiation are analyzed. • A large strain finite element model using an accurate logarithmic strain framework is proposed. • This large strain finite element model predicts well the strain measurements obtained by XRD. • Estimates of key material properties (plastic and fracture thresholds) are derived by comparison with experiments. • New closed-form expressions of the stresses experienced by the particles during lithiation are assessed. [ABSTRACT FROM AUTHOR]

The aim of this paper is to introduce the PLEIADES framework offering a set of services and numerical tools to model and simulate the behavior of nuclear fuels of different concepts of reactors. The framework provides in particular the following features: interfaces to manipulate meshes and fields, services to deal with different physical solvers, setup of coupling trees to realize multiphysics partitioned (accelerated) fixed point couplings, automated time-marching algorithm, checkpoint/restart strategies, capability to realize on-the-fly multiscale couplings. It is built with a permanent concern for sustainability, scalability and maintainability. PLEIADES framework supports multidimensional simulations, typically 1D, 2D and 3D, possibly multilayered. To date, this framework relies on the generic thermomechanical finite elements solver Cast3M to deal with partial derivative problems (mechanical, thermal or diffusion problems) at the scale of the structure or the (heterogeneous) microstructure. It also makes use of so-called point models (mainly based on ODE — ordinary differential equations) to describe the local (mesoscale) evolution of the material through the physics of irradiation. Several software applications are built on the PLEIADES framework, they can use its parallel features to use multiple processors. The multilayered calculation scheme provides a way to loosen the computations and access to a good parallel performance up to hundred cores. It is also possible to achieve efficient scalable concurrent multiscale simulations through finite element square (FE 2) computational homogenization algorithms. • Introduction of the PLEIADES framework, dedicated to nuclear fuel behavior simulation. • Structuring and computational choices of the PLEIADES framework are described. • The multiphysics coupling is solved through an accelerated block fixed-point strategy. • Multiscale global/local coupling and computational homogenization are also available. • Parallel PLEIADES features based on MPI communications are detailed. [ABSTRACT FROM AUTHOR]

Abstract: A detailed thermo-hydrodynamic analysis of a hot water cooled manifold microchannel heat sink for electronic chip cooling is presented. The hot water cooling enables efficient recovery of heat dissipated by the even hotter chip by using hot water recovered from a secondary application. Contrary to usual expectation of laminar flow in electronic cooling, high flow rate and high fluid temperatures result in turbulent flow conditions in the inlet and outlet manifolds of the heat sink with predominantly laminar flow conditions in microchannels. To simulate these complex flow conditions, a three dimensional (3D) conjugate heat transfer model with turbulent flow is developed. Microchannel heat transfer structure is modeled as porous medium with permeability parameters extracted from a 3D model for a single microchannel. The energetic performance of the heat sink is analyzed in terms of 2nd law efficiency and sources of exergy destruction are identified by detailed local entropy generation analysis at low and high Reynolds number conditions of 2400 and 11200 respectively. This analysis shows that entropy generation due to heat transfer dominates the net entropy generation in the heat sink for both conditions. Although entropy generation due to viscous dissipation increases significantly with increased Reynolds number, it still contributes less than a third to the total entropy generated at high Reynolds numbers. Use of hot water reduces the heat transfer component of entropy generation significantly, thus leading to higher 2nd law efficiency. [Copyright &y& Elsevier]

Abstract: This article is the second part of a study on flow boiling of R236fa and R245fa. This part presents the heat transfer coefficients obtained in a 12.7mm silicon evaporator composed of 135 microchannels with 85μm wide and 560μm high channels separated by 46μm wide fins. There were 35 local heaters and temperature measurements arranged in a 5×7 array. The heat transfer results were uniform in the lateral direction to the flow (attributable to the inlet restriction) and a function of the heat flux, vapor quality and mass flux. The steady-state standard deviation of the local base temperature was less than 0.2°C, inferring that the boiling process was very stable. For wall heat fluxes over 45kW/m2, the heat transfer coefficient curves were V-shaped, decreasing for intermittent flow regimes and increasing for annular flow. The three-zone model of was the best heat transfer prediction method when setting the dryout thickness equal to the channel roughness. [Copyright &y& Elsevier]

Abstract: Three micro-evaporator cooling cycles, one with a pump, one with a compressor and a hybrid of the two together, are proposed for cooling a computer blade server. The hybrid cycle is characterized by the interchangeability between the first two cycles, where the decision on the cycle to operate is based on the season (necessity or economical benefit for heat recovery) or the maintenance of cycle’s driver. The main characteristics of each cycle are presented as well as the details of the micro-evaporator cooler for the blade’s CPU. Analysis of the cycle overall efficiency and the potential for heat recovery shows that the best cycle to use depends mainly on the end application of the heat recovered. Four refrigerants were evaluated as the possible working fluids for cooling the microprocessors. HFC134a and HFC245fa were found to be the best choices for the desired application. [Copyright &y& Elsevier]

Plasticity of irradiated UO 2 is of major interest to improve the risk assessment of the nuclear fuel cladding failure in the case of design basis accidents. In this study, we investigate the main irradiation-hardening processes induced by {110} irradiation loops interacting with glissile dislocations of the primary slip system, 1/2 < 110 > {001}, of UO 2. The interactions are simulated at two scales using molecular dynamics and discrete dislocation dynamics, to characterise local interactions and identify strengthening configurations as a function of the dislocation-irradiation loop geometry. In particular, we show that 1/2 < 110 > {001} screw dislocations can be strongly pinned by helical turn configurations. Statistical large-scale discrete dislocation simulations are performed to investigate the collective behaviour of a large density of irradiation defects and quantify irradiation hardening. Several microstructural processes including loop drag and shovelling are observed and their involvement in clear band formation and hardening of UO 2 fuel at high temperature is discussed. • Interactions between {001} dislocations and {110} irradiation loops are investigated in UO 2. • Helical turn configurations are the strongest pinning reactions identified. • DDD simulations show the impact of irradiation loops on the mechanical response of UO 2. • Loop drag and shovelling constrain the defect microstructure and the stress response. [ABSTRACT FROM AUTHOR]

We present the design, optical characterization and full-system modeling of a novel 6-focus, high-concentration photovoltaic-thermal solar polygeneration system, aiming at an energy-efficient and cost-effective utilization of the solar resource. Essential to this system is a compact, modular solar dish concentrator design optimized for mass-production, structural rigidity, and scalability, with a high geometric concentration ratio of 1733× at each of its six receivers. Every receiver comprises 36 triple-junction CPV cells, interconnected in a unique hybrid parallel-serial scheme that mitigates mismatch losses caused by non-uniform irradiance distributions. Cogeneration is enabled by using high-performance microchannel heat exchangers, allowing the extraction of low grade heat for secondary thermal processes. The tested prototype achieves an average solar radiative flux of 1374 suns on each of the receivers. By optimizing several design parameters, the CPV-thermal system can deliver a solar-to-electricity conversion efficiency of 28.5% in PV-only mode and 26.6% in cogeneration mode while extracting heat at 89.8 °C, and a power of 12.1 kW el and 11.3 kW el /21.5 kW th respectively, matching the performance of state of the art CPV commercial systems, while striving towards a reduction of the investment costs. [ABSTRACT FROM AUTHOR]

We propose a facile, novel concept of mass transfer enhancement in flow batteries based on electrolyte guidance in rationally designed corrugated channel systems. The proposed fluidic networks employ periodic throttling of the flow to optimally deflect the electrolytes into the porous electrode, targeting enhancement of the electrolyte-electrode interaction. Theoretical analysis is conducted with channels in the form of trapezoidal waves, confirming and detailing the mass transport enhancement mechanism. In dilute concentration experiments with an alkaline quinone redox chemistry, a scaling of the limiting current with Re 0.74 is identified, which compares favourably against the Re 0.33 scaling typical of diffusion-limited laminar processes. Experimental IR-corrected polarization curves are presented for high concentration conditions, and a significant performance improvement is observed with the narrowing of the nozzles. The adverse effects of periodic throttling on the pumping power are compared with the benefits in terms of power density, and an improvement of up to 102% in net power density is obtained in comparison with the flow-by case employing straight parallel channels. The proposed novel concept of corrugated fluidic networks comes with facile fabrication and contributes to the improvement of the transport characteristics and overall performance of redox flow battery systems. [ABSTRACT FROM AUTHOR]

Recent years have witnessed a remarkable reduction in solar-panel costs, such that low-efficiency, low-cost photovoltaics (PV) currently prevail over more complex, high-efficiency technologies. Although solar-energy-generating installations provide a renewable energy source often considered emission-free, a number of externalities are frequently ignored that favor technologies with a reduced efficiency as long as they are available at lower cost. Whenever PV systems are installed, the absorption properties of the surface are changed and less sunlight from the Earth’s surface is reflected into space. By including this radiative forcing in the form of the Earth’s surface reflection coefficient albedo (α), we take these externalities into consideration in the overall equivalent global warming potential (GWP) of a PV system. Three different effects need to be considered when changing the absorption properties of the Earth’s surface: (1) global albedo impact, (2) regional atmospheric heat islands, and (3) locally heated surfaces. The unintended radiative forcing adversely affects the net efficiency of building-integrated solar installations in warm urban climates, as more energy is required for cooling to ensure human comfort. The total GWP of four different PV technologies was examined for three different urban climates, temperate, moderate, and warm. To minimize the system energy payback time (EPBT) it is most sensible to install high-efficiency solar-energy systems outside cities and urban developments in locations with high annual irradiance. Only when taking radiative forcing into environmental and economic considerations is it expected that solar-technology development will correct its trajectory away from low-cost systems and toward high-efficiency installations with lower overall GWP. [ABSTRACT FROM AUTHOR]

Membraneless flow cells for electrochemical energy conversion exploit the laminarity of microscale flows to avoid undesirable mixing of reactants. To increase the performance of microfluidic redox flow cells we employ herringbone-inspired flow promoters, thereby increasing convection of each individual species to the electrodes, while minimizing reactant mixing. Polarization curves from electrochemical discharge measurements with a dilute anthraquinone/iron redox system reveal that the presence of flow promoters substantially boosts device performance. Mass transfer enhancement for devices with flow promoters is demonstrated through both higher limiting currents and increased power density; the former is more than double compared to a plain reference microchannel for Reynolds numbers of Re >155. The chaotic mixing effect induced by the flow promoters also becomes apparent in the scaling regimes, the limiting currents are proportional to Re 0.58 instead of Re 1/3 (as for purely laminar flow). Further, we quantify the area specific resistance (ASR) of the electrolyte in our membraneless devices finding a reduction of more than one order of magnitude compared to the ASR of conventional membranes employed in redox flow cells. Overcoming mass transfer limitations, this work highlights the necessity of passive mixers in significantly raising the performance of microfluidic flow cells. [ABSTRACT FROM AUTHOR]

Adsorption processes are ubiquitous in nature as well as of great technological importance for gas separation, purification, storage and thermally driven heat pumps. This has led to a strong interest in the fundamental mechanisms governing adsorption phenomena and their exploitation to tailor adsorption systems for specific applications. In particular, the adsorption of water on porous silica exhibits remarkable properties due to the strong polarity of the adsorbate and moderate hydrophilicity of the adsorbent. It is generally accepted that the adsorption of water vapor on porous silica depends upon the concentration of surface silanols and the pore size. In fact, materials with ordered mesopores and a well-defined pore size have been used as model systems to demonstrate that water adsorption occurs predominantly through capillary condensation. While the pore surface chemistry is modified to become more hydrophilic after filling of the pores in these materials, the overall shape of the water adsorption isotherm (Type V) is not significantly affected in subsequent adsorption measurements. The present contribution shows that conservation of the isotherm shape is a unique phenomenon related to ordered mesopores but doesn't apply to materials with a more complex pore structure. For materials with wider pore size distributions including micropores, a synergistic effect of surface hydroxylation and pore size leads to a dramatic change in the water adsorption isotherm after the first adsorption/desorption cycle. In fact, we demonstrate that the water cycling capacity at relative pressures below the onset of capillary condensation increases significantly in these systems. These results contribute to a fundamental understanding of water adsorption in complex systems and have important implications in applications such as adsorption heat pumps where a large water cycling capacity in a specific relative pressure window is required. [ABSTRACT FROM AUTHOR]

Photovoltaic power generation is a growing renewable primary energy source, expected to assume a major role as we strive toward fossil fuel free energy production. However, the photovoltaic efficiencies limit the conversion of solar radiation into useful power output. Hybrid systems extend the functionality of concentrating photovoltaics (CPV) from simply generating electricity, to providing simultaneously electricity and heat. The utilization of otherwise wasted heat significantly enhances the overall system efficiency and boosts the economic value of the generated power output. The current system consists of a scalable hybrid photovoltaic–thermal receiver package, cooled with an integrated high performance microchannel heat sink. The package can be operated at elevated temperatures due to its overall low thermal resistance between solar cell and coolant. The effect of the harvested elevated coolant temperature on the photovoltaic efficiency is investigated. The higher-level available heat can be suitable for sophisticated thermal applications such as space heating, desalination or cooling (polygeneration approaches). A total hybrid conversion efficiency of solar radiation into useful power of 60% has been realized. The exergy content of the overall output power was increased by 50% through the exergy content of the extracted heat. An analysis based on the economic value of heat illustrates that the reused heat can double the economic value of such a system. [ABSTRACT FROM AUTHOR]

The shift to multicore microprocessor architecture is likely to result in higher coolant flow requirements and thus exacerbate the problem of increasing data center energy consumption, also with respect to hotspot elimination. We present and experimentally prove a novel concept, for embedded, hotspot-targeted and energy efficient cooling of heterogeneous chip power landscapes. The rationally distributed, embedded microstructures presented here are able to adapt the heat transfer capability to a steady but non-uniform chip power map by passively throttling the flow in low heat flux areas. For the industrially acceptable limit on pressure drop of approximately 0.4 bar, the hotspot-targeted embedded liquid cooling (HT-ELC) designs are evaluated against a conservatively chosen conventional embedded liquid cooling (C-ELC) design and existing heat sinks in the literature. For an average steady-state heat flux of 150 W/cm 2 in core areas (hotspots) and 20 W/cm 2 over the remaining chip area (background), the chip temperature variation is reduced from 10 °C under the conventional cooling to 4 °C under the current hotspot targeted heat sink – a reduction of 57%. For heat fluxes of 300 and 24 W/cm 2 , the temperature variation is reduced by 30%. We show that the HT-ELC designs consume less than 0.3% of total chip power as pumping power to achieve this thermal performance, which the C-ELC design cannot match under all feasible levels of pumping power. Moreover, the HT-ELC designs achieve at least 70% improvement over the existing hotspot targeted heat sinks in terms of normalized chip temperature non-uniformity, without the need for any additional system level complexity, reducing reliability risks. [ABSTRACT FROM AUTHOR]

Large data centers today already account for nearly 1.31% of total electricity consumption with cooling responsible for roughly 33% of that energy consumption. This energy intensive cooling problem is exacerbated by the presence of hotspots in multicore microprocessors due to excess coolant flow requirement for thermal management. Here we present a novel liquid-cooling concept, for targeted, energy efficient cooling of hotspots through passively optimized microchannel structures etched into the backside of a chip (embedded liquid cooling or ELC architecture). We adopt an experimentally validated and computationally efficient modeling approach to predict the performance of our hotspot-targeted ELC design. The design is optimized for exemplar non-uniform chip power maps using Response Surface Methodology (RSM). For industrially acceptable limits of approximately 0.4 bar (40 kPa) on pressure drop and one percent of total chip power on pumping power, the optimized designs are computationally evaluated against a base, standard ELC design with uniform channel widths and uniform flow distribution. For an average steady-state heat flux of 150 W/cm 2 in core areas (hotspots) and 20 W/cm 2 over remaining chip area (background), the optimized design reduces the maximum chip temperature non-uniformity by 61% to 3.7 °C. For a higher average, steady-state hotspot heat flux of 300 W/cm 2 , the maximum temperature non-uniformity is reduced by 54% to 8.7 °C. It is shown that the base design requires a prohibitively high level of pumping power (about 2000 fold for 150 W/cm 2 case and 600 fold for 300 W/cm 2 case) to match the thermal performance of the optimized, hotspot-targeting designs. The pumping power requirement for optimized designs is only 0.23% and 0.17% of the total chip power for 150 W/cm 2 and 300 W/cm 2 hotspot heat flux respectively. Moreover, the optimized designs distribute the coolant flow without any external flow control devices and the performance is only marginally affected by the manifold geometry used to supply the coolant to the microchannel heat transfer structure. This also attests to the robustness of the optimized embedded microchannel structures. [ABSTRACT FROM AUTHOR]

Abstract: This paper provides an exergetic analysis of a 10MW high concentration photovoltaic thermal (HCPVT) power plant case study located in Hammam Bou Hadjar, Algeria. The novel HCPVT multi-energy carrier plant converts 25% of the direct normal irradiance (DNI) into electrical energy and 62.5% to low grade heat for a combined efficiency of 87.5%. The HCPVT system employs a point focus dish concentrator with a cooled PV receiver module. The novel “hot-water” cooling approach is used for energy reuse purposes and is enabled by our state-of-the-art substrate integrated micro-cooling technology. The high performance cooler of the receiver with a thermal resistance of <0.12cm2 K/W enables the receiver module to handle concentrations of up to 5000suns. In the present study, a concentration of 2000suns allows using coolant fluid temperatures of up to 80°C. This key innovation ensures reliable operation of the triple junction PV (3JPV) cells used and also allows heat recovery for utilization in other thermal applications such as space cooling, heating, and desalination. Within this context, an exergoeconomics analysis of photovoltaic thermal co-generation for space cooling is presented in this manuscript. The valuation method presented here for the HCPVT multi-energy carrier plant comprises both the technical and economic perspectives. The proposed model determines how the cost structure is evolving in four different scenarios by quantifying the potential thermal energy demand in Hammam Bou Hadjar. The model pins down the influence of technical details such as the exergetic efficiency to the economic value of the otherwise wasted heat. The thermal energy reuse boosts the power station׳s overall yield, reduces total average costs and optimizes power supply as fixed capital is deployed more efficiently. It is observed that even though potential cooling demand can be substantial (19,490MWh per household), prices for cooling should be 3 times lower than those of electricity in Algeria (18USD/MWh) to be competitive. This implies a need to reach economies of scale in the production of individual key components of the HCPVT system. The net present value (NPV) is calculated taking growth rates and the system׳s modular efficiencies into account, discounted over 25 years. Scenario 1 shows that even though Algeria currently has no market for thermal energy, a break-even quantity (49,728MWh) can be deduced by taking into account the relation between fixed costs and the marginal profit. Scenario 2 focuses on the national growth rate needed to break even, i.e. +10.92%. Scenario 3 illustrates thermal price variations given an increase in the Coefficient of Performance (COP) of a thermally driven adsorption chiller after year 10. In this case, the price for cooling will decrease from 18USD/MWh to 14USD/MWh. Finally, scenario 4 depicts Hammam Bou Hadjar׳s potential cooling demand per household and the growth rate needed to break even if a market for heat would exist. [Copyright &y& Elsevier]

Highlights: [•] Experimentally evaluated a hybrid on-chip two-phase cooling cycle. [•] Steady-state and transient operation of two parallel pseudo-chips. [•] Control strategies evaluated by reference tracking and disturbance rejection tests. [•] Ø Energetic and exergetic comparison with two other cooling cycles. [ABSTRACT FROM AUTHOR]

Highlights: [•] We investigate next generation interlayer integrated water cooled 3D chips. [•] Confined vortex shedding in flow across various pin configurations is studied. [•] Effect of lateral, longitudinal and vertical confinement of pins is analyzed. [•] Nusselt number in shedding regime is enhanced 300% compared to steady conditions. [•] Results can be used to infer guidelines for best hydrothermal performance. [ABSTRACT FROM AUTHOR]

Abstract: Membrane distillation (MD), a thermally driven process involving hydrophobic micro-porous membranes has gained widespread interest in academic research and is set to become an alternative solution to other membrane separation processes such as reverse osmosis (RO). Although extensive experimental studies have been carried out since the 1980s [1,2], clear understanding of the heat and mass transport phenomena has yet to be established. This manuscript presents experimental results of direct contact membrane distillation (DCMD) with de-ionized water and aqueous salt solutions of NaCl with concentration levels of up to 15ppt as feed together with an experimentally optimized and validated model for the prediction of the permeate flux in DCMD for GE Aspire Membrane QL 833 (GE Energy). Different heat transfer prediction methods in combination with the three different forms of the Dusty Gas model for mass transport were used in the comparison of our experimental data in the laminar and turbulent flow regimes under steady-state conditions. The comparison between experimental and predicted results confirmed our expectation that the Knudsen-molecular diffusion transition model yielded the best prediction. We have also identified, based on the comparison of the data, the most accurate heat transfer correlation for the laminar and turbulent flow regimes, taking into account the experimental and permeate prediction uncertainties to optimally address the heat and mass transport equations used in DCMD studies. Hence, it is highly recommended that these heat transfer correlations and the Knudsen-molecular mass transport equation be used in the prediction of heat and mass transfer for flat sheet DCMD experiments. [Copyright &y& Elsevier]

Abstract: The cooling of three-dimensional electronic chip assemblies (stacks) is one of the most serious challenges facing the electronics industry as it moves toward fabrication approaches combining speed with energy efficiency. Here we show that by generating vortical microscale flows taking advantage of the inherent presence of Through Silicon Vias (TSV) in 3D integrated liquid (water) cooling of chip stacks, both large heat transfer enhancement as well as significantly better temperature uniformity can be accomplished. The approach is demonstrated experimentally in heat sinks consisting of a microcavity confining micropin fin arrays, mimicking TSV. Flow fluctuations and vortex shedding were triggered at specific Reynolds numbers, which are functions of the pin geometries and level of confinement. The resulting heat transfer enhancement due to the vortex-induced fluctuations and mixing, yields local Nusselt number increases up to 230% thereby reducing the chip temperature non-uniformity almost by a factor of three. The vortex shedding also induces a pressure drop increase. Remarkably, the effective improvement in the thermal performance due to vortex shedding, even after factoring in the rise in pumping power, reaches a peak value of 190%. Analysis of instantaneous liquid temperature signatures of shed microvortices using micron-resolution laser-induced fluorescence (μLIF), proved them to be the reason for both the elimination of liquid hotspots and the exceptional augmentation in heat transfer. These findings have important implications in the design of the new generation of integrated, out of plane electronics cooling with liquids. [Copyright &y& Elsevier]

Abstract: We report an experimental study on exergetically efficient electronics cooling using hot water as coolant. It is shown that water temperatures as high as 60°C are sufficient to cool microprocessors with over 90% first law (energy based) efficiency. The chip used in our experiment is kept at temperatures of 80°C or below so as not to exceed any allowable industrial specifications for maximum microprocessor chip temperature. The use of hot water as coolant will eliminate the requirement for chillers typically used in air-cooled data centers and, therefore, significantly reduce the power consumption. An exergy analysis shows that a six fold rise in second law (exergy based) efficiency is achieved by switching the water inlet temperature from 30°C to 60°C. The resulting high exergy at the heat sink outlet is a measure of the potential usefulness of the waste heat of data centers, thereby helping to design data centers with minimal carbon footprint. A new metric for the economic value of the recovered heat, based on costs for electricity and fossil fuels, heat recovery efficiency and an application specific utility function, is introduced to underscore the benefits of hot water cooling. This new concept shows that the economic value of the heat recovered from data centers can be much higher than its thermodynamic value. [Copyright &y& Elsevier]

Abstract: We report the energy and exergy efficiencies of Aquasar, the first hot water cooled supercomputer prototype. The prototype also has an air cooled part to help compare the coolants''s performances. For example, a chip/coolant temperature differential of only 15 °C was sufficient for chip cooling using water. The air cooled side, however, required air pre-cooling down to 23 °C and a chip/coolant temperature differential of 35 °C. Whereas extra exergy was expended for air pre-cooling, the higher thermal conductivity and specific heat capacity of water enabled coolant temperatures to be safely raised to 60 °C. Using such hot water not only eliminated the need for chillers, it also opened up the possibility of heat reuse. The latter was realized by using the hot water from Aquasar for building heating. A heat recovery efficiency of 80% and an exergetic efficiency of 34% were achieved with a water temperature of 60 °C. All these results establish hot water as a better coolant compared to air. A novel concept of economic value of heat was introduced to evaluate different reuse strategies such as space heating and refrigeration using adsorption chillers. It was shown that space heating offers the highest economic value for the heat recovered from data centers. [Copyright &y& Elsevier]

Abstract: A novel framework is developed to determine the optimal operating conditions of water cooled microprocessor chips through a tradeoff between heat recovery from the coolant and the chip thermal reliability. For illustration, a manifold microchannel heat sink is evaluated experimentally and computationally. First, a single objective optimization is demonstrated by combining the heat recovery and chip reliability into a single parameter. Then, multi-objective optimizations are performed by using Pareto optimality and Multi-Criteria Design Analysis. Using conservative guidelines, these approaches suggest that for an optimal coolant flow rate of 1l/m, optimal coolant inlet temperature lies between 40 and 50°C. [Copyright &y& Elsevier]

Abstract: This article is the first part of a study on flow boiling of R236fa and R245fa. This part presents pressure drop measurements obtained on a silicon multi-microchannel evaporator with 85μm wide and 560μm high channels separated by 46μm wide fins. The 135 microchannels were 12.7mm long. Dielectric refrigerants R236fa and R245fa were used as the evaporating test fluids. The inlet saturation temperature was maintained at 30.5°C while the mass fluxes were varied from 499 to 1100kg/m2 s and the base heat flux was tested from 130 to 1400kW/m2. A new experimental technique was developed to measure the outlet pressure losses, which represented up to 30% of the total pressure drop and thus cannot be neglected. The microchannel pressure drop measurements were very well predicted by the method of . [Copyright &y& Elsevier]

Abstract: New experimental critical heat flux results for saturated boiling conditions have been obtained for R236fa flowing in a silicon multi-microchannel heat sink composed of 67 parallel channels, 223μm wide, 680μm high and with 80μm thick fins separating the channels. The microchannel length was 20mm. The footprint critical heat fluxes measured varied from 112 to 250W/cm2 and the wall critical heat fluxes from 21.9 to 52.2W/cm2 for mass velocities from 276 to 992kg/m2s. When increasing the mass velocity, the wall critical heat flux was observed to increase. The inlet saturation temperatures (20.31⩽ T sat,in ⩽34.27°C) and the inlet subcoolings (0.4⩽ΔT sub ⩽15.3K) were found to have a negligible influence on the saturated CHF. The best methods for predicting the data were those of Wojtan et al. [L. Wojtan, R. Revellin, J. R. Thome, Investigation of critical heat flux in single, uniformly heated microchannels, Exp. Therm. Fluid Sci. 30 (2006) 765–774] and Revellin and Thome [R. Revellin, J. R. Thome, A theoretical model for the prediction of the critical heat flux in heated microchannels, Int. J. Heat Mass Transfer 50 (in press)]. They both predict the experimental CHF results with a mean absolute error of around 9%. Using the critical vapour quality, an annular-to-dryout transition is also proposed as a limit in a diabatic microscale flow pattern map. Pressure drop measurements were measured and analysed, showing that the homogeneous model could correctly predict the observed trends. [Copyright &y& Elsevier]

Abstract: This article is the first in a three part study on flow boiling of refrigerants R236fa and R245fa in a silicon multi-microchannel heat sink. The heat sink was composed of 67 parallel channels, which are 223μm wide, 680μm high and 20mm long with 80μm thick fins separating the channels. The base heat flux was varied from 3.6 to 221W/cm2, the mass velocity from 281 to 1501kg/m2 s and the exit vapour quality from 2% to 75%. The working pressure and saturation temperature were set nominally at 273kPa and 25°C, respectively. The present database includes 1217 local heat transfer coefficient measurements, for which three different heat transfer trends were identified, but in most cases the heat transfer coefficient increased with heat flux and was almost independent of vapour quality and mass velocity. Importantly, it was found for apparently the first time that the heat transfer coefficient as a function of vapour quality reaches a maximum at very high heat fluxes and then decreases with further increase of heat flux. [Copyright &y& Elsevier]