Bottom up approach toward prediction of effective thermophysical properties of carbon-based nanofluids

Carbon-based nanofluids, mainly suspensions of carbon nanotubes or graphene sheets in water, are typically characterized by superior thermal and optical properties. However, their multiscale nature is slowing down the investigation of optimal geometrical, chemical, and physical nanoscale parameters for enhancing the thermal conductivity while limiting the viscosity increase at the same time. In this work, a bottom up approach is developed to systematically explore the thermophysical properties of carbon-based nanofluids with different characteristics. Prandtl number is suggested as the most adequate parameter for evaluating the best compromise between thermal conductivity and viscosity increases. By comparing the Prandtl number of nanofluids with different characteristics, promising overall performances (that is, nanofluid/base fluid Prandtl number ratios equal to 0.7) are observed for semidilute (volume fraction  ⩽ 0.004) aqueous suspensions of carbon nanoparticles with extreme aspect ratios (larger than 100 for nanotubes, smaller than 0.01 for nanoplatelets) and limited defects concentrations (<5%). The bottom up approach discussed in this work may ease a more systematic exploration of carbon-based nanofluids for thermal applications, especially solar ones

Chapter Monitoring coastal areas: a brief history of measuring instruments for solar radiation

The first measuring instruments of solar radiation, for meteorological aims, were made only in 1800s. In 1896 OMI established a commission for radiometry which led, in 1905, to choose Ångström pyrheliometer as standard instrument. Later, radiometers were built with a chart recorder for measuring solar radiation components. Instruments using thermopile or photocell as sensitive element were made. From 1980s radiometers with data logger were built. In 2000s devices were developed for measuring solar radiation components in water column, for studies on physical and biological marine quantities

Thermal transmittance in graphene based networks for polymer matrix composites

Graphene nanoribbons (GNRs) can be added as fillers in polymer matrix composites for enhancing their thermo-mechanical properties. In the present study, we focus on the effect of chemical and geometrical characteristics of GNRs on the thermal conduction properties of composite materials. Configurations consisting of single and triple GNRs are here considered as representative building blocks of larger filler networks. In particular, GNRs with different length, relative orientation and number of cross-linkers are investigated. Based on results obtained by Reverse Non-equilibrium Molecular Dynamics simulations, we report correlations relating thermal conductivity and thermal boundary resistance of GNRs with their geometrical and chemical characteristics. These effects in turn affect the overall thermal transmittance of graphene based networks. In the broader context of effective medium theory, such results could be beneficial to predict the thermal transport properties of devices made of polymer matrix composites, which currently find application in energy, automotive, aerospace, electronics, sporting goods, and infrastructure industries

Characterisation and modelling of water wicking and evaporation in capillary porous media for passive and energy-efficient applications

Passive devices based on water wicking and evaporation offer a robust, cheap, off-grid, energy-efficient and sustainable alternative to a wide variety of applications, ranging from personal thermal management to water treatment, from filtration to sustainable cooling technologies. Among the available, highly-engineered materials currently employed for these purposes, polyethylene-based fabrics offer a promising alternative thanks to the precise control of their fabrication parameters, their light-weight, thermal and mechanical properties, chemical stability and sustainability. As such, both woven and non-woven fabrics are commonly used in capillary-fed devices, and their wicking properties have been extensively modelled relying on analytical equations. However, a comprehensive and flexible modelling framework able to investigate and couple all the heat and mass transfer phenomena regulating the water dynamics in complex 2-D and 3-D porous components is currently missing. This work presents a comprehensive theoretical model aimed to investigate the wetting and drying performance of hydrophilic porous materials depending on their structural properties and on the external environmental conditions. The model is first validated against experiments (R-2=0.99 for the wicking model; errors lower than 14% and 1% for the evaporation and radiative models, respectively), then employed in three application cases: the characterisation of the capillary properties of a novel textile; the assessment of the thermal performance of a known material for personal thermal management when used in different conditions; the model-assisted design of a porous hydrophilic component of passive devices for water desalination. The obtained results showed a deep interconnection between the different heat and mass transfer mechanisms, the porous structure and external working conditions. Thus, modelling their non-linear behaviour plays a crucial role in determining the optimal material characteristics to maximise the performance of porous materials for passive devices for the energy and water sector

A kinetic perspective on k-epsilon turbulence model and corresponding entropy production

In this paper, we present an alternative derivation of the entropy production in turbulent flows, based on a formal analogy with the kinetic theory of rarefied gas. This analogy allows proving that the celebrated k-epsilon model for turbulent flows is nothing more than a set of coupled BGK-like equations with a proper forcing. This opens a novel perspective on this model, which may help in sorting out the heuristic assumptions essential for its derivation, such as the balance between turbulent kinetic energy production and dissipation. The entropy production is an essential condition for the design and optimization of devices where turbulent flows are involved

Water transport control in carbon nanotube arrays

Based on a recent scaling law of the water mobility under nanoconned conditions, we envision novel strategies for precise modulation of water diffusion within membranes made of Carbon Nanotube Arrays - CNAs. In a rst approach, the water diusion coecient D may be tuned by nely controlling the size distribution of the pore size. In the second approach, D can be varied at will by means of externally induced electrostatic fields. Starting from the latter strategy, switchable molecular sieves are proposed, where membranes are properly designed with sieving and permeation features that can be dynamically activated/deactivates. Areas where a precise control of water transport properties is benecial range from energy and environmental engineering up to nanomedicine

Heat and mass transfer of water at nanoscale solid-liquid interfaces

A better physical understanding of heat and mass transfer of water at nanoscale solid interfaces is essential for the rational design of novel nanoconstructs for clean water and energy as well as for biomedical applications. Both nanoscale transfer phenomena are strongly influenced by solid-liquid nonbonded interactions occurring at the interface. First, classical Molecular Dynamics (MD) is used for investigating water transport in the proximity of several inorganic and biological solid surfaces, according to different surface functionalizations (i.e. hydrophobic/hydrophilic) and physical conditions. Results show that the self-diffusion coefficient D of water in nanoconfined geometries is reduced respect to bulk conditions. In fact, D scales with the dimensionless parameter θ, i.e. the ratio between the volume of confined water, which is defined by the solvent accessible surface and a characteristic length of confinement δ depending on surface chemistry, and the total one. The D(θ) relationship is then interpreted within the thermodynamics of supercooled water. Second, water diffusion in nanoconstructs also plays a fundamental role in nanoscale heat transfer phenomena. Non-equilibrium MD simulations are used to investigate the characteristic solid-liquid thermal boundary resistance of solvated nanoparticles with different degree of hydrophobicity, curvature or surface pegylation, where modeling guidelines are needed in order to optimize the design of nanofluids for novel coolants, solar collectors or ablation therapies. Results show that solid-liquid thermal boundary transmittance is proportional to the hydrophilicity of the nanoparticle surface. \ud Once a theoretical framework for the transport properties of nanoconfined water is established, the obtained scaling laws are applied to engineering and biomedical applications. Atomistic simulations are used for investigating the critical limitations of zeolite-based materials for filtering or thermal storage purposes, namely the limited water flux within the subnanometer pores and the low thermal transmittance, respectively. Infiltration isotherms of water in defective silicalite-I membranes are evaluated by MD simulations, and the water transport within the nanopores is interpreted in terms of solvent-structure and solvent-solvent nonbonded interaction energies. Large networks of carbon nanofillers, instead, may be introduced for enhancing the thermal transmittance of zeolite-based composite materials: non-equilibrium MD simulations show that CNTs with short overlap length and a few bonded interlinks already present a remarkable enhancement in the overall transmittance of the nanoconstructs, which also prove the importance of solid-solid interfaces for optimizing heat transfer at the nanoscale. Finally, water self-diffusivity has also a strong influence on the performances of contrast agents for Magnetic Resonance Imaging (MRI). In fact, lower mobility of water molecules close to MRI contrast agents enhances their longitudinal and transverse relaxivities. Here, MD simulations and the D(θ) relationship are shown to accurately predict the relaxometric responses of Gd(DOTA) or SPIOn MRI contrast agents confined within hydrated nanopores, proving that the D(θ) scaling law can help in tailoring nanostructures with precise modulation of water mobility

Anisotropic Electrostatic Interactions in Coarse-Grained Water Models to Enhance the Accuracy and Speed-Up Factor of Mesoscopic Simulations

Water models with realistic physical-chemical properties are essential to study a variety of biomedical processes or engineering technologies involving molecules or nanomaterials. Atomistic models of water are constrained by the feasible computational capacity, but calibrated coarse-grained (CG) ones can go beyond these limits. Here, we compare three popular atomistic water models with their corresponding CG model built using finite-size particles such as ellipsoids. Differently from previous approaches, short-range interactions are accounted for with the generalized Gay-Berne potential, while electrostatic and long-range interactions are computed from virtual charges inside the ellipsoids. Such an approach leads to a quantitative agreement between the original atomistic models and their CG counterparts. Results show that a timestep of up to 10 fs can be achieved to integrate the equations of motion without significant degradation of the physical observables extracted from the computed trajectories, thus unlocking a significant acceleration of water-based mesoscopic simulations at a given accuracy

Process optimization of osmotic membrane distillation for the extraction of valuable resources from water streams

The rising demand for sustainable wastewater management and high-value resource recovery is pressing industries involved in, e.g., textiles, metals, and food production, to adopt energy-efficient and flexible liquid separation methods. The current techniques often fall short in achieving zero liquid discharge and enhancing socio-economic growth sustainably. Osmotic membrane distillation (OMD) has emerged as a low-temperature separation process designed to concentrate valuable elements and substances in dilute feed streams. The efficacy of OMD hinges on the solvent’s migration from the feed to the draw stream through a hydrophobic membrane, driven by the vapor pressure difference induced by both temperature and concentration gradients. However, the intricate interplay of heat and mass processes steering this mechanism is not yet fully comprehended or accurately modeled. In this research, we conducted a combined theoretical and experimental study to explore the capabilities and thermodynamic limitations of OMD. Under diverse operating conditions, the experimental campaign aimed to corroborate our theoretical assertions. We derived a novel equation to govern water flux based on foundational principles and introduced a streamlined version for more straightforward application. Our findings spotlight complex transport-limiting and self-adjusting mechanisms linked with temperature and concentration polarization phenomena. Compared with traditional methods like membrane distillation and osmotic dilution, which are driven by solely temperature or concentration gradients, OMD may provide improved and flexible performance in target applications. For instance, we show that OMD—if properly optimized—can achieve water vapor fluxes 50% higher than osmotic dilution. Notably, OMD operation at reduced feed temperatures can lead to energy savings ranging between 5 and 95%, owing to the use of highly concentrated draw solutions. This study underscores the potential of OMD in real-world applications, such as concentrating lithium in wastewater streams. By enhancing our fundamental understanding of OMD’s potential and constraints, we aim to broaden its adoption as a pivotal liquid separation tool, with focus on sustainable resource recovery

Exergy analysis of solar desalination systems based on passive multi-effect membrane distillation

Improving the efficiency and sustainability of water treatment technologies is crucial to reduce energy consumption and environmental pollution. Solar-driven devices have the potential to supply off-grid areas with freshwater through a sustainable approach. Passive desalination driven by solar thermal energy has the additional advantage to require only inexpensive materials and easily maintainable components. The bottleneck to the widespread diffusion of such solar passive desalination technologies is their lower productivity with respect to active ones. A completely passive, multi-effect membrane distillation device with an efficient use of solar energy and thus a remarkable enhancement in distillate productivity has been recently proposed. The improved performance of this distillation device comes from the efficient exploitation of low-temperature thermal energy to drive multiple distillation processes. In this work, we analyze the proposed distillation technology by a more in-depth thermodynamic detail, considering a Second Law analysis. We then report a detailed exergy analysis, which allows to get insights on the production of irreversibilities in each component of the assembly. These calculations provide guidelines for the possible optimization of the device, since simple changes in the original configuration may easily yield up to a 46% increase in the Second Law efficiency. Keywords: Sustainability, Exergy analysis, Water treatment, Membrane distillation, Solar energ