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A new relationship on transport properties of nanofluids. Evidence with novel magnesium oxide based n-tetradecane nanodispersions
(Powder Technology, 397 (2022) 117082, 2022-02-03) Prado, José I.; Vallejo, Javier P.; Lugo, Luis
The worldwide increasing of thermal energy consumption fosters new technological solutions based on nanomaterials. The use of nanofluids enhances energy efficiency leading to eco-friendlier devices. Thus, researchers are encouraged to understand how modified thermophysical properties improve heat transfer capability. Magnesium oxide based n-tetradecane nanofluids are designed in terms of stability for cold storage application. Thermal conductivity, viscosity, density, and isobaric heat capacity were determined by transient hot wire, rotational rheometry, mechanical oscillation U-tube, and differential scanning calorimetry. Furthermore, a useful relationship on thermal conductivity and viscosity of nanofluids is proposed based on Andrade, Osida and Mohanty theories. Its reliability is checked with the here reported results and literature data of different nanofluids: titanium oxide within water, silver within poly(ethylene glycol), and aluminium oxide within (1-ethyl-3-methylimidazolium methanesulfonate + water). Similar trends have been found for all nanofluids excepting titanium oxide aqueous nanofluids, this differentiated behaviour being expected by the proposed relationship.
A rank-constrained coordinate ascent approach to hybrid precoding for the downlink of wideband massive MIMO systems
(2023-02-01) GONZÁLEZ-COMA, José P.; FRESNEDO, Óscar; CASTEDO, Luis
An innovative approach to hybrid analog-digital precoding for the downlink of wideband massive MIMO systems is developed. The proposed solution, termed Rank-Constrained Coordinate Ascent (RCCA), starts seeking the full-digital pre coder that maximizes the achievable sum-rate over all the frequency subcarriers while constraining the rank of the overall transmit covariance matrix. The frequency-flat constraint on the analog part of the hybrid precoder and the non-convex nature of the rank constraint are circumvented by transforming the original problem into a more suitable one, where a convenient structure for the transmit covariance matrix is imposed. Such structure makes the resulting full-digital precoder particularly adequate for its posterior analog-digital factorization. An addi tional problem formulation to determine an appropriate power allocation policy according to the rank constraint is also pro vided. The numerical results show that the proposed method outperforms baseline solutions even for practical scenarios with high spatial diversity.
Adaptative Integral Sliding Mode Based Course Keeping Control of Unnamded Surface Vehicle
(Journal of Marine Science and Engineering, 2022, 10, 68, 2022) González-Prieto, José Antonio; Pérez-Collazo, Carlos; Sing, Yogangh
This paper investigates the course keeping control problem for an unmanned surface vehicle (USV) in the presence of unknown disturbances and system uncertainties. The simulation study combines two different types of sliding mode surface based control approaches due to its precise tracking and robustness against disturbances and uncertainty. Firstly, an adaptive linear sliding mode surface algorithm is applied, to keep the yaw error within the desired boundaries and then an adaptive integral non-linear sliding mode surface is explored to keep an account of the sliding mode condition. Additionally, a method to reconfigure the input parameters in order to keep settling time, yaw rate restriction and desired precision within boundary conditions is presented. The main strengths of proposed approach is simplicity, robustness with respect to external disturbances and high adaptability to static and dynamics reference courses without the need of parameter reconfiguration.
Alternating Minimization for Wideband Multiuser IRS-aided MIMO Systems under Imperfect CSI
(IEEE, 2023-11) PÉREZ-ADÁN, Darian; JOHAM, Michael; FRESNEDO, Óscar; GONZÁLEZ-COMA, José P.; CASTEDO, Luis; UTSCHICK, Wolfgang
This work focuses on wideband intelligent reflecting surface (IRS)-aided multiuser MIMO systems. One of the major challenges of this scenario is the joint design of the frequency dependent base station (BS) precoder and user filters, and the IRS phase-shift matrix which is frequency flat and common to all the users. In addition, we consider that the channel state information (CSI) is imperfect at both the transmitter and the receivers. A statistical model for the imperfect CSI is developed and exploited for the system design. A minimum mean square error (MMSE) approach is followed to determine the IRS phase-shift matrix, the transmit precoders, and the receiving filters. The broadcast (BC)-multiple access channel (MAC) duality is used to solve the optimization problem following an alternating minimization approach. Numerical results show that the proposed approach leads to substantial performance gains with respect to baseline strategies that neglect the inter-user interference and do not optimize the IRS phase-shift matrix. Further performance gains are obtained when incorporating into the system design the statistical information of the channel estimation errors.
Analysis of Heat Transfer Characteristics of a GnP Aqueous Nanofluid through a Double-Tube Heat Exchanger
(Nanomaterials 2021, 11(4), 844, 2021) Calviño, Uxía; Vallejo, Javier P.; Buschmann, Matthias H.; Fernández-Seara, José; Lugo, Luis
The thermal properties of graphene have proved to be exceptional and are partly maintained in its multi-layered form, graphene nanoplatelets (GnP). Since these carbon-based nanostructures are hydrophobic, functionalization is needed in order to assess their long-term stability in aqueous suspensions. In this study, the convective heat transfer performance of a polycarboxylate chemically modified GnP dispersion in water at 0.50 wt% is experimentally analyzed. After designing the nanofluid, dynamic viscosity, thermal conductivity, isobaric heat capacity and density are measured using rotational rheometry, the transient hot-wire technique, differential scanning calorimetry and vibrating U-tube methods, respectively, in a wide temperature range. The whole analysis of thermophysical and rheological properties is validated by two laboratories. Afterward, an experimental facility is used to evaluate the heat transfer performance in a turbulent regime. Convective heat transfer coefficients are obtained using the thermal resistances method, reaching enhancements for the nanofluid of up to 13%. The reported improvements are achieved without clear enhancements in the nanofluid thermal conductivity. Finally, dimensionless analyses are carried out by employing the Nusselt and Péclet numbers and Darcy friction factor.
Channel estimation and hybrid precoding for frequency selective multiuser mmWave MIMO systems
(IEEE, 2018-02) GONZÁLEZ-COMA, José P.; GONZÁLEZ-PRELCIC, Nuria; CASTEDO, Luis; HEATH, Robert W. Jr
Configuring the hybrid precoders and combiners in a millimeter wave (mmWave) multiuser (MU) multiple-input multiple-output (MIMO) system is challenging in frequency selective channels. In this paper, we develop a system that uses compressive estimation on the uplink to configure precoders and combiners for the downlink (DL). In the first step, the base station (BS) simultaneously estimates the channels from all the mobile stations (MSs) on each subcarrier. To reduce the number of measurements required, compressed sensing techniques are developed that exploit common support on the different subcarriers. In the second step, exploiting reciprocity and the channel estimates, the base station designs hybrid precoders and combiners. Two algorithms are developed for this purpose, with different performance and complexity tradeoffs: 1) a factorization of the purely digital solution, and 2) an iterative hybrid design. Extensive numerical experiments evaluate the proposed solutions comparing to state-of-the-art strategies, and illustrating design tradeoffs in overhead, complexity, and performance.
Design of Linear Precoders for Correlated Sources in MIMO Multiple Access Channels
(IEEE, 2018) SUÁREZ-PASCAL, Pedro; GONZÁLEZ-COMA, José P.; FRESNEDO, Óscar, CASTEDO, Luis
This work focuses on distributed linear precod ing when users transmit correlated information over a fading Multiple-Input and Multiple-Output Multiple Access Channel. Precoders are optimized in order to minimize the sum-Mean Square Error (MSE) between the source and the estimated symbols. When sources are correlated, minimizing the sum-MSE results in a non-convex optimization problem. Precoders for an arbitrary number of users and transmit and receive antennas are thus obtained via a projected steepest-descent algorithm and a low-complexity heuristic approach. For the more restrictive case of two single-antenna users, a closed-form expression for the minimum sum-MSE precoders is derived. Moreover, for the scenario with a single receive antenna and any number of users, a solution is obtained by means of a semidefinite relaxation. Finally, we also consider precoding schemes where the precoders are decomposed into complex scalars and unit norm vectors. Simulation results show a significant improvement when source correlation is exploited at precoding, especially for low SNRs and when the number of receive antennas is lower than the number of transmitting nodes.
Development of paraffinic phase change material nanoemulsions for thermal energy storage and transport in low–temperature applications
(Applied Thermal Engineering, 2019-05-27) David Cabaleiro, Filippo Agresti, Simona Barison, Marco A. Marcos, Jose I. Prado, Stefano Rossi, Sergio Bobbo, Laura Fedele
In this study, new phase change material nanoemulsions (PCMEs) were designed and characterized as possible storage and heat transfer media for low–temperature thermal uses. Water– and (ethylene glycol+water)–based emulsions with fine droplets of n–heptadecane and RT21HC commercial paraffin were produced by a solvent–assisted emulsification method. No phase separation or significant growth in PCM drops were observed for the prepared emulsions through storage, after freeze–thaw cycles and under mechanical shear. Phase change transitions were characterized and a significant sub–cooling was observed, with solidification temperatures up to 13 K below the melting point. One pure alkane and two commercial paraffin waxes with higher melting points were considered as nucleating agents to reduce sub–cooling effect. Although the emulsions exhibited diminutions in thermal conductivity up to 9% with respect to the carrier fluids used as base fluid, enhancements in energy storage capacity (considering an operational temperature interval equal to the sub–cooling) reached 26% in the case of RT21HC nanoemulsion based on the (ethylene glycol+water) mixture that contained 10% in mass of paraffin. In addition, the thermal reliability of the nanoemulsions was verified analyzing the changes in latent heat after storage and throughout 1000 thermal cycles.
Enhancing the Thermal Performance of a Stearate Phase Change Material with Graphene Nanoplatelets and MgO Nanoparticles
(ACS Applied Materials & Interfaces, 2020-08-17) Jose I. Prado, Luis Lugo
The effectiveness of dispersed nanomaterials to improve the thermal performance of phase change materials (PCMs) is well-proven in the literature. The proposal of new engineered nanoenhanced phase change materials (NePCMs) with customized characteristics may lead to more efficient thermal energy storage (TES) systems. This work is focused on the development of new NePCMs based on dispersions of graphene nanoplatelets (GnPs) or MgO nanoparticles in a stearate PCM. The new proposed materials were developed using the two-step method and acetic acid was selected as surfactant to improve the stability of the dispersions. An extensive characterization of the constitutive materials and the developed dispersions through different spectroscopy techniques is reported. Also, the GnPs nanopowder was explored by using the XPS technique with the aim to characterized the used carbon nanomaterial. The obtained spectra were discussed in terms of the chemical bonds related to the found peaks. The thermophysical profile (density, thermal conductivity, isobaric heat capacity and thermal diffusivity) was experimentally determined once the main components of the NePCMs were characterized and dispersions were designed and developed. The differentiated and distinguished effect of the dispersed GnPs and MgO in the properties of the NePCMs have focused the discussion. A comprehensive analysis of the measurements to elucidate the mechanism that promoted higher improvements using GnPs instead of MgO was performed.
Enzymatic cometabolic biotransformation of organic micropollutants in wastewater treatment plants: A review
(Bioresource Technology Volume 344, Part B, January 2022, 126291, 2021-11-06) Kennes-Veiga, David M.; González-Gil, Lorena; Carballa, Marta; Lema, Juan M.
Biotransformation of trace-level organic micropollutants (OMPs) by complex microbial communities in wastewater treatment facilities is a key process for their detoxification and environmental impact reduction. Therefore, understanding the metabolic activities and mechanisms that contribute to their biotransformation is essential when developing approaches aiming to minimize their discharge. This review addresses the relevance of cometabolic processes and discusses the main enzymatic activities currently known to take part in OMPs removal under different redox environments in the compartments of wastewater treatment plants. Furthermore, the most common methodologies to decipher such enzymes are discussed, including the use of in vitro enzyme assays, enzymatic inhibitors, the analysis of transformation products and the application of several -omic techniques. Finally, perspectives on major challenges and future research requirements to improve OMPs biotransformation are proposed.
Experimental study on thermophysical properties of alumina nanoparticle enhanced ionic liquids
(Journal of Molecular Liquids, 2019-07-07) Elena Ionela Cherecheş, Jose I. Prado, Marius Cherecheş, Alina Adriana Minea, Luis Lugo
In this experimental study, several alumina Nanoparticle Enhanced Ionic Liquids were prepared and studied in regard to their stability, pH, density and thermal conductivity. These new fluids were manufactured by dispersing aluminium oxide nanoparticles in different mixtures based on water and 1-ethyl-3-methylimidazolium methanesulfonate ionic liquid. Furthermore, thermophysical properties (density, thermal conductivity) of pure and binary mixtures with water and 1-ethyl-3-methylimidazolium methanesulfonate were studied in order to select and propose base fluids to design new advanced heat transfer fluids. The pH of the dispersions was determined as around 8.0 - 8.5. In regard to density, the overall [C2mim][CH3SO3] density is higher by 25% than that of water and the influence of ionic liquid density over the mixtures was found to be much higher than that of water, while for the alumina Nanoparticle Enhanced Ionic Liquids the density respects classical equations. Evaluation of thermal conductivity revealed an increase of up to 13% in thermal conductivity when nanoparticles are added to the base fluids and new correlations based on mass fraction and temperature were proposed.
Feeding composition and sludge retention time both affect (co-)metabolic biotransformation of pharmaceutical compounds in activated sludge systems
(2021) González-Gil, Lorena; Fernández-Fontaina, Eduardo; Singh, Randolph R.; Lema, Juan M.; Carballa, Marta; Aga, Diana S.
The role of heterotrophic and nitrifying microorganisms in the (co-)metabolic biotransformation of 10 pharmaceutically active compounds (PhACs) was investigated. To this aim, biotransformation assays were performed with heterotrophic and nitrifying sludge developed separately in a two-stage full-scale activated sludge system. Each stage was operated at different inflow wastewater characteristics and sludge retention times (on average 8 d and 35 d). The biotransformation capacity of each sludge was evaluated in the absence of primary substrate and in the presence of acetate and ammonium, to independently elucidate the co-metabolic role of heterotrophs and nitrifiers present in both sludges. Trimethoprim, diclofenac and carbamazepine were recalcitrant (removal < 5% after 1 d; biotransformation rate < 50 μg/g VSS⋅d) under all the tested conditions. High concentrations of caffeine, acetaminophen and iopromide were quickly biotransformed (> 80% after 1 d; > 800 μg/g VSS⋅d) in the absence of primary substrates. The heterotrophic sludge only showed a co-metabolic effect towards erythromycin, which increased its biotransformation rate between 43% and 53% when acetate and ammonium were supplied. In contrast, when stimulated, nitrifiers and slow-growing heterotrophs present in the nitrifying sludge co-metabolically biotransformed acetaminophen, ibuprofen and naproxen to a significant extent. Sulfamethoxazole was recalcitrant, except when the nitrifying sludge was fed with acetate (> 800 μg/g VSS⋅d), suggesting that slow-growing heterotrophs co-metabolically biotransformed it. This study provides evidence that biotransformation of PhACs depends on several metabolic activities, as the heterotrophic activity of the nitrifying sludge, which are not only determined by the SRT but also by the feeding composition.
Hybrid LISA for Wideband Multiuser Millimeter Wave Communication Systems under Beam Squint
(IEEE, 2019) GONZÁLEZ-COMA, José P.; ULTSCHICK, Wolfgang; CASTEDO, Luis
This work jointly addresses user scheduling and precoder/combiner design in the downlink of a wideband mil limeter wave (mmWave) communications system. We consider Orthogonal frequency-division multiplexing (OFDM) modulation to overcome channel frequency selectivity and obtain a number of equivalent narrowband channels. Hence, the main challenge is that the analog preprocessing network is frequency flat and common to all the users at the transmitter side. Moreover, the effect of the signal bandwidth over the Uniform Linear Array (ULA) steering vectors has to be taken into account to design the hybrid precoders and combiners. The proposed algorithmic solution is based on Linear Successive Allocation (LISA), which greedily allocates streams to different users and computes the corresponding precoders and combiners. By taking into account the rank limitations imposed by the hardware at transmission and reception, the performance loss in terms of achievable sum rate for the hybrid approach is negligible. Numerical experiments show that the proposed method exhibits excellent performance with reasonable computational complexity.
Hybrid or mono nanofluids for convective heat transfer applications. A critical review of experimental research
(Applied Thermal Engineering 23, 25 february, 117926, 2021) Vallejo, Javier P.; Prado, José I.; Lugo, Luis
Research on nanofluids has increased markedly in the last two decades. Initial attention has focused on conventional or mono nanofluids, dispersions of one type of solid nano-sized particles in a base fluid. Despite various challenges such as dispersion stability or increased pumping power, nanofluids have become improved working fluids for various energy applications. Among them, convective heat transfer has been the main research topic since the very beginning. Hybrid nanofluids, dispersions of two or more different nanoadditives in mixture or composite form, have received attention more recently. Research on hybrid nanofluids aims to further enhance the individual benefits of each single dispersion through potential synergistic effects between nanomaterials. Multiple experimental studies have been conducted independently analysing the convective heat transfer performance of mono or hybrid nanofluids for single-phase and two-phase convective heat transfer applications. However, there are still no general conclusions about which nanofluids, mono or hybrid, present better prospects. This review summarizes the experimental studies that jointly analyse both hybrid and mono nanofluids for these applications and the results are classified according to the heat transfer device used. Based on this criterion, three large groups of devices were noticed for single-phase convective heat transfer (tubular heat exchangers, plate heat exchangers and minichannel heat exchangers/heat sinks), while one group was identified for two-phase convective heat transfer (heat pipes). The main outcomes of these studies are summarized and critically analysed to draw general conclusions from an application point of view.
Hybrid Precoding with Time-Modulated Arrays for mmWave MIMO Systems
(IEEE, 2018) GONZÁLEZ-COMA, José P.; MANEIRO-CATOIRA, Roberto; CASTEDO, Luis
We consider the utilization of Time-Modulated Arrays (TMAs) as a simple and cost-effective approach to hybrid digital-analog precoding in millimeter wave (mmWave) Multiple-Input Multiple-Output (MIMO) systems. Instead of conventional Variable Phase Shifters (VPSs), our proposed TMA hybrid precoders use Radio Frequency (RF) switches followed by 1-bit VPSs in the analog part. We study the insertion losses at mmWave of a TMA Analog Precoding Network (APN) in terms of Sideband Radiation (SR) and hardware efficiency. In addition, we present different algorithms for the design of both the baseband and RF parts of a TMA hybrid precoder. The proposed methods exhibit different trade-offs between performance, complexity, and power efficiency. Finally, TMA precoders are compared to those implemented with conventional VPSs in terms of insertion losses, chip area and cost, concluding that precoding with TMAs is a competitive solution for mmWave MIMO systems.
Low-Complexity Distance-Based Scheduling for Multi-User XL-MIMO Systems
(IEEE, 2021) GONZÁLEZ-COMA, José P.; LÓPEZ MARTÍNEZ, F.J.; CASTEDO, Luis
We introduce Distance-Based Scheduling (DBS), a new technique for user selection in downlink multi-user commu nications with extra-large (XL) antenna arrays. DBS categorizes users according to their equivalent distance to the antenna array. Such categorization effectively accounts for inter-user interfer ence while largely reducing the computational burden. Results show that (i) DBS achieves the same performance as the reference zero-forcing beamforming scheme with a lower complexity; (ii) a simplified version of DBS achieves a similar performance when realistic spherical-wavefront (SW) propagation features are considered; (iii) SW propagation brings additional degrees of freedom, which allows for increasing the number of served users.
Microwave Assisted Alkaline Pretreatment of AlgaeWaste in the Production of Cellulosic Bioethanol
(Energies, 2021, 14, 5891, 2021) Maceiras, Rocío; Alfonsín, Víctor; Seguí, Luis; González, Juan F.
Biomass pretreatment has an important role in the production of cellulosic bioethanol. In this study, the effectiveness of microwave assisted alkaline pretreatment of algae waste was analysed. After pretreatment, the product was hydrolysed using sulphuric acid. The effects of microwave power, irradiating time, solid–liquid ratio and NaOH concentration were examined. Under the best conditions, the fermentable sugars were converted to cellulosic bioethanol using Saccharomyces Cerevisiae with a bioethanol yield of 1.93 0.01 g/g and a fermentation efficiency of 40.4%. The reducing sugars concentration was 30% higher than that obtained from conventional hydrolysis without pretreatment. The obtained results suggest that microwave assisted alkaline pretreatment is effective in improving the production of cellulosic bioethanol of algae waste compared to that without microwave effect. Considering energy consumption, low microwave power and short microwave irradiation time are favourable for this pretreatment.
OrthoSpar, a novel substructure concept for floating offshore wind turbines: Physical model tests under towing conditions
(Ocean Engineering 245 (2022) 110508, 2022) Büttner, T.; Pérez-Collazo, C.; Abanades, J.; Hann, M.; Harper, P.; Greaves, D.; Stiesdal, H.
An important condition of any port-assembled floating offshore wind turbine concept is the de-ballasted trans-port stage. As the hydrostatic and dynamic stability may vary greatly from the operational condition, it needs to be carefully investigated in early stages of the design-phase. In this work, physical modelling of the transport of the de-ballasted OrthoSpar device was carried out to determine roll and pitch RAOs, as well as load charac-teristics of the towing line. Towing was simulated with a stationary model being subjected to currents. To examine the influence of wave direction, a range of model orientations towards the incident waves were tested in still water and together with the simulated towing state. Roll and pitch motions were found to be highly dependent on the wave frequency and a result of a low damping ratio. The towing load amplitude was found to be influenced by the towing direction regarding the wave direction.
Power Efficient Scheduling and Hybrid Precoding for Time Modulated Arrays
(IEEE, 2020) GONZÁLEZ-COMA, José P.; CASTEDO, Luis
We consider power efficient scheduling and precoding solutions for multiantenna hybrid digital-analog transmission systems that use Time-Modulated Arrays (TMAs) in the analog domain. TMAs perform beamforming with switches instead of conventional Phase Shifters (PSs). The extremely low insertion losses of switches, together with their reduced power consumption and cost make TMAs attractive in emerging technologies like massive Multiple-Input Multiple-Output (MIMO) and millimeter wave (mmWave) systems. We propose a novel analog processing network based on TMAs and provide an angular scheduling algorithm that overcomes the limitations of conventional approaches. Next, we pose a convex optimization problem to determine the analog precoder. This formulation allows us to account for the Sideband Radiation (SR) effect inherent to TMAs, and achieve remarkable power efficiencies with a very low impact on performance. Computer experiments results show that the proposed design, while presenting a significantly better power efficiency, achieves a throughput similar to that obtained with other strategies based on angular selection for conventional architectures.
Preparation of Synthetic Slays to Remove Phosphates and Ibuprofen in Water
(Water 2021, 13, 2394, 2021) Devesa-Rey, Rosa; Val, Jesús del; Feijoo, Jorge; González-Coma, José P.; Castiñeira, Gonzalo; González-Gil, Lorena
The main objective of this study consists in the synthesis of a layered double hydroxide (LDH) clay doped with magnesium and aluminum in order to test the removal of phosphates and ibuprofen in water. Two different LDH composites are assessed: oven-dried (LDHD) and calcined (LDHC). Single adsorptions of phosphate and ibuprofen showed up to 70% and 58% removal in water, when LDHC was used. A poorer performance was observed for LDHD, which presented adsorption efficiencies of 52% and 35%, respectively. The simultaneous removal of phosphate and ibuprofen in water showed that LDHC allows a greater reduction in the concentration of both compounds than LDHD. Phosphate adsorption showed a close agreement between the experimental and theoretical capacities predicted by the pseudo-second-order model, whereas ibuprofen fitted to a first-order model. In addition, phosphate adsorption showed a good fit to an intraparticle diffusion model and to Bangham model suggesting that diffusion into pores controls the adsorption process. No other mechanisms may be involved in ibuprofen adsorption, apart from intraparticle diffusion. Finally, phosphate desorption could recover up to 59% of the initial concentration, showing the feasibility of the recuperation of this compound in the LDH.

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