Universidad Politécnica de Cartagena
Recent publications
Objective This study aims to investigate the degree of concentration and the competitive positions of the Baltic cruise port network. Methods A set of 29 Baltic ports are analysed, market concentration is evaluated using the analytical technique of the Herfindahl–Hirschman index, and competitive positions are determined through portfolio analysis based on the Boston Consulting Group matrix from 2000 to 2019. Results The Herfindahl–Hirschman index indicates that the Baltic cruise port system is unconcentrated with an average score of 0.11 for the analysed period, suggesting that eight of the twenty-nine ports are the dominant ports in the Baltic. Portfolio analysis results suggest that the hierarchy picture of competitive positions is dynamic and has changed over time. The Baltic cruise port system has a wide range of competitive positions. Kiel and Rostock becoming mature leaders is one of the most relevant changes in competitive positions. Implications of the research This study contributes to the literature not only by investigating the competitive positions of the second most important European operational area for cruise ships but also by filling the gap in research on the concentration and competitive strategic positions of Baltic cruise ports. This research allows seaport operators to visualise the position and progress of selected ports and predict the possible future seaport developments.
Renewable energy and, in particular, solar PV power, will play a crucial role in achieving net-zero emissions scenarios over the coming decades. Increasing the share of renewables in countries’ energy mixes is therefore a major challenge that must be urgently addressed. Within this context, the present work proposes an innovative tool that may accelerate the commissioning process of new solar PV power plants, in the form of an aggregated simulation model to assess a power plant's responses under the technical requirements established in the Spanish grid code. Our analyses, based on the comparison of the proposed model with a detailed model -both representing the same installation, show, from a quantitative and qualitative viewpoint, the accuracy and usefulness of the dynamic simulation model proposed. The aggregated model is shown to be ten times faster than the detailed equivalent, while the errors encountered in the simulation responses of both models are below 0.1% in all cases. It must also be noted that the present paper has allowed for the introduction, in the most recent version of the Spanish grid code and following approval of the Spanish Transmission System Operator (TSO), Red Eléctrica de España (REE), of this new and advantageous PV power plant model. Finally, it is worth mentioning that this study could form the basis for encouraging other countries to also adopt this model in their grid codes as a new tool to help integrate PV power into power grids.
The integration of new renewable power capacity into the grid constitutes an important challenge. Control issues become more complex, and the behavior of the new installations must be carefully assessed. In this sense, countries are establishing strict technical requirements when integrating new generation facilities into networks. Within this framework, given the great importance of photovoltaic solar energy as a clean electricity generation technology that is experiencing an unstoppable increase, the present paper evaluates the compliance of a real solar photovoltaic power plant with a number of technical requirements established in the new Spanish grid code. The analyses are performed by means of a photovoltaic power plant simulation model representing the actual facility. Thus, the main contribution of the present work consists of mitigating the lack of information about the performance of real experiences on the commissioning process of new renewable power plants, as well as demonstrating the usefulness of simulation models towards the modernization of grid codes. The results reveal the compliance of the renewable facility with the requirements analyzed, showing a maximum deviation of 0.47% in the case of the power–frequency requirements, and very accurate responses of the power plant under the reactive power control requisites. The results also reveal the facility fully covers the reactive power capability requirements. This paper has served not only to deepen the process of compliance with grid codes, but also to obtain approval from the Spanish transmission system operator, Red Eléctrica de España, to commission the power plant under consideration, this being a contribution of special interest to industry.
Over the last decades, renewable energy sources have increased considerably their generation share in power systems. As a consequence, in terms of frequency deviations, both grid reliability and stability have raised interest. By considering the absence of a consensual set of models for frequency control analysis, both for the different generation units (conventional and renewables) and the power system itself, this paper provides extensive and significant information focused on the models and parameters for studies about frequency control and grid stability. An extensive analysis of supply-side and power system modeling for frequency stability studies over the last decade is presented and reviewed. Parameters commonly used and assumed in the specific literature for such simulations are also given and compared. Modeling of generation units are described as well, including both conventional and renewable power plants.
The intensification of agricultural systems has caused a noticeable impact on agro-ecosystem services. Thus, the adoption of more sustainable agricultural practices such as crop diversification and reduction of external inputs represent an alternative strategy to minimize the impacts of intensive agricultural systems to the environment. This study aimed at evaluating the effects of crop rotation, conservation tillage, and low-input strategies on soil quality and farming performance using a set of 7 indicators based on a fuzzy logic approach. Data were collected from three Mediterranean long-term field experiments (LTEs) mostly oriented on cereal-based and vegetables cropping systems, located in Spain and Italy. The selected agro-environmental indicators clearly discriminated both from a geographical point of view and between monoculture and diversification, showing their suitability for the evaluation of diversified cropping systems. Such indicators highlighted that implementing crop diversification and reducing soil disturbance and chemical inputs enhanced soil quality. In this context, the most significant effects of diversified cropping strategies were the increase of crucial variables such as soil organic carbon (SOC), total nitrogen (TN), available phosphorus (Pav), and bulk density (BD) maintaining a stability of yields in all the three LTEs. These results provide strong evidence for the benefits of crop diversification in Mediterranean areas, highlighting that diversification represents a very promising strategy for more sustainable land management. Simple and composite indicators calculated using fuzzy method can be proposed as tool to assess the effects of diversification strategies on cropping systems performance. This approach can be used to define local solutions to help the re-design of cropping system through crop diversification transition across Europe.
In many applications, it is useful to use piecewise polynomials that satisfy certain regularity conditions at the joint points. Cubic spline functions emerge as good candidates having C² regularity. On the other hand, if the data points present discontinuities, the classical spline approximations produce Gibbs oscillations. In a recent paper, we have introduced a new nonlinear spline approximation avoiding the presence of these oscillations. Unfortunately, this new reconstruction loses the C² regularity. This paper introduces a new nonlinear spline that preserves the regularity at all the joint points except at the end points of an interval containing a discontinuity, and that avoids the Gibbs oscillations.
Phosphoric fertiliser has enormously contributed to agriculture; however, it generates five tonnes of phosphogypsum per ton of phosphoric acid synthesised. Phosphogypsum houses heavy metals and long-lived radioactive elements that represents an environmental issue requiring remediation. This paper presents a methodology for characterising phosphogypsum deposits using geophysical, geochemical, and statistical tools. Gamma-ray probes determined the abnormal radioactive zones within the phosphogypsum deposits while electrical resistivity tomography provided the geometry and distribution of the phosphogypsum deposits. Chemical results confirmed the high presence of heavy metals in the waste determining chromium as the most concentrated metal. Radiological measures indicate that the effective ambient dose equivalent average in the study area surface is approximately 8.5 times higher than the average for Europe. While at 1.0 m depth, in the phosphogypsum layer, the ambient dose equivalent average surpasses approximately 27 times the European average. Statistical correlation analysis supports that the radiation increases due to the uranium presence. This methodology might reduce time and cost avoiding the use of expensive traditional methods, and it is exportable to any deposit.
Félix Candela was a celebrated builder of thin concrete hyperbolic paraboloidal (hypar) shells with a lasting legacy on the rationalist architectural movement of the 20th century. The iconic 4-edged square hypar umbrella became one of Candela most defining works due to its structural efficiency, economical constructability, and striking elegance. Yet, Candela’s propensity for geometric experimentation led to the development of hexagonal 6-edged umbrellas capable of supporting parabolic discontinuities bisecting each tympan into two separate leaves for projects such as the Villahermosa Cathedral. Despite their artistic appearance, the structural performance of such topologies has never been subjected to rigorous analysis. In response, this paper presents a parametric investigation into the behaviour of 6-edged umbrellas, with and without parabolic bisectors, based on the proposed Villahermosa Cathedral. Finite element modelling revealed that hexagonal umbrellas exhibit markedly reduced deflections and stresses relative to equivalent square and triangular geometries, which are inversely related to the rise-to-area ratio. Unlike square umbrellas, critical bending moments are not sensitive to the edge rise for hexagonal 6-leaved variants with a constant projected area. However, 12-leaved hexagonal umbrellas adopting parabolic discontinuities suffer from disruptions in the flow of internal forces which ultimately yield a less efficient design.
Gypsum plasters are often used in buildings to provide passive protection against fire. This paper studies the fire behaviour of lightweight plasters containing moderate and high doses of expanded perlite. A propane flame was used to study the heat transmission through the plaster from the exposed side to the back side (2 cm thickness) using an IR camera. The microstructural and mineralogical variations induced by fire were analysed by scanning electron microscopy (SEM), X-ray diffraction (XRD), thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) using micro-perforated lids to improve the resolution (separation) of endothermic peaks of gypsum and bassanite. The exposed side of the plasters was drastically affected by fire regardless the concentration of perlite. However, important differences were found in the non-exposed side depending on the used amount of perlite. The greatest alteration was observed for high doses of perlite (6.5 % w/w) followed by control samples (0 % w/w). By contrast, using moderate doses of perlite (2.0 % w/w) the plasters showed excellent fire performance and CaSO4 2H2O was well preserved. The results suggest that the incorporation of lightweight materials to plasters should be carefully adjusted to ensure appropriate fire protection.
An improved analytic model to accurately determine the geometry of structures manufactured by molten drop-on-drop deposition is presented. This deposition mode allows quasi-spherical deposited droplets to be achieved and precise control over the geometry of the final manufactured structures. The model exactly conserves the volume of the deposited material and matches the solidification contact angle between consecutive deposited droplets, providing a precise geometrical description of the manufactured structures. The proposed model is validated using results of experiments performed with several materials for the deposited droplets and substrate, and droplet radii ranging from 40 to 800 μm. A good degree of agreement was found between theoretical and experimental results. A comparison with the well-established Gao&Sonin model shows that the proposed model represents a major improvement, and may be of great practical interest in industrial applications.
Since the beginning of the 21st century, the fields of astronomy and astrophysics have experienced significant growth at observational and computational levels, leading to the acquisition of increasingly huge volumes of data. In order to process this vast quantity of information, artificial intelligence (AI) techniques are being combined with data mining to detect patterns with the aim of modeling, classifying or predicting the behavior of certain astronomical phenomena or objects. Parallel to the exponential development of the aforementioned techniques, the scientific output related to the application of AI and machine learning (ML) in astronomy and astrophysics has also experienced considerable growth in recent years. Therefore, the increasingly abundant articles make it difficult to monitor this field in terms of which research topics are the most prolific or novel, or which countries or authors are leading them. In this article, a text‐mining‐based scientometric analysis of scientific documents published over the last three decades on the application of AI and ML in the fields of astronomy and astrophysics is presented. The VOSviewer software and data from the Web of Science (WoS) are used to elucidate the evolution of publications in this research field, their distribution by country (including co‐authorship), the most relevant topics addressed, and the most cited elements and most significant co‐citations according to publication source and authorship. The obtained results demonstrate how application of AI/ML to the fields of astronomy/astrophysics represents an established and rapidly growing field of research that is crucial to obtaining scientific understanding of the universe. This article is categorized under: Algorithmic Development > Text Mining Technologies > Machine Learning Application Areas > Science and Technology
The main problems of the traditional foundry dewaxing processes in fine arts workshops are the emission of gases, the loss of 80% of the wax, the high electrical consumption, and the high risks for the operators. The introduction of the microwave technology for dewaxing of ceramic shell molds allows to minimize some of these problems, although the use of electromagnetic susceptors that capture the radiated energy and transform it into heat is required. This article describes different microwave dewaxing tests using TiO2 and graphite as susceptors. The results obtained show that this technique is viable, allowing the casting process to be carried out with a low percentage of breakage problems in the mold and significantly reducing the emitted gases and electricity consumption. The technique allows to recover in the same operation around 90% of the wax used in small and medium format objects. The tests show that the selection of the material used as a susceptor, the area of application and the power regimes, are fundamental to enable a controlled, soft and non-aggressive dewaxing, both for the art molds and for the environment, as opposed to the traditional Flash Dewaxing technique. In this way, it is possible to change the foundry of ceramic shells for artworks to achieve high levels of performance and safety, and to save energy, time and materials.
In this study, we have presented two new alternative definitions corresponding to the basic definitions of the discrete delta and nabla fractional difference operators. These definitions and concepts help us in establishing a relationship between Riemann-Liouville and Liouville-Caputo fractional differences of higher orders for both delta and nabla operators. We then propose and analyse some convexity results for the delta and nabla fractional differences of the Riemann-Liouville type. We also derive similar results for the delta and nabla fractional differences of Liouville-Caputo 18128 type by using the proposed relationships. Finally, we have presented two examples to confirm the main theorems.
In this article, a nonlinear binary three-point non-interpolatory subdivision scheme is presented. It is based on a nonlinear perturbation of the three-point subdivision scheme : A new three point approximating C2 subdivision scheme, where the convergence and the stability of this linear subdivision scheme are analyzed. It is possible to prove that this scheme does not present Gibbs oscillations in the limit functions obtained. The numerical experiments show that the linear scheme is stable even in the presence of jump discontinuities. Even though, close to jump discontinuities, the accuracy is loosed. This order reduction is equivalent to the introduction of some diffusion. Diffusion is a good property for subdivision schemes when the discontinuities are numerical, i.e., they appear when discretizing a continuous function close to high gradients. On the other hand, if the initial control points come from the discretization of a piecewise continuous function, it can be interesting that the subdivision scheme produces a piecewise continuous limit function. For instance, in the approximation of conservation laws, real discontinuities appear as shocks in the solution. The nonlinear modification introduced in this work allows to attain this objective. As far as we know, this is the first subdivision scheme that appears in the literature with these properties.
A large number of material and process parameters affect both the part quality and the process performance in pressure die-casting (PDC) processes. The complex relations between most of these variables make PDC process optimisation a difficult issue which has been widely studied for many years. Although there are several analytical and numerical models to optimise certain process parameters, it is difficult to establish a specific operational configuration for PDC machines that ensures the joint optimisation of these variables. Therefore, in this study, some of these optimisation models have been implemented in a Decision Support System (DSS) that allows us to define an operational region that establishes a setup of machine parameters that ensures the manufacture of quality parts. By using this DSS, the user can set the values of the input variables related to the casting material, the die, or the casting machine. Then the corresponding calculations are made by the system and the results are expressed in terms of certain output variables such as the maximum filling time, maximum filling fraction, or the plunger velocity profile among others. The DSS allows the user to estimate the influence between input and output variables and find proper values for the input variables to achieve an optimum operational range. Consequently, improved process performance can be achieved taking into account productivity, part quality, and economic aspects.
A novel configuration for heat transfer enhancement in parabolic trough solar collector absorbers by the use of a ferrofluid and an external magnetic field generated by a current-carrying wire is analyzed using numerical simulation. This new configuration consists of a wire which varies its position periodically along the tube. The analysis is focused on the study of the thermal hydraulic characteristics, Nusselt number and friction factor, and the average flow patterns in the turbulent flow regime (15·103⩽Re⩽250·103). In addition, different parameters of the periodic wire are analyzed: wire pitch and position angle. Ferrofluid Fe3O4/Therminol 66 is considered for this application. Firstly, the effect of an increase on the magnetic field intensity is studied. The periodic wire configuration shows a higher increment of the Nusselt number in comparison to the straight wire for the same magnetic field increase. The results reveal that the presence of a non-uniform magnetic field generates a disturbed flow with a high velocity region near the current-carrying wire. The periodic wire configuration leads to a spatially-periodic behavior that increases the average Nusselt number and the friction factor, in comparison to a straight wire. Among the studied cases, the position angle θ=30° and the pitch length l=3D are found to provide the maximum Nusselt number (Nu = 244.25 for Re = 15000).
We show that any compact surface of genus zero in $${\mathbb {R}}^3$$ R 3 that satisfies a quasiconformal inequality between its principal curvatures is a round sphere. This solves an old open problem by H. Hopf, and gives a spherical version of Simon’s quasiconformal Bernstein theorem. The result generalizes, among others, Hopf’s theorem for constant mean curvature spheres, the classification of round spheres as the only compact elliptic Weingarten surfaces of genus zero, and the uniqueness theorem for ovaloids by Han, Nadirashvili and Yuan. The proof relies on the Bers-Nirenberg representation of solutions to linear elliptic equations with discontinuous coefficients.
The numerical modeling of solar-driven melting in a shell-and-tube accumulator using paraffin RT70HC as PCM is presented. The coupling between an array of evacuated-tube solar collectors and the TES system is implemented in ANSYS Fluent, allowing for the proper evaluation of the time-dependent heat transfer fluid inlet temperature. A simplification of the TES geometry to a 2D section is accomplished, and an algorithm for the continuous evaluation of the heat transfer fluid temperature across the tubes in a serpentine-type configuration is developed. Validation with experimental results obtained in an outdoor test rig is carried out in terms of PCM temperature evolution. The model provides simulation results in a realistic configuration to be used in low-temperature applications of thermal energy storage for the residential sector. Energy performance is discussed, and the influence of the physics of melting and buoyancy forces leading to stratification are also analyzed, using temperature and liquid fraction contours during the solar irradiance cycle. The numerical results confirmed that natural convection plays an important role during melting of PCM: the motion of the melting front from the bottom to the top of the accumulator is described. Moreover, the results show that an increase of 50% of the solar surface improves the thermal storage by 28.8%, while doubling the solar surface increases the stored energy by 46.2%.
The objectives of this research are: (1) to study the process of formation of efflorescence salt crusts (ESC) on the surface of tailings dams, (2) to evaluate the possibility of recovering critical metals and raw materials, and (3) to study its effectiveness as a tailings (MT) decontaminant. To achieve these objectives, the evaporation process of 3 columns containing a mixture of MT and distilled water was monitored under laboratory conditions. Two MT of the flotation process from the extraction of Pb-Zn were used, one from the Mazarrón mining district (MDM) and the other from the Cartagena-La Unión mining district (CLUDM). MT and ESC were analysed by particle size analysis, mineralogical and chemical composition, thermal analysis, pH, electrical conductivity, potential redox, gravimetric water content and suction. It is important to note that some critical metals according to the EU 2020 CRWs, list are present in the MT sample. A significant growth in Zn ESC concentration was observed during the experiment. The concentration in MDM ESC is 8% higher than in the MT and in the CLUDM ESC it is 10% higher than in the MT. These results, together with the fact that ESC is water soluble, indicate the great potential of ESC treatment for metal recovery and MT decontamination.
In the last decades, a considerable number of studies have been conducted to find the optimal locations for renewable energy facilities. The reviewed literature demonstrates how the combination of spatial representation computer tools, such as geographic information systems (GIS), with multi-criteria decision making (MCDM) methodologies, has successfully solved the problem of identifying optimal locations. Furthermore, since the appearance of fuzzy logic, the combination approaches have extended to the field of fuzzy sets to consider the imprecision and vagueness that some criteria may involve. In this paper, we propose a comparative analysis among fuzzy versions of MCDM methodologies, including GIS technologies, for the optimal site selection of offshore wind power plants. With this aim, we combined a classical pair-wise comparison method (AHP) with two distance-based approaches (TOPSIS and VIKOR), applying GIS software and comparing the two most extended fuzzy membership functions: triangular and linear. As a case study, this optimal location problem was applied to offshore wind site selection in the Gulf of Maine (USA). Initially, 56 alternatives for potential locations were identified from 22,331 km2 study area. After applying the AHP methodology, the weights of the criteria were obtained, turning out to be the wind speed and bathymetry the most important criteria. The results demonstrate the robustness of the fuzzy TOPSIS methodology against potential variations in the criteria weights, since the best alternatives (optimal locations) and almost 90% of the 25 top–ranked alternatives were matching. Likewise, the rankings of alternatives illustrate that the use of triangular or linear fuzzy membership functions does not cause significant differences after applying the fuzzy VIKOR methodology and ArcGIS software. In fact, the most appropriate alternative is the same for both cases, and there is only an exchange of positions among the top–ranked alternatives. The proposed solutions can be applied to other locations and both onshore and offshore installations.
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Jose A. Martinez
  • Business Economics
Rafael Asorey Cacheda
  • Department of Information and Communication Technologies
Marta Doval
  • Chemical and Environmental Engineering
Bernardo Martin-Gorriz
  • Agricultural Engineering
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Beatriz Miguel Hernández
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