http://hdl.handle.net/10366/4102 Fri, 24 Apr 2026 12:21:44 GMT 2026-04-24T12:21:44Z http://hdl.handle.net/10366/170014 [EN]The annual performance, fuel consumption and emissions of a hybrid thermosolar central tower Brayton plant is analyzed in yearly terms by means of a thermodynamic model. The model constitutes a step forward over a previously developed one, that was satisfactorily validated for fixed solar irradiance and ambient temperature. It is general and easily applicable to different plant configurations and power output ranges. The overall system is assumed as formed by three subsystems linked by heat exchangers: solar collector, combustion chamber, and recuperative Brayton gas-turbine. Subsystem models consider all the main irreversibility sources existing in real installations. This allows to compare the performance of a real plant with that it would have in ideal conditions, without losses. Furthermore, the improved version of the model is capable to consider fluctuating values of solar irradiance and ambient temperature. Numerical calculations are presented taking particular parameters from a real installation and actual meteorological data. Several cases are analyzed, including plant operation in hybrid or pure combustion modes, with or without recuperation. Previous studies concluded that this technology is interesting from the ecological viewpoint, but that to be compelling for commercialization, global thermal efficiency should be improved (currently yearly averaged thermal efficiency is about 30% for recuperative plants). We analyze the margin for improvement for each plant subsystem, and it is concluded that, the Brayton heat engine, by far, is the key element to improve overall thermal efficiency. Numerical estimations of achievable efficiencies are presented for a particular plant and real meteorological conditions. Wed, 15 Feb 2017 00:00:00 GMT http://hdl.handle.net/10366/170014 2017-02-15T00:00:00Z http://hdl.handle.net/10366/169326 [EN]Moiré superlattices formed at the interface between stacked 2D atomic crystals offer limitless opportunities to design materials with widely tunable properties and engineer intriguing quantum phases of matter. However, despite progress, precise probing of the electronic states and tantalizingly complex band textures of these systems remain challenging. Here, we present gate-dependent terahertz photocurrent spectroscopy as a robust technique to detect, explore, and quantify intricate electronic properties in graphene moiré superlattices. Specifically, using terahertz light at different frequencies, we demonstrate distinct photocurrent regimes, evidencing the presence of avoided band crossings and tiny (∼1 to 20 meV) inversion-breaking global and local energy gaps in the miniband structure of minimally twisted graphene and hexagonal boron nitride heterostructures, key information that is inaccessible by conventional electrical or optical techniques. In the off-resonance regime, when the radiation energy is smaller than the gap values, enhanced zero-bias responsivities arise in the system due to the lower Fermi velocities and specific valley degeneracies of the charge carriers subjected to moiré superlattice potentials. In stark contrast, the above-gap excitations give rise to bulk photocurrents-intriguing optoelectronic responses related to the geometric Berry phase of the constituting electronic minibands. Besides their fundamental importance, these results place moiré superlattices as promising material platforms for advanced, sensitive, and low-noise terahertz detection applications. Wed, 01 Jan 2025 00:00:00 GMT http://hdl.handle.net/10366/169326 2025-01-01T00:00:00Z http://hdl.handle.net/10366/168013 [EN]As grid parity is reached in many countries, photovoltaic self-consumption is raising great interest. Currently, there is a big number of new projects being developed in Spain thanks to the new regulation. From the experience of the monitoring of one full year of operation of a self-consumption PV plant in a university building, a regulatory, energy, and economic analysis is made for this type of building. It has been carried out by simulating the behavior of the building with installations within the range of PV powers allowed in the Spanish regulation. The analysis shows the good fitting between the new Royal Decree of Self-Consumption and the new Building Code. The economic analysis proves that the new simplified compensation method gives the best economic return for this use of the buildings when the PV production is matched with the consumption. The time of return of investment is between 8 and 9 years, and the levelized cost of electricity (LCOE) is into the range of the pool market price of electricity. These results show the profitability of PV self-consumption for this type of building. Thu, 04 Feb 2021 00:00:00 GMT http://hdl.handle.net/10366/168013 2021-02-04T00:00:00Z http://hdl.handle.net/10366/168007 [EN]The deployment of distributed and affordable renewable energy has led to the development of the prosumer concept in the field of energy. To better understand its relevance and to analyse the main trends and research developments, a systematic literature review was performed. This work gathered 1673 articles related to this topic that were analysed following the PRISMA methodology with the help of VOSviewer 1.6.18 bibliometric software. These papers are classified into four clusters: smart grids, microgrids, peer to peer networks, and prosumers. The first two clusters show a certain degree of maturity, while the latter maintain a growing interest. The analysis of the articles provides a broad view of the prosumer’s role in energy and its potential, which is not limited to simple energy exchanges. Furthermore, this systematic review highlights the challenges, not only technical but also in terms of electricity market design and social aspects. The latter require further research, as society is undergoing a paradigm shift in the way in which energy is produced and used. How this shift occurred will determine whether it can lead to true prosumer empowerment and a fairer energy transition. Tue, 04 Jul 2023 00:00:00 GMT http://hdl.handle.net/10366/168007 2023-07-04T00:00:00Z http://hdl.handle.net/10366/166706 [EN] The photothermal conversion in plasmonic nanohelices is studied, unveiling how helical nanostructures made from metals with a notable interband activity -such as cobalt (Co) and nickel (Ni)- exhibit a remarkable temperature rise 𝚫T up to ≈1000 K under illumination. Such outstanding 𝚫T values exclusively occur at wavelengths close to their localized plasmon resonances (𝚫T is significantly lower off resonance), and therefore the photothermal conversion of these nanoparticles is spectrally selective. The exceptional and spectrally selective temperature rise is demonstrated at near infrared (NIR) wavelengths, which prompts the use of Co and Ni helical nanoparticles in a wide range of photothermal applications including solar energy conversion, seawater desalination, catalysis, or nanomedicine. Mon, 01 Jan 2024 00:00:00 GMT http://hdl.handle.net/10366/166706 2024-01-01T00:00:00Z http://hdl.handle.net/10366/163269 [EN]Solar cells rely on photogeneration of charge carriers in p-n junctions and their transport and subsequent recombination in the quasineutral regions. Several basic issues concerning the physics of the operation of solar cells remain obscure. This paper discusses some of those unsolved basic problems. In conventional solar cells, recombination of photogenerated charge carriers plays a major limiting role in the cell efficiency. High quality thin-film solar cells may overcome this limit if the minority diffusion lengths become large as compared to the cell dimensions, but, strikingly, the conventional model fails to describe the cell electric behavior under these conditions. A new formulation of the basic equations describing charge carrier transport in the cell along with a set of boundary conditions is presented. An analytical closed-form solution is obtained under the linear approximation. It is shown that the calculation of the open-circuit voltage of the solar cell diode does not lead to unphysical results in the new given framework. Sun, 01 Jan 2023 00:00:00 GMT http://hdl.handle.net/10366/163269 2023-01-01T00:00:00Z http://hdl.handle.net/10366/163234 [EN]In recent years, graphene field-effect-transistors (GFETs) have demonstrated an outstanding potential for terahertz (THz) photodetection due to their fast response and high-sensitivity. Such features are essential to enable emerging THz applications, including 6G wireless communications, quantum information, bioimaging and security. However, the overall performance of these photodetectors may be utterly compromised by the impact of internal resistances presented in the device, so-called access or parasitic resistances. In this work, we provide a detailed study of the influence of internal device resistances in the photoresponse of high-mobility dual-gate GFET detectors. Such dual-gate architectures allow us to fine tune (decrease) the internal resistance of the device by an order of magnitude and consequently demonstrate an improved responsivity and noise-equivalent-power values of the photodetector, respectively. Our results can be well understood by a series resistance model, as shown by the excellent agreement found between the experimental data and theoretical calculations. These findings are therefore relevant to understand and improve the overall performance of existing high-mobility graphene photodetectors. Mon, 01 Jan 2024 00:00:00 GMT http://hdl.handle.net/10366/163234 2024-01-01T00:00:00Z http://hdl.handle.net/10366/162271 [EN]Plasma waves in semiconductor gated 2-D systems can be used to efficiently detect Terahertz (THz) electromagnetic radiation. This work reports on the response of a strained-Si Modulation-doped Field-Effect Transistor (MODFET) under front and back sub-THz illumination. The response of the MODFET has been characterized using a two-tones solid-state continuous wave source at 0.15 and 0.30 THz. The DC drain-to-source voltage of 500-nm gate length transistors transducing the sub-THz radiation (photovoltaic mode) exhibited a non-resonant response in agreement with literature results. Two configurations of the illumination were investigated: (i) front side illumination in which the transistor was shined on its top side, and (ii) back illumination side where the device received the sub-THz radiation on its bottom side, i.e., on the Si substrate. Under excitation at 0.15 THz clear evidence of the coupling of terahertz radiation by the bonding wires was found, this coupling leads to a stronger response under front illumination than under back illumination. When the radiation is shifted to 0.3 THz, as a result of a lesser efficient coupling of the EM radiation through the bonding wires, the response under front illumination was considerably weakened while it was strengthened under back illumination. Electromagnetic simulations explained this behavior as the magnitude of the induced electric field in the channel of the MODFET was considerably stronger under back illumination. Wed, 01 Jan 2020 00:00:00 GMT http://hdl.handle.net/10366/162271 2020-01-01T00:00:00Z http://hdl.handle.net/10366/162266 [EN]An asymmetric dual-grating gate bilayer graphene-based field effect transistor (ADGG-GFET) with an integrated bowtie antenna was fabricated and its response as a Terahertz (THz) detector was experimentally investigated. The device was cooled down to 4.5 K, and excited at different frequencies (0.15, 0.3 and 0.6 THz) using a THz solid-state source. The integration of the bowtie antenna allowed to obtain a substantial increase in the photocurrent response (up to 8 nA) of the device at the three studied frequencies as compared to similar transistors lacking the integrated antenna (1 nA). The photocurrent increase was observed for all the studied values of the bias voltage applied to both the top and back gates. Besides the action of the antenna that helps the coupling of THz radiation to the transistor channel, the observed enhancement by nearly one order of magnitude of the photoresponse is also related to the modulation of the hole and electron concentration profiles inside the transistor channel by the bias voltages imposed to the top and back gates. The creation of local n and p regions leads to the formation of homojuctions (𝑛𝑝 , 𝑝𝑛 or 𝑝𝑝+) along the channel that strongly affects the overall photoresponse of the detector. Additionally, the bias of both back and top gates could induce an opening of the gap of the bilayer graphene channel that would also contribute to the photocurrent. Mon, 01 Jan 2024 00:00:00 GMT http://hdl.handle.net/10366/162266 2024-01-01T00:00:00Z http://hdl.handle.net/10366/162257 [EN]Herein, a simple terahertz (THz) receiver that uses subwavelength focusing of the THz beam on the detector area is proposed. As a proof of concept, a THz detection system with an original optical coupling scheme is implemented, where the signal to be detected is coupled to a THz detector through a mesoscale dielectric particle lens. Coherent detection is successfully demonstrated with an enhancement of the detector sensitivity of about 4.3 dB, compared with that of a direct detection system with the slight decreasing (≈1.67 times) of noise equivalent power value. The results show that the proposed method can reduce the size and increase the sensitivity of various THz systems, including imaging, sensing, and ranging, which would enable significant progress in different fields such as physics, medicine, biology, astronomy, security, etc. Wed, 01 Jan 2020 00:00:00 GMT http://hdl.handle.net/10366/162257 2020-01-01T00:00:00Z http://hdl.handle.net/10366/162235 [EN]A graphene-based field-effect-transistor with asymmetric dual-grating gates was fabricated and characterized under excitation of terahertz radiation at two frequencies: 0.15 THz and 0.3 THz. The graphene sheet was encapsulated between two flakes of h-BN and placed on a highly doped SiO2/Si substrate. An asymmetric dual-grating gate was implemented on the h-BN top flake. Even though no antenna was used to couple the incoming radiation, a clear gate-bias-dependent photocurrent was measured under excitation at 0.3 THz up to room temperature. We subsequently demonstrated that the device can be used for terahertz sensing and inspection of hidden metallic objects at room temperature. Wed, 01 Jan 2020 00:00:00 GMT http://hdl.handle.net/10366/162235 2020-01-01T00:00:00Z http://hdl.handle.net/10366/160682 [ES]Solar Photovoltaics has become one of the cheapest sources of electricity, with the potential to expand further through distributed generation. PV self-consumption can empower electricity customers, transforming them into prosumers, but its success relies on appropriate regulations, especially in the treatment of surplus electricity, being net-metering and net-billing the most common remuneration mechanisms. As the number of such PV systems increases, regulation needs to evolve and may affect profitability. The purpose of this research is to take advantage of the Spanish experience, establishing key points that can improve the regulation in Ecuador and other countries. The operation of these remuneration schemes is studied through an economic analysis for a wide range of residential customers, highlighting the low profitability of small PV installations, which are not profitable for the average consumer in Ecuador. Grid parity is reached in Spain over 3 kW and in Ecuador only for the highest electricity consumers. Although the net-billing model is more effective in promoting PV self-consumption in the medium and long term, it requires additional conditions, such as those in Spain. Some recommendations are proposed, e.g. additional support with progressive tax reductions, simplication of permits and the introduction of collective self-consumption. Sat, 01 Jan 2022 00:00:00 GMT http://hdl.handle.net/10366/160682 2022-01-01T00:00:00Z http://hdl.handle.net/10366/160559 [EN]An asymmetric dual-grating gate bilayer graphene-based field effect transistor (ADGG-GFET) with an integrated bowtie antenna was fabricated and its response as a Terahertz (THz) detector was experimentally investigated. The device was cooled down to 4.5 K, and excited at different frequencies (0.15, 0.3 and 0.6 THz) using a THz solid-state source. The integration of the bowtie antenna allowed to obtain a substantial increase in the photocurrent response (up to 8 nA) of the device at the three studied frequencies as compared to similar transistors lacking the integrated antenna (1 nA). The photocurrent increase was observed for all the studied values of the bias voltage applied to both the top and back gates. Besides the action of the antenna that helps the coupling of THz radiation to the transistor channel, the observed enhancement by nearly one order of magnitude of the photoresponse is also related to the modulation of the hole and electron concentration profiles inside the transistor channel by the bias voltages imposed to the top and back gates. The creation of local n and p regions leads to the formation of homojuctions ((Formula presented.), (Formula presented.) or (Formula presented.)) along the channel that strongly affects the overall photoresponse of the detector. Additionally, the bias of both back and top gates could induce an opening of the gap of the bilayer graphene channel that would also contribute to the photocurrent. Mon, 01 Jan 2024 00:00:00 GMT http://hdl.handle.net/10366/160559 2024-01-01T00:00:00Z http://hdl.handle.net/10366/159514 [EN]Nanometric spin structures such as skyrmions are covering a large area of applications related to new technologies. In this paper, we carry out a computational study on magnetic skyrmions behavior in Pt/CoFeB/Ru/CoFeB antiferromagnetic-exchange coupled systems. We aim to explore the possibilities of skyrmions nucleation in such multilayer. We demonstrated that skyrmions could be hosted in such system under specific conditions. We also studied the in-plane and out-of-plane reversal magnetization processes for these magnetic multilayers in order to check the skyrmion behavior under magnetic field loops. Fri, 01 Jan 2021 00:00:00 GMT http://hdl.handle.net/10366/159514 2021-01-01T00:00:00Z http://hdl.handle.net/10366/159428 [EN] The influence of thermal effects in AlGaN/GaN HEMTs is studied by means of Monte Carlo simulations. Measured output and transfer characteristics of a transistor are well reproduced using two techniques, a thermal-resistance method and an electrothermal model which solves the steady-state heat-conduction equation. The validity of the model to reproduce the experimental results is checked in two-terminal structures and transistors. Both methods are also employed to investigate in AC regime in terms of the elements of the small-signal equivalent circuit, providing a good agreement with experimental values, with no significant differences between the models. Apart from the expected decrease of transconductance and drain conductance, the gate to source capacitance is also found to be lowered by heating effects. Sat, 01 Jan 2022 00:00:00 GMT http://hdl.handle.net/10366/159428 2022-01-01T00:00:00Z http://hdl.handle.net/10366/159360 [EN]Manganite-based memristive devices have emerged as promising candidates for next-generation non-volatile memory and neuromorphic computing applications, owing to their unique resistive switching behavior and tunable electronic properties. This review explores recent innovations in manganite-based memristive devices, with a focus on materials engineering, device architectures, and fabrication techniques. We delve into the underlying mechanisms governing resistive switching in manganite thin films, elucidating the intricate interplay of oxygen vacancies, charge carriers, and structural modifications. This review underscores breakthroughs in harnessing manganite memristors for a range of applications, from high-density memory storage to neuromorphic computing platforms that mimic synaptic and neuronal functionalities. Additionally, we discuss the role of characterization techniques and the need for a unified benchmark for these devices. We provide insights into the challenges and opportunities associated with the co-integration of manganite-based memristive devices with more mature technologies, offering a roadmap for future research directions. Mon, 01 Jan 2024 00:00:00 GMT http://hdl.handle.net/10366/159360 2024-01-01T00:00:00Z