This paper presents the LED modelling work carried out in the AI-TWILIGHT H2020 Ecsel project. The new multi-domain LED model was developed based on measurement results of a comprehensive aging test carried out by an international collaboration of three academic laboratories. In addition to elapsed operating time, the new model also considers past stress conditions, i.e. forward current and temperature profiles. The predictive power was tested by fitting the model to measurement results performed during a pre-defined minimum test time, then comparing the resulting simulation results with real measurement data obtained at later times. The power of modelling different stress conditions was also verified by swapping the stress conditions for 2 x 2 sets of LED samples under test and continuing the aging test in this manner, thereby mimicking a sudden change in the LED samples’ mission profiles resulting also in a change of the consumption rate of their lifetime-budget.

Abstract Efficient modeling of PV systems is one of the key characteristics that enable this sector. The quality and robustness of the electrical model are strongly dependent on the accurate estimation of the physical characteristic parameters of the PV cell/module. However, considering the complexity of such a model, the computational cost is a significant concern. This paper presents effective techniques to minimize the computational cost by targeting multiple key aspects. One concept lies in the problem formulation phase, and the second one aims to optimize the number of the measured I-V points per curve, for which the physical parameters are to be extracted. The presented study has proved that using 20 I-V measured points per curve would reduce the computational cost for extracting the parameters by several folds compared to the typical situation in a measurement system that records 450 points per curve. Also, the suggested method shows its applicability in minimizing the error introduced by measurement systems.

Decreasing trend of thermal resistance was observed during the lifetime test of 2835 type mid-power LEDs. According to the results, the decrease in the radiant flux of the LEDs is mainly caused by the degradation of the wavelength converting phosphor. No significant changes could be observed in the electrical parameters, while the degradation of the main heat conduction path is not justified by any aging conditions. Moreover, the decrease in the value of the thermal resistance has a moderate correlation (with the value of 0.57) with the decrease in the radiant flux. In such a case, a suspicion arises that there could be a systematic error in the evaluation of the measurement results. This paper deals with investigation, revelation and possible elimination of this issue.

In this paper, a novel method is presented to estimate the parameters of the SPICE-like multi-domain model of light-emitting diode (LED) chips developed and proposed by the Delphi4LED project. The proposed estimation algorithm employs a modified Nelder-Mead method, as the gradient methods and the original version of Nelder-Mead fail to properly handle this problem. By using the new, modified Nelder-Mead method presented in this paper the parameters are estimated faster, compared to the previously used brute-force algorithm-based parameter extraction process, allowing the same precision of the SPICE-like multi-domain LED model. The modification of the parameter extraction procedure also allows speeding up and simplifying the multi-domain LED characterization method proposed earlier by the Delphi4LED project. The speed and robustness of the new model eliminate the need for time-consuming junction temperature control during measurements by employing a novel extraction strategy that seeks the global minimum, rather than relying on the composition of marginal minima.

This study investigates the transient flow dynamics and pressure interactions within Tesla valve configurations through comprehensive CFD simulations. Tesla valves offer efficient passive fluid control without the need for external power, making them favorable in various applications. Previous observations indicated that Tesla valves effectively reduce the amplitude of pressure transients, prolonging their duration and distributing energy over an extended timeframe. While suggesting a potential role for Tesla valves as pressure dampers during transient events, the specific mechanisms behind this behavior remain unexplored. This research focuses on elucidating the internal dynamics of Tesla valves during transient events, aiming to unravel the processes responsible for the observed attenuation in pressure transients. This study reveals the emergence of “pressure pockets” within Tesla valves, deviating from conventional uniform pressure fronts. These pockets manifest as discrete chambers with varying lengths and volumes, contributing to the non-uniform propagation of pressure throughout the system. This investigation employs advanced CFD simulations as a crucial tool to unravel the governing dynamics of transient flow within Tesla valve configurations. By elucidating underlying fluid dynamics, this study lays the groundwork for future Tesla valve design optimization, holding potential implications for applications where the control of transient flow events is crucial.

This paper briefly presents the LM80 based aging of a mid-power LED type, as well as the remote aging of the phosphor components used in its production. During the examination of the results, the effects of various stress conditions on aging are presented, such as temperature, forward current and blue light excitation. The tests are also supported by thermal imaging. The results of the case-level LEDs and the remote phosphor samples are presented separately, but the final conclusion is drawn by examining the separate measurement results together. Based on the test results, the possibility of an error in the real R th readings arises, the estimated value of which is derived in the paper.

In this paper we describe our contributions to a joint effort of three European academic laboratories within the AI-TWILIGHT H2020 ECSEL project, targeting an LED aging experiment that is aimed at gathering LED aging data sufficient enough to allow existing multi-domain LED models to be extended for describing mission-profile dependent aging. This experiment, referred to as the JointLab Project was defined as a mitigation action for the lack of sufficient aging data to support the LED aging modelling work within the AI-TWILIGHT project. Details and preliminary results of the new aging test are presented here along with an explanation of the likely causes of the observed degradation. The LED model fitted for the first 1000 hours of the aging test was verified for the test results of the first 2000 hours the results of which are also presented in this paper.

The Common Carotid Artery plays a vital role in supplying the brain, and its bifurcation is susceptible to vascular diseases. It is often analyzed using computational fluid dynamics (CFD) simulations, but it is challenging to prescribe boundary conditions that approach patient-specific flow conditions. We examined six boundary condition (BC) groups to determine the most accurate flow conditions aligning with available measured data. We conducted CFD simulations on a stenotic carotid bifurcation, using patient-specific Doppler ultrasound sonography velocity measurements at the inlet and both outlets. Three BC methods used defined inlet flow rate and either constant pressure (Basic), Windkessel model, or constant flow ratio (Murray) at the outlets. Three other methods were defined with flow rates at two boundaries and constant pressure at the third one. Defining two boundary flow rates shows the closest results to physiologically valid data. However, the difficult Doppler measurements on the outlet branches can inaccurately amplify velocity amplitudes and may detect a false flow direction. Therefore, cross-sectional corrections were implemented to fit the outlet and inlet flow rates, while keeping the measured velocity histories. Our results show that the Murray and Basic methods, while easily available, exclude carotid-specific flow conditions by disregarding downstream flow resistances. We conclude that a Windkessel-method can produce the most accurate results without forcing outflow conditions. However, usually unavailable measurements are necessary for its application. Simulations with outlet-defined volume flow can also produce physiologically valid solutions but require the application of cross-sectional geometry correction.

In this work the heat transfer of a PV solar module is investigated with CFD simulations. Conjugate heat transfer simulations are created, in which the air flow with the convective heat transfer around the solar panel and the heat transfer inside the PV module are both simulated. The radiative heat transfer is also modelled using the discrete ordinates method. First, the PV panel is simulated without wind flow, where the module induces a buoyant flow of warm air. Then, several cases are investigated with wind flow, considering different wind speeds and wind directions around the panel.

Accurate characterization of power LED chips is crucial for their efficient utilization. This research aims to determine the minimal number and handout of IVL measurements required for characterizing power LED chips. Through detailed IVL characterizations, we investigate the precision of the digital twin of the LED by selectively neglecting specific measurement points. With this method and with our novel parameter extraction method a remarkable reduction in the required number of measurements from 40 points to 15 is demonstrated while maintaining a low level of error within 1% for voltage and 5% for emitted light power. These findings carry significant implications, including substantial reductions in measurement time, cost savings, and enhanced efficiency in LED characterization. Based on our investigation of 60 power LED chips, we have determined that characterizing these chips can be achieved using a reduced set of measurements. Specifically, we found that measuring 15 points at three different temperatures and utilizing 4-5 forward currents is sufficient for effective characterization of the power LED chips.

A Magyarország számára legfontosabb megújulóenergia-termelési eszközöket tekinti át e tanulmány a magyarországi alkalmazásuk megtérülésének és potenciáljának figyelembevételével. Elemzi a napelem, a szélturbinák, a geotermikus erőművek, illetve a biomassza-erőművek megtérülési mutatóit. A szerző megállapítja, hogy magyar viszonylatban a napelemes termelés valóban a leggazdaságosabb megújulóenergia-előállítási módszer, ám további terjedését gátolja a piaci visszahatás, a hálózati problémák, továbbá az időjárás-függőség. E problémáknak egyik lehetséges megoldása a villamos energia tárolása. A tárolási módok közül áttekintjük a konvencionális megoldásokat, amelyek többé-kevésbé piacérettek; ezek a szivattyús tározós erőmű, az akkumulátor, a hidrogén-előállítás és a hőerőművel kombinált hőtároló. Ahhoz, hogy e tárolási módszerek megtérülését vizsgálni tudjuk, készítettünk egy egyszerű számítógépes szimulátort, amely képes meghatározni a tárolók használatából eredő hozzávetőleges jövedelmet a volatilisen változó árak mellett. A magyar adatokból kiindulva szimulációk segítségével meghatároztuk a különböző energiatárolók gazdasági mutatóit. A számítások eredménye alapján gazdaságilag javasolható legalább egy szivattyús tározós erőmű üzembe helyezése, vagy hőtározó kialakítása a Paks 2. atomerőműhöz kapcsolódóan.

The study reviews the most relevant renewable energy sources, focusing on their possible application, economic aspects and potential for Hungary. Feasibility and economic analysis is made for plant-sized photovoltaic devices, wind turbines, geothermal power plants and biomass power plants. It was found that solar cell technology has the highest revenue. However, its further spread is limited by several factors, such as the reactive effect on the energy market, grid problems, and weather dependency. A possible solution for these problems is to use energy storage systems. For the sake of simplicity, only the economically mature technologies are investigated, including pumped hydroelectric storage, batteries, green hydrogen production, and thermal energy storage connected to a heat power plant. The payback calculations require a simple simulation algorithm to calculate the revenue using Hungarian data. With the simulation, the most important economic indicators are estimated. As a result of these calculations, we suggest a pumped hydroelectric storage to be built, or if it is impossible, the Paks 2 nuclear plant should be completed with a thermal energy storage facility.

Background: The image reconstruction of stenotic carotid bifurcation can be managed by medical practitioners and non-medical investigators with semi-automatic or manual segmentation. The outcome of blood flow simulations may vary because of a single mean voxel difference along the examined section, possibly more in the stenotic lesions, which can lead to conflicting results regarding other research findings. The aim of our project is computational geometry reconstruction for blood flow simulations to make it suitable for comparison with plaque image analysis performed by commercially available software. In this paper, a comparison is made between the manual and semi-automatic segmentations performed by non-medical and medical investigators, respectively. Methods: 30 patients were classified into three homogeneous groups. Our group classification was based on the following parameters: plaque calcification score, thickness, extent, remodeling and plaque localization. The images in the first group were segmented individually by medical practitioners and experienced non-medical investigators, the second group was segmented collectively, and the last group was segmented individually again. Cross-sections along the centerline were extracted, then geometrical and statistical analyses were performed. Exploratory flow simulations were carried out on two patients to showcase the effect of geometrical differences on the hemodynamic flow field. Results: The largest centerline-averaged voxel difference between the medical and non-medical investigators occurred in the first group with a positive difference of 1.16 voxels. In the second and third groups, the average voxel difference decreased to 0.65 and 0.75, respectively. The example case from the first group showed that the difference in maximum wall shear stress in the middle of the stenosis is 30% with an average voxel difference of 1.73. Meanwhile, it can decrease to 4% when the average voxel difference is 0.64 for the example case from the third group. Conclusions: A collective review of the medical images should preceded the manual segmentations before applying them in computational simulations in order to ensure a proper comparison with plaque image analysis. Especially complex pathology such as calcifications should be segmented under medical supervision or after specific training. Non-significant differences in the segmentation can lead to significant differences in the computed flow field.

The phenomenon of nanoparticle aggregation is relevant for the applications in which magnetic nanoparticles are used and controlled by a magnetic field. In this work, the validity of an improved model for the magnetic nanoparticle aggregation in microchannels in an external magnetic field is investigated. The nanoparticles in the magnetic field are arranging into chains whose orientation is parallel to the external magnetic field in the absence of other forces. These chains were described in our previous work with a linear chain model, which was later used to establish a specialized macroscopic two-phase CFD model and solver. This solver is capable of modelling the MNP aggregation on the macroscopic scale without enormous computational demands. In this paper, the proposed linear chain model is compared to more detailed discrete particle model simulations, which were performed with an extended version of OpenFOAM’s discrete particle solver DPMFoam. In the simulations, several configurations were investigated, including single particle chains with different lengths or chain distributions at the different particle concentrations.

In this paper the magnetic nanoparticle aggregation procedure in a microchannel in the presence of external magnetic field is investigated. The main goal of the work was to establish a numerical model, capable of predicting the shape of the nanoparticle aggregate in a magnetic field without extreme computational demands. To that end, a specialized two-phase CFD model and solver has been created with the open source CFD software OpenFOAM. The model relies on the supposed microstucture of the aggregate consisting of particle chains parallel to the magnetic field. First, the microstructure was investigated with a micro-domain model. Based on the theoretical model of the particle chain and the results of the micro-domain model, a two-phase CFD model and solver were created. After this, the nanoparticle aggregation in a microchannel in the field of a magnet was modeled with the solver at different flow rates. Measurements with a microfluidic device were performed to verify the simulation results. The impact of the aggregate on the channel heat transfer was also investigated.

Large area multi-chip LED devices, such as chip-on-board (CoB) LEDs, require the combined use of chip-level multi-domain compact LED models (Spice-like compact models) and the proper description of distributed nature of the thermal environment (the CoB substrate and phosphor) of the LED chips. In this paper, we describe such a new numerical solver that was specifically developed for this purpose. For chip-level, the multi-domain compact modeling approach of the Delphi4LED project is used. This chip-level model is coupled to a finite difference scheme based numerical solver that is used to simulate the thermal phenomena in the substrate and in the phosphor (heat transfer and heat generation). Besides solving the 3D heat-conduction problem, this new numerical simulator also tracks the propagation and absorption of the blue light emitted by the LED chips, as well as the propagation and absorption of the longer wavelength light that is converted by the phosphor from blue. Heat generation in the phosphor, due to conversion loss (Stokes shift), is also modeled. To validate our proposed multi-domain model of the phosphor, dedicated phosphor and LED package samples with known resin-phosphor powder ratios and known geometry were created. These samples were partly used to identify the nature of the temperature dependence of phosphor-conversion efficiency and were also used as simple test cases to "calibrate" and test the new numerical solver. With the models developed, combined simulation of the LED chip and the CoB substrate + phosphor for a known CoB LED device is shown, and the simulation results are compared to measurement results.

The multi-domain operation of blue pump LEDs in a chip-on-board (CoB) device cannot be described properly by the existing chip level multi-domain compact models and a package compact thermal model because their operation is strongly affected by such parts of the environment as the substrate or the phosphor. In this case a mix of compact modelling and distributed modelling approaches need to be used. This paper presents a method that applies a chip level multi-domain LED compact model in a distributed way for the simulation of the LED chips and a Finite Volume Method based description for the whole package, including the temperature dependent light conversion taking place in the phosphor layer covering the entire LED chip array. The chip model describes the electrical, thermal and light output characteristics and their mutual dependence. This paper presents a new multi-domain phosphor model based on path tracing algorithms to describe the blue-to-yellow conversion, heat loss and light propagation in a distributed way for white CoB LEDs.

In a recent European H2020 project on LED characterisation and modelling (Delphi4LED, www.delphi4LED.eu)the major target was to represent physical LED package types by their digital twins in form of multi-domain compact models.In this project a specific task was devoted to CoB LEDs.Phosphor converted white CoB LEDs are large area devices on a ceramic substrate of high thermal conductivity, with a few dozens of LED chips mounted, covered by a phosphor layer.Such devices represent real technical challenges both in terms of their physical measurement and modelling.In this paper we report on our work regarding measurement and modelling of such devices performed in the context of the aforementioned project.

In this paper an extension of SYSRED (SYstem function Successive network REDuction) simulation tool is presented to handle the heat generated in the phosphor layer of a phosphor converted white LED device. The heating effect of light conversion and the attenuation of light flux is taken into account through a resistance network between the source (LED chip) and the active converting nodes. With this tool, the transfer function network for the lens of a Cree XP-E LED device is created and reduced so as to connect it to a similarly reduced LED model. The results from the reduced order model show a good agreement compared to measurement data obtained for devices examined in the Delphi4LED project.

In this paper a CFD model is presented to model the magnetic nanoparticle (MNP) suspension aggregation in a microfluidic device. The nanoparticles are injected to the microfluidic chip, which consists of serially connected microchambers. The device with the nanoparticle suspension is used to investigate enzyme-substrate reactions. The current work presents a dedicated two-phase solver for the nanoparticle suspension filling procedure of the chambers. The model relies on our measurements and a magnetorheological model, and it is created with the open source CFD software OpenFOAM.