During alkaline water electrolysis, gas-evolving reactions generate bubbles that contribute to ohmic losses and an increase in activation and concentration overpotentials affect cell efficiency. Building on prior studies of bubble behavior, this work introduces two new aspects: spatially resolved void fraction near the electrode surface and locally generated gas volume flow as an indicator of surface activity. Both quantities are correlated with measured bubble dynamics. To achieve this, the oxygen evolution reaction is studied on a vertical wire electrode using Laser-Marked Shadowgraphy over a wide current-density range and multiple heights. Statistical distributions of bubble position, size, and velocity are obtained, together with local void fraction and gas volume flow. Their dependence on current density and vertical position is analyzed. Results show that bubble size primarily governs void fraction and, thus, bubble-induced resistance. A feedback mechanism is observed in which bubble-induced electrolyte flow enhances bubble transport, while bubble-bubble interactions cause a slowdown at high current densities. Spatial variations in generated gas volume flow reveal inhomogeneous apparent surface activity linked to convective transport. Overall, the results demonstrate the interplay of bubble dynamics and the electrochemical process and underscore the need for quantitative, spatially resolved, and statistically converged data for bubble dynamics.
The detection of high-impedance ground faults (HIFs) in resonant-grounded grids is challenging due to the reduction of fault currents and high fault impedance. Convolutional neural networks (CNNs) have been successfully applied for detection at 14, 400 Hz sampling rate. The expensive hardware required for high sampling rates poses an obstacle for larger-scale deployment. These requirements arise from the fact that it has not yet been conclusively determined which information content is relevant for HIF detection. This investigation aims to find the optimal sampling rate that balances detection accuracy and hardware requirements. A large set of measurement data is downsampled to lower rates to simulate reduced sampling rate measurements and upsampled back to the sampling rate expected by the CNN. Results of this study indicate that lower sampling rates achieve comparable performance and and can be used to reduce the high hardware requirements of AI-based HIF detection.
To make a hydrogen economy feasible, it is crucial to identify efficient hydrogen storage technologies. While storing hydrogen by hydrogenation of aromatic and dehydrogenation of alicyclic hydrocarbons is technically feasible, it is associated with a high energy demand for hydrogen release that limits the energetic efficiency of the storage cycle if no suitable source of waste heat is available. This study investigates whether the release of hydrogen through the cyclization of diols forming lactones is energetically more favorable. To this end, equilibrium constants for the dehydrogenation reactions at 298.15 K, 400 K, and 500 K were determined using a combination of combustion calorimetry, vapor pressure measurements, and quantum chemical calculations. The results reveal that, compared to aromatic hydrocarbons, the equilibrium position for the dehydrogenation of diols is significantly more favorable for hydrogen release, and the reaction enthalpies are markedly lower. The most advantageous properties were observed for the 1,4-butanediol/gamma-butyrolactone system. The reaction enthalpy per mole of hydrogen for the dehydrogenation at 298.15 K is exceptionally low, at 42.5 kJ & sdot;mol-1. High hydrogen yields can already be expected at temperatures below 400 K at ambient pressure. Under such conditions, hydrogen release from 1,4-butanediol can be carried out with low energy input for heating the hydrogen carrier and for providing the enthalpy of reaction. This reduced thermal demand contributes to a higher prospective energy efficiency of the respective hydrogen storage cycle, which may represent in some application scenarios a significant economic advantage. Thermodynamics of reversible hydrogen storage with LOHC systems: 1,4-butanediol/gamma-butyrolactone, 1,5-pentanediol/delta-valerolactone and 1,6-hexanediol/s-caprolactone was studied in this work.
Context. Hot subdwarf B (sdB) and O (sdO) type stars are evolved helium-burning objects that lost their hydrogen envelope before the helium flash when their progenitors were close to the tip of the red giant branch (RGB). They populate the extreme horizontal branch (EHB) in the Hertzsprung-Russell diagram (HRD). The mass distribution of canonical hot subdwarfs is expected to peak at the core mass required for helium ignition under degenerate conditions in the 0.45-0.5 M⊙ range. However, non-degenerate helium ignition from intermediate-mass progenitors and non-canonical pathways, such as the merger of helium white dwarfs and delayed helium flashes, are also expected to contribute to the hot subdwarf population. Aims. Using high-quality, homogeneous spectra of 335 hot subluminous star candidates from the Arizona-Montréal Spectroscopic Survey, we aim to improve our understanding of the atmospheric and stellar properties of hot subdwarf stars. Our focus is on the mass distribution of the different types of hot subdwarfs and their connections to the various formation scenarios. Methods. We used large grids of model atmospheres to fit the observed spectra and derived their atmospheric parameters: effective temperature (Teff), surface gravity, and helium abundance. The model grids were further utilized to fit the spectral energy distribution of each star and the Gaia parallax was used to compute the stellar parameters radius, luminosity, and mass. Results. Our spectroscopic sample mostly consists of H-rich sdBs and sdOs, but also contains 41 He-rich sdOs. Additionally, the sample includes 11 intermediate-helium stars and 19 horizontal branch objects with Teff > 14 kK. We detected the presence of helium stratification in six sdB stars with Teff around 30 kK, making them good candidates for also showing 3He enrichment in their atmospheres. Our sdB distribution along the EHB shows a gap near 33 kK, visible in both the Kiel (logg-Teff) diagram and HRD, corroborating previous observations and predictions. The mass distributions of H-rich sdBs and sdOs are similar and centered around 0.47 M⊙, consistent with the canonical formation scenario of helium ignition under degenerate conditions. Among the H-rich hot subdwarfs, we found no difference between the mass distributions of close binaries and apparently single stars. The He-sdOs have a significantly wider mass distribution than their H-rich counterparts, with an average mass of about 0.78 M⊙. In the HRD, the He-sdOs lie on the theoretical helium main sequence for masses between 0.6 and 1 M⊙. This strongly favors a merger origin for these He-rich objects. We identified a small number of candidate low-mass (<0.45 M⊙) sdBs located below the EHB that might have originated from more massive progenitors. These low-mass sdBs preferentially show low helium abundances. Finally, we identified more than 80 pulsating stars in our sample and found that they fall into well-defined p- and g-mode instability regions.
Therapeutic hyperthermia is typically applied using radiative or capacitive heating devices. Capacitive heating applies electrode pairs positioned on the patient to increase the tumor temperature and is considered a user-friendly method for relatively easy application of hyperthermia to both superficial and deep-seated tumor sites. However, this heating technique also has some limitations that are mainly inherent to the physics of capacitively induced heating. This review provides an overview of the principles of capacitive heating devices and the factors influencing the power absorption and resulting temperature distribution in the patient. Device parameters that strongly influence the achieved temperature distribution include the electrode sizes, the water bolus temperature for skin cooling, the cooling medium, and the output power. These parameters vary in commercially available devices. Complete characterizations of most of the commercially available capacitive devices are still lacking. Sparse phantom measurements in the literature characterizing capacitive devices indicate that therapeutic heating is at least possible for superficial tumors and tumors at intermediate depth. The dominant E‑field orientation with capacitive heating induces preferential subcutaneous fat heating, which limits deep heating and makes it most effective for slender patients (i.e., fat layer thickness < 1.5–2 cm). Numerical simulations have been helpful in optimizing device design, particularly in terms of bolus cooling, and are increasingly used for patient-specific treatment planning. Based on the physical characteristics and present literature, it can be concluded that appropriate patient selection is important to ensure effective and responsible use of capacitive heating devices.
