High Output Power Research Articles

Efficiency of dry versus wet Er,Cr:YSGG laser debonding of lithium disilicate veneers using different power outputs

Since water is the chromophore for the erbium, chromium-yttrium scandium gallium garnet (Er,Cr:YSGG) laser, the laser energy reaching the restoration decreases as part of it is absorbed by water. Theoretically, reducing the water or implementing dry debonding could reduce the energy consumed by water, increasing laser efficiency. Studies on whether it is suitable for removing veneers without using coolant are lacking. The purpose of this in vitro study was to evaluate the debonding time, intrapulpal temperature, and translucency of veneers during wet versus dry debonding with an Er,Cr:YSGG laser using different power outputs. Sixty-three maxillary central incisors were flattened labially to receive ceramic specimens. After cementation, ceramic specimens were irradiated with an Er,Cr:YSGG laser for debonding with different power outputs and water percentages (N=70): subgroup A1, 4W and 1% water; A20, 4W and 20% water; A40, 4W and 40% water; B1, 5W and 1% water; B20, 5W and 20% water; B40, 5W and 40% water; C1, 6W and 1% water; C20, 6W and 20% water; C40, 6W and 40% water, and a control group of unbonded ceramic specimens. During debonding, the temperature rise and debonding time were evaluated, followed by the evaluation of the translucency and surface topography of the debonded specimens. Two-way analysis of variance (ANOVA) and the Dunnett test were used to analyze the data (α=.05). The mean intrapulpal temperature rise varied significantly among groups B and C (P<.001), with the highest mean temperature rise found in subgroup B1 (4.00 ±0.00 ºC) and the lowest mean temperature rise in subgroup C20 (1.20 ±0.45 ºC). For the debonding time, the mean values of time required for debonding varied significantly among different groups (P<.001), with the longest time recorded in subgroup A1 (333.4 ±74.8s) and the shortest time recorded in subgroup C20 (17.0 ±6.0s). Only subgroups C1 (18.89 ±0.2) and C40 (18.60 ±0.2) showed a significantly lower translucency than the control group (19.44 ±0.06) (P<.001). Dry Er,Cr:YSGG laser debonding resulted in increased intrapulpal temperature when using high power outputs, but without exceeding the critical threshold of dental pulp temperature. Dry debonding also limited the transmission of laser energy, affecting the debonding efficiency. A power output of 5W and 20% water can be considered efficient and safe laser parameters for debonding lithium disilicate veneers if their reuse is intended.

High-power diode laser spectrally narrowed with prism-etalon feedback.

A simple method for reducing the linewidth of a diode laser while maintaining high output power is described. It is based on a dispersive prism and a thin etalon for retroreflective feedback. The etalon creates two weak external cavities that provide spectral selectivity that is periodic with a period equal to the etalon's free spectral range. The method was applied to a multimode blue laser diode, which in the absence of feedback features a linewidth of several nanometers. The spectral properties of the laser were investigated for different etalon thicknesses and operating currents and tested in the presence of temperature fluctuations. With a SF11 equilateral uncoated prism near Brewster's angle and a 0.3 mm-thick uncoated fused silica etalon, the linewidth was reduced 20-fold to 70pm (3.6 cm-1) with an output power of 3W at a current of 2.15A. The largest diode current probed was 2.75A, which resulted in a linewidth of 100pm (5.1 cm-1) and an output power of 4W. In contrast to the use of, for example, a volume Bragg grating, a high degree of flexibility is afforded as the same prism-etalon pair can be used across the visible and near infrared.

High power spectral beam combining based on four long-wave infrared quantum cascade laser emitters

In this study, we experimentally investigate spectral beam combining based on four long-wave infrared quantum cascade lasers (QCLs) with three dichroic mirrors. The coating curves of the dichroic mirrors were finely designed according to the output spectrum of the four adopted QCLs, whose central wavelength locates in 7.5 μm, 8.3 μm, 9.0 μm and 10.2 μm, respectively. At room temperature the highest combined power reaches as high as 2.39 W, corresponding to a combining efficiency of 83.0 %, and meanwhile, the combined beam keeps a high beam quality with average M2 of 1.78. To the best of our knowledge, this is the highest output power among spectral beam combining for QCLs. Due to the advantage of wide spectral range, high power, high beam quality and compact size, this demonstration may meet the demands of medical diagnosis, remote sensing, free-space optical communication and military application.

Quantum dot fourth-harmonic colliding pulse mode-locked laser for high-power optical comb generation

Quantum-dot mode-locked lasers have advantages such as high temperature stability, large optical bandwidth, and low power consumption, which make them ideal optical comb sources, especially for wavelength-division multiplexing (WDM) telecommunications, and optical I/O applications. In this work, we demonstrate an O-band quantum dot colliding pulse mode-locked laser (QD-CPML) to generate optical frequency combs with 200 GHz spacing with maximum channels of 12 within 3 dB optical bandwidth. To achieve the high output power of individual comb lines, four channel conditions are implemented at central wavelength of 1310 nm for WDM transmission experiments. Each channel exhibits more than 10 dBm output power with 200 Gb/s PAM-4 and 270 Gb/s PAM-8 modulation capability via thin-film LiNbO3 Mach–Zehnder interferometer modulator without the requirement of any optical amplifications. This high-order QD-CPML is an ideal comb source for power-efficient optical interconnects and large bandwidth optical data transmission.

Facile and binder free nano-architecturing of anode with biocompatible g-C3N4-PPy for bacterial community enrichment and green energy generation in microbial fuel cells

The adoption of microbial fuel cells (MFCs) as waste-to-power solution for green energy generation while treating wastewater has attracted considerable attention. However, their wide-scale expansion requires further improvements, particularly by anode modifications which necessitates the development of anodes with large surface area, high electrical conductivity, and biocompatibility. Thus, herein, we present a novel and facile approach for the modification of carbon felt (CF) with graphitic carbon nitride (g-C3N4) and polypyrrole (PPy) to create a highly biocompatible anode following a binder-free route. Increased surface area with microporous rough surface of modified electrodes was confirmed through scanning electron microscopy (SEM) and atomic force microscopy (AFM). The g-C3N4@PPy-CF anode exhibited minimal water contact angle of 0.9°, in contrast to g-C3N4-CF (53.5°) and pristine CF (123.8°), indicating enhanced hydrophilicity essential for biofilm formation. The synergy between g-C3N4 and polypyrrole results in remarkable 80.7% higher power output (205.8 ± 7.36 mW/m2) and 47% higher current response (8.24 mA) as compared to pristine-CF. Efficient extracellular electron transport (EET) was enacted through better interfacial contact between electrode and electroactive bacteria (EAB), as evidenced by the minimal charge transfer resistance in g-C3N4@PPy-CF (4.21 Ω) and g-C3N4-CF (6.47 Ω), while 68% higher resistance (7.06 Ω) with pristine-CF. Microbial characterization of biofilms showed enrichment of electroactive bacteria, mainly the Proteobacteria phylum comprised 79–82% of total microbial community in modified electrodes while it was 74% in pristine CF. Acinetobacter (25.9%) and Desulfuromonas (17.9%) emerges as major electroactive genera on g-C3N4@PPy-CF contributing to its excellent bio-electrogenic performance. Thus, the low-cost, binder-free and facilely fabricated novel anode with stable electrogenic efficiency provides solution to longstanding challenge of suboptimal anode performance and opens new avenues for scale-up of sustainable energy generation from wastewater using MFCs.

Research on high beam quality 3 × 1 double-cone fiber signal combiner at 9 kW level

The fiber signal combiner with high output power and high beam quality is an urgent problem to be solved. In this paper, the structure of the double-cone (both the input fibers and the output fiber are tapered) is proposed to improve the beam quality of the fiber signal combiner, and it is verified by simulation and experiment, with the output fiber of 50 µm core tapered to 35 µm as an example. The prepared 3 × 1 double-cone fiber signal combiner reaches the output power of 8.64 kW, and the beam quality is measured as M2 = 2.91. Compared with the M2 = 3.41 of the fiber signal combiner with non-tapered output fiber, the M2 of the double-cone fiber signal combiner is reduced by 14.7%.

(Invited) The Impact ofin-Situ Activation of a C-Coated Catalyst on the Performance and Durability in Proton-Exchange Membrane Fuel Cells

Zero-emission electricity production for stationary or mobile applications can be realized by PEM fuel cells (PEMFCs), making it an important constituent for a renewable energy transition. Especially relevant in heavy-duty applications, a high power output at low current densities (related to a high mass activity, MA) under dry conditions is necessary to meet the efficiency targets.[1] In general, mass activity of a Pt/C catalyst can be negatively affected by ionomer poisoning, which describes the adsorption of ionomer endgroups onto the active Pt surface. This effect is especially pronounced for platinum supported on so-called solid carbon supports (e.g., Vulcan – Pt/V)[2] and by operating a fuel cell at low relative humidity (RH).[3] In order to minimize performance losses due to ionomer poisoning, concepts exist to shield the Pt particle from the ionomer, e.g., by alkanethiol masking[4] or by Pt-overlayers of porous SiO2 [5] and carbon derived from dopamine coatings.[6] In the latter case, the authors observed that it was possible to decrease ionomer poisoning and increase MEA performance under dry conditions. However, the transport of reactants to the Pt active sites is hindered by the carbon-overlayer, resulting in inferior high current density (HCD) performance.In this work, we developed multiple in-situ activation strategies for a carbon-coated Pt/V cathode catalyst, synthesized following an adapted protocol from Yamada et al.[6] Our results show that apart from a significantly improved MA, high current density performance is retained for an activated carbon-coated Pt/V (C-Pt/Vactivated, solid orange symbols in Figure 1). Compared to the Pt/V baseline catalyst (blue symbols), the MA can be increased by a factor of ~2 (C-Pt/Vactivated with im 0.9V = 248±9 A gPt -1 and Pt/V with im 0.9V = 110±13 A gPt -1) while simultaneously maintaining comparable high current density performance at 90% RH. However, as we will show, the beneficial effect of the activated carbon-coated Pt/V is particularly pronounced at operation under dry conditions, where C-Pt/Vactivated outperformed the Pt/V baseline over the entire current density range (e.g., by 80 mV at 1.7 A cm-2).Furthermore, possible limitations in terms of durability of the C-overlayer used in this work will be discussed. Figure 1: Differential-flow H2/Air performance at 80 °C/90% RH and 145 kPaabs for different cathode catalysts: baseline catalyst (Pt/V), C-coated Pt/V catalyst before activation (C-Pt/Vb efore activation), and activated C-coated Pt/V catalyst (C-Pt/Va ctivated). In the 5 cm2 active area single-cell, the catalyst loadings were 0.25 mgPt cm-2 and 0.05 mgPt cm-2 on cathode and anode, respectively; a 15 µm reinforced low-EW PFSA membrane was used.

High-power 940 nm DFB laser diode with large optical cavity.

High-power laser diodes with a stable wavelength and narrow linewidth are crucial for many applications. In this study, we introduce a first-order distributed feedback (DFB) grating into an asymmetric large-cavity laser diode operating around 940 nm. This design maintains high output power and offers a wide temperature locking range. The nearly sinusoidal shape first-order grating is fabricated by ultraviolet (UV) nanoimprint lithography, inductively coupled plasma (ICP) dry etching, and wet polishing. At a heat sink temperature of 25°C, the DFB laser diode, with a 200 µm stripe width and 4 mm cavity length, achieves a maximum output power of 24.8 W and a full width at half maximum (FWHM) of 0.4 nm under continuous-wave (CW) conditions. The maximum slope efficiency is calculated to be 1.04 W/A. At an output power of 10.7 W, the device reaches a peak wall-plug efficiency of 56%. Under quasi-continuous operation at 20 A, the laser output spectrum remains locked to the DFB grating over a temperature range from -10°C to 110°C, with a temperature coefficient of 0.062 nm/°C.

Theoretical Models for Performance Analysis of Spintronic THz Emitters

The terahertz (THz) region of the electromagnetic spectrum, spanning from 0.1 to 10 THz, offers unique opportunities for imaging, spectroscopy, and communication applications. However, the potential of THz technologies has been limited by the availability of efficient and versatile THz emitters. Spintronic THz emitters (STEs), leveraging the ultrafast dynamics of electron spins in magnetic materials, have emerged as a promising solution to this challenge. STEs offer significant advantages, including broad bandwidth, high power output, and room-temperature operation, positioning them at the forefront of THz technology development. Despite these advances, understanding the operational principles and improving the performance of STEs remain areas of active research. This review focuses on the theoretical models that describe the behavior of STEs, aiming to provide a comprehensive overview of the underlying physics and suggest directions for future enhancements. Through a detailed examination of these models, the review seeks to clarify the basics of the physics driving STE performance and highlight innovative strategies for their optimization and application expansion.

A review of single stage and multistage power converters for doubly fed induction generator integrated with power grid

The doubly fed induction generator (DFIG) is the most popular induction generator for onshore wind energy conversion system (WECS). Therefore, the purpose of research is to optimize power transfer from DFIG to power grid which requires frequency and voltage stability in power converters. Hence, the single-stage and multistage power converter topologies associated with grid-integrated DFIG are thoroughly examined in this work. In the single stage, a matrix converter and cycloconverter are used. Whereas, the multistage topologies include two-level back-to-back (2L-B2B) converter, Z-source converter, and multilevel converters. Furthermore, three-level neutral point clamped (3L-NPC) voltage source converters with battery energy storage systems (BESS) and modified superconducting fractional order terminal sliding mode controllers (MSTFOTSMC) have been used with multilevel converter both at rotor side converter (RSC) and grid side converter (GSC) for stabilized generated voltage and higher power output. The MATLAB simulation results demonstrate that the system effectively controls fluctuations in voltage and current thereby improving the power quality in unbalanced situations.

Conjointly active and passive modelings with deep neural networks as fully automated optimizations for upper-mid band 6G communications

Today wireless systems include the fifth and sixth generations (5G and 6G) technologies and are growing day by day that result in exponentially increasing data traffic. For providing a reliable and high performance radio frequency (RF) designs especially for 6G networks, amplifiers and antenna as active and passive components play important roles. In the 5G/6G communication systems, the propagation loss is considerably large and its compensation requires high output power generated from the amplifiers for guaranteeing the satisfied quality of transmitted signal. From another point of view, the installed antennas must be able to optimally manage the radiated signals and handle/compensate nonlinear performances of the RF circuitry. Hence, advanced modeling and multi-objective optimization algorithms are required for designing and optimizing high performance amplifiers and antennas in terms of output power, gain, efficiency, linearity, and bandwidth. Concurrently optimizing active and passive components is not straightforward and typically it requires additional efforts by the RF designers. To tackle this drawback, a two-step methodology is proposed: (1) configuring the initial structure of active and passive devices, and (2) sizing the configured devices. In this work, various methods are introduced for structuring the topology of circuits and then artificial intelligence, including machine learning and neural networks, is preferred among other surrogate modelling for sizing the designs. These neural networks are satisfied due to the accurate modeling responses and are able to provide an automated optimization process leads to employ multi-objective optimization methods. In this work, an automated optimization process for comprehensive design of high-performance amplifiers with antennas through bottom-up optimization (BUO) method and long short-term memory (LSTM)-based deep neural networks (DNNs) is proposed. At the output layer of DNNs, the multi-objective multi-verse optimizer (MOMVO) method is employed for optimizing various specifications of active device (i.e., amplifier), and passive device (i.e., antenna), concurrently. In the presented method, all the electromagnetic (EM) design rules are implemented which results in reducing simulation time in the harmonic balance simulation environment that also provides ready to fabricate layouts. The novelty consists of the all-inclusive style that (1) reduces the manual breaks, aka time-to-market, and (2) delivers ready-to-fabricate layouts of the device that exhibits global optimum performances, automatically. The validation of the proposed method is verified by designing and optimizing high power amplifier (HPA) with antenna in the frequency band from 9.0 GHz to 9.6 GHz, suitable for upper-mid band 6G communications.

Performance evaluation of photovoltaic-phase change material-thermoelectric system through parametric study

The dual-directional photovoltaic-thermoelectric generator (PV-TEG) system offers higher performance. However, there may be scenarios where the PV and TEG devices receive different levels of irradiation, leading to variations in performance. Under these conditions, the impact of using a PCM heat sink on the system's thermal management effectiveness has not yet been clearly established. Given these considerations, this study builds a coupled heat transfer-laminar flow-electromagnetic numerical model to explore the variations in system performance when subjected to different radiation intensities. The investigation also explores the implications of integrating PCM with varying thicknesses and melting points. Based on the obtained data, it can be concluded that when the PV and TEG devices are exposed to different radiation intensities, the system exhibits distinctly different performance. The highest power output and conversion efficiency are more than 0.16 W and 16 % for the PV device, and 0.08 W and 1 % for the TEG device. Furthermore, using thicker PCM allows the PV and TEG devices to maintain higher output power and efficiency for a longer period, surpassing the performance of systems with thinner PCM. Moreover, the PCM with a melting point of 301.15K has demonstrated the best potential for thermal management in the PV-TEG system.

Effects of acute simulated altitude on the maximal lactate steady state in humans.

We sought to determine the effects of acute simulated altitude on the maximal lactate steady state (MLSS) and physiological responses to cycling at and 10 W above the MLSS-associated power output (PO) (MLSSp and MLSSp+10, respectively). Eleven (4 females) participants (means [SD]; 28 [4] yr; V̇o2max: 54.3 [6.9] mL·kg-1·min-1) acclimatized to ∼1,100 m performed 30-min constant PO trials in simulated altitudes of 0 m sea level (SL), 1,111 m mild altitude (MILD), and 2,222 m moderate altitude (MOD). MLSSp, defined as the highest PO with stable (<1 mM change) blood lactate concentration ([BLa]) between 10 and 30 min, was significantly lower in MOD (209 [54] W) compared with SL (230 [56] W; P < 0.001) and MILD (225 [58] W; P = 0.001), but MILD and SL were not different (P = 0.12). V̇o2 and V̇co2 decreased at higher simulated altitudes due to lower POs (P < 0.05), but other end-exercise physiological responses (e.g., [BLa], ventilation [V̇e], heart rate [HR]) were not different between conditions at MLSSp or MLSSp + 10 (P > 0.05). At the same absolute intensity (MLSSp for MILD), [BLa], HR, and V̇E and all perceptual variables were exacerbated in MOD compared with SL and MILD (P < 0.05). Maximum voluntary contraction, voluntary activation, and potentiated twitch forces were exacerbated at MLSSp + 10 relative to MLSSp within conditions (P < 0.05); however, condition did not affect performance fatiguability at the same relative or absolute intensity (P > 0.05). As MLSSp decreased in hypoxia, adjustments in PO are needed to ensure the same relative intensity across altitudes, but common indices of exercise intensity may facilitate exercise prescription and monitoring in hypoxia.NEW & NOTEWORTHY This study demonstrates the power output and metabolic rate associated with the maximal lactate steady-state (MLSS) decline in response to simulated altitude; however, common indices of exercise intensity remained unchanged when cycling was performed at the work rate associated with MLSS at each simulated altitude. These results support previous studies that investigated the effects of hypoxia on alternative measures of the critical intensity of exercise and will inform exercise prescription/monitoring across altitudes.

High-power and efficiency W-band InAlGaN/AlN/GaN high-electron-mobility transistors for future high-capacity wireless communications

This study describes high-power and high-efficiency W-band InAlGaN/AlN/GaN high-electron-mobility transistors (HEMTs) for future sub-terahertz wireless communications. A low-thermal-budget selective-area growth (SAG) process was developed to obtain low contact resistance with low trap states. Transmission lines and substrate structures were optimized to obtain high-thermal conductivity and low substrate resonance. Consequently, a high output power of 28.7 dBm (742 mW), output power density of 4.6 W mm−1, and power-added efficiency (PAE) of 28.0% were achieved with pre-matched InAlGaN/AlN/GaN HEMTs at 90 GHz, which were superior combination of output power and PAE compared to the conventional high-temperature SAG process.

Regulation on reacting solvent engineering for scalable synthesis of Na3(VOPO4)2F@carbon nanotubes microsphere cathodes towards high-power and long-lifespan Na-ion batteries

Na3(VOPO4)2F (NVOPF) has emerged as a promising cathode candidate for commercial Na-ion batteries, ascribed to its elevated theoretical energy density and robust structure during the extraction/insertion of two Na+ ions. However, the intrinsic poor electronic conductivity and unscalable materials-preparation technologies make it unexpected to stand out in large-scale energy storage systems. In this study, a spray-drying process technology is implemented to synthesize Na3(VOPO4)2F@carbon nanotubes (NVOPF@CNTs) composite through the reduction of V5+ driven by methanol at room temperature, in which the shortened Na-ion transfer path and the rapid electron mobility are facilitated by the CNTs. The resulting NVOPF@CNTs electrode exhibits exceptional capabilities, including a high discharge capacity (122.1 mA h g−1 at 1 C= 120 mA h g−1), outstanding rate performance (103.1 mA h g−1 at 100 C), and an extended lifespan (71.8 % capacity retention after 6000 cycles at 20 C). Additionally, the Na3(VOPO4)2F//NaTi2(PO4)3 full battery system demonstrates impressive high-power output capabilities (93.3 mA h g−1, 20 C), versatile current switching adaptability, and robust cycling performance (75.3 % of 98.5 mA h g−1 for the initial capacity after 3000 cycles). The study underscores the potential of improving the preparation of vanadium-based phosphate materials to enhance the practical application of Na-ion batteries.

Polyphenol-Derived Carbonaceous Frameworks with Multiscale Porosity for High-Power Electrochemical Applications.

With the escalating global demand for electric vehicles and sustainable energy solutions, increasing focus is placed on developing electrochemical systems that offer fast charging and high-power output, primarily governed by mass transport. Accordingly, porous carbons have emerged as highly promising electrochemically active or supporting materials due to expansive surface areas, tunable pore structures, and superior electrical conductivity, accelerating surface reaction. Yet, while substantial research has been devoted to crafting various porous carbons to increase specific surface areas, the optimal utilization of the surfaces remains underexplored. This review emphasizes the critical role of the fluid dynamics within multiscale porous carbonaceous electrodes, leading to substantially enhanced pore utilization in electrochemical systems. It elaborates on strategies of using sacrificial templates for incorporating meso/macropores into microporous carbon matrix, while exploiting the unique properties of polyphenol moieties such as sustainable carbons derived from biomass, inherent adhesive/cohesive interactions with template materials, and facile complexation capabilities with diverse materials, thereby enabling adaptive structural modulations. Furthermore, it explores how multiscale pore configurations influence pore-utilization efficiency, demonstrating advantages of incorporating multiscale pores. Finally, synergistic impact on the high-power electrochemical systems is examined, attributed to improved fluid-dynamic behavior within the carbonaceous frameworks, providing insights for advancing next-generation high-power electrochemical applications.

Investigating the Role of Flow Plate Surface Roughness in Polymer Electrolyte Membrane Fuel Cells with the Use of Multiphysics Simulations

This study investigates the influence of surface roughness on the performance of polymer electrolyte membrane fuel cells (PEMFCs) through computational simulations using COMSOL Multiphysics. Two distinct gas flow channel (GFC) models of serpentine and parallel GFC structures were analysed, featuring various surface roughness levels to examine their impact on gas pressure and velocity dynamics. Rough surfaces are modeled using trigonometric functions to replicate machining-induced variations. Finite element simulations were conducted, assessing the time-dependent relationship between gas pressure and velocity while considering different electrode phase potentials as a function of surface roughness. Rough surfaces generally enhance mass transport, water management, and current distribution compared to smooth surfaces. The results indicated that a surface roughness of approximately 1 µm optimizes PEMFC performance by balancing pressure and velocity, enhancing electrochemical reactions, and reducing excessive pressure drops within the cell. Notably, the 0.7 V operating voltage was found to be the most efficient, achieving rapid stabilization of pressure and velocity levels swiftly. The findings underscore the importance of precise control over GFC roughness to enhance PEMFC performance gains in commercial applications, especially when multiple cells are stacked to achieve high power outputs.

Defying Gravity to Enhance Power Output and Conversion Efficiency in a Vertically Oriented Four-Electrode Microfluidic Microbial Fuel Cell.

High power output and high conversion efficiency are crucial parameters for microbial fuel cells (MFCs). In our previous work, we worked with microfluidic MFCs to study fundamentals related to the power density of the MFCs, but nutrient consumption was limited to one side of the microchannel (the electrode layer) due to diffusion limitations. In this work, long-term experiments were conducted on a new four-electrode microfluidic MFC design, which grew Geobacter sulfurreducens biofilms on upward- and downward-facing electrodes in the microchannel. To our knowledge, this is the first study comparing electroactive biofilm (EAB) growth experiencing the influence of opposing gravitational fields. It was discovered that inoculation and growth of the EAB did not proceed as fast at the downward-facing anode, which we hypothesize to be due to gravity effects that negatively impacted bacterial settling on that surface. Rotating the device during the growth phase resulted in uniform and strong outputs from both sides, yielding individual power densities of 4.03 and 4.13 W m-2, which increased to nearly double when the top- and bottom-side electrodes were operated in parallel as a single four-electrode MFC. Similarly, acetate consumption could be doubled with the four electrodes operated in parallel.

Performance analysis and optimization of an annular thermoelectric generator integrated with vapor chambers

To overcome the structural barriers of conventional annular thermoelectric generators (ATEGs), a new ATEG integrated with vapor chambers (VCs) is proposed in this paper, where the use of VCs enables a large hot-end contact area and a high temperature uniformity for traditional planar thermoelectric modules. Besides, a numerical model for the ATEG is built to conduct performance analysis and optimization under different parameters. Results suggest that the heat absorption of the ATEG increases with the increase of the number of VCs, thereby boosting the output power of the system. When the flow rate is lower than 35 g/s, the optimal number of VCs for the ATEG is 12, with the highest output power, conversion efficiency, net power, and net efficiency of 287.3 W, 5.36 %, 214.1 W, and 3.99 %, respectively, at the exhaust temperature of 800 K. Besides, the exhaust temperature does not affect the optimal result, while the ATEG should be designed with fewer VCs when applied to the waste heat recovery with a larger flow rate. The increase in the VC thermal conductivity is not related to heat absorption but can improve temperature uniformity, and the thermal conductivity of 4000 W m−1 K−1 already meets performance requirements. This study provides new perspectives on the structure design of ATEGs.

Combustion characteristics and flame development of ammonia in an optical spark-ignition engine

To meet the ever-increasing serious challenges of global energy shortages and environmental pollution, it is urgent to apply alternative carbon-free fuels in the transportation field to reduce global carbon dioxide emissions. Ammonia is considered a prospective carbon-free fuel for engines due to its advantages in production, storage and transportation. The combustion and flame development characteristics of ammonia at different excess air ratios (λ) were researched in an optical spark ignition (SI) engine employing high-speed natural flame luminosity (NFL) imaging, OH* chemiluminescence imaging and flame spectra measurement. The combustion performance and flame development of ammonia were compared with methane. Results indicate that ammonia has the optimal combustion performance at λ = 0.9, with the most concentrated heat release, the highest power output and the best combustion stability. The peak natural flame luminosity intensity is highest at λ = 0.9, and the flame speed of 13.7 m/s is the fastest among testing cases, which results in the shortest ignition delay and combustion duration. OH* and NH2* spectra intensity of the ammonia flame were quantified and analyzed under different λ. The peak OH* intensity is highest at λ = 0.9, while the largest intensity of NH2* is obtained at the fuel-rich case (λ = 0.8). The maximum overall OH* chemiluminescence intensity at the same crank angles is obtained at λ = 0.9. Finally, despite the delayed spark timing, the combustion performance of methane presents higher indicated mean effective pressure (IMEP) and better combustion stability as well as faster flame propagation speed than ammonia at the same λ. In summary, ammonia shows the best combustion and flame development characteristics at λ = 0.9. The combustion performance of ammonia in SI mode is significantly worse than that of methane.