Showing papers on "Power density published in 2008"

GaN-Based RF Power Devices and Amplifiers

Abstract: The rapid development of the RF power electronics requires the introduction of wide bandgap material due to its potential in high output power density, high operation voltage and high input impedance GaN-based RF power devices have made substantial progresses in the last decade This paper attempts to review the latest developments of the GaN HEMT technologies, including material growth, processing technologies, device epitaxial structures and MMIC designs, to achieve the state-of-the-art microwave and millimeter-wave performance The reliability and manufacturing challenges are also discussed

Solid oxide fuel cell with corrugated thin film electrolyte.

Abstract: A low temperature micro solid oxide fuel cell with corrugated electrolyte membrane was developed and tested. To increase the electrochemically active surface area, yttria-stabilized zirconia membranes with thickness of 70 nm were deposited onto prepatterned silicon substrates. Fuel cell performance of the corrugated electrolyte membranes released from silicon substrate showed an increase of power density relative to membranes with planar electrolytes. Maximum power densities of the corrugated fuel cells of 677 mW/cm2 and 861 mW/cm2 were obtained at 400 and 450 °C, respectively.

Microbial fuel cells based on carbon veil electrodes: Stack configuration and scalability

Abstract: SUMMARY The aim of this study was to compare the performance of three different sizes of microbial fuel cell (MFC) when operated under continuous flow conditions using acetate as the fuel substrate and show how small-scale multiple units may be best configured to optimize power output. Polarization curve experiments were carried out for individual MFCs of each size, and also for stacks of multiple small-scale MFCs, in series, parallel and series–parallel configurations. Of the three combinations, the series–parallel proved to be the more efficient one, stepping up both the voltage and current of the system, collectively. Optimum resistor loads determined for each MFC size during the polarization experiments were then used to determine the long-term mean power output. In terms of power density expressed as per unit of electrode surface area and as per unit of anode volume, the small-sized MFC was superior to both the medium- and large-scale MFCs by a factor of 1.5 and 3.5, respectively. Based on measured power output from 10 small units, a theoretical projection for 80 small units (giving the same equivalent anodic volume as one large 500 mL unit) gave a projected output of 10 W m � 3 , which is approximately 50 times higher than the recorded output produced by the large MFC. The results from this study suggest that MFC scale-up may be better achieved by connecting multiple small-sized units together rather than increasing the size of an individual unit. Copyright r 2008 John Wiley & Sons, Ltd.

Thermal Analysis of Radial-Flux Electrical Machines With a High Power Density

Abstract: A lumped-parameter-based thermal analysis applicable to radial-flux electrical machines with a high power density is presented. The modeling strategies using T-equivalent lumped-parameter blocks as well as conventionally defined thermal resistances are discussed. Special attention is paid to the modeling of the convective heat transfer in the air gap of radial-flux electrical machines at different rotational speeds. A brief overview of the evaluation of different loss components is given. The performance of the developed thermal model was verified by comparing the calculated temperature values with the measurements in three different applications.

Scaling up microbial fuel cells.

Abstract: The goal of this study was to quantify the relation between the surface area of the current-limiting electrode of a microbial fuel cell (MFC) and the power density generated by the MFC. Shewanella oneidensis (MR-1) was grown anaerobically in the anodic compartment of an MFC utilizing lactate as the electron donor. Graphite plate electrodes of various sizes were used as anodes. Commercially available air electrodes, composed of manganese-based catalyzed carbon bonded to a current-collecting screen made of platinum mesh, were used as cathodes, and dissolved oxygen was used as the cathodic reactant. The surface area of the cathode was always significantly larger than that of the anode, to ensure that the anode was the current-limiting electrode. The power density generated by the MFC decreased as the surface area of the anode increased, which fits well with the trend we detected comparing various published results. Thus, our findings bring into question the assertion that the overall power density generated ...

Poly(3,4-ethylenedioxythiophene) nanotubes as electrode materials for a high-powered supercapacitor

Abstract: We report the fast charging/discharging capability of poly(3,4-ethylenedioxythiophene) (PEDOT) nanotubes during the redox process and their potential application to a high-powered supercapacitor. PEDOT nanotubes were electrochemically synthesized in a porous alumina membrane, and their structures were characterized using electron microscopes. Cyclic voltammetry was used to characterize the specific capacitance of the PEDOT nanotubes at various scan rates. A type I supercapacitor (two symmetric electrodes) based on PEDOT nanotube electrodes was fabricated, and its energy density and power density were evaluated by galvanostatic charge/discharge cycles at various current densities. We show that the PEDOT-nanotube-based supercapacitor can achieve a high power density of 25?kW?kg?1 while maintaining 80% energy density (5.6?W?h?kg?1). This high power capability is attributed to the fast charge/discharge of nanotubular structures: hollow nanotubes allow counter-ions to readily penetrate into the polymer and access their internal surfaces, while the thin wall provides a short diffusion distance to facilitate the ion transport. Impedance spectroscopy shows that nanotubes have much lower diffusional resistance to charging ions than solid nanowires shielded by an alumina template, providing supporting information for the high charging/discharging efficiency of nanotubular structures.

Ambient RF Energy Scavenging: GSM and WLAN Power Density Measurements

Abstract: To assess the feasibility of ambient RF energy scavenging, a survey of expected power density levels distant from GSM-900 and GSM-1800 base stations has been conducted and power density measurements have been performed in a WLAN environment. It appears that for distances ranging from 25 m to 100 m from a GSM base station, power density levels ranging from 0.1 mW/m2 to 3.0 mW/m2 may be expected. First measurements in a WLAN environment indicate even lower power density values, making GSM and WLAN unlikely to produce enough ambient RF energy for wirelessly powering miniature sensors. A single GSM telephone however has proven to deliver enough energy for wirelessly powering small applications on moderate distances.

PWM Converter Power Density Barriers

Abstract: Power density of power electronic converters in different applications has roughly doubled every 10 years since 1970. Behind this trajectory was the continuous advancement of power semiconductor device technology allowing an increase of converter switching frequencies by a factor of 10 every decade. However, today's cooling concepts, and passive components and wire bond interconnection technologies could be major barriers for a continuation of this trend. For identifying and quantifying such technological barriers this paper investigates the volume of the cooling system and of the main passive components for the basic forms of power electronics energy conversion in dependency of the switching frequency and determines switching frequencies minimizing the total volume. The analysis is for 5 kW rated output power, high performance air cooling, advanced power semiconductors, and single systems in all cases. A power density limit of 28 kW/dm3@300 kHz is calculated for an isolated DC-DC converter considering only transformer, output inductor and heat sink volume. For single-phase AC-DC conversion a general limit of 35 kW/dm3 results from the DC link capacitor required for buffering the power fluctuating with twice the mains frequency. For a three-phase unity power factor PWM rectifier the limit is 45 kW/dm3@810 kHz just taking into account EMI filter and cooling system. For the sparse matrix converter the limiting components are the input EMI filter and the common mode output inductor; the power density limit is 71 kW/dm3@50 kHz when not considering the cooling system. The calculated power density limits highlight the major importance of broadening the scope of research in power electronics from traditional areas like converter topologies, and modulation and control concepts to cooling systems, high frequency electromagnetics, interconnection technology, multi-functional integration, packaging and multi-domain modeling and simulation to ensure further advancement of the field along the power density trajectory.

A Microfabricated PDMS Microbial Fuel Cell

Abstract: This paper presents a microfabricated polydimethylsiloxane (PDMS) microbial fuel cell (MFC) with embedded micropillar electrodes. This MFC is characterized by a flexible and biocompatible structure suitable for body implantation as a potential power source for implanted bioMEMS devices. The MFC is biocatalyzed by a microorganism, Saccharomyces cerevisiae, which converts chemical energy stored in glucose in the blood stream to electrical energy. The MFC is a laminate design, consisting of 0.2-mum-thick gold-evaporated PDMS anode and cathode separated by a Nafion 117 proton exchange membrane. These electrode surfaces feature more than 70 000 8- mum-high micropillar structures in a 1.2-cmtimes1.0-cm geometric area. The MFC is encapsulated by PDMS and has an overall size of 1.7 cm times 1.7 cm times 0.2 cm and a net weight of less than 0.5 g. Compared with recent silicon micromachined MFCs working in a phosphate buffer medium, the presented MFC with its micropillar structure showed a 4.9 times increase in average current density and a 40.5 times increase in average power density when operated at identical conditions. Using 15-muL droplet of human plasma, containing 4.2-mM blood glucose, the PDMS MFC demonstrated a maximum open circuit potential (OCP) of 488.1 mV, maximum current density of 30.2 muA/cm2, and a maximum power density of 401.2 nW/cm2. When the MFC operated continuously for 60 min, it showed an average OCP of 297.4 mV, average current density at 4.3 muA/cm2, and average power density of 42.4 nW/cm2 at 1-kOmega load. The coulombic efficiency of electron conversion from blood glucose was 14.7%.

Surface modification of aramid fiber by air DBD plasma at atmospheric pressure with continuous on-line processing

Abstract: Aramid fiber (AF) samples are modified by air dielectric barrier discharge (DBD) plasma at atmospheric pressure. Plasma discharge power density and sample treatment time are investigated as the major parameters. Modified AF is characterized by SEM, XPS and wettability tests. It is shown that the surface roughness is improved, the O/C atomic ratio is increased from 15.99% to 27.15%, and the surface wettability is also enhanced significantly. It is also found that the improvements of physical and chemical properties increased with increasing power density and treatment time. The experiment is operated in the case of continuous on-line processing with properly high speed of AF transmission. It is close to industrial production and application.

Efficient power scaling of laser radiation by spectral beam combining

Abstract: The possibility of achieving multikilowatt laser radiation by spectrally combining beams using volume Bragg gratings (VBGs) is shown. The VBGs recorded in a photothermorefractive glass exhibit long-term stability of all its parameters in high-power laser beams with power density >1 MW/cm2 in the cw beam of total power on a kilowatt level. We consider an architecture-specific beam-combining scheme and address the cross-talk minimization problem based on optimal channel positioning. Five-channel high efficiency spectral beam combining resulted in a >750 W near-diffraction-limited cw beam has been demonstrated experimentally.

High-Density Nanocrystalline Core Transformer for High-Power High-Frequency Resonant Converter

Abstract: A high-density transformer using nanocrystalline core is developed for a 30 kW, 200 kHz resonant converter. Loss models are established for nanocrystalline cores through experimental characterization. The important parasitic models are also developed considering litz wire effects. Following a minimum size design procedure, several transformers with both nanocrystalline and ferrite cores are designed and prototyped. While all transformers meet the converter performance requirement during testing, using nanocrystalline core can achieve a significantly higher power density even at 200 kHz.

A 9 keV electron-impact liquid-gallium-jet x-ray source

Abstract: The high brightness of synchrotron sources has enabled a significant growth of crystallography and microscopy in the 10 keV range. 1 Laboratory systems for such studies commonly rely on 8 keV emission from copper rotatinganode x-ray sources. The image quality of the laboratory systems is limited by the low brightness of the compact x-ray source. In the present paper we describe a compact liquidgallium-jet-anode electron-impact microfocus source with potential for three orders of magnitude higher brightness than present compact sources in this energy range. In electron-impact sources a multi-keV electron beam strikes a solid metal anode to produce x-rays via bremsstrahlung and line emission. The x-ray brightness basically scales with the electron-beam power density on the anode. The major factor limiting the brightness is the thermal load capacity of the anode since 99% of the kinetic energy of the electron beam is converted into heat. 2 For microfocus x-ray tubes with circular foci typically less than few tens of micrometers in diameter the maximum e-beam power loading is in the range of 0.4‐0.8 W per electron-beam diameter in micrometers full width at half maximum FWHM of Gaussian beam. 3 This corresponds to an e-beam power density at the anode of approximately 25‐50 kW/mm 2 for a1 0m source. For comparison, larger-spot systems based on modern rotating anodes are limited to 10‐20 kW/mm 2 for short exposures while large-spot stationary anodes usually operate at 1 kW/mm 2 . These numbers are not likely to increase significantly. 4 Angled viewing of a line focus can increase the apparent, but not the actual, power density for large-spot geometries. Here we demonstrate a 9 keV compact microfocus source which shows promise for three orders of magnitude higher brightness than present compact sources by operating at very high e-beam power density. This source is based on a liquid-gallium-jet anode and the spectrum is dominated by gallium line emission at 9.3 keV. The high e-beam power density capacity of this anode type is, in short, due to the different thermal properties of liquid-gallium versus solidmetal anodes and the possibility of a much higher anode speed with a liquid-jet anode. In addition and in contrast to existing techniques, the gallium-jet anode is completely regenerative and is therefore not as sensitive to anode damage as solid anodes. Electron-impact sources based on liquid-jet anodes have previously been demonstrated for 25 keV line emission using solder 5,6 and tin. 7 The high spatial coherence of this source allowed short-exposure-time x-ray phase imaging. 8 Gallium jets have been used as targets for femtosecond laser plasma 9 keV emission, although with much lower average brightness. 9

Characterisation of plasma induced during high power fibre laser welding of stainless steel

Abstract: The objective of this research is to obtain a fundamental knowledge of generation behaviour and ionised state of a plume or plasma induced during bead on plate welding of a 20 mm thick type 304 stainless steel plate with a 10 kW fibre laser beam of 0˙9 MW mm–2 power density, on the basis of 10 000 to 40 000 flames s–1 high speed video observation and spectroscopic analysis. The high power fibre laser produced a partial penetration weld of 12 mm in depth at 50 mm s–1 welding speed. According to the high speed observation pictures, the laser induced plume was repeatedly generated from a keyhole at the interval of about 0&dot5 ms period to reach 12 mm in maximum height. The spectroscopy indicated the line spectra of neutral atoms of alloying elements of type 304 such as iron (Fe), chromium (Cr) and manganese (Mn). However, ionised spectra of alloying elements and line spectra of argon (Ar) neutral atom were not apparently detected under these welding conditions. Furthermore, the temperature and the i...

High-power high-efficiency 2D VCSEL arrays

Abstract: We review recent results on high-power, high-efficiency two-dimensional vertical-cavity surface-emitting laser (VCSEL) arrays emitting around 980nm. Selectively oxidized, bottom-emitting single VCSEL emitters with 51% power conversion efficiency were developed as the basic building block of these arrays. More than 230W of continuous-wave (CW) power is demonstrated from a ~5mm x 5mm array chip. In quasi-CW mode, smaller array chips exhibit 100W output power, corresponding to more than 3.5kW/cm2 of power density. High-brightness arrays have also been developed for pumping fiber lasers, delivering a fiber output power of 40W. We show that many of the advantages of low-power single VCSEL devices such as reliability, wavelength stability, low-divergence circular beam, and low-cost manufacturing are preserved for these high-power arrays. VCSELs thus offer an attractive alternative to the dominant edge-emitter technology for many applications requiring compact high-power laser sources.

From Packaging to "Un"-Packaging - Trends in Power Semiconductor Modules

Abstract: Power semiconductor modules play a key role in power electronic systems. Their inherent advantage of integrating different power chips, circuits and sense, drive and protection functions into one sub-system with electrically insulated cooling has lead to a wide range of products, being different in size, power and function. This paper will provide an overview of today's power modules and packaging and interconnect technologies. Trends towards next generations of power modules will be highlighted. In the growing market of hybrid and electrical vehicles, products are emerging where power modules are "un-packaged" to arrive at highly integrated, compact sub-systems which are better suited for the harsh environmental conditions and the required power density than the classical power modules.

Fuel cell power generation from marine sediments: investigation of cathode materials.

Abstract: BACKGROUND: Marine sediment microbial fuel cells (MFC) utilise oxidisable carbon compounds and other components present in sediments on ocean floors and similar environments to produce power in conjunction with, principally, oxygen reduction at the cathode in the overlying water. The aim of the work was to investigate a range of cathode materials for sediment MFC, to achieve relatively high levels of power. RESULTS: Cell potential and power density performance data are reported for sediment MFC using cathodes of: carbon sponge, cloth and paper, graphite and reticulated vitreous carbon (RVC), Co and Fe-Co tetramethoxyphenyl porphyrin (FeCoTMPP) and platinised carbon and titanium. The anode was graphite cloth. After a period of stabilisation, open circuit voltages of 700 mV and maximum power densities of 62 mW m−2 were obtained, using FeCoTMPP. Relatively low cost carbon cathodes gave power densities of around 30 mW m−2. CONCLUSIONS: The study has shown that low level power can be produced from marine sediments using MFC without separators between the fuel and seawater containing dissolved oxygen. Cathode performance was an important factor determining the power output. Electrocatalyst at the cathode improved performance: FeCoTMMP gave power densities of 60 mW m−2 which was twice that achieved with the best un-modified carbon. Copyright © 2008 Society of Chemical Industry

Power control method for secondary batteries

Abstract: There is provided a power control method for secondary batteries constituting, in a grid connection system supplying electric power to a power system by combining a power generator where output power fluctuates with a power storage compensator, the power storage compensator and compensating fluctuation of output power of the power generator. The method includes the steps of: dividing the secondary batteries into a “constant power control” group and a “demand responsive” group, and distributing predetermined constant input-output power out of power to be input and output provided to all the secondary batteries in order to compensate fluctuation of output power of the power generator to the “constant power control group” and the remaining input-output power to the “demand responsive” group to control input-output power of the secondary batteries respectively depending on the belonging groups.

Modes in a pulse-modulated radio-frequency dielectric-barrier glow discharge

Abstract: This letter reports an experimental study of a pulse-modulated radio-frequency dielectric-barrier discharge in atmospheric helium. By controlling the duty cycle at a modulation frequency of 10 and 100kHz, the 13.56MHz discharge is shown to operate in three different glow modes: the continuum mode, the discrete mode, and the transition mode. By investigating plasma ignition, residual electrons during power off are found to affect different glow modes. Duty cycle dependences of power density, gas temperature, optical emission intensities at 706 and 777nm are used to capture clearly the characteristics of the three glow modes.

AlGaN Channel High Electron Mobility Transistors: Device Performance and Power-Switching Figure of Merit

Abstract: In this paper, AlGaN channels for high electron mobility transistors (HEMTs) have been evaluated based on a power device figure of merit. AlGaN-channel HEMTs grown on SiC substrates by plasma-assisted molecular beam epitaxy (PAMBE) were fabricated. Maximum saturation current of 0.55 A/mm was obtained at VGS=1 V. Current-gain cutoff ( ft) and power-gain cutoff ( fmax) frequencies obtained from small signal measurements were ft=13.2 GHz and fmax=41 GHz. Pulsed current–voltage (I–V) measurements at 200 ns showed no dispersion in I–V curves. Large signal continuous wave (CW) measurement yielded an output power density of 4.5 W/mm with power added efficiency (PAE) of 59% at 4 GHz. This work demonstrates the potential of AlGaN channel HEMTs for high voltage switching and microwave power applications.

Power architecture design with improved system efficiency, EMI and power density

Abstract: The optimized design of power architecture is discussed in this paper. The paper first discusses the asymmetrical interleaved multi-channel PFC technique and its benefits to system power density and the reduction of differential mode noise. A balance technique is then proposed to minimize the common mode noise of asymmetrical interleaved multi-channel PFC. Greatly reduced EMI leads to the size reduction of EMI filters. System power density is therefore improved. For DC/DC stage, a 1 MHz, LLC resonant converter with novel synchronous rectifier is proposed to reduce body diode conduction time. Both conduction loss and reverse recovery loss can be reduced. The whole system's efficiency, EMI and power density can be greatly improved by applying the techniques proposed in this paper.