Showing papers on "Power density published in 2011"

Iron-based cathode catalyst with enhanced power density in polymer electrolyte membrane fuel cells

Abstract: H(2)-air polymer-electrolyte-membrane fuel cells are electrochemical power generators with potential vehicle propulsion applications. To help reduce their cost and encourage widespread use, research has focused on replacing the expensive Pt-based electrocatalysts in polymer-electrolyte-membrane fuel cells with a lower-cost alternative. Fe-based cathode catalysts are promising contenders, but their power density has been low compared with Pt-based cathodes, largely due to poor mass-transport properties. Here we report an iron-acetate/phenanthroline/zeolitic-imidazolate-framework-derived electrocatalyst with increased volumetric activity and enhanced mass-transport properties. The zeolitic-imidazolate-framework serves as a microporous host for phenanthroline and ferrous acetate to form a catalyst precursor that is subsequently heat treated. A cathode made with the best electrocatalyst from this work, tested in H(2)-O(2,) has a power density of 0.75 W cm(-2) at 0.6 V, a meaningful voltage for polymer-electrolyte-membrane fuel cells operation, comparable with that of a commercial Pt-based cathode tested under identical conditions.

Graphene and carbon nanotube composite electrodes for supercapacitors with ultra-high energy density

Abstract: We describe a graphene and single-walled carbon nanotube (SWCNT) composite film prepared by a blending process for use as electrodes in high energy density supercapacitors. Specific capacitances of 290.6 F g−1 and 201.0 F g−1 have been obtained for a single electrode in aqueous and organic electrolytes, respectively, using a more practical two-electrode testing system. In the organic electrolyte the energy density reached 62.8 Wh kg−1 and the power density reached 58.5 kW kg−1. The addition of single-walled carbon nanotubes raised the energy density by 23% and power density by 31% more than the graphene electrodes. The graphene/CNT electrodes exhibited an ultra-high energy density of 155.6 Wh kg−1 in ionic liquid at room temperature. In addition, the specific capacitance increased by 29% after 1000 cycles in ionic liquid, indicating their excellent cyclicity. The SWCNTs acted as a conductive additive, spacer, and binder in the graphene/CNT supercapacitors. This work suggests that our graphene/CNT supercapacitors can be comparable to NiMH batteries in performance and are promising for applications in hybrid vehicles and electric vehicles.

Energy Management of a Fuel Cell/Supercapacitor/Battery Power Source for Electric Vehicular Applications

Abstract: This paper presents an energy management method in an electrical hybrid power source (EHPS) for electric vehicular applications. The method is based on the flatness control technique (FCT) and fuzzy logic control (FLC). This EHPS is composed of a fuel cell system as the main source and two energy storage sources (ESSs)-a bank of supercapacitors (SCs) and a bank of batteries (BATs)-as the auxiliary source. With this hybridization, the volume and mass of the EHPS can be reduced, because the high energy density of BAT and high power density of SC are utilized. In the proposed novel control strategy, the FCT is used to manage the energy between the main and the auxiliary sources, and the FLC is employed to share the power flow in the ESS between the SC and the BAT. The power sharing depends on the load power and the state of charge of the SC and the BAT. EHPS is controlled by the regulation of the stored electrostatic energy in the dc buses. The main property of this strategy is that the energy management in the power source is carried out with a single general control algorithm in different operating modes, consequently avoiding any algorithm commutation. An EHPS test bench has been assembled and equipped with a real-time system controller based on a dSPACE. The experimental results validate the efficiency of the proposed control strategy.

Doubled power density from salinity gradients at reduced intermembrane distance.

Abstract: The mixing of sea and river water can be used as a renewable energy source The Gibbs free energy that is released when salt and fresh water mix can be captured in a process called reverse electrodialysis (RED) This research investigates the effect of the intermembrane distance and the feedwater flow rate in RED as a route to double the power density output Intermembrane distances of 60, 100, 200, and 485 μm were experimentally investigated, using spacers to impose the intermembrane distance The generated (gross) power densities (ie, generated power per membrane area) are larger for smaller intermembrane distances A maximum value of 22 W/m2 is achieved, which is almost double the maximum power density reported in previous work In addition, the energy efficiency is significantly higher for smaller intermembrane distances New improvements need to focus on reducing the pressure drop required to pump the feedwater through the RED-device using a spacerless design In that case power outputs of more than 4 W per m2 of membrane area at small intermembrane distances are envisaged

Vanadium Oxide Nanowire–Carbon Nanotube Binder‐Free Flexible Electrodes for Supercapacitors

Abstract: Vanadium pentoxide (V2O5) layered nanostructures are known to have very stable crystal structures and high faradaic activity. The low electronic conductivity of V2O5 greatly limits the application of vanadium oxide as electrode materials and requires combining with conducting materials using binders. It is well known that the organic binders can degrade the overall performance of electrode materials and need carefully controlled compositions. In this study, we develop a simple method for preparing freestanding carbon nanotube (CNT)-V2O5 nanowire (VNW) composite paper electrodes without using binders. Coin cell type (CR2032) supercapacitors are assembled using the nanocomposite paper electrode as the anode and high surface area carbon fiber electrode (Spectracarb 2225) as the cathode. The supercapacitor with CNT-VNW composite paper electrode exhibits a power density of 5.26 kW Kg−1 and an energy density of 46.3 Wh Kg−1. (Li)VNWs and CNT composite paper electrodes can be fabricated in similar manner and show improved overall performance with a power density of 8.32 kW Kg−1 and an energy density of 65.9 Wh Kg−1. The power and energy density values suggest that such flexible hybrid nanocomposite paper electrodes may be useful for high performance electrochemical supercapacitors.

Impact-driven, frequency up-converting coupled vibration energy harvesting device for low frequency operation

Abstract: This paper presents experiments and models of an energy harvesting device in which a low frequency resonator impacts a high frequency energy harvesting resonator, resulting in energy harvesting predominantly at the system's coupled vibration frequency. Analysis shows that a reduced mechanical damping ratio during coupled vibration enables increased electrical power generation as compared with conventional technology. Experiments demonstrate that the efficiency of electrical power transfer is significantly improved with the coupled vibration approach. An average power output of 0.43 mW is achieved under 0.4g acceleration at 8.2 Hz, corresponding to a power density of 25.5 µW cm − 3. The measured power and power density at the resonant frequency are respectively 4.8 times and 13 times the measured peak values for a conventional harvester created from a low frequency beam alone.

Graphene surface-enabled lithium ion-exchanging cells: next-generation high-power energy storage devices.

Abstract: Herein reported is a fundamentally new strategy for the design of high-power and high energy-density devices. This approach is based on the exchange of lithium ions between the surfaces (not the bulk) of two nanostructured electrodes, completely obviating the need for lithium intercalation or deintercalation. In both electrodes, massive graphene surfaces in direct contact with liquid electrolyte are capable of rapidly and reversibly capturing lithium ions through surface adsorption and/or surface redox reaction. These devices, based on unoptimized materials and configuration, are already capable of storing an energy density of 160 Wh/kgcell, which is 30 times higher than that (5 Wh/kgcell) of conventional symmetric supercapacitors and comparable to that of Li-ion batteries. They are also capable of delivering a power density of 100 kW/kgcell, which is 10 times higher than that (10 kW/kgcell) of supercapacitors and 100 times higher than that (1 kW/kgcell) of Li-ion batteries.

Discharge physics of high power impulse magnetron sputtering

Abstract: High power impulse magnetron sputtering (HIPIMS) is pulsed sputtering where the peak power exceeds the time-averaged power by typically two orders of magnitude. The peak power density, averaged over the target area, can reach or exceed 10 7 W/m 2 , leading to plasma conditions that make ionization of the sputtered atoms very likely. A brief review of HIPIMS operation is given in a tutorial manner, illustrated by some original data related to the self-sputtering of niobium in argon and krypton. Emphasis is put on the current–voltage–time relationships near the threshold of self-sputtering runaway. The great variety of current pulse shapes delivers clues on the very strong gas rarefaction, self-sputtering runaway conditions, and the stopping of runaway due to the evolution of atom ionization and ion return probabilities as the gas plasma is replaced by metal plasma. The discussions are completed by considering instabilities and the special case of “gasless” self-sputtering.

Rebalancing of internally generated carriers for mid-infrared interband cascade lasers with very low power consumption

Abstract: Mid-infrared semiconductor lasers suffer from a high threshold power density, but interband cascade lasers may offer a more efficient alternative. Here, theory and experiments on such emitters demonstrate remarkably low thresholds and power consumption compared to state-of-the-art quantum cascade lasers.

Power dissipation, gas temperatures and electron densities of cold atmospheric pressure helium and argon RF plasma jets

Abstract: A set of diagnostic methods to obtain the plasma parameters including power dissipation, gas temperature and electron density is evaluated for an atmospheric pressure helium or argon radio frequency (RF) plasma needle for biomedical applications operated in open air. The power density of the plasma is more or less constant and equal to 1.3 ? 109?W?m?3. Different methods are investigated and evaluated to obtain the gas temperature. In this paper the gas temperatures obtained by rotational spectra of OH(A?X) and (B?X) are compared with Rayleigh scattering measurements and measurements of the line broadening of hydrogen and helium emission lines. The obtained gas temperature ranges from 300 to 650?K, depending on the gas. The electron densities are estimated from the Stark broadening of the hydrogen ? and ? lines which yield values between 1019 and 1020?m?3. In the case of helium, this is an overestimate as is shown by a power balance from the measured power density in the plasma jet. The obtained plasma parameters enable us to explain the radial contraction of the argon plasma compared with the more diffuse helium plasma. The accuracy of all considered diagnostics is discussed in detail.

V2O5 Nano-Electrodes with High Power and Energy Densities for Thin Film Li-Ion Batteries

Abstract: Nanostructured V2O5 thin films have been prepared by means of cathodic deposition from an aqueous solution made from V2O5 and H2O2 directly on fluorine-doped tin oxide coated (FTO) glasses followed by annealing at 500°C in air, and studied as film electrodes for lithium ion batteries. XPS results show that the as-deposited films contained 15% V4+, however after annealing all the vanadium is oxidized to V5+. The crystallinity, surface morphology, and microstructures of the films have been investigated by means of XRD, SEM, and AFM. The V2O5 thin film electrodes show excellent electrochemical properties as cathodes for lithium ion intercalation: a high initial discharge capacity of 402 mA h g−1 and 240 mA h g−1 is retained after over 200 cycles with a discharging rate of 200 mA g−1 (1.3 C). The specific energy density is calculated as 900 W h kg−1 for the 1st cycle and 723 W h kg−1 for the 180th cycle when the films are tested at 200 mA g−1 (1.3 C). When discharge/charge is carried out at a high current density of 10.5 A g−1 (70 C), the thin film electrodes retain a good discharge capacity of 120 mA h g−1, and the specific power density is over 28 kW kg−1.

A Vibration-Based Electromagnetic Energy Harvester Using Mechanical Frequency Up-Conversion Method

Abstract: This paper presents a new vibration-based electromagnetic energy harvester using a mechanical frequency up-conversion method for harvesting energy from external low-frequency vibrations within a range of 1-10 Hz. The structure consists of a magnet placed on a diaphragm, a polystyrene cantilever carrying a pick-up coil, and a mechanical barrier which converts low-frequency vibrations to a higher frequency, hence increasing the efficiency of the system. The tested structure proved to generate 88.6 mV and 544.7 μW rms power output by up-converting 10-Hz external vibration to 394 Hz. The obtained power density is 184 μW/cm3, with a device volume of 2.96 cm3. An analytical model is developed to analyze the behavior of the energy harvester prototypes with various dimensions. The model predicts the performance parameters of the structures within 5% error range. The effect of scaling down the device dimensions is investigated through the developed model and fabricated prototypes. It is shown that the power density of the energy harvester is increased as its dimensions are scaled down, proving that the proposed structure is a good candidate to be used in low-power wireless microsystems operating at low-frequency vibrations.

Improved solid oxide fuel cell performance with nanostructured electrolytes.

Abstract: Considerable attention has been focused on solid oxide fuel cells (SOFCs) due to their potential for providing clean and reliable electric power. However, the high operating temperatures of current SOFCs limit their adoption in mobile applications. To lower the SOFC operating temperature, we fabricated a corrugated thin-film electrolyte membrane by nanosphere lithography and atomic layer deposition to reduce the polarization and ohmic losses at low temperatures. The resulting micro-SOFC electrolyte membrane showed a hexagonal-pyramid array nanostructure and achieved a power density of 1.34 W/cm2 at 500 °C. In the future, arrays of micro-SOFCs with high power density may enable a range of mobile and portable power applications.

Improved Solid Oxide Fuel Cell Performance with Nanostructured

Abstract: Considerableattentionhasbeenfocusedonsolidoxidefuelcells(SOFCs)duetotheir potential for providing cleanand reliable electric power. However, the high operating temperatures of current SOFCs limit their adoption in mobile applications. To lower the SOFC operating temperature,wefabricatedacorrugatedthin-filmelectrolytemembranebynanospherelithography and atomic layer deposition to reduce the polarization and ohmic losses at low temperatures. The resulting micro-SOFC electrolyte membrane showed a hexagonal-pyramid array nanostructure and achieved a power density of 1.34 W/cm 2 at 500 C. In the future, arrays of micro-SOFCs with high

Auger recombination in III-nitride nanowires and its effect on nanowire light-emitting diode characteristics.

Abstract: We have measured the Auger recombination coefficients in defect-free InGaN nanowires (NW) and InGaN/GaN dot-in-nanowire (DNW) samples grown on (001) silicon by plasma-assisted molecular beam epitaxy. The nanowires have a density of ∼1 × 1011 cm−2 and exhibit photoluminescence emission peak at λ ∼ 500 nm. The Auger coefficients as a function of excitation power have been derived from excitation dependent and time-resolved photoluminescence measurements over a wide range of optical excitation power density. The values of C0, defined as the Auger coefficient at low excitation, are 6.1 × 10−32 and 4.1 × 10−33 cm6·s−1 in the NW and DNW samples, respectively, which are in reasonably good agreement with theoretical predictions for InGaN alloy semiconductors. Light-emitting diodes made with the NW and DNW samples exhibit no efficiency droop up to an injection current density of 400 A/cm2.

Thinned-PZT on SOI process and design optimization for piezoelectric inertial energy harvesting

Abstract: This paper presents the design, fabrication, and testing of a thinned-PZT/Si unimorph for vibration energy harvesting. It produces a record power output and has state-of-the-art efficiency. The harvester utilizes thinning of bulk-PZT pieces bonded to an SOI wafer, and takes advantage of the similar thermal expansion between PZT and Si to minimize beam bending due to residual stress. Monolithic integration of a tungsten proof mass lowers the resonance frequency and increases the power output. The harvester dimensions, including the PZT/Si thickness ratio and the proof-mass/total-beam length ratio, are optimized via parametric multi-physics FEA. Additionally, a fabrication process for hermetic packaging of the harvester is introduced. It uses vertical Si vias for electrical feed-throughs. An unpackaged harvester with a tungsten proof mass produces 2.74 µW at 0.1 g (167 Hz), and 205 µW at 1.5 g (154 Hz) at resonance (here, g = 9.8 m/s2). The active device volume is 27 mm3 (7 × 7 × 0.55 mm3). We report the highest power output, Normalized Power Density (N.P.D.), and Figure of Merit (N.P.D. × Bandwidth) amongst reported microfabricated vibration energy harvesters.

Direct comparison of silicon and silicon carbide power transistors in high-frequency hard-switched applications

Abstract: RECENT progress in wide bandgap power (WBG) switches shows great potential. Silicon carbide (SiC) is a promising material for power devices with breakdown voltages of several hundred volts up to 10 kV. SiC Schottky power diodes have achieved widespread commercial acceptance. Recently, much progress has been made on active SiC switches, including JFETs, thyristors, BJTs, IGBTs, and MOSFETs. Many a great promise has been made, and wondrous claims abound, but the question remains: will they live up to the hype? We explore this question for the class of high-frequency, hard-switched converters with input voltages of up to 600 VDC and power throughputs in the kilowatt range. Experimental evidence shows that both superior efficiency and higher power density may be obtained via the use of SiC MOSFETs. A direct comparison is made using silicon power devices (IGBTs and MOSFETs) and SiC MOSFETs in a 200 kHz, 6 kW, 600 V hard-switched converter. The losses are measured and conduction and switching losses of the active devices are estimated. Total losses can be reduced by a factor of 2–5 by substitution of SiC MOSFETs for Si active power devices.

Nanostructured cathode materials: a key for better performance in Li-ion batteries

Abstract: Battery technology plays critical role under clean energy systems because it contributes for the reduction of greenhouse gas emissions. Breakthroughs in cathodes, anodes, and electrolyte materials are needed to reach high power and energy density in lithium ion batteries. The electrochemical performance (cycling stability, power density, and reversibility) depends on the morphological and compositional characteristics of the electrode materials, which could be controlled during the synthesis and also the annealing process. Enormous leverage can result from the advances in nanostructured electrode materials. Nano sized electrodes exhibit minimal impedance growth, which means no substantial loss of power with cycling, also increase the electrode/electrolyte contact area, which in turn increase the charge/discharge rate to achieve maximum power. This report reviews novel nanostructured cathode materials and their influence on the electrochemical performance of lithium ion batteries.

Cyclic energy harvesting from pyroelectric materials

Abstract: A method of continuously harvesting energy from pyroelectric materials is demonstrated using an innovative cyclic heating scheme. In traditional pyroelectric energy harvesting methods, static heating sources are used, and most of the available energy has to be harvested at once. A cyclic heating system is developed such that the temperature varies between hot and cold regions. Although the energy harvested during each period of the heating cycle is small, the accumulated total energy over time may exceed traditional methods. Three materials are studied: a commonly available soft lead zirconate titanate (PZT), a pre-stressed PZT composite, and single-crystal PMN-30PT. Radiation heating and natural cooling are used such that, at smaller cyclic frequencies, the temporal rate of change in temperature is large enough to produce high power densities. The maximum power density of 8.64 μW/cm3 is generated with a PMN-30PT single crystal at an angular velocity of 0.64 rad/s with a rate of 8.5°C/s. The pre-stressed PZT composite generated a power density of 6.31 μW/cm3, which is 40% larger than the density of 4.48 μW/cm3 obtained from standard PZT.

Regenerative Performance of the NASA Symmetrical Solid Oxide Fuel Cell Design

Abstract: The NASA Glenn Research Center is developing both a novel cell design (BSC) and a novel ceramic fabrication technique to produce fuel cells predicted to exceed a specific power density of 1.0 kW/kg. The NASA Glenn cell design has taken a completely different approach among planar designs by removing the metal interconnect and returning to the use of a thin, doped LaCrO3 interconnect. The cell is structurally symmetrical. Both electrodes support the thin electrolyte and contain micro-channels for gas flow-- a geometry referred to as a bi-electrode supported cell or BSC. The cell characteristics have been demonstrated under both SOFC and SOE conditions. Electrolysis tests verify that this cell design operates at very high electrochemical voltage efficiencies (EVE) and high H2O conversion percentages, even at the low flow rates predicted for closed loop systems encountered in unmanned aerial vehicle (UAV) applications. For UAVs the volume, weight and the efficiency are critical as they determine the size of the water tank, the solar panel size, and other system requirements. For UAVs, regenerative solid oxide fuel cell stacks (RSOFC) use solar panels during daylight to generate power for electrolysis and then operate in fuel cell mode during the night to power the UAV and electronics. Recent studies, performed by NASA for a more electric commercial aircraft, evaluated SOFCs for auxiliary power units (APUs). System studies were also conducted for regenerative RSOFC systems. One common requirement for aerospace SOFCs and RSOFCs, determined independently in each application study, was the need for high specific power density and volume density, on the order of 1.0 kW/kg and greater than 1.0 kW/L. Until recently the best reported performance for SOFCs was 0.2 kW/kg or less for stacks. NASA Glenn is working to prototype the light weight, low volume BSC design for such high specific power aerospace applications.