Showing papers on "Power density published in 2009"

Use of Carbon Mesh Anodes and the Effect of Different Pretreatment Methods on Power Production in Microbial Fuel Cells

Abstract: Flat electrodes are useful in microbial fuel cells (MFCs) as close electrode spacing improves power generation. Carbon cloth and carbon paper materials typically used in hydrogen fuel cells, however, are prohibitively expensive for use in MFCs. An inexpensive carbon mesh material was examined here as a substantially less expensive alternative to these materials for the anode in an MFC. Pretreatment of the carbon mesh was needed to ensure adequate MFC performance. Heating the carbon mesh in a muffle furnace (450 degrees C for 30 min) resulted in a maximum power density of 922 mW/m2 (46 W/m3) with this heat-treated anode, which was 3% more power than that produced using a mesh anode cleaned with acetone (893 mW/ m2; 45 W/m3). This power density with heating was only 7% less than that achieved with carbon cloth treated by a high temperature ammonia gas process (988 mW/m2; 49 W/m3). When the carbon mesh was treated by the ammonia gas process, power increased to 1015 mW/m2(51 W/m3). Analysis of the cleaned or heated surfaces showed these processes decreased atomic O/C ratio, indicating removal of contaminants that interfered with charge transfer. Ammonia gas treatment also increased the atomic N/C ratio, suggesting that this process produced nitrogen related functional groups that facilitated electron transfer. These results show that low cost heat-treated carbon mesh materials can be used as the anode in an MFC, providing good performance and even exceeding performance of carbon cloth anodes.

Design and Synthesis of Hierarchical Nanowire Composites for Electrochemical Energy Storage

Abstract: Nanocomposites of interpenetrating carbon nanotubes and vanadium pentoxide (V 2 O 5 ) nanowires networks are synthesized via a simple in situ hydrothermal process. These fibrous nanocomposites are hierarchically porous with high surface area and good electric conductivity, which makes them excellent material candidates for supercapacitors with high energy density and power density. Nanocomposites with a capacitance up to 440 and 200 F g -1 are achieved at current densities of 0.25 and 10 A g -1 , respectively. Asymmetric devices based on these nanocomposites and aqueous electrolyte exhibit an excellent charge/discharge capability, and high energy densities of 16 W h kg -1 at a power density of 75 W kg -1 and 5.5 W h kg -1 at a high power density of 3750 W kg -1 . This performance is a significant improvement over current electrochemical capacitors and is highly competetive with Ni-MH batteries. This work provides a new platform for high-density electrical-energy storage for electric vehicles and other applications.

Nanotubular metal–insulator–metal capacitor arrays for energy storage

Abstract: Nanostructured devices have the potential to serve as the basis for next-generation energy systems that make use of densely packed interfaces and thin films 1 . One approach to making such devices is to build multilayer structures of large area inside the open volume of a nanostructured template. Here, we report the use of atomic layer deposition to fabricate arrays of metal–insulator–metal nanocapacitors in anodic aluminium oxide nanopores. These highly regular arrays have a capacitance per unit planar area of 10 m Fc m 22 for 1-mm-thick anodic aluminium oxide and 100 m Fc m 22 for 10-mm-thick anodic aluminium oxide, significantly exceeding previously reported values for metal–insulator–metal capacitors in porous templates 2–6 . It should be possible to scale devices fabricated with this approach to make viable energy storage systems that provide both high energy density and high power density. The nanocapacitor structures in this Letter are formed of metal electrodes separated by a dielectric film; therefore they behave in the same manner as conventional electrostatic capacitors, in which charge is stored on opposing electrode surfaces. A characteristic feature of electrostatic capacitors is high power. This is because charge can be moved rapidly, with speeds limited only by external circuit RCs. However, energy storage is limited because only surface charge is used. In contrast, conventional electrochemical supercapacitors store charge in electric double layers or in faradic reactions, permitting larger energy density storage on the electrode surfaces. Power density is limited in these devices because of the requirement for mass transport of ions and/or redox reactions 7 . The use of highly regular nanostructures promises both high energy and high power density. For the nanocapacitors described in this Letter, the nanostructure significantly enhances capacitance density. The nanocapacitors demonstrate the high power (up to � 1 � 10 6 Wk g 21 ) typical of electrostatic capacitors while achieving the much higher energy density (� 0.7 Wh kg 21 ) characteristic of electrochemical supercapacitors. As a result, electrostatic nanocapacitors are attractive for high-burst-power applications requiring the energy density of supercapacitors. To obtain energy devices that achieve dense packing of active interfaces and thin films, nanoporous structures are required that have internal surfaces on which highly uniform films can be reproducibly formed. We make use of the self-assembly of regular nanopore arrays available from anodic aluminium oxide (AAO) formation together with multilayer atomic layer deposition (ALD) to form highly controlled, self-aligned nanocapacitors. ALD has become the leading process used to achieve such coatings, yielding a high degree of thickness control and conformality in the most demanding nanostructures, with features that are either highly ordered 8–10 or are random porous networks 11–13 . Nanostructures fabricated with AAO and ALD show a high degree of uniformity across massive arrays, imparting the regularity that is a key factor in the viability of any technology 8 . Our fabrication strategy makes use of AAO nanopore templates in combination with metal–insulator–metal (MIM) structures deposited in the nanopores by ALD. The anodization process produces an ultrahigh density (� 1 � 10 10 cm 22 ) of hexagonally arranged, uniform, self-assembled nanopores in AAO film on Al tens of micrometres deep, which provides a good template for

Erratum: Microfibre–nanowire hybrid structure for energy scavenging

Abstract: Nature 451, 809–813 (2008) It has been drawn to our attention that an error was made in calculating the output power of our fibre nanogenerator. The correct output power density of the fabric is expected to be 4–16 mW per square metre rather than 20–80 mW per square metre as originally claimed. Thiscorrection does not affect the main conclusions of our paper.

Micromachined PZT cantilever based on SOI structure for low frequency vibration energy harvesting

Abstract: A PZT piezoelectric cantilever with a micromachined Si proof mass is designed and fabricated for a low frequency vibration energy harvesting application. The SiO2 layer in the SOI wafer promotes accurate control of the silicon thickness that is used as a supporting layer in the cantilever beam structure. The entire effective volume of the fabricated device is about 0.7690 mm3. When excited at 0.75g (g = 9.81 m/s2) acceleration amplitude at its resonant frequency of 183.8 Hz, the AC output measured across a resistive load of 16 kΩ connecting to the device in parallel has an amplitude of 101 mV. The average power and power density determined by the same measurement conditions are, respectively, 0.32 μW and 416 μW/cm3.

Analysis and improvement of a scaled-up and stacked microbial fuel cell

Abstract: Scaling up microbial fuel cells (MFCs) is inevitable when power outputs have to be obtained that can power electrical devices other than small sensors. This research has used a bipolar plate MFC stack of four cells with a total working volume of 20 L and a total membrane surface area of 2 m(2). The cathode limited MFC performance due to oxygen reduction rate and cell reversal. Furthermore, residence time distribution curves showed that bending membranes resulted in flow paths through which the catholyte could flow from inlet to outlet, while leaving the reactants unconverted. The cathode was improved by decreasing the pH, purging pure oxygen, and increasing the flow rate, which resulted in a 13-fold power density increase to 144 W m(-3) and a volumetric resistivity of only 1.2 mOmega m(3) per cell. Both results are major achievements compared to results currently published for laboratory and scaled-up MFCs. When designing a scaled-up MFC, it is important to ensure optimal contact between electrodes and substrate and to minimize the distances between electrodes.

Exploring the pareto front of multi-objective single-phase PFC rectifier design optimization - 99.2% efficiency vs. 7kW/din 3 power density

Abstract: Up to now, in the development of power electronics systems, the reduction of the initial costs or the increase of the power density have been of primary concern. However, with increasing energy costs also the power conversion efficiency is gaining higher and higher importance. Accordingly, while maintaining high power density, an efficiency as high as possible must be obtained. In this paper the maximum attainable efficiency and the dependency of the efficiency limit on technological parameters is determined for single-phase PFC boost rectifiers. In a first step basic PFC boost rectifier topologies are briefly compared with regard to high efficiency and a dual-boost PFC rectifier with integral common-mode filtering is selected as basis for the investigations. Next, simple approximations of the technological limits of the system performance are calculated in the efficiencypower density plane. With this, the Feasible Performance Space and the reduction in power density which has to be accepted for increasing the efficiency are clarified, and the trade-off limit curve (Pareto Front) of a multi-objective, i.e. efficiency and power density design optimization is determined. Furthermore, a comprehensive numerical efficiency optimization is carried out which identifies an efficiency limit of 99.2% for a 3.2kW system. The theoretical considerations are verified by experimental results from a laboratory prototype of the ultra-high efficiency system achieving 99.1% efficiency at a power density of 1.1kW/din3, as well as those firom an ultra-compact dual-boost PFC rectifier (95.8%, 5.5kW/dn3) and a very low switching freluency (3kHz) conventional PFC boost rectifier (96.7%, 2kW/din3). Finally, the sensitivity of the efficiency optimum with regard to various technological parameters is analyzed and an outlook on the further course of the research is given.

Impact of Power Density Maximization on Efficiency of DC–DC Converter Systems

Abstract: The demand for decreasing costs and volume leads to a constantly increasing power density of industrial converter systems. In order to improve the power density, further different aspects, like thermal management and electromagnetic effects, must be considered in conjunction with the electrical design. Therefore, a comprehensive optimization procedure based on analytical models for minimizing volume of DC-DC converter systems has been developed at the power electronic systems laboratory of the Swiss Federal Institute of Technology (ETH Zurich). Based on this procedure, three converter topologies-a phase-shift converter with current doubler and with capacitive output filter and a series-parallel resonant converter-are optimized with respect to power density for a telecom supply (400 V/48 V). There, the characteristic of the power density, the efficiency, and the volume distribution between the components as functions of frequency are discussed. For the operating points with maximal power density, the loss distribution is also presented. Furthermore, the sensitivity of the optimum with respect to junction temperature, cooling, and core material is investigated. The highest power density is achieved by the series-parallel resonant converter. For a 5-kW supply, a density of approximately 12 kW/L and a switching frequency of ca. 130 kHz are obtained.

Multicomponent Polymeric Film for Red to Green Low Power Sensitized Up-Conversion

Abstract: The realization of red to green photon energy up-conversion in a multicomponent polymeric organic solid film with good photochemical stability is presented. Up-converted light is obtained by using an ultralow excitation power density in the range of 1 mW cm(-2), suitable to recover the low energy tail of the solar emission spectrum.

Analysis of Supercapacitor as Second Source Based on Fuel Cell Power Generation

Abstract: This paper presents the utilization of a supercapacitor as an auxiliary power source in a distributed generation system, composed of a polymer electrolyte membrane fuel cell (PEMFC) as the main energy source. The main weak point of fuel cells (FCs) is slow dynamics because one must limit the FC current slope in order to prevent fuel starvation problems, to improve its performance and lifetime. The very fast power response and high specific power of a supercapacitor can complement the slower power output of the main source to produce the compatibility and performance characteristics needed in a load. The FC and supercapacitor characteristics are clearly presented. Experimental results with small-scale devices (supercapacitor bank: 292-F, 30-V, 400-A; PEMFC: 500-W, 40-A) illustrate excellent performance during a motor drive cycle.

Microwave engineering of plasma-assisted CVD reactors for diamond deposition.

Abstract: The unique properties of CVD diamond make it a compelling choice for high power electronics. In order to achieve industrial use of CVD diamond, one must simultaneously obtain an excellent control of the film purity, very low defect content and a sufficiently rapid growth rate. Currently, only microwave plasma-assisted chemical vapour deposition (MPACVD) processes making use of resonant cavity systems provide enough atomic hydrogen to satisfy these requirements. We show in this paper that the use of high microwave power density (MWPD) plasmas is necessary to promote atomic hydrogen concentrations that are high enough to ensure the deposition of high purity diamond films at large growth rates. Moreover, the deposition of homogeneous films on large surfaces calls for the production of plasma with appropriate shapes and large volumes. The production of such plasmas needs generating a fairly high electric field over extended regions and requires a careful design of the MW coupling system, especially the cavity. As far as MW coupling efficiency is concerned, the presence of a plasma load represents a mismatching perturbation to the cavity. This perturbation is especially important at high MWPD where the reflected fraction of the input power may be quite high. This mismatch can lead to a pronounced heating of the reactor walls. It must therefore be taken into account from the very beginning of the reactor design. This requires the implementation of plasma modelling tools coupled to detailed electromagnetic simulations. This is discussed in section 3. We also briefly discuss the operating principles of the main commercial plasma reactors before introducing the reactor design methodology we have developed. Modelling results for a new generation of reactors developed at LIMHP, working at very high power density, will be presented. Lastly, we show that scaling up this type of reactor to lower frequencies (915 MHz) can result in high density plasmas allowing for fast and homogeneous diamond deposition on up to 160 mm diameter surfaces.

Comprehensive Design and Optimization of a High-Power-Density Single-Phase Boost PFC

Abstract: The design of a single-phase boost power-factor-correction (PFC) circuit is associated with a large variety of considerations, such as the following questions. Which operation mode should be selected (e.g., continuous or discontinuous operation)? How many interleaved boost cells are advantageous? Which switching frequency should be selected? What is the optimum number of EMI input filter stages? Which semiconductor technology should be chosen? All these issues have a significant influence on the converter efficiency and power density. In this paper, the aforementioned questions are addressed for exemplary specifications of the PFC (300-W output power, 400-V output voltage, and 230-V mains voltage), whereby the focus in the design is mainly put on very high power density. As a result, different design points are identified and comparatively evaluated. By considering different aspects such as volume, losses, capacitor lifetime, and also cost issues (e.g., by additional current sensors or expensive silicon carbide devices), a dual-interleaved PFC operated in discontinuous conduction mode at 200 kHz is selected. With an experimental prototype, a superior power density of 5.5 kW/L and a system efficiency of 96.4% are achieved, which is close to the values predicted by the design procedure. Furthermore, measurements verify a near-unity power factor (PF = 99.7%) and the compliance with electromagnetic compatibility conducted noise emission standards. Finally, it is investigated to which extent the power density could be further increased by an integration of the input filter in the PCB.

Imaging by Modification: Numerical Reconstruction of Local Conductivities from Corresponding Power Density Measurements

Abstract: We discuss the reconstruction of the impedance from the local power density. This study is motivated by a new imaging principle which allows us to recover interior measurements of the energy density by a noninvasive method. We discuss the theoretical feasibility in two dimensions, and propose numerical algorithms to recover the conductivity in two and three dimensions. The efficiency of this approach is documented by several numerical simulations.

Impact of EMC Filters on the Power Density of Modern Three-Phase PWM Converters

Abstract: Electromagnetic compatibility (EMC) filters are typically included in offline power converters, controlling electromagnetic emissions, but adding volume to power electronic systems. During the last decades, one of the main objectives of the power electronics industry has been the increase of the power density. Thus, it is reasonable to analyze how filters affect power density values, imposing limits or ldquobarriersrdquo to it. The impact of the EMC filters on the overall volume of three-phase pulsewidth modulation (PWM) converters is studied here for converters in the range of 5-10 kW. An analytical procedure based on the volume minimization of the EMC filters is proposed to estimate the total filter volume as function of the converters' rated power and switching frequency. With this, the minimum volume for EMC filters that allow the converters to comply with EMC standards regarding conducted emissions can be estimated and volume limitations identified. A discussion about the limits of power density for the considered three-phase PWM converters for state-of-the-art power semiconductors is performed, and optimum switching frequencies are identified. An experimental verification is carried out, which validates the achieved results.

Design of a 5-kW, 1-U, 10-kW/dm $^{\hbox{3}}$ Resonant DC–DC Converter for Telecom Applications

Abstract: The demand for decreasing cost and volume and also for increasing efficiency leads to a constantly increasing power density of converter systems. For maximizing the power density of a 5 kW telecom supply, an optimization procedure that automatically balances the switching frequency, semiconductor and passive losses, and thermal performance has been developed. This procedure and the belonging analytical converter and transformer models are presented in this paper. Moreover, the resulting optimized design, which has a power density of 10 kW/dm 3 and an efficiency of 94.5% at a height of 1 U, is presented.

SiC power semiconductors in HEVs: Influence of junction temperature on power density, chip utilization and efficiency

Abstract: With SiC, junction temperatures of power semiconductors of more than 700 _C are theoretically possible due to the low intrinsic charge carrier concentration of SiC. Hence, a lot of research on package configurations for power semiconductor operation above 175 _C is currently carried out, especially within the automotive industry due to the possible high ambient temperatures occurring in hybrid electric vehicles (HEVs). This paper shows, that a higher junction temperature though does not necessarily guarantee a higher utilization of the SiC chips with respect to the current that the device can conduct without overheating. The reason is, that for most power devices the power losses start to increase very rapidly at high junction temperatures while the power that can be dissipated always increases linearly with the junction temperature. The junction temperature, where the device current starts to decrease at, is derived for different SiC chips using measured onstate conduction and switching losses in this paper. This paper furthermore analyzes in detail, how the junction temperature on the one hand is influenced by boundary conditions and on the other hand influences itself the core parameters of a converter such as efficiency, the required chip area (i. e. cost) as well as the volumetric power density and thus forms an additional degree of freedom in the design of a power electronic converter. While calculating the optimum junction temperature and analyzing its impact on the system performance, it is demonstrated, how these results can help to find the best suited power semiconductor device for the particular application. The performance of the calculations is shown on a design applied to a drive inverter for hybrid electric vehicles with normally-off SiC JFETs. Operated close to the optimum junction temperature of the SiC JFETs, it reaches a power density of 51 kW/l for the power modules and the air-cooling system, which is shown to be doubled by increasing chip size and using an advanced power semiconductor package with a lower thermal resistance from junction to ambient than the for this case assumed 1 K/W.

Thermal Expansion Studies of Selected High-Temperature Thermoelectric Materials

Abstract: Radioisotope thermoelectric generators (RTGs) generate electrical power by converting the heat released from the nuclear decay of radioactive isotopes (typically plutonium-238) into electricity using a thermoelectric converter. RTGs have been successfully used to power a number of space missions and have demonstrated their reliability over an extended period of time (tens of years) and are compact, rugged, radiation resistant, scalable, and produce no noise, vibration or torque during operation. System conversion efficiency for state-of-practice RTGs is about 6% and specific power less than or equal to 5.1 W/kg. Higher specific power would result in more on-board power for the same RTG mass, or less RTG mass for the same on-board power. The Jet Propulsion Laboratory has been leading, under the advanced thermoelectric converter (ATEC) project, the development of new high-temperature thermoelectric materials and components for integration into advanced, more efficient RTGs. Thermoelectric materials investigated to date include skutterudites, the Yb14MnSb11 compound, and SiGe alloys. The development of long-lived thermoelectric couples based on some of these materials has been initiated and is assisted by a thermo-mechanical stress analysis to ensure that all stresses under both fabrication and operation conditions will be within yield limits for those materials. Several physical parameters are needed as input to this analysis. Among those parameters, the coefficient of thermal expansion (CTE) is critically important. Thermal expansion coefficient measurements of several thermoelectric materials under consideration for ATEC are described in this paper. The stress response at the interfaces in material stacks subjected to changes in temperature is discussed, drawing on work from the literature and project-specific tools developed here. The degree of CTE mismatch and the associated effect on the formation of stress is highlighted.

Wideband vibration energy harvester with high permeability magnetic material

Abstract: A vibration energy harvester based on a high permeability cantilever beam was demonstrated, which overcomes the limitation of the existing approaches in output power and working bandwidth. Magnetostatic coupling between the vibrating highly permeable beam and bias magnetic field leads to maximized flux change and large induced voltage. The coexistence of magnetostatic and elastic potential energy results in the nonlinear oscillation with wide bandwidth. The harvester showed a maximum power of 74 mW and power density of 1.07 mW/cm3 at 54 Hz under acceleration of 0.57 g (with g=9.8 m/s2), and bandwidth of 10 Hz (or 18.5% of the operating frequency).

Electron energy distributions and plasma parameters in high-power pulsed magnetron sputtering discharges

Abstract: We report on the results obtained using time-resolved Langmuir probe measurements in high-power pulsed dc magnetron sputtering discharges. Time evolutions of the electron energy distribution and the local plasma parameters were investigated at a substrate position of 100 mm from a planar target of 100 mm diameter during a high-rate deposition of copper films. The average target power density in a pulse was 500 W cm−2 at a repetition frequency of 1 kHz, a voltage pulse duration of 200 µs and an argon pressure of 1 Pa. The electron energy distributions with two energy groups and sharply truncated high-energy tails were observed during a pulse. After a fast rise in a 50 µs initial stage of the pulse, the kinetic temperature of electrons, defined using the mean electron energy, remained in the range from 10 500 to 12 200 K till the pulse termination. The temperature of weakly populated hot electrons decreased rapidly in the initial stage of the pulse approaching the kinetic temperature approximately 100 µs after a pulse initiation. High plasma densities, being in the range 1 × 1012–2 × 1012 cm−3 for 100 µs, were achieved at the substrate position with a 50 µs delay after establishing the 125 µs steady-state discharge regime with the target power density of 650–680 W cm−2 during a pulse. The plasma potential slowly increased from 0.5 to 3.5 V during the pulse and 25 µs after its termination.

Reaching High Power Density in Multikilowatt DC–DC Converters With Galvanic Isolation

Abstract: In this paper, the possibility of reaching high power densities in multikilowatt dc-dc converters with galvanic isolation is demonstrated and the main design issues are discussed. The issues related to converter topology, transformer design, and thermal management are addressed, and new conceptual solutions are proposed. Implementing zero-voltage-switching quasi-zero-current-switching topology, optimized transformer design with leakage layer, and thermal management based on conduction enhanced by heat pipes at critical places resulted in very high power density and efficiency. The power density reached by the converter prototype is 11.13 kW/L with water cooling and 6.6 kW/L with air cooling. In the same time, the measured efficiency exceeded 97% in a broad load range. The new design concepts are demonstrated on a 50-kW converter prototype that was successfully tested at full-load conditions.

InAlN/GaN MOSHEMT With Self-Aligned Thermally Generated Oxide Recess

Abstract: We report on lattice-matched InAlN/GaN MOSHEMTs with an oxide-filled recess, self-aligned to the gate prepared by thermal oxidation at 800degC in oxygen atmosphere. The device delivered a maximum current density of 2.4 A/mm. Pulse measurements showed no apparent lag effects, indicating a high-quality native oxide. This was confirmed by monitoring the radio-frequency load lines in the time domain. The MOSHEMT yielded a power density of 6 W/mm at a drain voltage as low as 20 V and at 4 GHz, a power added efficiency of 32% and an ft and f max of 61 and 112 GHz, respectively, illustrating the capability of such MOSHEMT to operate at high frequencies.

20W continuous wave reliable operation of 980nm broad-area single emitter diode lasers with an aperture of 96μm

Abstract: High power broad area diode lasers provide the optical energy for all high performance solid state and fiber laser systems. The maximum achievable power density from such systems is limited at source by the performance of the diode lasers. A crucial metric is the reliable continuous wave optical output power from a single broad area laser diode, typically for stripe widths in the 90-100 μm range, which is especially important for users relying on fibered multi-mode pumps. We present the results of a study investigating the reliable power limits of such 980nm sources. We find that 96μm stripe single emitters lasers at 20°C operate under continuous wave power of 20W per emitter for over 4000 hours (to date) without failure, with 60μm stripe devices operating reliably at 10W per stripe. Maximum power testing under 10Hz, 200μs QCW drive conditions shows that 96μm stripes reach 30W and 60μm stripes 21W per emitter, significantly above the reliable operation point. Results are also presented on step-stress-studies, where the current is step-wise increased until failure is observed, in order to clarify the remaining reliability limits. Finally, we detail the barriers to increased peak power and discuss how these can be overcome.

Optimal energy management for a hybrid energy storage system combining batteries and double layer capacitors

Abstract: Many mobile vehicular applications like hybridelectrical cars and autonomous rail vehicles require an onboard energy storage for operation. High demands concerning power and energy density, small volume and weight at the same time cannot be satisfied solely by batteries or double layer capacitors. A suitable approach is to combine storage technologies with complementary characteristics as a hybrid energy storage system. Thus, long term storage like batteries featuring high energy density can be combined with short term storage like double layer capacitors offering high power density and high cycliability. To control the power flows of the system, we propose the use of self-optimization methods involving multi-objective and discrete optimization to design an operating strategy which is able to adapt the system behavior to different conditions not only by adapting its parameters but also its objectives, offering an optimal operation in different situations.