Showing papers on "Power density published in 2001"

Electrochemical Properties of High-Power Supercapacitors Using Single-Walled Carbon Nanotube Electrodes

Abstract: We have investigated the key factors determining the performance of supercapacitors constructed using single-walled carbon nanotube (SWNT) electrodes. Several parameters, such as composition of the binder, annealing temperature, type of current collector, charging time, and discharging current density have been optimized for the best performance of the supercapacitor with respect to energy density and power density. We find a maximum specific capacitance of 180 F/g and a measured power density of 20 kW/kg at energy densities in the range from 7 to 6.5 Wh/kg at 0.9 V in a solution of 7.5 N KOH (the currently available supercapacitors have energy densities in the range 6‐7 Wh/kg and power density in the range 0.2‐5 kW/kg at 2.3 V in non-aqueous solvents).

Trapping effects and microwave power performance in AlGaN/GaN HEMTs

Abstract: The dc small-signal, and microwave power output characteristics of AlGaN/GaN HEMTs are presented. A maximum drain current greater than 1 A/mm and a gate-drain breakdown voltage over 80 V have been attained. For a 0.4 /spl mu/m gate length, an f/sub T/ of 30 GHz and an f/sub max/ of 70 GHz have been demonstrated. Trapping effects, attributed to surface and buffer layers, and their relationship to microwave power performance are discussed. It is demonstrated that gate lag is related to surface trapping and drain current collapse is associated with the properties of the GaN buffer layer. Through a reduction of these trapping effects, a CW power density of 3.3 W/mm and a pulsed power density of 6.7 W/mm have been achieved at 3.8 GHz.

Very-high power density AlGaN/GaN HEMTs

Abstract: Research work focusing on the enhancement of large-signal current-voltage (I-V) capabilities has resulted in significant performance improvement for AlGaN/GaN HEMT's. 100-150 /spl mu/m wide devices grown on SiC substrates demonstrated a record power density of 9.8 W/mm at 8 GHz, which is about ten times higher than GaAs-based FETs; similar devices grown on sapphire substrates showed 6.5 W/mm, which was thermally limited, 2-mm-wide devices flip-chip mounted on to AlN substrates produced 9.2-9.8 W output power at 8 GHz with 44-47% PAE. A flip-chip amplifier IC using a 4-mm device generated 14 W at 8 GHz, representing the highest CW power obtained from GaN-based integrated circuits to date.

An implantable power supply with an optically rechargeable lithium battery

Abstract: A novel power supply for medical implants has been developed. A wireless near-infrared power transmission recharges a lithium secondary battery in the power supply. A photovoltaic cell array embedded under skin receives near-infrared light through the skin and charges the battery directly powering an implanted device. The authors have shown that, for a photodiode area of 2.1 cm/sup 2/, 17 min of near-infrared irradiation at a 810-mn wavelength with a power density of 22 mW/cm/sup 2/ can send enough energy to allow regular commercial cardiac pacemakers to run for 24 h. The temperature rise of the skin during the light irradiation was 1.4/spl deg/C.

Power supply system, fuel pack constituting the system, and device driven by power generator and power supply system

Abstract: There is provided a power supply system which can cause a predetermined device to stably and excellently operate by only directly connecting an electrode terminal to the device, suppress waste of a power generation fuel, and achieve effective use of an energy resource.

Vehicular power supply apparatus and engine-drive-regulation supporting apparatus

Abstract: A power feed portion includes a power storage portion and a power generating portion. The power storage portion includes a storage battery. The power feed portion feeds electric power to a plurality of on-vehicle loads. In cases where the sum of feedable electric power is smaller than the sum of required electric power or in cases where an electric quantity related to the sum of feedable electric power is smaller than an electric quantity related to the sum of required electric power, a control portion increases the sum of feedable electric power or decreases the sum of required electric power. The sum of feedable electric power is equal to electric power which can be fed from the power feed portion to the on-vehicle loads, and which contains electric power generated by the power generating portion and electric power feedable from the storage battery.

Thermodynamic analysis of thermophotovoltaic efficiency and power density tradeoffs

Abstract: This report presents an assessment of the efficiency and power density limitations of thermophotovoltaic (TPV) energy conversion systems for both ideal (radiative limited) and practical (defect-limited) systems. Thermodynamics is integrated into the unique physics of TPV conversion, and used to define the intrinsic tradeoff between power density and efficiency. The results of the analysis reveal that the selection of diode band gap sets a limit on achievable efficiency well below the traditional Carnot level. In addition it is shown that filter performance dominates diode performance in any practical TPV system and determines the optimum band gap for a given radiator temperature. It is demonstrated that for a given radiator temperature, lower band-gap diodes enable both higher efficiency and power density when spectral control limitations are included. The goal of this work is to provide a better understanding of the basic system limitations that will enable successful long-term development of TPV energy conversion technology.

Efficiency and power density potential of combustion-driven thermophotovoltaic systems using GaSb photovoltaic cells

Abstract: The renewed interest in thermophotovoltaic (TPV) energy conversion, based on recent progress in materials and photovoltaic (PV) cell technology, requires a new evaluation of the TPV efficiency and power density potential. In this paper, we address some important points in TPV system design. We proceed in three steps, analyzing 1) the thermodynamic limit, 2) an idealized, and 3) a realistic model, based on an extrapolation of the current state of technology. In a TPV system, the radiation converted to electricity is adapted to the spectral response of the PV cell. This can be achieved by different means, which are examined in detail. Broadband and selective radiators, and optical filters are considered. We focus on combustion driven systems using low bandgap GaSb PV cells. For a system containing GaSb cells and a radiator at 1500 K, we find a thermodynamic limit efficiency of 60.5% and an output power density of 3 W/cm/sup 2/. For an idealized system model, an efficiency of 34% and a power density of 2.2 W/cm/sup 2/ are determined. For a realistic system with a broadband radiator and a filter, 9% and 1.2 W/cm/sup 2/ are estimated; using a selective radiator without filter, 16% and 1 W/cm/sup 2/ are expected. Performance values of this order should be achievable with a sufficient development effort.

Development and characterization of vacuum plasma sprayed thin film solid oxide fuel cells

Abstract: The vacuum plasma spraying (VPS) process allows the production of thin solid oxide fuel cells (SOFCs) with low internal resistances. This enables the reduction of the cell operating temperature without a significant decrease in power density. Consequently, the long-term stability of the cells can be improved and low-cost materials can be used.

Anode supported solid oxide fuel cells with screen-printed cathodes

Abstract: To increase power density at reduced temperature operation of SOFC, thin film 8YSZ electrolytes were deposited on Ni–YSZ anode support plates by tape casting and cofiring. Cathode behaviour, limiting cell output at lower temperature, was studied in more detail. Cathodes were deposited by screenprinting and firing. With consecutive layers of doped lanthanum manganite and cobaltite, power density of 0.5 W cm−2 at 750°C was obtained using hydrogen fuel. On cells of 100 cm2, no fuel diffusion limitation above 70% conversion occurred, and electrical efficiency of 35% was achieved. Actual cell temperature increases significantly above a current density of 0.3 A cm−2. This effect causes erroneous electrode and cell characteristics.

Activated carbon for use in electric double layer capacitors

Abstract: Activated carbon adapted for electric double layer capacitors is provided, which capacitors can give a large power density per unit volume and which capacitors, even when charge-and-discharge cycle is repeated under a large current or a constant voltage is continuously applied for a long time, undergo less decrease in output density. That is, the present invention relates to the activated carbon manufactured by carbonization of coconut shell, which has a BET specific surface area of 2000 m2/g to 2500 m2/g, an average pore diameter of 1.95 nm (19.5 Å) to 2.20 nm (22 Å) and a pore volume of pores having a pore diameter calculated according to a Cranston-Inkley method of 5.0 nm (50 Å) to 30.0 nm (300 Å), of 0.05 cm3/g to 0.15 cm3/g.

Amplified spontaneous emission in active channel waveguides produced by electron-beam lithography in LiF crystals

Abstract: In this letter we report the observation of amplified spontaneous emission of the red light from LiF:F2 centers in active channel waveguides realized by electron-beam lithography in lithium fluoride crystals. Low pumping power densities have been used in quasi-continuous-wave regime at room temperature; the appreciable values of the gain coefficients, 4.67 cm−1 with an exciting power density of 0.31 W/cm2 at 458 nm, make this material a good candidate for the realization of active integrated optical devices.

GaN/AlGaN HEMTs operating at 20 GHz with continuous-wave power density >6 W/mm

Abstract: MBE-grown GaN/AlGaN HEMTs have been fabricated on a 2/spl deg/ SiC wafer, where the source-drain spacing was 2 /spl mu/m and the gate length was 0.15 /spl mu/m. A peak extrinsic transconductance of 350 mS/mm and a maximum drain current density greater than 1.5 A/mm were obtained. Small-signal S-parameter measurement showed f/sub r/ of 85 GHz and f/sub MAN/ approaching 140 GHz. At 20 GHz, a continuous-wave output power density of 6.6 W/mm was obtained with power-added-efficiency of 35%, yielding the highest reported power performance at 20 GHz.

Bias-dependent performance of high-power AlGaN/GaN HEMTs

Abstract: Large-signal behavior with a fixed load and varying supply voltages was proposed for characterizing the quality of AlGaN/GaN HEMTs. Improved devices demonstrated constantly high PAEs of 56-62% at 8 GHz throughout a wide voltage range from 10 to 40 V. These 300-/spl mu/m-wide devices also generated 3.1-W output power with only 3.4-dB gain compression at 45 V, which translates to 10.3-W/mm power density; the highest for any FET of the same size.

A highly efficient NiO-Doped MgO matched emitter for thermophotovoltaic energy conversion

Abstract: A novel, thermally excited emitter material for thermophotovoltaic (TPV) energy conversion is described in this work. The new emitter is called a ‘matched emitter’ because its emissive power spectrum is matched very efficiently to the response of GaSb photovoltaic cells that convert the infrared radiation into electricity. It has been shown that doping concentrations of 2–4 wt.% Co 3 O 4 or NiO within a low infrared emissivity MgO host will produce matched emitters with continuous, strong radiant emissions in the optimal energy range between 1 and 2 μm and minimal radiation at non-convertible wavelengths. The experimental measurements show that power densities of almost 9 W cm −2 , in the convertible wavelength range between 1 and 1.9 μm, and temperatures of 1400°C or more are easily achieved with NiO-doped MgO tape cast emitters in an air–propane flame. The first reported emittance measurements for NiO-doped MgO tape cast emitters show that remarkably high peak emissivities, up to 0.9, at near optimal wavelengths for TPV energy conversion are achievable. Both the measured power density (9 W cm −2 ) and peak emissivity (nearly 0.9) appear to greatly exceed all previous values reported for any selective emitter at the given temperatures and wavelengths.

Optimum distribution of heat exchanger inventory for power density optimization of an endoreversible closed Brayton cycle

Abstract: In this paper, the power density (defined as the ratio of the power output to the maximum specific volume in the cycle) is taken as the objective for performance optimizations of an endoreversible closed Brayton cycle coupled to constant-temperature heat reservoirs in the viewpoint of finite-time thermodynamics (FTT) or entropy generation minimization (EGM). The optimum heat conductance distribution corresponding to the optimum power density of the hot- and cold-side heat exchangers for the fixed heat exchanger inventory is analysed using numerical examples. The influence of some design parameters on the optimum heat conductance distribution and the maximum power density and the optimum pressure ratio corresponding to the maximum power density are provided. The power plant design with optimization leads to higher efficiency and smaller size.} \fnm{3}{Author to whom correspondence should be addressed.

Silicon micromachining using in situ DC microplasmas

Abstract: This paper reports on the generation of spatially confined plasmas and their application to silicon etching. The etching is performed using SF/sub 6/ gas and dc power applied between thin-film electrodes patterned on the silicon wafer to be etched. The electrodes also serve as a mask for the etching. The typical operating pressure and power density are in the range of 1-20 Torr and 1-10 W/cm/sup 2/, respectively. The plasma confinement can be varied from 1 cm by varying the electrode area, operating pressure, and power. High power densities can be achieved at moderate currents because the electrode areas are small. Etch rates of 4-17 /spl mu/m/min., which enable through-wafer etching and varying degrees of anisotropy, have been achieved. The etch rate increases with power density, whereas the etch rate per unit power density increases with operating pressure. Scaling effects are explored for varying sized mask openings. Plasma resistance measurements and electric field modeling are used to provide an initial assessment of the microplasmas.

High-power VCSEL arrays for emission in the watt regime at room temperature

Abstract: Selectively oxidized InGaAs vertical-cavity surface emitting lasers (VCSELs) at an emission wavelength of /spl lambda/=980 mm are investigated for high-power applications. Densely packed arrays consisting of 19 single devices with an active diameter of 50 /spl mu/m emit 1.08 W of continuous-wave (CW) optical output power at room temperature. At 10/spl deg/C, heat sink temperature the output power increases to 1.4 W, which corresponds to a chip size averaged power density of 1 kW/spl middot/cm/sup 2/. Low divergence angle of less than 16/spl deg/ full-width at half-maximum (FWHM) and the circularly symmetric far-field pattern allow for simple focusing of the beam with power densities above 10 kW/spl middot/cm/sup 2/.

Effect of light power density on development of elastic modulus of a model light-activated composite during polymerization.

Abstract: Purpose: Elastic modulus development during polymerization of a composite is a measure of the polymerization maturity and the restoration's ability to transfer stress to enamel and dentin. The characteristics of elastic modulus development in real time during cure are largely unknown. The purpose of this study was to evaluate the effect of light power density and total energy density on the early development of elastic modulus for a light-activated composite. Methods: Cylindrical specimens of a model hybrid composite were tested in flexure in a dynamic mechanical analyzer (DMA). Specimens were light-activated (Variable Intensity Polymerizer, Bisco, Itasca, Illinois) for 60 seconds. Elastic modulus was measured continuously for 5 minutes from the start of light activation. Development of elastic modulus was assessed for six different light power densities and two reduced power density levels given at longer exposure duration to provide similar energy density values. One-way analysis of variance with Tukey's post hoc comparison test was used to evaluate significant differences of elastic modulus at p= .05. Results: The rates of elastic modulus development and final moduli were dependent on the light power density applied. Composite specimens cured by equivalent energy densities using short times and high power density or long times and low power density produced equivalent elastic moduli. Elastic moduli for emitted power densities between 400 and 600 mW/cm2 (160–260 mW/cm2 measured at the specimen surface) were not significantly different (p > .05). Conclusions: Light power densities greater than 160 mW/cm2 measured at the specimen surface resulted in elastic moduli that were not significantly different. Equivalent energy densities produced comparable elastic moduli.

Fabrication of Light weight Flexible CIGS solar Cells for Space Power Applications

Abstract: A novel, non-vacuum and a patented process developed by ISET for manufacturing terrestrial CIGS solar cells was successfully applied to fabricate CIGS solar cells on flexible 50 μm thick Mo foil for space power applications. Solar cells of area 5.0 cm 2 were fabricated with their AM0 efficiency approaching 9.0% with an estimated power density of 250 watts/kg. With further improvements it is possible to fabricate CIGS solar cells with AM0 efficiencies approaching 13%. This process can be used to fabricate low cost, light-weight CIGS solar cells on a variety of flexible substrates much lighter than Mo foil to achieve power densities approaching 1.0 kW/kg.

CMOS design challenges to power wall

Abstract: CMOS power dissipation has been increasing due to the increase in power density. The power dissipation increased fourfold every three years until the early 1990's, due to a constant voltage scaling. Recently, a constant field scaling has been applied to reduce power dissipation, where the power density is increased proportional to the 0.7th power of scaling factor, resulting in power increase by twice every 6.5 years. It is considered that the power dissipation of CMOS chips will steadily be increased as a natural result of device scaling. Technology scaling will become difficult due to the power wall. On the other hand, future computer and communications technology will require further reduction in power dissipation. Since no new energy efficient device technology is on the horizon, low power CMOS design should be challenged. This paper discusses what and how much designers can do for CMOS power reduction.

Electrochemical double-layer energy storage cells with high energy density and high power density

Abstract: The invention concerns a method for preparing activated carbons based on wood, preferably softwood and in particular pine wood, for making electrodes for energy storage cells, particularly for super-capacitors. Said activated carbons have a volume of mesopores less than 75 % of the total pore volume and a volume of micropores less than 57 % of the total pore volume. The invention also concerns a method for making an electrode for energy storage cell, comprising the application of such an activated carbon on a support, preferably by coating derived from a slurry. The energy storage cells using said activated carbons advantageously provide a better compromise between energy density and power density.

Development and operation of a 150 W air-feed direct methanol fuel cell stack

Abstract: A five-cell 150 W air-feed direct methanol fuel cell (DMFC) stack was demonstrated. The DMFC cells employed Nafion 117® as a solid polymer electrolyte membrane and high surface area carbon supported Pt-Ru and Pt catalysts for methanol electrooxidation and oxygen reduction, respectively. Stainless steel-based stack housing and bipolar plates were utilized. Electrodes with a 225 cm2 geometrical area were manufactured by a doctor-blade technique. An average power density of about 140 mW cm−2 was obtained at 110 °C in the presence of 1 M methanol and 3 atm air feed. A small area graphite single cell (5 cm2) based on the same membrane electrode assembly (MEA) gave a power density of 180 mW cm−2 under similar operating conditions. This difference is ascribed to the larger internal resistance of the stack and to non-homogeneous reactant distribution. A small loss of performance was observed at high current densities after one month of discontinuous stack operation.

Experimental results on the direct electrochemical oxidation of methanol in PEM fuel cells

Abstract: The cell performance of direct methanol fuel cells (DMFC) is 0.5 V at 0.5 A cm−2 under high pressure oxygen operation (3 bar abs.) at 110 °C. However, high oxygen pressure operation at high temperatures is only useful in special market niches. Therefore, our work has now focused on air operation of a DMFC under low pressure (up to 1.5 bar abs.). At present, a power density of more than 100 mW cm−2 can be achieved at 0.5 V on air operation at 110 °C. These measurements were carried out in single cells with an electrode area of 3 cm2 and the air stoichiometry only amounted to 10. The effects of methanol concentration and temperature on the anode performance were studied by pseudo half cell measurements and the results are presented together with their impact on the cell voltage. A cell design with an electrode area of 550 cm2, which is appropriate for assembling a DMFC stack, was tested. A three-celled stack based on this design revealed nearly the same power densities as in the small experimental cells at low air excess pressure and the voltage–current curves for the three cells were almost identical. At 110 °C a power output of 165 W at a stack voltage of 1.5 V can be obtained in the air mode.

Performance Comparisons of Conventional and Line-Focused Surface Raman Spectrometers

Abstract: Four Raman spectrometer configurations were compared for obtaining spectra of surface monolayers, including a new design that employed a line rather than point focus. Each spectrometer used a 514.5 nm laser and charge-coupled device (CCD) detector, but they differed in collection efficiency and sampling optics. Previously defined figures of merit for Raman signal and signal-to-noise ratio (SNR) were determined for each spectrometer, to aid quantitative comparison. The figure of merit for SNR, FSNR, is demonstrated to be useful for comparisons because it permits prediction of SNR for a given spectrometer, sample, and measurement conditions. A rigorous definition of FSNR is based on power density and takes into account the laser damage threshold of the sample. A simpler but less rigorous definition is based on laser power at the sample rather than power density and may be more useful to users who rarely determine the laser spot size. A new spectrometer design employing line focusing and collection is presented, with f/2 optics and a 6 mm slit image at the CCD. A proprietary aberration correction prevents slit image curvature common to most spectrographs with low f/#, and permits full height binning of the CCD. The line-focused spectrometer yielded an SNR and FSNR which are comparable to those for a point focus using the same collection optics and slightly lower than those for the most efficient spectrograph examined. However, the line focus permitted much lower power densities to be employed, or yielded much larger signal for the same power density at a point focus. In quantitative terms, the new line-focused design yielded an SNR which is 67 times that of the best point-focused system, for the same sample, measurement time, and laser power density.

Power Density Optimization for an Irreversible Closed Brayton Cycle

Abstract: In this paper, the power density, defined as the ratio of power output to the maximum specific volume in the cycle, is taken as objective for performance optimization of an irreversible closed Brayton cycle coupled to constant-temperature heat reservoirs in the viewpoint of finite time thermodynamics (FTT) or entropy generation minimization (EGM). The analytical formulas about the relations between power density and pressure ratio are derived with the heat resistance losses in the hot- and cold-side heat exchangers and the irreversible compression and expansion losses in the compressor and turbine. The maximum power density optimization is performed by searching the optimum heat conductance distribution corresponding to the optimum power density of the hot- and cold- side heat exchangers for the fixed heat exchanger inventory. The influence of some design parameters on the optimum heat conductance distribution, the maximum power density, and the optimum pressure ratio corresponding to the maximum power density are provided. The power plant design with optimization leads to a higher efficiency and smaller size including the compressor, turbine, and the hot- and cold-side heat exchangers.