Maximum Attainable Efficiency Research Articles

Comments on "Class-E, Class-C, and Class-F Power Amplifier Based Upon a Finite Number of Harmonics"

In this comment, we would like to call attention to a qualitative fact that the maximum attainable efficiencies of finite harmonic class C, contrary to the claim suggested in the above paper, differ from maximum attainable efficiencies of finite harmonic class-F power amplifiers (PAs). The difference is not significant and could be easily overlooked in numerical treatment of the problem. This probably explains how the claim came into being. The best way to prove that difference between efficiencies of finite harmonic class-C and class-F exists is to solve the problem in closed analytical form. In this comment, we provide closed form for maximum attainable efficiency of finite harmonic class-C using a famous Fejer's result which goes back to 1915.

Spectroscopic and lasing performance of Tm^3+-doped bulk TZN and TZNG tellurite glasses operating around 19 μm

We report spectroscopic and bulk laser performance characteristics for Tm(3+)-doped tellurite glasses when used as gain media operating around 1.9 microm. Two glass hosts studied are TZN and TZNG and their performances have been compared. In each case, well-characterized cw laser performance was obtained and this has been related to detailed spectroscopic measurements of the important lasing parameters of the laser transitions around 1900 nm when pumped at 793 nm. The maximum output power achieved was 124 mW from the TZNG sample with an associated slope efficiency of 28 % with a tuning range of 135 nm. Efficiency and loss analyses yielded a calculated maximum attainable efficiency of 48 % in Tm(3+):TZN compared to 28 % for the TZNG host.

InGaAlN heterostructures for LEDs grown on patterned sapphire substrates

One of the main factors that limit the maximum attainable efficiency of InGaAlN-based light-emitting diode (LED) structures grown on standard sapphire substrates is the low efficacy of extracting light from devices. A promising solution of this problem consists in using specially profiled (patterned) sapphire substrates. A method for the formation of a special surface microrelief on the sapphire substrates is described. The properties of GaN epilayers and InGaAlN-based LED heterostructures grown on such substrates are presented.

Efficiency potential of organic bulk heterojunction solar cells

AbstractIn order for organic bulk heterojunction solar cells to compete with the traditional inorganic cells, power conversion efficiencies of more than 10% are desirable. Nowadays, efficiencies up to 5% are reached and the question about the limits for the attainable efficiency of organic cells arises. In this paper, we study the efficiency potential of organic bulk heterojunction solar cells. We make realistic assumptions to predict efficiencies obtainable in the near future, and calculate the upper‐limit. We study the influence of the difference between the lowest unoccupied molecular orbital (LUMO)‐energy levels of donor and acceptor, and the absorption window on the efficiency. Ideal material characteristics are obtained from these calculations, giving an idea how the ideal organic solar cell should look like. The calculations show that nowadays an efficiency of 5·8% for the single junction bulk heterojunction solar cell should be possible. Considering parameters which are credible to be achieved in the future, an organic solar cell of 15·2% is in reach, with an optimal bandgap of 1·5 eV for the absorber. We also consider the situation where both the n‐ and p‐type materials are absorbers. All calculations are not only done for a single junction cell, but also for tandem solar cells. For a tandem structure of organic cells, we find in a realistic scenario a maximum attainable efficiency of 10·1% and an efficiency of 23·2% in an optimistic scenario with optimal bandgaps Eg1 = 1·7 eV and Eg2 = 1·1 eV. Copyright © 2007 John Wiley & Sons, Ltd.

Strong Enhancement of the Radiative Decay Rate of Emitters by Single Plasmonic Nanoantennas

We demonstrate a strong, 5-fold enhancement of the radiative decay rate from highly efficient fluorescent dye molecules around resonant optical nanoantennas. The plasmonic modes of individual gold dimer antennas are tuned by the particle length and the antenna gap, providing control over both the spectral resonance position and the near-field mode profile of the nanoantenna. Resonant enhancement of the radiative and nonradiative decay rates of a fluorescent dye is observed, resulting in an increase of the internal quantum efficiency from 40% up to 53% for single antennas, and up to 59% for antenna clusters. This improvement of the already high quantum efficiency of the dye molecules is in agreement with electrodynamic model calculations that predict a maximum attainable efficiency around 80% due to nonradiative losses in the metal.

Device model for the operation of polymer/fullerene bulk heterojunction solar cells

We have developed a numerical device model that consistently describes the current-voltage characteristics of polymer:fullerene bulk heterojunction solar cells. Bimolecular recombination and a temperature- and field-dependent generation mechanism of free charges are incorporated. It is demonstrated that in poly[2-methoxy-5-(3('),7(')-dimethyloctyloxy)-p-phenylene vinylene]- (OC1C10-PPV-) and [6,6]-phenyl C-61-butyric acid methyl ester- (PCBM-) (1:4 wt. %) based solar cells space-charge effects only play a minor role, leading to a relatively constant electric field in the device. Furthermore, at short-circuit conditions only 7% of all free carriers are lost due to bimolecular recombination. The model predicts that an increased hole mobility together with a reduction of the acceptor strength of 0.5 eV will lead to a maximum attainable efficiency of 5.5% in the PPV/PCBM-based solar cells.

Geiger mode operation of ultraviolet 4H-SiC avalanche photodiodes

We report single photon counting in p-n junction 4H-SiC avalanche photodetectors. At 325 nm, the unity-gain external quantum efficiency was 10% and the single photon detection efficiency was 2.9%. This result represents /spl sim/30% of the maximum attainable detection efficiency.

Maximum Attainable Performance of Stirling Engines and Refrigerators

The flow through the regenerator of a Stirling engine is driven by differences of pressure in the compression and expansion spaces. These differences lead to power dissipation in the regenerator. Using linearized theory, it is shown that this dissipation severely limits the maximum attainable thermal efficiency and nondimensional power output. The maximum attainable values are independent of the value of the regenerator conductance. For optimized nondimensional power output, the thermal efficiency equals only half the Carnot value. The power dissipated in the regenerator is removed as part of the heat withdrawn at the regenerator’s cold side. Analogous results are presented for the Stirling refrigerator. At optimized nondimensional rate of refrigeration, its coefficient of performance is less than half the Carnot value.

Quantum dot solar cells

Quantum dot (QD) solar cells have the potential to increase the maximum attainable thermodynamic conversion efficiency of solar photon conversion up to about 66% by utilizing hot photogenerated carriers to produce higher photovoltages or higher photocurrents. The former effect is based on miniband transport and collection of hot carriers in QD array photoelectrodes before they relax to the band edges through phonon emission. The latter effect is based on utilizing hot carriers in QD solar cells to generate and collect additional electron–hole pairs through enhanced impact ionization processes. Three QD solar cell configurations are described: (1) photoelectrodes comprising QD arrays, (2) QD-sensitized nanocrystalline TiO 2, and (3) QDs dispersed in a blend of electron- and hole-conducting polymers. These high-efficiency configurations require slow hot carrier cooling times, and we discuss initial results on slowed hot electron cooling in InP QDs.

Improvement of washout flow in a centrifugal blood pump by a semi-open impeller.

To reduce the possible thrombogenicity of the pump studied, pump characteristics and washout conditions were compared between a pump with a semi-open and a pump with a full-open impeller. A difference in hydrodynamic performance was observed between the semi-open impeller and the full-open impeller; the pressure in the former was less by approximately 10%, and the maximum attainable efficiency decreased from 0.41 to 0.34. The flow pattern, as visualized by the oil film method, showed that the washout condition was enhanced by addition of the shroud, especially at the bottom region of the pump where the blood flow tended to be stagnant. The stagnant area was observed in the suction side of the impeller in both models, where the vortices shed from the impeller tip contributed to the washout. It was also shown that the flow entering the bottom region was circumferentially uniform in the full-open impeller, whereas in the semi-open impeller the flow was not uniform and entered primarily from the vicinity of the outlet port. The semi-open impeller, thus, was demonstrated to have better washout conditions than the full-open impeller regardless of a slight decrease in hydrodynamic efficiency.

High-efficiency diode-pumped Q-switched Yb:Er:glass laser

Diode-pumped lasers using Yb–Er phosphate glass with 3×10 19 and 5.6×10 19 cm −3 of Er concentrations are investigated. It is concluded that upconversion plays significant role even for so low doped glasses. The upconversion parameters are estimated as well as maximum attainable efficiency for a diode-pumped Yb:Er:glass laser. Using a single LD bar, 14.5 mJ of Q-switched output pulse energy is obtained under 113 W×5 ms of transversal pumping. Maximum “light-to-light” efficiency obtained is 2.5%, while the maximum slope efficiency is as high as 7.1%. At free-running mode maximum slope efficiency obtained is 31%.

The limiting efficiency of band gap graded solar cells

Two fundamental mechanisms limit the maximum attainable efficiency of solar cells, namely the radiative recombination and Auger recombination. We show in this paper that proper band gap grading of the solar cell localizes the Auger recombination around the metallurgical junction. Two beneficial effects result from this Auger recombination localization; first the cell is less sensitive to the surface conditions, and second, the previous estimates for the limiting efficiency of solar cells by Shockley, Tiedje, and Green are revised upwardly. We calculate the optimum bandgap grading profile for several real material systems, including GaInAsP lattice matched to InP, and a-SiGe on a-Si substrate.

Trade-off between speed and efficiency of silicon metal– i– n photodetectors

In this paper, an optical model is presented to develop an electrical model for a silicon metal– i– n photodetector in the ultraviolet range of the spectrum. Closed form equations for the quantum efficiency and short circuit current density of the device are derived. It is shown that in typical silicon metal– i– n photodetectors, the response speed and the maximum attainable quantum efficiency are the most interactive parameters. In this regard, the maximum attainable quantum efficiency vs response time is obtained. Using the results, one can design the most efficient photodetector for the desired response speed. The sensitivity analysis and the optical bandwidth of the optimum device are also presented.

Experimental performance evaluation of a 99.0 mm radial inflow nozzled turbine with different stator throat areas

The performance of a single-stage, radial inflow turbine was measured with seven vaned stators of different throat areas. The turbine test facility is described and details of the turbine rotor and stator geometry are provided. Efficiency, mass flowrates and rotor static pressure ratios are presented for each turbine configuration at a range of speeds and pressure ratios. The experimental data clearly shows the turbine mass flowrate increasing approximately in proportion to the stator throat area, while the maximum attainable efficiency decreased as the stator throat area decreased.

Maximum attainable efficiency of a cubic nonlinear response of matter

It is shown that the efficiency of a cubic nonlinear optical response of matter, defined as the ratio of the cubic nonlinear susceptibility to its relaxation time, may reach ∼ 108 cm2 J-1 for electronic mechanisms of this response. This is 103—104 times higher than the value achieved so far. The conditions for attaining the maximum efficiency are found.

Performance Evaluation of Deep Well Turbine Pumps

Complete energy audits of 64 tubewells, installed in the metropolitan area of Islamabad, Pakistan, were carried out in order to evaluate the performance of the pumping system and assess the possible potential for energy savings through a programme of routine energy audits and retrofits. The results of 64 tubewell energy audits may be summarized as follows: (1) The average measured overall efficiency was 39% (ranging from 8 to 73%) against a maximum average attainable level of 70%. Fifty out of 64 tubewells were below the minimum acceptable overall efficiency of 50%. This low overall efficiency was mainly attributed to over-size pumps and motors for their application and worn pumping parts. (2) More than 30 electric motors had efficiencies of less than 83% against the maximum attainable efficiency of 90%. The major cause of this low efficiency was underloading since most of the motors were found to be operating below 60% loading. (3) The average measured transmission efficiency was 98% which is 99% of the maximum attainable level of 99%. The transmission lines were generally found to be satisfactory except a few which had a transmission efficiency of 95% or less due to high initial frictional torque. It has been estimated that, if overall efficiency is improved to an average of 65% through a comprehensive retrofit and repair programme, total energy savings of about 4·8 million kWh can be made in the current estimated energy usage of about 12 million kWh per year. The savings would be valued at more than Rs. 7 million (21 pumping hours per day at Rs. 1·50 per kWh) in the metropolitan area of Islamabad, Pakistan.

Deflection network: Principles, implementation, services

AbstractThis paper deals with slotted data networks routing packets with a deflection technique. In particular regular mesh topologies that can be mapped on toroidal surfaces are analyzed. First an introduction to the concept of deflection routing is given, then many routing algorithms are considered and compared by means of their performance in terms of maximum attainable throughput and efficiency. Afterwards the architecture of a node currently under development, which implements one of the described algorithms, is described and analyzed; the switching fabric, a full custom VLSI, the interface of the node towards LANs and workstations, and the network management algorithm are described in detail. Finally the possibility of providing special services, like guaranteed bandwidth, circuit oriented connections, etc. is taken into account.

Design of a 35 GHz gyrotron

The conceptual design of a 35 GHz gyrotron has been developed consistently with the complex formulation of the electric field longitudinal distribution in the resonant cavity. Some models of magnetron injection guns able to produce laminar beams have been investigated leading to the design of an electron gun capable of generating a current of 5 A with a perpendicular velocity dispersion of 0.5%. The device includes three magnetic systems producting flat axial magnetic induction profiles of 1.05 kG, 13.2 kG and 0.65 kG in the cathode, cavity and collector regions, respectively. The gyrotron has been designed for pulsed operation in the TE021 mode. Under the soft self-excitation condition, the maximum attainable efficiency is 40% with an output power of 100 kW. An analysis of the collector thermal behaviour has been carried out as well as a study of the thermophysical properties of the alumina window to be used.

Lasers operating on cascade transitions of triatomic molecules

The general conditions necessary for the operation of gas lasers based on cascade vibrational-rotational transitions of molecules in the pulse-periodic regime are formulated. The features of such triatomic-molecule lasers with arbitrary excitation are discussed. A computation model is proposed for determining the energy characteristics (maximum attainable efficiency, average and relative lasing power, specific energy output) of free-flow cascade lasers. Systems are analyzed with various excitation methods: gasdynamic (including those with optical feedback), electric-discharge, and chemical (with the DF-CO2 laser as the example). Practical recommendations with respect to the choice of mixture composition, optimum cavity lengths along the gas stream, and Q-switching frequency are made for the cascade lasers. A molecular gasdynamic laser emitting three wavelengths simultaneously is proposed.

High-power conversion efficiency in a strained InGaAs/AlGaAs quantum well laser

A strained quantum well laser with a front-end power conversion efficiency exceeding 33% under continuous-wave operation is demonstrated. The laser structure, grown by atmospheric pressure organometallic vapor-phase epitaxy, consists of a 70-Å In0.2Ga0.8As quantum well active region with graded index separate confinement heterostructure. Lasing wavelength is 930 nm, and the front-end differential quantum efficiency is 58% for broad-area oxide stripe lasers with a high-reflection coating on the rear facet. Front-end, continuous power outputs greater than 1 W are available. Although these strained quantum well lasers have threshold currents as low as lattice-matched GaAs quantum well lasers, their internal quantum efficiencies appear to be reduced, thus limiting the maximum attainable conversion efficiency.