Maximum Generation Efficiency Research Articles

A numerical procedure to obtain ideal piston trajectories and key design parameters for a hydrogen-fuelled resonating free piston generator

A new numerical procedure is presented to enable a hydrogen-fuelled two-stroke resonating free-piston generator target ideal Otto cycles to provide tracking paths for feedback control to maximise efficiency. Piston trajectories and design parameters are obtained for specified power, compression-ratio, and air-fuel-ratio. These parameters include scavenge port locations, heat release position, equivalent bottom-dead-centre position, and an electrical machine constant which is generally power-switched to take two values within a cycle. The main objectives of the paper are to develop the numerical procedure, and to demonstrate its use in finding a generator with maximum efficiency, and a generator with a single electrical machine constant, avoiding in-cycle power switching. To develop the procedure, a multi-physics generator model provides kinematics to an energy equation, culminating in a novel two-stage procedure. Stage-1 involves iteration and minimisation; Stage-2 just involves minimisation. Testing by simulation involves a 14-kW generator case study with compression ratio of 9, lean air-fuel equivalence ratio of 0.4, plus friction and electrical machine losses. An overall maximum generator efficiency of 54.9% is obtained using the procedure, compared to 51.3% without power switching. Computationally, the two-stage procedure proves highly efficient, typically taking around 11 min in total to complete on a desktop computer. The novelty of the procedure is its ability to optimally design and operate a hydrogen-fuelled resonating free-piston generator to meet a target specification, thereby offering a potentially inexpensive way to decarbonize transport, particularly in heavy duty applications.

Controlling the time shape of terahertz pulses from two-color plasma by combining wavelength dispersion and laser chirp

Using a two-color plasma in air or gas allows for the generation of ultra-broadband and highly intense terahertz (THz) pulses. Precisely controlling the shape of THz pulses, particularly the carrier envelope phase, is crucial for a wide range of technological and scientific applications. Recent research has revealed that the phase difference between the fundamental laser wave and its second harmonic influences the shape of the emitted THz pulse. However, the combined impact of laser chirp and the air-dispersion-induced phase difference between the two colors has not been explored in detail. This study aims to fill this gap. The results show that both of these factors have a significant impact on the THz waveform and emitted pulse energy. Positive and negative chirps lead to distinct effects on the pulse shape. A positively chirped pump laser produces a THz waveform with a negative monopolar shape, while a negatively chirped pump laser results in a positive THz pulse shape. Additionally, our findings indicate that the relationship between THz pulse energy and chirp changes with variations in the specific phase difference between the first and second harmonics. These results underscore the importance of carefully tuning both parameters to achieve maximum generation efficiency for a given waveform.

High-performance polarization-controlled perfect optical vortex generation via all-dielectric metasurfaces in the visible region

Metasurfaces, with superiority in integration and flexibility compared to conventional bulky optical elements, open a perspective in generating perfect optical vortex (POV). However, the generation of POV beams with simultaneous high-efficiency, multi-functionality, and broadband is still challenging. Here, we design POV generators based on highly efficient all-dielectric metasurfaces and produce POV beams in simulation in the visible region with maximum generation efficiency of 74%. Such high efficiency originates from the stimulation of the resonance modes of polarization-dependent parity in the well-designed unit cell. By superposing two sets of phase profiles, polarization-controlled POV generators are obtained. Furthermore, the POV generator exhibits broadband property due to the geometric phase independent of the incident wavelengths. Our work provides an effective approach for generating controllable POV beams, promoting the construction of integrated and multi-functional optical systems.

High order modes of intense second harmonic light produced from a plasma aperture

Because of their ability to sustain extremely high-amplitude electromagnetic fields and transient density and field profiles, plasma optical components are being developed to amplify, compress, and condition high-power laser pulses. We recently demonstrated the potential to use a relativistic plasma aperture—produced during the interaction of a high-power laser pulse with an ultrathin foil target—to tailor the spatiotemporal properties of the intense fundamental and second-harmonic light generated [Duff et al., Sci. Rep. 10, 105 (2020)]. Herein, we explore numerically the interaction of an intense laser pulse with a preformed aperture target to generate second-harmonic laser light with higher-order spatial modes. The maximum generation efficiency is found for an aperture diameter close to the full width at half maximum of the laser focus and for a micrometer-scale target thickness. The spatial mode generated is shown to depend strongly on the polarization of the drive laser pulse, which enables changing between a linearly polarized TEM01 mode and a circularly polarized Laguerre–Gaussian LG01 mode. This demonstrates the use of a plasma aperture to generate intense higher-frequency light with selectable spatial mode structure.

Effective Direct Power Control for a Sensor-Less Doubly Fed Induction Generator with a Losses Minimization Criterion

The present paper is concerned with introducing an effective direct power control (DPC) approach for a sensor-less doubly fed induction generator (DFIG). The derivation of the proposed DPC approach is performed in a systematic manner in which the design of the rotor current controllers is well analyzed, which clarifies the real base of the control system as when and why it works properly. The operation of the proposed DPC approach is based on the stator voltage-oriented control principle in which the stator voltage is aligned with the quadrature axis of the rotating reference frame. To obtain maximum generation efficiency, the reactive power reference value is derived based on a loss minimization criterion (LMC) that is described and analyzed in detail. To enhance the robustness of the control system, an effective rotor position estimator is proposed that is robust against the system uncertainties, such as the parameters’ variation. To validate the effectiveness of the proposed sensor-less DPC approach, the DFIG dynamic performance is tested for a wide range of operating speeds. The obtained results confirm and emphasize the feasibility of the proposed control approach and its LMC methodology in improving the generation efficiency and in obtaining high dynamic performance from the DFIG.

A Control Scheme for Variable-Speed Micro-Hydropower Plants

The aim of this work was to design and build a control system to control the performance of the Pelton wheel and synchronous generator system at different upstream water flow and electrical load conditions. The turbine output power is determined by the upstream water flow and spear valve, whilst the generator output power is determined by the turbine output power and the electrical load. A spear valve is used to control the generator output power at different water and load conditions. An autotuning proportion integration (PI) arithmetic-based controller was built using a relay feedback tuning method. An on–off relay was used in the program in order to oscillate the system. The optimal PI gains can be estimated via the Ziegler–Nichols method. A fully open test was used to test the tuned PI gains. The performance of the original gains and the new tuned gains were discussed. A controller was used to maintain the frequency or voltage of the output power by automatic regulation of the turbine valve. The program could search for the maximum generation efficiency by entering the output current value of the generator into the program manually.

Giant spin-torque generation by heavily oxidized Pt

We report a giant spin-torque generation originated from the ferromagnet/heavily oxidized Pt interface. The heavily oxidized Pt was fabricated by magnetron sputtering in a mixture of oxygen and argon gases with different oxygen flow rate $Q$ from $50%$ to $100%$. The dominant structure of the oxidized Pt is confirmed to be ${\mathrm{PtO}}_{2}$ in this $Q$ range. We show that even in this heavily oxidized range, robust dampinglike and fieldlike spin torques can still be generated from the ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$/oxidized Pt interface. By increasing the oxidation level of the oxidized Pt, the generation efficiencies of both dampinglike and fieldlike torques increase drastically, and noticeably, a maximum generation efficiency of 0.92 for the dampinglike torque is obtained. Our study further demonstrates that the generation efficiency of the dampinglike torque has a much larger value than that of the fieldlike torque, which indicates a dominant generation of the dampinglike torque from the interface. Since no energy dissipation occurs in the bulk of the spin-torque generator, this study provides a piece of information for the development of the energy-efficient spin-orbit devices based on insulating metallic oxides.

Optimization of terahertz generation from LiNbO3 under intense laser excitation with the effect of three-photon absorption.

We proposed a three-dimensional model to simulate terahertz generation from LiNbO3 crystal under intense laser excition (up to ~50 mJ/cm2). The impact of three-photon absorption, which leads to free carrier generation and free carrier saturation (when pump fluence above ~10 mJ/cm2) on terahertz generation was investigated. And further with this model, we stated the optimized experimental conditions (incident postion, beam diameter, and pulse duration, etc) for maximum generation efficiency in commonly-used tilted-pulse-front scheme. Red shift of spectrum, spatial distribution "splitting" effects of emitted THz beam, and primilary experimental verification under intense laser excitation are given.

Response to “Comment on ‘Effective thermal conductivity in thermoelectric materials’” [J. Appl. Phys. 113, 204904 (2013)

It is commonly claimed that achieving maximum power from a thermoelectric generator necessitates electrical load matching conditions instead of the operating condition derived for maximum generator efficiency. Here, we explain why the electrical load matching claim for maximum power in a design optimization is flawed and show that the load condition derived for maximum efficiency always produces more power. Finally, we consider a CPM generator, and prove that the electrical condition for maximum efficiency is indeed the electrical condition for maximum power, maximum power density, maximum power/cost of thermoelectric material, and maximum power/weight of thermoelectric material, when the leg length of the thermoelectric generator is a design variable.

The influence of operating and device parameters on the maximum efficiency and the maximum output power of thermoelectric generator

Thermoelectric power generators are one of the promising green energy sources. The operating and the generator parameters influence the generator output performance. In the present study, the influence of the operating and the generator parameters on the maximum output power and the efficiency of the thermoelectric power generator are examined. The output power corresponding to the maximum efficiency and the maximum attainable output power of the generator are compared. It is found that the maximum power of the thermoelectric generator corresponding to the high Figure of Merit is very sensitive to the operating temperature. The maximum power attainable is larger than that its counterpart corresponding to the maximum generator efficiency. Copyright © 2010 John Wiley & Sons, Ltd.

Čerenkov radiation excited by an ultrashort laser pulse with oblique amplitude front

We develop the theory of generation of terahertz Cerenkov radiation in a homogeneous nonlinear optical crystal if the amplitude and phase fronts of the excitation pulse do not coincide. It is shown that in the case of the nonlinear optical interaction, the use of laser pulses with oblique amplitude front makes it possible to increase the efficiency of generation of terahertz radiation. In contrast to the case where the amplitude and wave fronts coincide, radiation propagates in a strictly determined direction specified by the amplitude-front plane. It is established that the maximum generation efficiency is reached if the angle of inclination of the amplitude front coincides with that of the terahertz-radiation wave vector.

Polarisation dependences of harmonic generation in the plasma produced in the ionisation of excited-state hydrogen-like atoms

An analytic theory of harmonic generation in the plasma produced from the gas of hydrogen-like atoms in excited states is considered for relatively intense radiation. The consideration of l-degeneracy of the electrons in these excited states allowed deriving the dependence of generation efficiency on the principal quantum number. In the context of the Bethe model of gas ionisation, we revealed the threshold nonlinear dependence of the maximum generation efficiency on the degree of circular polarisation of the pump field for its given intensity. Analytic calculations were performed for the fifth and seventh harmonics. The results of these calculations allowed generalising to the case of excited atoms the previously obtained results for the third harmonic in the plasma arising from hydrogen-like atoms in the ground state.

Ozone Generator with Cylindrical Type of Rotating Electrode

An ozone generator using a rotating electrode to improve ozone generation efficiency is proposed. The ozone generator electrode unit consists of a rotating electrode and fixed electrode. The rotating electrode has the grounded 36 pieces of tungsten wires fixed in parallel to the rotation axis on the rotating cylinder surface. A dielectric electrode is used as a fixed electrode located on the inside of the tube of the electrode unit. The width of the apparent discharge gap is 1mm. Alternating current with a frequency of 50 Hz is applied to the electrode unit. The rotation speed can be adjusted from 0 rpm to 1200 rpm by a variable speed motor. Oxygen gas is used as the material gas. Higher ozone concentration and higher ozone generation efficiency are obtained compared with that when the rotation speed is 0 rpm. The gas temperature is measured at the inlet and outlet of the ozone generator, and the rotation speed for the cooling effect is most effective at about 500 rpm. The maximum generation efficiency is estimated to be 61 g/kWh at 800 rpm, and this value is twice as large as in the case of 0 rpm.

Background of gravitational waves from pre-galactic black hole formation

We study the generation of a gravitational wave (GW) background produced from a population of core-collapse supernovae, which form black holes in scenarios of structure formation of the Universe. We obtain, for example, that a pre-galactic population of black holes, formed at redshifts z ≃ 30–10, could generate a stochastic GW background with a maximum amplitude of hBG ≃ 10−24 in the frequency band νobs ≃ 30–470 Hz (considering a maximum efficiency of generation of GWs, namely, εGW = 7 × 10−4). In particular, we discuss what astrophysical information could be obtained from a positive, or even a negative, detection of such a GW background produced in scenarios such as those studied here. One of them is the possibility of obtaining the initial and final redshifts of the emission period from the observed spectrum of GWs.

Gravitational wave background from Population III black hole formation

We study the generation of a stochastic gravitational wave (GW) background produced from a population of core-collapse supemovae, which form black holes in scenarios of structure formation. We obtain, for example, that the formation of a population (Population III) of black holes, in cold dark matter scenarios, could generate a stochastic GW background with a maximum amplitude of h B G ≃ 10 - 2 4 and corresponding closure energy density of Ω G W 10 - 7 , in the frequency band ν o b s ≃ 30-470 Hz (assuming a maximum efficiency of generation of GWs, namely, eGw m a x = 7 X 10 - 4 ) for stars forming at redshifts z ≃ 30-10. We show that it will be possible in the future to detect this isotropic GW background by correlating the signals of a pair of 'advanced' LIGO observatories (LIGO III) at a signal-to-noise ratio of ≃40. We discuss what astrophysical information could be obtained from a positive (or even a negative) detection of such a GW background generated in scenarios such as those studied here. One of them is the possibility of obtaining the initial and final redshifts of the emission period from the observed spectrum of GWs.

Induction MHD Generator Using Alternating Magnetic Field

The induction MHD generator using an alternating magnetic field is proposed. The characteristics of the machine are analyzed theoretically and also compared with those of the induction MHD generator using a traveling magnetic field. Following conclusions are obtained for the fundamental characteristics of the present machine: 1. This type of the machine is possibly operated not only as the generator but also as the pump or as the damper.2. The optimum condition for the maximum generator efficiency exists among the relations of the frequency, the fluid velocity and the inner core radius because of the eddy current loss due to an alternating magnetic field.3. The power ratio of the reactive power of the machine to the gross output power can be reduced to a much smaller value than that of the traveling wave MHD generator. Therefore, even in the case of the working fluid with a relative low electrical conductivity such as two-phase liquid metal flow with high void fraction, the acceptable power ratio can be expected.4. For the working fluid with higher electrical conductivity the skin effect is also able to be reduced to the acceptable level in the present machine, while it is a serious problem in the traveling wave MHD generator.

Induction MHD Generator Using Alternating Magnetic Field

The induction MHD generator using an alternating magnetic field is proposed. The characteristics of the machine are analyzed theoretically and also compared with those of the induction MHD generator using a traveling magnetic field. Following conclusions are obtained for the fundamental characteristics of the present machine: 1. This type of the machine is possibly operated not only as the generator but also as the pump or as the damper. 2. The optimum condition for the maximum generator efficiency exists among the relations of the frequency, the fluid velocity and the inner core radius because of the eddy current loss due to an alternating magnetic field. 3. The power ratio of the reactive power of the machine to the gross output power can be reduced to a much smaller value than that of the traveling wave MHD generator. Therefore, even in the case of the working fluid with a relative low electrical conductivity such as two-phase liquid metal flow with high void fraction, the acceptable power ratio can be...

A Device Characterization and Circuit Design Procedure for Realizing High-Power Millimeter-Wave IMPATT-Diode Amplifiers

A measurement and characterization technique is presented which allows design and realization of IMPATT amplifiers operating at maximum generation efficiency. Diodes mounted in a stable reduced-height waveguide circuit are characterized by their complex reciprocal scattering coefficient as a function of frequency, dc bias, and RF drive power level. In particular, terminal conditions which correspond to a maximum power exchange between the active one-port network and the source are identified and then used to design and realize controlled-gain maximum generation-efficiency amplifiers. Simple equalization networks are shown to provide a wide range of available amplifier gains between limits set by stability requirements. As an example, the technique is effectively used with silicon diodes at K/sub a/-band (33-40 GHz) to realize a 7-dB gain 250-mW power amplifier with 5-GHz bandwidth.