Time-averaged Power Density Research Articles

Performance optimization and comparison of vertical motion-based triboelectric nanogenerators.

The vertical motion configuration is a common design in triboelectric nanogenerators (TENGs) for energy harvesting; however, the performance optimization and comparison are still vague between various vertical motion-based structures. In this paper, time-averaged power density is defined as a metric to compare the power output performances of vertically structured TENGs, including contact mode and freestanding mode. To ensure comparisons under the same circumstances, a novel sandwich-structured dielectric layer was designed to maintain a stable and consistent surface charge density, with an extra rotating triboelectric nanogenerator working as a charge pump. We also investigated the impact of parasitic capacitance, which is a primary source of error in theoretical optimization. The freestanding TENG (FTENG) with a single dielectric layer demonstrates superior power performance, even when accounting for the influence of parasitic capacitance. This work provides valuable insights and guidelines for the design of high-performance mechanical energy harvesting devices.

A comparison of the power potential for surface- and seabed-deployed tidal turbines in the San Juan Archipelago, Salish Sea, WA

The San Juan archipelago lies along the axis of tidal movement between Straits of Juan de Fuca and Strait of Georgia in the Salish Sea. The amplitude of the tidal exchange produces significant tidal currents between the islands, as well as in Rosario Strait to the east and Haro Strait to the west. These currents are of interest as a future source of electrical power generation, given the archipelago’s dependence on electricity supply by a subsea cable from the United States mainland. Here, we evaluate the tidal current energy potential in this region through a re-analysis of measurements collected by the National Ocean Service (NOS) and a high-resolution numerical model. Given the considerable variations in water depth and vertical velocity profiles across candidate tidal energy sites, we consider the trade-offs between tidal turbines deployed from a floating platform and those anchored to the seabed. Measurement re-analysis indicates several locations that could support tidal current power generation by MW-scale turbines with an acceptable balance between turbine size, rated power, and capacity factor. Even for relatively large (30 m) turbine diameters, surface-deployed turbines would be expected to produce up to 30 % more electricity than the same turbine deployed near the seabed due to vertical shear, with this difference increasing for smaller diameter turbines. As anticipated for sparse measurements, the regional simulation identifies several locations with power generation potential more than twice as high as locations in the measurement re-analysis. These sites were either not surveyed or excluded due to data quality issues. A benchmark comparison at the measurement locations with the highest power generation potential shows relatively good model fidelity, though time-average power density disagreements of ± 50 % persist throughout the water column. In addition to the case study, we also treat three general considerations for measurement data: (1) the accuracy of energy generation estimates from hub height speed relative to rotor-averaged power density, (2) the feasibility of extrapolating profiles towards the surface and seabed, and (3) trends in energy generation with rotor size and position in the water column. Overall, these results demonstrate that models and measurements can be used in a complementary manner for tidal energy site assessment and that tidal currents could be an important source of electricity generation in the San Juan archipelago.

On the relationship between the plasma characteristics, the microstructure and the optical properties of reactively sputtered TiO2 thin films

A titanium target was reactively sputtered in an Ar/O2 atmosphere by (i) conventional direct current magnetron sputtering (DCMS), (ii) high-power impulse magnetron sputtering (HiPIMS), and (iii) bipolar HiPIMS (BPH) discharges for the deposition of titanium dioxide thin films without intentional heating and biasing. In the HiPIMS and BPH cases, the peak current density was set to either 0.32 A cm−2 or 0.86 A cm−2. The time-averaged power density delivered to the plasma was set to ≈1.2 W cm−2 in each case. For the BPH discharge, a positive pulse of +300 V was applied after the negative pulse to accelerate the positive ions toward the substrate. Energy-resolved mass spectrometry analysis has shown a low-energy peak for DCMS while a high energy tail extending in the range of several tens of eV was observed with HiPIMS. In the BPH discharge, the energy reached ≈300 eV for Ti+ ions, which is in agreement with the applied positive potential. According to the x-ray diffraction patterns, amorphous coatings were obtained with DCMS, while rutile TiO2 was obtained with both HiPIMS and BPH. However, in the BPH case, diffractograms were characterized by more intense high-angle diffraction peaks highlighting a modification of the growth process for these conditions. Rutherford backscattering spectrometry analysis has shown that a higher amount of argon atoms were incorporated into the TiO2 films which are found to be slightly understoichiometric for the BPH discharge. It was also found that the refractive index, n, varied as a function of the sputtering regime with the highest value obtained in the HiPIMS case (n = 2.73 at 550 nm). Finally, the experimentally determined optical data were compared to the ones extracted from NASCAM simulations which are found in good agreement except for the BPH case.

Power Density and Temperature Rise Estimation at End-turn Stress Grading System of Large Rotating Machines under PWM Waveform

In this paper, the time-averaged power densities as well as the resulting temperature rise, on an end-turn stress grading (SG) system under PWM waveforms as functions of the carrier and the fundamental frequencies are systematically investigated by finite element method-based computations. A novel analytical approximation method to estimate the power densities and the temperature rises without time-consuming numerical computations are proposed. As a result, the maximum power density and the temperature rise increase nearly linearly with increasing fundamental and the carrier frequencies. It is suggested that power dissipations in the SG layer by the fundamental frequency component of a PWM waveform and those by the carrier frequency component can be clearly separated. Moreover, the estimated power densities and temperature rises based on the analytical approximations show reasonable agreement with the numerically computed ones, which indicates the validity of the proposed estimation method.

Skin-attachable and biofriendly chitosan-diatom triboelectric nanogenerator

Abstract Skin-attachable wearable devices including triboelectric nanogerators (TENGs) and self-powered sensors will have biofriendly and biocompatible issues, because they can directly interact with the human body. Based on the diatom frustule and chitosan, known as biocompatible ocean biomaterials, we developed a chitosan-diatom TENG for biofriendly and skin-attachable wearable devices. The highly porous diatom frustule, which is naturally abundant and mass-producible from mega-scale ocean environments, can be used as a biocompatible additive to greatly change electro-positivity and surface properties of chitosan films, showing bio-affinity and much higher output power. The time averaged power density of the chitosan-diatom triboelectric nanogenerator is measured as 15.7 mW/m2, which is 3.7 times higher than that of the pure chitosan TENG. For practical applications, the electrode-encapsulated chitosan-diatom film is successfully applied for a self-powered watch and a skin-attachable motion sensor. We believe that the diatom bio-silica can be used to make a synergy with other biomaterials for achieving the high power of TENGs for the skin-attachable wearable devices.

Compact Unidirectional Conformal Antenna Based on Flexible High-Permittivity Custom-Made Substrate for Wearable Wideband Electromagnetic Head Imaging System

An approach toward designing and building of a compact, low-profile, wideband, unidirectional, and conformal imaging antenna for electromagnetic (EM) head imaging systems is presented. The approach includes the realization of a custom-made flexible high-permittivity dielectric substrate to achieve a compact sensing antenna. The developed composite substrate is built using silicon-based poly-di-methyl-siloxane (PDMS) matrix and microscale of aluminium oxide (Al2O3) and graphite (G) powders. Al2O3 and G powders are used as fillers with different weight-ratio to manipulate and control the dielectric properties of the substrate for attaining better matched with the human head and reducing antenna’s physical size while keeping the PDMS flexibility feature. Using the custom-made substrate, a compact, wideband, and unidirectional on-body matched antenna for wearable EM head imaging system is realized. The antenna is configured as a multi-slot planar structure with four shorting pins, working as electric and magnetic dipoles at different frequency bands. The measured reflection coefficient (S11) shows an operating frequency band of 1–4.3 GHz. The time-average power density and the amplitude of the received signal inside the MRI-based realistic head phantom demonstrate a unidirectional propagation and high-fidelity factor (FF) of more than 90%. An array of 13 antennas are fabricated and tested on a realistic 3-D head phantom to verify the imaging capability of the proposed antenna. The reconstructed images of different targets inside the head phantom demonstrate the possibility of utilizing the conformal antenna arrays to detect and locate abnormality inside the brain using multistatic delay-multiply-and-sum beamforming algorithm.

Numerical investigation of co-axial DBD: Influence of relative permittivity of the dielectric barrier, applied voltage amplitude, and frequency

In this work, a one-dimensional numerical fluid model is developed for co-axial dielectric barrier discharge in pure helium and a parametric study is performed to systematically study the influence of relative permittivity of the dielectric barrier and the applied voltage amplitude and frequency on the discharge performance. Discharge current, gap voltage, and spatially averaged electron density profiles are presented as a function of relative permittivity and voltage parameters. For the geometry under consideration, both the applied voltage parameters are shown to increase the maximum amplitude of the discharge current peak up to a certain threshold value, above which it stabilized or decreased slowly. The spatially averaged electron density profiles follow a similar trend to the discharge current. Relative permittivity of the dielectric barrier is predicted to have a positive influence on the discharge current. At lower frequency, it is also shown to lead to a transition from Townsend to glow discharge mode. Spatially and time averaged power density is also calculated and is shown to increase with increasing relative permittivity, applied voltage amplitude, and frequency.

The phenomena of the super energy flows with an electric dipole in the air and the ALHM waveguide

In this paper, the phenomena of the super energy flows have been investigated theoretically in the waveguide, which is filled with the air and the anisotropic left-handed materials(ALHM). For TM waves, the super energy flows can be excited and transmitted in the waveguide, if the electromagnetic parameters in the ALHM satisfied ϵrϿϿ=⿿1/(k(1+δ)), ϵrzz=⿿k/(1+δ), μrϿϿ=⿿k(1+δ), where k control anisotropy degree by the ratio of the two layers thickness. Numerical simulations are performed to demonstrate the super energy flows for different choices of parameters in the ALHM layer. Although the loss in the ALHM decrease the amplitude of the time-average power density, the super energy flows are still existence and far larger than that of normal waveguide.

Simulation of electron beam from two strip electron guns and control of power density by rotation of gun

Electron beam is preferably used for large scale evaporation of refractory materials. Material evaporation from a long and narrow source providing a well collimated wedge shaped atomic beam has applications in isotopic purification of metals relevant to nuclear industry. The electron beam from an electron gun with strip type filament provides a linear heating source. However, the high power density of the electron beam can lead to turbulence of the melt pool and undesirable splashing of molten metal. For obtaining quiet surface evaporation, the linear electron beam is generally scanned along its length. To further reduce the power density to maintain quiet evaporation the width of the vapour source can be controlled by rotating the electron gun on its plane, thereby scanning an inclined beam over the molten pool. The rotation of gun has further advantages. When multiple strip type electron guns are used for scaling up evaporation length, a dark zone appears between two beams due to physical separation of adjacent guns. This dark zone can be reduced by rotating the gun and thereby bringing two adjacent beams closer. The paper presented here provides the simulation results of the electron beam trajectory and incident power density originating from two strip electron guns by using in-house developed code. The effect of electron gun rotation on the electron beam trajectory and power density is studied. The simulation result is experimentally verified with the image of molten pool and heat affected zone taken after experiment. This technique can be gainfully utilized in controlling the time averaged power density of the electron beam and obtaining quiet evaporation from the metal molten pool.

Long period oscillations and tidal level in the Port of Ferrol

A new container terminal will soon be inaugurated in the Port of Ferrol (Spain). Sea level observations show the occurrence of seiche events in the basin. The objective of this work is to investigate the long wave oscillations and their dependence on the tidal level. Two analysis techniques, fast Fourier transform (FFT) and short time Fourier transform (STFT), are applied. Time-averaged spectra corresponding to different tidal levels are obtained with the FFT, whereas seiche events are identified on spectrograms computed with the STFT. The time-averaged power density spectra features eleven well-marked peaks, with moderate to high amplification. A clear influence of the tide on the spectral peaks is found, with most peaks presenting higher frequencies and greater power densities at high tide. The analysis of the individual seiche events shows that the behavior of long wave energy in the basin varies across the spectrum; on these grounds, three frequency bands are proposed: LF (low frequency), VLF (very low frequency), and ULF (ultra low frequency). The LF band exhibits a high correlation with the offshore swell energy, both outside and inside the harbor. At the other end of the long wave spectrum, the ULF band is only weakly correlated with the swell; it responds to a different forcing, possibly related to atmospheric disturbances. Finally, the intermediate VLF band presents a mixed character, with influences both from the swell and the other driving agent. The contributions of the paper are as follows. First, the long wave behavior at the Port of Ferrol, a major port in Spain, is characterized for the first time. Second, two contrasting behaviors affecting different frequency ranges are identified—one is proven to be swell-driven, whereas the other is proven, on the contrary, to not be swell-related. And, finally, the tidal oscillation is found to be relevant to the long wave behavior within the port, for it affects both the frequencies and power densities of the spectral peaks—but not to the generation of long waves outside the harbor.

Hyperthermic effects on behavior

This review focuses upon the past 8 years of research on hyperthermic effects on behavior. Heat stress and heat stoke become severe conditions when body temperatures exceed 40°C as this can lead to delirium, convulsions, coma, and death. The animal literature indicates that hyperthermia can increase glutamatergic and decrease GABAergic neurotransmission. Interestingly, µ-opiate receptor antagonists can attenuate the morphological and biochemical changes in brain, as well as, ameliorate some behavioral deficits induced by heart stress. In humans, heat stress can produce detrimental effects on motor and cognitive performance. Since most cognitive tasks require a motor response, some cognitive deficiencies may be attributed to decreased motor performance. Although hyperthermia may exert more deleterious effects on complex than simple cognitive tasks, systematic studies are needed to examine the effects of different levels and durations of hyperthermia (irrespective of dehydration) on cognition. Additionally, body temperatures should be carefully monitored where controls are run for baseline or brief exposures to a hyperthermic environment.Acute radiofrequency exposure can disrupt behavior when body temperatures increase >1°C with whole body SAR between 3.2–8.4 W/kg and time-averaged power densities at 8–140 mW/cm2. Effects of lower levels of radiation are conflicting and some experiments fail to replicate even with the original investigators. This suggests either that brief exposure to the radiation is at a threshold where some individuals are affected while others are not, or that these levels are innocuous. Nevertheless, thermal changes appear to account for almost all of the behavioral effects reported.

Multiple Fractional Dual Solutions in PEMC Backed Chiral Nihility Slab

Inside a chiral nihility slab which is backed by perfect electromagnetic conductor, all valid fractional dual solutions are determined using the fractional curl operator. Fractional dual solutions may be regarded as intermediate step between the original and dual to the original solution. Corresponding fields outside the nihility slab for each case are also determined and analyzed. It is noted that only for one out of three cases the corresponding field outside chiral nihility slab is also fractional dual, while for other two cases, corresponding fields are not fractional dual. It is noted that time average power density vanishes inside the PEC‐backed chiral nihility slab, while it exists outside the nihility slab.

Spherical, rolling magnet generators for passive energy harvesting from human motion

In this work, non-resonant, vibrational energy harvester architectures intended for human-motion energy scavenging are researched. The basic design employs a spherical, unidirectionally magnetized permanent magnet (NdFeB) ball that is allowed to move arbitrarily in a spherical cavity wrapped with copper coil windings. As the ball rotates and translates within the cage, the time-varying magnetic flux induces a voltage in the coil according to Faraday's Law. Devices ranging from 1.5 cm3 to 4 cm3 in size were tested under human activity scenarios—held in the user's hand or placed in the user's pocket while walking (4 km h−1) and running (14.5 km h−1). These harvesters have demonstrated rms voltages ranging from ∼80 mV to 700 mV and time-averaged power densities up to 0.5 mW cm−3.

Numerical study of lateral displacements of Gaussian beams reflected from weakly absorbing media near the Brewster dip and reflected from strongly absorbing media

Using the angular spectrum method, we numerically investigate the reflection of a Gaussian beam reflected from weakly absorbing media near the Brewster dip and from strongly absorbing media. Numerical simulations are presented and the field values are computed as well as the time-averaged power densities. Besides, the number of effective propagation modes is defined. Apart from that, beamsplitting is presented and is interpreted by the missing of the propagation mode when the light is reflected from a weakly absorbing medium near the Brewster dip. Beamsplitting, however, is not observed, therefore both beam shift and angular shift have been calculated when the light is reflected from a strongly absorbing medium. The conditions for Artmann's formula to be valid in the case of the Brewster dip are discussed. Our results do not support the validity of Artmann's formula applied to the case when a Gaussian beam is reflected from weakly absorbing media near the Brewster dip, but indicate that Artmann's formula is valid and very good for the case of a beam reflected from strongly absorbing media.

Lateral displacement of a Gaussian beam reflected from absorbing media

Lateral displacements of well-focused beams reflected from absorbing media are demonstrated in this paper. The incident beam is modeled as a tapered wave with a Gaussian spectrum. The field solutions are obtained on both sides of the interface by solving analytically from Maxwell's equations and by matching the boundary conditions at the interface. Numerical simulations are presented and the field values as well as the time-averaged power densities are computed. The influences of the polarization, the angle of incidence and the beam width on the displacement are discussed. The origin of the displacement lies in the absorption of the media, for which there is a phase shift between the incident and reflected waves. Because of this phase shift, the center of the reflected beam does not coincide with that of the incident one, similar to the Goos–Hänchen shift.

Array and element design for complementary large-volume power deposition patterns in microwave heating

In interstitial hyperthermia cancer treatment and other invasive microwave heating applications, arrays of implanted coaxial dipoles have been used to provide regional heating of human tissue and a variety of materials. Phase modulation of the array-element excitations has been proposed as a means of obtaining uniform, time-average power densities. In the paper an insulated monopole at its second resonance is described as an alternative to the large-diameter coaxial dipole, and a pair of complementary excitations is proposed as a means of obtaining uniform, time-average power density. The array configuration considered consists of four monopoles inserted into a homogeneous medium having a high, complex permittivity. The cases of bare metal and insulated monopoles are analyzed using the method of moments and the theory of insulated antennas in matter.

Surface modes along two parallel circular ferrite rods magnetized longitudinally

A method is presented for solving the problem of propagation along many parallel ferrite rods magnetized axially. The characteristic determinant is derived from the reciprocity theorem where auxiliary electric and magnetic current sources are suitably selected. Using this method, the dispersion relations and the mode configurations of surface modes along two rods are shown and discussed in detail for both the cases of parallel and antiparallel magnetizations. For each case there generally exists a pair of modes with the same mode number which are backward waves. For the parallel magnetization, the field patterns in the transverse plane are symmetric and antisymmetric, respectively, about the point bisecting the straight-line segment that connects the centers of two rods. For the antiparallel case, the respective modes have the symmetrical and antisymmetrical electric field patterns about the vertical bisector of the same segment; and the results are reversed for the magnetic field patterns. Therefore, the field patterns of the latter first (or second) mode in the half-space exactly coincide with those of the corresponding mode along a single rod over a plane magnetic (or electric) conductor. The latter first mode disappears as the spacing between two rods becomes narrower than a limit. The distributions of the time-averaged power density for all the above modes are found to be symmetric about the above-mentioned vertical bisector. For some modes, there exists a slight local forward power flow in rods, accompanied with a much larger backward flow in the other region. This region with forward flow is found to appear when the strong coupling between two rods occurs. The coupling effects between two rods are more remarkable for the antiparallel case than for the parallel case, and they tend to become weaker for higher modes and for the larger spacing.

Diameter Optimization for Maximum Vertical Penetration of a Beam of Collimated Optical Radiation Normally Incident on the Ocean Surface

The optimum diameter of a circular beam of collimated optical radiation normally incident on the ocean surface, in order to obtain a maximum time-average power density vertically downward, is determined for (1) the case of a given constant beam power density and (2) the case of a given constant total beam power. The resulting optima are functions of the roughness of the ocean surface (wind velocity), the depth of the point of observation, and the index of refraction.

Time-Average Power-Density Probability Distribution Below the Ocean Surface of a Beam of Collimated Optical Radiation Incident on the Surface*

The time-average power-density probability distribution below the ocean surface of a circular beam of collimated optical radiation incident on the surface is developed as a function of angle of incidence, beam diameter, and wind velocity and direction by means of geometrical optics.