Energy Extraction Efficiency Research Articles

An adaptive plate at flapping wing’s trailing edge in promoting energy extraction performance

As is known, the maximum thickness position plays a key role in determining the energy extraction properties for flapping wing airfoils. Previous work demonstrated that the energy extraction efficiency might increase when we move the maximum thickness position towards the leading-edge direction. Inspired from this phenomenon, we propose in this work the addition of a flat plate at the tail of the flapping wings. Under two different Reynolds numbers (Re = 1100 and Re = 5×105), the effect of adding a rigid flat plate or a flexible flat plate to the trailing edge of the flapping wing on the flapping wing’s energy efficiency was investigated. The corresponding results were also compared with that of the original flapping wing. Results show that the addition of a rigid flat plate is sufficient for the amelioration of the flapping wing’s energy extraction efficiency in low Reynolds number cases. At high Reynolds number cases, the flapping wing with flexible flat plate trailing edge can maintain a relatively high energy extraction efficiency over a wide range of frequency, while the flapping wing with rigid flat plate trailing edge can only improve the energy extraction efficiency in the low frequency region. Therefore, we propose the application of flapping wings with flexible flat plate trailing edge for increasing the energy extraction efficiency under high Reynolds number conditions.

Wave power absorption by an oscillating water column (OWC) device of annular cross-section in a combined wind-wave energy system

This paper deals with a new combined concept consisting of an oscillating water column (OWC) device and an offshore wind turbine for the multi-purpose utilization of offshore renewable energy resources. The wind turbine is supported by a monopile foundation, and the attached OWC is coaxial with the foundation. Within the chamber, the exterior shell of the OWC and the monopile foundation are connected by four vertical stiffening plates. Correspondingly, the whole chamber is divided into four equivalent fan-shaped sub-chambers. A higher-order boundary element method is then adopted to model the wave interaction with the combined system. Numerical models based on two different approaches, namely ‘Direct’ and ‘Indirect’, are both developed in this study. In addition, a self-adaptive Gauss integration method is developed to treat the nearly singular integration that occurs when the field and source points are very close to each other. A detailed numerical analysis is then conducted for the case of an OWC integrated into a NREL 5 MW wind turbine in both regular and irregular sea states. Numerical results illustrate that a significant energy extraction efficiency is attained when remarkable piston-like fluid motion is induced within each sub-chamber, and the wave power absorption by the OWC is not restricted by wave direction. The air compressibility makes a negative effect on the wave power absorption especially when the wave frequency is less than the resonance frequency of the piston-mode motion of the fluid in the chamber. In addition, the wave forces on the OWC and the monopile can balance each other at specific wave conditions, leading to a nearly zero net wave force on the whole system. The results also illustrate that by using an optimal turbine parameter, the wave power production by the OWC can be an important supplement to the combined system in operational sea states.

Unified quasi-optical theory of short-wavelength radiation amplification by relativistic electron beams moving near the impedance surfaces

We combine impedance approximation with a quasi-optical approach to describe the amplification of short-wavelength radiation by rectilinear relativistic electron beams (REBs) moving near the impedance surfaces. We consider a number of physical systems in which wave propagation and amplification by REBs under certain conditions can be described within the developed unified approach. These include metal surfaces with shallow periodical corrugations, the surface of the isotropic plasma, and metals with finite conductivity. In the latter case, resistive instability arises. For the specified class of systems, universal (differing only in the definition of impedance) linear and nonlinear equations are obtained, which allow for finding the instability increments, the spatial profiles of excited fields, and the efficiency of energy extraction.

Rotating dyonic black hole in , U(1)2 gauged supergravity as natural laboratory for high energy particle collisions

In the present work, we explore several gravitational aspects such as energy extraction (via the Penrose process and superradiance), particle collisions around a , U(1)2 rotating dyonic black hole (BH) in the gauged supergravity model. The influence of the rotation parameter (a) and the gauge coupling constant (g) on the behavior of the horizon and ergoregion of the BH is investigated. In comparison to the extremal Kerr BH, the gauge coupling constant, under certain constraints, can interestingly enhance the maximum efficiency of energy extraction through the Penrose process by almost twice. Under the same constraints, we can extract approximately 60.75% of the initial mass-energy from the BH which is noticeably higher and far different from that of the Kerr BH. The limit of energy extraction in terms of the local speeds of the fragments is also determined with the help of the Wald inequality. We discover an upper limit on the gauge coupling constant up to which superradiance is likely to occur. Finally, we estimate the center-of-mass energy (E CM) of two particles with the same rest mass moving in the equatorial plane of the BH. Our study also aims to sensitize E CM to the parameters a and g for both extremal and nonextremal spacetime. Especially, for the extremal case, an infinitely large amount of E CM can be achieved closer to the event horizon confirming a basic point of view that an extreme supergravity BH with dyons could serve as ultimate particle accelerators as compared to Kerr and any other generalized BHs in this family and other alternative theories of gravity. However, E CM for the nonextremal spacetime is shown to be finite and has an upper bound.

Particle dynamics around a dyonic charged black hole

In this article, we study the circular motion of particles and the well-known Penrose mechanism around a Kerr-Newman-Kasuya black hole spacetime. The inner and outer horizons, as well as ergosurfaces of the said black hole, are briefly examined under the effect of spin and dyonic charge. Moreover, by limiting our exploration to the equatorial plane, we discuss the characteristics of circular geodesics and investigate both photons, as well as marginally stable circular orbits. It is noted that black hole charge diminishing the radii of photon and marginally stable circular orbits. To investigate the nature of particle dynamics, we studied the effective potential and Lyapunov exponent. While inspecting the process of energy extraction, we derived the Wald inequality, which can help us to locate the energy limits of the Penrose process. Furthermore, we have found expressions for the negative energy states and the efficiency of energy extraction. The obtained result illustrates that both black hole rotation and dyonic charge contributes to the efficiency of energy extraction.

Energy extraction characteristic of the flapping wing type vertical axis turbine

Flapping wings are known to be used not only for propulsion, but also for energy extraction from the surrounding fluid environment. In this work, the authors propose a vertical axis turbine based on flapping wings. Numerical experiments have been conducted. Results show (i) In a certain range of frequency, the vertical axis turbine equipped with flapping wings of circular or reversed-D motion trajectory demonstrates better energy extraction performance when compared to the traditional vertical axis wind turbine. (ii) For the reversed-D motion trajectory, as two airfoils begin to move apart from each other, the downstream flapping-wing takes advantage of the trailing edge vortex of the upstream flapping-wing to enhance its own energy extraction ability. However, for the circular motion trajectory, the two airfoils are usually located far away from each other. This leads to the result that the mutual influence between the two airfoils is very weak. Generally speaking, the application of flapping wings in the vertical axis turbine can improve the energy extraction efficiency in a certain range, which can provide another method for the design of a new type of vertical axis turbine in the research and industry.

A high-stability triboelectric nanogenerator with mechanical transmission module and efficient power management system

The triboelectric nanogenerator (TENG) has been demonstrated to be a promising technology for harvesting mechanical energy and powering electronic devices. However, it is difficult to make TENG output stable because of the irregular movement of actual mechanical sources in the environment, and the inefficient energy utilization limits its application. In this study, we propose a contact-separation TENG (CS-TENG) integrated with a mechanical transmission module and its optimal power management system. Firstly, a gear and cylindrical cam mechanism is designed in the mechanical transmission module, which respectively increases the working frequency of CS-TENG and makes the output stable. Secondly, a control module is developed in the PMS to accurately control a switch for extracting maximum output energy, and a series of simulation and experiments have been done to further study the effect of control parameters on the energy extraction process. In the control module, a differential circuit is used to detect the peak of the output voltage. It is found that an optimum differential resistance can increase the detection accuracy and reduce external interference. A time-delay circuit is used to accurately control the switch-on time. As the switch-on time increases, the energy extraction efficiency first increases and then decreases. Finally, the energy extraction efficiency of PMS reaches to 37.8% with the optimal control parameters. We not only solve the problem of the irregular output of TENG, but also provide a guidance for optimizing the design of self-powered system.

Revisiting collisional Penrose processes in terms of escape probabilities for spinning particles

We first study the escape probability of a spinning particle emitted from a Kerr black hole and find that the escape probability increases with the spin of the particle around the extreme Kerr black hole; in contrast, the escape probability decreases at the position near the horizon but increases at the position far from the horizon with the increasing spin of the particle. We then probe the relation between the escape probabilities and the energy extraction efficiencies of collisional Penrose processes for particles with varying spin. For an extreme Kerr black hole, the efficiency increases with the escape probability; for a nonextreme Kerr black hole, the near-horizon efficiency decreases with the escape probability, while the efficiency may increase with the escape probability in the ergosphere. In the event horizon limit, we also find that the average escape probability of the spinning particle produced in the collisional Penrose process decreases with the rotation parameter of the Kerr black hole.

Piezoelectric Energy-Harvesting Interface Using Split-Phase Flipping-Capacitor Rectifier With Capacitor Reuse for Input Power Adaptation

This article proposes a split-phase flipping-capacitor rectifier (SPFCR) to resolve the hard tradeoff between the number of capacitors and the energy-extraction efficiency for capacitive piezoelectric energy-harvesting (PEH) interfaces. By splitting the capacitor usage into multiple phases, this article can achieve the most number of flipping phases using the same number of capacitors when compared with the state of the art. We also relaxed the implementation complexity by removing insignificant SFPCR flipping phases without sacrificing the energy-harvesting efficiency. To improve further the input power adaptation, the flipping capacitors are reconfigured as a multiple-voltage-conversion-ratio (MVCR) switched-capacitor dc–dc converter during the non-flipping period without using extra passives. Maximum-power-point tracking (MPPT) is also accomplished using the full-bridge rectifier (FBR) fractional open-circuit voltage ( $V_{{\text {OC}, \text {FBR}}}$ ) to relax the voltage-tolerance requirements. Fabricated in 0.18- $\mu \text{m}$ CMOS, the proposed 21-phase SPFCR PEH interface demonstrates a measured maximum output power improving rate (MOPIR) of up to 9.3 $\times $ with $V_{D}= 0.12$ V (6.5 $\times $ with $V_{D}= 0$ V) when compared with the conventional FBR interface over an equivalent FBR input power range ( $P_{{\text {in, FBR}}}$ ) from 0.15 to 5.57 $\mu \text{W}$ .

Do Unconventional Oil and Gas Wells Generate More Wastewater? A Lifespan Perspective

The rapidly increasing oil and gas wastewater has caused widespread concerns about its adverse environmental and health effects. Using panel data of 49,050 wells from 2008 to 2016, we estimate a difference-in-difference-in-differences model to study the wastewater discharge patterns across wells’ lifespan and cohorts for unconventional wells, i.e., horizontal wells in shale formations, compared with conventional wells, i.e., vertical wells in shale or non-shale regions. Our analysis shows that since the 2010 cohort an average unconventional well has generated more wastewater than a conventional well in the initial six months of production while this gap increased in more recent well cohorts. However, the contemporaneous gap always decreased with the age of a well and reversed eventually regardless of cohorts, leading to less lifespan cumulative water discharge of an unconventional well than a conventional one. Moreover, unconventional oil wells had a lower lifespan wastewater-to-energy ratio than conventional counterparts, whereas no such efficiency difference existed among gas wells. Our findings show the trade-off between higher near-term wastewater discharge and lower lifespan cumulative generation as well as higher long-run efficiency of shale energy extraction compared with conventional energy production.

A Review of Power Electronics Applications for Wind Energy Systems in Microgrids

Wind energy conversion systems continue to show promises as energy source of the future. To champion the supply for future industrial and domestic energy demand, it is imperative to improve on their reliability, energy extraction efficiency and security as these factors would enormously affect their viability and deployment. This paper presents an overview of the applications of power electronics to speed control of generators in wind energy conversion systems. Wind energy conversion system, the basis on which they are classified and their various components are concisely reviewed. Further, an up-to-date highlight of wind turbines speed control schemes is presented with focus on the roles of power electronics in these schemes. Concluding, necessary technological and economic development to aid applications of power electronics devices in emerging wind energy systems and their significance to the microgrid is understudied.

Asymmetric effects of a modelled tidal turbine on the flow and seabed

The extraction of power from the flow of water has become an important potential source of clean energy. In spite of significant interest in the interaction between energy extraction devices and water currents, comparatively little work has focused on flow asymmetry. Indeed, unusual wake behaviour and limits of turbine array efficiency have typically been attributed to boundary effects rather than the particular turbine geometry. The aim of the present study was to reveal the asymmetries in the hydrodynamic wake and the interactions with the sediment bed due to the presence of a hydrokinetic turbine. We combined: (i) computational fluid dynamics simulations; (ii) optical flow measurements from a series of flume experiments above a fixed rough bed; and (iii) acoustic measurements from a further series of flume experiments above a mobile sand bed. Results showed flow asymmetry due to the presence of the rotor which appeared to be related to the development of the wake and potentially to the gyre of the blades. Suspended sediments in the flume also exhibited asymmetrical characteristics due to the flow asymmetry. This imbalance in the flow field and sediment transport may decrease energy extraction efficiency in turbine arrays and also could have important environmental consequences.

Maximum power exploitation for grid-connected PV system under fast-varying solar irradiation levels with modified salp swarm algorithm

With the rapid development of photovoltaic (PV) power generation technology, PV generation system has been applied into planes, cars, boats and trains. Solar irradiation levels will change rapidly in these PV systems. Fast-varying solar irradiation can invalidate the traditional configurations of PV system and maximum power point tracking (MPPT) control strategy, which will reduce the energy conversion efficiency. In order to extract maximum power exploitation, this paper proposes a novel maximum power exploitation configuration combined with the proposed control scheme for grid-connected PV system under fast-varying solar irradiation levels. In the proposed configuration, each PV panel cascades a step-up boost converter, hence allowing independent control corresponding to the irradiation levels, which generates multiple-levels of dc-voltage and may be converted to ac via an active neutral point clamped (ANPC) inverter. Meanwhile, a modified salp swarm algorithm with dynamic w factor (DWSSA) based MPPT method is proposed for each boost converter units, which can effectively accelerate convergence velocity and stability of iteration to make the searching process rapidly adapting to different fast-varying solar irradiation levels. In order to express the dynamic variation of the PV system, the mathematical model of the proposed configuration is established. Moreover, the performance of the proposed DWSSA algorithm is investigated by the stability analysis based Lyapunov theory and convergence analysis on 23 benchmark functions and CEC 2005 benchmark functions. Finally, the constructed simulation test platform is implemented and the results demonstrate that the configuration can exploit the variable converter ratios, and the proposed DWSSA method has faster tracking speed and higher energy extraction efficiency compared with the previous MPPT methods in most cases, especially, the power extraction percentage of 97.89% and the tracking time of 0.761s under the most severe uneven solar irradiation levels.

A High Thermal Conductivity Cement for Geothermal Exploitation Application

In geothermal reservoir exploitation, the thermal conductivity of casing cement material is a key parameter that may affect the heat exchange and utilization efficiency of geothermal reservoirs. Based on a systematic approach of theoretical analysis, experimental research and microscopic analysis, this paper presents graphite composite cement (GC) with high thermal conductivity for potential application in geothermal reservoir exploitation. The GC consists of ordinary Portland cement (P.O 42.5), natural flake graphite, water-reducing agent (GB), early strength agent (CaCl2) and defoamer (HB-03). The physical properties of the GC, such as fluidity, pumpability, setting time, compressive strength and thermal conductivity, were obtained by carefully prepared laboratory tests that met our expectation. Finally, X-ray diffractometry and environmental scanning electron microscopy were used to analyze the micro-characteristics for an interpreted thermal conduction mechanism of GC. The results of analyses show that the formation of thermal conductivity network of natural flake graphite in cement is a key to improve thermal conductivity. The research results provide a new solution to the problems of low thermal energy extraction efficiency and poor cementing quality of geothermal reservoirs.

Optimization of series-parallel connection of PV array to mitigate negative influence of partial shading conditions

Rapid development in the photovoltaic systems demanding more and more efficient solutions, not only in micro-fabrication techniques but in energy extraction methods as well. In recent decades large number of Maximum Power Point Tracking algorithms with different complexity accompanied by the capability to efficiently locate global maximum under partial shading were developed. However, a deeper look at the problem reveals that to further enhance the performance of the PV system under non-uniform irradiance there is a need for hardware intervention to the interconnect, or in other words adjustments to the panel configuration. In this paper, we propose a simple technique for optimizing the electrical scheme of the solar panel (connection of individual cells) by using a simple algorithm which disperses shadow across all structure. The method allows to mitigate partial shading effect on solar panels of any size and shape, therefore rise efficiency of energy extraction. Thus static optimization of the PV panel configuration disperses partial shading across all structure and ensures more synchronous functioning of the PV cells. The resulting structure not only capable of extracting more available power but, straighten ups power curve to loosen requirements for MPPT computations.

Effects of the downstream spatial configuration on the energy extraction performance of tandem/parallel combined oscillating hydrofoils

Unsteady 2D laminar flow simulations were performed on tandem oscillating hydrofoils to study the influence of the downstream spatial location on the power extraction efficiency of a tandem/parallel combined configuration. A sinusoidal plunging/pitching motion of NACA 0015 profile was used. Inspired by insect inter-wing spacing, different configurations of the downstream foil (y)-position were first studied at a fixed (L/c) horizontal inter-foil spacing and reduced frequencies within the range of 0.02 to 0.32. It was found that the energy extraction efficiency improved by around 23 % as compared to the conventional tandem configuration by changing the vertical inter-foil spacing. An interesting finding is that, at specified frequencies and inter-foil spacing, the power efficiency extracted from the downstream foil, associated with the new configuration, exceeds the upstream foil efficiency. The horizontal inter-foil spacing was then investigated for an optimal (y)-position of the downstream foil, and the results show that the optimal horizontal spacing was reduced compared to the conventional tandem configuration, resulting in a gain in space and about 15 % of increment in the power extraction efficiency. Therefore, a total of 38 % of power efficiency enhancement was obtained at f * = 0.24 by using the new configuration in which the horizontal inter-foil spacing was reduced five times compared to the conventional configuration. An appropriate inter-foil spacing/frequency synchronization resulted in a favorable foil-vortex interaction that improves the power extraction from the oscillating foil system. The frequency and the inter-foil spacing were found to be critical factors that determine whether the interaction is favorable or unfavorable.

Cs-Ar optical amplifier with a saturation intensity of 10 kW-cm-2 and single-pass extraction efficiency of 28% at 852.2 nm.

Optical amplification by the stimulated emission of Cs(6p2P3/2)-Ar atomic pairs, observed in pump-probe experiments over a ∼290 GHz-wide spectral region lying to the red of the Cs D2 line (852.1 nm), has been realized by photoexciting thermalized, ground state Cs-Ar atoms in the 834-849 nm wavelength interval. When the gain medium is pumped at the peak of the CsAr B2Σ1/2+←X2Σ1/2+ transition at 836.7 nm, maximum gain occurs between 852.2 nm and 852.3 nm and >28% of the energy stored in the upper laser level is extracted with 8 ns (FWHM) probe pulses in a single pass. From the measured rate of saturation of the extracted pulse energy with increasing probe intensity, the product of γ0L and Esat, the saturation pulse energy, is measured directly to be 400 ± 20 µJ and the lower limit for the saturation intensity (Isat) of this amplifier is estimated to be 10 kW-cm-2 at 852.2 nm. Circularly polarizing the optical pump beam increases the optical-to-optical conversion efficiency by 20%, and the storage lifetime of the upper laser level is observed from temporally-resolved gain spectra to be 5 ± 1 ns. Pump excitation spectra also reveal a significant contribution from Ar-Cs-Ar (3-body) photoassociation and suprathermal Ar atoms generated by the dissociation of the CsAr B2Σ1/2+ complex. Multipass-amplifier geometries with broad-bandwidth probe signals are expected to yield upper state energy extraction efficiencies above 50%. This alkali-rare gas amplifier demonstrates the efficiencies available with the storage of energy in, and optical extraction from, excited atomic collision pairs.

Effects of oxyhydrogen gas induction on the performance of a small-capacity diesel engine.

Compression ignition engines are one of the world's largest consumers of fossil oil but have energy extraction efficiency limited to 35%. Addition of hydrogen alongside diesel fuel has been found to improve engine performance and efficiency; however, after a certain limit, hydrogen begins to show adverse effects, mainly because the ratio of oxygen to fuel decreases. This can be overcome by using oxyhydrogen, which is a mixture of hydrogen and oxygen gas. In this study, effects of addition of oxyhydrogen generated by electrolysis, with varying flows at the intake manifold, on a 315 cc compression ignition engine alongside diesel were analyzed. The engine was mounted on a Thepra test bed and torque measurements were taken at predetermined test points for diesel and 6 and 10 standard cubic feet per hour flowrates of oxyhydrogen. H10 showed the maximum improvement in engine performance equating to a 22.4% increase in both torque and power at 3000 r/min, and a 19.4% increase in efficiency at 2600 r/min was recorded. The large increase in engine performance as compared to previous results is because of high oxyhydrogen flowrate to displacement volume ratio. The oxyhydrogen flowrate to displacement ratio is the most important factor as it is directly impacts engine performance. The difference in engine performance because of oxyhydrogen becomes prominent at higher engine speed due to high suction pressure. No experimental flowrates of oxyhydrogen showed any adverse effect on the engine performance.

Effect of convergent duct geometry on the energy extraction performance of tandem oscillating hydrofoils system

In the present study, a new hydrokinetic energy harvester concept based on the tandem oscillating hydrofoils configuration is numerically investigated. Unsteady turbulent two dimensional flow simulations were performed, by using the commercial finite volume computational fluid dynamics code FLUENT, to study the power extraction from tandem hydrofoils oscillating inside a convergent duct. A sinusoidal function was used to interpret the plunging/pitching motion of NACA 0015 hydrofoil. Computations were carried out for the different configurations of area ratios (AR) that is defined as the ratio of the upstream hydrofoil’s inlet cross section (A1) to the downstream hydrofoil’s inlet cross section (A2) ranging from 1.0 (without the convergent section) to AR = 2.6. The study revealed that the power extraction was enhanced by the increased incoming flow velocity enabled by the geometry of the convergent duct, which positively affected the total pressure as well as the hydrofoil-vortex interaction time and resulted the generation of larger vertical hydrodynamic force. Upon analysis of the obtained results, it is found that the energy extraction efficiency is improved, compared to that of the conventional tandem configuration where the hydrofoils move in an open flow space, and, as the area ratio (AR) increased, the amount of extracted power becomes higher in a proportion ranging from 4% to 80%. The concept was numerically tested for tandem and single configurations and for non-dimensional frequencies in the range of f∗=0.04 - 0.20. One interesting finding is that, due to some positive effect of hydrofoil-vortex interaction, and at a specified area ratio and frequency, the downstream hydrofoil efficiency exceeds the efficiency of a single hydrofoil located at exactly same position as the downstream hydrofoil in the tandem configuration. The frequency and the area ratio are found to be critical factors that determine whether the downstream hydrofoil-vortex interaction is favorable or unfavorable. An optimal area ratio/frequency combination results in a positive hydrofoil-vortex interaction effect and allows the downstream hydrofoil to contribute positively to the total power extraction as a single hydrofoil. Based on the present results, and for an optimal efficient energy harvester design, a new correlation is suggested in order to properly control the area ratio in accordance with the variation of the oscillating frequency leading to more power production and high performance of the oscillating tandem hydrofoils. It is anticipated that this new innovative technique will be applied to improve the existing energy harvester devices.

LGS electro-optically Q-switched Er,Pr:YAP crystal laser operated at 2.7 µm

We demonstrate an optimized 2.7 µm Er,Pr:YAP laser in both free-running and Q-switched modes. In free running mode, a maximum energy of 346 mJ is obtained with repetition frequency of 5 Hz. In Q-switched mode, a giant pulse with 63.4 mJ energy and 40 ns pulse width is realized at 5 Hz, which corresponds to an energy extraction efficiency of 47% and a peak power of 1.59 MW. Additionally, the M2 factor and laser spectrum are also measured in two modes. These results indicate that the LGS Q-switched 2.7 µm Er,Pr:YAP laser is a promising candidate for the mid-infrared pulse laser device.