High Efficiency Regime Research Articles

Oscillating-Foil Turbine Performance Improvement by the Addition of Double Gurney Flaps and Kinematics Optimization

Refinement of the performance of a fully constrained oscillating-foil turbine is carried out via the addition of passive double Gurney flaps. Flaps ranging from hGF=0.005c to 0.075c are added at the trailing edge of the NACA 0015 blade of turbines operating in high-efficiency regimes without leading-edge vortex shedding (LEVS). Performance improvements are determined using 2D numerical simulations with an unsteady Reynolds-averaged Navier–Stokes (URANS) approach. Based on a recent study of the double Gurney flaps on stationary foils, instantaneous power-extraction coefficients are analyzed and modifications of the foil’s kinematics are tested in order to fully benefit from the Gurney flaps’ performance improvements. Modifications to the pivot point location of the foil, to the pitch-heave phase, and to the pitching amplitude of the turbine are considered. Improvements are found for all turbine cases studied, including some of the previously optimal cases reported in the literature. The double Gurney flaps, being a simple and passive device, offer great practical application potential. They represent an efficient refinement to already robust and high-performance oscillating-foil turbines operating without the perceived benefit of leading-edge vortex shedding, an essential characteristic for actual, finite-span applications.

Using Relativistic Self-Trapping Regime of a High-Intensity Laser Pulse for High-Energy Electron Radiotherapy

—Full-3D particle-in-cell Monte Carlo simulation of a new scheme of electron radiotherapy based on electron acceleration by high-power femtosecond laser pulse propagating in plasma of sub-critical density in the relativistic self-trapping regime (V. Yu. Bychenkov et al., Plasma Phys. Control. Fusion 61, 124004 (2019)) was carried out. Based on the results of simulation of distribution of energy deposited by electron bunches accelerated in such high-efficiency regime, it is demonstrated that a laser facility of gtrsim {kern 1pt} 100 TW class is capable of providing therapy of deep soft-tissue lesions in soft biotissue and this approach has a number of advantages relative to traditional methods of beam therapy.

Scaling Laws for Three-Dimensional Combined Heaving and Pitching Propulsors

We present three-dimensional scaling relations for the thrust production and power consumption of combined heaving and pitching hydrofoils by extending the three-dimensional pitching scaling laws introduced by Ayancik et al. (“Scaling Laws for the Propulsive Performance of Three-Dimensional Pitching Propulsors,” Journal of Fluid Mechanics, Vol. 871, July 2019, pp. 1117–1138). Self-propelled inviscid simulations and previously published experimental data are used to validate the scaling laws over a wide range of motion amplitudes, Strouhal numbers, heave ratios, aspect ratios, and pitch axis locations. The scaling laws are shown to predict inviscid numerical and experimental data well, within and of the thrust and power data, respectively. It reveals that both the circulatory and added mass forces are important when considering a wide range of motion amplitudes and that nonlinear corrections to the classic linear theory are essential to modeling the power performance across a wide amplitude and aspect ratio range. By using the scaling laws as a tool, it is obtained that peak efficiency occurs when dimensionless amplitude and for these large-amplitude motions there is an optimal nondimensional heave ratio , where the efficiency maximizes in the narrow range of . Finally, the scaling laws show that to further improve efficiency in this high-efficiency regime, the aspect ratio and dimensionless amplitude should be increased, whereas the Lighthill number should be decreased (lower drag and/or a larger propulsor planform area to wetted surface area ratio), and the pitch axis should be located behind the leading edge. This scaling model can be used to guide the design of the next generation of high-efficiency bio-inspired machines.

Synthesis and Enhanced Capture Properties of a New BioMOF@SWCNT‐BP: Recovery of the Endangered Rare‐Earth Elements from Aqueous Systems

AbstractHuman society is facing—among other environmental threats—an enormous challenge due to human activities. The extensive use of high‐tech devices and electronics equipment in the daily life makes, among others, rare‐earth elements (REEs) recovery from secondary sources highly required. Here, a novel bioMOF‐based single‐walled carbon nanotube buckypaper (SWCNTBP) is presented as a new and efficient composite material (BioMOF@SWCNT‐BP). The flexible and highly crystalline metal–organic framework (MOF), prepared from the natural amino acid L‐threonine, has been homogeneously dispersed within the tangled net of a self‐standing SWCNT‐BP for lanthanides recovery from water. This MOF‐carbon‐based membrane exhibits high efficiency, either in static or dynamic regimes, in the recovery of lanthanides from aqueous streams outperforming the state‐of‐the‐art. The capture performances of BPs are successfully improved after incorporation of such MOF featuring hexagonal functional channels decorated with the threonine amino acid residues, pointing toward the accessible void spaces, which boosts the capture properties of the final membrane, providing the adaptable functional environment to interact with lanthanides. This material's preparation presents also a potential for large‐scale applications with a potential benefit on natural aquatic ecosystems as well. It is highly demanded because REEs from non‐recycled waste materials are potential pollutants for surface waters.

Transient Loss and Recovery of Platinum Fuel Cell Cathode Catalyst at High Voltage Efficiency Regimes

The performance of oxygen reduction reaction (ORR) catalysts, typically evaluated by potential scanning techniques, fails to capture the significant activity decay occurring at longer time scales. To meet the 65% peak energy efficiency target, the continuous decay of Pt/C ORR catalyst, commonly referred to as “transient performance loss”, is studied at 0.8 V by chronoamperometry under membrane electrode assembly (MEA) testing conditions. Based on the results from the time-resolved cyclic voltammetry (CV), surface oxidation was identified as the primary cause of the transient loss. The reduction of surface oxide was observed to occur at 0.6 V, and the recovery of cathode performance can hence be achieved at equal or lower potentials. In addition, the effects of operating temperature and cathode humidity were also studied. The coverage of Pt surface oxides and the extent of the transient loss were both significantly reduced as temperature decreased. However, the benefit of a lower operating temperature came at the cost of slower recovery kinetics. In terms of the impact of humidity, the presence of liquid water was identified as the critical factor leading to a much more severe performance loss over time.

Compress-and-Forward Via Multilevel Coding and Trellis Coded Quantization

Compress-forward (CF) relays can improve communication rates even when the relay cannot decode the source signal. Efficient implementation of CF is a topic of contemporary interest, in part because of its potential impact on wireless technologies such as cloud-RAN. There exists a gap between the performance of CF implementations in the high spectral efficiency regime and the corresponding information theoretic achievable rates. We begin by re-framing a dilemma causing this gap, and propose an approach for its mitigation. We utilize trellis coded quantization (TCQ) at the relay together with multi-level coding at the source and relay, in a manner that facilitates the calculation of bit LLRs at the destination for joint decoding. The contributions of this work include designing TCQ for end-to-end relay performance, since a distortion-minimizing TCQ is suboptimum. The reported improvements include a 1dB gain over prior results for PSK modulation.

Does Scale and Efficiency of Government Health Expenditure Promote Development of the Health Industry?

Macro-economic development of China’s health industry is essential to the sustainable development and growth momentum of the national economy. Strategies to promote the development and rebalancing of the industrial structure need to be improved in order to transform China’s health industry and drive development. Based on panel data of 25 regions in China from 2004 to 2016, this paper analyzes the linear and non-linear relationship between Chinese government health expenditure (GHE), GHE efficiency, and the macro-economic development of the health industry. It uses a novel index of industrial structure to measure the transformation of industrial sectors in China, based on a semi-parametric generalized additive model. The model shows that per capita GHE and its efficiency have a significant positive linear and comprehensive non-linear effect on the development of health industry structure. By analyzing the interaction of GHE and its efficiency, we show that high expenditure with low-efficiency regimes and high expenditure with high-efficiency regimes have a positive impact on the development of industrial structure. Following the empirical results, the paper puts forward corresponding policy suggestions for the role of fiscal policy in promoting the development of the health industry in China.

Electrical model and optimal design scheme for low work-function tethers in thrust mode

A low work-function tether (LWT), a subclass of bare electrodynamic tether made of a conductor partially coated with a low work function material, can exchange momentum and energy with planetary magnetospheres without any consumable. If fed by an onboard power source to reverse the natural direction of the current given by the motional electric field, a LWT can produce an useful thrust in drag compensation and re-boost scenarios. In the considered scheme, the LWT has an anodic bare segment for passive electron collection, followed by an insulated segment, a power source, and a cathodic segment that emits electrons passively through thermionic and photoelectric effects. Current and voltage profiles along the LWT are obtained and used to compute the system efficiency, i.e. the electrical power to mechanical power conversion rate, and the minimum electrical power to avoid space-charge effects in the cathodic segment. The design conditions to reach a high-efficiency regime are presented. For a given orbit and required thrust, optimal values for the tether geometry, including the fractional lengths of all three tether segments, are found. The results are applied to the preliminary design of a LWT system that would compensate the aerodynamic drag on the International Space Station. The analysis shows that the mission can be performed by a LWT fed with a power source of 4.7 kW, length, width and thickness equal to 5.1 km, 2 cm and 30μm, work function 1.5 eV, and temperature 600 K.

Drivers of Carbon Export Efficiency in the Global Ocean.

The export of organic carbon from the surface ocean forms the basis of the biological carbon pump, an important planetary carbon flux. Typically, only a small fraction of primary productivity (PP) is exported (quantified as the export efficiency: export/PP). Here we assemble a global data synthesis to reveal that very high export efficiency occasionally occurs. These events drive an apparent inverse relationship between PP and export efficiency, which is opposite to that typically used in empirical or mechanistic models. At the global scale, we find that low PP, high export efficiency regimes tend to occur when macrozooplankton and bacterial abundance are low. This implies that a decoupling between PP and upper ocean remineralization processes can result in a large fraction of PP being exported, likely as intact cells or phytoplankton‐based aggregates. As the proportion of PP being exported declines, macrozooplankton and bacterial abundances rise. High export efficiency, high PP regimes also occur infrequently, possibly associated with nonbiologically mediated export of particles. A similar analysis at a biome scale reveals that the factors affecting export efficiency may be different at regional and global scales. Our results imply that the whole ecosystem structure, rather than just the phytoplankton community, is important in setting export efficiency. Further, the existence of low PP, high export efficiency regimes imply that biogeochemical models that parameterize export efficiency as increasing with PP may underestimate export flux during decoupled periods, such as at the start of the spring bloom.

A high-efficiency regime for gas-phase terahertz lasers

We present both an innovative theoretical model and an experimental validation of a molecular gas optically pumped far-infrared (OPFIR) laser at 0.25 THz that exhibits 10× greater efficiency (39% of the Manley-Rowe limit) and 1,000× smaller volume than comparable commercial lasers. Unlike previous OPFIR-laser models involving only a few energy levels that failed even qualitatively to match experiments at high pressures, our ab initio theory matches experiments quantitatively, within experimental uncertainties with no free parameters, by accurately capturing the interplay of millions of degrees of freedom in the laser. We show that previous OPFIR lasers were inefficient simply by being too large and that high powers favor high pressures and small cavities. We believe that these results will revive interest in OPFIR laser as a powerful and compact source of terahertz radiation.

Efficiency versus instability in plasma accelerators

Plasma wake-field acceleration is one of the main technologies being developed for future high-energy colliders. Potentially, it can create a cost-effective path to the highest possible energies for e+e- or {\gamma}-{\gamma} colliders and produce a profound effect on the developments for high-energy physics. Acceleration in a blowout regime, where all plasma electrons are swept away from the axis, is presently considered to be the primary choice for beam acceleration. In this paper, we derive a universal efficiency-instability relation, between the power efficiency and the key instability parameter of the trailing bunch for beam acceleration in the blowout regime. We also show that the suppression of instability in the trailing bunch can be achieved through BNS damping by the introduction of a beam energy variation along the bunch. Unfortunately, in the high efficiency regime, the required energy variation is quite high, and is not presently compatible with collider-quality beams. We would like to stress that the development of the instability imposes a fundamental limitation on the acceleration efficiency, and it is unclear how it could be overcome for high-luminosity linear colliders. With minor modifications, the considered limitation on the power efficiency is applicable to other types of acceleration.

Effect of intake manifold water injection on a natural gas spark ignition engine: an experimental study

Design and development of gas CHP (combined heat and power) engines are strongly influenced by the progressively more severe European NOx emissions normative. Water injection represents a promising approach to reduce these emissions while attaining high engine efficiency. In this work, the effect of intake manifold water injection on combustion parameters and performance of a single-cylinder naturally aspirated natural gas spark ignition engine is presented. First, the most appropriate injector was selected, using a spray test bed. Subsequently, engine experiments at constant indicated mean effective pressure (IMEP) and engine speed were conducted with water-fuel ratios of 0.1 to 0.3. IMEP was kept constant at about 6.3 bar by adjusting both air-fuel ratio and spark timing. A NOx reduction of 0.2 g/kWhi (15 %) for a constant ISFC of about 204 g/kWhi was achieved. In the low NOx regime, water injection allows for an improvement of the NOx-ISFC trade-off, while leading to poor fuel consumption at same NOx in the high efficiency regime. Furthermore, water injection implies a reduction of intake mixture temperature, lengthened burning delay and combustion duration and a moderate increase of combustion instability.

Pulse sequences for efficient multi-cycle terahertz generation in periodically poled lithium niobate.

The use of laser pulse sequences to drive the cascaded difference frequency generation of high energy, high peak-power and multi-cycle terahertz pulses in cryogenically cooled (100 K) periodically poled Lithium Niobate is proposed and studied. Detailed simulations considering the coupled nonlinear interaction of terahertz and optical waves (or pump depletion), show that unprecedented optical-to-terahertz energy conversion efficiencies > 5%, peak electric fields of hundred(s) of mega volts/meter at terahertz pulse durations of hundred(s) of picoseconds can be achieved. The proposed methods are shown to circumvent laser induced damage limitations at Joule-level pumping by 1µm lasers to enable multi-cycle terahertz sources with pulse energies >> 10 milli-joules. Various pulse sequence formats are proposed and analyzed. Numerical calculations for periodically poled structures accounting for cascaded difference frequency generation, self-phase-modulation, cascaded second harmonic generation and laser induced damage are introduced. The physics governing terahertz generation using pulse sequences in this high conversion efficiency regime, limitations and practical considerations are discussed. It is shown that varying the poling period along the crystal length and further reduction of absorption can lead to even higher energy conversion efficiencies >>10%. In addition to numerical calculations, an analytic formulation valid for arbitrary pulse formats and closed-form expressions for important cases are presented. Parameters optimizing conversion efficiency in the 0.1-1 THz range, the corresponding peak electric fields, crystal lengths and terahertz pulse properties are furnished.

In Situ Enhancement of Flow-through Porous Electrodes with Carbon Nanotubes via Flowing Deposition

This study describes a novel flowing deposition method which can be used in situ on a fully assembled electrochemical cell to enhance the electrochemical active surface area of flow-through porous electrodes. As a test case, vanadium redox reaction rates are enhanced by flowing deposition of carbon nanotubes on carbon paper electrodes. The deposition is characterized using a recently developed analytical flow cell technique and shown to increase the electrochemical active surface area and therefore the exchange current for the limiting V2+/V3+ reaction by over an order of magnitude. When applied to the anode of an assembled vanadium redox flow cell, the method nearly eliminates the overpotential associated with this reaction, thereby significantly improving the power output of the cell by up to 70% in the high voltage efficiency regime. As a surface area treatment, this flowing deposition method could be used to similarly enhance the energy conversion efficiency of many other electrochemical flow cells such as fuel cells, flow batteries and capacitive deionization cells.

A hot-electron thermophotonic solar cell demonstrated by thermal up-conversion of sub-bandgap photons.

The direct conversion of solar energy to electricity can be broadly separated into two main categories: photovoltaics and thermal photovoltaics, where the former utilizes gradients in electrical potential and the latter thermal gradients. Conventional thermal photovoltaics has a high theoretical efficiency limit (84%) but in practice cannot be easily miniaturized and is limited by the engineering challenges of sustaining large (>1,000 K) temperature gradients. Here we show a hot-carrier-based thermophotonic solar cell, which combines the compact nature of photovoltaic devices with the potential to reach the high-efficiency regime of thermal photovoltaics. In the device, a thermal gradient of 500 K is established by hot electrons, under Stokes illumination, rather than by raising the temperature of the material itself. Under anti-Stokes (sub-bandgap) illumination we observe a thermal gradient of ∼20 K, which is maintained by steady-state Auger heating of carriers and corresponds to a internal thermal up-conversion efficiency of 30% between the collector and solar cell.

Exploring packaging strategies of nano-embedded thermoelectric generators

Embedding nanostructures within a bulk matrix is an important practical approach towards the electronic engineering of high performance thermoelectric systems. For power generation applications, it ideally combines the efficiency benefit offered by low dimensional systems along with the high power output advantage offered by bulk systems. In this work, we uncover a few crucial details about how to embed nanowires and nanoflakes in a bulk matrix so that an overall advantage over pure bulk may be achieved. First and foremost, we point out that a performance degradation with respect to bulk is inevitable as the nanostructure transitions to being multi moded. It is then shown that a nano embedded system of suitable cross-section offers a power density advantage over a wide range of efficiencies at higher packing fractions, and this range gradually narrows down to the high efficiency regime, as the packing fraction is reduced. Finally, we introduce a metric - \emph{the advantage factor}, to elucidate quantitatively, the enhancement in the power density offered via nano-embedding at a given efficiency. In the end, we explore the maximum effective width of nano-embedding which serves as a reference in designing generators in the efficiency range of interest.

High-efficiency tandem perovskite solar cells

Abstract

High-efficiency second-harmonic generation in doubly-resonant χ^(2) microring resonators

By directly simulating Maxwell's equations via the finite-difference time-domain (FDTD) method, we numerically demonstrate the possibility of achieving high-efficiency second harmonic generation (SHG) in a structure consisting of a microscale doubly-resonant ring resonator side-coupled to two adjacent waveguides. We find that ≳ 94% conversion efficiency can be attained at telecom wavelengths, for incident powers in the milliwatts, and for reasonably large bandwidths (Q ∼ 1000s). We demonstrate that in this high efficiency regime, the system also exhibits limit-cycle or bistable behavior for light incident above a threshold power. Our numerical results agree to within a few percent with the predictions of a simple but rigorous coupled-mode theory framework.

Kinetic optimisation of the reversed phase liquid chromatographic separation of proanthocyanidins on sub-2 μm and superficially porous phases

Phenolic compounds, and proanthocyanidins in particular, are important natural molecules which are of significant importance due to their sensory and biological activities. The analysis of proanthocyanidins in natural products is very challenging due to their complex nature. In this study, the kinetic performance of a range of recently developed C18 columns, including sub-2μm fully porous and 2.6μm superficially porous particle-packed columns, was evaluated for improved proanthocyanidin analysis. The kinetic plot method was employed to compare the ultimate performance limits of each column in terms of efficiency and speed for different maximum pressures and temperatures using representative proanthocyanidins comprising a range of molecular weights and functionalities as test analytes. By combining plate height data with relevant parameters such as column permeability and mobile phase viscosity, plots of practically attainable efficiencies as a function of analysis time for specific experimental configurations were obtained and performance limits for all investigated supports could accurately be compared. Both fully- and superficially porous particles provided significant speed and/or efficiency gains compared to conventional 5μm particle packed columns. Analyte properties, particle size and packing quality as well as analysis temperature were all found to have a significant influence on the performance of the presently investigated chromatographic supports. For smaller compounds, higher optimal linear velocities and better performance in the low-efficiency range were observed, while the lower diffusion coefficients of larger procyanidins resulted in lower optimal linear velocities and better performance in the high-efficiency regime. Faster analyses become possible at higher temperatures due to decreased eluent viscosity and faster mass transfer, which was especially beneficial for larger compounds and resulted in dramatic improvement in efficiency. A possible explanation of the abnormal behaviour of oligomeric procyanidins is presented. Our findings indicate that new column formats, when used under optimal conditions, significantly improve the speed and/or efficiency of reversed phase liquid chromatographic analyses of proanthocyanidins.

Electron bunching in the optimal operating regime of a carcinotrode

Electron bunching processes in a carcinotrode (backward-wave oscillator with self-modulation of electron emission) operating in the high-efficiency regime determined previously are investigated. The possibility of obtaining an efficiency of about 80% is explained from the physical viewpoint.