Theoretical Performance Limits Research Articles

THERMAL AND EXERGETIC ANALYSIS OF A ROTARY-TYPE MAGNETIC COOLING SYSTEM

Despite the high theoretical performance limits of magnetic refrigeration systems, coefficient of performance (COP) of these systems can be lower due to abortive magnet and magnetocaloric material (MCM) assembly orientation and space limitation. To address both of these problems, a rotary-type magnetic cooling system with varied geometric properties was modelled and simulated. Heat transfer from the MCM, which was selected to be gadolinium (Gd), to the working fluid, was considered as volumetric energy generation. Three different geometries of disc-MCM assembly were studied where the inner and outer radii were selected as 5, 10, 15 mm, and 10, 15, 20 mm, respectively. It was observed that the gradient between steady-state temperature and the room temperature decreases with decreasing gadolinium-to-disc area ratio. Conductive heat transfer through the thickness of the disc was found to be inconsiderable compared with the convective heat transfer through the top surface.

Effect of operation conditions on the second law analysis of a desiccant cooling system

In order to evaluate the potential of a desiccant based evaporative air conditioning system, a second law analysis of the systems was investigated. In this work, a wide range of working parameters that are regeneration temperature from 90°C to 110°C, volume flow rate from 2000m3 to 4000m3 and ambient air conditions which are relative humidity, ambient air temperature and wet-bulb temperature is carried out for performance calculation of the system. It is observed from results that the Carnot COP and the thermal COP of the system varies vice versa during the day. The highest value of Carnot COP and the thermal COP is about 25 and 0.62, respectively. Also, the cooling capacity has significant effect on the second law efficiency. The importance of this study shows that the second law analysis can provide useful information with respect to the theoretical upper limit of the system performance, which cannot be obtained from the first law of thermodynamic analysis alone.

Porous metal produced by selective laser melting with effective isotropic thermal conductivity close to the Hashin–Shtrikman bound

Additive manufacturing may be a novel method for fabricating porous materials. These materials can achieve effective performance because of their internal geometries. Metal-additive manufacturing is expected to utilize thermal conduction materials and devices. We have developed a porous metal with effective isotropic thermal conductivity by using metal-selective laser melting additive manufacturing. The internal pore structure was designed by topology optimization, which is the most effective structural optimization technique to maximize effective thermal conductivity. The designed structure was converted to a three-dimensional STL model, which is a native digital format of additive manufacturing, and assembled as a test piece. Effective thermal conductivity was measured by a steady-state method in which the effective thermal conductivity was calculated from a one-dimensional temperature gradient and the heat flux of the test pieces. The test pieces showed an effective thermal conductivity close to the Hashin–Shtrikman or Maxwell–Eucken bound, which is the theoretical limit of effective performance with an error less than 10%.

Benchmarking the Performance of Boost-Derived Converters Under Start-Up and Load Transients

The control of boost-derived converters presents a special challenge due to its non-minimum phase behavior, forcing a slow transient response when traditional linear techniques are employed. In order to improve the dynamic behavior, more complex controllers have been proposed (e.g., adaptive, nonlinear, boundary, etc.), but the improvements introduced by each one of them cannot be objectively assessed in the absence of a theoretical performance limit standard reference. This work introduces a benchmarking tool based on the evaluation of the converters’ performance under start-up, load current, and input voltage step-up/step-down transients. Six performance indices are defined using the theoretical performance limits as reference. In this way, the converters’ dynamic performance can be quantitatively assessed in six different aspects, enabling a thorough benchmarking procedure. The ideal parameters are found by exploring the performance limits of boost converters, providing valuable insight into the behavior of the topology. These reference parameters are found in a normalized domain and characterized by closed-form expressions, providing simple, intuitive, and general results valid for any combination of LC parameters and voltage levels. The characterization of the dynamic performance limits is validated by experimental results. The proposed benchmarking procedure is illustrated by evaluating the dynamic performance of two different dc–dc boost converters.

Giant spin-torque diode sensitivity in the absence of bias magnetic field.

Microwave detectors based on the spin-torque diode effect are among the key emerging spintronic devices. By utilizing the spin of electrons in addition to charge, they have the potential to overcome the theoretical performance limits of their semiconductor (Schottky) counterparts. However, so far, practical implementations of spin-diode microwave detectors have been limited by the necessity to apply a magnetic field. Here, we demonstrate nanoscale magnetic tunnel junction microwave detectors, exhibiting high-detection sensitivity of 75,400 mV mW−1 at room temperature without any external bias fields, and for low-input power (micro-Watts or lower). This sensitivity is significantly larger than both state-of-the-art Schottky diode detectors and existing spintronic diodes. Micromagnetic simulations and measurements reveal the essential role of injection locking to achieve this sensitivity performance. This mechanism may provide a pathway to enable further performance improvement of spin-torque diode microwave detectors.

Focal Plane Phase Masks for PIAA: Design and Manufacturing

The Phase Induced Amplitude Apodization Complex Mask Coronagraph (PIAACMC) is a coronagraph architecture for the direct detection of extrasolar planets, which can achieve close to the theoretical performance limit of any direct detection system. The primary components of a PIAACMC system are the Phase Induced Amplitude Apodization (PIAA) optics and the complex phase-shifting focal plane mask. PIAA optics have been produced and demonstrated with high coronagraph performance. In this paper, we describe the design process for the phase-shifting focal plane mask, and strategies for smoothing the mask profile. We describe the mask manufacturing process and show manufacturing results. Errors in the fabricated mask profile degrade the system performance, but we can recover performance by refining the manufacturing process and implementing wavefront control.

A Miniaturized FBG Accelerometer Based on a Thin Polyurethane Shell

A miniaturized fiber bragg grating accelerometer based on a thin polyurethane shell is designed and tested. The FBG accelerometer is based on spring-mass structure. The elastic body is a thin polyurethane cylindrical shell. By inserting the mass into the inner of the shell, the accelerometer axial size is reduced by half compared with traditional ones in which masses are exposed outside. The theoretical performance limit of the miniaturized accelerometer is discussed. Test results show the miniaturized accelerometer has similar sensing performance as the traditional one. Size of the miniaturized accelerometer is Φ23 mm × 17 mm, sensitivity over the flat frequency response zone is 54 pm/g, and resonant frequency is 480 Hz. Furthermore, orthogonal crosstalk of the miniaturized accelerometer is -17.75 dB which is 3 dB lower than the traditional one. Thus, this miniaturized design can reduce both size and orthogonal crosstalk of accelerometers based on elastic shells. This miniaturized structure can also be used in other fiber optic accelerometers based on shells.

Exergy analysis of solid desiccant-vapour compression hybrid air conditioning system

This paper presents energy and exergy analysis of a solid desiccant vapour compression hybrid air conditioning system. Parametric study has been conducted at varying dead state properties and operating conditions to investigate their effects on the overall system performance. The obtained results show that the exergy efficiency of hybrid air conditioning system is 11.45%. The energy-based performance parameters such as the coefficient of performance (COP) and components effectiveness are also presented. The rotary desiccant dehumidifier and heater are found as major contributors to exergy performance of the system. The analysis provides us beneficial knowledge to determine theoretical upper limit of the system performance.

Estimating the Theoretical Performance Limits of a Biogas Powered Dual Fuel Diesel Engine Using Emulsified Rice Bran Biodiesel as Pilot Fuel

The present study is an attempt to estimate the energy and the exergy potential of a biogas run dual fuel diesel engine using emulsified rice bran biodiesel (RBB) as pilot fuel at varying compression ratio (CR) and injection timing (IT). The objective is to arrive at an optimum setting of the engine based on dual fuel characteristics using energy and exergy analysis. The pilot fuel considered for this study is a two-phase stable water emulsion of RBB having water content (5%), surfactants (3%), and hydrophilic lipophilic balance value of 6. For experimentation, a 3.5 kW single cylinder, direct injection (DI), natural aspirated water-cooled, variable CR (VCR) diesel engine is converted into a dual fuel engine. Experiments are conducted for 12 different combinations of CR of 18, 17.5, and 17 and IT of 23 deg, 26 deg, 29 deg, and 32 deg bottom top dead center (BTDC) at full load conditions of brake mean effective pressure (BMEP) of 4.24 bar. The parameters analyzed are the energy and exergy potential of fuel input, shaft work, cooling water, exhaust gas, exergy destruction, peak cylinder pressure (PCP), peak heat release rate (PHRR), brake thermal efficiency (BTE), exergy efficiency, exhaust gas temperature (EGT), entropy generation rate, and emission analysis. The results indicate that the combination of CR = 18 and IT = 29 deg BTDC gives a better thermodynamic performance for this particular range of the operating parameters for a raw biogas run dual fuel diesel engine using emulsified RBB as pilot fuel.

Introduction to the Issue on Advances in Hyperspectral Data Processing and Analysis

The articles in this special issue represent advances in hyperspectral image-processing research. Specifically, it focuses on novel algorithmic approaches related to the representation and analysis of hyperspectral images, as well as their application to real-world image-analysis needs. It also delves into theoretical insights, performance limits, and trade-offs on the class of emerging image-processing approaches tailored to hyperspectral images. This issue covers a gamut of broad topics, from representation and sensing (image coding, compressive sensing), feature extraction and channel selection, to image analysis (spectral unmixing, classification, and detection.

Theoretical Performance Limits of a Biogas–Diesel Powered Dual Fuel Diesel Engine for Different Combinations of Compression Ratio and Injection Timing

AbstractThe present work attempts to investigate the effect of compression ratio (CR) and injection timing (IT) on the energy and the exergy potential of a biogas-run dual fuel diesel engine. For experimentation, a 3.5 kW single cylinder, four-stroke, direct injection, naturally aspirated, water cooled, variable compression ratio diesel engine is converted into a dual fuel engine. Experiments are conducted for 16 different combinations comprising of ITs of 23, 26, 29, and 32° before top dead center (BTDC) and CRs of 18, 17.5, 17, and 16 at full load condition of 4.24 bar of brake mean effective pressure. The parameters analyzed are the energy and exergy potential of fuel input, shaft work, cooling water, exhaust gas, exergy destruction, peak cylinder pressure, peak heat release rate, brake thermal efficiency, exergy efficiency, exhaust gas temperature and entropy generation rate. The thermodynamic analysis indicates that the combination of IT = 29° BTDC and CR = 18 gives an improved performance of the bio...

Quantifying Losses in Open-Circuit Voltage in Solution-Processable Solar Cells

To compare and improve solar cells, merely knowing their percent efficiencies is insufficient; we need to be able to understand and quantify their different physical mechanisms of loss. The authors use the reciprocity relation between light absorption and emission to explore theoretical and practical performance limits for emerging technologies based on organics and perovskites, and compare them to state-of-the-art systems based on GaAs, c-Si, and CIGS. This study indicates the potential of the newer technologies, and shows the factors that limit present-day performance.

Electrocatalysis of lithium polysulfides: current collectors as electrodes in Li/S battery configuration.

Lithium Sulfur (Li/S) chemistries are amongst the most promising next-generation battery technologies due to their high theoretical energy density. However, the detrimental effects of their intermediate byproducts, polysulfides (PS), have to be resolved to realize these theoretical performance limits. Confined approaches on using porous carbons to entrap PS have yielded limited success. In this study, we deviate from the prevalent approach by introducing catalysis concept in Li/S battery configuration. Engineered current collectors were found to be catalytically active towards PS, thereby eliminating the need for carbon matrix and their processing obligatory binders, additives and solvents. We reveal substantial enhancement in electrochemical performance and corroborate our findings using a detailed experimental parametric study involving variation of several kinetic parameters such as surface area, temperature, current rate and concentration of PS. The resultant novel battery configuration delivered a discharge capacity of 700 mAh g−1 with the two dimensional (2D) planar Ni current collectors and an enhancement in the capacity up to 900 mAh g−1 has been realized using the engineered three dimensional (3D) current collectors. The battery capacity has been tested for stability over 100 cycles of charge-discharge.

What price of speed? A critical revision through constructal optimization of transport modes

The use of energy by the major modes and the environmental impact of freight transportation is a problem of increasing importance for future transportation policies. This paper aims to study the relative energy efficiency of the major transport modes, setting up an impartial analysis, improving previous literature substantially. Gabrielli and von Karman have studied the relationship between speed and energy consumption of the most common transport modes. From this pioneering activity different methods for evaluating the energetic performance of vehicles have developed. Initially the maximum vehicle power and theoretical performance limits have been calculated in terms of weight and payload. Energy efficiency has then been evaluated in terms of the first principle of thermodynamics as the mass of the vehicle times distance moved divided by thermal energy used. A more effective analysis can be performed both in terms of vehicle life cycle and in terms of second principle considering the quality and the amount of dissipated amount of useful energy. This paper defines an LCA based model, which could allow an effective comparison between different transport modes classifying them in terms of exergy destruction. In this case, an effective comparison, which considers the quality of used energy, can be performed allowing precise politics for a future more effective evaluation of the transport modes.

Information theoretical performance limits of single‐carrier underwater acoustic systems

In this study, the authors investigate the information theoretical limits on the performance of point-to-point single-carrier acoustic systems over frequency-selective underwater channels with intersymbol interference. Under the assumptions of sparse and frequency-selective Rician fading channel and non-white correlated Gaussian ambient noise, the authors derive an expression for channel capacity and demonstrate the dependency on channel parameters such as the number, location and power delay profile of significant taps, as well as environmental parameters such as distance, temperature, salinity, pressure and depth. Then, the authors use this expression to determine the optimal carrier frequency, input signalling and bandwidth for capacity maximisation.

A dynamic graph optimization framework for multihop device-to-device communication underlaying cellular networks

With emerging demands for local area and popular content sharing services, multihop device-to-device communication is conceived as a vital component of next-generation cellular networks to improve spectral reuse, bring hop gains, and enhance system capacity. Ripening these benefits depends on fundamentally understanding its potential performance impacts and efficiently solving several main technical problems. Aiming to establish a new paradigm for the analysis and design of multihop D2D communications, in this article, we propose a dynamic graph optimization framework that enables the modeling of large-scale systems with multiple D2D pairs and node mobility patterns. By inherently modeling the main technological problems for multihop D2D communications, this framework benefits investigation of theoretical performance limits and studying the optimal system design. Furthermore, these achievable benefits are demonstrated by examples of simulations under a realistic multihop D2D communication underlaying cellular network.

Stable Lithium Electrodeposition in Liquid and Nanoporous Solid Electrolytes

High energy and safe electrochemical storage are critical components in multiple emerging fields of technology where portability is a requirement for performance and large-scale deployment. From advanced robotics, autonomous aircraft, to hybrid electric vehicles, the number of technologies demanding advanced electrochemical storage solutions is rising. The rechargeable lithium ion battery (LIB) has received considerable attention because of its high operating voltages, low internal resistance and minimal memory effects. Unfortunately LIBs are currently operating close to their theoretical performance limits due to the relatively low capacity of the anode LiC6 and the lithiated cathode materials (LiCoO2 and LiFePO4) in widespread use. It has long been understood that a rechargeable lithium metal battery (LMB), which eschewed the use of a carbon host at the anode can lead to as much as a ten-fold improvement in anode storage capacity (from 360 mAh g-1 to 3860 mAh g-1) and would open up opportunities for high energy un-lithiated cathode materials such as sulfur and oxygen, among others. Together, these advances would lead to rechargeable batteries with step-change improvements in storage capacity relative to today’s state of the art LIBs.A grand challenge in the field concerns the development of electrolytes, electrode, and battery systems configurations that prevent uneven electrodeposition of lithium and other metal anodes, and thereby eliminate dendrites at the nucleation step. It is understood that without significant breakthroughs in this area, the promise of LMBs, as well as of storage platforms based on more earth abundant metals such as Na and Al metal cannot be realized. Specifically, after repeated cycles of charge and discharge growing metal dendrites can bridge the inter-electrode space, producing internal short circuits in the cell. In a volatile electrolyte, ohmic heat generated by these shorts may lead to thermal runaway and catastrophic cell failure, which places obvious safety and performance limitations on the cells. The ohmic heat generated during a short-circuit may also locally melt dendrites to create regions of “orphaned” or electrically disconnected metal that results in a steady decrease in storage capacity as a battery is cycled. Lithium-ion batteries (LiB) are designed to remove these risks by hosting the lithium in a conductive carbon host at the anode. However, the small potential difference that separates lithium insertion into versus plating onto carbon can potentially lead to similar failure modes in an overcharged or too quickly charged LiB. Thus, the need for materials that prevent non-uniform electrodeposition of metals such as Li is also implicit in new fast charging LIB technology targeted for electric-drive vehicles. We have investigated electrodeposition of lithium in simple liquid electrolytes such as propylene carbonate, ethylene carbonate and diethyl carbonate, and in nanoporous solids infused with liquid electrolytes. We find that simple liquid electrolytes reinforced with LiXA blends exhibit stable long-term cycling at room temperature, often with no signs of deposition instabilities over hundreds of cycles of charge and discharge and thousands of operating hours. We rationalize these observations with the help of surface energy data for the electrolyte/lithium interface and impedance analysis of the interface during different stages of cell operation. Our findings provide support for an important recent theoretical prediction that the surface mobility of lithium is significantly enhanced in the presence of LiXA.

Exergy analysis of a novel configuration of desiccant based evaporative air conditioning system

In this work, a process is developed for exergy analyses of a novel configuration of desiccant based an evaporative air conditioning system. The exergy transfer and destruction between the components of the system are defined for the average measured variables obtained from the experimental results. The exergy formulations are carried out to the experimental system using the data collected during a typical operation of the system. The exergy output, specific flow exergy, exergy destruction, exergy input and exergy efficiency are determined. Furthermore, the sustainability assessment and relative irreversibility of components are obtained. It is found that the exergy efficiency of the entire experimental unit is 40.7% at a reference temperature of 15°C. It is also observed that the exergy efficiencies of the entire system varies between 56% and 25% for reference temperature of 0–30°C, respectively. The effects of reference temperature on the performance of the studied system are investigated. Based on the investigation, it is seen that an exergy analysis can provide beneficial knowledge with respect to the theoretical upper limit of the system performance.

Monoporous micropillar wick structures, II-optimization & theoretical limits

This article, the second of a two-part study, is aimed at finding optimal micropillar wick geometries and their theoretical performance limits. For the first time, the theoretical optimization results are experimentally verified. The permeability and capillary pressure models determined in the first part of this study are utilized in the optimization process. First, the existence of optimality is ascertained through constrained parametric sweeps of the individual variables in the wick performance model, which reveal the presence of optimal values of each variable for a specific combination of the other variables. Then, the overall optimization of wicks is accomplished through a genetic algorithm. It is established that a unique optimal geometry, i.e. a combination of diameter, spacing and height, exists at each wicking length. Finally, these results are used to determine the theoretical performance limits of micropillar array wicks. The results suggest that the theoretical performance limit is 3–4 times greater than the highest performance reported in literature.

SIMULATION AND PERFORMANCE ANALYSIS OF BLAST STRUCTURE IN MIMO ENVIRONMENT

This paper presents the modeling and simulation of the most innovative MIMO-VBLAST technique. Wireless communication has suffered from the fading problem ever since its first appearance in 1897, when Guglielmo Marconi transmitted a wireless signal to a ship in the English Channel. The following century witnessed the remarkable development of wireless communication, especially in the last decade. The capacity of traditional communication systems along with 1x1 antenna system would be very low because of multi path propagation within radio channels. The multi-path signals add up constructively or destructively at the receiver antenna to give a fluctuating signal, which can vary widely in amplitude and phase. When the amplitude of the signal experiences a low value it is termed fading and the capability of the wireless channel is severely limited. Several efforts into this direction have been carried out to make the radio channel more efficient and this has got remarkable progress. Efficient techniques, such as frequency reuse and OFDM, have been invented to increase the bandwidth efficiency; on the other hand, advances in coding techniques, such as Reed Solemn codes, Convolution codes and low density parity check (LDPC) codes make it possible to almost reach Shannon capacity the theoretical performance limit of the channel. However, the development of the techniques for a single channel has yet to catch up with the increasing demand for the capacity The most promising solution to combat this effect is to have antenna diversity scheme which can be implemented with many of ways such as BLAST structure, STBC coding technique, etc. With the implementation of antenna diversity i.e. MIMO, the capacity of the communication system as well as channel will increase in terms of improved error rate performance. Also with the implementation of BLAST structure the data rate of the communication can be enhanced at the same amount of bandwidth. Ultimately this paper concludes effect of MIMO-BLAST to provide array gain i.e high data rate for modern wireless scenario.