Maximum Attainable Efficiency Research Articles

Impact of Temperature and Optical Error on the Combined Optical and Thermal Efficiency of Solar Tower Systems for Industrial Process Heat

Concentrating solar thermal (CST) power towers can provide high flux concentrations at commercial scale. As a result, CST towers exhibit potential for high-temperature solar industrial process heat (SIPH) applications. However, at higher operating temperatures, thermal radiation losses can be significant. This study explores the trade-off between thermal and optical losses for SIPH applications using a collection of three case studies at operating temperatures that range from 900-1,550 °C. We assume blackbody radiation to represent the thermal losses at the receiver and we use ray tracing to estimate the optical losses. The results show the impact of process temperature on the maximum attainable system efficiency, as well as the higher flux concentration requirements as the temperature increases.

(Invited) Atmospheric Plasmas Synthesized Nanocrystals with Quantum Confinement and Quantum Hybrids in Photovoltaics

Nanocrystals share lot of advantages of organics namely scalable and controlled synthesis, an ability to be processed in solution while additionally retaining the broadband absorption and superior transport properties of traditional photovoltaic semiconductors. Nanocrystal solar cells have the potential to considerably increase the maximum attainable thermodynamic conversion efficiency (> 50%). Nanocrystal solution-processed can be used in solar cell structure not only as an absorber but also as electron and hole transport layer where the HOMO and LUMO levels can be efficiently controlled by size and/or plasma induced surface engineering directly in colloidal solution. Solution-processed and surface engineered nanocrystals with quantum confinement can be then further used to fabricate new class of quantum hybrids when blended for instance with polymers or perovskites and serves as absorbing and/or e-h transporting material. In this presentation, we overview the atmospheric plasma-based approaches to synthesis and surface engineering of nanocrystals with quantum confinement. We will compare surface engineering by fs laser processing in liquid solutions and synthesis of nanocrystals with strong quantum confinement by atmospheric plasmas. Moreover, to understand the thermal stability of nanocrystals observed experimentally, we calculate the cohesive and the formation energies of nanocrystals by means of first-principle calculations. Finally, we overview our recent progress in integration of surface engineered nanocrystal as a quantum hybrids incorporated within perovskites solar cells.

Optimal design of a hybrid thermal- and membrane-based desalination unit based on renewable geothermal energy

• A renewable-based hybrid desalination system consisting of HDH and RO is evaluated. • Two different operating modes of the introduced system are investigated. • Exergoeconomics is used to identify the optimal working condition of the system. • Maximum exergy efficiency of the plant is 28.86% for the second operating mode. • The first operating mode can provide freshwater at a maximum rate of 22070 m 3 /day. In this article, exergoeconomic analysis of a hybrid desalination system driven by renewable geothermal energy source is investigated. Freshwater and electrical power are the products of this system. Humidification – dehumidification (HDH) unit works in parallel with reverse osmosis (RO) unit to produce freshwater and Kalina cycle is the power generator unit. Thermoelectric generator (TEG) is responsible for providing the energy required by RO unit. As both HDH and TEG can operate with low-grade energy, two different modes are suggested for the proposed system. In the first mode, the geothermal low grade energy stream is passed to TEG unit, while in the second, it goes through the HDH unit. The effect of different parameters on four main objective functions is examined in parametric study. Moreover, with the aid of selection maps, the objectives of modes can be compared in different operating conditions. The maximum attainable exergy efficiency is calculated to be 28.86% for mode 2. Single objective optimization studies revealed that the first mode produces the largest amount of freshwater at a rate of 22,072 (m 3 /day), while the minimum achievable freshwater cost and total product cost rate can be obtained in the second mode, which is calculated to be 21.94 (¢/m3) and 37.19 ($/GJ). Besides, Sankey diagrams for exergy flow and mass flow of saline water are presented to unravel the impact of each sub-system on exergy and salinity.

The impact of spectral variation on the thermodynamic limits to photovoltaic energy conversion

Research into the fundamental limitations to photovoltaic power conversion has historically used a single predetermined set of conditions to define device performance limitations. This fails to account for the many variables involved in real-world situations. Previous work describing thermodynamic losses in solar energy conversion has typically used an analytical approach, precluding the use of real-world spectra. This paper describes a model which marries the advantages of the analytical approach with a numerical detailed balance calculation, enabling analysis of maximum attainable power conversion efficiency and associated loss mechanisms in photovoltaics under more representative conditions.Input spectra in the model are treated as separate beam and diffuse components, both in terms of power and subtended angle. Differences in conversion show that diffuse light is effectively under maximum concentration. This does not result in an efficiency gain since the equivalent energy is instead accounted for in the Carnot loss. The Carnot limit for the diffuse portion of the spectrum is therefore lower than that for direct light.Simulated hourly “clear sky” spectra across a year were analysed for five geographically disparate locations. Results showed that at higher latitudes narrower band gap devices have a similar maximum efficiency to those with wider band gaps, whilst at lower latitudes wider band gap devices have a slightly higher maximum efficiency. This is compounded by increased irradiance at lower latitudes. Irrespective of band gap, annual energy conversion shows little variation at lower latitudes, with greater conversion in summer being offset by reductions in winter at higher latitudes.

Numerical Insights into the Influence of Electrical Properties of n-CdS Buffer Layer on the Performance of SLG/Mo/p-Absorber/n-CdS/n-ZnO/Ag Configured Thin Film Photovoltaic Devices

A CdS thin film buffer layer has been widely used as conventional n-type heterojunction partner both in established and emerging thin film photovoltaic devices. In this study, we perform numerical simulation to elucidate the influence of electrical properties of the CdS buffer layer, essentially in terms of carrier mobility and carrier concentration on the performance of SLG/Mo/p-Absorber/n-CdS/n-ZnO/Ag configured thin film photovoltaic devices, by using the Solar Cell Capacitance Simulator (SCAPS-1D). A wide range of p-type absorber layers with a band gap from 0.9 to 1.7 eV and electron affinity from 3.7 to 4.7 eV have been considered in this simulation study. For an ideal absorber layer (no defect), the carrier mobility and carrier concentration of CdS buffer layer do not significantly alter the maximum attainable efficiency. Generally, it was revealed that for an absorber layer with a conduction band offset (CBO) that is more than 0.3 eV, Jsc is strongly dependent on the carrier mobility and carrier concentration of the CdS buffer layer, whereas Voc is predominantly dependent on the back contact barrier height. However, as the bulk defect density of the absorber layer is increased from 1014 to 1018 cm−3, a CdS buffer layer with higher carrier mobility and carrier concentration is an imperative requirement to a yield device with higher conversion efficiency and a larger band gap-CBO window for realization of a functional device. Most tellingly, simulation outcomes from this study reveal that electrical properties of the CdS buffer layer play a decisive role in determining the progress of emerging p-type photo-absorber layer materials, particularly during the embryonic device development stage.

Optimal regulatory choices in the organization of solid waste management systems: Empirical evidence and policy implications

The paper deals with the definition of the territorial partitions within which a specific sub-service of the waste cycle should be entrusted to a monopolistic provider—service-specific optimal territorial areas (SOTAs). In fact, the Italian legislation requires SOTAs to be of an appropriate size (presumed to be at least as large as a province) to ensure the maximum attainable efficiency level, but it gives no guidance on how to empirically identify the optimal size. A range of possible sizes of SOTAs for urban waste collection services is tested in the paper through a multi-stage DEA-based procedure aimed at assessing the impact of regulatory choices on the efficiency of a regulated service. The procedure is applied to data relative to the set of Italian municipalities with a population of over 20,000 residents for the years 2013, 2015 and 2017. A key finding is that, in the case of waste collection services, areas with a population under 57,000 provide the best solution for the size of SOTA. This suggests a possible change in the Italian approach to the regulation of waste services. According to the conclusions obtained, efficiency could be increased by limiting the size of SOTAs (even subdividing the bigger municipalities into more SOTAs) and promoting, instead, the growth of service providers in order to fully exploit technical economies of scale. The system would have big service providers competing for the awarding of services in a high number of small SOTAs, so intensifying the positive effects of competition for the market.

Beam Steering Efficiency in Resonant Reflective Metasurfaces

Beam steering is one of the prevailing functions performed by electromagnetic metasurfaces. Its efficiency depends on a large number of physical parameters associated with resonant elements comprising the metasurface and is thus notoriously difficult to optimize. Here we formulate a theoretical model for evaluating the diffraction efficiency of an array of lossy resonant elements whose spectral response is dominated by the coupling between a leaky eigenmode and a single incoming/outgoing channel. We use it to deduce a formula for the maximum attainable diffraction efficiency and the gradient parameter profile for which it is achieved. The optimization procedure is demonstrated on the example of an electrically tunable liquid-crystal terahertz beam steering metasurface. Finally, the proposed model is benchmarked against rigorous metasurface simulations.

Analysis and optimal design of a vibration isolation system combined with electromagnetic energy harvester

This work investigates a vibration isolation energy harvesting system and studies its design to achieve an optimal performance. The system uses a combination of elastic and magnetic components to facilitate its dual functionality. A prototype of the vibration isolation energy harvesting device is fabricated and examined experimentally. A mathematical model is developed using first principle and analyzed using the output frequency response function method. Results from model analysis show an excellent agreement with experiment. Since any vibration isolation energy harvesting system is required to perform two functions simultaneously, optimization of the system is carried out to maximize energy conversion efficiency without jeopardizing the system’s vibration isolation performance. To the knowledge of the authors, this work is the first effort to tackle the issue of simultaneous vibration isolation energy harvesting using an analytical approach. Explicit analytical relationships describing the vibration isolation energy harvesting system transmissibility and energy conversion efficiency are developed. Results exhibit a maximum attainable energy conversion efficiency in the order of 1%. Results suggest that for low acceleration levels, lower damping values are favorable and yield higher conversion efficiencies and improved vibration isolation characteristics. At higher acceleration, there is a trade-off where lower damping values worsen vibration isolation but yield higher conversion efficiencies.

A novel solar assisted vacuum thermionic generator-absorption refrigerator cogeneration system producing electricity and cooling

A novel solar assisted cogeneration system composed of a parabolic dish collector (PDC), a vacuum thermionic generator (VTIG), and an absorption refrigerator (APR) is proposed, in which the APR harvests the rejected heat from the VTIG for further cooling production. The equivalent power output and efficiency of the proposed system are obtained by considering various irreversible losses including radiation and convection heat losses of the PDC, radiation heat losses between the cathode and anode of the VTIG, heat transfer irreversibility, irreversible effects inside the working fluid of the APR. The maximum attainable efficiency increases from 17.21% to 27.06% as the bottoming APR is hybridized into the system. Meanwhile, the corresponding power density gains a 13.47% increase. Comprehensive sensitivity analyses are conducted to examine the dependence of the proposed system performance on some decisive design parameters and operating conditions such as absorber temperature of the solar collector, solar concentration ratio, solar irradiance, work function and anodic temperature of the VTIG, heat transfer area, heat transfer coefficient, and internal irreversibility factor of APR. The obtained results may provide some new insights into development and operation of such a solar powered cogeneration system.

CFD analysis of the dispersed phase behavior for micropowders production via spray drying and ultrasonic atomization

This work is devoted to the use of modern computer simulation tools based on the principles and methods of computational fluid dynamics that are applied to develop processes and equipment for producing spray dried micropowders. The feed solution was atomized into small droplets using an ultrasonic nozzle that exploits the principle of liquid disintegration caused by impact of high-frequency wave oscillations. The use of this device for spraying a liquid allowed obtaining spherical particles with diameters less than 20 μm and with a narrow size distribution. The process development is based on a mathematical model incorporating the hydrodynamics, heat and mass transfer mechanisms, which has been formulated to numerically predict particles trajectories and temporal changes of their mass and temperatures. The generated data were used to analyze the trajectories of discrete phase particles considering various process parameters and to estimate adhesion of wet particles to the drying chamber and nozzle. Also, predicted were the degree of particles deposition and precipitation in a cyclone, since these are useful for selection of process parameters to ensure maximum quality of particles and maximum attainable efficiency of the system collecting finished products.

Thermodynamic limits to energy conversion in solar thermal fuels

Solar thermal fuels (STFs) are an unconventional paradigm for solar energy conversion and storage which is attracting renewed attention. In this concept, a material absorbs sunlight and stores the energy chemically via an induced structural change, which can later be reversed to release the energy as heat. An example is the azobenzene molecule which has a cis-trans photoisomerization with these properties, and can be tuned by chemical substitution and attachment to templates such as carbon nanotubes, small molecules, or polymers. By analogy to the Shockley–Queisser limit for photovoltaics, we analyze the maximum attainable efficiency for STFs from fundamental thermodynamic considerations. Microscopic reversibility provides a bound on the quantum yield of photoisomerization due to fluorescence, regardless of details of photochemistry. We emphasize the importance of analyzing the free energy, not just enthalpy, of the metastable molecules, and find an efficiency limit for conversion to stored chemical energy equal to the Shockley–Queisser limit. STF candidates from a recent high-throughput search are analyzed in light of the efficiency limit.

Simulation of the ohmic loss in photovoltaic laser-power converters for wavelengths of 809 and 1064 nm

The method of mathematical simulation is used to examine the influence exerted by the characteristics of the epitaxial structure and contact grid of photovoltaic laser-power converters on their ohmic loss. The maximum attainable photoconverter efficiency at a Gaussian distribution of the laser-beam intensity on the surface of a photovoltaic converter and at dark-current densities of p–n junctions typical of structures grown by the metal-organic vapor-phase epitaxy (MOVPE) technique are determined. An approach to finding the optimal parameters of GaAs and In0.24Ga0.76As/GaAs photovoltaic converters in relation to the optical power being converted is suggested, and the structural parameters for incident-power values of 5, 20, and 50 W at wavelengths of 809 and 1064 nm are determined. It is found that, at laser-light intensities of up to 5 W, >60% efficiency can be achieved in laser-light conversion at a wavelength of 809 nm and >55% efficiency, at a wavelength of 1064 nm.

Theoretical Energy and Exergy Analyses of Direct Methanol Fuel Cell

This study studied the energy and exergy of direct methanol fuel cell (DMFC) through computer-assisted simulations. This was achieved through the development of mathematical models that represented the performance of a DMFC followed by simulations with a developed model using MATLAB. It was discovered that the simulated results conformed to the literature values with a correlation coefficient of 0.99 and standard deviation of 0.12. Simulations conducted with the developed model regarding the influence of operating parameters on the performance of the fuel cell indicated that maximum cell output voltage was obtained at a methanol concentration of 2 M. The results also showed that increases in temperature and operating pressure influenced the performance of the fuel cell. From the simulation results, it was seen that the maximum attainable energy efficiency of 95% was obtained at an operating temperature of 353 K, anode pressure of 1 atm, cathode pressure of 15 atm, and methanol concentration of 2 M, while the maximum exergy efficiency of 42% was obtained at the same parameters. It can therefore be inferred from the simulated results that improved performance, energy, and exergy efficiencies of a DMFC can be obtained by operating at controlled methanol concentrations and at temperatures that do not favour methanol cross-over or activation and concentration losses. Operating at a cathode pressure that is higher than the anode pressure can also enhance the performance of a fuel cell. Simulated results showed that cell performance depended on operating conditions; however, care must be taken in actual use, as high operating pressure will affect the safety and cost of operation of the cell because auxiliary equipment will be required to operate the cell at higher pressure.

Enhanced absorption in two-dimensional materials via Fano-resonant photonic crystals

The use of two-dimensional (2D) materials in optoelectronics has attracted much attention due to their fascinating optical and electrical properties. However, the low optical absorption of 2D materials arising from their atomic thickness limits the maximum attainable external quantum efficiency. For example, in the visible and near-infrared regimes monolayer MoS2 and graphene absorb only ∼10% and 2.3% of incoming light, respectively. Here, we experimentally demonstrate the use of Fano-resonant photonic crystals to significantly boost absorption in atomically thin materials. Using graphene as a test bed, we demonstrate that absorption in the monolayer thick material can be enhanced to 77% within the telecommunications band, the highest value reported to date. We also show that the absorption in the Fano-resonant structure is non-local, with light propagating up to 16 μm within the structure. This property is particularly beneficial in harvesting light from large areas in field-effect-transistor based graphene photodetectors in which separation of photo-generated carriers only occurs ∼0.2 μm adjacent to the graphene/electrode interface.

Closed form of optimal current waveform for class-F PA up to fourth harmonic

In this paper, rigorous analytical derivation of the coefficients of optimal current waveform for class-F power amplifier (PA) up to fourth harmonic (dc, 1st, 2nd and 4th harmonic) is presented. The coefficients of the optimal current waveform along with related maximum attainable efficiency are provided in closed form. The results obtained are also of interest for the inverse class-F, in the case when optimal voltage waveform consists of dc, 1st, 2nd and 4th harmonic.

Modelling and optimization of fully flexible assembly systems (F‐FAS)

PurposeThe paper aims to address the modelling and optimization of fully flexible assembly systems (F‐FAS), a new concept in flexible automation recently introduced by the authors.Design/methodology/approachThe paper presents a mathematical model of the F‐FAS, which makes it possible to predict its efficiency, throughput and unit direct production costs, correlating such values with system and production variables. The mathematical model proposed in the paper was derived from experimental and simulation data, which were analysed for a wide range of different productions and system settings.FindingsCorrelation analysis revealed that there are three main determinants of the efficiency of the F‐FAS: the number of components (types of parts) used to assemble the models (production variable); the average complexity of the models to be assembled (production variable); the ratio of the average perimeter of components (production variable) over a significant dimension of the working plane (system variable). Such parameters makes it possible to estimate the maximum attainable efficiency of the F‐FAS, and to calculate the optimal setting of the feeder which makes it possible to obtain such efficiency during the execution of the whole production order.Originality/valueThe model presented in the paper makes it possible to quantify in advance the real potential of the F‐FAS, according to the characteristics of the production mix and type of components to be assembled. By using the methodologies presented in the paper, one can first evaluate the convenience of the F‐FAS approach with respect to traditional FAS technology and manual assembly, then identify the optimal design and settings of the F‐FAS, according to the needs of a specific application. As a result, not only can the investment on the automated assembly system be accurately evaluated in advance, but also the return on investment can be maximized.

High-speed and high-efficiency travelling wave single-photon detectors embedded in nanophotonic circuits

Ultrafast, high-efficiency single-photon detectors are among the most sought-after elements in modern quantum optics and quantum communication. However, imperfect modal matching and finite photon absorption rates have usually limited their maximum attainable detection efficiency. Here we demonstrate superconducting nanowire detectors atop nanophotonic waveguides, which enable a drastic increase of the absorption length for incoming photons. This allows us to achieve high on-chip single-photon detection efficiency up to 91% at telecom wavelengths, repeatable across several fabricated chips. We also observe remarkably low dark count rates without significant compromise of the on-chip detection efficiency. The detectors are fully embedded in scalable silicon photonic circuits and provide ultrashort timing jitter of 18 ps. Exploiting this high temporal resolution, we demonstrate ballistic photon transport in silicon ring resonators. Our direct implementation of a high-performance single-photon detector on chip overcomes a major barrier in integrated quantum photonics.

Recent progress in the numerical modeling for organic thin film solar cells

Device modeling is constructive in finding the dependency of devices efficiency on structure parameters and material properties. For the sake of looking into the physics mechanism of organic solar cells (OSCs), as well as predicting their maximum attainable efficiency, numerical modeling is widely utilized to simulate the behavior of OSCs. Although some indispensable parameters are neglected or hypothesized because of inexplicitness in simulation models for OSCs, numerical modeling can describe the kinetic process in OSCs intuitively. This paper summarizes the optical/electrical models in the BHJ solar cell, as well as addresses their corresponding development in recent years on the basis of device physics and its working principle. Applications of numerical modeling and comments on modeling results are summarized. Meanwhile, precision and open questions about every model are discussed.

Thermodynamic Optimization of Advanced Steam Power Plants Retrofitted for Oxy-Coal Combustion

Thermodynamic optimization of power plants based on supercritical (SupC) and ultrasupercritical (USC) steam parameters is reported in this article. The objective is to compute the maximum attainable power plant efficiency in Indian climatic conditions using high ash (HA) indigenous coal. A unit size of 800 MWe presently under development in India is considered for energy and exergy analysis of power plants. Commercially established steam turbine parameters are used for the optimization of SupC power plant, whereas advanced steam turbine parameters currently under research and development are used for the optimization of USC power plant. The plant energy efficiency of the optimized SupC and USC power plant based on air-coal combustion (ACC) show considerable increases of 2.8 and 5.2% points, respectively compared with the current SupC ACC power plant (reference plant) being commissioned in India. The increases in plant exergy efficiency for the same power plants are 2.6 and 4.8% points and the corresponding CO2 reductions are about 6 and 11%, respectively. The maximum possible plant energy efficiency in Indian climatic conditions using HA Indian coal is about 42.7% (USC power plant). The effect of low ash coal on plant energy and exergy efficiencies compared with HA coal is also presented. Further, the effect of oxy-coal combustion (OCC) on the plant energy and exergy efficiencies compared with the ACC is studied for the double reheat SupC and USC power plants to account for the impact of CO2 capture. A significant reduction of 8.8 and 6.6% points in plant energy efficiency is observed for SupC and USC OCC power plants, respectively compared with the reference SupC ACC power plant.

Estimating the Maximum Attainable Efficiency in Dye‐Sensitized Solar Cells

AbstractFor an ideal solar cell, a maximum solar‐to‐electrical power conversion efficiency of just over 30% is achievable by harvesting UV to near IR photons up to 1.1 eV. Dye‐sensitized solar cells (DSCs) are, however, not ideal. Here, the electrical and optical losses in the dye‐sensitized system are reviewed, and the main losses in potential from the conversion of an absorbed photon at the optical bandgap of the sensitizer to the open‐circuit voltage generated by the solar cell are specifically highlighted. In the first instance, the maximum power conversion efficiency attainable as a function of optical bandgap of the sensitizer and the “loss‐in‐potential” from the optical bandgap to the open‐circuit voltage is estimated. For the best performing DSCs with current technology, the loss‐in‐potential is ∼0.75 eV, which leads to a maximum power‐conversion efficiency of 13.4% with an optical bandgap of 1.48 eV (840 nm absorption onset). Means by which the loss‐in‐potential could be reduced to 0.4 eV are discussed; a maximum efficiency of 20.25% with an optical bandgap of 1.31 eV (940 nm) is possible if this is achieved.