Electrical Conversion Efficiency Research Articles

Tabulated values of the Shockley–Queisser limit for single junction solar cells

The detailed balance limit for solar cells presented by Shockley and Queisser in 1961 describes the ultimate efficiency of an ideal p–n junction solar cell illuminated by a black body with a surface temperature of 6000K. Today the AM 1.5G spectrum is the standard spectrum for non-concentrated photovoltaic conversion, taking light absorption and scattering in the atmosphere into account. New photovoltaic materials are investigated every day, but tabulated values to estimate their performance limits are difficult to find. Here values of the maximum short circuit current density (Jsc), open circuit voltage (Voc), light to electric power conversion efficiency (η) as well as current density (Jmpp) and voltage (Vmpp) at the maximum power point are presented as a function of the light absorbers’ band gap energy.

Graphene modified vanadium pentoxide nanobelts as an efficient counter electrode for dye-sensitized solar cells

Efficient dye-sensitized solar cells (DSSCs) were fabricated using graphene modified vanadium pentoxide nanobelts (GVNBs) as a cost-effective counter electrodes (CEs). GVNBs were synthesized by facile one-step hydrothermal method without using any reducing agent and harmful solvents. These novel nanocomposite have been tested as a counter electrode in DSSCs for the first time, which showed high solar to electrical energy conversion efficiency. The GVNBs showed the excellent electrocatalytic activity for the reduction of triiodide to iodide due to the synergetic effect between one-dimensional shape of vanadium pentoxide nanobelts and reduced graphene oxide. The DSSCs based on the GVNBs CEs have reached a high power conversion efficiency of 6.17%, which is comparable to that (7.98%) of the conventional platinum (Pt)-based CEs. This result indicate that GVNBs can be used as an alternative to conventional Pt-based CEs for the highly efficient DSSCs.

Performance validation and optimization of a dual coaxial-cylinder ocean-wave energy extractor

A point-absorber wave-energy extractor is developed, consisting of a dual coaxial-cylinder system, with the inner cylinder tension-tethered and an outer cylinder (floater) oscillating vertically. A permanent magnet linear generator (PMLG) is used as a power take-off (PTO) capturing wave energy from the relative motion of the two cylinders. The mathematical modeling of the system includes the coupling effects of the cylinder hydrodynamics and the PMLG behavior. It gives a rational and effective way of providing performance predictions and directions for optimization. The flat bottom shape of the floater is modified into a needle-like curved shape to minimize viscous losses, which leads to three-times increase in floater response, compared with the flat-bottom geometry and thus improved wave-energy capture. The behavior of the PTO in the presence of an appropriate supporting structure for the coaxial cylinders are investigated, and optimal operating conditions for energy extraction and mechanical to electrical conversion efficiency are determined. Experimental results of this coupled system in regular waves confirm the validity of the theoretical predictions and soundness of the engineering design. Optimizing the floater bottom shape and the operating conditions for energy extraction lead to a two-times increase in overall efficiency, even without any active control.

Inverted Ultrathin Organic Solar Cells with a Quasi-Grating Structure for Efficient Carrier Collection and Dip-less Visible Optical Absorption

We propose a metallic-particle-based two-dimensional quasi-grating structure for application to an organic solar cell. With the use of oblate spheroidal nanoparticles in contact with an anode of inverted, ultrathin organic solar cells (OSCs), the quasi-grating structure offers strong hybridization between localized surface plasmons and plasmonic gap modes leading to broadband (300~800 nm) and uniform (average ~90%) optical absorption spectra. Both strong optical enhancement in extreme confinement within the active layer (90 nm) and improved hole collection are thus realized. A coupled optical-electrical multi-physics optimization shows a large (~33%) enhancement in the optical absorption (corresponding to an absorption efficiency of ~47%, AM1.5G weighted, visible) when compared to a control OSC without the quasi-grating structure. That translates into a significant electrical performance gain of ~22% in short circuit current and ~15% in the power conversion efficiency (PCE), leading to an energy conversion efficiency (~6%) which is comparable to that of optically-thick inverted OSCs (3–7%). Detailed analysis on the influences of mode hybridization to optical field distributions, exciton generation rate, charge carrier collection efficiency and electrical conversion efficiency is provided, to offer an integrated understanding on the coupled optical-electrical optimization of ultrathin OSCs.

Electrostatically tunable lateral MoTe2 p-n junction for use in high-performance optoelectronics.

Because of their ultimate thickness, layered structure and high flexibility, pn junctions based on layered two-dimensional semiconductors have been attracting increasing attention recently. In this study, for the first time, we fabricated lateral pn junctions (LPNJs) based on ultrathin MoTe2 by introducing two separated electrostatic back gates, and investigated their electronic and photovoltaic performance. Pn, np, nn, and pp junctions can be easily realized by modulating the conductive channel type using gate voltages with different polarities. Strong rectification effects were observed in the pn and np junctions and the rectification ratio reached ∼5 × 10(4). Importantly, we find a unique phenomenon that the parameters for MoTe2 LPNJs experience abrupt changes during the transition from p to n or n to p. Furthermore, a high performance photovoltaic device with a filling factor of above 51% and electrical conversion efficiency (η) of around 0.5% is achieved. Our findings are of importance to comprehensively understand the electronic and optoelectronic properties of MoTe2 and may further open up novel electronic and optoelectronic device applications.

Design of a Load Torque Based Control Strategy for Improving Electric Tractor Motor Energy Conversion Efficiency

In order to improve the electrical conversion efficiency of an electric tractor motor, a load torque based control strategy (LTCS) is designed in this paper by using a particle swarm optimization algorithm (PSO). By mathematically modeling electric-mechanical performance and theoretical energy waste of the electric motor, as well as the transmission characteristics of the drivetrain, the objective function, control relationship, and analytical platform are established. Torque and rotation speed of the motor’s output shaft are defined as manipulated variables. LTCS searches the working points corresponding to the best energy conversion efficiency via PSO to control the running status of the electric motor and uses logic and fuzzy rules to fit the search initialization for load torque fluctuation. After using different plowing forces to imitate all the common tillage forces, the simulation of traction experiment is conducted, which proves that LTCS can make the tractor use electrical power efficiently and maintain agricultural applicability on farmland conditions. It provides a novel method of fabricating a more efficient electric motor used in the traction of an off-road vehicle.

Application of infiltrated LSCM–GDC oxide anode in direct carbon/coal fuel cells

Hybrid direct carbon/coal fuel cells (HDCFCs) utilise an anode based upon a molten carbonate salt with an oxide conducting solid electrolyte for direct carbon/coal conversion. They can be fuelled by a wide range of carbon sources, and offer higher potential chemical to electrical energy conversion efficiency and have the potential to decrease CO2 emissions compared to coal-fired power plants. In this study, the application of (La, Sr)(Cr, Mn)O3 (LSCM) and (Gd, Ce)O2 (GDC) oxide anodes was explored in a HDCFC system running with two different carbon fuels, an organic xerogel and a raw bituminous coal. The electrochemical performance of the HDCFC based on a 1-2 mm thick 8 mol% yttria stabilised zirconia (YSZ) electrolyte and the GDC-LSCM anode fabricated by wet impregnation procedures was characterized and discussed. The infiltrated oxide anode showed a significantly higher performance than the conventional Ni-YSZ anode, without suffering from impurity formation under HDCFC operation conditions. Total polarisation resistance (Rp) reached 0.8-0.9 Ω cm(2) from DCFC with an oxide anode on xerogel and bituminous coal at 750 °C, with open circuit voltage (OCV) values in the range 1.1-1.2 V on both carbon forms. These indicated the potential application of LSCM-GDC oxide anode in HDCFCs. The chemical compatibility of LSCM/GDC with carbon/carbonate investigation revealed the emergence of an A2BO4 type oxide in place of an ABO3 perovskite structure in the LSCM in a reducing environment, due to Li attack as a result of intimate contact between the LSCM and Li2CO3, with GDC being stable under identical conditions. Such reaction between LSCM and Li2CO3 was not observed on a LSCM-YSZ pellet treated with Li-K carbonate in 5% H2/Ar at 700 °C, nor on a GDC-LSCM anode after HDCFC operation. The HDCFC durability tests of GDC-LSCM oxide on a xerogel and on raw bituminous coal were performed under potentiostatic operation at 0.7 V at 750 °C. The degradation mechanisms were addressed, especially on raw coal.

Nanostructured Anatase Titania Spheres as Light Scattering Layer in Dye-sensitized Solar Cells

Incorporation of scattering layer on the nanocrystalline titania coating is an effective way to increase the efficiency of Dye sensitized solar cell. In this work, nanostructured titania spheres of ∼400nm size were synthesized through modified sol gel method using ethylene glycol as coordinating ligand. The anatase titania spheres were characterized using XRD, SEM and TEM. The performance of the synthesized nanostructured spherical anatase titania were studied by coating additional layers of scattering layer over the conventional photo anode. The photovoltaic performances of the dye sensitized solar cells were measured under AM1.5G solar spectrum with light intensity of 100 mW/cm2. DSSC with scattering layer achieved photovoltaic parameters such as, Voc of 0.690V, Jsc of 10.44mA/cm2, fill factor of 0.68 and solar to electric power conversion efficiency of 4.92%.

IGBTレグとMOSFETレグを備えたオールメタル対応誘導加熱システム用フルブリッジ・ハーフブリッジ切り替えインバータ

An induction heating system is proposed as a new circuit topology for induction heating cookers that can be used with all metals. This system has the mechanism to drive an IGBT and a MOSFET in parallel that switch the inverter between full-bridge or modified half-bridge configuration according to the cookware material. When cookware made of a low-resistivity material such as copper or aluminum is heated, the inverter is switched to the half-bridge configuration, which turns off the IGBT leg earlier than the MOSFET leg. When cookware made of a high-resistivity material such as iron or magnetic stainless steel is heated, the inverter is switched to the full-bridge configuration. We confirmed a turn-off timing for the IGBT where the loss of the switching element in the parallel drive was minimized by an experiment. The proposed inverter effectively improved the electricity conversion efficiency for all metals pan heating with the utilized IGBT and MOSFET legs.

Optimizing Photovoltaic Response by Tuning Light-Harvesting Nanocrystal Shape Synthesized Using a Quick Liquid–Gas Phase Reaction

The electron recombination lifetime in a sensitized semiconductor assembly is greatly influenced by the crystal structure and geometric form of the light-harvesting semiconductor nanocrystal. When such light harvesters with varying structural characteristics are configured in a photoanode, its interface with the electrolyte becomes equally important and directly influences the photovoltaic efficiency. We have systematically probed here the influence of nanocrystal crystallographic structure and shape on the electron recombination lifetime and its eventual influence on the light to electricity conversion efficiency of a liquid junction semiconductor sensitized solar cell. The light-harvesting cadmium sulfide (CdS) nanocrystals of distinctly different and controlled shapes are obtained using a novel and simple liquid-gas phase synthesis method performed at different temperatures involving very short reaction times. High-resolution synchrotron X-ray diffraction and spectroscopic studies respectively exhibit different crystallographic phase content and optical properties. When assembled on a mesoscopic TiO2 film by a linker molecule, they exhibit remarkable variation in electron recombination lifetime by 1 order of magnitude, as determined by ac-impedance spectroscopy. This also drastically affects the photovoltaic efficiency of the differently shaped nanocrystal sensitized solar cells.

A Case of Study of a Concentrating Solar Power Plant with Unfired Joule-Brayton Cycle

A solar closed air Brayton cycle, with rated power of 50 MW, was considered. The system is composed of a concentrating solar tower with volumetric receiver, an intercooling and regenerating gas turbine and an evaporative tower cooling system. The characteristic feature of the system is a control strategy able to adjust the plant in a large range of load, maintaining net electric conversion efficiency almost constant. The concentrating solar power (CSP) plant operates without adding fuel and can heat air up to a maximum temperature of 850°C, at the solar tower outlet. The numerical analysis was performed by SAM for the solar tower and by Thermoflex © for the assessment of the performance of the whole system. The thermal energy input was calculated on the basis of the DNI of the TMY from Seville. Results show an electricity production greater than 75GWh per year, with a significant sparing fossil fuel consumption and avoided CO2 emissions.

Small scale biomass CHP: Techno-economic performance of steam vs gas turbines with bottoming ORC

Small scale Combined Heat and Power (CHP) plants present lower electric efficiency in comparison to large scale ones, and this is particularly valid when biomass fuels are used. In most cases, the use of both heat and electricity to serve on site energy demand is a key to achieve acceptable global energy efficiency and investment profitability. However, the heat demand follows a typical daily and seasonal pattern and it is influenced by climatic conditions. During low heat demand periods, a lot of heat produced by the CHP plant is discharged. In order to increase the electric conversion efficiency of small scale biomass CHP plants, a bottoming ORC system can be coupled to the cycle, however this decreases the temperature and quantity of cogenerated heat available to the load. In this perspective, the paper proposes a thermo-economic analysis of small scale CHP plants based on steam turbine (ST) or externally fired micro gas turbine (EFGT) coupled to different typologies of bottoming Organic Rankine Cycles (ORC). The research assesses the influence of the thermal energy demand and CHP plant operational strategies on the global energy efficiency and profitability of the proposed cogeneration options, taking into account the part load efficiency and the heat to electricity ratio flexibility that could be achieved through a switch on-off of the bottoming ORC. The thermodynamic cycles and their part load efficiency are modeled by Gate-Cycle (Brayton cycles) and Cycle-Tempo (Rankine cycles). The research explores the profitability of bottoming ORC in view of the higher efficiency and electricity generation revenues but higher costs and reduced heat available for cogeneration in the case of bottoming ORC. The results indicate the optimal CHP technology and configuration for each energy demand segment and the relative key technical and economic factors in the Italian legislative framework.

Low-temperature templated synthesis of porous TiO2 single-crystals for solar cell applications

A facile low-temperature synthetic method of growing semiconductor mesoporous single-crystal of anatase TiO2 directly on FTO substrate was developed. The templated hydrothermal synthesis approach was employed to make mesoporous single-crystal TiO2 that contains pores tens to hundreds of nanometres in size under low temperature, which opens a potential way to produce useful functional thin film photoanodes by one-pot approach for fabricating cheap and highly efficient optoelectronic devices. This method is based on seeded nucleation and growth inside a pre-formed mesoporous silica film template immersed in diluted precursor solution. The electrochemical characterizations showed that the directly grown mesoporous single-crystal thin film on FTO substrate has substantially higher conductivity and electron mobility than conventionally deposited TiO2 thin films by printing techniques. Hence, using the as-synthesized mesoporous single-crystal thin film baking at 150°C as photoanodes, an encouraging 5.83% solar to electricity conversion efficiency was achieved. It is expected that the developed mesoporous single-crystals on FTO substrate may find broader applications in many different technologies. This generic synthetic strategy extends the possibility of mesoporous single-crystal films directly growing to a range of substrates. Moreover, this approach could work at lower temperatures below 150°C, which could considerably minimize the environmental impact and production costs of high performance mesoporous materials.

Formation of Semimetallic Cobalt Telluride Nanotube Film via Anion Exchange Tellurization Strategy in Aqueous Solution for Electrocatalytic Applications.

Metal telluride nanostructures have demonstrated several potential applications particularly in harvesting and storing green energy. Metal tellurides are synthesized by tellurization process performed basically at high temperature in reducing gas atmosphere, which makes the process expensive and complicated. The development of a facile and economical process for desirable metal telluride nanostructures without complicated manipulation is still a challenge. In an effort to develop an alternative strategy of tellurization, herein we report a thin film formation of self-standing cobalt telluride nanotubes on various conducting and nonconducting substrates using a simple binder-free synthetic strategy based on anion exchange transformation from a thin film of cobalt hydroxycarbonate nanostructures in aqueous solution at room temperature. The nanostructured films before and after ion exchange transformation reaction are characterized using field emission scanning electron microscope, energy dispersive X-ray analyzer, X-ray photoelectron spectroscopy, thin film X-ray diffraction technique, high resolution transmission electron microscope, and selected area electron diffraction analysis technique. After the ion exchange transformation of nanostructures, the film shows conversion from insulator to highly electrical conductive semimetallic characteristic. When used as a counter electrode in I3(-)/I(-) redox electrolyte based dye-sensitized solar cells, the telluride film exhibits an electrocatalytic reduction activity for I3(-) with a demonstration of solar-light to electrical power conversion efficiency of 8.10%, which is highly competitive to the efficiency of 8.20% exhibited by a benchmarked Pt-film counter electrode. On the other hand, the telluride film electrode also demonstrates electrocatalytic activity for oxygen evolution reaction from oxidation of water.

Charge separation in subcells of triple-junction solar cells revealed by time-resolved photoluminescence spectroscopy.

We measure the excitation-wavelength and power dependence of time-resolved photoluminescence (PL) from the top InGaP subcell in a InGaP/GaAs/Ge triple-junction solar cell. The wavelength-dependent data reveals that the PL decays are governed by charge separation. A fast single-exponential PL decay is observed at low excitation power densities, which is the charge separation under short-circuit condition. Under strong excitation a bi-exponential PL decay is observed. Its slow component appears at early times, followed by a faster component at late times. The slow decay is the carrier recombination of the subcell. The following fast component is the charge separation process under reduced built-in potential near the operating point. The subcells electrical conversion efficiency close to the operating point is evaluated using this decay time constant.

Experimental study on the operating characteristics of a novel photovoltaic/thermal integrated dual-source heat pump water heating system

This paper proposed a novel solar photovoltaic/thermal (PV/T) integrated dual-source heat pump (DSHP) water heating system. This system used the circulating water in solar photovoltaic thermal system and air source to provide domestic hot water for domestic usage, and meanwhile decreased the cooling water temperature for the PV/T panel to improve the electrical conversion efficiency. An experimental PV/T integrated DSHP unit was established in Shanghai to examine its performance. The influences of the inlet temperature of the evaporator and the hot water temperature under water–water mode and the influence of the outdoor temperature under air–water mode on the system performance were studied. Energy analysis was conducted between water–water mode and air–water mode. The cooling effect and photovoltaic efficiency of the PV/T panel were evaluated. The results showed that when water side evaporator was used, the PV/T panel operating temperature can be decreased up to 45 °C, and the electrical conversion efficiency yielded a surplus of 10.3%.

Improvements in Sunlight to Electricity Conversion Efficiency: above 40% for Direct Sunlight and over 30% for Global

Improvements in Sunlight to Electricity Conversion Efficiency: above 40% for Direct Sunlight and over 30% for Global

Assessing the Efficiency of Advanced Multijunction Solar Cells in Real Working Conditions: A Theoretical Analysis

Multijunction (MJ) solar cells are currently seen as one of the most promising options toward achieving solar energy to electricity conversion efficiencies exceeding 50%. The detailed balance theory provides a practical tool for tailoring MJ solar cells, taking several strong assumptions regarding the spectral input or the cell temperature. However, solar cells in real working operation may meet conditions that differ significantly from the ideal conditions for which they were designed. Assessing the extent to which advanced MJ solar cell architectures are able to withstand changes in the operating conditions is thus crucial toward ensuring an efficient use of MJ solar cells in real working operation. In this paper, the sensitivity of current-constrained MJ solar cells to operating conditions was studied by quantifying the decrease in the conversion efficiency associated with a change in the spectral input, the illumination level, and the cell working temperature. The sensitivity of MJ solar cells to the spectral content of the light was shown to be significant for cell architecture involving four subcells and more. On the contrary, the gain in efficiency achieved by tailoring the combination of bandgap to a specific value of the working temperature or the illumination level was demonstrated to be low. The implications for future generations of MJ solar cells comprising up to ten subcells are also addressed.

The Effect of Electric Load Profiles on the Performance of Off-Grid Residential Hybrid Renewable Energy Systems

This paper investigates the energy performance of off-grid residential hybrid renewable electric power systems, particularly the effect of electric load profiles on the ability to harvest available solar energy and avoid the consumption of auxiliary energy in the form of propane. The concepts are illustrated by an analysis of the energy performance of electric and propane-fired refrigerators. Off-grid electric power systems frequently incorporate a renewable source, such as wind or solar photovoltaic (PV), with a back-up power provided by a propane fueled motor/generator. Among other design decisions, residential consumers face the choice of employing an electric refrigerator with a conventional vapor compression refrigeration system, or a fuel-fired refrigerator operating as an absorption refrigeration system. One interesting question is whether it is more advantageous from an energy perspective to use electricity to run the refrigerator, which might be provided by some combination of the PV and propane motor/generator, thereby taking advantage of the relatively higher electric refrigerator Coefficient of Performance (COP) and free solar energy but having to accept a low electrical conversion efficiency of the motor/generator, or use thermal energy from the combustion of propane to produce the refrigeration effect via an absorption system, albeit with a much lower COP. The analysis is complicated by the fact that most off-grid renewable electrical power systems utilize a battery bank to provide electrical power when it is not available from the wind turbine or PV system, so the state of charge of the battery bank will have a noticeable impact on what energy source is available at any moment in time. Daily electric load profiles combined with variable solar energy input determine the state of charge of the battery bank, with the degree of synchronization between the two being a critical factor in determining performance. The annual energy usage and fuel input depend strongly on the ability to make use of the renewable sources in real time to avoid battery bank conversion losses and dumping of excess electrical power, as well as to have sufficient battery storage capacity to minimize the need for operation of the motor/generator to meet electric loads which occur during periods when the renewable energy is not available.

A theoretical study of the coupling between a vortex-induced vibration cylindrical resonator and an electromagnetic energy harvester

This paper presents a theoretical study of the coupling between a vortex-induced vibration (VIV) cylindrical resonator and its associated linear electromagnetic generator. The two-equation mathematical model is based on a dual-mass formulation in which the dominant masses are the stator and translator masses of the generator. The fluid–structure interaction implemented in the model equations follows the so-called ‘advanced forcing model’ whose closure relies on experimental data. The rationale to carry out the study is the fact that in these types of configurations there is a two-way interaction between the moving parts in such a way that their motions influence each other simultaneously, thereby affecting the energy actually harvested. It is believed that instead of mainly resorting to complementary numerical simulations, a theoretical model can shed some light on the nature of the interaction and, at the same time, provide scaling laws that can be used for practical design and optimization purposes. It has been found that the proposed configuration has a maximum hydrodynamic to mechanical to electrical conversion efficiency (based on the VIV resonator oscillation amplitude) of 8%. For a cylindrical resonator 10 cm long with a 2 cm diameter, this translates into an output power of 20 to 160 mW for water stream velocities in the range from 0.5 to 1 m s−1.