Nanoscale FeS₂ was synthesized via a simple hydrothermal method and was decorated by hydrothermal carbonization (FeS₂@C). The structural properties of the synthesized materials detected by X-ray diffraction (XRD), together with the morphologies characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) indicated that the hydrothermal carbonization only had an impact on the morphology of pyrite. Additionally, the electrochemical performance of the coated pyrite in Li/FeS₂ batteries was evaluated by galvanostatic discharge-charge tests and electrochemical impedance spectroscopy (EIS). The results showed that the initial capacity of FeS₂@C was 799.2 mAh·g−1 (90% of theoretical capacity of FeS₂) and that of uncoated FeS₂ was only 574.6 mAh·g−1. XRD and ultraviolet (UV) visible spectroscopy results at different depths of discharge-charge for FeS₂ were discussed to clarify the electrochemical mechanism, which play an important part in Li/FeS₂ batteries.

The conversion efficiency of thin-film silicon solar cells needs to be improved to be competitive with respect to other technologies. For a more efficient use of light across the solar spectrum, multi-junction architectures are being considered. Light-management considerations are also crucial in order to maximize light absorption in the active regions with a minimum of parasitic optical losses in the supportive layers. Intrinsic and doped silicon oxide alloys can be advantageously applied within thin-film Si solar cells for these purposes. Intrinsic a-SiOx:H films have been fabricated and characterized as a promising wide gap absorber for application in triple-junction solar cells. Single-junction test devices with open circuit voltage up to 950 mV and ~1 V have been demonstrated, in case of rough and flat front electrodes, respectively. Doped silicon oxide alloys with mixed-phase structure have been developed, characterized by considerably lower absorption and refractive index with r... [more]

The quest for single-stage deposition of CuInGaSe₂ (CIGS) is an open race to replace very effective but capital intensive thin film solar cell manufacturing processes like multiple-stage coevaporation or sputtering combined with high pressure selenisation treatments. In this paper the most recent achievements of Low Temperature Pulsed Electron Deposition (LTPED), a novel single stage deposition process by which CIGS can be deposited at 250 °C, are presented and discussed. We show that selenium loss during the film deposition is not a problem with LTPED as good crystalline films are formed very close to the melting temperature of selenium. The mechanism of formation of good ohmic contacts between CIGS and Mo in the absence of any MoSe₂ transition layers is also illustrated, followed by a brief summary of the measured characteristics of test solar cells grown by LTPED. The 17% efficiency target achieved by lab-scale CIGS devices without bandgap modulation, antireflection coating or K-dop... [more]

The paper presents the results of a transient numerical investigation of the melting and solidification process of sodium nitrate (NaNO₃), which is used as phase change material. For enhancing the heat transfer to the sodium nitrate an aluminum wire matrix is used. The numerical simulation of the melting and solidification process was done with the enthalpy-porosity approach. The numerical analysis of the melting process has shown that apart from the first period of the charging process, where heat conduction is the main heat transfer mechanism, natural convection is the dominant heat transfer mechanism. The numerical investigation of the solidification process has shown that the dominant heat transfer mechanism is heat conduction. Based on the numerical results, the discharging process has been slower than the charging process. The performance of the charged and discharged power has shown that the wire matrix is an alternative method to enhance the heat transfer into the phase change... [more]

The challenge of developing effective separation and purification technologies that leave much smaller energy footprints is greater for carbon dioxide (CO₂) than for other gases. In addition to its involvement in climate change, CO₂ is present as an impurity in biogas and bio-hydrogen (biological production by dark fermentation), in post-combustion processes (flue gas, CO₂-N₂) and many other gas streams. Selected phenol-formaldehyde resin-based activated carbons prepared in our laboratory have been evaluated under static conditions (adsorption isotherms) as potential adsorbents for CO₂ separation at sub-atmospheric pressures, i.e., in post-combustion processes or from biogas and bio-hydrogen streams. CO₂, H₂, N₂, and CH₄ adsorption isotherms at 25 °C and up to 100 kPa were obtained using a volumetric equipment and were correlated by applying the Sips model. Adsorption equilibrium was then predicted for multicomponent gas mixtures by extending the multicomponent Sips model and the Ideal... [more]

The design of efficient Organic Rankine Cycle (ORC) units for the usage of industrial waste heat at high temperatures requires direct contact evaporators without intermediate thermal oil circuits. Therefore, the thermal stability of high-temperature working fluids gains importance. In this study, the thermal degradation of hexamethyldisiloxane (MM) is investigated in an electrically heated tube. Qualitative results concerning remarks on degradation products as well as quantitative results like the annual degradation rate are presented. It is shown that MM is stable up to a temperature of 300 °C with annual degradation rates of less than 3.5%. Furthermore, the break of a silicon⁻carbon bond can be a main chemical reaction that influences the thermal degradation. Finally, it is discussed how the results may impact the future design of ORC units.

In this article, we describe an effective method to tether Pt and PtSn nanoparticles (NPs) on polyelectrolyte modified multi-walled carbon nanotubes (MWCNTs) for ethanol electrooxidation. By using a polymer wrapping technique, positively charged polyethyleneimine (PEI) was attached onto carbon nanotubes (CNTs) to provide preferential linking sites for metal precursors. Well-dispersed Pt and PtSn nanocrystals (2⁻5 nm) were subsequently decorated on PEI-functionalized MWCNTs through the polyol reduction method. The successful non-covalent modification of MWCNTs was confirmed by Fourier transform infrared spectroscopy (FTIR) and Zeta potential measurements. Energy dispersive X-ray (EDX) spectrum indicates approximately 20 wt % Pt loading and a desirable Pt:Sn atomic ratio of 1:1. Electrochemical analysis demonstrated that the as-synthesized PtSn/PEI-MWCNTs nanocomposite exhibited improved catalytic activity and higher poison tolerance for ethanol oxidation as compared to Pt/PEI-MWCNTs and... [more]

Partial discharge (PD) detection is essential to the operation of high-voltage systems. In this context, we investigate the basic characteristics of light emission during PDs in SF₆ gas from the perspective of insulation diagnosis. A synchronous system is constructed using three optical photoelectric instruments with separate wavelength responses in the ultraviolet (UV, 189⁻352 nm), visible (VIS, 381⁻675 nm), and near-infrared (NIR, 737⁻920 nm) spectral ranges and a wide-band PD current pulse detector with a response of 1 pC. The results indicate that light emission depends upon the type of insulation defect and discharge energy. An increase in PD charge gives rise to more components in the spectral range from UV to VIS, and the presence of an insulator surface in discharges yields a more complex VIS-to-NIR spectrum. The phase-resolved partial discharge pattern (PRPD) of UV light pulses can reasonably reflect the electroluminescence process in the presence of the insulator surface and... [more]

Focusing on the influence of fine metal particles on the insulation characteristics of outdoor insulators, spherical micrometer-level iron powders were used to represent fine metal particles of different parameters on a polymer insulator specimen surface. Dynamic movement and lift-off behavior of fine particles, as well as the triggered surface discharges under AC voltage were investigated in a uniform electric field under different experimental conditions. The results reveal that the inception, propagation and intensity of surface discharges are significantly affected by the particle parameters, including particle size, amount and distributing characteristic. Based on the measurement of light emission during the flashover process using a high-speed camera, the process of surface discharge to flashover triggered by the fine metal particles were investigated to obtain a relationship between flashover voltage, discharge light intensity and particle parameters. It is suggested that partic... [more]

Electric vehicle (EV) manufacturers are employing cylindrical format cells in the construction of the vehicles’ battery systems. There is evidence to suggest that both the academic and industrial communities have evaluated cell degradation due to vibration and other forms of mechanical loading. The primary motivation is often the need to satisfy the minimum requirements for safety certification. However, there is limited research that quantifies the durability of the battery and in particular, how the cells will be affected by vibration that is representative of a typical automotive service life (e.g., 100,000 miles). This paper presents a study to determine the durability of commercially available 18,650 cells and quantifies both the electrical and mechanical vibration-induced degradation through measuring changes in cell capacity, impedance and natural frequency. The impact of the cell state of charge (SOC) and in-pack orientation is also evaluated. Experimental results are presented... [more]

In this study, we investigate power generation by reverse electrodialysis in a dense silica membrane that is between two NaCl solutions with various combinations of concentrations. Each silica membrane is fabricated by depositing a silica layer on a porous alumina substrate via chemical vapor deposition. The measured potential-current (V-I) characteristics of the silica membrane are used to obtain the transference number, diffusion potential, and electrical resistance. We develop empirical correlations for the transference number and the area-specific resistance, and present the results of power generation by reverse electrodialysis using the fabricated silica membranes. The highest measured power density is 0.98 mW/m². In addition, we develop a contour map of the power density as a function of NaCl concentrations on the basis of the empirical correlations. The contour map shows that a power output density of 1.2 mW/m² is achievable with the use of silica membranes and is sufficient to... [more]

Stopping the plasma-enhanced chemical vapor deposition (PECVD) once and maintaining the film in a vacuum for 30 s were performed. This was done several times during the formation of a film of i-layer microcrystalline silicon (μc-Si:H) used in thin-film silicon tandem solar cells. This process aimed to reduce defect regions which occur due to collision with neighboring grains as the film becomes thicker. As a result, high crystallinity (Xc) of μc-Si:H was obtained. Eventually, a solar cell using this process improved the conversion efficiency by 1.3% (0.14 points), compared with a normal-condition cell. In this paper, we propose an easy method to improve the conversion efficiency with PECVD.

A series of mono- and di-branched donor-π-acceptor charge-separated dyes incorporating triphenylamine as a donor and either Dalton’s or benzothiadiazole group as strong acceptors was synthesized and its fundamental properties relevant to the sensitization of nanocrystalline NiO investigated. The dyes exhibited an intense visible absorption band with a strong charge transfer character favorable to NiO sensitization, shifting the electron density from the donor to the acceptor branches. Nevertheless, the computed exciton binding energy is circa twice that of a common literature standard (P1), suggesting a more difficult charge separation. When tested in p-type dye-sensitized solar cells the dyes successfully sensitized NiO electrodes, with photocurrent densities about half than that of the reference compound. Being recombination kinetics comparable, the larger photocurrent generated by P1 agrees with the superior charge separation capability originating by its smaller exciton binding ene... [more]

The viscosity and the thermal conductivity of slag are among two of the most important material properties that need to be studied. In this paper we review the existing theoretical and experimental correlations for the thermal conductivity of slag. However, since, in general, slag behaves as a non-linear fluid, it is the heat flux vector which must be studied. Both explicit and implicit approaches are discussed and suggestions about the form of the heat flux vector and the thermal conductivity and their dependence on shear rate, porosity, deformation, etc. are provided. The discussion of the constitutive modeling of the heat flux vector for slag is from a theoretical perspective.

Nanofluids are considered a promising choice for several heat transfer applications. With the increasing awareness for energy saving and efficiency improvement in various thermal systems, including refrigeration systems, there is a growing interest in the refrigerant-based nanofluids owing to their superior thermo-physical properties. Nanorefrigerants are a class of nanofluid, which consist of suspended nanoparticles in a base refrigerant. In this paper, it is intended to include many articles on refrigeration systems that use nanorefrigerants, published in the period from 2005 to 2015. Although this is an extensive review, it could not include all the papers, and only some major research works were selected. It is believed that the dependency of thermal conductivity and other properties on temperature will make the thermal systems more efficient while operating at a high temperature. The literature reviews associated with the performance characteristics of nanorefrigerants in refriger... [more]

A nanocomposite polymer membrane based on quaternized poly(vinyl alcohol)/fumed silica (QPVA/FS) was prepared via a quaternization process and solution casting method. The physico-chemical properties of the QPVA/FS membrane were investigated. Its high ionic conductivity was found to depend greatly on the concentration of fumed silica in the QPVA matrix. A maximum conductivity of 3.50 × 10−2 S/cm was obtained for QPVA/5%FS at 60 °C when it was doped with 6 M KOH. The permeabilities of methanol and ethanol were reduced with increasing fumed silica content. Cell voltage and peak power density were analyzed as functions of fumed silica concentration, temperature, methanol and ethanol concentrations. A maximum power density of 96.8 mW/cm² was achieved with QPVA/5%FS electrolyte using 2 M methanol + 6 M KOH as fuel at 80 °C. A peak power density of 79 mW/cm² was obtained using the QPVA/5%FS electrolyte with 3 M ethanol + 5 M KOH as fuel. The resulting peak power densities are higher than the... [more]

We review several examples of how spectrally-selective photonic structures may be used to improve solar cell systems. Firstly, we introduce different spectrally-selective structures that are based on interference effects. Examples shown include Rugate filter, edge filter and 3D photonic crystals such as artificial opals. In the second part, we discuss several examples of photovoltaic (PV) concepts that utilize spectral selectivity such as fluorescence collectors, upconversion systems, spectrum splitting concepts and the intermediate reflector concept. The potential of spectrally selective filters in the context of solar cells is discussed.

Since its appearance in 1991, the Li ion battery has been the major power source driving the rapid digitalization of our daily life; however, much of the processes and mechanisms underpinning this newest battery chemistry remains poorly understood. As in any electrochemical device, the major challenge comes from the electrolyte/electrode interfaces, where the discontinuity in charge distribution and extreme disequality in electric forces induce diversified processes that eventually determine the kinetics of Li⁺ intercalation chemistry. This article will summarize the most recent efforts on the fundamental understanding of the interphases in Li ion devices. Emphasis will be placed on the formation chemistry of the so-called “SEI” on graphitic anode, the effect of solvation sheath structure of Li⁺ on the intercalation energy barrier, and the feasibility of tailoring a desired interphase. Biologically inspired approaches to an ideal interphase will also be briefly discussed.

It is customary, for convenience, to use relative permeability data produced at room temperature. This paper shows that this practice underestimates oil recovery rates and ultimate recovery from chalk rocks for high temperature reservoirs. Above a certain temperature (80°C in this work) a reduction of oil recovery was observed. The reduction in oil recovery is reflected by the shift of relative permeability data towards more oil-wet at high temperature (tested here 130°C). However, both IFT and contact angle measurements indicate an increase in water wetness as temperature increases, which contradict the results obtained by relative permeability experiments. This phenomenon may be explained based on the total interaction potential, which basically consists of van der Waals attractive and short-range Born repulsive and double layer electrostatic forces. The fluid/rock interactions is shown to be dominated by the repulsive forces above 80°C, hence increase fine detachment enhancing oil t... [more]

Theoretical investigations of the ejector refrigeration system using hydrofluoroolefins (HFOs) and hydrochlorofluoroolefin (HCFO) refrigerants are presented and discussed. A comparative study for eight olefins and R134a as the reference fluid was made on the basis of a one-dimensional model. To facilitate and extend the possibility of comparing our results, three different levels of evaporation and condensation temperature were adopted. The generator temperature for each refrigerant was changed in the range from 60 °C to the critical temperature for a given substance. The performed analysis shown that hydrofluoroolefins obtain a high efficiency of the ejector system at low primary vapor temperatures. For the three analyzed sets of evaporation and condensation temperatures (te and tc equal to 0 °C/25 °C, 6 °C/30 °C, and 9 °C/40 °C) the maximum Coefficient of Performance (COP) was 0.35, 0.365, and 0.22, respectively. The best performance was received for HFO-1243zf and HFO-1234ze(E). How... [more]

Cryogenic fracturing is a type of thermal shocking in which a cold liquid or gas is injected into a hot formation to create fractures. Research has shown that like traditional hydraulic fracturing, cryogenic fracturing could improve oil/gas recovery from unconventional reservoirs. Research has also shown, though, that, unlike traditional hydraulic fracturing, which uses water-based fluids, cryogenic fracturing limits and can even heal damage that is near the wellbore. Previous studies on thermal shocking, however, have generally examined only a few parameters at a time. To provide a more complete overview of the process, this study examines the effects of thermal shocking with low-temperature nitrogen gas on the porosities, permeabilities, and rock mechanical properties of unconventional reservoirs. Three cycles of thermal shocking were applied to a core sample and an outcrop sample from an unconventional reservoir. Each sample was heated at 82 °C for 1 h, and then nitrogen at −18 °C w... [more]

The ZnO@ZnS nanorod is synthesized by solvothermal method as an anode material for lithium ion batteries. ZnS is deposited on ZnO and assembles in nanorod geometry successfully. The nanosized rod structure supports ion diffusion by substantially reducing the ion channel. The close-linking of ZnS and ZnO improves the synergetic effect. ZnS is in the middle of the ZnO core and the external environment, which would greatly relieve the volume change of the ZnO core during the Li⁺ intercalation/de-intercalation processes; therefore, the ZnO@ZnS nanorod is helpful in maintaining excellent cycle stability. The ZnO@ZnS nanorod shows a high discharge capacity of 513.4 mAh g−1 at a current density of 200 mA g−1 after 100 cycles, while a reversible capacity of 385.6 mAh g−1 is achieved at 1000 mA g−1.

The lifetime of an electrical machine mainly depends on the thermal overloading. Modern day applications of electrical machines on one hand require compact machines with high power density, while on the other hand force electrical machines to undergo frequent temperature cycling. Until recently, in the case of electrical machines, the main factor related to the degradation of the winding insulation was thought to be the thermal oxidization of the insulation materials. It has now been revealed that thermal overloading can also induce mechanical stress in the windings of electrical machines, which over time could lead to fatigue and degradation. In this paper, a comprehensive study of the thermally induced mechanical stress in the windings of an electrical machine is presented. The study is performed using combined thermo-mechanical models. The numerical results are validated by experiments on a segmented stator winding set-up.

Medium frequency transformers (MFTs) are a key component of DC⁻DC dual active bridge (DAB)-type converters. These technologies are becoming a quintessential part of renewable energy solutions, such as photovoltaic systems and wind energy power plants, as well as in modern power grid interfaces functioning as solid-state transformers in smart-grid environments. The weight and physical dimensions of an MFT are key data for the design of these devices. The size of an MFT is reduced by increasing its operating frequency. This reduction implicates higher power density through the transformer windings, as well as other design requirements distinct to those used for conventional 60/50 Hz transformers; therefore, new MFT design procedures are needed. This paper introduces a novel methodology for designing MFTs, using nanocrystalline cores, and tests it using an MFT⁻DAB lab prototype. Different to other MFT design procedures, this new design approach uses a modified version of the area-product... [more]

Positive and negative streamer inception voltages from ultra-sharp needle tips (with tip radii below 0.5 μm) are measured in TiO₂, SiO₂, Al₂O₃, ZnO and C60 nanofluids. The experiments are performed at several concentrations of nanoparticles dispersed in mineral oil. It is found that nanoparticles influence positive and negative streamers in different ways. TiO₂, SiO₂ and Al₂O₃ nanoparticles increase the positive streamer inception voltage only, whilst ZnO and C60 nanoparticles augment the streamer inception voltages in both polarities. Using these results, the main hypotheses explaining the improvement in the dielectric strength of the host oil due to the presence of nanoparticles are analyzed. It is found that the water adsorption hypothesis of nanoparticles is consistent with the increments in the reported positive streamer inception voltages. It is also shown that the hypothesis of nanoparticles reducing the electron velocity by hopping transport mechanisms fails to explain the resu... [more]