Entropy production due to free convection in partially heated isosceles triangular enclosures

A numerical analysis of the entropy production has been performed due to natural convection heat transfer and fluid flow in isosceles triangular enclosures with partially heated from below and symmetrically cooled from sloping walls. Governing equations are solved by finite difference method. Governing parameters on flow and temperature fields are Rayleigh number (10³  Ra  8.8 × 10⁵), dimensionless length of heater (0.25  (ℓ′ = ℓ/L)  1.0), dimensionless location of heater (0.25  (c′ = c/L)  0.75) and inclination angle of slopping walls (30°  β  60°). Heat transfer results are presented in terms of local and mean Nusselt numbers (Nu) while entropy production results are shown with entropy production number (N_s) and Bejan number (Be). Isotherms, streamlines, contours of entropy production due to heat transfer and fluid friction irreversibility are plotted. It is observed that entropy production number increases but Bejan number decreases with increasing of Rayleigh number. However, both entropy production due to heat transfer and fluid friction irreversibility are affected by higher inclination angle of triangle and length of heater.     

X-ray scattering measurements on nanosized TiO2 micelles

Titanium dioxide, micelles on glass substrate were generated in situ in a water-in-oil (w/o) microemulsion composed of water, dioctyl sulfosuccinate sodium salt (AOT), and cyclohexane, by controlled hydrolysis of TiCl₄. The average grain size R, obtained by grazing-incidence small-angle X-ray scattering (GISAXS), was 6.3±0.8 nm. “Corrected specific surface” of TiO₂ micelles was determined as 5.0×10⁵ cm⁻¹. The average grain size R of 5.0±1.3 nm obtained by grazing-incidence wide-angle X-ray scattering (GIWAXS) agrees with GISAXS value. GIWAXS can detect smaller amounts of additional phases or impurities than classical X-ray diffraction equipment.     

Enhanced heat transfer in inclined solar chimneys by electrohydrodynamic technique

Heat transfer enhancement of natural convection inside the inclined solar chimneys is investigated using electrohydrodynamic technique. The interactions between electric field, flow field, and temperature field are analyzed. The ranges of parameters considered are 10⁴Ra10⁷, 7.5 kVV₀17.5 kV, 30°θ120°, and 2aspect ratio14. Flow and heat transfer enhancements are significantly influenced at low Rayleigh number. The optimum inclined angle which obtains maximum volume flow rate and heat transfer is found to be at θ=60°. A maximum volume flow rate enhancement is expressed in relation with the number of electrodes. The relation between aspect ratio of chimney and number of electrodes that performs the optimum condition between efficiency and economy is analyzed incorporating with all concerning parameters.     

The growth kinetics and optical confinement studies of porous Si for application in terrestrial Si solar cells as antireflection coating

The mesoporous porous-silicon (PS) layers were grown on 1 0 0, 1 1 0, and 1 1 1 oriented wafers at constant current density of 20 mA cm⁻². The pore sizes and surface morphologies were measured by atomic force and scanning electron microscopes. The thickness x of the PS formed and the refractive index were measured by an ellipsometer as a function of time duration t (in min) of anodization. The x vs. t data were fitted into a power law x=at^c where c is a dimensionless constant and growth kinetics was established. The growth is practically independent of orientation. This is due the reason that the growth rate is controlled largely by the availability of holes which exchange their charge with oxidizing species and desirably large concentrations of holes were available at current density of 20 mA cm⁻². For a similar reason the growth of PS layer on the front surface of the n⁺ region of n⁺–p solar cells could also be done at current density of 20 mA cm⁻² nearly at the same rate. A large concentration of holes could be injected from p region into the n⁺ region because the positive contact was made on the p side and thus the junction was forward biased. The PS ARC of thickness 70 nm showed increase 26% in the short circuit current density J_sc and 24% in efficiency of the cells. However, the improvement in the values of the open circuit voltage V_oc were lower than the expected value indicating that the PS layers had enhanced recombination of minority carriers at the front surface or in the front emitter region immediately below the PS layer.     

Analytic solutions for the geometric and optical properties of stationary compound parabolic concentrators with fully illuminated inverted V receiver

Stationary low concentrator collectors (C < 2), of the CPC type, are of great interest to supply thermal energy for industrial processes, at temperatures below or around 90 °C. In particular, concentrators with fully illuminated inverted V absorbers have attractive properties for thermal energy production. Two classes of CPC’s with inverted V absorber are identified, according to the relationship between the vertex angle of the absorber (γ) and the acceptance angle of the cavity (θ_a), (γ  θ_a) or (γ < θ_a). The first class of CPC’s (γ  θ_a) converge to the fully illuminated CPC with horizontal flat receiver when (γ = 90°). The second class of CPC’s (γ < θ_a) converge to the fully illuminated CPC with vertical flat receiver when (γ = 0°). Both limiting cases have been published in the technical literature. This paper analyzes the class of concentrators satisfying (γ  θ_a). The ideal concentrator corresponding to a fully illuminated wedge absorber and (γ  θ_a) is a circular involute plus three parabolic segments. Closed-form analytic solutions are derived for its geometric and optical properties: reflector geometry, aperture, height, reflector length, angular acceptance function and average number of reflections for any degree of truncation. The equations obtained can be used as important design tools, for simulation techniques and optimization purposes. The collectible energy for North–South and East–West oriented collectors, for various receiver vertex angle and acceptance angle, was calculated. A cost-benefit figure, given by the relationship between collectible energy and reflector surface, is also estimated. Numerical results for any degree of cavity truncation are presented. As the degree of truncation varies, a clear minimum of the length over aperture ratio (L/A) occurs. The geometric and optical characteristics of different low concentration CPC’s (C, between 1 and 2, range of interest of stationary concentrators) show that the cavities with the minimal relationship between the length or height of the reflector surface and the aperture, (L/A) and (H/A), and the lower average number of reflections n correspond to the lowest angular acceptance concentrator (highest nominal concentration). If a concentration of 1.2 is desired, then the smallest ratios of (L/A), (H/A) and n, within the set of concentrators with maximal concentration (C) between 1 and 2, occur for (C = 2) (nominal acceptance half angle θ_a = 30°). Collectible energy results together with a cost-benefit relationship enable to conclude that a good choice for a well-designed collector for the city of Recife-PE, Brazil is: (a) East–West orientation; (b) receiver vertex angles (γ) of the order of (65°); (c) acceptance angle of the CPC (θ_a = 30°) and (d) concentration of the truncated cavity (C_t) in the interval (1.0–1.2).     

Organic photovoltaic devices based on polythiophene films electrodeposited on FTO substrates

We investigated photovoltaic devices based on electrochemically deposited monolayer of neat polythiophene (PT) films onto fluorine-doped tin oxide (FTO)/glass substrates. The photo-electrical behavior of these devices, using FTO and aluminum as electrodes, presented symbatic and antibatic response. These devices presented V_oc700 mV, under monochromatic irradiation (λ=610 nm; 1 W/m²) and the Incident Photon Converted to Electron Efficiency (IPCE) around 5%, with illumination through the FTO electrode (λ=610 nm; 1 W/m²). Cyclic voltammogramms and optical measurements were used to estimate the PT HOMO and LUMO energy levels, as well to demonstrate that the potential synthesis did not produce any polymer degradation. Using the Schottky model expression in the dark current voltage characteristics it was possible to obtain the barrier height value (_b), for the interface PT/Al. The _b was quite near to the difference between the aluminum work function and the PT electronic affinity and coherent with the V_oc values.     

Three-dimensional convection flow adjacent to inclined backward-facing step

Simulations of three-dimensional laminar forced convection adjacent to inclined backward-facing step in rectangular duct are presented to examine effects of step inclination on flow and heat transfer distributions. The step height is maintained as constant while its inclination angle is changed from 15° to 90°. The inlet flow is hydrodynamically steady and fully developed with uniform temperature. The bottom wall is heated with constant heat flux, while other walls are maintained as being thermally adiabatic. Velocity, temperature, Nusselt number, and friction coefficient distributions are presented. The “jet-like” flow and its impingement do not appear as the inclination angle of backward-facing step is small (α = 15°). At the center width of the duct and close to the stepped wall, the location where the streamwise velocity component is zero changes from a saddle point to a nodal point as the step inclination angle decreases. The downwash adjacent to the sidewall becomes stronger as the step inclination angle increases. The maximum Nusselt number on the stepped wall is located near the sidewall for α  30° and it appears near the center width of duct for small step inclination angle (α = 15°). The friction coefficient inside the primary recirculation region increases with the increase of the step inclination angle. Downstream of the primary recirculation region, increase of the friction coefficient becomes slower as the step inclination angle increases.     

Three-stage growth of Cu–In–Se polycrystalline thin films by chemical spray pyrolysis

Structural, optical and electrical properties of polycrystalline Cu–In–Se films, such as CuInSe₂ and ordered vacancy compounds (OVC), prepared by three-stage process of sequential chemical spray pyrolysis (CSP) of In–Se (first stage), Cu–Se (second stage) and In–Se (third stage) solutions have been studied in terms of substrate temperature at the second stage (T_S2). The films grown at T_S2420 °C exhibited larger grains in comparison with the Cu–In–Se films grown by the usual CSP method. Optical gap energy was approximately 1.06 eV for 360 °CT_S2420 °C, but increased dramatically from 1.06 to 1.35 eV when the T_S2 rose from 420 to 500 °C. Conductivity type was p-type for T_S2<420 °C, but n-type for T_S2>420 °C.     

Influence of hydrogen dilution on structural, electrical and optical properties of hydrogenated nanocrystalline silicon (nc-Si:H) thin films prepared by plasma enhanced chemical vapour deposition (PE-CVD)

Hydrogenated nanocrystalline silicon (nc-Si:H) thin films were deposited from pure silane (SiH₄) and hydrogen (H₂) gas mixture by conventional plasma enhanced chemical vapour deposition (PE-CVD) method at low temperature (200 °C) using high rf power. The structural, optical and electrical properties of these films are carefully and systematically investigated as a function of hydrogen dilution of silane (R). Characterization of these films with low angle X-ray diffraction and Raman spectroscopy revealed that the crystallite size in the films tends to decrease and at same time the volume fraction of crystallites increases with increase in R. The Fourier transform infrared (FTIR) spectroscopic analysis showed at low values of R, the hydrogen is predominantly incorporated in the nc-Si:H films in the mono-hydrogen (SiH) bonding configuration. However, with increasing R the hydrogen bonding in nc-Si:H films shifts from mono-hydrogen (SiH) to di-hydrogen (SiH₂) and (SiH₂)_n complexes. The hydrogen content in the nc-Si:H films decreases with increase in R and was found less than 10 at% over the entire studied range of R. On the other hand, the Tauc's optical band gap remains as high as 2 eV or much higher. The quantum size effect may responsible for higher band gap in nc-Si:H films. A correlation between electrical and structural properties has been found. For optimized deposition conditions, nc-Si:H films with crystallite size 7.67 nm having good degree of crystallinity (84% ) and high band gap (2.25 eV) were obtained with a low hydrogen content (6.5 at%). However, for these optimized conditions, the deposition rate was quite small (1.6 Å/s).     

Mixed convective heat transfer in rectangular enclosures driven by a continuously moving horizontal plate

The fluid flow and heat transfer induced by the combined effects of the mechanically driven lid and the buoyancy force within rectangular enclosures were investigated in this work. The fluid filled enclosures are heated and lid-driven either on the upper or on the lower horizontal wall, thermally isolated on the right vertical wall, and cooled on the other walls. The basis of the investigation was the numerical solutions of the equations for the conservation of mass, momentum, and energy transport using the finite difference method. The effects of the flow governing parameters including the Richardson and the Prandtl numbers, and the length-to-height aspect ratio, respectively, in the range 10⁻²  Ri  10², 10⁻³  Pr  10, and 1  AR  4 for a fixed Reynolds number, Re = 100, were studied. The results are presented in the form of the hydrodynamic and thermal fields, and the profiles for vertical and horizontal components of velocity, temperature, and the local heat flux. The fluid flow and energy distributions within the enclosures and heat flux on the heated wall are enhanced by the increase in the Richardson number. While an increase in the Prandtl number improves the heat flux on the heated wall, an increase in aspect ratio suppresses it. The results can be used as base line data in the design of systems in which mixed convection heat transfer in rectangular enclosures occurs.     

Energy analysis of silicon solar cell modules based on an optical model for arbitrary layers

An optical model for arbitrary layers is developed and a one-dimensional steady-state thermal model is applied to analyze the energy balance of silicon solar cell modules. Experimental measurements show that simulations are in good agreement, with a maximum relative error of 8.43%. The wind speed v_wind, ambient temperature T_amb and irradiance G are three main factors influencing the temperature of a photovoltaic panel. Over the course of a day the electrical output is reduced by the module temperature to only 32.5% of the rated value. Optical studies reveal that before 8:00 hours and after 16:00 hours, significant incident energy is lost by reflection because of the large angle of incidence θ_in, while at other times of day optical losses are nearly the same due to only small changes of transmission for θ_in < 45°. In addition, some optical losses result from the mismatched refractive indexes of encapsulating materials, especially at the ethylene-vinyl-acetate (EVA)/anti-reflection coating (ARC) and the ARC/Si interfaces. The uses of SiO₂ and TiO₂ as ARC materials for un-encapsulated and encapsulated Si solar cells are investigated by simulation. Comparing the results indicates that TiO₂ as ARC reduces the reflective optical loss within λ = 0.4–1.1 μm after encapsulation, while SiO₂ as ARC increases the loss by 5%. Energy allotment analysis shows that from 9:00 to 15:00, the reflective and transmissive optical losses are relatively steady at 26% and 13% of the incident energy, while the convective and radiative heat losses account for a further 30% and 24%, respectively. Thus, only 7% of incident energy is converted to electrical power.     

Natural convection in a vertically divided square enclosure by a solid partition into air and water regions

Numerical analyses of the flow and heat transfer due to buoyancy forces in a square enclosure divided by an impermeable partition between air and water filled chests were carried out using a finite difference technique. The enclosure was heated from left wall and cooled from right, isothermally. Horizontal walls were adiabatic. The partition divided the enclosure into air and water regions. Thus, two cases were examined: left side of partition was filled with air and right side was filled with water (Case I, air-partition-water) and left side was filled with water and right side with air (Case II, water-partition-air). Epoxy was chosen as partition material. Results were obtained for different Grashof numbers (10³  Gr  10⁶), thickness of the partition (0.05  ε  0.2) and location of the partition (0.25  c  0.75). An analytical treatment has been performed for low Grashof numbers. Numerical and analytical results gave an acceptable agreement. It was found that filling of fluid into chests is important for obtaining maximum heat transfer and energy saving. When left chest was filled with air (Case I), higher heat transfer was formed. It was an interesting result that heat transfer decreased with increasing of location of the partition for all values of partition thickness at Case I. On the contrary, heat transfer was a decreasing function of increasing value of location of the partition.     

Ruthenium oxide films for selective coatings

Although commercial selective surfaces are already available, investigation on different deposition methods and materials still goes on at many laboratories. In this work, ruthenium oxide films upon metallic substrates are assessed for this usage. Deposition of the films was made at room temperature by either spraying or dipping method in a ruthenium chloride alcoholic solution. After deposited on titanium substrates, the films were heat-treated at temperatures between 450 and 500 °C. When deposited on no-polished substrates, such films not only exhibit a high solar absorptance (α0.98), but also a high infrared emittance (ε0.8), which yield a low selectivity (S=α/ε=1.2). By deposition of similar films on polished substrates, absorptance decreases (α0.74), but emittance significantly decreases as well (ε0.12), resulting in a net selectivity increase (S6). On the other hand, evaporating a thin (20 nm) gold film upon the surface of a coating on a no-polished substrate also improves noticeably its emittance value (ε0.16) and a lower decrease in absorptance is achieved (α0.91), resulting in a selectivity increase (S5.7). These preliminary promising results indicate the high potential for using these films as solar selective coatings, but in order to optimize such selectivity values, further work to establish a close control on the deposition parameters and the substrate roughness value, should be done.     

Technical note Liquid chromatographic analysis of the main pungent principles of solar dried West Indian ginger (Zingiber officinale Roscoe)

The main pungent principles and essential oils of West Indian ginger (Zingiber officinale Roscoe) were identified and quantified by liquid chromatography. The stationary phase used was (5.0 g silica gel 70–230 mesh) and the mobile phase [petroleum ether (60–80°C) : diethyl ether (3 : 7 vv)]. The first 15 ml contained the very volatile and least polar compounds present in the extracted oleoresin from the solar dried ginger rhizome. These compounds are the essential oils (R_f = 0.90) and 25.63% (ww) of the total oleoresin charge to the column. The next 5 ml aliquot was without any compounds. The following 25 ml contained the shogaol fraction (R_f = 0.42) and 47.74% (ww) of the total oleoresin charge to the column. The next 5 ml aliquot was without any compounds. The following 35 ml contained the gingerol fraction (R_f = 0.20) and 27.13% (ww) of the total oleoresin charge to the column. This simple liquid chromatography method can be used to investigate the essential oils and pungent principles of the extracted oleoresin from the solar dried ginger rhizomes.     

Technical note Copper oxide coatings for use in a linear solar Fresnel reflecting concentrating collector

Electroplating selective copper oxide coatings were prepared on the surface of a rectangular absorber (α = 0.92 and ₁₀₀°C = 0.18) for operation in conjunction with a prototype linear solar Fresnel reflecting concentrating collector. Overall heat loss coefficients (U_L) of the selectively coated absorber at temperatures up to 300°C, optical efficiency (η_o) and stagnation temperature of the concentrator–absorber system were determined experimentally. The results have been compared with those obtained from an identical black painted absorber. Thermal efficiency (η_c) computed from the results of U_L and η_o measurements at a temperature of 250°C provides a value of 49% with the selectively coated absorber and 48% with the black painted absorber.     

Effect of the Cu underlayer on the optoelectrical properties of ITO/Cu thin films

Sn doped indium oxide (ITO) single layer films and ITO/Copper (Cu) bi-layer films were prepared on polycarbonate substrates by DC and RF magnetron sputtering without intentional substrate heating. In order to consider the influence of the Cu underlayer on the optoelectrical properties and microstructures of the films, the thickness of the Cu bottom layer in the ITO/Cu films was varied from 5 to 20 nm.Conventional ITO films had a constant optical transmittance of 74% and an electrical resistivity of 3.1 × 10⁻³ Ω cm, while ITO/Cu films had different optoelectrical properties that were influenced by the thickness of the Cu bottom layer. The lowest electrical resistivity, 5.7 × 10⁻⁵ Ω cm, was obtained from ITO 80 nm/Cu 20 nm films and the highest optical transmittance of 72%, was obtained from the ITO 95 nm/Cu 5 nm films. From the figure of merit (_TC) which is defined by _TC = T¹⁰/R_s, where T is the optical transmittance at 550 nm and R_s is the sheet resistance, it can be concluded that the most effective Cu thickness in the ITO/Cu films on the optoelectrical properties was 5 nm.     

Miniaturization limits for single-chamber micro-solid oxide fuel cells with coplanar electrodes

Single-chamber solid oxide fuel cells with coplanar microelectrodes were operated in methane–air mixtures (R_mix = 2) at 700 °C. The performance of cells with one pair of NiO–YSZ (yttria stabilized zirconia) anode and (La_0.8Sr_0.2)_0.98MnO₃–YSZ cathode, arranged parallel on a YSZ electrolyte substrate, was found to be significantly dependent on the electrode width. For an interelectrode gap of 250 μm, cells with average electrode widths exceeding 850 μm could establish a stable open circuit voltage (OCV) of 0.8 V, while those with widths less than 550 μm could not establish any OCV. In the intermediate range, the cells exhibited significant fluctuations in voltage and power under our testing conditions. This behavior suggests that a lower limit to electrode dimensions exists for cells with single electrode pairs, below which neither a stable difference in oxygen partial pressure, nor an OCV, can be established. Conversely, increasing the electrode width imposes a penalty in the form of an increase in the cell resistance. However, both size limits can be circumvented by employing multiple pairs of microscale electrodes in an interdigitated configuration.     

Effect of the grain boundaries on the conductivity and current transport in II–VI films

The paper presents an investigation of the effect of the grain boundaries on the conductivity and current transport in polycrystalline ZnTe, CdS and CdTe films. The temperature dependence of the conductivity of films in parallel σ and perpendicular σ directions to the substrate was studied. It is shown that σ slightly increases with increasing of the temperature for all temperature range considered. We have shown that σ linearly decreases with increasing of the reverse value of temperature 1/T, which indicates that the barrier height does not depend on temperature. We have found that diffusion theory of the conductivity for bicrystals can be applied only at low voltages not exceeding the net voltage on the grain boundaries. At higher voltages, theory of current transport for drift approximation can be used for large grain films. It is concluded that the influence of the intergrain energy barrier height on the electrical properties of II–VI films is negligible at room temperature.     

Structure, morphology and electrochemical behaviour of manganese oxides prepared by controlled decomposition of permanganate

Hydrothermal decomposition of permanganate, conducted in a range of pH-controlled solutions (from strongly acidic to strongly basic), is used to prepare manganese dioxides that are well-suited for use as supercapacitor electrode materials. While permanganate is thermodynamically unstable, the kinetics of its decomposition in an aqueous environment are very slow, until the temperature is raised to 200 °C. Although the resultant materials are relatively crystalline and have low total pore volume, their prominent meso-porosity leads to good electrochemical performance. Best behaviour is obtained for material from permanganate decomposition in 0.01 M H₂SO₄ solution, for which composite electrodes (150 μm thick) yield 150 F g⁻¹ at 5 mV s⁻¹ in a 9 M KOH electrolyte.     

Purification and characterization of [Fe]-hydrogenase from high yielding hydrogen-producing strain, Enterobacter cloacae IIT-BT08 (MTCC 5373)

Fe-hydrogenase from Enterobacter cloacae IIT-BT08 was purified 1284 fold with specific activity of 335 μmol H₂/min/mg protein for hydrogen evolution using reduced methyl viologen as an electron-donor at 25 °C. The molecular weight of the monomeric enzyme was determined to be 51 kDa by MALDI-ToF mass spectrometry. The PI of the enzyme was 5.6 displaying its acidic nature. The optimal temperature and pH for hydrogen evolution was 37 °C and 7–7.2 respectively. The affinity constant, K_m for reduced methyl viologen was 0.57 ± 0.03 mM and that of reduced ferredoxin was 0.72 ± 0.04 μM. The enzyme contained 11.47 gm-atom Fe/mol of Fe-hydrogenase. Electron paramagnetic resonance analysis ascertained the existence of iron molecules as [4Fe–4S] clusters. The internal amino acid sequences of trypsin digested peptides of hydrogenase as determined by ESI MS/MS Q-ToF showed 80-87% identities with the respective sequences of Clostridium sp. and Trichomonas sp. hydrogenase.