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.     

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.     

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.     

Lattice Boltzmann simulation of collision dynamics of two unequal-size droplets

The lattice Boltzmann method for two-phase fluid flows with large density ratios is applied to the simulations of the collision dynamics of two unequal-size droplets with the diameter ratios of λ=0.5 and 0.25 for various Weber numbers of 30<We<140 and impact parameters of 0B0.75 at Reynolds numbers of 3900Re4900. The density ratio of the liquid to the gas is fixed at 50. Coalescence collision and two different types of separating collisions, namely reflexive and stretching separations, are simulated. The boundaries between the coalescence collision and both of the separating collisions are found and compared with available theoretical predictions. The mixing processes during separating collisions with λ=0.5 for various Weber numbers are also simulated by tracing colored particles embedded in the droplets, and the relation between the mixing rate and the Weber number is obtained.     

Heat and fluid flow resulting from the chimney effect in a symmetrically heated vertical channel with adiabatic extensions

An experimental study on a channel-chimney system was carried out in order to elucidate the behavior of heat transfer and fluid flow. The results are presented in terms of local air temperature measurements inside the symmetrically heated channel and between the adiabatic extensions. Different fluid motion regions are observed inside the chimney. Inflows of air are detected in the lower extension ratio, particularly for large values of the ratio of the width of chimney to that of the heated channel. Some typical configurations show the presence of a vortex structure for an expansion ratio greater than one close to the corner regions in the chimney. Some monomial correlation equations between the local Nusselt number, the channel Rayleigh number and the geometric parameters are proposed. The dimensionless parameters are in the following ranges: 10²Ra^*(B/b)10⁶; 1.5L/L_h4.0; 1.0B/b4.0, in which L is the total height of the system, L_h is the height of the heated channel, B is the width of the chimney and b is the width of the heated channel. A good agreement between the correlation and the experimental data is observed.     

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.     

Predictions of turbulent flow and heat transfer in gas–droplets flow downstream of a sudden pipe expansion

Droplets-laden turbulent flow downstream of a sudden pipe expansion has been investigated by using Euler/Euler two-fluid model for the gaseous and dispersed phases. Significant increase of heat transfer in separated flow at the adding of evaporating droplets has been demonstrated (more than 2 times compare with one-phase flow at the value of mass concentration of droplets M_L1  0.05). Addition of dispersed phase to the turbulent gas flow results in insignificant increase of the reattachment length. Low-inertia droplets (d₁  50 μm) are well entrained into the circulation flow and present over the whole pipe section. Large particles (d₁ ≈ 100 μm) go through the shear layer not getting into the detached area. Comparison with experimental data on separated gas–droplets flows behind the plane backward-facing step has been carried out.     

Properties of brush plated CdxZn1−xTe thin films

Cd_xZn_1−xTe (0  x  0.5) thin films were deposited for the first time by the brush plating technique using cadmium sulphate, zinc sulphate and tellurium dioxide precursors. The deposition current density was maintained at 100 mA cm⁻². X-ray diffraction studies indicated the formation of cubic phase with (1 1 1), (2 2 0), (3 1 1) orientations. From optical absorption measurements the band gaps of the films are found to be direct. AFM studies indicate a surface roughness around 54 Å. Density of the films of different composition has been estimated. Laser Raman studies indicated CdTe like LO and TO phonons.     

Computation of high Prandtl number turbulent thermal fields by the analytical wall-function

The extended version of the analytical wall-function (AWF) for rough wall turbulence by Suga et al. [K. Suga, T.J. Craft, H. Iacovides, An analytical wall-function for turbulent flows and heat transfer over rough walls. Int. J. Heat Fluid Flow 27 (2006) 852–866] is improved for high Prandtl number flows. The original AWF assumes a linear profile of turbulent viscosity near a wall though it is widely recognised that a theoretically correct cubic profile of the turbulent viscosity is essential for heat transfer of high Prandtl number flows. In order to predict thermal boundary layer of high Prandtl number fluid flows, the present approach thus employs a correct limiting profile of the turbulent viscosity in the analytical integration process. The presently proposed version of the AWF proves its good performance for predicting turbulent high Prandtl number thermal flows at Pr  4 × 10⁴ for smooth wall cases, and at least at Pr  10 for rough wall cases.     

Solid-state dye-sensitized photocell based on pentacene as a hole collector

A solid-state dye-sensitized photovoltaic cell consisting of vacuum deposited pentacene onto ruthenium dye-coated TiO₂ electrode doped with iodine was fabricated. Cell delivers a short-circuit current of 3.6 mA cm⁻² and an open-circuit voltage of 415 mV at 100 mW cm^–2 (1.5 air mass). The efficiency and the fill factors of the above cell are 0.8% and 0.5%, respectively. Studies of the photocurrent action spectra showed that the dye is mainly responsible for this photocurrent generation. Preliminary results under extended illumination suggested that “long term” stability of the cell is promising.     

The thermochromic properties of La1−xSrxMnO3 compounds

La_1−xSr_xMnO₃ (LSMO) compounds (0.175x0.30) were prepared by conventional solid-state reaction method. Temperature dependence of the total hemispherical emittance (ε_H) of the compounds from 173 to 373 K was measured on a calorimetric emissometer (CE) which was constructed based on the steady-state calorimetric method. The compounds show thermochromic properties and ε_H's have low value at low temperature and have high value at high temperature, because the compounds are dominated by metallic phase and insulator phase, respectively. We use the phase separation model to interpret the temperature dependence of ε_H.     

Electrical and optical properties of highly conducting CdO:F thin film deposited by sol–gel dip coating technique

Highly conducting fluorine-doped cadmium oxide (CdO:F) thin films were deposited by sol–gel dip coating technique on glass and Si substrates. F concentration in the films was varied from 2.0% to 13.8% as determined from energy dispersive X-ray analysis. X-ray diffraction pattern showed that the films were polycrystalline in nature. The optimum F concentration for obtaining maximum conductivity was found to be 9.7%. The corresponding electrical conductivity was found to be 1.088×10⁴ S/cm and mobility 60.41 V/cm². Analysis of UV-VIS-NIR spectrum of the film with F concentration 9.7% showed a direct band gap energy of 2.3 eV.     

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.     

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.     

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.     

Dye-sensitized titania aerogels as photovoltaic electrodes for electrochemical solar cells

We describe the fabrication and performance of dye-sensitized photoanodes derived from TiO₂ aerogel. Nanocrystalline titania aerogel is a bicontinuous, nanostructured pore–solid architecture featuring specific surface areas of 85–150 m²/g and a continuous mesoporous network, allowing chemisorption of high concentrations of sensitizing dye and rapid mass-transport of electron-transfer mediators. Considerable design and processing flexibility arises with aerogels because the continuous pore–solid networks are fixed by the supercritical drying process, allowing the creation of multifunctional, nanostructured films of single or multiple layers. Titania aerogels can be processed as powders and deposited as nearly opaque films from 2 μm to >35-μm thick while retaining their bicontinuous nanoscale networks. Two-layer, 30-μm-thick TiO₂ aerogel films yield incident photon-to-electron conversion efficiency (IPCE) values of 85% in the 500–600 nm range and 52% at 700 nm with N719 as a sensitizing dye and after correcting for transmittance of the 3.2-mm-thick FTO-coated glass substrates at these wavelengths.     

XPS study of amorphous carbon nitride (a-C:N) thin films deposited by reactive RF sputtering

Amorphous carbon nitride (a-C:N) thin films were deposited by reactive radiofrequency (RF) sputtering. The a-C:N films were deposited, at room temperature, onto silicon substrates, from a graphite target of very high purity, in an atmosphere of pure nitrogen. The chemical properties of these films were studied by X-ray photoelectron spectroscopy (XPS). The XPS spectra of the a-C:N films reveal that nitrogen is well incorporated in the amorphous carbon network. The atomic percentage of nitrogen in the a-C:N films, calculated from the XPS spectrum, is about 32%. In addition to C–C and CC bonds, the analysis of the chemical shifts of C 1 s and N 1 s core level peaks show that nitrogen is bonded to carbon in CN double bonding and CN triple bonding configurations. The content of the CN triple bonds is found to be more important than the CN double bonds.     

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).     

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.     

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.