Structural, thermal and electrochemical properties of layered perovskite SmBaCo2O5+d, a potential cathode material for intermediate-temperature solid oxide fuel cells

The synthesis, conductivity properties, area specific resistance (ASR) and thermal expansion behaviour of the layered perovskite SmBaCo₂O_5+d (SBCO) are investigated for use as a cathode material for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The SBCO is prepared and shows the expected orthorhombic pattern. The electrical conductivity of SBCO exhibits a metal–insulator transition at about 200 °C. The maximum conductivity is 570 S cm⁻¹ at 200 °C and its value is higher than 170 S cm⁻¹ over the whole temperature range investigated. Under variable oxygen partial pressure SBCO is found to be a p-type conductor. The ASR of a composite cathode (50 wt% SBCO and 50 wt% Ce_0.9Gd_0.1O_2−d, SBCO:50) on a Ce_0.9Gd_0.1O_2−d (CGO91) electrolyte is 0.05 Ω cm² at 700 °C. An abrupt increase in thermal expansion is observed in the vicinity of 320 °C and is ascribed to the generation of oxygen vacancies. The coefficients of thermal expansion (CTE) of SBCO is 19.7 and 20.0 × 10⁻⁶ K⁻¹ at 600 and 700 °C, respectively. By contrast, CTE values for SBCO:50 are 12.3, 12.5 and 12.7 × 10⁻⁶ K⁻¹ at 500, 600 and 700 °C, that is, very similar to the value of the CGO91 electrolyte.     

Conductive electrique de sels doubles du type iodure d'argent-diiodures de piperazinium et de N, N′-alklylpiperazinium

The electrical conductivity has been studied for four binary systems of the type silver iodide-piperazinium or N, N′-disubstituted piperazinium diiodides. At 25 °C, the conductivity shows a maximum of 0.1 (ohm cm)⁻¹ for the system silver iodide-N, N′ tetramethylpiperazinium diiodide at 93.25% mol silver iodide. The ionic nature of the conductivity has been proven.     

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.     

Heat losses in Grätzel solar cells

The absorption and reflection spectra of a Grätzel-type solar cell were evaluated in terms of heat losses. Using these experimental data, the temperature distribution inside the solar cell was calculated. The temperature-controlling process in this cell is the heat transfer from the cell to the environment. Assuming the environment temperature of 25°C, the natural air convection leads to the temperature of 46.9°C inside the TiO₂ layer. Forced air convection causes the decrease, up to 30.1°C at the wind speed of 10 m s⁻¹. Variations in the cell dimensions, except the active layer length, have only a small effect on the temperature field, compared to the variations in the heat transfer coefficients.     

Simulation and model validation of a horizontal shallow basin solar concentrator

Salt removal from drainage water is becoming increasingly important for sustainable irrigated arid land agriculture, where inadequate drainage infrastructure exists. Solar evaporation and concentration systems are currently in development in California for this purpose. The thermal behavior and evaporation rates of a horizontal shallow basin solar concentrator were modeled for design purposes and investigated experimentally in order to validate the model. Three different evaporation rate models were evaluated and compared. Measured and predicted peak brine temperatures differed by as much as 5 °C when using prescribed literature coefficients without calibration. Model prediction was improved by calibration so that peak brine temperature deviated less than 3 °C when tested against independent data sets.Minimum root mean square error was used to calibrate the mass transfer coefficient and absorptance of the collector surface for solar radiation, which are the main factors affecting the heat transfer associated with the solar concentrator. Calibrated collector surface absorptance for solar radiation declined while mass transfer coefficients were increased from reported literature values. Under calibration, the absorptance of the collector surface was adjusted from 0.8 to 0.61, and mass transfer coefficients estimated by Newell et al. [Newell, T.A., Smith, M.K., Cowie, R.G., Upper, J.M., Cler, C.L., 1994. Characteristics of a solar pond brine reconcentration system. Journal of Solar Energy Engineering 116 (2), 69–73] from 1.36 × 10⁻⁶(1.9 + 1.065V) to 1.70 × 10⁻⁶(1.84 + 1.0V) kg m⁻² s⁻¹ mm Hg⁻¹, by Manganaro and Schwartz [Manganaro, J.L., Schwartz, J.C., 1985. Simulation of an evaporative solar salt pond. Industrial & Engineering Chemistry Process Design and Development 24, 1245–1251] from 0.0208(1 + 0.224V) to 0.0233(1 + 0.214V) kg m⁻² h⁻¹ mm Hg⁻¹, and by Alagao et al. [Alagao, F.B., Akbarzadeh, A., Johnson, P.W., 1994. The design, construction, and initial operation of a closed-cycle, salt-gradient solar pond. Solar Energy 53 (4), 343–351] from 2.8 + 3.0V to 3.0 + 3.33V W m⁻² °C⁻¹. The calibrated models were tested using an independent data set. Maximum deviation between measured and predicted brine temperatures differed by less than 3 °C. The measured and predicted peak evaporation rates were between 1.2 and 1.4 kg m⁻² h⁻¹.The calibrated Newell model was used to predict the monthly productivity and daily maximum evaporation rates at Five Points, California for the year 2004. The productivity from April to September and from March to October was 80.7% and 94.3% of the total annual productivity, respectively.     

Determination of glazing area in direct gain systems for three different climatic zones

This study determines the glazing area in direct gain passive systems needed to ensure thermal comfort inside a building (room air temperature 20 ± 2°C). A 4 m × 4 m × 3 m single zone isolated house is analyzed in three different types of climates namely composite (8°C to 20°C, New Delhi), cold-cloudy (−2°C to 5°C, Srinagar), and cold-sunny (−14°C to −3°C, Leh). The analysis is based on the periodic solution of the heat conduction equations describing heat transmission in the building components, floor, walls, and roof, and the Fourier representation of the ambient temperature vnd the total solar radiation intercepted by the building envelope. Two types of construction are analyzed: the first type is a traditional construction with 22-cm-thick brick wall, plastered 15 mm on both the sides (U = 2.0 W m⁻² K⁻¹); and the second one is of the same type but with 10 cm of expanded polystyrene insulation on all the four walls and the roof (U = 0.31 W m⁻² K⁻¹). It is found that for traditional construction with U = 2.0 W m⁻² K⁻¹, the glazing U value has almost no effect on the room temperature even for large variation of the glazing area (10% to 40%, expressed in terms of percentage of floor area). For a well-insulated house (U = 0.31 W m⁻² K⁻¹), the glazing U value has no effect upon the room air temperature if the glazing area is small (less than 10%). The position of the insulation on the external surfaces is more effective in reducing large inroom air temperature. Finally, for an insulated house, we recommended glazing is 30%, 20%, and 10% for cold-sunny, cold-cloudy, and composite climates, respectively.     

Optical, structural and photoelectron spectroscopic studies on amorphous and crystalline molybdenum oxide thin films

Optical, structural and electronic properties of amorphous and crystalline molybdenum oxide thin films have been investigated. As-deposited amorphous films got crystallized into a layered orthorhombic phase on annealing at 350°C. Refractive index (n) and extinction coefficient (k) of as-deposited films and films annealed at 150°C, 240°C and 350°C have been calculated using reflectance and transmittance data. Spectral dependence of absorption coefficient has been explained on the basis of charge transfer transition mechanism. Optical band gap of amorphous MoO_3−x is 3.16 eV and it has increased by 0.11 eV on crystallization. XPS core level analysis reveals the presence of Mo⁺⁴, Mo⁺⁵ and Mo⁺⁵ oxidation states in amorphous films, proving oxygen deficiency in as-deposited films. Same studies on crystalline films show the presence of only Mo⁺⁶ states. Valence band spectrum of amorphous films reveal emission from Mo4d levels, which is absent in crystalline films. Complete correlation is seen between the optical properties and XPS data.     

Effects of incidence angle on endwall convective transport within a high-turning turbine rotor passage

Effects of incidence angle on the endwall convective transport within a high-turning turbine rotor passage have been investigated. Surface flow visualizations and heat/mass transfer measurements at off-design conditions are carried out at a fixed inlet Reynolds number of 2.78 × 10⁵ for the incidence angles of −10°, −5°, 0, 5°, and 10°. The result shows that the incidence angle has considerable influences on the endwall local transport phenomena and on the behaviors of various endwall vortices. In the negative incidence case, convective transport is less influenced by the leading edge horseshoe vortex and by the suction-side corner vortex along their loci but is increased along the pressure-side corner vortex. In the case of positive incidence, however, convective transport is augmented remarkably along the leading edge horseshoe vortex, and is much influenced by the suction-side corner vortex. Moreover, heat/mass transfer is enhanced significantly along the pressure-side leading edge corner vortex. Local endwall convective transport in the area other than the endwall vortex sites is influenced significantly by the cascade inlet-to-exit velocity ratio which depends strongly on the incidence angle.     

Numerical analysis of natural convection in an inclined trapezoidal enclosure filled with a porous medium

A theoretical study of buoyancy-driven flow and heat transfer in an inclined trapezoidal enclosure filled with a fluid-saturated porous medium heated and cooled from inclined walls has been performed in this paper. The governing non-dimensional equations were solved numerically using a finite-difference method. The effective governing parameters are: the orientation or inclination angle of the trapezoidal enclosure , which varies between 0° and 180°, the Rayleigh number Ra, which varies between 100 and 1000, the side wall inclination angle θ_s and the aspect ratio A. The side wall inclination parameter θ_s is chosen as 67°, 72° and 81° and the calculations are tested for two different values of A=0.5 and 1.0. Streamlines, isotherms, Nusselt number and flow strength are presented for these values of the governing parameters. The obtained results show that inclination angle is more influential on heat transfer and flow strength than that of the side wall inclination angle θ_s. It is also found that a Bénard regime occurs around =90°, which depends on the inclination of the side wall, Rayleigh number and aspect ratio.     

Structural and electrical properties of CuGaS2 thin films by electron beam evaporation

Single phase CuGaS₂ thin film with a highest diffraction peak of (1 1 2) at a diffraction angle (2θ) of 28.8° was made at a substrate temperature of 70°C, an annealing temperature of 350°C and an annealing time of 60 min. Second highest (2 0 4) peak was shown at diffraction angle of (2θ) 49.1°. Lattice constant of a and c of that CuGaS₂ thin film was 5.37 and 10.54 Å, respectively. The greatest grain size of the thin film was about 1 μm. The (1 1 2) peak of single phase of CuGaS₂ thin film at an annealing temperature of 350°C with excess S supply appeared at a little higher about 10% than that of no excess S supply. The resistivity, mobility and hole density at room temperature of p-type CuGaS₂ thin film was 1.4 Ω cm, 15 cm²/V s and 2.9×10¹⁷ cm⁻³, respectively. It was known that carrier concentration had considerable effect than mobility on a variety of resistivity of the fabricated CuGaS₂ thin film, and the polycrystalline CuGaS₂ thin films were made at these conditions were all p-type.     

Phosphorus-doped, silver-based pastes for self-doping ohmic contacts for crystalline silicon solar cells

Undoped and phosphorus-doped Ag-based pastes were applied as circular contacts to the (1 1 1) surface of dendritic web n-type Si. Current–voltage characteristics of as-deposited contacts and contacts annealed at 780°C for 10 min, 950°C for 5 min, 1000°C for 10 min were measured and compared. Annealing above the Ag–Si eutectic temperature (835°C) yielded Si precipitation within the Ag matrix, resulting in increased current across the metal/semiconductor interface. The contact resistivity was significantly lower for P-doped (<0.04 Ω cm²) than for undoped (1.90 Ω cm²) Ag contacts, both of which were annealed at 1000°C. As supported by secondary ion mass spectrometry analyses, these results are attributed to an enhanced P doping level in the Si substrate after annealing the P-doped contacts. A p–n junction diode was demonstrated by alloying the Ag–P paste with p-type Si at 1000°C. The contact resistance was inferred from diode I–V data to be 0.013 Ω cm², a value which is comparable to the 0.010 Ω cm² target value for solar cell contacts.     

Transient thermo-solutal convection of water near 4 °C with opposing gradients in a square cavity

A numerical study of two-dimensional thermo-solutal convection of water in a square cavity heated from below and salted from above for various value of Lewis number is conducted. The maximum density associated with water around 4 °C occurs inside the cavity, as the top wall is maintained at 0 °C while the bottom wall temperature varies in the range 8–12 °C. The maximum density region acts as an obstacle to prohibit convectional heat, mass, and momentum transfer. These effects are investigated numerically in the domain −5 × 10² < Ra_T < 2 × 10⁴, 1 × 10⁵ < Ra_S < 8 × 10⁶ and L = 0.015 m length of square cavity where Ra is the Rayleigh number of the fluid. The effect of Lewis number on the heat, mass, and momentum transfer is also systematically studied. For certain range of parameters, it is interesting to find that the flow pattern may change inversely from rolling (fluid particles raise along both vertical side walls and fall along the vertical center line) to plume motion (fluid particles raise along the vertical center line and fall along both side walls) as the bottom wall temperature and top wall concentration increase. Further increase in the value of Rayleigh number results in oscillating two cell flow structure in the cavity. It is found that there is a temporal maximum absolute value of average Nusselt and Sherwood number followed by a temporal minimum absolute value of average Nusselt and Sherwood number in a small time interval (0 < t < 300 s) and the steady state is reached after a certain time interval at the bottom wall. These time intervals are reduced with increasing Lewis number. Also, critical Grashof number which accounts for oscillatory heat and mass transfer with Lewis number is studied and it reveals that an increase in Lewis number results in slowing down oscillation and oscillation cycle becomes shorter with increasing species Grashof number.     

Partitioning of rare earths and some major elements in the Kizildere geothermal field, Turkey

Rare earth elements and yttrium (REY), Na⁺, K⁺, Cl⁻, and Ca²⁺ were determined in water, steam, mineral scale and rock samples from the Kizildere geothermal field, Turkey. The CO₂-rich parent fluid originates from a sequence of mica schists with marble intercalations. The chemistry of the parent fluid varies with location and time. The average REY composition of the fluids is derived by extrapolation to the lowest Ca concentrations. The apparent vapor–liquid partitioning factors for REY at 145 °C and 5 bar total absolute pressure are about 0.2, whereas for Ca²⁺, Na⁺, K⁺ and Cl⁻ they are <0.05, about 0.0005, 0.0005 and about 0.02, respectively. Apparent scale-liquid distribution coefficients for REY at 145 and 190 °C are about 0.15 and 0.55, whereas at 100 °C they increase from 0.3 (La) to 1.5 (Lu).     

Thermal efficiency of solid electrolyte fuel cells with mixed conduction

The effect of mixed anionic and n-type electronic conduction in solid electrolytes on the thermal efficiency of a fuel cell system was analyzed quantitatively. The mixed conduction observed when electrolytes based on ceria are used in H₂/air fuel cell applications lowers the maximum attainable cell thermal efficiency to below 40%. Neither the zirconia nor the ceria based solid oxide electrolytes studied to date can be used in a low temperature (700 °C) system that meets simultaneously the requirements of power density and thermal efficiency for electric utility power plants. The material properties required for an advanced fuel cell power plant solid electrolyte were derived in terms of the ionic conductivity and the Schmalzried parameters P and P : σ_ion > 0.033 (Ω-cm)⁻¹, P > 10³ atm., P < 10⁻²³ atm. at 700 °C.     

Natural Convection Heat Transfer From a Hot Rectangular and a Square Corrugated Plate to a Cold Flat Plate

IntroductionReduction of heat loss from the absorber plate of asolar collector through the cover plates improvescollector efficiency. Therefore, the natllral convectionheat loss across air layers bounded by tWo parallel platesis of special interest to the designers of solar collectors.Most of the investigations on heat transfer in aconfined space have been cAned out with parallel platesin horizontal and inclined positions. Hollands, et al.[l]experimentally investigated the heat trallsferchara…     

Kinetics of temperature-dependent photoinduced redox reactions in a photoelectrochemical cell

The effect of temperature on the electrode kinetics of photovoltage generation in photoelectrochemical (PEC) cells consisting of a phenazine dye-EDTA system, separated from an aqueous solution of an electron acceptor like iodine by a salt bridge has been studied. The phenazine dyes used are phenosafranin, safranin-O, and safranin-T. The maximum photovoltages (V_oc) generated and the sunlight engineering efficiency (SEE) have been found to increase with increasing temperature, but there is a fixed critical temperature for each dye above which the V_oc decreases: 29°C for phenosafranin, 35°C for safranin-T, and 40°C for safranin-O. The photovoltage growth and decay follow the functional forms related to the relaxation times. The rate constants for the forward and backward reactions have been calculated from these relaxation times at different temperatures. The rate of the photoinduced chemical reaction increases with an increase in temperature from 20°C–50°C for all the dyes, with concomitant decrease for the backward reaction. The free energies of electron transfer across the electrode/electrolyte interface have been calculated. The activation energies calculated from the rate constants at different temperatures for phenosafranin-EDTA, safranin-T-EDTA, and safranin-O-EDTA reactions are 5.14, 5.60, and 5.63 kJ mol⁻¹ respectively.     

Heat transfer to a horizontal tube bundle located in the freeboard of a bubbling fluidized bed combustor

The characteristics of heat transfer from bubbling gas-fired fluidized bed to a horizontal staggered water-tube bundle located in the freeboard region is experimentally investigated. The purpose is to demonstrate the effect of bed temperature on the coefficients of heat transfer by the different modes to each of the four rows of the bundle, which experiences heat transfer by convection from flue gases, luminous radiation from bed material and non-luminous radiation from gases. The bed temperature itself is varied and controlled through the fuel–air mass ratio. Sixteen runs have been conducted with bed temperature ranging from 1114 to 1429 K, resulting in an overall heat transfer coefficient in the range 74·0–105·0 W m⁻² K⁻¹ for the first row and 58·0–65·0 W m⁻² K⁻¹ for the last. An overall convective heat transfer coefficient from gases, and possible carried over sand particles, to the bundle is formulated. © 1997 by John Wiley & Sons, Ltd.     

The effect of surface blocking on mass transfer from a stagnant cap drop

Numerical results for single-drop exterior mass transport of a solute from a surfactant covered drop to the continuous phase are presented. In particular the effect of physicochemical surface blocking is determined by considering the case in which the adsorbed surfactant accumulates at the rear of the translating drop. The stagnant cap velocity profiles are used to describe convective transport. Surface blocking is incorporated through the choice of a zero flux boundary condition on those portions of the drop where surfactant is present. Finite element numerical results for the Sherwood numbers as a function of Peclet number (Pe 10⁴) and stagnant cap angle, φ, show that for surface coverages greater than 0.1π, the effect of surface blocking cannot be ignored. For a Peclet number equal to 10⁴ and φ = 0.5π, the mass transfer coefficients calculated under the assumption that the presence of surfactant reduced convection in the vicinity of the drop without inhibiting the interfacial transport of solute, are found to overestimate the rate of solute mass transfer by as much as 20%.     

Thermal loss coefficients of low-density silica aerogel tiles

The thermal losses of a monolithic low-density SiO₂ aerogel tile (thickness d = 9 mm, density = 80 kg/m³) have been determined in our evacuable guarded hot plate system LOLA II. The measurements were performed for two values of boundary emissivities ( ≈ 0.05 and 0.9) under variation of temperature and of gas pressure for several types of gases. From our results we conclude that low-density silica aerogel tiles about 10 mm thickness provide excellent thermal insulation with overall loss coefficients k 0.7 W/(m² · K) at ambient temperatures (10°C or 283 K) and pressures 10 mbar. Their use in window systems and covers for passive use of solar energy thus is extremely promising. From the change of thermal losses with gas pressure, a mean free path within the (evacuated) skeleton of about 120 nm can be derived.     

Conjugate natural convection heat transfer in an inclined square cavity containing a conducting block

The present work is concerned with computation of natural convection flow in a square enclosure with a centered internal conducting square block both of which are given an inclination angle. Finite volume method through the concepts of staggered grid and SIMPLE algorithm have been applied. Deferred QUICK scheme has been used to discretize the convective fluxes and central difference for diffusive fluxes. The problem of conjugate natural convection has been taken up for validating the code. The abrupt variation in the properties at the solid/fluid interface are taken care of with the harmonic mean formulation. Solution has been performed in the computational domain as a whole with proper treatment at the solid/fluid interface. Computations have been performed for Ra = 10³–10⁶, angle of inclination varying from 15° to 90° in steps of 15° and ratio of solid to fluid thermal conductivities of 0.2 and 5.0. Results are presented in terms of streamlines, isotherms, local and average Nusselt number.