Mixed nonstationary problem of heat conduction for a microperiodic two-layered half-space

The paper deals with a mixed nonstationary problem of heat conduction for a microperiodic two-layered half-space. The body is assumed to be initially at zero temperature, the boundary plane is kept at a known temperature for time 0 ≤ t ≤ t₀ and is insulated for t > t₀. The half-space is composed of periodically repeated two-layered laminae with layering parallel to the boundary. The problem is solved within the framework of homogenized model with microlocal parameters [Cz. Woźniak, A nonstandard method of modelling of thermoelastic periodic composites, International Journal of Engineering Science 25, (1987), pp. 483–499, S.J. Matysiak Cz. Woźniak, On the modelling of heat conduction problem in laminated bodies, Acta Mechanica, 65, (1986), pp. 223–238.]. The influence of thermal and geometric properties of the composite components on the temperature distribution for some special case of the boundary condition is analyzed.     

Thermochemical hydrogen production from a two-step solar-driven water-splitting cycle based on cerium oxides

A new thermochemical cycle for H₂ production based on CeO₂/Ce₂O₃ oxides has been successfully demonstrated. It consists of two chemical steps: (1) reduction, 2CeO₂ → Ce₂O₃ + 0.5O₂; (2) hydrolysis, Ce₂O₃ + H₂O → 2CeO₂ + H₂. The thermal reduction of Ce(IV) to Ce(III) (endothermic step) is performed in a solar reactor featuring a controlled inert atmosphere. The feasibility of this first step has been demonstrated and the operating conditions have been defined (T = 2000 °C, P = 100–200 mbar). The hydrogen generation step (water-splitting with Ce(III) oxide) is studied in a fixed bed reactor and the reaction is complete with a fast kinetic in the studied temperature range 400–600 °C. The recovered Ce(IV) oxide is then recycled in first step. In this process, water is the only material input and heat is the only energy input. The only outputs are hydrogen and oxygen, and these two gases are obtained in different steps avoiding a high temperature energy consuming gas-phase separation. Furthermore, pure hydrogen is produced (it is not contaminated by carbon products like CO, CO₂), thus it can be used directly in fuel cells. The results have shown that the cerium oxide two-step thermochemical cycle is a promising process for hydrogen production.     

Effect of temperature dependence of electrical resistivity on the cooling performance of a single thermoelectric element

The coefficients of performance (COP) φ₀ and φ for a single thermoelectric (TE) element welded with two metal plates were calculated as functions of temperature difference (ΔT) and thermoelectric figure of merit (ZT) from the conventional thermal rate equations and the new thermal rate ones proposed here, respectively. We made an attempt to take the differences in the Seebeck coefficient , electrical resistivity ρ and thermal conductivity κ of TE materials at the hot and cold sides of a TE element into the thermal rate equations on the assumption that their TE properties change linearly with temperature. However, the difference in κ was neglected even in the new thermal rate equations because its temperature dependence was too small when φ was applied to the high-performance Bi–Te alloys. The normalized temperature dependences at 300 K of and ρ were denoted by A and B, respectively. The term of A in the thermal rate equations was canceled out by the Thomson coefficient, but that of B remained. When B > 0 K⁻¹, φ/φ₀ is enhanced more significantly with an increase of B at larger ΔT and lower ZT, and it reached about 1.20 at ΔT = 80 K for Bi–Te alloys with B ≈ 5 × 10⁻³ K⁻¹. It was thus found that the COP of a cooling module is also affected strongly by B as well as ZT.     

Entropy generation due to conjugate natural convection in enclosures bounded by vertical solid walls with different thicknesses

Entropy generation due to conjugate natural convection heat transfer and fluid flow has been studied inside an enclosure with bounded by two solid massive walls from vertical sides at different thicknesses. Enclosure is differentially heated from vertical walls and horizontal walls are adiabatic. Governing equations which are written in streamfunction-vorticity form solved by finite difference technique for the governing parameters as Rayleigh number, 10³ ≤ Ra ≤ 10⁶, length ratio of solid walls as ₁ (for left vertical wall) and ₂ (for right vertical wall) and thermal conductivity ratio of solid to fluid (k), 1 ≤ k ≤ 10. Entropy generation contours due to fluid friction and heat transfer irreversibility, isotherms, streamlines, Nusselt numbers and velocity profiles were obtained. It is found that entropy generation increases with increasing of thermal conductivity ratio and thicknesses of the walls. Entropy generation due to heat transfer is more significant than that of fluid flow irreversibility for all values of thickness of the solid vertical walls.     

Importance of the band gap energy and flat band potential for application of modified TiO2 photoanodes in water photolysis

The influence on water photolysis of two important parameters of the electronic structure of photocatalytic semiconductors: the forbidden band gap, E_g, that decides about the absorption spectrum and the flat band potential, V_Fb, that affects the recombination probability, was studied. The photoelectrochemical experiments were performed in a three-electrode cell PEC with a TiO₂ thin film photoanode immersed in liquid electrolyte of variable pH. Titanium dioxide photoanodes doped with chromium (up to 16 at.%) and tin (up to 50 at.%) were prepared by rf reactive sputtering. Different methods of flat band potential determination: Mott–Schottky plots and photocurrent versus voltage characteristics were used. The energy band gap was derived from the spectrophotometric measurements of optical transmittance and reflectance coefficients of thin films. For TiO₂ + 7.6 at.% Cr high and negative flat band potential V_Fb = −0.72 eV (at pH 4) has been found but the recombination time τ = 8 s was the shortest of all TiO₂ modifications. Despite additional absorption feature at about 2.8 eV, i.e., at wavelength corresponding to visible range of the light spectrum, the photoconversion efficiency of TiO₂ + 7.6 at.% Cr was found to be much smaller (η_c = 0.1%) than that of undoped TiO₂ (η_c = 1.8%) and TiO₂ doped with 8 at.% of Sn (η_c = 1.0%).     

Analysis of mixed convection in a vertical porous layer using non-equilibrium model

The problem of two-dimensional steady mixed convection in a vertical porous layer is investigated numerically in the present paper using the thermally non-equilibrium model. The vertical porous layer is assumed to have a finite isothermally heated segment on one vertical wall which is otherwise adiabatic and the other vertical wall is cooled to a constant temperature. Non-dimensionalization of the governing equations results in four parameters for both aiding and opposing flows: (1) Ra, Rayleigh number (2) Pe, Péclet number (3) K_r, thermal conductivity ratio parameter, and (4) H, heat transfer coefficient parameter. The numerical results are presented for 0.01  H  100, 0.01  K_r  100, 0.01  Pe  100 and Ra = 10, 50 and 100. The results show that, the thermal equilibrium model cannot predict the average Nusselt number correctly for small values of H × K_r. In both the aiding and opposing flows, the total average Nusselt number is decreasing with increasing the heat transfer coefficient parameter at low values of Pe, while for high values of Pe, higher H will enhance the total heat transfer rate. Increasing the thermal conductivity ratio leads to increase in the total average Nusselt number. It is found also that the total average Nusselt number depends strongly on the thermal conductivity ratio parameter and depends slightly on the heat transfer coefficient parameter.     

Triple vacuum glazing: Heat transfer and basic mechanical design constraints

Given the major role played by windows with regard to energy losses from buildings in cold climates, low thermal transmittance is an indispensable property of glazing in low-energy buildings. Evacuation offers the only means of achieving negligible gaseous conduction in glazing cavities. Application of low-emittance coatings to glass sheet surfaces inside the cavity reduces the radiative heat transfer. The feasibility of double vacuum glazing using arrays of support pillars between the glass sheets has been shown by other authors. This type of glazing is commercially manufactured today. Based on these achievements, our study set out to investigate heat transfer in triple vacuum glazing by means of (i) an analytical thermal network model and (ii) a numerical finite difference model. The study focused on the impact of the following parameters on thermal transmittance: emittances of glass sheet surfaces inside the cavity, support pillar radius, support pillar separation and thermal conductivity of support pillar material. The design procedure for triple vacuum glazing taking into account not only thermal but also mechanical stresses due to atmospheric pressure, i.e., to enable identification of favourable parameter sets, is presented. Our findings suggest that use of the triple vacuum glazing concept can significantly reduce the thermal transmittances achieved by the best insulation glazing units currently on the market. E.g., a centre-of-glazing thermal transmittance of less than 0.2 W m⁻² K⁻¹ is achievable using stainless steel support pillars, 6 mm/4 mm/6 mm sheets of untempered soda-lime glass and four low-emittance coatings (ε = 0.03).     

Effect of Co content on performance of LiAl1/3−xCoxNi1/3Mn1/3O2 compounds for lithium-ion batteries

Layered LiAl_1/3−xCo_xNi_1/3Mn_1/3O₂ (0  x  1/3) compounds were studied via the combination of computational and experimental approach. The calculated voltage curve of LiNi_1/3Al_1/3Mn_1/3O₂ compound is presented, indicating it is of great potential for a cathode material of lithium-ion batteries. Unfortunately, it was found that the LiNi_1/3Al_1/3Mn_1/3O₂ compound without impurity phase could not be synthesized via a sol–gel process. To obtain a layered compound without impurity phase, partial of Al is replaced by Co in LiNi_1/3Al_1/3Mn_1/3O₂ compound in this study. Layered LiAl_1/3−xCo_xNi_1/3Mn_1/3O₂ (0  x  1/3) compounds were synthesized via sol–gel reaction at 900 °C under a oxygen stream. Single phase of the LiAl_1/3−xCo_xNi_1/3Mn_1/3O₂ in 1/6  x  1/3 region could be prepared successfully. The discharge capacity and conductivity increased with an increase in the Co-substitution content. The enhancement of the conductivity and phase purity by the introduction of Co content shows profound influence on the performance of the LiAl_1/3−xCo_xNi_1/3Mn_1/3O₂ compounds.     

Turbulent natural convection in a porous square cavity computed with a macroscopic κ–ε model

Detailed numerical computations for laminar and turbulent natural convection within a square cavity filled with a fluid saturated porous medium are presented. Heated vertical walls are maintained at constant but different temperatures, while horizontal surfaces are kept insulated. The macroscopic κ–ε turbulence model with wall function is used to handle turbulent flows in porous media. In this work, the turbulence model is first switched off and the laminar branch of the solution is found when increasing the Rayleigh number, Ra_m. Computations covered the range 10 < Ra_m < 10⁶ and 10⁻⁷ < Da  < 10⁻¹⁰ and made use of the finite volume method. Subsequently, the turbulence model is included and calculations start at high Ra_m, merging to the laminar branch for a reducing Ra_m and for Ra_m less than a certain critical Rayleigh number, Ra_cr. This convergence of results as Ra_m decreases can be seen as a characterization of the laminarization phenomenon. For Ra_m values less than around 10⁴, both laminar and turbulent flow solutions merge, indicating that such critical value for Ra_m was reached. Results further indicate that when the parameters porosity, Pr, conductivity ratio between the fluid and the solid matrix and the Ra_m are kept fixed, the lower the Darcy number, the higher the average Nusselt number at the hot wall.     

Effect of electricity tariff on the optimum insulation-thickness in building walls as determined by a dynamic heat-transfer model

Thermal insulation is one of the most effective energy-conservation measures in buildings. Despite the widespread use of insulation materials in recent years, little is known regarding their optimum thickness under dynamic thermal conditions. Insulated concrete blocks are among the units most commonly used in the construction of building walls in Saudi Arabia. Typically, the insulation layer thickness is fixed at a value in the range 2.5–7.5 cm, regardless of the climatic conditions, type and cost of insulation material, and other economic parameters. In the present study, a numerical model based on a finite-volume, time-dependent implicit procedure, which has been previously validated, is used to compute the yearly cooling and heating transmission loads under steady periodic conditions through a typical building wall, for different insulation thicknesses. The transmission loads, calculated by using the climatic conditions of Riyadh for a west-facing wall, are fed into an economic model in order to determine the optimum thickness of insulation (L_opt). The latter corresponds to the minimum total cost, which includes the cost of insulation material and its installation plus the present value of energy consumption cost over the lifetime of the building. The optimum insulation thickness depends on the electricity tariff as well as the cost of insulation material, lifetime of the building, inflation and discount rates, and coefficient of performance of the air-conditioning equipment. In the present study, the effect of electricity tariff on the computed optimum insulation thickness is investigated. Different average electricity tariffs are considered; namely, 0.05, 0.1, 0.2, 0.3 and 0.4 SR/kWh (designated as Cases 1–5, respectively; 1 US$ = 3.75 Saudi Riyals). Results using moulded polystyrene as an insulating material show that the values of L_opt are: 4.8, 7.2, 10.9, 13.7 and 16.0 cm for Cases 1–5. Under the conditions of optimal insulation thickness for each electricity tariff, Case 1 gives the lowest total cost of 17.4 SR/m², while Case 5 gives the highest total cost of 53.1 SR/m². Corresponding thermal performance characteristics in terms of yearly total and peak transmission loads, R-value, time lag and decrement factor are presented.     

Sm0.5Sr0.5CoO3 + Sm0.2Ce0.8O1.9 composite cathode for cermet supported thin Sm0.2Ce0.8O1.9 electrolyte SOFC operating below 600 °C

The cathode is a key component in low temperature solid oxide fuel cells. In this study, composite cathode, 75 wt.% Sm_0.5Sr_0.5CoO₃ (SSC) + 25 wt.% Sm_0.2Ce_0.8O_1.9 (SDC), was applied on the cermet supported thin SDC electrolyte cell which was fabricated by tape casting, screen-printing, and co-firing. Single cells with the composite cathodes sintered at different temperatures were tested from 400 to 650 °C. The best cell performance, 0.75 W cm⁻² peak power operating at 600 °C, was obtained from the 1050 °C sintered cathode. The measured thin SDC electrolyte resistance R_s was 0.128 Ω cm² and total electrode polarization R_p(a + c) was only 0.102 Ω cm² at 600 °C.     

Cadmium telluride films prepared by pulsed electrodeposition

Cadmium telluride polycrystalline films were deposited on various transparent semiconductors on glass using periodic pulse electrolysis from an aqueous solution of Cd²⁺ and HTeO₂⁺ ions. Substrates included fluorine doped tin oxide/glass, tin oxide/indium tin oxide/glass and on those substrates with an electrodeposited cadmium sulphide film on the oxide. The properties of the deposited films were determined as a function of variables, viz. initial cathodic voltage V₁, on-time t₁, second cathodic voltage V₂, on-time t₂, solution concentration and type of substrate. Film quality was judged by adherence, continuity, optical quality, composition and morphology. The preferred deposition conditions (versus SCE) were V₁ = −0.76 V, t₁ = 1 s, V₂ = −0.60 V, t₂ = 0.1 s using a stirred 90°C solution with composition 2.5 M Cd²⁺, 160 ppm HTeO₂⁺ and pH of 1.7. Films deposited under those conditions were cubic polycrystals. X-ray diffraction spectra showed reflections from the (111) (220) and (311) planes with the most intense being the (111) reflection. As the concentration of species in the solution decreased, the reflection intensities from the (220) and (311) planes decreased relative to the (111) reflection. After annealing under conditions to type convert the n-CdTe to p-CdTe, the crystallinity improved and if a CdS layer was present, the (220) and (311) planes were further developed relative to the (111) plane. Resistivity through the film was (3.0±0.8)×10⁸) Ω cm but reduced to (1.0±0.3) × 10³ Ω cm after annealing. The band gap was 1.48±0.03 eV for both deposited and annealed films. There was a limited range over which the deposition variables could be altered. The pulse duration for the more cathodic phase needed to be longer than the less cathodic phase for adherent films. Better adherence was achieved when pulse durations were greater than 0.1 s, especially for the more cathodic phase. The magnitude of the pulse duration and potential in each phase of a particular cycle determined whether the deposited film was rich or deficient in cadmium and whether the film adhered.     

Charge and discharge characteristics of a commercial LiCoO2-based 18650 Li-ion battery

We studied the charge and discharge characteristics of commercial LiCoO₂-based 18650 cells by using various electrochemical methods, including discharging at constant power, ac impedance spectroscopy, and dc-voltage pulse. At 20 °C, these cells deliver 8.7–6.8 Wh of energy when discharged at a power range of 1–12 W between 2.5 and 4.2 V. Ragone plots show that the effect of discharge power on the energy is significantly increased with decreasing of the temperature. For example, energy of the cell is entirely lost when the temperature downs to −10 °C and the discharge rate still remains at 10 W. Impedance analyses indicate that the total cell resistance (R_cell) is mainly contributed by the bulk resistance (R_b, including electric contact resistance and electrolytic ionic conductivity), solid electrolyte interface resistance (R_sei), and charge-transfer resistance (R_ct). Individual contribution of these three resistances to the cell resistance is greatly varied with the temperature. Near room temperature, the R_b occupies up to half of the cell resistance, which means that the rate performance of the cell could be improved by modifying cell design such as employing electrolyte with higher ionic conductivity and enhancing electric contact of the active material particles. At low temperature, the R_ct, which is believed to reflect cell reaction kinetics, dominates the cell resistance. In addition, galvanosatic cycling tests indicate that the charge and discharge processes have nearly same kinetics. The performance discrepancy observed during charging and discharging, especially at low temperatures, can be attributed to these two factors of: (1) substantially higher R_ct at the discharged state than at the charged state; (2) asymmetric voltage limits pre-determined for the charge and discharge processes.     

Technical Note Transient behavior of vertical buoyancy layer in a stratified fluid

horizontal length scale of the vertical channelg acceleration of gravityRa Rayleigh number [ ≡ gβT_oD⁴ν]T temperaturew velocity component in the z-directionx horizontal coordinatez vertical coordinate.Greek symbols coefficient of thermometric expansionδ thermal perturbation thermal diffusivityν kinematic viscosityσ Prandtl number [ ≡ νχ].     

Structural, magnetic and electrochemical properties of lithium iron orthosilicate

We report the structural and electronic characterization of Li₂FeSiO₄ synthesized by solid-state reaction. X-ray diffraction, Raman scattering, Fourier transform infrared (FTIR) spectroscopy, electron paramagnetic resonance (EPR) spectroscopy and magnetization measurements are analyzed. Magnetic susceptibility experiments give evidence that Li₂FeSiO₄ powders possess an antiferromagnetic ordering below T_N = 25 K due to long range Fe–O–Li–O–Fe interactions. Analysis of the paramagnetic region giving the Curie–Weiss parameters θ_p = −93.5 K and C_p = 4.13 emu K mol⁻¹ shows the divalent state of Fe cations. Electron paramagnetic resonance experiments confirm this electronic configuration. Electrochemical measurements were carried out in lithium cells with LiTFSI in a poly(ethylene oxide) (PEO) polymer electrolyte at 80 °C. The resulting cyclic voltammogram indicates a stable structure for the first cycle with redox peaks at 2.80 and 2.74 V versus Li⁰/Li⁺.     

Smart model for accurate estimation of solar radiation

Prediction of solar radiation has drawn increasing attention in the recent years. This is because of the lack of solar radiation measurement stations. In the present work, 14 solar radiation models have been used to assess monthly global solar radiation on a horizontal surface as function of three parameters: extraterrestrial solar irradiance (G₀), duration sunshine (S) and daylight hours (S₀). Since it has been observed that each model is adequate for some months of the year, one model cannot be used for the prediction of the whole year. Therefore, a smart hybrid system is proposed which selects, based on the intelligent rules, the most suitable prediction model of the 14 models listed in this study. For the test and evaluation of the proposed models, Tamanrasset city, which is located in the south of Algeria, is selected for this study. The meteorological data sets of five years (2000–2004) have been collected from the Algerian National Office of Meteorology (NOM), and two spatial databases. The results indicate that the new hybrid model is capable of predicting the monthly global solar radiation, which offers an excellent measuring accuracy of R² values ranging from 93% to 97% in this location.     

真空隔热油管传热性能研究

真空隔热油管是稠油注蒸汽开采的主要设施之一,其隔热性能直接影响热采效果,因此,分析隔热油管隔热层内部的传热过程,研究隔热层各种结构参数对隔热油管隔热性能的影响,对改善隔热油管隔热性能有重要的指导作用。本文在测试隔热油管视导热系数的实验模型基础上,建立了隔热层传热的物理及数学模型,计算得到不同结构参数下隔热油管的视导热系数;研究了玻璃丝布孔隙比、隔热层层数、支撑材料导热系数以及铝箔发射率对隔热油管隔热性能的影响,研究表明:隔热层层数宜选为4~6层之间;选用的铝箔发射率应在0.01~0.05之间;在缠绕玻璃丝布工艺中,尽量不要让玻璃丝布对角线方向受力,以确保较大的孔隙比;在隔热油管加工过程中,应尽量使玻璃丝布和铝箔保持干燥。     

Evaluation of CPC-collector designs for stand-alone, roof- or wall installation

An asymmetrically truncated non-tracking compound parabolic concentrator type collector design concept has been developed. The collector type has a bi-facial absorber and is optimised for northern latitudes. The concept is based on a general reflector form that is truncated to fit different installation conditions. In this paper collectors for stand-alone, roof and wall mounting are studied. Prototypes of six different collectors have been built and outdoor tested. The evaluation gave high annual energy outputs for a roof mounted collector, 925 MJ/m², and a stand-alone collector with Teflon, 781 MJ/m², at an operating temperature of T_op = 75 °C. A special design for roofs facing east or west was also investigated and gave an annual energy output of 349 (east) and 436 (west) MJ/m² at T_op = 75 °C. If a high solar fraction over the year is the objective, a load adapted collector with a high output during spring/fall and a low output during summer can be used. Such a collector had an output of 490 MJ/m² at T_op = 75 °C. Finally a concentrating collector for wall mounting was evaluated with an estimated annual output of 194 MJ/m² at T_op = 75 °C. The concentrator design concept can also be used for concentrators for PV-modules.     

Probing the reaction pathway of dehydrogenation of the LiNH2 + LiH mixture using in situ H NMR spectroscopy

Using variable temperature in situ ¹H NMR spectroscopy on a mixture of LiNH₂ + LiH that was mechanically activated using high-energy ball milling, the dehydrogenation of the LiNH₂ + LiH to Li₂NH + H₂ was investigated. The analysis indicates NH₃ release at a temperature as low as 30 °C and rapid reaction between NH₃ and LiH at 150 °C. The transition from NH₃ release to H₂ appearance accompanied by disappearance of NH₃ confirms unambiguously the two-step elementary reaction pathway proposed by other workers.     

Optimizing insulation thickness for buildings using life cycle cost

A systematic approach for optimization of insulation material thickness is developed in this paper and then applied to Palestine. The optimization is based on the life cycle cost analysis. Generalized charts for selecting the optimum insulation thickness as a function of degree days and wall thermal resistance are prepared. Life cycle savings of the insulated buildings are computed for Palestine. Savings up to 21 $/m² of wall area are possible for rock wool and polystyrene insulation. Payback periods between 1 and 1.7 years are possible for rock wool and payback periods between 1.3 and 2.3 years for polystyrene insulation, depending on the type of wall structure.