Thermoconvective instability in a horizontal porous cavity saturated with cold water

The onset of convection in a horizontal porous cavity with regard to the density maximum of water at 3.98°C is studied using a linear stability analysis. In the formulation of the problem use is made of the Brinkman-extended Darcy model which is relevant to sparsely packed porous media. A parabolic density-temperature relationship is used to model the effect of density inversion. The perturbation equations are solved with the aid of the Galerkin and finite element methods. The onset of motion is found to be dependent of the aspect ratio A of the cavity, the Darcy number Da, the inversion parameter γ and the hydrodynamic boundary conditions applied on the horizontal walls of the porous layer. The results for a viscous fluid (Da→∞) and the Darcy porous medium (Da→0) emerge from the present analysis as limiting cases. Numerical results for finite amplitude convection, obtained by solving the full governing equations, indicate that subcritical convection is possible when the upper stable layer extends over more than the half depth. Also, the existence of multiple solutions for a given range of governing parameters is demonstrated.     

Crystallographic analysis of high quality poly-Si thin films deposited by atmospheric pressure chemical vapor deposition

Crystallinity of thin film polycrystalline silicon (poly-Si) grown by atmospheric pressure chemical vapor deposition has been investigated by X-ray diffraction measurement and Raman spectroscopy. Poly-Si films deposited at high temperatures of 850–1050°C preferred to 2 2 0 direction. By Raman spectroscopy, the broad peak of around 480–500 cm⁻¹ belonged to microcrystalline Si (μc-Si) phase was observed even for the poly-Si deposited at 950°C. After high-temperature annealing (1050°C) 3 3 1 direction of poly-Si increased. This result indicates that the μc-Si phase at grain boundary became poly-Si phase preferred to 3 3 1 direction by high-temperature annealing. Effective diffusion length of poly-Si films deposited at 1000°C was estimated to be 11.9–13.5 μm and 10.2–12.9 μm before and after annealing, respectively.     

Natural convection near 4°C in a water saturated porous layer heated from below

This paper reports a numerical study of two-dimensional natural convection in a horizontal porous layer heated from below and saturated with cold water. The density maximum of water at 3.98°C and atmospheric pressure occurs inside the layer, as the top surface is maintained at 0°C and the bottom surface is varied from 4 to 8°C. Three separate series of numerical simulations document the effect of Rayleigh number, bottom surface temperature, and horizontal length of the porous layer on the overall heat transfer rate vertically through the layer. The range of these numerical experiments is 200 < Ra_p < 10000, 0.167 < H/L < 1 and °C < T_H < 8°C, where Ra_p, H/L and T_H are the Darcy-modified Rayleigh number for a fluid with density maximum, the geometric ratio height/length, and the bottom wall temperature. The numerical results agree with published linear stability results regarding the onset of convection.     

Thin CdS films prepared by metalorganic chemical vapor deposition

Polycrystalline CdS thin films have been deposited on borosilicate glass substrates coated with ITO film by metalorganic chemical vapor deposition using dimethyl cadmium and diethyl sulfide as source materials. The growth of CdS film occurred at substrate temperatures within the range of 280–360°C. The deposition rate increased with increasing VI/II molar ratio at any substrate temperature and showed a maximum value at the VI/II molar ratio of 4. The grain size of as-deposited CdS film prepared at substrate temperatures from 300°C to 360°C was about 0.1 μm. The CdS films consist of hexagonal form with a preferential orientation of the (0 0 2) plane parallel to the substrate. Thin CdS film with high optical transmittance was prepared at 350°C with the VI/II molar ratio of 4. The CdS film deposited by MOCVD may be used as a window layer for CdS/CdTe solar cell.     

Prevention of supercooling and stabilization of inorganic salt hydrates as latent heat storage materials

Various inorganic salt hydrates have been studied as a latent heat storage medium. A super-absorbent polymer (SAP) made from an acrylic acid copolymer is proposed as an effective thickener to prevent undesirable phase separation of the high hydrate inorganic salts (Na₂SO₄· 10H₂O, Na₂HPO₄·12H₂O, Na₂CO₃·10H₂O). Most of these materials can be stabilize by the addition of 3 to 5 wt% SAP as a thickener. For the low hydrate inorganic salts (CH₃COONa· 3H₂O, Na₂S₂O₃·5H₂O), carboxymethyl cellulose (CMC) is found to be an effective thickener. Similarly, the phase separation of the low hydrate salts can be prevented by the addition of 2 to 4 wt% thickener. To overcome the supercooling of the thickened phase change materials, various potential nucleators have been evaluated. For the thickened Glauber's salt, borax reduces supercooling of the salt from 15 to 3–4°C. Three different powders of carbon (1.5–6.7 μm), copper (1.5–2.5 μm) and titanum oxide (2–200 μm) are found to reduce the supercooling of thickened Na₂HPO₄·12H₂O. Also, the supercooling of thickened CH₃COONa·3H₂O is reduced from 20 to 2–3°C by adding 2 wt% potassium sulfate. New compositions for preventing supercooling and phase separation of PCMs are developed in the temperature range 30–60°C: Glauber's salt/SAP/borax (94/3/3 wt%, T_m = 35°C), Na₂CO₃·10H₂/SAP/Sr(OH)₂ (93/3/4 wt%, T_m = 32°C), Na₂HPO₄·12H₂O (92.8/3.5/3.7 wt%, T_m = 35°C), Na₂S₂O₃·5H₂O/CMC/ SrSO₄ (92/3/5 wt%, T_m = 48°C), CH₃COONa·3H₂O/CMC/K₂SO₄ (95/3/2 wt%, T_m = 58°C).     

Black cobalt selective coatings by spray pyrolysis for photothermal conversion of solar energy

A low cost technique of spray pyrolysis has been described for the growth of black cobalt selective surfaces on commercially available aluminum and galvanized iron substrates. Parameters of growth have been optimized by a comparative analysis of opto-thermal, structural and optical properties of these films. Optimized films on aluminum were 0.21 μm thick and had α = 0.92 and _100°C = 0.13, Films on galvanized iron substrates gave best results (α=0.91, _100°C=0.12) for film thickness 0.24 μm. Accelerated life test studies indicate that these films have good adhesion to the substrates and are stable up to 220°C.     

Routine instrumental procedures to characterise the mineralogy of modern and ancient silica sinters

Tightly constrained determinative methods can be used to characterise the silica minerals (opal-A, opal-CT, opal-C, quartz, moganite) and physical properties of silica sinters. Optimal X-ray powder diffraction operating parameters indicate silica lattice order/disorder using untreated, dry, <106 μm powders scanned at 0.6° 2θ/min with a step size of 0.01° from 10–40° 2θ and an internal Si standard. Simultaneous differential thermal and thermogravimetric analysis of 15.0±0.1 mg sinter samples of <106 μm grain size, at a heating rate of 20°C/min in dry air, identify thermal events associated with dehydration, organic combustion, and changes of state. Where abundant organic matter is present, nitrogen is the preferred atmosphere for thermal analysis. Thermogravimetric-determined water contents of sinters differ from Penfield determinations reflecting the differing nature of the two techniques. Laser Raman microprobe techniques can be used to explore the mineralogy of particular sinter morphologies and habits down to 10 μm diameter. The nature of the silica species present can assist in characterising individual sinter deposits and, combined with textural, density and/or porosity determinations, can lead to a better understanding of the hydrology and paleohydrology of a geothermal prospect.     

Layered GdBa0.5Sr0.5Co2O5+δ as a cathode for intermediate-temperature solid oxide fuel cells

The layered GdBa_0.5Sr_0.5Co₂O_5+δ (GBSC) perovskite oxides are synthesized by Pechini method and investigated as a novel cathode material for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The single cell of NiO–SDC (Sm_0.2Ce_0.8O_1.9)/SDC (20 μm)/GBSC (10 μm) is operated from 550 to 700 °C fed with humidified H₂ as fuel and the static air as oxidant. An open circuit voltage of 0.8 V and a maximum power density of 725 mW cm⁻² are achieved at 700 °C. The interfacial polarization resistance is as low as 0.88, 0.29, 0.13 and 0.05 Ω cm² at 550, 600, 650 and 700 °C, respectively. The ratio of polarization resistance to total cell resistance decreases with the increase in the operating temperature, from 60% at 550 °C to 21% at 700 °C, respectively. The experimental results indicate that GBSC is a promising cathode material for IT-SOFCs.     

Clear separation of seasonal effects on the performance of amorphous silicon solar modules by outdoor I/V-measurements

The causes of seasonal variations on the performance of an amorphous silicon solar module were clearly separated using long-term outdoors I(V)-measurements. We normalized the data to a standard temperature, by using measured temperature coefficients of the characteristic parameters of the I(V)-curve, rather then extrapolating the curve itself. The resulting data were interpreted using a new model containing an effective μτ-product in the i-layer of the device (Merten et al. 1997). This μτ-product is accessed by variable illumination measurements (VIM) of the I(V) characteristic, which can be easily performed outdoors, making use of the natural variation in the illumination levels. The effective μτ-product of the module remains constant throughout its second year of outdoor exposure. We conclude that the enhanced efficiency in summer is, therefore, mainly a spectral effect, and operating temperatures exceeding the winter value of 60°C do not further increase the module's performance.     

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.     

Measurement and interpretation of terrestrial heat flow in Israel

Sixty-eight new determinations of terrestrial heat flow in Israel have a range of 0.17-11.07 μcal/cm²s. The average value of deep conductive heat flow in the undisturbed complex of the Arabo-Nubian Massif is 0·94 μcal/cm²s; it is least affected by circulation of groundwater. This value is only slightly higher than the heat flow of 0·88 μcal/cm²s in the Levantine Basin of the Mediterranean Sea. Several values that exceed 2·0 μcal/cm²s are due either to (probable) deep hydrothermal activity or to small domal structures of the basement.Within the sedimentary sequence which blankets the crystalline massif, terrestrial heat flow is often redistributed by circulating groundwater. Recharge regions, particularly Judean-Samarian Galilee, where cool meteoric waters percolate into the subsurface have anomalously low heat flow, ranging from 0·17 to about 1·0 μcal/cm²s. Part of the original deep thermal flux in those regions is intercepted at moderate depths by the recharge flow, and is carried into deeper aquifers of the Foothills, Coastal Plain, or the Jordan-Dead Sea Rift. Movement of groundwater occurs mainly along faults.Deep faults associated with the Jordan-Dead Sea Rift system act as conduits for hot waters ascending from deep confined aquifers. The most tangible surface expression of the convective hydrothermal system are the numerous warm to hot springs, emerging along the margins of the Rift. However, the waters emerging on the surface as the warm and hot springs are a minor fraction of the convective system. Most of the ascending thermal waters are absorbed by shallow aquifers with lower hydraulic potential. Such regions are characterized by anomalously high heat flow; several values exceed 2 and one value is 11 μcal/cm²s.     

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.     

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.     

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.     

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.     

Hydrogeological and isotopic survey of geothermal fields in the Buyuk Menderes graben, Turkey

The main high and low enthalpy geothermal fields in the Buyuk Menderes graben (Western Anatolia) and their reservoir temperatures are as follows: Kizildere (242 °C), Germencik (232 °C), Aydin-Ilicabasi (101 °C), Yılmazkoy (142 °C), Salavatli (171 °C), Soke (26 °C), Denizli -Pamukkale (36 °C), Karahayit (59 °C), Golemezli (101 °C) and Yenice (70 °C). The geothermal systems are controlled by active graben faults. The reservoir rocks in the geothermal fields are the limestone and conglomerate units within Neogene sediments and the marble-quartzite units within Paleozoic metamorphic formations. There are clear δ¹⁸O shifts from the Mediterranean Meteoric Water Line (MMWL) in the Kizildere, Germencik and Aydin fields, where a good relation between high temperatures and δ¹⁸O shift has also been observed, indicating deep circulation and water rock interactions. In the Pamukkale, Karahayit, Golemezli and Yenice fields and in Soke region, low temperatures, small isotope shifts, shallow circulations and mixing with shallow cold water have been noted.     

A computational method for determining segmental and overall geothermal gradients and geothermal heat flow values

A new computational method is presented which calculates geothermal heat flow values and geothermal gradients with more precision than permitted by previously published techniques. The data required are: geothermal temperature at a known depth, mean surface temperature, the rock types in the stratigraphic column and the thermal resistivity values for the different types of rocks. This method is valuable in areas that have no measured gradient values. Basic equation used was the Fourier heat transfer equation where is heat flux in μcal/(cm² s), ρ_i is thermal resistivity (°C s cm/μcal) and ∂T/∂x is the x component of the temperature gradient (°C/cm). The thermal resistivity was allowed to vary linearly with temperature ρ_i = ρ_i^o [1 + K_i (T − 30)] where ρ_i is thermal resistivity of the lithographic segment «iå at a temperature T, ρ_i^o is thermal resistivity at 30°C and K_i is the temperature coefficient of thermal resistivity. The procedure consisted of integrating the combined equation for heat flux in terms of temperature dependent resistivity.Two iterative solutions were used to simplify the calculations: exact and approximate. The heat flux for each well was assumed to be 1.0 HFU and segmental temperatures were calculated from the bottom (arbitrarily) up, until a surface temperature was obtained. The calculated surface temperature could then be compared with the mean surface temperature (MST). Correction in the heat flux value was made until the calculated surface temperature and MST agreed. An analysis of three deep Appalachian test wells was made and the results showed the critical importance of lithographic ordering and the temperature dependence of thermal resistivity upon calculated geothermal quantities.     

Effect of annealing on the electrical, optical and structural properties of hydrogenated amorphous silicon films deposited in an asymmetric R.F. plasma CVD system at room temperature

This paper reports the effect of annealing on hydrogenated amorphous silicon films (a-Si : H) deposited by r.f. self-bias technique on cathode in an asymmetric r.f. plasma CVD system at room temperature. Detailed study of the variation of the dark and photoconductivity (σ_D and σ_ph) as a function of temperature and light intensity, surface morphology, hydrogen evolution, optical absorption, subgap absorption and related parameters, thermal and structural disorder on the optical-absorption edge, IR vibrational modes and bonded hydrogen content have been carried out on unannealed and annealed samples at different temperatures (T_a) from 100°C to 550°C. It is found that the values of σ_ph increase and that of Urbach energy (E_o), subgap defect density (N_d) and the polyhydride to monohydride ratio decrease upto T_a=250°C and beyond 250°C the values of σ_ph decrease and that of E_o, N_d and the polyhydride to monohydride ratio increase. The best opto-electronic properties with much improved σ_ph and σ_ph/σ_D and dominant monohydride bonding are obtained after annealing the room temperature deposited film at 250°C for 1 h. The σ_D data obeys a Meyer Neldel rule in annealed a-Si : H films. The value of optical band gap is found to be related to the E_o and the hydrogen content. The Urbach energy (E_o) which is a measure of the disorder is the sum of structural and thermal disorder. The structural disorder part decreases with the annealing temperature upto 300°C and thereafter it increases. The curves of optical absorption coefficient versus photon energy at different T_a converge to a common point.     

Screen printed conductive CuS-poly(acrylic acid) composite coatings

Copper sulfide (CuS) powder precipitated from two different chemical baths (citrate and triethanolamine baths) was dispersed in a poly(acrylic acid) aqueous solution and the resulting mixtures were screen-printed over glass slides. Compared to the coatings of the same composite materials obtained by casting, the screen-printed coatings show better homogeneity and an improved thermal stability (up to 300°C). Sheet resistances of 50 Ω/□ were typical in 20 μm thick coatings and these values remained stable even after baking in nitrogen or vacuum at 200–300°C. The X-ray diffraction pattern indicates degradation of CuS to Cu_1.8S and Cu_1.96S, particularly in air-baked samples at 250–300°C. The XRD, TGA and R_□ results show CuS(Cit)–PAA samples as superior conductive coatings.     

Au–Cu/p–CdTe/n–CdO/glass-type solar cells

The heterostructure design proposed by us for the photovoltaic (PV) solar cell is: Au–Cu/p–CdTe:Sb/n–CdO:F/glass. The CdO:F films were grown by the sol–gel method, in conditions in order to get low resistivity 4.5×10⁻⁴ Ω-cm and an optical transmission higher than 85%. The CdTe:Sb films were prepared by means of the RF sputtering technique, in conditions to get resistivity value around 10⁶ Ω-cm, high crystalline quality and higher grain size. The Au–Cu contacts were thermally evaporated. For the study of PV-heterostructure a systematic variation of the preparation parameters were carried out. The parameters involved in the manufacture of the cell, in order to look for the highest efficiency were: (A) For the deposit of the p-CdTe:Sb films, a low argon pressure of 2.5 m Torr and high substrate temperature of 450 °C. The CdTe:Sb film thickness was varied in the interval 4.5–11 μm. (B) For the activation of the heterostructure: (i) The treatment temperature in vacuum, after the CdTe is deposited, was varied in the 350–550 °C range and (ii) the treatment temperature in Ar atmosphere, after the heterostructure is dipped in CdCl₂ solution, was studied in the 400–510 °C range. (C) Optimization of the Cu–Au contact with the adequate Cu-film thickness. The highest energy conversion efficiency (η) value was 5.48%. This work reports a systematic study of the parameters involved in the solar cell manufacture, for the search of a better value of η.