The effect of crystal size on the thermal energy storage capacity of thickened Glauber's salt

Data are presented which demonstrate that a significant increase in thermal energy storage with thermal cycling can be obtained in thickened Glauber's salt phase change mixtures by controlling the size of Na₂SO₄ and Na₂SO₄·10H₂O crystals. This has been done in two ways: (1) by the addition of crystal habit modifiers such as an acid copolymer (AACP) or sodium hexametaphosphate [SHMP (NaPO₃)₆] and (2) by using a polymeric polycarboxylic acid (PCA) thickener that is more effective than attapulgite clay. Thermal energy storage capacities ranging from 67 to 82 per cent of ideal have been obtained for as many as 1600 thermal cycles by using the habit modifiers and PCA as a thickener.     

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.     

Assessment of reservoir temperatures of thermal springs of the northern areas of Pakistan by chemical and isotope geothermometry

Chemical and isotope geothermometers, i.e. the Na–K, K–Mg, quartz and δ¹⁸O(SO₄–H₂O), have been applied to estimate the reservoir temperature of the thermal springs in the northern areas of Pakistan. The chemical types of the thermal waters and the effects of mixing of shallow cold water with the thermal end-members are discussed. These waters are neutral to slightly alkaline and have low dissolved contents. Sodium is the dominant cation in almost all the cases. In terms of anions, the hot waters of Budelas are of the SO₄ type, those of Tatta Pani are of mixed character (SO₄ and HCO₃), and the waters from the remaining areas show HCO₃ domination. An absence of tritium in Tatta Pani and Tato thermal springs indicates that they do not have any contribution of shallow young water. In the case of the Murtazabad springs, the wide range of tritium concentrations, negative correlations with surface temperature and Cl, and positive correlation between Na and Cl show that the shallow cold groundwater is mixing with thermal water in different proportions. For the mixed water of Murtazabad thermal springs, ‘isochemical modelling’ using the Na–K, K–Mg and quartz geothermometers indicates an equilibrium temperature in the range 185–200 °C. The δ¹⁸O(SO₄–H₂O) geothermometer gives relatively low temperatures for three springs, whereas two samples are close to the 185–200 °C temperature interval. The reservoir temperatures of Tatta Pani springs (100–120 °C), determined by Na–K and quartz geothermometers, are in good agreement. The δ¹⁸O(SO₄–H₂O) geothermometer gives a relatively higher range (140–150 °C) for most of the Tatta Pani springs. For Tato spring, the isotope and chemical geothermometers (except for the K–Mg) agree on an equilibrium temperature of about 170 °C. Reservoir temperatures of the remaining minor fields are not conclusive due to the lack of sufficient data.     

Prediction of mineral scale formation in geothermal and oilfield operations using the Extended UNIQUAC model: Part II. Carbonate-scaling minerals

Two parameters have been added to the Extended UNIQUAC model of Thomsen and Rasmussen [Thomsen, K., Rasmussen, P., 1999. Modeling of vapor–liquid–solid equilibrium in gas-aqueous electrolyte systems. Chem. Eng. Sci. 54, 1787–1802] to account for the pressure dependency of mineral solubility. The improved model has been used for correlating and predicting vapor–liquid–solid equilibrium for different carbonate systems (CaCO₃, MgCO₃, BaCO₃ and SrCO₃) causing mineral scaling problems. The solubility of NaCl and CO₂ in pure water and the solubility of CO₂ in NaCl and Na₂SO₄ solutions have also been correlated. The results show that the Extended UNIQUAC model, with the added pressure parameters, is able to represent binary (NaCl–H₂O, CaCO₃–H₂O, BaCO₃–H₂O, SrCO₃–H₂O, MgCO₃–H₂O, Mg(OH)₂–H₂O and CO₂–H₂O), ternary (CaCO₃–CO₂–H₂O, BaCO₃–CO₂–H₂O, SrCO₃–CO₂–H₂O, MgCO₃–CO₂–H₂O, CO₂–NaCl–H₂O and CO₂–Na₂SO₄–H₂O), and quaternary (CO₂–NaCl–Na₂SO₄–H₂O) solubility data within the experimental accuracy in the range of temperatures and pressures considered in the study, i.e. from 0 to 250 °C, and from 1 to 1000 bar, respectively.The modified Extended UNIQUAC model will be a useful tool for predicting and quantifying the scaling problems that may occur in wells and surface equipment during geothermal operations. This would allow adequate preventive measures to be taken before mineral deposition becomes troublesome.     

Deposition of transparent and conductive Al-doped ZnO thin films for photovoltaic solar cells

The effect of the substrate temperature on the optoelectronic properties of ZnO-based thin films prepared by rf magnetron sputtering has been studied. Three different targets (Zn/Al 98/2 at%, ZnO:Al 98/2 at% and ZnO:Al₂O₃ 98/2 wt%) have been investigated in order to compare resulting samples and try to reduce the substrate temperature down to room temperature. From the ZnO:Al₂O₃ target, transparent conductive zinc oxide has been obtained at 25°C with the average optical transmission in the 400–800 nm wavelength range, T = 80–90% and resistivity, = 3−5 × 10⁻³ Ωcm. In Al:Zn0 layers, the spatial distribution of the electrical properties across the substrate placed parallel to the target has been improved by depositing at high substrate temperatures, above 200°C. Besides, owing to diffusion processes of CuInSe₂ and CdS take place at 200°C, an AI:ZnO/CdS/CuInSe₂ polycrystalline solar cell made with the Al:ZnO deposited at 25°C as the transparent conductive oxide, has shown a more efficient photovoltaic response, η = 6.8%, than the one measured when the aluminium-doped zinc oxide has been prepared at 200°C, η = 1.8%.     

Phase change performance assessment of salt mixtures for thermal energy storage material

The phase transition performance of the CaCl₂ · 6H₂O–Ca(NO₃)₂ · 4H₂O composite salt system with nucleating and thickening agents was investigated in this paper. The CaCl₂ · 6H₂O–Ca(NO₃)₂ · 4H₂O composite salt system was prepared by adding Ca(NO₃)₂ · 4H₂O (12 wt%) to CaCl₂ · 6H₂O. Different nucleating agents including SrCl₂ · 6H₂O, SrCO₃, BaCl₂, BaCO₃, Na₂B₄O₇ · 10H₂O, H₃BO₃ and NH₄Cl were used to address the problems of phase segregation and supercooling phenomena during the phase change process. The results show that the single nucleating agent SrCl₂ · 6H₂O or the mixture of nucleating agents with 2 wt% SrCl₂ · 6H₂O, 1 wt% BaCl₂ and 0.5 wt% of thickening agent carboxyl methyl cellulose is the most suitable for this system. The latent heat remained constant at about 116 J/g before and after adding the agents. Density functional theory was used to further investigate the microstructure‐related reason for the salt–water separation and supercooling phenomena. It can be deduced that the hydrogen bond is the vital factor involved during the phase transition. The aim of adding thickener was to form more hydrogen bonds which encapsulated the hydrated species and made it difficult to lose the hydrated waters. The main purpose of adding nucleating agent was to break the metastable state among microscopic species. The results of this work indicate that the CaCl₂ · 6H₂O–Ca(NO₃)₂ · 4H₂O salt mixture has potential as a thermal energy storage material. Copyright © 2017 John Wiley & Sons, Ltd.     

The behavior of saturated solutions of trisodium phosphate dodecahydrate as heat storage media

Trisodium phosphate dodecahydrate can be considered for solar heat storage if it is used as a saturated solution rather than as pure material. However, even with a saturated solution it is necessary to take certain precautions to avoid deterioration of the heat storage capacity: either a cold finger must be used to maintain the presence of dodecahydrate crystal nuclei at all times or the temperature must not be allowed to exceed the liquidus temperatures for prolonged periods (e.g. 48 hr) or 85°C for brief periods, these treatments leading to the subsequent appearance of a lower hydrate. Enthalpy was measured as a function of temperature (to 85°C) and concentration (72–87% dodecahydrate), and the thermal cycling behavior of solutions was studied. Since crystalline trisodium phosphate dodecahydrate (as purchased or separating from solutions having Na/P 3) does not conform to the formula Na₃PO₄·12 H₂O, being more correctly considered as an alkaline complex salt approximating NaPO₄·12 H₂O·1/9 to 1/4 NaOH, the role of the Na/P ratio is examined in the context of heat storage in a saturated solution.     

Properties of RF sputtered zinc oxide based thin films made from different targets

The effect of deposition parameters on optoelectronic and properties of ZnO based thin films prepared by RF magnetron sputtering have been studied. Different targets (pure Zn, ZnO, Zn---Al (98/2 at 2%), ZnO---Al (98/2 at%), and ZnO---Al₂O₃ (98/2 wt%)) have been investigated to compare resulting samples and establish the best target composition. From reactive sputtering, using a Zn---Al target, transparent conductive zinc oxide has been obtained at 380°C with E_g = 3.25–3.35 eV and = 4.8 × 10⁻⁴ ω cm. Reduction of substrate temperature at 200°C has been possible by nonreactive sputtering from ZnO---Al and ZnO---Al₂O₃ targets. The values of the energy gap and resistivity under these conditions are 3.30–3.35 eV and 1 × 10⁻³ ω can 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.     

Sprayed CeO2 thin films for electrochromic applications

Cerium dioxide (CeO₂) thin films were prepared by spray pyrolysis using hydrated cerium chloride (CeCl₃·7H₂O) as source compound. The films prepared at substrate temperatures below 300°C were amorphous, while those prepared at optimal conditions (T_s=500°C,s=5 ml/min) were polycrystalline, cubic in structure, preferentially oriented along the (2 0 0) direction and exhibited a transmittance value greater than 80% in the visible range. The cyclic voltammetry study showed that films of CeO₂ deposited on ITO pre-coated glass substrates were capable of charge insertion/extraction when immersed in an electrolyte of propylene carbonate with 1 M LiClO₄.These films also remained fully transparent after Li+ intercalation/deintercalation.     

Sodium/lithium ratio in water applied to geothermometry of geothermal reservoirs

We propose here a new geothermometer for natural waters. Analyses from many explored geothermal fields allow us to define two empirical thermometric relationships.One is for waters of low to moderate salinity (Cl⁻< 0·3 M) log Na/Li = 1000/T −0·38 and one for marine waters and brines (Cl⁻ > 0·3 M) log Na/Li = 1195/T + 0·38 These relationships, which at present are not well understood, result mainly from the increase of Li concentrations in waters with temperature.Equation (a) proved to be adequate for spring waters from mostly known geologic origin; this is an important feature in geochemical surveys for geothermal prospecting.Furthermore, when comparison between springs and drillhole chemistry of a given geothermal field is possible, the Na/Li geothermometer gives more reliable temperature estimates from the spring compositions than do classical geothermometers.     

New Li ion-conducting ormolytes

The preparation and characterization of two new families of lithium-conducting solid-state electrolytes is reported. Both systems are silica (SiO₂) – polyethyleneglycol (PEG_n) hybrid materials with (type I) or without (type II) covalent organic-inorganic chemical bonds. Their electrical conductivity has been studied by complex impedance spectroscopy between 20°C and 100°C in the frequency range 1 Hz to 10 MHz as a function of the polymer chain length (200<n<1900), polymer concentration and lithium concentration (4<[O]/[Li]<80). The highest room-temperature ionic conductivity (σ6×10⁻² S cm⁻¹) has been found for type II material for ratios [O]/[Li]=15 and PEG₃₀₀/TEOS=1.0. The effect of the chain length on the polymer mobility has been studied by nuclear magnetic resonance by measuring the Li⁺ line widths and the spin-lattice relaxation time T₁ between -100°C and +100°C. The bonded chain mobility increases with the chain length (type II) while the opposite occurs with unbonded chain material (type I). Both types of materials present high ionic conductivity at room temperature and are adequate as Li⁺-conducting electrolyte in all solid-state electrochemical devices.     

Investigations on ZnxCd1−xO thin films obtained by spray pyrolysis

Zn_xCd_1−xO thin films were prepared on glass substrates by spray pyrolysis technique. The precursor solutions were obtained by varying the concentration of Zn(NO₃)₂·6H₂O and Cd(NO₃)₂·4H₂O in bi-distilled water. The structural properties have been studied using X-ray diffraction spectra. All the structures include the basic compounds, i.e. ZnO and CdO. The orientation and the crystalline phases of the deposited films were specified. With the addition of Zn to the precursor solution, we can observe the preferential orientation of the CdO in the [2 0 0] direction. The electrical measurements were performed using method of four contacts. Thin films transmittances, in the 1.5–4.3 eV range, for different compositions have been measured and the optical gaps have been determined. The variations are explained considering the gaps of the two pure films. The influence of increased Cd concentration in the films on the structural, electrical and optical properties is investigated in this study.     

Intergration of evacuated tubular solar collectors with lithium bromide absorption cooling systems

By surrounding the absorber-heat exchanger component of a solar collector with a glass-enclosed evacuated space and by providing the absorber with a selective surface, solar collectors can operate at efficiencies exceeding 50 per cent under conditions of ΔT/H_T = 75°C m²/kW (ΔT = collector fluid inlet temperature minus ambient temperature, H_T = incident solar radiation on a tilted surface). The high performance of these evacuated tubular collectors thus provides the required high temperature inputs (70–88°C) of lithium bromide absorption cooling units, while maintaining high collector efficiency. This paper deals with the performance and analysis of two types of evacuated tubular solar collectors intergrated with the two distinct solar heating and cooling systems installed on CSU Solar Houses I and III.     

Texturization of monocrystalline silicon with tribasic sodium phosphate

Tribasic sodium phosphate (Na₃PO₄·12H₂O) was successfully introduced to texture monocrystalline silicon for the first time. A series of comparative experiments were made to indicate the dependence of hemispherical surface reflectance on the tribasic sodium phosphate (Na₃PO₄·12H₂O) concentration, reaction temperature and etching time. Meanwhile, the effects of other agents, such as isopropyl alcohol (IPA), sodium hydroxide (NaOH) and bi-sodium hydrogen phosphate (Na₂HPO₄) on average reflectance were also investigated. The results showed that IPA and NaOH have great detrimental effects on texture, and the average reflectance slightly increased with the addition of Na₂HPO₄. On the basis of our experiments, it is concluded that the effect of phosphatidate (PO₄³⁻) or its compounds on the texturization is the same as that of the mixture of alkaline and IPA. Furthermore, this method is economical, has low pollution and good reproducibility. We feel that it is suitable for the large-scale production.     

The preparation of TiO2–nitrogen doped by calcination of TiO2·xH2O under ammonia atmosphere for visible light photocatalysis

The investigation on incorporating nitrogen group into titanium dioxide in order to obtain powdered visible light-active photocatalysts is presented. The industrial hydrated amorphous titanium dioxide (TiO₂·xH₂O) obtained directly from sulphate technology installation was modified by heat treatment at temperatures of 100–800 °C for 4 h in an ammonia atmosphere. The photocatalysts were characterized by UV–VIS–DR and XRD techniques. The UV–VIS–DR spectra of the modified catalysts exhibited an additional maximum in the VIS region (, ) which may be due to the presence of nitrogen in TiO₂ structure. On the basis of XRD analysis it can be supposed that the presence of nitrogen does not have any influence on the transformation temperature of anatase to rutile. The photocatalytic activity of the modified photocatalysts was determined on the basis of decomposition rate of phenol and azo-dye (Reactive Red 198) under visible light irradiation. The highest rate of phenol degradation was obtained for catalysts calcinated at 700 °C (6.55%), and the highest rate of dye decomposition was found for catalysts calcinated at 500 and 600 °C (ca. 40–45%). The nitrogen doping during calcination under ammonia atmosphere is a very promising way of preparation of photocatalysts which could have a practical application in water treatment system under broader solar light spectrum.     

Pyrheliometric determination of atmospheric turbidity in the harmattan season over Ile-Ife, Nigeria

Measurements of direct solar radiation intensity, using an Angström compensation pyrheliometer carried out over three harmattan seasons (1985–1987) at Ile-Ife (7°29′N, 4°34′E), Nigeria, have been used to determine atmospheric turbidity based on five different models of turbidity viz:: Schüepp (B), Angstrom (β), Kastrov (C), Unsworth (τ_a) and Linke (T). The five parameters indicate high aerosol loading of the atmosphere during the period and high correlation is established between them: (0.919 r 0.999). An inverse relationship has been noticed between horizontal visibility and atmospheric turbidity: (−0.80 r −0.76).     

Microcrystalline-phase p-type a-Si:O:H windows prepared by Cat-CVD

Different amounts of oxygen, boron-doped hydrogenated amorphous silicon (a-Si:H) and hydrogenated microcrystalline silicon (μc-Si:H) deposition were carried out using catalytic chemical-vapor deposition (Cat-CVD) process. Pure silane (SiH₄), hydrogen (H₂), oxygen (O₂), and diluted diborane (B₂H₆) gases were used at the deposition pressure of 0.1–0.5 Torr. The tungsten catalyst temperature (T_fil) was varied from 1700 to 2100 °C. Sample transmittance measurement shows an optical-band gap (E_gopt) variation from 1.45 to 2.1 eV X-ray diffraction (XRD) spectra have revealed silicon microcrystalline phases for the samples prepared at the temperature greater than T_fil1900 °C. For the used silicon oxide deposition conditions, no strong tungsten filament degradation was observed after a number of sample preparations.     

Pyrite (FeS2) thin films prepared by spray method using FeSO4 and (NH4)2Sx

A simple spray method for the preparation of pyrite (FeS₂) thin films has been studied using FeSO₄ and (NH₄)₂S_x as precursors for Fe and S, respectively. Aqueous solutions of these precursors are sprayed alternately onto a substrate heated up to 120°C. Although Fe–S compounds including pyrite are formed on the substrate by the spraying, sulfurization of deposited films is needed to convert other phases such as FeS or marcasite into pyrite. A single-phase pyrite film is obtained after the sulfurization in a H₂S atmosphere at around 500°C for 30 min. All pyrite films prepared show p-type conduction. They have a carrier concentration (p) in the range 10¹⁶–10²⁰ cm⁻³ and a Hall mobility (μ_H) in the range 200–1 cm²/V s. The best electrical properties (p=7×10¹⁶ cm⁻³, μ_H=210 cm²/V s) for a pyrite film prepared here show the excellence of this method. The use of a lower concentration FeSO₄ solution is found to enhance grain growth of pyrite crystals and also to improve electrical properties of pyrite films.     

Phase change stability of CaCl2·6H2O

A CaCl₂·6H₂O composition with a slight excess of water was found very stable on repeated phase change (over 1000 cycles) in a heat cycle test (35 18°C), using vertical glass tubes of 950 mm height. The mole ratio of water/CaCl₂, n, was 6.11, less than that of the peritectic composition (n = 6.14). Dissolved NaCl had superior nucleating ability to that of common barium salts under certain experimental conditions. This result was attributed to a “memory effect”. The dissolved NaCl decreased the heat of vaporization of water about 15 per cent. NaF had a similar effect to that of NaCl, because NaF powder reacted with CaCl₂ in solution yielding NaCl and CaF₂. A brief discussion of CaCl₂·4H₂O formation is presented.