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.     

Mathematical modelling of silica deposition in Tongonan-I reinjection wells, Philippines

Mathematical models of silica deposition are derived using the method of characteristics for the problem of variable rate injection into a well producing radially symmetric flow. Solutions are developed using the first order rate equation of silica deposition suggested by Rimstidt and Barnes (1980). The changes in porosity and permeability resulting from deposition are included in the models. The models developed are successfully applied in simulating the changes in injection capacity in some of the reinjection wells in Tongonan geothermal field, Philippines.     

Chemical and isotopic changes in shallow groundwater caused by exploitation of the Wairakei geothermal field, New Zealand

Chemical data obtained in 1983/84 and 1994 from groundwater monitor wells at Wairakei are reviewed and interpreted in conjunction with physical changes in the deeper system. The composition of the waters is controlled by the proportion of deep chloride, shallow meteoric and steam heated components. The deep component dominates the central production area, a bicarbonate meteoric water is predominant in the peripheral areas, and steam-heated water is more evident in between, particularly near surface steam discharge and hot ground. The major change observed is a decrease of the deep component. Some wells show effects of evaporation.     

“FRAC ” — A simulation code for forced fluid flow and transport in fractured, porous rock

The 3D Finite Element Program FRACTure was developed with the specific aim of studying the coupling of interactive mechanisms in geoscience and in particular those relevant to the long term behaviour of a Hot Dry Rock reservoir. The flexible modular structure facilitates the addition of further processes and elements to the existing library and the handling of linear and non-linear constitutive laws and the calculation of their interactions. The Hot Dry Rock applications involve essentially forced fluid flow of cool fluid injected into a hot fractured rock matrix. A study of the relevant processes required finite element solutions for the hydraulic, thermal and elastic fields and especially their interactions. Particular attention has been paid to modelling the perturbations arising from poro-elastic and thermo-elastic effects in the rock matrix and of a non-linear, stress dependent joint closure law. The fracture network is designed in a way to represent nature as realistically as possible: fracture flow takes place along planes or in pipes (fracture intersections), porous matrix flow in 3D bodies. In devising the coupling scheme careful thought has been given to the different time constants for each physical process. Apart of simulating the coupled Hot Dry Rock behaviour, the code has been successfully practised in a variety of applications in geoscience: analysis of geoelectric measurements, radon transport, solute tracer tests, simulations of heat probe operations and hydrogeologic system modelling.     

Measurement and use of the vertical gravity gradient in correcting repeat microgravity measurements for the effects of ground subsidence in geothermal systems

Changes in mass resulting from production and reinjection in geothermal fields can be monitored using repeat microgravity measurements. The measured changes in gravity, however, need to be corrected for the effects of any exploitation-induced ground subsidence. The correction required at each measurement point is the product of the amount of ground subsidence and the vertical gravity gradient at that point. Measurements of the gravity gradient, made using a portable tower, show that it varies from place to place depending mainly on the local topography. Measured gradients, in areas of high subsidence rate, range between −276 and −339 μgal/m at 30 sites in the Wairakei–Tauhara field and between −296 and −321 μgal/m at nine sites in the Ohaaki field (New Zealand). These values are similar to those used previously. At 37 sites measured in the Yanaizu–Nishiyama field (Japan) the gradient varied between −244 and −352 μgal/m. Measurements at six sites showed no significant change in gravity gradient with height above the ground surface; a single value for the gradient can therefore be used at each site to correct for the gravitational effect of any ground movement. Calculations show the effects on the vertical gravity gradient of groundwater level variations, mass changes associated with the ground subsidence, and reservoir mass changes are negligible at Wairakei.     

Trace metal chemistry and silicification of microorganisms in geothermal sinter, Taupo Volcanic Zone, New Zealand

As part of a pilot study investigating the role of microorganisms in the immobilisation of As, Sb, B, Tl and Hg, the inorganic geochemistry of seven different active sinter deposits and their contact fluids were characterised. A comprehensive series of sequential extractions for a suite of trace elements was carried out on siliceous sinter and a mixed silica-carbonate sinter. The extractions showed whether metals were loosely exchangeable or bound to carbonate, oxide, organic or crystalline fractions. Hyperthermophilic microbial communities associated with sinters deposited from high temperature (92–94°C) fluids at a variety of geothermal sources were investigated using SEM. The rapidity and style of silicification of the hyperthermophiles can be correlated with the dissolved silica content of the fluid. Although high concentrations of Hg and Tl were found associated with the organic fraction of the sinters, there was no evidence to suggest that any of the heavy metals were associated preferentially with the hyperthermophiles at the high temperature (92–94°C) ends of the terrestrial thermal spring ecosystems studied.     

Strategic cogeneration — fresh horizons for the development of cogeneration in Brazil

An earlier study (Szklo A, Soares J, Tolmasquim M. Economic potential of natural gas-fired cogeneration in Brazil: two case studies. Applied Energy 2000;67(3):245–67) that assessed the economic feasibility of gas-fired cogeneration systems in Brazil, indicated that the use of cogeneration in Brazilian malls tends to be small in the short-term. However, current experience is opening up the possibility of investments in cogeneration plants — despite their lack of economic feasibility — as part of the strategic actions of utilities in the energy market. The earlier study made no attempt to analyze this possibility, focusing solely on economic feasibility. This article supplements its predecessor by analyzing a type of cogeneration rated as strategic. This is a recent phenomenon prompted by the deregulation of Brazil's energy market. Within this new context, power utilities are taking up a defensive behavior as they battle to preserve their markets, in parallel to aggressive behaviors designed to expand and diversify their activities. This means that they may well invest in cogeneration for strategic reasons that may not be clearly reflected through an economic-feasibility assessment.     

Creep crack growth in welded components — a numerical study and comparison with the R5 procedures

In the present study, creep crack growth (CCG) in a circumferentially welded low alloyed pipe is numerically investigated for a number of different combinations of weldment constituent material properties. A creep ductility based damage model describes the accumulation of creep damage ahead of the crack tip where a constraint parameter and the creep strain rate perpendicular to the crack plane are used as characterising parameters. It is assumed that a fully circumferential creep crack, located in the heat affected zone with a depth of one quarter of the pipe thickness, is growing at a constant rate from the outer surface towards the inside. The numerical results reveal that not only the properties of the zone containing the crack, but also the deformation properties of the surrounding material influence the CCG behaviour. This influence can be noted on the characterising parameters used for the CCG rate predictions as well as on the CCG rate itself. The mismatch influence on corresponding C^* values is, however, marginal. This indicates that determination of the CCG rate in weldments, based on the C^* value only, may result in uncertain estimates.The numerically investigated cases are also assessed by use of the R5 procedures for the sake of comparison. Considering the stress re-distribution, due to the mismatch effect, the CCG rate is determined for the different weldment configurations. The comparison shows that the assumption of plane strain or plane stress conditions in the R5 analysis is essential for the agreement of the results between R5 and the two-parameter approach. Assuming plane stress conditions at the crack tip results in a relatively good agreement for the axial stress dominated cases investigated. However, for the hoop stress dominated cases, the R5 procedures predict higher CCG rates by an order of magnitude.     

Creep behaviour and lifetime of large welds in X 20 CrMOV 12 1—results based on simulation and inspection

Weld creep performance in steam lines of X 20 CrMoV 12 1 (X20) has been studied by simulation and plant inspection. By finite-element simulations, the influences of (i) the heat affected zone creep rate, (ii) the material constant α, related to the multiaxial rupture criterion, and (iii) axial system stresses on creep life, have been studied. The results show that an increase of each factor (i)-(iii) reduces the creep life, especially when two of them act at the same time. The inspection study covers over two decades of replica inspection of steam line welds in Danish power plants with service times up to 190.000 h. Analysis of the results shows that large butt joints and T-butt joints in X20 perform exceptionally well. They are unlikely to develop detectable creep damage within service times up to 200.000 h. One of the reasons for this good performance of the steel is its high time safety margin.     

Deep levels in the band gap of CdTe films electrodeposited from an acidic bath—PICTS analysis

The deep traps detected in a CdTe thin film electrodeposited from an acidic bath are discussed. CdTe thin films were developed on flexible metallic substrates by electro deposition. The films were nearly stiochiometric, highly uniform and exhibit good crystallinity. The films were characterized using XRD, SEM, AUGER and AFM. The crystallites exhibited a strong preference for the (1 1 1) plane. The grain size of the film was in the range 0.2–0.4 μm. The photoinduced current transient spectroscopic technique was effectively used to identify the electron and hole traps. Two hole traps and one electron trap was detected. The activation energies of those deep traps were 0.37 and 0.42 eV for the hole traps and 0.41 eV for the electron trap.     

Elucidating the Effects of Cu‐Nanoparticles in a Porous Medium vis‐à‐vis Heat Transfer Phenomena

In this paper, a numerical simulation technique is developed to investigate the qualitative and quantitative behaviour of Cu‐nanoparticles in a porous medium vis‐a‐vis the heat transfer enhancements—buoyancy driven flow in a two‐dimensional square cavity, with moving walls is presented. The model utilizes the finite volume approach to solve the Brinkman–Darcy equations for Cu‐nanoparticles in a porous media. Discretization is carried out for convective and diffusive fluxes using Quadratic Upwind Interpolation for Convective Kinematics (QUICK) and central difference schemes, respectively. Tri‐Diagonal Matrix Algorithm is invoked to solve the set of algebraic equations. The Darcy number (Da), Prandtl number (Pr), and volume fraction (χ) are varied from 10^?3 to 10^?1, 3 to 7, and 0% to 20%, respectively. Insight into the cause of variations in isotherms, streamlines, Nusselt number (Nu), and mid‐plane velocities is explicated. The present numerical results are compared with the existing literature and found to be in good agreement. Even though nanoparticles slightly hinder the activity of the fluid, they can augment the average Nu by 90% for Pr = 7, Da = 0.1, and χ = 20% as compared to the absence of nanoparticles. Their efficacy is more prominent for flows with higher Da and Pr. Quantitative values for Nu were obtained for various combinations of Pr, Da, and χ.     

Evidence of electron trapping in undoped hydrogenated amorphous silicon (a-Si:H) Schottky barriers by the surface photovoltage (SPV) technique — A study of stability in a-Si:H solar cells

Surface photovoltage (SPV) measurements on a-Si:H/metal Schottky devices under forward bias are presented. The space-charge density decreases with applied forward bias while both the space-charge width and the apparent diffusion length are essentially constant. These are all attributed to electron trapping in Schottky barrier region. Furthermore, we explore electron trapping effect in a-Si:H on light-induced degradation of a-Si:H solar cells.     

Study of the miscibility gap in H2SO4/HI/I2/H2O mixtures produced by the Bunsen reaction – Part I: Preliminary results at 308 K

In the framework of the optimization of the sulfur–iodine thermochemical cycle for massive hydrogen production, investigations were performed in order to characterize the liquid phase (HIx and H₂SO_4(aq) phases) separation of solutions resulting from Bunsen reaction. Quaternary H₂SO₄/HI/I₂/H₂O mixtures were prepared at 308 K with different relative proportions of reactants and the chemical composition of each of the two phases formed was analyzed. An increase in iodine concentration and a decrease in water concentration appeared to improve the liquid–liquid equilibrium phase separation. However, a too low concentration of water also promoted the formation of byproducts. An increase in the [H₂SO₄]/[HI] ratio tended to favor the separation and seemed to lead to a dehydration of the HIx phase.