29,443 research outputs found
Projection of distributed-collector solar-thermal electric power plant economics to years 1990-2000
A preliminary comparative evaluation of distributed-collector solar thermal power plants was undertaken by projecting power plant economics of selected systems to the 1990 to 2000 time frame. The selected systems include: (1) fixed orientation collectors with concentrating reflectors and vacuum tube absorbers, (2) one axis tracking linear concentrator including parabolic trough and variable slat designs, and (3) two axis tracking parabolic dish systems including concepts with small heat engine-electric generator assemblies at each focal point as well as approaches having steam generators at the focal point with pipeline collection to a central power conversion unit. Comparisons are presented primarily in terms of energy cost and capital cost over a wide range of operating load factors. Sensitvity of energy costs for a range of efficiency and cost of major subsystems/components is presented to delineate critical technological development needs
Turbulent Mixing in the Outer Solar Nebula
The effects of turbulence on the mixing of gases and dust in the outer Solar
nebula are examined using 3-D MHD calculations in the shearing-box
approximation with vertical stratification. The turbulence is driven by the
magneto-rotational instability. The magnetic and hydrodynamic stresses in the
turbulence correspond to an accretion time at the midplane about equal to the
lifetimes of T Tauri disks, while accretion in the surface layers is thirty
times faster. The mixing resulting from the turbulence is also fastest in the
surface layers. The mixing rate is similar to the rate of radial exchange of
orbital angular momentum, so that the Schmidt number is near unity. The
vertical spreading of a trace species is well-matched by solutions of a damped
wave equation when the flow is horizontally-averaged. The damped wave
description can be used to inexpensively treat mixing in 1-D chemical models.
However, even in calculations reaching a statistical steady state, the
concentration at any given time varies substantially over horizontal planes,
due to fluctuations in the rate and direction of the transport. In addition to
mixing species that are formed under widely varying conditions, the turbulence
intermittently forces the nebula away from local chemical equilibrium. The
different transport rates in the surface layers and interior may affect
estimates of the grain evolution and molecular abundances during the formation
of the Solar system.Comment: To appear in the Astrophysical Journal; 20 pages, 9 figure
Structure of deformed silicon and implications for low cost solar cells
The microstructure and minority carrier lifetime of silicon were investigated in uniaxially compressed silicon samples. The objective of the investigation was to determine if it is feasible to produce silicon solar cells from sheet formed by high temperature rolling. The initial structure of the silicon samples ranged from single crystal to fine-grained polycrystals. The samples had been deformed at strain rates of 0.1 to 8.5/sec and temperatures of 1270-1380 C with subsequent annealing at 1270-1380 C. The results suggest that high temperature rolling of silicon to produce sheet for cells of high efficiency is not practical
Palaeolimnological assessment of trace element inputs to lakes in the Athabasca Oil Sands Region, Alberta, Canada
Undisturbed lake sediment records provide a robust natural archive of conditions within
waterbodies. They have been used successfully over a number of decades to determine
temporal trends of surface water acidification and to follow the effects of eutrophication.
However, lake sediments also provide an archive of changes occurring within lake catchments
and of atmospheric pollutants deposited onto lake and catchment surfaces. In August 2006,
parallel lake sediment cores were collected from 22 lakes in the Athabasca Oil Sands Region of
Alberta by UCL staff as part of the RAMP regional lakes survey. Twelve of these lakes were
selected for study covering a range of locations from around the Fort McMurray area to sites in
the Caribou Mountains and the Canadian Shield. The main aim of this initial study was to
assess the evidence for lake acidification in the region, but analysis also revealed changes in
nutrient input and, at one site, mercury (Hg) analysis showed an indication of industrial
contamination. This work was reported in Curtis et al. (2010).
The analysis undertaken in this initial project was focussed on single radiometrically dated
sediment cores (hereafter the ‘A’ cores) from each of the 12 selected lakes. The parallel cores
from each lake (the ‘B’ cores) were stored dark, at 4ºC, following their transfer to UCL and
hence were available for further analysis. Both sediment cores (A and B) from the remaining 10
lakes remain unstudied.
The aim of this current project was to use the stored ‘B’ sediment cores to assess temporal
trends and rates of change in trace element input to a subset of the lakes cored in the
Athabasca Oil Sands Region of Alberta and compare these with the sediment records of two
reference lakes in the Caribou Mountains.
This interim report contains details of progress on this work up to end March 2012 and a
summary of remaining work under this contract. As a consequence this report focusses on data
collected so far. Only limited interpretation is provided and will be undertaken fully when the
dataset is complete. A final report will be produced upon completion of the study
Making a financial time machine:a multitouch application to enable interactive 3-D visualization of distant savings goals
Financial planning and decision making for the general public continues to vex and perplex in equal measure. Whilst the tools presented by a typical desktop computer should make the task easier, the recent financial crisis confirms the increasing difficulty that people have in calculating the benefits of deferring consumption for future gains (i.e. Saving). We present an interactive concept demonstration for Microsoft SurfaceTM that tackles two of the key barriers to saving decision making. Firstly we show an interface that avoid the laborious writing down or inputting of data and instead embodies the cognitive decision of allocation of resources in a physical gesture based interface, where the scale of the investment or expenditure correlates with the scale of the gesture. Second we show how a fast-forward based animation can demonstrate the impact of small increments in savings to a long term savings goal in a strategy game-based, interactive format. The platform uses custom software (XNATM format) as opposed to the more usual WPFTM format found on Surface applications. This enables dynamic 3-D graphical icons to be used to maximize the interactive appeal of the interface. Demonstration and test trial feedback indicates that this platform can be adapted to suit the narrative of individual purchasing decisions to inform educate diverse user groups about the long term consequences of small financial decisions
Predicted efficiency of Si wire array solar cells
Solar cells based on arrays of CVD-grown Si nano- or micro-wires have attracted interest as potentially low-cost alternatives to conventional wafer-based Si photovoltaics [1-6], and single-wire solar cells have been reported with efficiencies of up to 3.4% [7]. We recently presented device physics simulations which predicted efficiencies exceeding 17%, based on experimentally observed diffusion lengths within our wires [8]. However, this model did not take into account the optical properties of a wire array device - in particular the inherently low packing fraction of wires within CVD-grown wire arrays, which might limit their ability to fully absorb incident sunlight. For this reason, we have combined a device physics model of Si wire solar cells with FDTD simulations of light absorption within wire arrays to investigate the potential photovoltaic efficiency of this cell geometry. We have found that even a sparsely packed array (14%) is expected to absorb moderate (66%) amounts of above-bandgap solar energy, yielding a simulated photovoltaic efficiency of 14.5%. Because the wire array comprises such a small volume of Si, the observed absorption represents an effective optical concentration, which enables greater operating voltages than previously predicted for Si wire array solar cells
Turbulence and Steady Flows in 3D Global Stratified MHD Simulations of Accretion Disks
We present full 2 Pi global 3-D stratified MHD simulations of accretion
disks. We interpret our results in the context of proto-planetary disks. We
investigate the turbulence driven by the magneto-rotational instability (MRI)
using the PLUTO Godunov code in spherical coordinates with the accurate and
robust HLLD Riemann solver. We follow the turbulence for more than 1500 orbits
at the innermost radius of the domain to measure the overall strength of
turbulent motions and the detailed accretion flow pattern. We find that regions
within two scale heights of the midplane have a turbulent Mach number of about
0.1 and a magnetic pressure two to three orders of magnitude less than the gas
pressure, while outside three scale heights the magnetic pressure equals or
exceeds the gas pressure and the turbulence is transonic, leading to large
density fluctuations. The strongest large-scale density disturbances are spiral
density waves, and the strongest of these waves has m=5. No clear meridional
circulation appears in the calculations because fluctuating radial pressure
gradients lead to changes in the orbital frequency, comparable in importance to
the stress gradients that drive the meridional flows in viscous models. The net
mass flow rate is well-reproduced by a viscous model using the mean stress
distribution taken from the MHD calculation. The strength of the mean turbulent
magnetic field is inversely proportional to the radius, so the fields are
approximately force-free on the largest scales. Consequently the accretion
stress falls off as the inverse square of the radius.Comment: Accepted for publication in Ap
Photon Bubbles in the Circumstellar Envelopes of Young Massive Stars
We show that the optically-thick dusty envelopes surrounding young high-mass
stars are subject to the photon bubble instability. The infrared radiation
passing through the envelope amplifies magnetosonic disturbances, with growth
rates in our local numerical radiation MHD calculations that are consistent
with a linear analysis. Modes with wavelengths comparable to the gas pressure
scale height grow by more than two orders of magnitude in a thousand years,
reaching non-linear amplitudes within the envelope lifetime. If the magnetic
pressure in the envelope exceeds the gas pressure, the instability develops
into trains of propagating shocks. Radiation escapes readily through the
low-density material between the shocks, enabling accretion to continue despite
the Eddington limit imposed by the dust opacity. The supersonic motions arising
from the photon bubble instability can help explain the large velocity
dispersions of hot molecular cores, while conditions in the shocked gas are
suitable for maser emission. We conclude that the photon bubble instability may
play a key role in the formation of massive stars.Comment: Accepted by the Astrophysical Journal; 18 pages, 4 figure
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