46 research outputs found
Electronic damping of molecular motion at metal surfaces
A method for the calculation of the damping rate due to electron-hole pair
excitation for atomic and molecular motion at metal surfaces is presented. The
theoretical basis is provided by Time Dependent Density Functional Theory
(TDDFT) in the quasi-static limit and calculations are performed within a
standard plane-wave, pseudopotential framework. The artificial periodicity
introduced by using a super-cell geometry is removed to derive results for the
motion of an isolated atom or molecule, rather than for the coherent motion of
an ordered over-layer. The algorithm is implemented in parallel, distributed
across both and space, and in a form compatible with the
CASTEP code. Test results for the damping of the motion of hydrogen atoms above
the Cu(111) surface are presented.Comment: 10 pages, 3 figure
Comparison of pyranometers vs. PV reference cells for evaluation of PV array performance
As the photovoltaics (PV) industry has grown, the need for accurately monitoring the solar resource of PV power plants has increased. Historically, the PV industry has relied on thermopile pyranometers for irradiance measurements, and a large body of historical irradiance data taken with pyranometers exists. However, interest in PV reference devices is increasing. In this paper, we discuss why PV reference devices are better suited for PV applications, and estimate the typical uncertainties in irradiance measurements made with both pyranometers and PV reference devices. We assert that the quantity of interest in monitoring a PV power plant is the equivalent irradiance under the IEC 60904-3 reference solar spectrum that would produce the same electrical response in the PV array as the incident solar radiation. For PV-plant monitoring applications, we find the uncertainties in irradiance measurements of this type to be on the order of +/-5% for thermopile pyranometers and +/-2.4% for PV reference devices
Phase-controlled, heterodyne laser-induced transient grating measurements of thermal transport properties in opaque material
The methodology for a heterodyned laser-induced transient thermal grating
technique for non-contact, non-destructive measurements of thermal transport in
opaque material is presented. Phase-controlled heterodyne detection allows us
to isolate pure phase or amplitude transient grating signal contributions by
varying the relative phase between reference and probe beams. The phase grating
signal includes components associated with both transient reflectivity and
surface displacement whereas the amplitude grating contribution is governed by
transient reflectivity alone. By analyzing the latter with the two-dimensional
thermal diffusion model, we extract the in-plane thermal diffusivity of the
sample. Measurements on a 5 {\mu}m thick single crystal PbTe film yielded
excellent agreement with the model over a range of grating periods from 1.6 to
2.8 {\mu}m. The measured thermal diffusivity of 1.3 \times 10-6 m2/s was found
to be slightly lower than the bulk value.Comment: 19 pages, 6 figure
Effect of bending test on the performance of CdTe solar cells on flexible ultra-thin glass produced by MOCVD
The development of lightweight and flexible solar modules is highly desirable for high specific power applications, building integrated photovoltaics, unmanned aerial vehicles and space. Flexible metallic and polyimide foils are frequently used, but in this work an alternative substrate with attractive properties, ultra-thin glass (UTG) has been employed. CdTe solar cells with average efficiency reaching 14.7% AM1.5G efficiency have been produced on UTG of 100 ÎĽm thickness. Little has been reported on the effects on PV performance when flexed, so we investigated the effects on J-V parameters when the measurements were performed in 40 mm and 32 mm bend radius, and in a planar state before and after the bend curvature was applied. The flat J-V measurements after 32 mm bending test showed some improvement in efficiency, Voc and FF, with values higher than the first measurement in a planar state. In addition, two CdTe solar cells with identical initial performance were subjected to 32 mm static bending test for 168 hours, the results showed excellent uniformity and stability and no significant variation on J-V parameters was observed. External quantum efficiency and capacitance voltage measurements were performed and showed no significant change in spectral response or carrier concentration. Residual stress analysis showed that no additional strain was induced within the film after the bending test and that the overall strain was low. This has demonstrated the feasibility of using CdTe solar cells on UTG in new applications, when a curved module is required without compromising performance
Ab initio Quantum and ab initio Molecular Dynamics of the Dissociative Adsorption of Hydrogen on Pd(100)
The dissociative adsorption of hydrogen on Pd(100) has been studied by ab
initio quantum dynamics and ab initio molecular dynamics calculations. Treating
all hydrogen degrees of freedom as dynamical coordinates implies a high
dimensionality and requires statistical averages over thousands of
trajectories. An efficient and accurate treatment of such extensive statistics
is achieved in two steps: In a first step we evaluate the ab initio potential
energy surface (PES) and determine an analytical representation. Then, in an
independent second step dynamical calculations are performed on the analytical
representation of the PES. Thus the dissociation dynamics is investigated
without any crucial assumption except for the Born-Oppenheimer approximation
which is anyhow employed when density-functional theory calculations are
performed. The ab initio molecular dynamics is compared to detailed quantum
dynamical calculations on exactly the same ab initio PES. The occurence of
quantum oscillations in the sticking probability as a function of kinetic
energy is addressed. They turn out to be very sensitive to the symmetry of the
initial conditions. At low kinetic energies sticking is dominated by the
steering effect which is illustrated using classical trajectories. The steering
effects depends on the kinetic energy, but not on the mass of the molecules.
Zero-point effects lead to strong differences between quantum and classical
calculations of the sticking probability. The dependence of the sticking
probability on the angle of incidence is analysed; it is found to be in good
agreement with experimental data. The results show that the determination of
the potential energy surface combined with high-dimensional dynamical
calculations, in which all relevant degrees of freedon are taken into account,
leads to a detailed understanding of the dissociation dynamics of hydrogen at a
transition metal surface.Comment: 15 pages, 9 figures, subm. to Phys. Rev.
Incidence angle and diffuse radiation adaptation of soiling ratio measurements of indirect optical soiling sensors
Comparison of Pyranometers vs. PV Reference Cells for Evaluation of PV Array Performance
Abstract -As the photovoltaics (PV) industry has grown, the need for accurately monitoring the solar resource of PV power plants has increased. Historically, the PV industry has relied on thermopile pyranometers for irradiance measurements, and a large body of historical irradiance data taken with pyranometers exists. However, interest in PV reference devices is increasing. In this paper, we discuss why PV reference devices are better suited for PV applications, and estimate the typical uncertainties in irradiance measurements made with both pyranometers and PV reference devices. We assert that the quantity of interest in monitoring a PV power plant is the equivalent irradiance under the IEC 60904-3 reference solar spectrum that would produce the same electrical response in the PV array as the incident solar radiation. For PV-plant monitoring applications, we find the uncertainties in irradiance measurements of this type to be on the order of ±5% for thermopile pyranometers and ±2.4% for PV reference devices
Comparison of Pyranometers vs. PV Reference Cells for Evaluation of PV Array Performance
Abstrac
Scattering of vibrationally and electronically excited co molecules from a lif(100) surface
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