148 research outputs found
Soiling of solar collectors – Modelling approaches for airborne dust and its interactions with surfaces
No full textThis literature review deals with the well-known problem of soiling in solar plants, which it severely affects the energy yield of solar power plants. A loss of reflectivity due to soiling reduces the entire productivity of the plant by limiting the energy harvested (i.e. the incoming direct normal irradiance is not properly reflected towards the right focus). On the other hand, the costs of maintenance and cleaning of the collectors represent a significant component of the plant operational costs. Therefore, in this paper, a multi-disciplinary literature review is conducted with the aim of collecting existing models for the key processes, organising them into a ‘dust life cycle’. This cycle is divided into four steps: Generation, Deposition, Adhesion, and Removal; with emphasis on the interaction between dust particles and solar collectors’ surfaces. Generation deals with the loading of atmosphere with dust particles, deposition concerns the processes that actually bring airborne dust onto the collectors’ surface, adhesion and removal represent the competing forces whose balance determine which particles remains adherent on the collectors and which are detached. The intent is to provide a complete framework for the development of a future physical model for the prediction and estimation of the actual soiling of the solar collectors, which engineers can implement in order to maximize the revenues of CSP plant, pushing towards more clean and sustainable energy production technologies
Infrared emission spectrum and potentials of 0(u)(+) and 0(g)(+) states of Xe-2 excimers produced by electron impact
No full textWe present an investigation of the Xe2 excimer emission spectrum observed in
the near-infrared range about 7800 cm−1 in pure Xe gas and in an Ar (90%)–Xe
(10%) mixture for P = 0.1 MPa at T = 300 K obtained by exciting the gas
with energetic electrons. The Franck–Condon simulation of the spectrum shape
suggests that the emission stems from a bound–free molecular transition never
studied before. The states involved are assigned as the bound (3)0+
u state related
to the 6p[1/2]0 atomic limit and the dissociative (1)0+
g state with the 6s[3/2]1
limit. Acomparison with the spectrum simulated by using theoretical potentials
for internuclear distances over which the vibrational eigenfunctions of the
bound state have non-negligible amplitude shows that the dissociative potential
does not reproduce correctly the spectrum features. A best fit, purely repulsive
potential is thus proposed to accurately reproduce the observed spectrum
Environmental influence on the IR fluorescence of Xe-2* molecules in electron beam excited Ar-Xe mixture at high density
No full textWe report new measurements of the near infrared (NIR) Xe2* excimer fluorescence in an electron beam excited Ar (90%) Xe (10 %) mixture at room temperature. Previous measurements up to a density N≈ 2× 1026 m-3 discovered a broad excimer fluorescence band at ≈ 7800 cm-1, whose center is red shifted by increasing N [A.F. Borghesani, G. Bressi, G. Carugno, E. Conti, D. Iannuzzi, J. Chem. Phys. 115, 6042 (2001)]. The shift has been explained by assuming that the energy of the optical active electron in the molecule is shifted by the density dependent Fermi shift and by accounting for the solvation effect due to the environment. We have extended the density range up to N≈ 6× 1026 m-3, confirming the previous measurements and extending the validity of the interpretative model. A detailed analysis of the width of the fluorescence band gives a value of 2.85 nm for the size of the investigated excimer state. Such a large value lends credibility to the validity of the proposed explanation of the experimental findings
Solute - solute - solvent interactions in dilute ternary aqueous solutions of urea and α,ω-aliphatic diols
No full textThe enthalpies of dilution and osmotic coefficients of ternary solutions of urea and α,ω-aliphatic diols in water have been determined at 298 K through microcalorimetric and vapour pressure osmometric measurements, respectively. The experimental virial coefficients of the power expansion series of the excess Gibbs energies and enthalpies have been evaluated and compared with the literature data for similar aqueous systems containing urea and alkanols, ketones and cyclic ethers. The following diols have been considered: propane-diol-1,3; butane-diol-1,4; pentane-diol-1,5. The results are discussed in terms of the hydrophilic and hydrophobic properties of the solutes, and in the light of previous data on the excess thermodynamic properties of binary water-aliphatic diol solutions
Infrared and visible scintillation of Ho3+-doped YAG and YLF crystals
No full textIn our effort to develop a new kind of detector for low-energy, low-rate energy deposition events we have investigated the cathodo- and radioluminescence of Ho:YAG and Ho:YLF single crystals in an extended wavelength range from 200 nm to 2200 nm. The emission spectra of both crystals show a much more intense emission in the infrared range than in the visible one. We estimate an infrared light yield of several tens of photons/keV when exciting the crystals with X-rays of energy ≈ 30 keV. The main reason of this high value is due to the Ho3+ ions energy levels scheme that allows efficient cross relaxation processes to occur even at low dopant concentration
Near-infrared cathodoluminescence of Xe2 excimers in dense gaseous mixtures
No full textWe report new spectroscopic measurements of the near-infrared uorescence
band of electron-beam excited Xe2 excimers as a function of the gas density in
several gaseous mixtures of different Xe concentrations at room temperature. We
have used He, Ar, and N2 as buffer gases, with the Xe concentrations in the
mixtures ranging from approximately 8% up to 80%. The investigated density
range extends up to 10 times the density Nig of the ideal gas at standard
temperature and pressure (Nig ≈ 2.5 × 1025m−3). In all mixtures, we have
observed that the wavelength of the centroid of the infrared excimer band is
shifted toward longer wavelengths as the gas density is increased, in a manner
similar to what was originally observed in pure Xe gas and an Ar–Xe mixture. The
strength of the redshift depends on the Xe concentrations in the mixture and the
nature of the buffer gas and is very well rationalized by taking into account both
the classical dielectric screening effect due to the gaseous environment and the
density-dependent quantum shift of the energy of the optically active electron in
the Xe2 molecule, provided that the contributions of the two gaseous species in
the mixture are weighted by applying the laws of ideal gaseous mixtures
INFLUENCE OF THE DEMAGNETIZATION ON THE POLARIZATION OF THE THERMAL RDIATION EMITTED BY HOT COBALT WIRES
No full textThe polarization P of thermal radiation emitted by a hot cobalt wire in the temperature range from T � 400. K up to melting is studied for the first time. The radiation is linearly polarized perpendicularly to the wire. P decreases from 30 %. just above room temperature down to 6:5 % near melting and is continuous across the martensitic hcp fcc transition at Tm � 700 K and across the Curie point at Tc � 1400K. However, P shows a rapid decrease for T≳1000K and, contrary to previous measurements with tungsten wires, it hysteretically behaves if the temperature change is reversed. This behavior is rationalized by accounting for the thermal demagnetization of the wire with magnetic domain size change
Dilution Enthalpies of Alkanols in Concentrated Aqueous Solutions of Urea at 25° C
No full textEnthalpies of dilution of some aliphatic alcohols were determined at 25°C in aqueous 7M urea solutions by flow microcalorimetry. The excess enthalpies were expressed as power expansion series in molalities referred to 1 kg of constant composition urea-water mixture. This urea-water mixture was utilized throughout as a mixed solvent. The values of the second enthalpic virial coefficients were all found to be positive and generally lower than the corresponding values in water. Large differences were encountered, as in water, by comparing normal and branched isomeric propanols and butanols. For one system it was possible to measure the third coefficients, which were also positive. The second enthalpic coefficients were found to increase with the molecular weight of the alkanols. These facts suggest that in the presence of a large concentration of urea, the excess enthalpies are mainly determined by apolar interactions. This is surprising and potentially rich in consequences for a better understanding of the interactions among amino acid residues distantly situated in the primary sequences but topologically near in the loops of globular proteins. An analysis, carried out using the Savage-Wood additivity group method, shows that the enthalpic contributions (that appear to play a crucial role in water in making the polar interaction to be favorable) become essentially unfavorable in urea-water solvent. The hypothesis that the peptide-peptide interactions are prevented by the preferential solvation of urea is also discussed
Electron thermalization length in solid para-hydrogen at low-temperature
No full textWe report the first ever measurements of the thermalization length of
low-energy electrons injected into solid para-hydrogen at a temperature
K. The use of the pulsed Townsend photoinjection technique
has allowed us to investigate the behavior of quasi-free electrons rather than
of massive, slow negative charges as reported in all previous literature. We
have found an average thermalization length \AA\ which is 3 to 5 times longer than that in liquid helium at the same
temperature.Comment: submitted to and accepted by The Journal of Chemical Physic
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