1,517 research outputs found
Techniques to Measure Solar Flux Density Distribution on Large-Scale Receivers
Flux density measurement applied to central receiver ystems delivers the spatial distribution of the concentrated solar radiation on the receiver aperture, measures receiver
input power, and monitors and might control heliostat aimpoints. Commercial solar tower plants have much larger aperture surfaces than the receiver prototypes tested in
earlier research and development (R&D) projects. Existing methods to measure the solar flux density in the receiver aperture face new challenges regarding the receiver size.
Also, the requirements regarding costs, accuracy, spatial resolution, and measuring speed are different. This paper summarizes existent concepts, presents recent research
results for techniques that can be applied to large-scale receivers and assesses them against a catalog of requirements. Direct and indirect moving bar techniques offer high measurement accuracy, but also have the disadvantage of large moving parts on a solar tower. In the case of external receivers, measuring directly on receiver surfaces avoids moving parts and allows continuous measurement but may be not as precise. This promising
technique requires proper scientific evaluation due to specific reflectance properties of current receiver materials. Measurement-supported simulation techniques can also be
applied to cavity receivers without installing moving parts. They have reasonable uncertainties under ideal conditions and require comparatively low effort
Heliostat Testing according to SolarPACES Task III Guideline
The SolarPACES Guideline for Heliostat Performace Testing finally provides a solid base for standardized
testing and comparison as well as the definitions of essential heliostat parameters such as slope and tracking errors. SBPS is running an extensive test program for their 4 Stellio preseries heliostats at the DLR Solar Tower in Juelich, Germany until summer 2019. Additional objective is to accumulate operating hours and evaluate long-term effects on the Stellio performance quality. Slope error measurement has been performed by CSPS and is repeated every 3 months. First results show 1D slope errors of 0.7 to 1.2 mrad. Tracking performance could not have been concluded due to missing final measurements of the kinematic system of each heliostat which is necessary for calibration. However, beam centroid evaluation software has been tested with first uncalibrated tracking hours and is prepared for normal operation. First
photogrammetric measurements have been performed to characterize the dead weight deflection of the heliostat in 15 different azimuth and elevation combinations. This has been prepared and implemented in Rhino CAD. Adaptions may be necessary to include pylon deflection as well
From Research to Industry: Development of a High-Resolution Measurement System for Mirrored Heliostats in Series Production
A new measurement system able to measure the shape accuracy of complete mirrored heliostat modules in series production was developed. The applied deflectometry measurement method is based on the reflection of regular
patterns in the mirror surface and their distortions due to mirror surface deviations. The measurement system’s key
features are its high spatial resolution, its low global measurement uncertainty of less than 0.2 mrad and its total measurement and evaluation time of few minutes. The measurement process is contact-free and completely automatic, which allows a 100% optical quality control of the production of the heliostat modules for a typical solar tower power plant. The system is validated by measuring a flat reference surface and by comparison to manual photogrammetry measurements. This makes the new measurement system, called QDec-M-Helio, a valuable tool for final geometric quality control of heliostats in series production, especially for new generation heliostats which obtain its final curvature during a bonding process between mirror and support structure from a precise jig
Probing Intermediate Mass Black Holes With Optical Emission Lines from Tidally Disrupted White Dwarfs
We calculate the emission line spectrum produced by the debris released when
a white dwarf (WD) is tidally disrupted by an intermediate-mass black hole
(IMBH; M\sim 10^{2}-10^{5}\msun) and we explore the possibility of using the
emission lines to identify such events and constrain the properties of the
IMBH. To this end, we adopt and adapt the techniques developed by Strubbe &
Quataert to study the optical emission lines produced when a main sequence (MS)
star is tidally disrupted by a supermassive black hole. WDs are tidally
disrupted outside of the event horizon of a < 10^{5}\msun black hole, which
makes these tidal disruption events good signposts of IMBHs. We focus on the
optical and UV emission lines produced when the accretion flare photoionizes
the stream of debris that remains unbound during the disruption. We find that
the spectrum is dominated by lines due to ions of C and O, the strongest of
which are \ion{C}{4} 1549 at early times and [\ion{O}{3}]
5007 at later times. Furthermore, we model the profile of the emission
lines in the [\ion{O}{3}] 4959, 5007 doublet and find that it
is highly asymmetric with velocity widths of up to , depending on the properties of the WD-IMBH system and the
orientation of the observer. Finally, we compare the models with observations
of X-ray flares and optical emission lines in the cores of globular clusters
and propose how future observations can test if these features are due to a WD
that has been tidally disrupted by an IMBH.Comment: 19 pages, 9 figures, accepted for publication in The Astrophysical
Journa
Fast calibration of heliostats
We present the HelioPoint method - a fast airborne method for calibrating entire heliostat fields
An ultraviolet-optical flare from the tidal disruption of a helium-rich stellar core
The flare of radiation from the tidal disruption and accretion of a star can
be used as a marker for supermassive black holes that otherwise lie dormant and
undetected in the centres of distant galaxies. Previous candidate flares have
had declining light curves in good agreement with expectations, but with poor
constraints on the time of disruption and the type of star disrupted, because
the rising emission was not observed. Recently, two `relativistic' candidate
tidal disruption events were discovered, each of whose extreme X-ray luminosity
and synchrotron radio emission were interpreted as the onset of emission from a
relativistic jet. Here we report the discovery of a luminous
ultraviolet-optical flare from the nuclear region of an inactive galaxy at a
redshift of 0.1696. The observed continuum is cooler than expected for a simple
accreting debris disk, but the well-sampled rise and decline of its light curve
follows the predicted mass accretion rate, and can be modelled to determine the
time of disruption to an accuracy of two days. The black hole has a mass of
about 2 million solar masses, modulo a factor dependent on the mass and radius
of the star disrupted. On the basis of the spectroscopic signature of ionized
helium from the unbound debris, we determine that the disrupted star was a
helium-rich stellar core.Comment: To appear in Nature on May 10, 201
Luminous Thermal Flares from Quiescent Supermassive Black Holes
A dormant supermassive black hole lurking in the center of a galaxy will be
revealed when a star passes close enough to be torn apart by tidal forces, and
a flare of electromagnetic radiation is emitted when the bound fraction of the
stellar debris falls back onto the black hole and is accreted. Here we present
the third candidate tidal disruption event discovered in the GALEX Deep Imaging
Survey: a 1.6x10^{43} erg s^{-1} UV/optical flare from a star-forming galaxy at
z=0.1855. The UV/optical SED during the peak of the flare measured by GALEX and
Palomar LFC imaging can be modeled as a single temperature blackbody with
T_{bb}=1.7x10^{5} K and a bolometric luminosity of 3x10^{45} erg s^{-1},
assuming an internal extinction with E(B-V)_{gas}=0.3. The Chandra upper limit
on the X-ray luminosity during the peak of the flare, L_{X}(2-10 keV)< 10^{41}
erg s^{-1}, is 2 orders of magnitude fainter than expected from the ratios of
UV to X-ray flux density observed in active galaxies. We compare the light
curves and broadband properties of all three tidal disruption candidates
discovered by GALEX, and find that (1) the light curves are well fitted by the
power-law decline expected for the fallback of debris from a tidally disrupted
solar-type star, and (2) the UV/optical SEDs can be attributed to thermal
emission from an envelope of debris located at roughly 10 times the tidal
disruption radius of a ~10^{7} M_sun central black hole. We use the observed
peak absolute optical magnitudes of the flares (-17.5 > M_{g} > -18.9) to
predict the detection capabilities of upcoming optical synoptic surveys.
(Abridged)Comment: Accepted for Publication in ApJ, 19 pages, 10 figures, 2 tables,
emulateapj, corrections from proofs adde
Dark Matter Candidates: A Ten-Point Test
An extraordinarily rich zoo of non-baryonic Dark Matter candidates has been
proposed over the last three decades. Here we present a 10-point test that a
new particle has to pass, in order to be considered a viable DM candidate: I.)
Does it match the appropriate relic density? II.) Is it {\it cold}? III.) Is it
neutral? IV.) Is it consistent with BBN? V.) Does it leave stellar evolution
unchanged? VI.) Is it compatible with constraints on self-interactions? VII.)
Is it consistent with {\it direct} DM searches? VIII.) Is it compatible with
gamma-ray constraints? IX.) Is it compatible with other astrophysical bounds?
X.) Can it be probed experimentally?Comment: 29 pages, 12 figure
SolarPACES Task III Project: Analyze Heliostat Field
In recent years, great efforts have been made to reach a consensus on heliostat testing best practices. A specific SolarPACES task was launched to provide a Heliostat Testing Guidelines document for single heliostat evaluation with a focus on prototype validation and qualification. Such guidelines are not well-suited for heliostat evaluation in operating commercial heliostat fields. The commercial implementation of the Central Receiver technology is burdened by the lack of a demonstrated cost-effective methodology to test solar fields, particularly during the commissioning and operation phases of the plant. To address heliostat characterization challenges, the SolarPACES funded Project Analyze
Heliostat Field aims to set the basis towards a SolarPACES guideline for Heliostat Field Performance testing under a common framework. This is by means of a review of the existing methodologies, R&D and industrial stakeholders information sharing and preparation of a future quantitative comparison and validation plan. As part of the development of this project, several meetings and a workshop involving the SolarPACES community was organized to share knowledge and experience in the measurement and characterization of heliostat fields using a range of technologies and
procedures. Research centers and companies from 5 different and distant countries have actively participated in these meetings, sharing their experiences, needs and interests. This paper summarizes the outcome of this international collaborative effort and the prospects for future close collaborations sustained over time
SolarPACES Guideline for Heliostat Performance Testing - Release v1.0
Based on national drafts, a group of R&D and industry experts as members of the SolarPACES task III heliostat working group has been working since 2012 on the creation of a guideline for heliostat performance testing. It contains a well-defined list of parameters to describe heliostats and their performance, as well as a list for deriving these parameters. After applying the draft to several industrial and research heliostats (e.g. [1]) and iterative improvements, version 1.0 of the guideline has been released
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