8,064 research outputs found
Investigation of reliability attributes and accelerated stress factors on terrestrial solar cells
Three tasks were undertaken to investigate reliability attributes of terrestrial solar cells: (1) a study of the electrical behavior of cells in the second (reverse) quadrant; (2) the accelerated stress testing of three new state-of-the-art cells; and (3) the continued bias-temperature testing of four block 2 type silicon cells at 78 C and 135 C. Electrical characteristics measured in the second quadrant were determined to be a function of the cell's thermal behavior with breakdown depending on the initiation of localized heating. This implied that high breakdown cells may be more fault tolerant when forced to operate in the second quadrant, a result contrary to conventional thinking. The accelerated stress tests used in the first (power) quadrant were bias-temperature, bias-temperature-humidity, temperature-humidity, thermal shock, and thermal cycle. The new type cells measured included an EFG cell, a polycrystalline cell, and a Czochralski cell. Significant differences in the response to the various tests were observed between cell types. A microprocessed controlled, short interval solar cell tester was designed and construction initiated on a prototype
Investigation of reliability attributes and accelerated stress factors on terrestrial solar cells
The results of accelerated stress testing of four different types of silicon terrestrial solar cells are discussed. The accelerated stress tests used included bias-temperature tests, bias-temperature-humidity tests, thermal cycle and thermal shock tests, and power cycle tests. Characterization of the cells was performed before stress testing and at periodic down-times, using electrical measurement, visual inspection, and metal adherence pull tests. Electrical parameters measured included short-circuit current, open circuit voltage, and output power, voltage, and current at the maximum power point. Incorporated in the report are the distributions of the prestress electrical data for all cell types. Data were also obtained on cell series and shunt resistance
Discovery of 36 eclipsing EL CVn binaries found by the Palomar Transient Factory
We report the discovery and analysis of 36 new eclipsing EL CVn-type
binaries, consisting of a core helium-composition pre-white dwarf and an
early-type main-sequence companion, more than doubling the known population of
these systems. We have used supervised machine learning methods to search 0.8
million lightcurves from the Palomar Transient Factory, combined with SDSS,
Pan-STARRS and 2MASS colours. The new systems range in orbital periods from
0.46-3.8 d and in apparent brightness from ~14-16 mag in the PTF or
filters. For twelve of the systems, we obtained radial velocity
curves with the Intermediate Dispersion Spectrograph at the Isaac Newton
Telescope. We modelled the lightcurves, radial velocity curves and spectral
energy distributions to determine the system parameters. The radii (0.3-0.7
) and effective temperatures (8000-17000 K) of the
pre-He-WDs are consistent with stellar evolution models, but the masses
(0.12-0.28 ) show more variance than models predicted. This
study shows that using machine learning techniques on large synoptic survey
data is a powerful way to discover substantial samples of binary systems in
short-lived evolutionary stages
Two dimensional dynamical systems which admit Lie and Noether symmetries
We prove two theorems which relate the Lie point symmetries and the Noether
symmetries of a dynamical system moving in a Riemannian space with the special
projective group and the homothetic group of the space respectively. The
theorems are applied to classify the two dimensional Newtonian dynamical
systems, which admit a Lie point/Noether symmetry. Two cases are considered,
the non-conservative and the conservative forces. The use of the results is
demonstrated for the Kepler - Ermakov system, which in general is
non-conservative and for potentials similar to the H\`enon Heiles potential.
Finally it is shown that in a FRW background with no matter present, the only
scalar cosmological model which is integrable is the one for which 3-space is
flat and the potential function of the scalar field is exponential. It is
important to note that in all applications the generators of the symmetry
vectors are found by reading the appropriate entry in the relevant tables.Comment: 25 pages, 17 table
Agile Course Design: Multi-University Faculty Collaboration to Design the MIS Course for an Online MBA Program
This paper outlines a collaborative course design process to develop and implement a required IS course in an online cohort-based MBA program using principles of The Agile Manifesto. The primary goal of this study is to analyze how students in traditionally developed courses and those in collaboratively developed courses differ. Specifically, we sought to reduce variability in student satisfaction across multiple sections offered by instructors who hail from different universities. We compared three semesters of students who took the course before (n = 101) and after (n = 162) use of the agile course development process. We found less variability in student evaluations in the ‘after’ group as compared to the ‘before’ group, providing support that the agile course development process provided a more consistent and similar experience for students. The second goal is to evaluate changes in student evaluations, comparing ‘before’ and ‘after’ groups. We did not expect to see substantial improvement since all instructors already received very high evaluations. Scores for all questions on the student evaluations increased after using the agile process, but the increases were not statistically significant. The final goal is to prepare an agenda for future research on agile course development based on components of The Agile Manifesto that were not used in the course development process. Opportunities include: comparing the agile course development process to other methods; adding more targeted questions to the student survey to better gauge changes in student satisfaction; partnering with alumni, current students, and industry to develop more relevant course material; and extending the process to other contexts
Extreme Mass-Ratio Inspirals in the Effective-One-Body Approach: Quasi-Circular, Equatorial Orbits around a Spinning Black Hole
We construct effective-one-body waveform models suitable for data analysis
with LISA for extreme-mass ratio inspirals in quasi-circular, equatorial orbits
about a spinning supermassive black hole. The accuracy of our model is
established through comparisons against frequency-domain, Teukolsky-based
waveforms in the radiative approximation. The calibration of eight high-order
post-Newtonian parameters in the energy flux suffices to obtain a phase and
fractional amplitude agreement of better than 1 radian and 1 % respectively
over a period between 2 and 6 months depending on the system considered. This
agreement translates into matches higher than 97 % over a period between 4 and
9 months, depending on the system. Better agreements can be obtained if a
larger number of calibration parameters are included. Higher-order mass ratio
terms in the effective-one-body Hamiltonian and radiation-reaction introduce
phase corrections of at most 30 radians in a one year evolution. These
corrections are usually one order of magnitude larger than those introduced by
the spin of the small object in a one year evolution. These results suggest
that the effective-one-body approach for extreme mass ratio inspirals is a good
compromise between accuracy and computational price for LISA data analysis
purposes.Comment: 21 pages, 8 figures, submitted to Phys. Rev.
System development and early biological tests in NASA's biomass production chamber
The Biomass Production Chamber at Kennedy Space Center was constructed to conduct large scale plant growth studies for NASA's CELSS program. Over the past four years, physical systems and computer control software have been continually upgraded and the degree of atmospheric leakage from the chamber has decreased from about 40 to 5 percent of the total volume per day. Early tests conducted with a limited degree of closure showed that total crop (wheat) growth from the best trays was within 80 percent of reported optimal yields for similar light levels. Yields from subsequent tests under more tightly closed conditions have not been as good--up to only 65 percent of optimal yields. Yields appear to have decreased with increasing closure, yet potential problems exist in cultural techniques and further studies are warranted. With the ability to tightly seal the chamber, quantitative data were gathered on CO2 and water exchange rates. Results showed that stand photosynthesis and transpiration reached a peak near 25 days after planting, soon after full vegetative ground cover was established. In the final phase of testing when atmospheric closure was the highest, ethylene gas levels in the chamber rose from about 10 to nearly 120 ppb. Evidence suggests that the ethylene originated from the wheat plants themselves and may have caused an epinastic rolling of the leaves, but no apparent detrimental effects on whole plant function
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