315 research outputs found
Hybrid Power System Topology and Energy Management Scheme Design for Hydrogen-Powered Aircraft
The electrification of the aviation industry is a major challenge to realizing net-zero in the global energy sector. Fuel cell (FC) hybrid electric aircraft (FCHEV) demonstrate remarkable competitiveness in terms of cruise range and total economy. However, the process of simply hybridizing different power supplies together does not lead to an improvement in the aircraft economy, since a carefully designed power system topology and energy management scheme are also necessary to realize the full benefit of FCHEV. This paper provides a new approach towards the configuration of the optimal power system and proposes a novel energy management scheme for FCHEA. Firstly, four different topologies of aircraft power systems are designed to facilitate flexible power flow control and energy management. Then, an equivalent model of aircraft hydrogen consumption is formulated by analyzing the FC efficiency, FC aging, and BESS aging. Using the newly established model, the performance of aircraft can be quantitatively evaluated in detail to guide FCHEA design. The optimal aircraft energy management is realized by establishing a mathematical optimization model with the reduction of hydrogen consumption and aging costs as objectives. An experimental aircraft, NASA X-57 Maxwell, is used to provide a detailed performance evaluation of different power system topologies and validate the effectiveness of the energy management scheme. The new approach represents a guide for future power system design and energy management of electric aircraft.</p
Unsupervised Anomaly Detection via Variational Auto-Encoder for Seasonal KPIs in Web Applications
To ensure undisrupted business, large Internet companies need to closely
monitor various KPIs (e.g., Page Views, number of online users, and number of
orders) of its Web applications, to accurately detect anomalies and trigger
timely troubleshooting/mitigation. However, anomaly detection for these
seasonal KPIs with various patterns and data quality has been a great
challenge, especially without labels. In this paper, we proposed Donut, an
unsupervised anomaly detection algorithm based on VAE. Thanks to a few of our
key techniques, Donut greatly outperforms a state-of-arts supervised ensemble
approach and a baseline VAE approach, and its best F-scores range from 0.75 to
0.9 for the studied KPIs from a top global Internet company. We come up with a
novel KDE interpretation of reconstruction for Donut, making it the first
VAE-based anomaly detection algorithm with solid theoretical explanation.Comment: 12 pages (including references), 17 figures, submitted to WWW 2018:
The 2018 Web Conference, April 23--27, 2018, Lyon, France. The contents
discarded from the conference version due to the 9-page limitation are also
included in this versio
Prospects for detection rate of very-high-energy {\gamma}-ray emissions from short {\gamma}-ray bursts with the HADAR experiment
The observation of short gamma ray bursts (SGRBs) in the TeV energy range
plays an important role in understanding the radiation mechanism and probing
new areas of physics such as Lorentz invariance violation. However, no SGRB has
been observed in this energy range due to the short duration of SGRBs and the
weakness of current experiments. New experiments with new technology are
required to detect sub-TeV SGRBs. In this work, we observe the very high energy
(VHE) -ray emissions from SGRBs and calculate the annual detection rate
with the High Altitude Detection of Astronomical Radiation HADAR (HADAR)
experiment. First, a set of pseudo-SGRB samples is generated and checked using
the observations of Fermi-GBM, Fermi-LAT, and SWIFT measurements. The annual
detection rate is calculated from these SGRB samples based on the performance
of the HADAR instrument. As a result, the HADAR experiment can detect 0.5 SGRB
per year if the spectral break-off of -rays caused by the internal
absorption is larger than 100 GeV. For a GRB09010-like GRB in HADAR's view, it
should be possible to detect approximately 2000 photons considering the
internal absorption. With a time delay assumption due to the Lorentz invariance
violation effects, a simulated light curve of GRB090510 has evident energy
dependence. We hope that the HADAR experiment can perform the SGRB observations
and test our calculations in the future
The mHz quasi-regular modulations of 4U 1630--47 during its 1998 outburst
We present the results of a detailed timing and spectral analysis of the
quasi-regular modulation (QRM) phenomenon in the black hole X-ray binary 4U
1630--47 during its 1998 outburst observed by Rossi X-ray Timing Explore
(RXTE). We find that the 50-110 mHz QRM is flux dependent, and the QRM
is detected with simultaneous low frequency quasi-periodic oscillations
(LFQPOs). According to the behavior of the power density spectrum, we divide
the observations into four groups. In the first group, namely behavior A,
LFQPOs are detected, but no mHz QRM. The second group, namely behavior B, a QRM
with frequency above 88 mHz is detected and the 5 Hz and 7
Hz LFQPOs are almost overlapping. In the third group, namely behavior C, the
QRM frequency below 88 mHz is detected and the LFQPOs are significantly
separated. In the forth group, namely behavior D, neither QRM nor LFQPOs are
detected. We study the energy-dependence of the fractional rms, centroid
frequency, and phase-lag of QRM and LFQPOs for behavior B and C. We then study
the evolution of QRM and find that the frequency of QRM increases with
hardness, while its rms decreases with hardness. We also analyze the spectra of
each observation, and find that the QRM rms of behavior B has a positive
correlation with / . Finally, we give
our understanding for this mHz QRM phenomena.Comment: 14pages, 15 figure
Effect of working fluids on the performance of phase change material storage based direct vapor generation solar organic Rankine cycle system
Working fluids can play a critical role in the working of an organic Rankine cycle system. A direct vapor generation solar organic Rankine cycle embedded with phase change material storage is analyzed in this study. The system comprised of an array of evacuated flat plate collectors, phase change material based thermal storage, expander, condenser, and organic working fluid pump. The storage tank model is modeled using a finite difference method in MATLAB programming environment while the 1D model of ORC system is used to evaluate the system performance. After a careful screen, 12 dry and isentropic working fluids were selected and their impact on the performance of the heat storage tank and the overall system is evaluated. The results show that the system efficiencies increase and decrease with the increment and decrement in the critical temperature of the working fluid. Moreover, the rise and fall of working fluid temperature, phase change material temperature, and the quantity of energy stored and released generally increase with an increase in the critical temperature of the working fluid. At the evaporation temperature of 10 Β°C higher and lower than the melting point temperature of the phase change material, Benzene has achieved the highest system efficiencies of 10.7% & 10.4% during charging and discharging mode, respectively. However, the maximum the rise and fall of working fluid temperature, phase change material temperature, and the quantity of energy stored and released during charging and discharging mode is attained by Heptane which is found to be 5.35 Β°C & 7.34 Β°C, 0.48 Β°C & 0.44 Β°C and 13.81 MJ & 23.04 MJ, respectively. Heptane has shown overall best performance among the selected working fluids and found to be feasible for phase change material storage based direct vapor generation solar ORC system
Synthesis and catalysis of chemically reduced metalβmetalloid amorphous alloys
This is the published version. Copyright 2012 Royal Society of ChemistryAmorphous alloys structurally deviate from crystalline materials in that they possess unique short-range ordered and long-range disordered atomic arrangement. They are important catalytic materials due to their unique chemical and structural properties including broadly adjustable composition, structural homogeneity, and high concentration of coordinatively unsaturated sites. As chemically reduced metalβmetalloid amorphous alloys exhibit excellent catalytic performance in applications such as efficient chemical production, energy conversion, and environmental remediation, there is an intense surge in interest in using them as catalytic materials. This critical review summarizes the progress in the study of the metalβmetalloid amorphous alloy catalysts, mainly in recent decades, with special focus on their synthetic strategies and catalytic applications in petrochemical, fine chemical, energy, and environmental relevant reactions. The review is intended to be a valuable resource to researchers interested in these exciting catalytic materials. We concluded the review with some perspectives on the challenges and opportunities about the future developments of metalβmetalloid amorphous alloy catalysts
Flexible transit routing model considering passengersβ willingness to pay
This paper proposes an alternative flexible transit model with two levels of bus stops, A level and B level. A-level bus stops are fixed, while B-level bus stops are flexible and provide service only when passengers indicate a strong willingness to pay (WTP). This fare structure encourages passengers to choose bus stops with their mobile phones or computers. An optimization model of 0-1 integer-programming is formulated based on whether certain B-level stops can be serviced. With a numerical example, we compare the performance of the proposed traversing method and a tabu search algorithm, both of which are adapted to solve the model. Finally, a real case is provided to evaluate the proposed transit system against comparable systems (e.g., a fixed-route transit system and a taxi service), and the result shows that the flexible transit routing model will help both passengers and bus companies, thus creating a win-win situation
Bioinformatics in China: A Personal Perspective
Biochemical Research MethodsMathematical & Computational BiologySCI(E)PubMed3EDITORIAL MATERIAL4e1000020
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