Araştırma Çıktıları | TR-Dizin | WoS | Scopus | PubMed
https://hdl.handle.net/20.500.12573/393
Research Outputs | TR-Dizin | WoS | Scopus | PubMed2024-03-29T08:36:23ZAnalysis of electret-based vibration energy harvesting devices with curved-beam hinges
https://hdl.handle.net/20.500.12573/2055
Analysis of electret-based vibration energy harvesting devices with curved-beam hinges
Hah, Dooyoung
Recently, vibration energy harvesting devices have gained growing interests. One of the main requirements for them is a broad bandwidth owing to stochastic spectral characteristics of the general vibration sources. Among various approaches for wide bandwidth, curved-beam hinges are quite attractive due to their simple structures. Although there have been several reports on curved beams, a more detailed study is needed. The device under study is an electret-based one with balanced comb-drive configuration. The whole system is modeled by using nonlinear stochastic differential equations. The numerical analysis results show that there is an optimum curve height for maximum power output, which depends on various conditions, such as external vibration strength, comb-drive dimensions, and initial electret charges. At the external acceleration magnitude of 0.02g and 0.05g, the device with curved beams can produce up to 2.9 times and 4.8 times higher power output, respectively, than one with straight beams for given device geometries. To the contrary, at lower and higher vibration magnitudes, straight-beam devices harvest more energy than curved-beam ones. Therefore, it can be concluded that the curved beam height needs to be carefully determined based on the conditions of the application, especially on the characteristics of the external vibration sources.
2023-01-01T00:00:00ZA dynamic connectedness analysis between rare earth prices and renewable energy
https://hdl.handle.net/20.500.12573/2054
A dynamic connectedness analysis between rare earth prices and renewable energy
Madaleno, Mara; Taskin, Dilvin; Dogan, Eyup; Tzeremes, Panayiotis
Current world environmental challenges put pressure on clean energy produced mostly through renewables.
There is an undeniably important role of rare earth minerals in renewable energy technologies. This study aims
to infer the relationship between rare earth, clean energy, renewable energy technologies, and carbon emissions,
focusing on daily stock price index data and applying the novel quantile time-frequency connectedness model,
and the cross-quantilogram dependence approach during 2012–2022. Results show that spillovers among rare
earth minerals and renewable energy are dependent on market conditions, time horizons, and analyzed quantiles. They also highlight the net receiver role of rare earth, especially in the short term. Findings might help
investors understand diversification benefits and support policymakers in developing strategies for lessening
import dependence on rare earth metals, as important as they are for renewable technology adoption to ensure
green growth.
2023-01-01T00:00:00ZOptimization of carbon dioxide absorption in a continuous bubble column reactor using response surface methodology
https://hdl.handle.net/20.500.12573/2053
Optimization of carbon dioxide absorption in a continuous bubble column reactor using response surface methodology
Gul, Ayşe; Derakhshandeh, Masoud; Un, Umran Tezcan
Carbon dioxide absorption using amine based solvents is a well-known approach for carbon dioxide removal. Especially with the increasing concerns about greenhouse gas emissions, there is a need for an optimization approach capable of multifactor calibration and prediction of interactions. Since conventional methods based on empirical relations are not efficiently applicable, this study investigates use of Response Surface Methodology as a strong optimization tool. A bubble column reactor was used and the effect of solvent concentration (10.0, 20.0 and 30.0 vol%), flow rate (4.0, 5.0 and 6.0 L min−1), diffuser pore size (0.5, 1.0 and 1.5 mm) and temperature (20.0, 25.0 and 30.0°C) on the absorption capacity and also overall mass transfer coefficient was evaluated. The optimization results for maintaining maximum capacity and overall mass transfer coefficient revealed that different optimization targets led to different tuned operational factors. Overall mass transfer coefficient decreased to 34.7 min−1 when the maximum capacity was the desired target. High reaction rate along with the highest absorption capacity was set as desirable two factor target in this application. As a result, a third scenario was designed to maximize both mass transfer coefficient and absorption capacity simultaneously. The optimized condition was achieved when a gas flow rate of 5.9 L min−1, MEA solution of 29.6 vol%, diffuser pore size of 0.5 mm and temperature of 20.6°C was adjusted. At this condition, mass transfer coefficient reached a maximum of 38.4 min−1, with a forecasted achievable absorption capacity of 120.5 g CO2 per kg MEA.
2023-01-01T00:00:00ZAttack-Aware Dynamic Upstream Bandwidth Assignment Scheme for Passive Optical Network
https://hdl.handle.net/20.500.12573/2052
Attack-Aware Dynamic Upstream Bandwidth Assignment Scheme for Passive Optical Network
Butt, Rizwan Aslam; FAHEEM, MUHAMMAD; Ashraf, M. Waqar; Khawaja, Attaullah; Raza, Basit
Network security is an important component of today’s networks to combat the security attacks. The passive optical network (PON) works at the medium access layer (MAC). A distributed denial of service (DDOS) attack may be launched from the network and transport layers of an Optical Network unit (ONU). Although there are various security techniques to mitigate its impact, however, these techniques cannot mitigate the impact on the MAC Layer of the PON and can cause an ONU to continuously drain too much bandwidth. This will result in reduced bandwidth availability to other ONUs and, thus, causing an increase in US delays and delay variance. In this work we argue that the impact of a DDOS attack can be mitigated by improving the Dynamic bandwidth assignment (DBA) scheme which is used in PON to manage the US bandwidth at the optical line terminal (OLT). The present DBA schemes do not have the capability to combat a security attack. Thus, this study, uses a machine learning approach to learn the ONU traffic demand patterns and presents a security aware DBA (SA-DBA) scheme that detects a rogue (attacker) ONU from its traffic demand pattern and limits its illegitimate bandwidth demand and only allows it the bandwidth assignment to it as per the agreed service level agreement (SLA). The simulation results show that the SA-DBA scheme results in up to 53%, 55% and 90% reduced US delays and up to 84%, 76% and 95% reduced US delay variance of T2, T3 and T4 traffic classes compared to existing insecure DBA schemes.
2023-01-01T00:00:00Z