Lifetime-aware cloud data centers: models and performance evaluation

We present a model to evaluate the server lifetime in cloud data centers (DCs). In particular, when the server power level is decreased, the failure rate tends to be reduced as a consequence of the limited number of components powered on. However, the variation between the different power states triggers a failure rate increase. We therefore consider these two effects in a server lifetime model, subject to an energy-aware management policy. We then evaluate our model in a realistic case study. Our results show that the impact on the server lifetime is far from negligible. As a consequence, we argue that a lifetime-aware approach should be pursued to decide how and when to apply a power state change to a server

The Untold Story of the Clones: Content-agnostic Factors that Impact YouTube Video Popularity

Video dissemination through sites such as YouTube can have widespread impacts on opinions, thoughts, and cultures. Not all videos will reach the same popularity and have the same impact. Popularity differences arise not only because of differences in video content, but also because of other "content-agnostic" factors. The latter factors are of considerable interest but it has been difficult to accurately study them. For example, videos uploaded by users with large social networks may tend to be more popular because they tend to have more interesting content, not because social network size has a substantial direct impact on popularity. In this paper, we develop and apply a methodology that is able to accurately assess, both qualitatively and quantitatively, the impacts of various content-agnostic factors on video popularity. When controlling for video content, we observe a strong linear "rich-get-richer" behavior, with the total number of previous views as the most important factor except for very young videos. The second most important factor is found to be video age. We analyze a number of phenomena that may contribute to rich-get-richer, including the first-mover advantage, and search bias towards popular videos. For young videos we find that factors other than the total number of previous views, such as uploader characteristics and number of keywords, become relatively more important. Our findings also confirm that inaccurate conclusions can be reached when not controlling for content.Comment: Dataset available at: http://www.ida.liu.se/~nikca/papers/kdd12.htm

Sensitivity of solar cells

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Pendekatan psikologi dakwah menangani histeria remaja muslim di Selangor

Gejala histeria dalam kalangan remaja di Malaysia merupakan epidemik sosial yang telah lama berlaku dalam masyarakat. Kejadian tersebut mengganggu proses pengajaran dan pembelajaran, psikologi, jasmani dan spiritual remaja. Pelbagai pendekatan pencegahan dilakukan oleh remaja dan pihak sekolah dalam menangani gejala yang berlaku, namun tidak menunjukkan pengurangannya. Oleh itu kajian ini bertujuan mengenal pasti konsep histeria, menganalisis faktor dan simptom histeria remaja, mengenal pasti pendekatan psikologi dakwah untuk menangani histeria, mengkaji kaedah pencegahan yang dilakukan remaja terhadap gejala histeria serta menganalisis hubungan antara faktor, simptom dan langkah pencegahan. Reka bentuk kajian adalah kajian tinjauan dengan menggunakan instrumen soal selidik dan temu bual. Seramai 122 orang responden yang mengalami histeria dari 10 buah sekolah di Selangor dipilih secara persampelan kelompok dan seramai 9 orang pelajar yang mengalami histeria ditemu bual. Temu bual turut dijalankan terhadap kaunselor, pakar psikiatri dan perawat perubatan Islam bagi menyokong dapatan kajian. Hasil kajian mendapati terdapat lima faktor histeria dalam kalangan remaja Muslim iaitu faktor psikologi, tekanan keluarga dan persekitaran, kepercayaan dan kebudayaan dan amalan keagamaan. Manakala dari aspek simptom, terdapat tiga bentuk simptom iaitu simptom gangguan emosi, psikiatri dan gangguan makhluk halus. Dari sudut pencegahan, remaja yang mengalami histeria kerap menggunakan pencegahan secara tazkiyah al-nafs, perawatan Islam dan psikiatri. Hasil analisis hubungan antara faktor dan simptom histeria mendapati faktor yang berhubung secara signifikan dengan ketiga-tiga simptom histeria adalah faktor psikologi, tekanan persekitaran dan kepercayaan dan kebudayaan. Manakala dari aspek pencegahan terdapat hubungan yang signifikan antara pencegahan tazkiyah al-nafs dan perawatan Islam dengan simptom gangguan emosi. Implikasi kajian dapat membantu pelbagai pihak dalam menangani punca dan simptom histeria remaja dengan menggunakan pendekatan psikologi 3 dakwah

Lifetime Estimation of IGBTs in a Grid-connected STATCOM

Lifetime estimation of power semiconductor devices, and IGBT devices in particular, used in the power electronics integrated with power systems has gained technical importance in recent times with increased scope of distributed generation, renewable energy systems and FACTS. Since most of the common failures (wire bond and solder fatigue) are caused by thermo-mechanical stresses, the methodology of lifetime estimation starts with temperature estimation, cycle counting based on rainflow algorithm, and finally degradation calculation based on linear accumulation model. Different number of RC cells for each packaging layer in the module for the thermal model, including the influence of encapsulant is proposed for temperature estimation of IGBTs in power modules. A modified rainflow algorithm with faster execution time and time dependent temperature calculation is introduced for cycle counting. Finally, the lifetime of the IGBT is estimated during STATCOM operation using real-time load profiles for power factor variation. For a power factor variation data for a building, the lifetime is estimated to be about 3 years. Similarly, a month long arc furnace load data is considered to compare the equivalent temperature based calculation to conventional tests. 4% more degradation is observed in the equivalent temperature based calculation than compared with conventional rainflow algorithm. A simulation study on the operation parameter dependence on the stresses in a wire is considered to estimate lifetime from Finite Element Analysis (FEA) in COMSOL. Power cycling tests are conducted on two different modules (600 V, 50 A H-bridge module and a 1200 V, 150 A phase leg module) to validate the lifetime model for four months. The low power module was tested without any protection circuits and hence failed catastrophically. Wire melt-off or fusing failure was dominantly observed, following by dielectric based short circuit failure. The high power module was tested with protection circuits to prevent catastrophic damage for a maximum of 4 months. A maximum of 20% degradation in static characteristics, with decreased on state resistance was observed in the modules. The degradation is attributed to increased junction temperature as the thermal resistance increases owing to solder fatigue

Diagnostic and prognostic analysis tools for monitoring degradation in aged structures

This research addresses the problem of prolonging the life of aged structures of historical value that have already outlived their original designed lives many times. While a lot of research has been carried out in the field of structural monitoring, diagnostics and prognostics for high tech industries, this is not the case for historical aged structures. Currently most maintenance projects for aged structures have focused on the instrumentation and diagnostic techniques required to detect any damage with a certain degree of success. This research project involved the development of diagnostic and prognostic tools to be used for monitoring and predicting the ‘health’ of aged structures. The diagnostic and prognostic tools have been developed for the monitoring of Cutty Sark iron structures as a first application. The concept of canary and parrot sensor devices are developed where canary devices are small, accelerated devices, which will fail according to similar failure mechanisms occurring in an aged structures and parrot devices are designed to fail at the same rate as the structure, thus mimicking the structure. The model-driven prognostic tool uses a Physics-of-Failure (PoF) model to predict remaining life of a structure. It uses a corrosion model based on the decrease in corrosion rate over time to predict remaining life of an aged iron structures. The data-driven diagnostic tool developed uses Mahalanobis Distance analysis to detect anomalies in the behaviour of a structure. Bayesian Network models are then used as a fusion method, integrating remaining life predictions from the model-driven prognostic tool with information of possible anomalies from data-driven diagnostic tool to provide a probability distribution of predicted remaining life. The diagnostics and prognostic tools are validated and tested through demonstration example and experimental tests. This research primarily looks at applying diagnostic and prognostic technologies used in high-tech industries to aged iron structures. In order to achieve this, the model-driven and data-driven techniques commonly used had to be adapted taking into consideration the particular constraints of monitoring and maintaining aged structures. The fusion technique developed is a novel approach for prognostics for aged structures and provides the flexibility often needed for diagnostic and prognostic tools

Improvements in Testing and Performance of Batteries for Automotive Applications

Batteries are increasingly important in modern technologies. This is particularly true in the automotive sector, with hybrid vehicles using batteries to augment the traction power traditionally provided by the internal combustion engine. In such applications, one of the most important factors is the Dynamic Charge Acceptance (DCA) performance of the battery. This study investigates the standard method for establishing DCA performance and determines how the individual parameters of the test procedure and external factors influence the performance of lead-acid cells. This work identifies shortcomings of the standard test, which result in the true DCA performance being better than the standard test suggests. A series of modifications are proposed, which are shown to produce a more representative result. An investigation is performed to determine the effect of cell degradation on charge acceptance. This shows that the DCA test itself is not well suited to assessing the effects of degradation on DCA, and causes the results to appear worse than reality. The work also demonstrates that the usual methods of characterising degradation do not correlate well with DCA performance, and there is very little reduction in charge acceptance over the operational life of the cell. Investigations are undertaken into methods by which DCA performance may be improved. This shows that the application of ac ripple currents to batteries causes a significant increase in charge acceptance, and demonstrates how the frequency of the ripple is important in achieving the best results. This study also shows that the ripple currents have no detrimental effects on the health of the battery. Finally, the work is extended to cover lithium cells. This shows that whilst the DCA performance of lithium is more consistent, maximum charge acceptance is less than lead. It is shown that, by reducing maximum charge voltage, cycle life of cells can be extended without significant loss of stored energy

Lessons Learned and Next Steps in Energy Efficiency Measurement and Attribution: Energy Savings, Net to Gross, Non-Energy Benefits, and Persistence of Energy Efficiency Behavior

This white paper examines four topics addressing evaluation, measurement, and attribution of direct and indirect effects to energy efficiency and behavioral programs: Estimates of program savings (gross); Net savings derivation through free ridership / net to gross analyses; Indirect non-energy benefits / impacts (e.g., comfort, convenience, emissions, jobs); and, Persistence of savings