Desiccation cracking of soils: A review of investigation approaches, underlying mechanisms, and influencing factors

Because of their intermittency and inter-temporal variations, the integration of renewable energy resources is usually very challenging. Some of the previously proposed solutions, such as the use of energy storage units and fast-responding generators, are costly and may not be desirable in practice. Instead, in this paper, we argue that due to the workload flexibility in the Internet and Cloud Computing data centers they can offer a unique opportunity to tackle the challenges in integrating renewable energy resources. Moreover, by running data centers with behind-the-meter renewable generators, we can significantly reduce not only data centers' energy cost but also their Carbon Footprint. To achieve these objectives, we propose to optimally distribute the workload among geographically dispersed data centers such that we can benefit from the location diversity of different types of available renewable energy resources. The quality-of-service is guaranteed by real-time monitoring of the queue lengths and effectively stabilizing short queue lengths in each data center. Simulation results, based on various experimental data, show that the proposed design can significantly reduce data centers' cost of electricity while maintaining quality-of-service.

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Optimal integration of renewable energy resources in data centers with behind-the-meter renewable generator

With a growing concern on the considerable energy consumed by data centers, research efforts are targeting toward green data centers with higher energy efficiency. In particular, server virtualization is emerging as the prominent approach to consolidate applications from multiple applications to one server, with an objective to save energy usage. However, little understanding has been obtained about the potential overhead in energy consumption and the throughput reduction for virtualized servers in data centers. In this research, we take the initiative to characterize the energy usage on virtualized servers. An empirical approach is adopted to investigate how server virtualization affects the energy usage in physical servers. Through intensive data collection and analysis, we identify a fundamental trade-off between the energy saving from server consolidation and the detrimental effects (e.g., energy overhead and throughput reduction) from server virtualization. This characterization lays a mathematical foundation for server consolidation in green data center architecture.

Energy efficiency and server virtualization in data centers: An empirical investigation

Energy consumption represents a significant cost in data center operation. A large fraction of the energy, however, is used to power idle servers when the workload is low. Dynamic provisioning techniques aim at saving this portion of the energy, by turning off unnecessary servers. In this paper, we explore how much gain knowing future workload information can bring to dynamic provisioning. In particular, we develop online dynamic provisioning solutions with and without future workload information available. We first reveal an elegant structure of the offline dynamic provisioning problem, which allows us to characterize the optimal solution in a &#x201C;divide-and-conquer&#x201D; manner. We then exploit this insight to design two online algorithms with competitive ratios $(2-\alpha)$ and $(e/(e-1+\alpha ))$, respectively, where $(0\le \alpha \le 1)$ is the normalized size of a look-ahead window in which future workload information is available. A fundamental observation is that future workload information beyond the full-size look-ahead window (corresponding to $(\alpha =1)$) will not improve dynamic provisioning performance. Our algorithms are decentralized and easy to implement. We demonstrate their effectiveness in simulations using real-world traces.

https://researchbank.swinburne.edu.au/file/cc028dd0-8734-4c53-bf4a-2264bb73f890/1/PDF (Accepted manuscript).pdf

Simple and Effective Dynamic Provisioning for Power-Proportional Data Centers

Energy consumption represents a significant cost in data center operation. A large fraction of the energy, however, is used to power idle servers when the workload is low. Dynamic provisioning techniques aim at saving this portion of the energy, by turning off unnecessary servers. In this paper, we explore how much performance gain can knowing future workload information brings to dynamic provisioning. In particular, we study the dynamic provisioning problem under the cost model that a running server consumes a fixed amount energy per unit time, and develop online solutions with and without future workload information available. We first reveal an elegant structure of the off-line dynamic provisioning problem, which allows us to characterize and achieve the optimal solution in a {}"divide-and-conquer" manner. We then exploit this insight to design three online algorithms with competitive ratios $2-\alpha$, $(e-\alpha)/(e-1)\approx1.58-\alpha/(e-1)$ and $e/(e-1+\alpha)$, respectively, where $0\leq\alpha\leq1$ is the fraction of a critical window in which future workload information is available. A fundamental observation is that \emph{future workload information beyond the critical window will not} \emph{improve dynamic provisioning performance}. Our algorithms are decentralized and are simple to implement. We demonstrate their effectiveness in simulations using real-world traces. We also compare their performance with state-of-the-art solutions.

Soil electrical resistivity is an important parameter in grounding system design. In this paper, laboratory measurements of soil electrical resistivity are carried out using a soil box. Two measurement methods are used and compared, namely, a two-electrode method and a four-electrode method. The measurements indicate that the results reported by the two-electrode method are affected by the contact resistance between the electrodes and the soil, which increases as the soil water content increases. Moreover, the soil electrical resistivity was observed to decrease as the test signal frequency increases. Using the four-electrode method and a 50-Hz ac signal, the effects of soil water content, soil porosity, pore fluid composition, and temperature on the soil electrical resistivity are then investigated. The results show that increasing the soil saturation level results in a power function decrease in the soil electrical resistivity. For a given gravimetric water content, as the soil porosity decreases, the resistivity decreases. In addition, owing to the mobility of the ions, different electrolytes in the pore fluid contribute differently to the soil electrical resistivity. Finally, the dependence of soil electrical resistivity on soil temperature is found to be divided into three stages: above 0 °C, around 0 °C, and below 0 °C. An abrupt change in the soil electrical resistivity is observed at around 0 °C.

Laboratory Investigations on Factors Affecting Soil Electrical Resistivity and the Measurement

Far-field measurements of lightning-produced electromagnetic fields are of high importance for lightning location systems. Data from recorded lightning strokes can be, for example, used to assess the risk of lightning-induced damage in some geographical areas. Artificially triggered lightning, usually generated using rocket-wire technology, plays an important role in the study of lightning physics. Multiple-station observations of far (typically larger than 20 km) electric field waveforms produced by rocket-triggered lightning are quite rare. In 2014, electric field waveforms were measured for return strokes through the rocket-and-wire technique at Conghua. They were measured by electric field sensors at nine sites belonging to the lightning locating system in Foshan, Guangzhou, China. A total of 38 events were recorded and overall statistics of those waveforms are presented. The results were compared to previous experimental data related to natural and artificially triggered lightning to validate their similarity.

Multiple-Station Measurements of a Return-Stroke Electric Field From Rocket-Triggered Lightning at Distances of 68–126 km

This paper presents the induced voltage measured at the terminal of a 10 kV distribution line when triggered lightning taking place at a distance of 40 m. It is shown that the induced voltage waveforms exhibit three components including a slow increase preceding the initial negative peak, following a rapid polarity reversal and the attenuated oscillations. The waveform parameters are defined in this paper. The observed negative peak value ranges from −47.1 kV to −7.5 kV. The positive peak value ranges from 5.3 kV to 20.9 kV, and the peak to peak value ranges from 12.8 kV to 63.2 kV. The test results show that the negative peak value of the induced voltage at the terminal of distribution generated by nearby negative triggered lightning generally exceeds the positive peak value, and in some cases the negative peak value even reaches twice the positive peak value. The negative 10%-90% rise time ranges from 2.3 μs to 88.5 μs, and the average oscillation period is about 18 μs. Both negative and positive peaks of induced voltages show good linear relationship with the peaks of lightning return stroke current.

Observation of Induced Voltage at the Terminal of 10 kV Distribution Line by Nearby Triggered Lightning

This paper deals with problems arising at Electromagnetic Compatibility (EMC) in relation to the interference caused by surge voltages and currents. For this purpose, IEC 61000–4–5 Standard has been instituted, describing the procedure of testing and verification of a device for immunity against surge. Specifically, measurements and tests were performed on an investigation of surge immunity of a personal computer at power lines. Response of the testing computer under different coupling modes of the surges and different test levers were presented. The results implied that when the surges were applied between phases or the phase shifting was 90° or 270°, the tested personal computer's surge immunity was lower. The information in this paper can also be used in the lightning protection design by the computer manufacturer.

Surge immunity test of personal computer at power lines

This paper presents the analysis and observation results of the induced voltage waveforms caused by nearby natural lightning on overhead power lines in the Guangzhou field experiment site during the summer of 2008. Peak values of the induced voltage for 36 natural lightning return strokes ranged from 0.21 to 3.87 kV. And the double-peak oscillation phenomenon can be found in the waveforms. The interval time of adjacent return strokes ranged from 17.1 to 430.1 ms. The average 10%-10% peak value duration time of induced voltage waveform of the first return strokes is 244.6 μs, and that of the subsequent return strokes is 83.2 μs. The induced voltage waveforms on the live and neutral conductors are almost the same, while the induced voltage amplitude on the live conductor is slightly larger than that on the neutral conductor. The lightning location system data indicate that the induced voltage on the live and neutral conductor may be of either positive or negative polarity, depending on the lightning ground strike points. However, the amplitude of the induced voltage calculated by the Rusck model is smaller than those of the observation results, with the former being 60% of the latter.

Li Cai

Papers

Laboratory Investigations on Factors Affecting Soil Electrical Resistivity and the Measurement

Multiple-Station Measurements of a Return-Stroke Electric Field From Rocket-Triggered Lightning at Distances of 68–126 km

Observation of Induced Voltage at the Terminal of 10 kV Distribution Line by Nearby Triggered Lightning

Surge immunity test of personal computer at power lines

Observation of Natural Lightning-Induced Voltage on Overhead Power Lines