Abstract

Fractured vuggy carbonate reservoirs usually develop several large-scale caves which are the main reservoir space of oil-gas. The volume and position of caves are important parameters for production guidance, but there are no effective well test methods of calculating the cave volume of multi-cave reservoirs at present. This paper establishes a multi fracture-vug composite model for fractured vuggy carbonate reservoirs, divides reservoirs into various fracture-vug seepage zones according to the cave distribution, establishes a mathematical model through the coupling of seepage zones, and then obtains analytical solutions of the mathematical model using dimensionless and Laplace transformation. By plotting the log-log type curves of well test, the paper analyzes the flow characteristics of the model. It is found that the pressure derivative of inter-porosity flow segment from caves to fractures is presented as a V-shaped concave, and the pressure and pressure derivative curves at the fracture flow segment take on the parallel lines with the slope of 0.5. A parameter sensitivity analysis indicates: the bigger the cave is, the steeper the V-shaped concave of the pressure derivative curve goes; the farther the distance between cave and wellbore is, the more hysteretic the V-shaped concave becomes. Finally, the paper demonstrates the field example analysis and application by fitting measured pressure data to calculate the volume and distance of caves.

Abstract

Reducing the conservatism of robust optimization in an efficient way has always been a challenging problem for microgrid day-ahead dispatch with wind power integrated. In this paper, we address this problem by formulating a multi-ellipsoidal uncertainty set (MEUS) that is able to capture the strong temporal correlation of forecast error of wind power (WPFE) as well as the conditional correlation of WPFE with the forecast power, and combining it with a box uncertainty set. The dimension of each ellipsoid is optimized based on a comprehensive evaluation index to reduce the invalid region, so as to improve the conservatism of the model. A two-stage robust optimization model of microgrid is built based on the improved MEUS, which is cast into a mixed-integer second-order cone programming problem and solved by column and constraint generation algorithm. The effectiveness of the proposed method is verified by numerous simulations with actual data.

Abstract

The economic feasibility of Vehicle-to-Grid (V2G) technology is subject to a lot of debate due to the battery degradation costs associated with it. In this paper, the daily operational cost of a V2G system is calculated for three different types of Li-ion batteries and one new Li-S battery, and a comparative study is conducted to understand the conditions required to deploy a successful V2G system. Different models are applied in estimating the degradation of batteries with different battery chemistries. The best Li-ion battery for this system is found to be LiFePO4 even though it is the most expensive battery considered. It is also noted that the starting state-of-charge (SOC) relates to the operational costs non-linearly and an optimal starting SOC is calculated for minimizing operational cost. When these Li-ion batteries are replaced by the new double SEI layer Li-S batteries, additional cost benefits of about 17% are expected.

Abstract

This paper analyzes the application of a novel Lithium-Sulfur (Li-S) battery with the bilateral solid electrolyte interface to the cruise control of plug-in hybrid electric vehicles (PHEVs). An optimal cruise control strategy based on the dynamic programming algorithm is modified to include battery degradation cost during energy management. The degradation cost of a Lithium-ion Battery is used as a benchmark to analyze the degradation cost of Li-S battery of PHEVs. The comparison results demonstrated that the application of the new Li-S battery significantly reduces the cost of PHEVs.

Abstract

Remote and temporary military installations often do not have grid power connectivity despite the large electrical loads they have to support. In order to ensure adequate power supply for such needs, microgrids (MGs) composed of a variety of mobile power sources are often utilized. This paper studies the component sizing problem for such microgrids by formulating an optimization model with various budgetary, power, and packaging constraints. The optimization problem is solved to obtain a design that minimizes the operating cost of the MG while meeting all the microgrid design objectives and operational constraints.

Abstract

In order to realize the rapid detection and isolation of faults on the transmission lines of high voltage DC transmission system based on modular multilevel converters, a protection scheme based on the overvoltage transient characteristic signal of the faulted line is proposed. The dynamic fluctuation amplitude of the line voltage is detected to realize the rapid capture of the fault signal. The characteristic signal injection principle is adopted to suppress the fault current and line overvoltage, and create a reliable environment for the protection of the equipment against short-circuit faults. PSCAD/EMTDC simulation results show that the protection scheme can quickly and reliably transmit fault occurrence signals without complex communication systems and data processing units, reduce the impact of sudden changes in electrical quantities on the equipment, and meet the requirements of system protection reliability and rapid mobility.

Abstract

The rapid increase of the renewable energy affects the power system in such way that could reduce the reliability of the system. The replacement of fossil fuel driven vehicles with electric vehicles (EVs) would also change the underlying demand and make the stabilization of the system frequency to become more difficult. This paper addresses the availability of electric vehicles for the provision of frequency response at different time of the day. A low frequency response is provided by stop charging the domestic EV batteries where applicable. Simulation results indicates that EVs are good elements for the provision of frequency response if a proper design of a control scheme is implemented considering the EV’s charging behaviour.

Abstract

While phase change materials have been widely used for latent heat storage, water/ice is promising for cold energy storage due to its large latent heat. Characterizing phase change, either ice melting or water evaporation, within porous media is of great significance for efficient cold energy charging/discharging and enhanced energy storage, but it remains challenging because of the opaque nature of porous matrix. In this work, by combining the non-destructive NMR-MRI technology and pore-scale numerical simulation, we investigate the dynamics of water-vapor transition in porous medium with an average grain size of 150 μm. The transient water content and distribution are obtained from NMR transversal relaxation time 𝑇2. The shifting of 𝑇2 curve and MRI imaging results show uniform evaporation over the entire device, which implies cavitation-induced evaporation in homogenous porous medium. This is further elucidated from the COMSOL simulation, where we look into the water-vapor interface evolution during the evaporation process in a representative pore. Our insights on water phase change in porous media are also valuable to geothermal energy extraction.

Abstract

Numerical simulations are carried out to evaluate the performance of different reduced n-heptane mechanisms in dual-fuel combustion. Three reduced nheptane mechanisms with a size <70 species are tested in the present study. The ignition characteristics of methane and n-heptane for these mechanisms are first evaluated by performing zero-dimensional (0-D) homogeneous reactor (HR) calculations for a pressure range of 36 – 48 atm and equivalence ratio of 0.4. Results from all three mechanisms show good agreement with measurements. Next, computational fluid dynamic simulations of a n-heptane spray in a premixed methane/air mixture are carried out using the three mechanisms, in which the ambient temperature and density are set to 900 K and 14.8 kg/m3, respectively. Dual-fuel is considered by varying the equivalence ratio of methane (𝜙CH4) in the ambient gas from 0 to 0.5. The ignition delay time of the pure n-heptane spray is predicted to be within 21% as compared to measured data. It is shown that despite having reasonable performance in 0-D HR calculations, the ignition process in dual-fuel spray combustion varies for different mechanisms which may affect the conclusion of a study. Therefore, an extensive model evaluation is required prior to actual application.

Abstract

Most of existing fuel cell hybrid electric vehicle (FCHEV) powertrain models ignore the real physical processes such as heat and mass transfer, and the researches of individual power sources do not consider the whole vehicle level. A comprehensive transient FCHEV model is developed, including an integrated fuel cell system model, a one-dimensional transient LiFePO4 battery model, and a vehicle dynamics model. A rule-based energy management strategy is developed, and simulations are carried out under China standard operating conditions of light vehicles. Output performances and dynamic responses of the hybrid power system are investigated. The results show that both fuel cell system and lithium battery can successfully work within a suitable range and simultaneously achieve good responsiveness.