The science and technology of ultracapacitors are reviewed for a number of electrode materials, including carbon, mixed metal oxides, and conducting polymers. More work has been done using microporous carbons than with the other materials and most of the commercially available devices use carbon electrodes and an organic electrolytes. The energy density of these devices is 3¯5 Wh/kg with a power density of 300¯500 W/kg for high efficiency (90¯95%) charge/discharges. Projections of future developments using carbon indicate that energy densities of 10 Wh/kg or higher are likely with power densities of 1¯2 kW/kg. A key problem in the fabrication of these advanced devices is the bonding of the thin electrodes to a current collector such the contact resistance is less than 0.1 cm2. Special attention is given in the paper to comparing the power density characteristics of ultracapacitors and batteries. The comparisons should be made at the same charge/discharge efficiency.

Ultracapacitors: Why, How, and Where is the Technology

http://ieeexplore.ieee.org/iel5/5610941/5624686/05624745.pdf

Power electronics

The significant benefits associated with microgrids have led to vast efforts to expand their penetration in electric power systems. Although their deployment is rapidly growing, there are still many challenges to efficiently design, control, and operate microgrids when connected to the grid, and also when in islanded mode, where extensive research activities are underway to tackle these issues. It is necessary to have an across-the-board view of the microgrid integration in power systems. This paper presents a review of issues concerning microgrids and provides an account of research in areas related to microgrids, including distributed generation, microgrid value propositions, applications of power electronics, economic issues, microgrid operation and control, microgrid clusters, and protection and communications issues.

State of the Art in Research on Microgrids: A Review

Renewable energy, such as hydro power, photovoltaics and wind turbines, has become the most widely applied solutions for addressing issues associated with oil depletion, increasing energy demand and anthropogenic global warming. Solar and wind energy are strongly dependent on weather resources with intermittent and fluctuating features. To filter these variabilities, battery energy storage systems have been broadly accepted as one of the potential solutions, with advantages such as fast response capability, sustained power delivery, and geographical independence. During the implementation of battery energy storage systems, one of the most crucial issues is to optimally determine the size of the battery for balancing the trade-off between the technical improvements brought by the battery and the additional overall cost. Numerous studies have been performed to optimise battery sizing for different renewable energy systems using a range of criteria and methods. This paper provides a comprehensive review of battery sizing criteria, methods and its applications in various renewable energy systems. The applications for storage systems have been categorised based on the specific renewable energy system that the battery storage will be a part. This is in contrast to previous studies where the battery sizing approaches were either arranged as an optimised component in renewable systems or only accounted for one category of renewable system. By taking this approach, it becomes clear that the critical metrics for battery sizing, and by extension the most suitable method for determining battery size, are determined by the type of renewable energy system application, as well as its size. This has important implications for the design process as the renewable energy system application will drive the battery energy storage system sizing methodology chosen.

Battery energy storage system size determination in renewable energy systems: A review

Probability, Reliability and Statistical Methods in Engineering Design

The vanadium redox battery (VRB) has proven to be a reliable and highly efficient energy storage system (ESS) for microgrid applications. However, one challenge in designing a microgrid system is specifying the size of the ESS. This selection is made more complex due to the independent power and energy ratings inherent in VRB systems. Sizing a VRB for both required power output and energy storage capacity requires an in-depth analysis to produce both optimal scheduling capabilities and minimum capital costs. This paper presents an analytical method to determine the optimal ratings of VRB energy storage based on an optimal scheduling analysis and cost-benefit analysis for microgrid applications. A dynamic programming (DP) algorithm is used to solve the optimal scheduling problem considering the efficiency and operating characteristics of the VRBs. The proposed method has been applied to determine the optimal VRB power and energy ratings for both isolated and grid-connected microgrids, which contain PV arrays and fossil-fuel-based generation. We first consider the case in which a grid-tie is not available and diesel generation is the backup source of power. The method is then extended to consider the case in which a utility grid tie is available.

Optimal Sizing of a Vanadium Redox Battery System for Microgrid Systems

The vanadium redox flow battery (VRB) is well-suited for applications with renewable energy devices. This paper presents a practical analysis of the VRB for use in a microgrid system. The first part of the paper develops a reduced order circuit model of the VRB and analyzes its experimental performance efficiency during deployment. The model parameters of the various VRB system components were estimated from experimental field data. The parasitic losses of the circulation pumps power consumption were predicted during different operating situations. The second part of the paper addresses the implementation issues of the VRB application in a photovoltaic-based microgrid system. Commercially available chargers designed for lead-acid battery systems were shown to be non-optimal for VRB systems and a new dc–dc converter control was proposed to provide improved charging performance. The system model was validated with field-obtained experimental data.

A Field Validated Model of a Vanadium Redox Flow Battery for Microgrids

The integration of photovoltatics (PV) and vanadium redox batteries (VRB) in microgrid systems has proven to be a valuable, environmentally friendly solution for reducing the dependency on conventional fossil fuel and decreasing emissions. The integrated microgrid system must be characterized to develop appropriate charging strategies specifically for VRBs, sizing microgrid systems to meet a given load, or comparing the VRB to other energy storage technologies in different applications. This paper provides a performance characterization analysis in a PV-VRB microgrid system for military installations under different conditions of load and weather. This microgrid system is currently deployed at the Fort Leonard Wood army base in Missouri, USA.

Performance Characterization for Photovoltaic-Vanadium Redox Battery Microgrid Systems

Performance evaluation of energy efficient lighting associated with renewable energy applications

Vanadium redox batteries (VRBs) have proven to be a viable energy storage technology for portable microgrids due to their rechargeability and high energy density. VRBs exhibit parasitic load loss during operation due to pumping of electrolyte across the membrane during charging and discharging cycles, as well as required temperature control in the form of heating, ventilation and air conditioning. This paper focuses on empirically characterizing VRB efficiency based on known climatic operating conditions and load requirements. A model is created to determine system performance based on known climatic and load data collected and analyzed over an extended time period. A case study is performed using known data for a week time period to characterize system performance, which was compared to actual system performance observed during this same time period. This model allows for appropriate sizing of the PV array and discretionary loads based on required energy density of the system.

Andrew Curtis Elmore

Papers

Optimal Sizing of a Vanadium Redox Battery System for Microgrid Systems

A Field Validated Model of a Vanadium Redox Flow Battery for Microgrids

Performance Characterization for Photovoltaic-Vanadium Redox Battery Microgrid Systems

Performance evaluation of energy efficient lighting associated with renewable energy applications

Performance Prediction of a Vanadium Redox Battery for Use in Portable, Scalable Microgrids