Electric Energy Storage

The paper presents the operational results of a real life residential microgrid which includes six
apartments, a 20 kWp photovoltaic plant, a solar based thermal energy plant, a geothermal heat pump,
a thermal energy storage, in the form of a 1300 l water tank and two 5.8 kW h batteries supplying, each,
a couple of apartments. Thanks to the thermal energy storage, the solar based thermal energy plant is
able to satisfy the 100% of the hot water summer demand. Therefore the thermal energy storage represents
a fundamental element in the management of the residential demand of thermal energy. It collects
renewable thermal energy during day-time to release it during night-time, effectively shaving the peak of
the thermal energy demand. The two electric storages, on the other hand, provide the hosted electrical
subsystems with the ability to effectively increase the self-consumption of the local energy production,
thus lowering the amount of energy surplus to be sold back to the grid, and increasing the self-sufficiency
of the microgrid. For instance, the storage has supported self-consumption up to the 58.1% of local energy
production with regard to the first battery, and up to the 63.5% with regard to the second one. Also, 3165
and 3365 yearly hours of fully autonomous activity have been recorded thanks to the first, and the second
battery respectively. On the other hand, the yearly average efficiency amounts to 63.7%, and 65.3%
respectively, for the first and second battery. In the second part of the paper we propose a computational
framework to evaluate the overall performance of the microgrid system, while accounting different operating
conditions and energy management policies. From this perspective, the framework acts as a useful
modeling and design tool, to assess the opportunity of employing alternative energy management system
topologies and strategies. Eight different configurations, with growing complexity, have been derived
from the original system on purpose. The simulations, carried out based on real data related to one-year
time period, have provided results showing that, the higher the integration level of electrical and thermal
storage is, the higher degree of self-sufficiency can be achieved by the microgrid, and, in turn, the more
consistent the yearly energy saving become. Nevertheless, despite the energy cost reduction achievable with the availability of storage systems in the Leaf House, their high investment cost made them not
really profitable at the current price conditions for devices and energy purchase.

Electrochemical capacitors, also known as supercapacitors, are becoming increasingly important components in energy storage, although their widespread use has not been attained due to a high cost/ performance ratio. Fundamental research is contributing to lowered costs through the engineering of new materials. Currently the most viable materials used in electrochemical capacitors are biomassderived and polymer-derived activated carbons, although other carbon materials are useful research tools. Metal oxides could result in a step change for electrochemical capacitor technology and is an exciting area of research. The selection of an appropriate electrolyte and electrode structure is fundamental in determining device performance. Although there are still many uncertainties in understanding the underlying mechanisms involved in electrochemical capacitors, genuine progress continues to be made. It is argued that a large, collaborative international research programme is necessary to fully develop the potential of electrochemical capacitors.

The concept of microgrid was first introduced in 2001 as a solution for reliable integration of distributed generation and for harnessing their multiple advantages. Specific control and energy management systems must be designed for the microgrid operation in order to ensure reliable, secure and economical operation; either in grid-connected or stand-alone operating mode. The problem of energy management in microgrids consists of finding the optimal or near optimal unit commitment and dispatch of the available sources and energy storage systems so that certain selected criteria are achieved. In most cases, energy management problem do not satisfy the Bellman's principle of optimality because of the energy storage systems. Consequently, in this paper, an original fast heuristic algorithm for the energy management on stand-alone microgrids, which avoids wastage of the existing renewable potential at each time interval, is presented. A typical test microgrid has been analysed in order to demonstrate the accuracy and the promptness of the proposed algorithm. The obtained cost of energy is low (the quality of the solution is high), the primary adjustment reserve is correspondingly assured by the energy storage system and the execution runtime is very short (a fast algorithm). Furthermore, the proposed algorithm can be used for real-time energy management systems.

This paper surveys the existing literature on the effects of population age structure on saving, capital flows and the real exchange rate. We build on previous surveys of age structure and saving behaviour by including extensive discussion of international linkages, specifically the impact on capital flows and real exchange rates. We also point to unresolved issues that need to be addressed given that population aging has serious policy implications in the developed economies these days.

Electrochemical capacitors, also known as supercapacitors, are becoming increasingly important components in energy storage, although their widespread use has not been attained due to a high cost/ performance ratio. Fundamental research is contributing to lowered costs through the engineering of new materials. Currently the most viable materials used in electrochemical capacitors are biomassderived and polymer-derived activated carbons, although other carbon materials are useful research tools. Metal oxides could result in a step change for electrochemical capacitor technology and is an exciting area of research. The selection of an appropriate electrolyte and electrode structure is fundamental in determining device performance. Although there are still many uncertainties in understanding the underlying mechanisms involved in electrochemical capacitors, genuine progress continues to be made. It is argued that a large, collaborative international research programme is necessary to fully develop the potential of electrochemical capacitors.

This paper presents a distributed energy storage device (DESD) based on a novel isolated bidirectional DC-DC converter with 650V GaN transistors. The device integrates a low-voltage (13.2V) Li-ion battery pack, an embedded bidirectional DC-DC converter and wireless communication system. The three parts are packaged together, thus it can be directly connected to high-voltage (380V) DC grid, enabling a modular approach for battery energy storage systems. Two 650V enhancement mode GaN transistors are used at the high voltage side. Compared with Si device, three improvements can be achieved in the application: expanding the operation range to light load, reducing switching loss and EMI, increasing the total efficiency of charging and discharging operation. The power stage design as well as a loss analysis of GaN is based on a steady state analysis and PSpice simulation. A 400V to 12V DC, 1kW converter for 1kWh DESD prototype is designed, fabricated, and tested. Experimental results verify the validity of the proposed DESD and the performance improved by using GaN transistors.

The ocean represents a huge energy reservoir since waves can be exploited to generate clean and renewable electricity; however, a hybrid energy storage system is needed to smooth the fluctuation. In this paper a hybrid energy storage system using a superconducting magnetic energy system (SMES) and Li-ion battery is proposed. The SMES is designed using Yttrium Barium Copper Oxide (YBCO) tapes, which store 60 kJ electrical energy. The magnet component of the SMES is designed using global optimization algorithm. Mechanical stress, coupled with electromagnetic field, is calculated using COMSOL and Matlab. A cooling system is presented and a suitable refrigerator is chosen to maintain a cold working temperature taking into account four heat sources. Then a microgrid system of direct drive linear wave energy converters is designed. The interface circuit connecting the generator and storage system is given. The result reveals that the fluctuated power from direct drive linear wave energy converters is smoothed by the hybrid energy storage system. The maximum power of the wave energy converter is 10 kW.

This paper presents a generalized linear model based on LMI state-feedback with integral action, applicable to the control of Electric Energy Storage Systems (EESS) such as Superconducting Magnetic Energy Storage (SMES) and Supercapacitor Energy Storage (SCES). A Voltage Source Converter (VSC) and a Pulse-Width modulated Current Source Converter (PWM-CSC) are respectively used to integrate the SCES and the SMES systems to the electrical distribution system. To represent the dynamics between the EESS and the power distribution system a reduced general linear model in the state-space representation is introduced. The proposed control scheme regulates independently the active and reactive power flow between the EESS and ac the grid. Three case scenarios comparing a conventional PI controller and the proposed technique are conducted considering grid voltage fluctuations. Extensive time-domain simulations demonstrate the robustness and proper performance of the proposed controller to Universidad EAFIT 147| A Generalized Model and Control for Supermagnetic and Supercapacitor Energy Storage operate the EESS as power compensator, in order to improve the operative conditions of electrical distribution systems.

"The secure operation of the electric power system is a challenging task for the system operator which is responsible of its coordination, control and monitoring. In most power systems security is typically handled on a multi–stateapproach. In which case, security criteria are incorporated by means of additional constraints, modifying the initial dispatch calculation. In this papera generation dispatch model for competitive energy markets considering security constraints is presented. The proposed approach combines the generation dispatch related to pool and bilateral markets with coupled post–contingency optimal power flows into a single optimal dispatch model, avoiding economic inefficiencies that appear in conventional multi–stage dispatch approaches.The proposed model is linear, and as such, it is based on a DC model of the network. A 6–bus didactic system and the IEEE RTS–24 bus test systemare used in order to show the operation and effectiveness of the proposed approach and to compare it with the basic pool/bilateral dispatch with no security constraints. Results show that the inclusion of security constraints leadto a higher dispatch cost. Furthermore, it was found that the enforcement offirmed bilateral contracts might lead to system congestion"