Energy Storage Systems in Microgrid

Abstract

The microgrid represents a controllable electric entity that contains different loads into distributed energy resources. All typical microgrids use two or more sources by which electricity is generated, at least one of which is a renewable source. In this respect the main issues of the energy storage systems (ESS) are the enhancing of the stability of microgrid and power balance. Also the insertion of the energy storage systems is beneficial for both operation modes of microgrids, grid connected and islanded. This chapter begins with an overview of the current state of microgrids and ESS. The island operation mode of microgrids is based on the energy storage system. At the first level the control tasks during this mode of operation are to regulate the voltage and to maintain the frequency at the constant value. The power in each unit is shared among the storage units by secondary control of the energy storage system taking into account the energy level of each of them. Further, the authors present storage technologies of the electrical energy, i.e. converting it into mechanical, chemical, electrochemical, electromagnetic and thermal energy. The widespread mechanical energy storage technology is the pumped hydro (99% of the world total storage capacity) followed by the compressed air energy and flywheel. Afterwards, the fuel cell, biomass and fossil fuel compose the chemical storage domain. Batteries constitute the electrochemical storage technology. Electromagnetic and thermal energy storage technologies are represented by the super capacitors respectively the heat pumps. Both the features of the energy Storage technologies and the main properties will be presented. A comparison of the discharge time of the storage technologies by application is taken into account. The advantages and disadvantages of the storage technologies will be highlighted. The lack of the appropriate standards of interconnecting different kinds of energy sources and ESS to the microgrid is a disadvantage in technology developing. Thereby the IEC/ISO 62264 standards refers to wind turbine technology, while the IEEE 2000 standard refers to photovoltaic interconnection power systems. In order to reduce air pollution and mitigate climate change, in recent years the need to use renewable energy sources in microgrids has become important. In fact, technological development, public and political policies, availability of different renewable energy resources to be used in microgrids, are elements that create real perspectives in widespread development of renewable energies and their required ESS. The environmental impact and the costs of renewable energy resources have been estimated through the life cycle assessment (LCA) methodology, which determines whether the use of renewable sources and ESS is sustainable. A case study regarding a PV system with and without batteries used into a microgrid that supply a wastewater treatment plant situated in Romania will be presented. Life cycle assessment was used to quantify ecological sustainability and costs in both cases compared to a conventional supply of electricity from the grid. Conclusions and measures are drawn in order to increase the utilization of renewable sources and ESS in microgrids. Also the technically, economic and environmental benefits of inserting microgrids are discussed. The bibliographic references will be presented at the end of the chapter.