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compensations schemes available for the benefits of battery storage systems. Costcompetitiveness, validated performance and safety are others, as is a regulatory environment built around a legacy system of centralised Similarly, general lack of industry/ utility acceptance is also a barrier, according to the U.S Department of Energy and Sioshansi et al. in 2012. As with all less established forms of technology, traditional project financing may be difficult or costly to obtain. Furthermore, common approaches and standards for quality, testing and manufacturing must be established to ensure product data reliability and transparency. Section 3.1 presents an overview and analysis of the key concepts underlying battery storage. Overview of Battery Storage Batteries are not a new technology. The Italian physicist Alessandro Volta is credited with their invention in 1799. Leadacid batteries provided electricity at night time in New York in the 1880s according to the Electric Power Research Institute (EPRI) and DOE in 2013. The cathode (the positive part) is separated from the anode (the negative part) by a porous separator, and ions are allowed to flow between the two charges via an electrolyte. The chemical reaction creates current and voltage (which together create power) that can be supplied to a load. In flow batteries, the electrolyte is stored in external tanks and is pumped through a central reaction unit. This consists of a cathode and anode through which a current is either taken in (charged) or supplied (discharged) to the external demand/supply source. Since batteries are composed of chemicals, the manner and conditions under which they are used affects their performance, cost and life time. For instance, in many cases the amount of a battery’s capacity used, also known as depth of discharge (DoD), dramatically affects its operational life. This is measured in charge cycles (see below). A battery’s capacity is often referred to in energy terms as power over a specified time. Megawatt hours (MWh) or kilowatt hours (kWh) are examples. Another important metric is power capability, which is the amount of power an installation can provide. Power capability is denoted in MW or kW. Ambient conditions like temperature also have an important effect in many battery types. Definitions of these concepts RENEWABLE ENERGY must thus be understood when approaching the topic of battery storage. These are presented below and are based on studies by IRENA, EPRI and DOE. It is important to note that different battery types have unique attributes. In addition, manufacturers of batteries differ. Calendar and cycle life The cycle life of a battery is the number of charge and discharge cycles a battery can complete before losing considerable performance. It is specified at a certain DoD and temperature. The necessary performance depends on the application and relative size of the installation. However, a fully charged battery that can only deliver 60-80% of its original capacity may be considered at the end of its cycle life. Calendar life is the number of years the battery can operate before losing considerable performance capability. The primary parameters are temperature and time. Depth of discharge This refers to the amount of the battery’s capacity that has been utilised. It is expressed as a percentage of the battery’s full energy capacity. The deeper a battery’s discharge, the shorter the expected life time. This is true of several cell-based batteries due to cell degradation, including lead-acid and lithium-ion. For example, if a battery discharges 10% of its full energy capacity, 90% of the full capacity is unused. This corresponds to 10% DoD. This battery will be able to complete more charging cycles (defined above) than a battery cycled at deep discharge. Deep cycle is often defined as 80% or more DoD. Each battery type and chemistry is affected differently. Other conditions, such as temperature, also play a role. Flow batteries are not affected by DoD to the same extent as some cell-based batteries. Ambient temperature Ambient temperature may have an important effect on battery performance. High ambient temperatures cause internal reactions to occur, and many batteries lose capacity more rapidly in hotter climates. High temperatures may also cause corrosion and and the creation of gases requiring ventilation. Reactions may be sluggish in very cold climates, and discharge may stop altogether. The electrolyte may also Figure 1: Illustration of depth of discharge versus cycle life The battery is only one part of a larger battery storage system, displayed and described below. A battery storage system contains several primary components, including the battery, monitoring and control systems, and a power conversion system 51 energetica INDIA · MAY | JUN16


energetica-india-57_asiapowerweek
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