Energy Storage
Electricity is more versatile in use because it is a highly ordered form of energy that can be converted efficiently into other forms. For example, it can be converted into mechanical form with efficiency near 100 percent or into heat with 100 percent efficiency. The heat energy, on the other hand, cannot be converted into electricity with high efficiency, because it is a disordered form of energy in atoms. For this reason, the overall thermal to electrical conversion efficiency of a typical fossil thermal power plant is under 40 percent. The energy storage, therefore, is a desired feature to incorporate with renewable power systems, particularly in stand-alone plants. It can significantly improve the load availability, a key requirement for any power system. The present and future energy storage technology that may be considered for stand-alone photovoltaic or wind power systems falls in the following broad categories: • electrochemical battery • flywheel • compressed air • superconducting coil Battery The battery stores energy in the electrochemical form, and is the most widely used device for energy storage in a variety of applications. The electrochemical energy is a semi-ordered form of energy, which is in between the electrical and thermal forms. It has one-way conversion efficiency of 85 to 90 percent. There are two basic types of electrochemical batteries: • the primary battery, which converts the chemical energy into the electrical energy. The electrochemical reaction in the primary battery is non-reversible, and the battery after discharge is discarded. For this reason, it finds applications where high energy density for one time use is needed. • the secondary battery, which is also known as the rechargeable battery. The electrochemical reaction in the secondary battery is reversible. After a discharge, it can be recharged by injecting direct current from an external source. The cell capacity, denoted by C, is measured in Ampere-hours (Ah), meaning it can deliver C amperes for one hour or C/n amperes for n hours. The battery is made of numerous electrochemical cells connected in a series-parallel combination to obtain the desired operating voltage and current. The higher the battery voltage, the higher the number of cells required in series. The battery rating is stated in terms of the average voltage during discharge and the Ah capacity it can deliver before the voltage drops below the specified limit. There are at least six major rechargeable electro-chemistries available today. They are as follows: • lead-acid (Pb-acid). • nickel-cadmium (NiCd). • nickel-metal hydride (NiMH). • lithium-ion (Li-ion). • lithium-polymer (Li-poly). • zinc-air. Solar Storage Battery New electro-chemistries are being developed by the United States Advance Battery Consortium for a variety of applications, such as electric vehicles, spacecraft, utility load leveling and, of course, for renewable wind power systems. The average voltage during discharge depends on the electrochemistry, as listed in Table 10-1. The energy density of various batteries, as measured by Wh capacity per unit mass and per unit volume, are compared in Figure 10-2. The selection of the electrochemistry for a given application is a matter of performance and cost optimization. Some construction and operating features of the above electro-chemistries are presented in the proceeding parts 2 and 3 of this ‘Scientific Series’. Cell Voltage Table
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