Energy Storage – Part 1

This part 1 of our comprehensive ‘Scientific Series’ covers all aspects of ‘Energy Storage’ which is required in the process of harnessing the energy generated from Wind Power and Solar Power.

 

energy storage system2 Energy Storage   Part 1

ENERGY STORAGE SYSTEM

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 the electrical conversion efficiency of a typical fossil thermal power plant is under 40 per cent. A disadvantage of electricity is that it cannot be easily stored on a large scale. Almost all electrical energy used today is consumed as it is generated. This poses no hardship in conventional power plants, where the fuel consumption is varied with the load requirements. The photovoltaic and wind, being intermittent sources of power, cannot meet the load demand all of the time, 24 hours a day, 365 days of the year. The energy storage, therefore, is a desired feature to incorporate with renewable power systems, particularly in stand-alone plants. It can significantly improve 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 fall into 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 devices for energy storage in a variety of applications. Electrochemical energy is a semi-ordered form of energy, which is in between the electrical and thermal forms. It has a one-way conversion efficiency of 85 to 90 per cent. There are two basic types of electrochemical batteries:

the primary battery, which converts the chemical energy into 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. This type of battery converts the chemical energy into electrical energy in the discharge mode. In the charge mode, it converts electrical energy into chemical energy. In both the charge and the discharge modes, a small fraction of energy is converted into heat, which is dissipated to the surrounding medium. The round trip conversion efficiency is between 70 and 80 per cent. The internal construction of a typical electrochemical cell is shown in Figure 10-1. It has positive and negative electrode plates with insulating separators and a chemical electrolyte in-between. The two groups of electrode plates are connected to two external terminals mounted on the casing.

figure 1042 Energy Storage   Part 1

The cell stores electrochemical energy at low electrical potentials, typically a few volts. 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 the 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. The product of the voltage and the Ah forms the Wh energy rating it can deliver to a load from the fully-charged condition. The battery charge and discharge rates are stated in the unit of its capacity in Ah. For example, charging a 100 Ah battery at C/10 rate means charging at 10 A rate. Discharging that battery at C/2 rate means draining 50 A, at which rate the battery will be fully discharged in 2 hours. The State of Charge (SOC) of the battery at any time is defined as the following:

formula2 Energy Storage   Part 1

Types of Batteries

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 battery2 Energy Storage   Part 1

 

New electro-chemistries are being developed by the United States Advanced Battery Consortium for a variety of applications, such as electric vehicles, spacecraft, utility load leveling and, of course, for renewable 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’.

table 10 12 Energy Storage   Part 1
Cell Voltage Table
figure 1022 Energy Storage   Part 1
Energy Density Graph

Continued in Part 2 ‘Storage Battery Types’