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1. Introduction
A battery converts chemical energy into electrical energy. There are two types of batteries, these are primary and secondary batteries. Primary batteries are designed to be used only once. Secondary batteries (rechargeable batteries indeed) can be used many times [Toyota, 2012].
In general terms, all electrochemical devices, i.e. batteries, are composed of two parts: called the electrode and the electrolyte. Usually, electrodes store the energy and the electrolyte serves as a medium through which ions flow (see section 2 for a more detailed explanation). An exception is flow batteries, which store the energy in the electrolyte. The useful energy stored in the device is the product of the power output and the discharge time [PIER, 2011].
2. Flow Batteries
Flow batteries are secondary batteries composed of one or more liquid electrolytes which flow through electrochemical cells to convert chemical energy into electrical energy. In contrast to common batteries, flow batteries store the energy in the electrolyte [PIER, 2011]. A Redox Flow Battery is a good example of flow batteries (see section 5).
3. Lead–acid batteries
The most common example of a battery is the Lead–Acid battery (a secondary battery). This technology is the most mature EES that exist currently [PIER, 2011]. The generation of electricity in these devices is commonly performed into two processes. First, electrons are obtained by oxidizing a metal (called the anode). The second process performs the opposite, a metal is reduced (this means by accepting electrons) – it is called cathode. Both anode and cathode are immersed in an acid solution, which is called electrolyte [Toyota, 2012], see Fig. 1.
Fig. 1: General diagram of a Lead–Acid Cell (adapted from [Toyota, 2012]).
In the upper left side of Fig. 1, a load is connected between the anode and cathode (discharging process), therefore the voltaic cell is discharged (a battery is the combination of many cells connected in series or parallel). The conventional direction of the current flow is shown. However, in reality, electrons flow in the opposite direction. While discharging, the cathode and anode are chemically modified [Energizer–Website, 2012]. In a rechargeable battery this process can be reversed by sending current in the opposite direction as shown in the charging process section of Fig. 1 [Toyota, 2012].
Referring to Fig. 1, from the chemical point of view, an ion is an atom in which the total number of protons is not equal to the total number of electrons. There are two types of ions, namely cations and anions. A cation is an atom in which the total number of protons is more than the total numbers of electrons (positively charged atom). In an anion, the total numbers of protons is less than the total number of electrons (negatively charged atom) as exemplified in Fig. 2.
Fig. 2: General representation of the atom, cation and anion.
This model is the Rutherford atom model. Although it is not the most adequate model for the atom, it provides an easier way to explain the differences between cations and anions.
An example of the voltage variation of a lead–acid battery is shown in Table 1; where the nominal voltage, voltages under charging and after discharging are given. The main advantages of lead–acid batteries are that they have a low cost and are easy to manufacture. However, lead–acid batteries have been used only in a few network applications due to their cycle life limitations and their heavy weight. The cycle life of a lead–acid batteries system is directly related to the application: i.e. the discharge rate and number of deep discharge cycles to which it is subjected [PIER, 2011].
Table 1: Nominal Voltage, final voltage after discharge and voltage under charging of some batteries (adapted from [Xie et al., 2011])
Battery Type | Final Voltage after discharge (V) | Nominal Voltage (V) | Voltage under Charging (V) |
ZEBRA | 1.72 | 2.58 | 2.85 |
NaS | 1.82 | 2.08 | 2.3 |
Ni–Cd | 1.0 | 1.2 | 1.5 |
Lead–Acid | 1.79 | 1.94 | 2.3 |
Li–Ion | 2.7 | 3.6 | 4.0 |
4. Sodium–Sulphur (Na–S) Battery Storage
This type of battery was proposed by J. T. Kummer and N. Weber in 1967 [DTI, 2006]. In contrast to common batteries, it has a liquid metal negative electrode and a solid electrolyte. Also, its cell reaction varies according to the temperature of operation.
The Na–S battery has been widely commercialized in Japan, where, it has been used for peak lopping, emergency power and power quality. In USA, a Na–S system has been implemented at a substation in Charleston (West Virginia): a NGK Na–S 20–module 1MW/7.2MWh system. Fig. 3 gives a schematic idea of the NGK Na–S 20–Module System and its connection with the electric network.
Fig. 3: A schematic idea of the NGK Na–S 20–Module System and its connection with the electric network (adapted from [DTI, 2006])
The NGK Na–S 20–Module System at Charleston has been used for peak–shaving and power quality. Table 2 shows a summary of its main characteristics under these applications. Also, it has shown an efficiency of 85% [DTI, 2006].
Table 2: NGK Na–S 20–module Characteristics (adapted from [DTI, 2006]). Approximate Battery System weight: 100 tones.
Application | Max Power | Duration | Cycle Frequency |
Peak Shaving Load | 1MW | 7.2 hours | 1/Day |
Power Quality Transmission and Distribution Short Duration Pulse | 5 MW | 30 seconds | 1/h |
However, on charging a Na–S battery develops a very high resistance and if charging is continued, because of its internal reaction, battery failure could occur. Therefore a Na–S battery must have a very well controlled charging process [DTI, 2006].
5. Redox Flow Batteries (Redox Flow Cell)
Redox flow (RF) batteries were developed first in the USA. In contrast to common batteries, RF batteries have two separated reservoirs to store two different liquid electrolytes, see Fig. 4. Pumps are used to send the electrolytes through two half–cells and return them to the reservoirs again. A membrane is used to separate these two half–cells. This membrane allows the interchange of ions and avoids the contact between electrodes. An electrochemical reaction, between the electrolytes and electrodes, is performed which allows the output or input of current into the cell [Bartolozzi, 1989].
Fig. 4: General schematic representation of a redox flow cell (adapted from [Bartolozzi, 1989])
In the USA, two types of redox flow battery systems have been commercialized: the zinc–bromine battery ZBB system and the vanadium– vanadium battery system [DTI, 2006]. A ZBB 500 kWh storage battery system (developed by ZBB Energy Corp) has been tested at the Distributed Utility Integration Test facility in San Ramon, California, USA [PIER, 2012]. It has been used to provide peak–shaving capability, energy efficiency, reliability and maintenance requirements [DTI, 2006]. The 500 kWh is made of two strings of five 50 kWh battery modules, as is shown in Fig. 5 [DTI, 2006; PIER, 2012].
Fig. 5: The ZBB 50 and 500 KWh battery models: a schematic representation (adapted from [ZBB, 2012; PIER, 2012])
The 50 kWh battery model is composed by three batteries connected in parallel. Every battery has 60 cells connected in series (voltage per cell is about 1.81 volts) [DTI, 2006; ZBB, 2012; PIER, 2012]. Therefore, its open circuit voltage is approximately 108 Volts (DC). The 50 kWh battery–system can provide 50 kW peak power in less than 2 minutes [DTI, 2006].
The main disadvantages of the 500 kWh storage battery system are the noise (similar to a medium–sized diesel generator) due to fans used to control the temperature of the system, and they require more frequent maintenance due to the presence of pumping circuits. The system efficiency has been estimated to be about 70% [DTI, 2006].
6. Other types of batteries
Other types of batteries are [PIER, 2011]:
A) Nickel–cadmium (Ni–Cad) Battery: Nickel oxide hydroxide is used for the positive electrode and metallic cadmium is used for the negative electrode.
B) Nickel–Metal Hydride (Ni–MH) Battery: This is similar to the Ni–Cad battery, however, instead of cadmium, it uses hydrogen–containing alloy for the negative electrode. It is widely used for applications in hybrid electric vehicles.
C) Lithium–ion (Li–ion) Battery: the most common, power–dense commercial battery and frequently found in consumer electronics products. Commercially available for electric vehicle applications from 15 to 50 kWh capacities.
D) Metal Air Battery: where the electricity is generated by the oxidation of chemical elements (such as zinc) to generate electricity.
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