Download 14 – Energy Storage Devices for UPS

Energy Storage Devices for UPS
In this section, three dominant energy storage devices for the existing and future UPS
systems are described. These energy storage devices are battery, flywheel, and fuel cell.
Battery
Battery is the energy storage component of current static UPS systems. It determines the
capacity and run-time of the UPS. For small units, it is the size of battery that determines the
size of the UPS. Different types of batteries are used in UPS systems but the most commonly
used types are lead-acid, nickel–cadmium, and lithium ion. The lead acid batteries used in this
application are the same as the ones used in the cars. However, there is one small difference. Car
batteries generate electricity by the reaction of sulfuric acid on lead plates that are drowned
under the liquid. These types of battery cells are not suitable for UPS applications because there
is a chance of acid spillage from them. In addition, during the charging process, they release
hydrogen that is explosive and dangerous in a closed environment. Lead acid batteries used in
UPS systems are a special kind called sealed or valve-regulated. The nickel–cadmium batteries
are another popular type of batteries used in UPS systems. They usually provide higher energy
and power density compared to lead-acid batteries. The nominal voltage of nickel–cadmium cells
is 1.2 V, which is smaller than 1.5 V of lead-acid batteries. However, the cell voltage variation
throughout different charge levels is less than lead-acid batteries. These batteries also have less
series resistance and can provide higher surge currents. Lithium-ion batteries have much higher
energy density. This kind of battery can be molded into different shapes. They have a nominal
voltage of 4.2 V. The main disadvantage of lithium-ion battery is that they lose their capacity
from the time of manufacturing regard-less of their charge level and conditions of use. Table
24.2 shows a comparison between different kinds of batteries for UPS application.
The traditional method of charging batteries is to apply constant current and constant
voltage in two consecutive periods. Constant current is applied at the beginning of a typical fullcharge cycle, when the battery voltage is low. When the battery voltage rises to a specified limit,
the charger switches to constant voltage and continues in that mode until the charging current
declines to nearly zero. At that time, the battery is fully charged. During the constant-voltage
phase, the current drops exponentially due to the sum of battery resistance and any resistance in
series with the battery (much like charging a capacitor through a resistor). Because current drops
exponentially, a complete, full charge takes a long time.
Flywheel
Flywheel is simply a mechanical mass that is placed on the shaft of a motor–generator set
and stores mechanical energy in the form of kinetic energy. When the electrical power is
required, this kinetic energy is converted to electricity by the genera-tor coupled with the
flywheel. Flywheels are the oldest type of energy storage devices. The advantages of flywheel
energy storage systems are high efficiency, high energy and power density, and long life. On the
other hand, flywheels are more expensive and require more space than batteries and fuel cells.
There are also some safety concerns about flywheels rotating at high speeds.
Fuel Cell
Due to high efficiency and low emissions, fuel cell systems have been gaining popularity
in recent years. A fuel cell uses hydrogen as fuel and produces electricity, heat, and water from
the reaction between hydrogen and oxygen. Each cell consists of an electrolyte and two
electrodes as anode and cathode. Figure 24.17 shows the configuration of a typical fuel cell system. There are different kinds of fuel cell system depending on the types of electrolyte and
hydrogen sources. Some fuel cell systems have an on-board fuel reformer and generate hydrogen
from natural gas, methanol, and other hydrocarbons. Recent technology development in this field
has made fuel cells a more reliable and cost-effective alternative for batteries. Fuel cells
currently have a variety of applications in automotive, electric utility, and portable power
industries. Table 24.3 provides a comparison between the most popular types of fuel cells.