Page 157 - Power Quality in Electrical Systems

Uninterruptible Power Supplies 139

separated from it through a vent that incorporates a flash arresting
device. Note in Figure 9.9 that the flooded lead acid battery has the
lowest specific energy and lowest energy density compared to other
batteries.
■ Valve-regulated lead-acid (VRLA) batteries: These have seen
tremendous growth in standby usage in the last two decades [9.11].
Note their approximately two-to-one advantage over flooded batter-
ies in Figure 9.9 in specific energy and energy density. In the VRLA
cell, the vent for the gas space incorporates a pressure relief valve to
minimize the gas loss and prevent direct contact of the headspace
with the outside air.

Standard VRLA battery warranties range from 5 to 20 years depend-
ing upon their construction, manufacturer-based requirements con-
cerning proper charging and maintenance, and whether the battery is
kept in a 25 C (77 F) environment compared to a 40 to 65 C outdoor
environment. When placed in an outdoor environment, the batteries
must be heated to prevent freezing, or loss of capacity. At 6 C (20 F),
battery capacity is reduced by 30 percent. At 16 C (4 F), battery capac-
ity is reduced by 55 percent. [9.12]

Flywheels
Flywheels were the original means for energy storage in early designs
of “no-break” engine-generator sets. (See Figure 9.2a.) They are return-
ing to serve for short-time supply in standby systems as an alternative
to batteries, and in other applications.
The energy stored in a flywheel is given by the classical equation:

W (1/2)I 2

2
2
where W energy, joules or watt seconds (m kg/s )
2
I moment of inertia (N m s )
rotational velocity (rad/s)
Note that the energy W stored in the flywheel is always known by the
speed .
Sample ratings are given by Weissback [9.13] of low speed systems
(less than 10,000 rpm) capable of delivering power over 1 MVA, with
energy storage below 10 kWh. Reiner [9.14] describes a flywheel plant
concept that can supply power peaks of 50 MW for about 13 s, equiva-
lent to energy storage of 181 kWh.
For perspective, consider a UPS that requires 1000 kW at its DC bus
for 10 s to insure time for start up and transfer to back-up engine gen-
erators. The calculated energy is 2.78 kWh. Assume that the flywheel