Page 156 - Advances in Renewable Energies and Power Technologies

2. PV System Under Nonshading Conditions 129

(no-solar radiation in the location, assume it is 2 days). So, the amount of energy
storage required can be obtained from Eq. (4.19). For safety, we divide the previous
value by allowable level of discharge, SOC L (90%)
Average kWh days of autonomy 5:5 2
E rough ¼ ¼ ¼ 14:67 kWh (4.19)
SOC L 0:75
The capacity of the battery bank in ampere-hours assuming we select a battery
voltage of 72 V¼(6*12), C ¼ 14,667/72 ¼ 204 Ah, and according to the selected
battery (UB-8D AGM-250 Ah, 12-V dc and a price of $475) [65], the number of bat-
teries needed is six batteries in series.
The battery block implements a generic model parameterized to represent the
most popular types of rechargeable batteries.
Q
E ¼ E 0 K þ A x e B i T (4.20)
Q i T
where E ¼ battery no-load voltage (V); E 0 ¼ battery constant voltage (72 V);
K ¼ battery polarization voltage (0.02 V); Q ¼ battery-rated capacity (250 Ah);
A x ¼ battery exponential voltage (0.01 V); B ¼ battery exponential capacity
1
(2.55 (Ah) )
The State-of-Charge (SOC) of the battery is between 0% and 100%. The SOC for
a fully charged battery is 100% and for an empty battery is 0%. The SOC is calcu-
lated as shown in the following equation:
Q

(4.21)
SOCE ¼ 100 1 R
idt
All the parameters of the equivalent circuit can be modified to represent a partic-
ular battery type, based on its discharge characteristics. A typical discharge curve is
composed of three sections:
The initial State-Of-Charge (SOC i %) of the battery: This parameter is used as an
initial condition for the simulation and does not affect the discharge curve.
The voltage factor (% of the nominal voltage) corresponding to the fully charged
voltage, for a given nominal discharge current: For example, a battery cell with a
nominal voltage of 1 V and a fully charged voltage factor of 105% has a fully
charged voltage of 1.1 V. Note that the fully charged voltage is not the no-load
voltage.
The internal resistance of the battery (U) is a generic value and is loaded, corre-
sponding to the nominal voltage and the rated capacity of the battery. The resistance
is supposed to be constant during the charge and the discharge cycles and does not
vary with the amplitude of the current.
The capacity, Q, extracted from the battery until the voltage drops under the
nominal voltage: This value should be between 0% and 100%, which is the voltage
(% of the nominal voltage) corresponding to the end of the exponential zone.
Control switches are necessary to control the charging and discharging of the bat-
tery as shown in Fig. 4.11. These switches are necessary to keep the battery from