Civil Engineering Reference
In-Depth Information
DC-DC buck converter that charges the battery, the maximum available
battery-side voltage is 70 volts for it to work efficiently in the PWM mode.
For the DC-DC boost converter discharging the battery, the minimum
required battery voltage is 45 volts. Assuming that we are using NiMH
battery, the cell voltage can vary from 1.55 when fully charged to 1.1 when
drained to the maximum allowable DOD. Then, the number of cells needed
in the battery is less than 70/1.55 = 45 cells and more than 45/1.1 = 41. Thus,
the number of cells required in the battery from the voltage considerations
is between 41 and 45. It is generally more economical to use fewer cells of
higher capacity than more lower-capacity cells. We, therefore, select 41 cells
in the battery design.
Now again for an example, let us assume that the battery is required to
discharge 2 kW load for 14 hours (28,000 Wh) every night for five years
before replacement. The life requirement is, therefore, 5
365 = 1,825 cycles
of deep discharge. For the NiMH battery, the cycle life at full depth of
discharge is 2,000. Since this is greater than the 1,825 cycles required, we can
fully discharge the battery every night for five years. If the discharge effi-
ciency is 80 percent, the average cell discharge voltage is 1.2 V, and we desire
three batteries in parallel for reliability, each battery Ah capacity calculated
from the above equation is as follows:
×
28000
080 41 12 10 3
Ah
=
=
237
(12-5)
[
]
⋅
.
⋅
⋅
.
.
⋅
Three batteries, each having 41 series cells of 237 Ah capacity, therefore,
will meet the system requirement. Margin must be allowed to account for
the uncertainty in estimating the loads.
12.5.3
pv Array Sizing
The basic tenet in sizing the stand-alone “power system” is to remember
that it is really the stand-alone “energy system.” It must, therefore, maintain
the energy balance over the specified period. The energy drained during
lean times must be made up by the positive balance during the remaining
time of the period. A simple case of a constant load on the pv system using
solar arrays perfectly pointing toward the sun normally for 10 hours of the
day is shown in
Figure 12-17
to illustrate the point. The solar array is sized
such that the two shaded areas on two sides of the load line must be equal.
That is, the area
. The system losses in
the round trip energy transfers, e.g., from and to the battery, adjust the
available load to a lower value as shown by the dotted line.
In general, the stand-alone system must be sized so as to satisfy the fol-
lowing energy balance equation over one period of repetition.
oagd
must be equal to the area
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