Environmental Engineering Reference
In-Depth Information
(4.104)
Therefore, diodes with low forward voltages such as Schottky diodes (
V
D
~
0.55 V) are often used. Cables cause further losses; a connection cable with
cross-section
A
, specific resistance
and cable lengths
l
1
and
l
2
for the cables
from the photovoltaic generator to the battery and back, respectively, causes
the following losses:
ρ
(4.105)
mm
2
/m) with cable lengths (
l
1
=
l
2
= 10 m),
a cross-section of
A
= 1.5 mm
2
and a current
I
PV
= 6 A causes cable losses
of
P
L,cable
= 8.4 W. Assuming a photovoltaic generator power of 100 W, these
cable losses plus the blocking diode losses of 3.3 W are considerable at 12
per cent of the power generated. To minimize losses, cables should be as short
as possible and the cable cross-section appropriately large. For a 12-V battery
system, a voltage drop of 3 per cent, or 0.35 V, is acceptable in the cables from
the photovoltaic generator to the battery and 7 per cent, or 0.85 V, from the
battery to the load. For the above example, the cable cross-section must
therefore be 6 mm
2
.
For systems with higher power, the losses can be reduced if some batteries
are connected in series. This increases the battery voltage
V
Bat
and decreases
the current flow and thus the losses.
The photovoltaic generator has the voltage:
A copper cable (
ρ
Cu
= 0.0175
Ω
(4.106)
The diode voltage
V
D
is nearly constant, the cable voltage drop
V
C1
and
V
C2
are proportional to the photovoltaic current
I
PV
. The battery voltage
V
Bat
depends on the charge current and state of charge. Hence, the voltage at the
photovoltaic generator increases slightly with rising currents and increasing
Figure 4.45
Simple Photovoltaic System with Battery Storage
