Environmental Engineering Reference
In-Depth Information
Table 5.4
Monthly and yearly totals of solar radiation in kWh/m
2
for different locations and orientations.
Location
Berlin
Madrid
Los Angeles
Cairo
Orientation
30 ° south
60 ° south
30 ° south
60 ° south
30 ° south
30 ° south
February
28
31
100
113
139
157
April
117
106
172
141
201
204
June
151
127
214
163
175
214
August
153
135
209
175
223
220
October
68
87
105
137
164
201
December
21
25
73
82
128
146
Year
1139
1042
1855
1675
2089
2310
PV Power Needed for Island Systems
The required MPP power
P
MPP
of a photovoltaic module can be roughly calculated
based on the solar radiation
H
solar,m
during the worst month in kWh/m
2
(Table 5.4 ),
the electricity requirement
E
demand,m
for the same month, a safety margin
f
S
of at least
50% and the performance ratio
PR
(on average 0.7):
1
kW
m
solar,m
(
1
+
fE
PR
)
⋅
2
S
demand,m
P
=
⋅
.
MPP
H
The battery should be dimensioned so that it is planned to be discharged to only
half its power and can supply the total electricity requirements for a number of
reserve days. In Central Europe up to fi ve reserve days are enough to ensure reliable
operation in the winter; in countries that get more sun only two to three reserve days
would be enough. The required battery capacity is calculated on the basis of the
battery voltage
V
bat
(e.g. 12 volts):
2
⋅
E
V
d
demand,m
r
C
=
⋅
31
bat
For example, a photovoltaic system should be capable of operating an 11-watt low-
energy light bulb in a summerhouse for three hours every day in the winter. The monthly
electricity requirement for one month is then
E
demand,m
= 31 · 11 W · 3 h = 1023 Wh.
With a safety margin of 50% = 0.5 and a performance ratio of 0.7, a module in Berlin
orientated towards the south and tilted 60° then has a required MPP power of
1
25
kW
m
kWh
m
(
) ⋅
1
+
0 5
.
1023
07
Wh
2
P
MPP
=
⋅
=
87 7
.
W.
.
2
With four reserve days and a battery voltage of 12V, the battery capacity is calcu-
lated as follows:
2 1023
12
⋅
Wh
V
4
31
C
=
⋅
=
22
Ah.
