CPD Programme
Sunlight
In order to compare the performance of
modules, standard test conditions (STC):
1000W/m
2
irradiance at air mass 1.5 and cell
-ve electrode
Load
Nonreflecting shield
temperature of 25C are used. The results are
N-type silicon
presented as a current/voltage (I-V) curve,
shown in Figure 3. Typical modules have a
P-type silicon
V of about 20V and an I of 5A at STC, with
oc sc
a maximum power point of 16 – 18V.
+ve electrode
Figure 4 shows relative efficiencies for the
Current
various materials and types of PV cell and
also indicates the surface area of module
Figure 1 Principle of operation for PV cell
required per kW of power produced.
Photovoltaic arrays
Frame
The power output of a PV system can be
increased by connecting modules together
to form an ‘array’. The number of modules
A connected in series (a series string)
determines the system voltage and the
B
current generated depends on the number
of series strings connected in parallel. The
C
PV array produces DC electricity which is
D
converted to AC using an inverter, before
it is used in parallel with the mains or
E
exported to the grid. There will be a small
drop in system efficiency as a result of the
conversion from DC to AC. In remote areas,
A) EVA Encapsulant
or if grid electricity were not available, the
B) Crystalline Cells
energy-storage system would be a bank of
C) EVA Encapsulant
batteries charged by the photovoltaic cells
D) White glass (Tempered)
during the day.
E) Opaque back film
Protection devices are also needed for the
Figure 2 Based on Mitsubishi polycrystalline TD series construction
DC side. The necessary devices will depend
on the system but could include:
• Bypass diodes (to provide a current path
Short
circuit
5
around a module or part of a module and
current
prevent possible overheating due to localised
(I
sc
)
Maximum
shading);
4
power
point
• Blocking diodes (connected in series with
(MPP)
a string to protect the modules from reverse
current flow); and
3
(I)
• overvoltage/lightning protection devices
and circuit breakers.
Current
Photovoltaic production has been doubling
2
every two years, increasing by an average of 48
per cent each year since 2002, making it the
world’s fastest-growing energy technology. At
1
the end of 2008, the cumulative global PV
installations reached 15,200 MW. Roughly
90 per cent of this generating capacity
0
0 5 10 15 20 25
consists of grid-tied electrical systems.
Voltage (V)
Open circuit Such installations may be ground-mounted
voltage (V
oc
)
(and sometimes integrated with megawatts
Figure 3 Typical I-V curve
farming and grazing) or built into the roof
or walls of a building, known as Building
> in a step batch process involving wafers thin-film alloy cells is 100 times less than that Integrated Photovoltaics or BIPV for short.
of silicon semiconductor material. It is of crystalline cells and can be formed using Solar PV power stations today have capacities
expensive, because of the high cost of wafers low temperature vapour-deposition. So a-Si is ranging from 10-60 MW, although proposed
and the lengthy production stages. The cell cheaper and the panels, more frequently rolls solar PV power stations will have a capacity
thickness of amorphous silicon (a-Si) alloy of photovoltaic strips, are relatively flexible. of 150 MW or more.
60 CIBSE Journal January 2010
www.cibsejournal.com
CIBSEjan10 pp59-62 cpd.indd 60 17/12/09 16:30:13
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