Samlex America SCC-30AB - Manuel (Page 7)

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General Description of solar system
I-V Curve for a typical 12V Solar Panel is shown in Fig. 2.2. The Open Circuit Voltage
V
oc
is 21V and the Short Circuit Current I
sc
= 2.7V.
Maximum Power Point in the example curve given above is where Vmp is 17V, and the
current Imp is 2.5A. Therefore, the rated or the maximum power Pmax in watts is 17V
times 2.5A, or 42.5 Watts.
Standard Test Conditions (STC) for Specifying Solar Panels
The I-V curve is also used to compare the performance of Solar Panel. The curve is,
therefore, generated based on the performance under Standard Test Conditions (STC)
of sunlight and device temperature of 25°C. It assumes there is no shading on the
device. Standard sunlight conditions on a clear day are assumed to be 1,000 Watts of
solar energy per square meter (1000 W/m
2 or 1 kW/m 2 ). This is sometimes called one
sun, or a peak sun. Less than one sun will reduce the current output of the PV device
by a proportional amount. For example, if only one-half sun: (500 W/m
2 ) is available,
the amount of output current is roughly cut in half.
Factors Affecting Voltage and Current Output of Solar Cell
The amount of electric current generated by photon excitation in a Solar Cell at a
given temperature is affected by the incident light in two ways:
• By the intensity of the incident light.
• By the wavelength of the incident rays.
The materials used in Solar Cells have different spectral responses to incident light,
and exhibit a varying sensitivity with respect to the absorption of photons at given
wavelengths. Each semiconductor material will have an incident radiation threshold
frequency, below which no electrons will be subjected to the photovoltaic effect.
Above the threshold frequency, the kinetic energy of the emitted photoelectron
varies according to the wavelength of the incident radiation, but has no relation
to the light intensity. Increasing light intensity will proportionally increase the rate
of photoelectron emission in the photovoltaic material. In actual applications, the
light absorbed by a solar cell will be a combination of direct solar radiation, as well
as diffused light bounced off of surrounding surfaces. Solar Cells are usually coated
with anti-reflective material so that they absorb the maximum amount of radiation
possible.
The output current of the Solar Panel can increase due to what is known as the “Edge
of the Cloud Effect”. As the sun moves into a hole between the clouds, your solar
panels will see full direct sunlight combined with reflected light from the clouds! They
will absorb more energy than they could on a cloudless day! Thus, a factor of 1.25
times the Short Circuit Current Isc is recommended when sizing the current capacity
of the Charge Controller.