Photovoltaics - Understanding Renewable Energy Systems - page 137

characteristic parameters can be defined. This section describes the most
common parameters.
The voltage of a short-circuited solar cell is equal to zero, in which case,
the short circuit current I SC is approximately equal to the photocurrent I Ph .
Since the photocurrent is proportional to the irradiance E , the short circuit
current also depends on the irradiance:

The short circuit current rises with increasing temperature. The standard
temperature for reporting short circuit currents I SC is usually ϑ = 25°C. The

temperature coefficient α ISC of the short circuit current allows its value to be

estimated at other temperatures:

(4.41)

For silicon solar cells, the temperature coefficient of the short circuit current is
normally between

α ISC = +10 -4 /°C.
If the cell current I is equal to zero, the solar cell is in open circuit
operation. The cell voltage becomes the open circuit voltage V OC . The I-V
equation of the simple equivalent circuit, see Equation (4.34), provides V OC
when setting I to zero:

= +10 -3 /°C and

α ISC

(4.42)

Since the short circuit current I SC is proportional to the irradiance E , the open
circuit voltage dependence is:

(4.43)

The temperature coefficient α VOC of the open circuit voltage is obtained

similarly to the short circuit current. It commonly has a negative sign. For
silicon solar cells, the temperature coefficient is between

α VOC =-3 • 10 -3 /°C

and α VOC = -5 • 10 -3 /°C. In other words, the open circuit voltage decreases

faster with rising temperature than the short circuit current increases (see
section on temperature dependence, p139).
The solar cell generates maximum power at a certain voltage. Figure 4.21
shows the current-voltage as well as the power-voltage characteristic. It shows
clearly that the power curve has a point of maximal power. This point is called
the maximum power point (MPP) .
The voltage at the MPP, V MPP , is less than the open circuit voltage V OC , and
the current I MPP is lower than the short circuit current I SC . The MPP current
and voltage have the same relation to irradiance and temperature as the short
circuit current and open circuit voltage. The maximum power P MPP is:

(4.44)