Page 35 - Renewable Energy Devices and System with Simulations in MATLAB and ANSYS

22 Renewable Energy Devices and Systems with Simulations in MATLAB and ANSYS ®
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associated series resistance, R , and series inductance, L . These parasitic components are often
s
s
ignored when only a simple representation of a PV cell or panel is needed, but they should be taken
into consideration when more accurate modeling is required.
2.2.2 PV Operating Characteristics

While there are many environmental factors that affect the operation characteristics of a PV cell and
its power generation, the two main factors are solar irradiance G, measured in W/m , and tempera-
2
ture T, measured in degree Celsius (°C). The relation between these two factors and the PV operat-
ing characteristics can be modeled mathematically. First, we examine the ideal model that consists
of just the photocurrent source I and a diode, as it is shown in Figure 2.6.
ph
The photocurrent I depends on both the irradiance and temperature according to
ph
I ph ( GT) =  I scn + ( − G (2.1)
K TT n) 
,
 i  G n
where
I is the nominal short-circuit current
scn
K is the current temperature coefficient
i
G is the nominal solar irradiance, which is typically 1000 W/m 2
n
T is the nominal cell temperature, which is typically 25 °C
n
These values can be determined from the ratings listed for commercial PV cells or panels. Also, the
current I and voltage V of the diode are expressed by an exponential relation, and they are represented as
d
d
   
d (
V d
,
I TV d) = ()exp   aV T ()  −  1 (2.2)
IT

s
   t   
where
I is the diode saturation current
s
a is the diode ideality constant
V is the diode voltage
d
V is the thermal voltage of the semiconductor junction
t
The diode saturation current depends on temperature and can be defined as
I scn + ( −
K TT n)
i
I T () =  K TT n)  (2.3)
v (
s
exp   V ocn + aV T () −  −1

 t 
P I pv +
I ph D V pv
N –

FIGURE 2.6 Ideal PV model with a current source and diode.