Page 164 - A Comprehensive Guide to Solar Energy Systems

Chapter 8 • Photovoltaics: The Basics 165

FIGURE 8.12 The influence of series resistance R s on PV cell efficiency in dependence on irradiance.

Figs. 8.12 and 8.13 demonstrate that operating temperature is a very important param-
eter for the efficiency of PV energy conversion. In a linear approximation, the temperature
dependence of cell efficiency can be described by
η T() = η STC + K T η T( STC −T ), (8.22) η(T)=ηsTC+KTη(TsTC−T),

where η sTC is the PV cell efficiency at sTC, K Tη is the thermal coefficient of efficiency, and
T sTC = 25°C. The thermal coefficient K Tη depends on the material bandgap, as shown in
Fig. 8.14.
The PV cell equivalent circuit and the I–V characteristics have been discussed for the
case of a constant or slowly varying irradiance. For fast changes (e.g., using a flash simu-
lator), it is also necessary to take into account the Pn junction capacitance that can be
(under conditions of a forward biased junction) relatively large and that can influence the
measured I–V characteristic shape.

8.3.3 In-Series and In-Parallel Connection of PV Cells
The open-circuit voltage V oC of individual cells is usually lower than 1.5 V and the photo-
generated current density is in order of tens of milliampere per square centimeter. solar
cells can be connected in series to obtain a higher output voltage or in parallel to obtain
higher output currents. The case of a series connected cell is demonstrated in Fig. 8.15A.
In this case, the same current flows through all solar cells. If the current generated in in-
dividual cells differs, the short-circuit current I sC of the series connected cells will be the
minimum I sC of all in-series connected cells and the voltage for currents I ≤ I sC will be the
sum of voltages at individual cells at the current I, as indicated in Fig. 8.15B. The maximum
power current I mp of a series connected cell will be limited by a cell with the minimum I sC
and, consequently, the output power of the series connected cells will be lower than the
sum of output power of individual cells. only in the case when the current I mp of a series
connected cells is the same, will the output power be the sum of output power of indi-
vidual cells, as indicated in Fig. 8.15C.