Page 176 - A Comprehensive Guide to Solar Energy Systems

Chapter 8 • Photovoltaics: The Basics 177

FIGURE 8.27 An equivalent circuit of a module consisting of m in-series connected cells.

From the module characteristics, it is possible to apply the same parameters as for PV
cells, that is, short-circuit current I sC , open-circuit voltage V oC , maximum power P m , volt-
age V mp , and current I mp at the maximum power point, FF, and efficiency η. As follows from
the rules for connecting cells (section 8.3.3), in the case of in-series cell connections, the
output current of the string will be limited by the minimum current of all in-series con-
nected cells; in the case of in-parallel connection the output voltage will be influenced by
the minimum voltage of all in-parallel connected cells.
It is difficult to ensure exactly the same characteristics for cells connected in a module.
The problem comes from unavoidable parameter dispersion during cell fabrication and
causes so-called mismatch losses, so that the module output power is less than the sum of
the powers of the individual cells.
The module is the building unit for the PV generator and it is the PV product on the
market. The parameter list contains information measured at the output under sTC (the
sTCs are defined in section 8.3.2). The following are the parameters usually derived from
the module I–V characteristic measured using a flash simulator:

• Maximum power P m
• Maximum power voltage V mp
• Maximum power current I mp
• Open-circuit voltage V oC
• Short-circuit current I sC
• Module efficiency
• Temperature coefficient of I sC
• Temperature coefficient of V oC
• Temperature coefficient of maximum power (or efficiency)
• Maximum system voltage
• NOCT (nominal operating cell temperature)

8.5.2 PV Module Optical, Mechanical, and Thermal Properties

The encapsulation is a very important part of the module. The front should be transpar-
ent, allowing the solar radiation to penetrate the PV cell surface with a minimum of opti-
cal loss. The output contacts should enable easy and reliable connection to an external