Solar modules

Modules come in a variety of sizes, types and ratings. The electrical performance of PV modules is given for Standard Test Conditions (STC). The common types of panel technologies used are mono-crystalline, multi-crystalline and thin film. The related cost of silicon-type panels depends on the layers used. The variation of temperature highly affects the efficiency of the module, for e.g. in Si-type, as temperature increases, the efficiency increases resulting in higher output and lower space site.

Traditionally, solar modules had 36 cells in series that could only be able to provide overvoltage to charge a 12V battery. With the advent of time, the number of cells in series has been increased to produce larger voltages such that there is compatibility with inverters for AC grid-tied systems.

Mounting components of a PV module:

Fig.1. Global PV market share


Table 1: Solar cell base material parameters [1]
Material base Band gap, Eg (eV) Direct, D or Indirect, I Example of cell Efficiency of commercial cell (%)
Ge 0.6 I Not used 2.1
Mono-crystalline Si 1.1 I p/n 15
Multi-crystalline Si - - Commonest commercial cell 13
Amorphous Si - - Thin film or ribbon ~8
GaAs 1.4 D p/n 26.2
CdTe 1.4 D Thin film (heterojunction with CdS) ~8
CdS 2.4 D Only in heterojunctions ~8

Single crystals are made from single base materials particularly produced by the Czochralski process. They are available at a relatively cheap cost and their use has been encouraged in solar cells. On the other hand, the most common type of panel technology used commercially is the polycrystalline. This is due to the presence of boundaries between crystal grains that increases the recombination of electron-hole pairs. While for amorphous, the range of order within the material is short and is mostly used in thin film solar cells. With development, the amorphous cell can have an efficiency up to ~10% by having multiple junctions within 1┬Ám. However, the efficiency tends to lower with time, normally within the first few years of operation. Still, a remarkable merit of the a-Si as compared to other Si cells is that the efficiency does not decrease as temperature increases [1].

Quality marks of solar modules help customers in developing a sense for the quality of product they are buying. The quality of the module mainly focuses on the mechanical, electrical and safety characteristics.

  1. IEC 61215 (Testing procedures and requirements)

    • Power output at Standard Test Conditions

    • Continuous light simulation test including UV radiation tests (determines decrease of efficiency (lifetime)

    • Climate tests (heat damp, thermal cycling, humidity etc.)

    • Mechanical resilience (hail, snow, twist)

    • Thin-film related aging qualities / output degradation

  2. IEC 61730 (PV Module Safety Qualification)

    • Minimum requirements for materials, components and construction (part 1)

    • Minimum requirements for testing, defining of application and safety class (part 2) - types of solar cells used (Grade A to D) and voltage and fire resistance (Class 1 to 3)


References:

[1] J. Twidell and T. Weir, Renewable Energy Resources, 2nd Edition, Taylor and Francis Group, London and New York, 2006, pp. 200-220.