Solar Module Selection

The common factors that can be influenced by an installer (or a user/designer) in order to improve the performance of a PV array are location, tilt angle and orientation. For every solar module, more light means more output energy, but this can further be enhanced when shading is eliminated, the module is optimally oriented, and it is mounted at an appropriate angle of attack to the horizontal. Unlike some solar thermal collectors, a PV module’s output is almost linearly dependent on the incident radiation, so even small enhancements in incident radiation can make a positive big difference (Ross & Royal, 1999:25).

Optimum location: The caveat is to avoid shading as much as possible (The Renewable Energy Hub). Shading is one of the most detrimental factors to be put into check when designing a system and when calculating the available solar radiation. Every module in a solar array must be evenly and equally illuminated – shading of the solar array decreases its output greatly (Ross & Royal, 1999:25). Merely shading one cell in a module will reduce the output of the module very significantly (Ross & Royal, 1999:26). To minimize shading, the solar array should be located as high above the ground and as far away from trees and other shading objects as possible (Ross & Royal, 1999:25). Solar Arrays consisting of a field of sub-arrays arranged in parallel must be spaced such that the front rows will not shade the rows behind them (Ross & Royal, 1999:25). However, when shadows cannot be avoided, the array must be enlarged so that it still produces the required amount of electricity (Ross & Royal, 1999:26). This increases the cost and leads to underutilization of the system. Hence, other techniques have been recommended to proffer good design that can mitigate the effect of shading, where it is inevitable, without unnecessarily oversizing the array (Falk, et al, 2010:126). They are discussed below.

• The use of single-string or multi-string inverters: Shading in any form, whether partial, small shadows and dirt such as leaves and bird droppings, can become problematic especially where central inverter is used (Falk, et al, 2010:129). Central inverter configuration has little ability to equally process the output of both the shaded and the shade-free modules, effectively causing mismatching (Falk, et al, 2010:126). However, the use of several single-string inverters or a multi-string inverter is recommended to mitigate effect of any form of shading (Falk, et al, 2010:129).
• Bypass diodes: The use of bypass diodes has potential to mitigate the effect of shading and avoid hot spot. They are connected in parallel to modules or string of modules thereby providing a short circuit in the affected areas and allowing current to bypass the shaded module or string of modules (Falk, et al, 2010:129).
• Optimum arrangement of modules: It is also possible to mitigate the effect of shading by physically positioning the modules in a more appropriate arrangement and by appropriate string configuration (Falk, et al, 2010:130).

Optimum direction: For fixed arrays, the optimum surface azimuth angle of the array is usually due south in the northern hemisphere and due north in the southern hemisphere, i.e., the array should face the equator.

Optimum tilt angle: Several practical installations have shown that mounting array on a pitched roof maximises area more than array on flat roof (Falk, et al, 2010:122). Designers should try to explore the feasibility of seasonal adjustment of the array tilt angle as a means of optimising system performance (Messenger & Ventre, 2010:300).