PV plants on partially shaded roofs, with suboptimal layout or non-optimal tilt angles are deemed low yielders. Regions with lower irradiation values and a high portion of indirect or diffuse light are seen in the same dim light. Thin-film modules made of copper, indium and selenium can show their mettle even under these conditions, however, and contradict this view.
While the ideal roof for a PV installation in the UK is south-facing with a conventional roof pitch of 30 to 40 degrees, many roofs in reality have an east-west orientation and are partially or temporally affected by shade caused by chimneys, trees or neighbouring houses. Fog, clouds or pollution are also factors which can minimise the energy yield of PV installations.
Crystalline silicon modules, in particular, quickly reach their limit under such challenging conditions resulting in a comparatively low energy harvest. By contrast, thin film modules deploying a PV layer formed from copper, indium and selenium (abbreviated to CIS) offer a rewarding alternative.
Low-light performance
A decisive advantage of the CIS thin film modules produced by Solar Frontier is their good low-light performance which allows generating a maximum energy harvest even at times of low irradiation such as in the early morning or late evening hours as well as in autumn and winter. This also applies to diffuse light conditions usually caused by fog or heavy cloud. Compared to other module technologies, CIS modules can use a broader light spectrum which allows them to continue generating electricity under conditions in which amorphous or crystalline silicon modules have long stopped producing energy.
The performance of Solar Frontier’s CIS modules in cloudy or rainy weather, for example, was the determining factor for a 41.4 kWp installation on a barn roof near Blandford, Dorset. Installed in March 2011 in a farm environment, the ammonia resistance of the Solar Frontier modules was a helpful additional feature. Installed by the Save Energy Group, the system achieved very strong yields even during the first two months of operation despite a fault in the local grid that limited the system’s peak performance and the wettest April in England since records began.
Shadow tolerance
CIS modules are also less affected by shade than crystalline modules. While shadows or partial covering by dirt or leaves on silicon module installations may stop the energy generation by the affected panel and thus the entire string, CIS modules still generate electricity under the same conditions. The reason for this is the different cell structure of the two module technologies. Crystalline silicon modules usually consist of square cells, whereas Solar Frontier’s CIS modules comprise very narrow CIS cells stretching along the entire length of the module. This allows the CIS module to produce energy even if it is partially covered by shade.
The technological advancements in the CIS thin film sector create new opportunities to build profitable PV installations on roofs in sub-ideal conditions. By ensuring a maximum energy yield at times of low irradiation, CIS modules are particularly well suited for the UK weather as well as for roofs affected by partial shading. As a consequence, roofs with East-West orientation, partial or temporary shading or regions with low light conditions are now able to play a significant role in growing the share of PV energy in the future energy mix.