Photovoltaic 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, and contradict this view.
Plant planners and those wishing to invest in photovoltaic plants have been concentrating to date mainly on roofs facing south, since these have the best exposure to sunlight, and therefore the best yields.
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The potential yield of a plant is also influenced by the tilt angle it has, which helps maintain the best irradiation angle, disregarding the time of the year. In an ideal world, the weather should be permanently sunny and cold, and the plant free from shadows and dirtying. In reality, the conditions for a solar plant are more often than not anything but ideal. The roof faces in the wrong direction, say eastwards or westwards, trees cast shadows, or chimneys, or the neighbor’s roof, it’s hot, or foggy, or cloudy, or dusty – these are factors that can reduce the yields of a solar plant.
Advances in the thin-film module technology have rendered even roofs with unfavorable preconditions interesting as far as harvesting solar energy is concerned. CIS modules capture yields that even crystalline modules cannot, even with partially shadowed roofs, unfavorable layout, or suboptimal tilt angle because of their specific physical characteristics.
Because of their good low-light behavior CIS thin-film modules produce high yields even with suboptimal irradiation angles, such as are present during early morning or late evening, or in winter and fall. The same is true for diffuse light (fog or cloudiness).
The difference in low-light behavior between CIS-modules and other technologies is easily explained: a deciding factor is CIS-modules’ use of more of the available light spectrum. On looking more closely at the spectral response (SR), which is the ability to absorb light of different frequencies, this becomes clear. CIS has a much higher spectral response than crystalline modules, and therefore a higher efficiency. The result: CIS-modules generate energy long after crystalline or amorphous modules have stopped production.
Solar harvest even with shading
A further advantage of CIS modules is their high shadow tolerance. Crystalline modules have the disadvantage that shading or partial shading through dirt or leaves causes failure of the entire module or indeed the entire string. The reason for this is the serial connection of cells in a module, and the connection of several modules to one string. CIS modules are a lot less sensitive: Shading or partial shading doesn’t cause failure of the module, but only of the covered or shaded part. CIS modules consist of long stripped cells, in the case of Solar Frontier’s modules 170 per module, as opposed to the square cells of crystalline modules. Because of this cell-shape, only that part of the module fails that is shaded, the rest continues generating. This is especially important in the residential sector, since there we more often encounter shading through trees or chimneys, which can have an effect on the efficiency of the plant.
Suited for climate conditions in the BeNeLux
Their product properties, especially the good lowlight behavior of CIS thin-film modules make them perfectly suited for the weather conditions to be typically found in the BeNeLux countries. This fact together with the environmentally friendly character of Solar Frontier modules were two of the main reasons for Solarclarity, an expert in the area of solar power, project developer and wholesaler of PV systems in the BeNeLux countries, to include Solar Frontier in its product portfolio. Regarding the ecological aspect, the refusal to use cadmium and lead mean the absence of environmentally hazardous materials contained in the modules and is the reason why although not yet a requirement in the photovoltaic industry, Solar Frontier is one of the very few PV manufacturers that comply with the stringent RoHS directive (Restriction of Hazardous Substances).
Since 2011 a great variety of installations have been constructed all over the region using the CIS thin-film modules distributed by SolarClarity, both in the residential as well as in the industrial sector. One example is a 31kWp PV system using Solar Frontier CIS thin-film modules designed by SolarClarity and installed as part of a renovation of a barn rooftop. It was connected to the grid in October 2012 in the Dutch town of Lelystad. The 200 CIS modules were installed on a 245 m² subarea of the southeast-facing rooftop of the building. The system is expected to produce more than 25,000 kWh of energy per year and will offset more than 21 tons of CO2 emission.
On top of the new black rooftop cladding, the black Solar Frontier modules match perfectly with the overall appearance of the building. The Solar Frontier modules were also selected due to the high output of CIS thin-film technology, as their good lowlight behavior leads them to start production earlier in the morning and stop producing energy later in the evening than other available technologies. The ammonia resistance of the CIS modules was taken into account as an additional benefit in an agricultural environment. All these advantages confirmed the agricultural entrepreneur and owner of the barn rooftop in his decision for Solar Frontier’s cadmium- and lead free CIS thin-film modules.
More yields with CIS modules
Long-term tests have shown that CIS modules maintain a higher power output than the label claims. The reason for this is the so-called ‘light-soaking effect’. This describes a special characteristic of CIS modules: after exposure to light their efficiency and power output climb. A suitable comparison might be with a car motor – once the motor has run warm, it reaches its most efficient and powerful state.
The technological advances in the CIS thin-film area make even unfavorable roofs interesting from the power generation point-of-view as they as CIS thin-film modules yield high harvests even under difficult conditions. With the right technology east-west roofs, roofs with shading, or roofs in regions with high levels of indirect or low light irradiation will play an increasingly important role in solar power generation.