Research and practice: Photovoltaics A - Z
The following abbreviations for solar-cell technology are commonly used: • a-Si: amorphous silicon; used for thin-film solar cells. • c-Si: crystalline silicon • CI(G)S: collective term for copper-indium-diselenide and copper-indium-disulphide; common for thin-film solar cells. These semiconductors can also be alloyed with gallium (G). • CdTe: cadmium telluride; the third main semiconductor material for thin-film cells. Cost There is an unwritten rule in the PV sector: With every doubling of output from installations worldwide, module prices will fall by around 20 %. Recently this has been the case every two years, resulting in costs falling annually by 10%. Experts expect the downward trend in prices to continue for at least another one to two decades, so that modules and installations in 2020 will cost only half what they do today. Energy payback times With today's production technology the energy payback time for optimally oriented installations with crystalline modules in Central Europe, given global horizontal radiation of 1000 kWh/m2a, is around three years; with thin-film modules the figure is around 1.4 to 2.4 years (Ill. 8). In southern Europe (which at 1700 kWh/m2a is also representative for the worldwide average) that time period is reduced to around 0.8 to 1.7 years. For facades the maximum period is between 1.2 and 5 years. As a general trend, the energy payback time for the modules is likely to fall in the coming years, as the manufacturing of modules is becoming ever more energy-efficient. Operating temperatures Solar cell efficiency declines with rising temperature - in the case of crystalline modules, up to 0.5 % for each °C of temperature rise. Even with ventilated facades the modules can heat up to 80 °C; reducing their power output by over 25 % as compared to the specified rating. Over a period of a year they have a 5 - 6 % lower energy yield than free-standing modules (e.g. on fields or flat roofs); with non-ventilated facades the figure is up to 11 %. With thin-film modules these losses are at a lower level. Yields In Central Europe building-integrated PV installations that are properly oriented, well ventilated and virtually unshaded produce an average of 860 - 980 kWh of solar electricity per kWp and year; vertical south-facing facades produce around 550 - 650 kWh. In the case of crystalline modules with an efficiency ratio of 13 - 14%, this corresponds to around 110 - 140 kWh/m2a for unshaded roofs, or 70 - 85 kWh/m2a for facades; with amorphous thin-film modules the figure is 50 - 70, or 40 - 50 kWh/m2a.
