Solar cells brave desert heat and snow

Solar modules must stand up to UV radiation, rain, snow and heat. To find out where their weak points lie, outdoor weathering stations are being set up at sites exposed to extreme climatic conditions: in the tropics, in the desert, and on top of Germany's highest mountain.

Manufacturers usually provide a warranty of 20 or 25 years on solar modules. But there is little reliable data on which to estimate the life of more recent designs. How resistant are they to snow, salt-laden sea winds, arid deserts or humid tropical climates? Researchers from the Fraunhofer Institute for Solar Energy Systems ISE in Freiburg have set up outdoor weathering stations at a variety of sites where solar cells can be exposed to extreme climatic conditions: heat and a wide fluctuation between day and night temperatures in the Negev desert, Israel; snow, high winds and intense UV radiation at the top of the Zugspitze, Germany's highest mountain; combined heat and humidity in Serpong, Indonesia; and corrosive sea air on the island of Gran Canaria, Spain.

'We are using these sites to test new materials for photovoltaic modules, such as alternative encapsulation techniques for the semiconductor layer or new reflective backing films,' says Michael Koehl, who heads the ISE's photovoltaics test center. 'Because solar modules have such a long warranty period, manufacturers are reluctant to try out new materials. Accelerated weathering tests might persuade them to be more open to innovation.' Sophisticated measuring instruments record the levels of UV radiation, temperature and humidity to which the modules are exposed. 'The secret is to choose easily measured parameters that exhibit significant changes after only two or three years. One such parameter is the UV transmittance of the encapsulation material: This changes well before any detectable drop in output,' Koehl explains. The researchers employ a mathematical model to calculate the mean statistical load averaged over all measured variables for each set of climatic conditions.

The outdoor stations will also be used to validate results of tests in a unique environmental simulation chamber currently under construction in Freiburg. Tests on solar modules are due to start here in spring 2008. The chamber intensifies the climatic conditions, enabling weak points in the modules to be detected sooner. It works on the basis of fluorescent lamps, which simulate the sun's UV radiation without generating as much heat as conventional Xenon lamps. 'This solves the main problem of UV testing: The lamps employed in a standard controlled-environment chamber raise the temperature of the modules, which in turn affects their lifetime - and distorts the results.' The new chamber, by contrast, enables the temperature of the module to be regulated to a constant value while at the same time setting the humidity to a high level.