Author: Benedikt Vogel, on behalf of the Swiss Federal Office of Energy (SFOE)

******** 

Much of Austria and Switzerland lies within the Alps. This means that both countries face similar challenges when it comes to the use of photovoltaics (PV). Austria aims to increase renewable energy production at high altitudes, for example to power tourist resorts. In the ski resort of Sölden (Tyrol), an alpine solar plant comprising 800 ‘solar trees’ is due to be built this year. Each of these structures consists of four ‘wings’ mounted on a central column, each of which houses bifacial photovoltaic modules. Once completed, expectedly in October, the plant is expected to reach a capacity of up to 6.3 MW.

Switzerland is promoting the development of solar energy in the Alpine region, giving a decisive boost to the process through an urgent federal law known as the “Solarexpress”. This incentive scheme has enabled the construction of the first large-scale Alpine plants, MadrisaSolar (11 MW, Graubünden) and Sidenplangg (8 MW, Uri), which from autumn 2025 will contribute, amongst other things, to the production of valuable winter energy. Both plants are currently being expanded. Other large-scale projects are under construction in Graubünden, including SedrunSolar (20 MW) and NalpSolar (8 MW). To stimulate new projects, the so-called “winter bonus” has come into force, replacing Solarexpress as the new incentive scheme.

Heavy loads, freezing temperatures

Large-scale solar power plants in the Alps are still relatively rare. However, precisely because energy generation above the fog line offers great potential – particularly for supplying electricity during the winter – the sector is now the focus of intensive research. Photovoltaic modules designed for the Alpine environment must meet specific requirements: they must withstand heavy snow and wind loads, as well as very low temperatures and high exposure to ultraviolet radiation. The aim of the Swiss-Austrian PV-DETECT project was precisely to identify which modules were actually capable of withstanding these conditions. As part of the study, the University of Applied Sciences and Arts of Southern Switzerland (SUPSI) collaborated with the Austrian Research Institute for Chemistry and Technology (OFI) in Vienna. In Switzerland, the project was funded by the Swiss Federal Office of Energy as part of the European research network SOLAR-ERA.NET.

Thanks to PV-DETECT, researchers have been able to define more precisely the characteristics that a photovoltaic module intended for the Alpine environment should possess: the use of framed double-glazed modules, with glass panes at least 3 mm thick each, is recommended. As regards encapsulation, particular attention is paid to materials belonging to the polyolefin family. “Polyolefin elastomers (POE) retain their elasticity even at low temperatures and offer better protection for the cells against mechanical stress than ethylene vinyl acetate (EVA), which is commonly used in standard modules,” explains Anika Gassner, who wrote her PhD thesis on this topic at the OFI.

Amended test conditions

Currently, manufacturers of photovoltaic modules must subject new products to certification testing before they can be placed on the market. These tests (see Box 1) ensure the safety and robustness of photovoltaic systems and are based on the standards of the International Electrotechnical Commission (IEC). However, these standards are designed for conventional applications, such as residential rooftops or ground-mounted systems. “When modules are used in ‘non-conventional’ contexts, such as in alpine installations or building-integrated photovoltaic (BIPV) systems, standard tests are less suitable,” notes Ebrar Özkalay, a researcher at the PVLAB of the Institute for Applied Sustainability in the Built Environment at SUPSI. 

As part of the PV-DETECT project, test procedures have been recalibrated to reflect the specific operating conditions of modules installed in alpine environments and those integrated into buildings. BIPV modules, for example, tend to overheat more, as they dissipate heat less effectively from the rear side than traditional panels mounted on elevated structures. To address this issue, test temperatures were increased in the PV-DETECT project: in UV tests, for example, from 60 °C (according to the IEC standard) to 110 °C (PV-DETECT). Test conditions were also specifically adapted for modules intended for alpine environments. To simulate winter conditions, OFI researchers carried out static mechanical load tests in a climate chamber at temperatures as low as -40 °C, rather than at 25 °C as required by IEC standards. These tests led, among other things, to the conclusion – already mentioned above – that, for use in alpine environments, polyolefin-based encapsulation materials are more suitable than ethylene vinyl acetate.

Shorter delivery times

The Swiss-Austrian research team has also set itself a second objective: to modify the tests so that photovoltaic modules are subjected to particularly harsh conditions, such as extremely high or low temperatures. In the long term, this approach reduces testing times: fewer cycles are needed to observe any damage or signs of wear. Such acceleration is particularly desirable, given that testing of newly designed modules can currently take several months. “If we can reduce the duration of individual tests, we create the conditions for manufacturers to bring their modules to market more quickly,” explains Ebrar Özkalay of SUPSI. In the PV-DETECT project, for example, it was possible to accelerate the thermal cycling test, which analyses the impact of daily temperature fluctuations on photovoltaic modules. In the standard IEC procedure, modules are subjected to 200 cycles with temperatures ranging from -40 °C to 85 °C. By increasing the maximum temperature to 110 °C, the duration of the test was reduced by a third. Mechanical load tests and the so-called critical point test could also be shortened, whilst for other tests the acceleration was only partial or impossible.

Benefits for manufacturers and installers

The findings of the PV-DETECT project are relevant to companies developing photovoltaic modules for use in alpine environments or for building-integrated photovoltaic systems. But they are equally significant for installers working in mountainous areas: thanks to the project’s findings, they can assess more accurately which modules are actually suitable for use in an alpine environment. Installers also benefit from the new testing procedures, which they can apply directly, where necessary, to carry out their own checks and reduce the risks associated with module installation. As Ebrar Özkalay points out: “In alpine environments, photovoltaic systems involve high installation costs. This is precisely why it is essential to use high-quality modules capable of ensuring a long operational life and thus avoiding subsequent maintenance costs or, in the worst-case scenario, replacement costs.”

******** 

Test procedures

To ensure safe and reliable photovoltaic (PV) power generation, manufacturers must subject newly designed modules to a series of tests. Here are a few:
UV testing : this test assesses the extent of damage caused by ultraviolet radiation to photovoltaic modules. UV rays are part of sunlight, but have a shorter wavelength than visible light.
Thermal cycle test : these tests analyse the impact of temperature fluctuations on the modules. Temperature variations, for example between day and night, cause varying degrees of material expansion, which can lead to the failure of solder joints or the separation of the module layers (delamination).
Humidity testing : these tests measure the effects of high air humidity on the performance of photovoltaic modules.
Mechanical load testing : these tests assess the modules’ ability to withstand static loads, such as the weight of snow, and dynamic loads, such as wind action.
Hotspot testing : this test examines the formation of so-called ‘hotspots’, i.e. areas of localised overheating within the module. Damage typically occurs when parts of the module are temporarily shaded, for example by chimneys or trees, causing certain cells or areas of cells to overheat, as the shaded cells consume some of the energy generated by the module rather than sending it to the inverter. In alpine systems, shading phenomena can also occur between modules installed at a steep angle to maximise winter production (shading between parallel rows or self-shading).
Bypass diode testing : assesses the ability of bypass diodes to withstand high currents and temperatures. These safety components integrated into the photovoltaic modules prevent current flow through shaded cells, diverting the current and thus preventing damage from overheating.

Photovoltaic systems with a lifespan of over 30 years

As part of the PV-DETECT project, in collaboration with Bern University of Applied Sciences, CSEM and the Helmholtz Institute in Erlangen-Nuremberg, the long-term performance of photovoltaic systems was also analysed. The focus was on six photovoltaic installations in operation since the late 1980s or early 1990s, equipped with AM55 and SM55 solar modules (Jungfraujoch, Birg, Mont-Soleil, Burgdorf-Fink, Burgdorf-Tiergarten and Möhlin). Analysis of the available data revealed an average annual power loss rate (PLR) of -0.24%, a figure significantly lower than that generally reported in the literature (between -0.75% and -1% per year). After 30–35 years of operation, most of the modules still retained at least 80% of their nominal power.

It should be noted, however, that the modules analysed represent earlier generations of technology compared to current systems. The degradation rates observed are therefore not directly applicable to more recent systems. Modern modules, developed since the 1990s, offer greater efficiency, but are also more complex in design and, in some cases, less reliable than earlier solutions. “The excellent long-term performance of older modules demonstrates that the photovoltaic sector is capable of producing extremely reliable products even in harsh environmental conditions such as those found in the Alps,” notes Ebrar Özkalay. “At the same time, they highlight the crucial role played by material selection and manufacturing quality in ensuring a service life of several decades.”

Note: Three decades, three climates: environmental and material impacts on the long-term reliability of photovoltaic modules. DOI 10.1039/D4EL00040D.