Powered up, not heated up – the tram window of the future
What if climate-friendly mobility and heat mitigation could be combined in public transport? Does that sound too good to be true? It really does. But this might soon be an engineering reality in modern, sustainable public transport systems. Researchers at Fraunhofer ISE and the University of Freiburg published a study on organic semitransparent photovoltaic modules in the August edition of the Joule scientific journal. This technology could turn what sounds too good to be true into reality. By Johann Rauhaus
Some background information: semitransparent photovoltaic modules are not entirely new. Two technologies currently dominate the market here: crystalline thick-film cells and what are known as solar films. Photovoltaic modules based on crystalline thick-film cells achieve semi-transparency through gaps between the active areas. The modules therefore consist of opaque active areas that convert light into electrical energy, separated by gaps that allow light to pass through. The light transmission depends on the ratio of transparent to opaque areas and can be tailored almost freely. Light transmission is often between 40% and 50%. Solar films use very thin semiconductors on a transparent substrate. The thinner the semiconductor layer, the higher the light transmittance, typically ranging from 10% to 20%.
The organic semitransparent photovoltaic modules presented in the study allow just over 50% of the visible light to pass through while also performing the energy conversion, which is enabled through the use of organic semiconductors. Their unique optical properties allow them to strongly absorb radiation in the near-infrared range while maintaining a high degree of transparency in the visible spectrum. Thanks to the absorption of infrared radiation, heat mitigation comes as standard with these multifunctional semitransparent photovoltaic modules. Windows equipped with this technology would be barely distinguishable from commercially available solar-control glazing.
By reducing the amount of heat building up inside passenger compartments, this technology would lower the electricity demand of the air-conditioning units. In addition, it could also provide electrical power for other auxiliary equipment, such as information displays and interior lighting. Solar modules integrated into windows would not affect vehicle design or aerodynamics (unlike roof-mounted solar panels) and are also much lighter than traditional photovoltaic modules.
However, while conventional photovoltaic systems are readily available thanks to global mass production, there is still a lack of market-ready solutions in the case of organic photovoltaic modules. Nevertheless, the technology offers significant economic and environmental potential. Organic photovoltaic modules are made from carbon polymers. The cost of these raw materials is significantly lower than that of conventional solar-cell materials such as silicon, silver and copper, as well as cadmium and tellurium used in solar films. The challenges associated with geopolitical and geological scarcity are also becoming increasingly prominent and could be addressed with the use of carbon polymers.
The technology for organic photovoltaic modules has significant potential to enable clean, sustainable mobility in the future – not just in daily deployment in our cities, but along the entire value chain.