Fraunhofer ISE brightens up solar car roofs

By Almut Otto

The International Motor Show (IAA) will take place in Frankfurt from the 12th to the 22nd of September. Most of it will revolve around the mobility of the future. In addition to the range of self-charging electric vehicles, Fraunhofer ISE is now also bringing the subject of optics into play: the Fraunhofer Institute for Solar Energy Systems (ISE) is presenting two solar car roofs at the joint stand of the Fraunhofer Gesellschaft (Hall 4.1, Stand C 12). What makes them so special is their colorful design.

Two years ago, the Fraunhofer scientists developed a Morpho-Color® glass coating. This was originally intended for buildings. But now ISE went one step further and built colored panels for curved surfaces such as car roofs.

“In order to achieve a CO2-free energy supply across all sectors, we must make substantial progress with regards to the expansion of photovoltaics, even beyond house roofs and open spaces. Solar modules will be integrated even more into our already built-up environment in the future, for example in vehicles,” explains Dr. Andreas Bett, Director of Fraunhofer ISE.

The Freiburg researchers are using shingle wiring in order to integrate photovoltaics into solar roofs. The monocrystalline silicon solar cells are arranged so that they overlap. They are then stuck together with a conductive adhesive. As a result, inactive surfaces caused by gaps between cells are avoided. The module surface can consequently be used for generating electricity to the full.

Behold the wings of the Morpho butterfly

Another special feature of the solar roof: The solar cells are completely hidden by a single color coating and are thereby invisible. The efficiency loss due to the Morpho-Color® glass coating is in relative terms just seven percent. The color effect was modelled on the bright blue wings of the Morpho butterfly. The unique, scale-like surface structure ensures a high level of color saturation combined with good and stable visual angles. Dr. Martin Heinrich, Head of PV for Mobility at Fraunhofer ISE, explains:

“The range of colors is almost infinite.”

The roof has an overall homogeneous, aesthetic appearance as a result of the range of colors in all spectral colors. Moreover, lower resistance losses, the absence of any shading from overlying cell connectors and a particularly high tolerance for shade ensure module efficiency that is up to two percent higher than that of conventional solar modules.

Support Us!

With a rated output of around 210 W/m², the roof of a mid-range electric car is expected to supply electricity for around ten km on a sunny day. That would mean an extended vehicle range of around 10 percent or a corresponding reduction in fuel consumption over a period of one year. This calculation is based on the (unshaded) energy gain from sunlight in Freiburg im Breisgau, an electric car consumption of 17 kWh per 100 km and an annual mileage of 15,000 km. This solar polar would be useable also for consumers who would normally limit the range of an electric vehicle (e.g. due to air conditioning, heating). Fraunhofer ISE sees the potential for research in the integration of photovoltaics into other vehicle surfaces.

According to Heinrich, the project is in its first stage of development. In future, research will be conducted into bringing e.g. the solar matrix to the engine and rear, which would further increase the range. And in a further step, consideration could then be given to installing the panels on the sides of cars as well. However, this would not be easy to implement due to the stringent safety regulations.

Areas of application range from cars to aircraft to shipbuilding

The solar cell matrix developed by ISE is laminated in a foil laminator between the glass panels of a commercially available, spherically curved panoramic car roof. The lamination process can also be done in a conventional laminator using a specially manufactured mould.

The functionality of the solar roof matches that of a standard metal car roof: The solar cells convert incoming sunlight into electricity. This also helps to reduce overheating in the car. Module voltage is higher than with a conventional module due to the shingle configuration. Moreover, it is easier to transform it to the battery voltage. The bonded shingle cells are able to cope well with the high thermal and mechanical stresses on traffic routes too.

The untapped potential lying dormant on vehicle roofs has already been researched by Fraunhofer ISE in cooperation with several transportation companies in a 2016-2017 survey campaign. Six trucks were equipped with sunlight and temperature sensors along with GPS. Their routes were then recorded in eastern USA as well as in central and southern Europe. For Europe, the researchers determined 5000-7000 kilowatt hours of annual power generation potential on a typical truck roof. This is equivalent to a mileage of 5000- 7000 kilometers. As part of the proposed Citizen Science project “PV2Go”, scientists at Fraunhofer ISE want to determine the solar energy potential for cars with the support of interested car owners.

Heinrich also sees further potential for the modules in aircraft, drones and shipbuilding.

The first of these solar roofs could go into series production by the end of next year. The surcharge for the solar cells currently in use should not be much more expensive than a panorama roof.