Fresh energy from the roof

Photovoltaic modules are integrated into the roofs of these trucks.

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Text: Oliver Schoenfeld
Photos: Fraunhofer ISE

The large roof surfaces of box bodies, with their attractive sunny position, are ideal for generating renewable energy. A Fraunhofer research team is therefore testing new concepts for vehicle-integrated photovoltaics.

Harnessing the power of the sun directly in the vehicle thanks to photovoltaics (PV) is not a new idea: the first solar sunroofs were already available in the passenger car sector around 20 years ago. At that time, however, the technology was nowhere near as powerful and efficient as it is today – the energy yield was at best sufficient to ventilate the car when it was stationary and to cool it somewhat. Today, vehicle-integrated photovoltaics, or VIPV for “Vehicle-Integrated Photovoltaics”, can do much more and is also suitable for trailers and refrigerated bodies, among other things. If used consistently, it helps to increase the range of electrically powered vehicles and improve the CO2 balance of commercial vehicle fleets. The potential is considerable: “If every truck in Germany were equipped with a solar roof, an installed capacity of eight gigawatts would be possible in the optimum case,” describes Christoph Kutter, project manager at the Fraunhofer Institute for Solar Energy Systems ISE.

Subsequent on-roof mounting or full integration

For electrified commercial vehicles, the starting conditions are particularly promising due to the high battery capacities. However, the extra boost of solar energy can also be worthwhile for commercial vehicles with combustion engines. In the course of the “Lade-PV” project funded by the German Federal Ministry of Economics, the Freiburg scientists are therefore developing lightweight and robust PV modules for two applications: retrofitting on the roof and full integration in the vehicle body.

»An installed capacity of eight gigawatts would be possible on truck roofs in Germany alone.«

Christoph Kutter, Project Manager Fraunhofer ISE

High energy yield with low weight

The lightweight modules developed at Fraunhofer ISE are particularly suitable for box bodies with their large roof areas. The requirements for VIPV modules in commercial vehicles are high: They should achieve a surface utilization factor of more than 90 percent and at the same time be vibration-stable, resistant to shear and bending, and easy to assemble. According to the project plan, their weight is limited to a maximum of 2.6 kilograms per square meter. “Lightweight PV modules are indispensable for the practicality of the solution so as not to restrict the payload,” continues Kutter.

Schematic structure of a lightweight module for refrigerated box trucks

Electric cooling of cargo in the future

In addition to the increased vehicle range, other advantages speak in favor of the development: The relief of the power grid and the charging infrastructure is just as much a part of this as cost savings in the purchase of charging power. In refrigerated trucks, the PV energy can also be used to cool the load electrically. This allows the identical power to be generated with less use of the cooling unit – and reduces diesel consumption. The integration of the PV modules into the heat sink requires particularly lightweight modules so as not to impair the thermal insulation.

First prototype in daily practical use

The concept developed by Fraunhofer ISE enables the area-wide use of integrated PV modules on electric and other commercial vehicles (over 3.5 tonnes total weight). A first demonstration vehicle, an 18-tonne E-truck, is already being tested at Alexander Bürkle GmbH & Co KG in daily distribution operations in the Freiburg area. The entire roof surface of the box truck is being used. On sunny days, the PV integration can produce up to 30 kilowatt hours of energy. In the accompanying measurement campaign, the irradiation potential of the routes driven is recorded, and the performance development and stability of the modules under real conditions are also regularly checked.
Commercial vehicles need particularly light and thin photovoltaic modules so as not to reduce the payload. Fraunhofer ISE has developed such lightweight modules, which are particularly suitable for the large roof areas of box bodies.

Range gain of up to 6,000 kilometres per year

The experience so far has been very positive, and the technology has proven to be robust and reliable. “We not only want to develop this technology, but also demonstrate that trucks can cover more than five percent of their drive energy with solar energy. 4,000 to 6,000 kilometres of additional range per year are mathematically possible. The VIPV will be worthwhile for manufacturers and operators of solar electric commercial vehicles,” Christoph Kutter continues. By the end of the project in 2022, the technology should be optimized to such an extent that it can be transferred to product development. The researchers expect scaling effects that will make solar cells in commercial vehicle integration highly attractive not only technically and ecologically, but also economically.
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