Harvesting the Sun Twice: How Researchers in Freiburg Are Advancing Agrivoltaics

Mr Trommsdorff, how would you define agrivoltaics?

Max Trommsdorff: The idea is that AV combines agricultural production and photovoltaic electricity generation on the same piece of land. Fields, orchards or pastures are fitted with solar modules in such a way that farming can continue underneath or between them. The exact layout depends on the crop: For orchards and vineyards, roof‑like structures are used to allow tractors to drive through the rows. On arable fields, modules are installed in spaced rows to let enough light reach the soil.  In vegetable production some greenhouses are partially covered with PV modules. Above pastures, modules can be arranged in a way that sheep or cattle are able to graze beneath them.  But there exist as well specialised concepts for other settings. In shrimp farms in Vietnam, where I have conducted some parts of my research, the modules span water basins, supply power for pumps and simultaneously cool the water – a configuration that is called “aquavoltaics”.

How did you become involved in this field?

Max Trommsdorff: I studied economics at the University of Freiburg and have been interested in renewable energies from early on. In 2012 I attended a talk by Stephan Schindele of Fraunhofer ISE on AV. Back then it was a niche, almost futuristic topic. I became a student research assistant at Fraunhofer ISE, wrote my master’s thesis on the subject, and have continued the research ever since. The breakthrough happened in 2018 with an AV facility on a community owned organic Demeter farm by Lake Constance. That spring and summer were exceptionally dry, and together with the University of Hohenheim we observed that three of the four arable crops grown under the semi‑shaded PV modules produced higher yields than the control plot in full sun—on top of the electricity that they generated. Since then, our project proposals have been far more successful and the team has steadily expanded; today the AV research group includes more than 40 staff members.

Tim Krieger: As an institutional economist, one of my research topics is “land grabbing” – analysing the conflicts that arise when investors want to lease or buy land that is governed more by customary law than by formal land registries. A research project in Mali and Gambia led me to AV, because the region has abundant sunshine and dry soils – in theory ideal conditions for generating photovoltaic electricity, reducing evaporation and boosting crop yields. In practice, however, many questions arose immediately: Who owns the modules? Who is allowed to use them? How do village communities react? The project has now been running for about five years, and almost everything that could go wrong has gone wrong. That experience taught me that AV is far from being just a technological issue; it is a complex interplay of engineering, natural sciences, law, economics and societal acceptance.

Where do we stand today with AV – in research and in the field?

Max Trommsdorff: From a research point of view the sector has exploded over the past few years. Demonstration sites are emerging worldwide, dedicated conferences have taken place and the number of peer‑reviewed publications has risen from virtually zero a decade ago to more than 150 per year. An important milestone was the release of the German pre-standard DIN SPEC 91434 in 2021, which for the first time clearly defined what may be called an AV system. One indicator is that after installation at least 66 % of the agricultural yield of the land must be retained compared to cultivation without any energy harvesting. Germany currently hosts only a few hundred megawatts of AV capacity – a drop in the ocean compared to roughly 100 gigawatts of total PV capacity. Nevertheless, promising pilot projects exist, for example a site installed by a winemaker at Tuniberg near Freiburg that was realised without a big project developer. On the other hand, there are schemes where the agricultural component is merely a fig leave. As scientists we must stay vigilant and provide robust, transparent methods for evaluating the quality of AV projects.

Tim Krieger: For AV to become mainstream, suitable political and economic frameworks are essential. These include clear rules on fiscal treatment and planning certainty for both agronomists and investors. Subsidies should be structured so that economically sound projects are attractive without creating a subsidy bubble. In my opinion this also offers an opportunity for the domestic economy. Standard silicon solar modules are now produced far more cheaply in China, whereas Germany focuses on specialised technologies – for instance organic solar modules that can be fabricated as thin films and can filter specific parts of the light spectrum. Such technologies create completely new applications, like simultaneously optimising electricity output and plant growth. In that respect, Freiburg is one of the world’s leading research hubs.

What role does the new FRIAS Project Group Agrivoltaics play?

Tim Krieger: Max and I quickly agreed that we needed to combine the strengths of the university and Fraunhofer ISE. Fraunhofer ISE excels at application‑oriented research, prototype development and highly accurate measurements. On experimental plots, for example, we test different module spacings or tilt angles and systematically compare the resulting electricity and crop yields.  The university provides expertise in answering the “why” questions. While previous work at the University of Freiburg has focused on building up soil organic matter and economic aspects, the collaboration with biological institutes will allow us to address questions regarding plant growth. Why does a plant behave differently under a particular module? What is the role of light spectrum, temperature or soil moisture? And why do for example radishes grown under AV sometimes become elongated rather than round, complicating their marketing?
The Project Group Agrivoltaics brings together the application focus with experts from biology, computer science, forest science, economics and – hopefully soon – legal and behavioural sciences, in order to find answers and pose new questions.

The Project Group Agrivoltaics brings together the application focus with experts from biology, computer science, forest science, economics and – hopefully soon – legal and behavioural sciences, in order to find answers and pose new questions.

Max Trommsdorff: What impressed me most at our first meeting in November 2025 was the reaction of the biologists. When they heard about organic solar cells – thin, flexible films that might eventually be tuned to transmit only specific light spectra – a flood of research questions manifested immediately: How does an altered light spectrum affect photosynthesis, flowering or pest infestation? So far, solar cells are being optimised mainly for maximal power output. In AV systems we could design them to create the optimal light environment for particular crops. That opens an entirely new playground for collaboration of physics and biology.

Tim Krieger: The trans‑disciplinary impulse was equally exciting. Arnim Wiek, who currently holds a Humboldt Professorship for Sustainable Food Economics at the Faculty of Economics, suggested ways to involve regional companies and the public early on. From other areas of the energy transition we know there is a critical tipping point: if people feel that something is being forced on them, trust quickly erodes and is hard to regain. We observed that the topic touches so many facets that it naturally connects to various other disciplines – law, behavioural science, etc. – which are still under‑represented. The FRIAS Project Group gives us an excellent platform to build new alliances.