Solar power on top – potatoes down below

Solar energy is considered the new renewable energy with the greatest growth potential in Switzerland. Primarily, solar plants can be built on roof surfaces or over car parks. In contrast, space is limited in Switzerland for ground-mounted systems. However, powerful ground-mounted solar plants can also be built on agricultural land. One approach that is being explored worldwide is agrivoltaics: Photovoltaic modules generate electricity from above, and vegetables or grain grow underneath.

The world population is growing and, hence the need for food. At the same time, land areas are needed to produce green electricity in the fight against climate change. The combination of agriculture and photovoltaics, also referred to as agrivoltaics, could alleviate this conflict. In the magazine Scientific Reports, a team from Oregon State University recently calculated that less than one per cent of global agricultural areas would have to be equipped with solar modules to quench world power demand.

Pilot system in Baden-Württemberg

What agrivoltaics looks like and how it functions can be seen about 30 kilometres north of Lake Constance. The pilot plant on the Demeterhof farm of the Herwangen-Schönach farming community was installed three years ago. Together with the largest solar research institute in Europe, the Fraunhofer Institute for Solar Energy Systems, farmers have been testing the possibilities on 2500 square metres: The steel construction for the solar panels with an output of 194 kWp is five metres high, so that tractors still pass underneath. Winter wheat, potatoes, celery and clover grass are cultivated. 

The farmers have had three harvests so far. Results have been quite positive. In the rainy summer, the wheat and potato harvest was one fifth less than that of comparable areas without photovoltaic systems. In the very dry year of 2018, the yield was 12 per cent higher for celery and 3 percent higher for winter wheat thanks to the shade proved by the solar plant – the system lets less light through.

Higher costs

Despite the smaller crop, the efficiency of land use increased by 60 percent overall thanks to the PV system and the generated solar power. In comparison to normal solar plants, the agrivoltaic system on the Demeterhof farm produces 15 per cent more electricity thanks to solar modules on the upper and under side that use the sunlight reflected from the ground. However, investments for the steel structure have quite an impact on power generation costs. For an average plant with an output of 2MWp these costs are between 8 and 10 Euro-cents per kWh. As a result, agrivoltaic plants are cheaper than private roof-mounted systems, but more expensive than ground-mounted PV arrays. 

Combating climate warming

"Agrivoltaic systems make sense wherever in the world land is at a premium," says Tabea Obergfell, a geoecologist at Fraunhofer ISE, in the RWE-Blog. Such land-use conflicts occur in various densely populated countries, such as Germany and, in particular, Switzerland. According to the researchers, agrivoltaics and the shade it provides on agricultural surfaces could also be a response to increasing climate warming with higher temperatures during the summer. 

It's clear: In very hot regions with high levels of solar radiation, agrivoltaics is particularly advantageous. Solar production is higher as is the yield thanks to shade. According to the "Süddeutsche Zeitung" chilli, cherry tomatoes and jalapeño peppers were planted on a test area in Arizona. The chilli harvest was three times and the tomato harvest two times higher as compared to areas without photovoltaics. Jalapeño peppers grew well in both environments. After watering, moisture stayed in the ground longer and reduced water consumption. The solar modules also benefited and were about 9 degrees centigrade cooler during the day in comparison to the those on ground-mounted systems in the same region. The cooling effect improved efficiency and increased the annual electricity yield by about one per cent.

Agrivoltaics as a win-win situation

More information on this topic is available here.