Berry crop shelters were among the earliest agri-PV systems to be put into practice, and they offer several clear advantages. The modules provide reliable protection for sensitive fruit against severe weather, giving growers peace of mind during hail or thunderstorms. In most cases, these modules replace conventional plastic film covers.
Yet for much of the growing season, the modules also cast shade. Jocelyn Widmer of Agroscope in Switzerland has examined how this shading impacts berry yields and fruit quality. His team at the Institute for Agricultural, Food and Environmental Research conducted scientific monitoring of agri-PV installations at three Swiss farms.
Mirroring real-world agricultural practice
Widmer’s team analysed both yield and fruit quality under agri-PV modules and conventional plastic tunnels, with particular attention to the daily light integral (DLI). The DLI measures the amount of photosynthetically active light, in moles, that reaches each square metre per day. “We are comparing agri-PV with the typical production methods used by Swiss farmers,” Widmer says. “Our aim is to replace existing structures with agri-PV solutions, not to introduce entirely new systems. That’s why our comparisons always involve covered approaches.”
The research sites are located in different regions of Switzerland. One 2,000-square-metre plantation is in the Berner Seeland near the Swiss capital, while a second site, covering more than 4,000 square metres, is in southern Switzerland. At both sites, farmers grow several strawberry varieties alongside different types of raspberries. The third installation, located in the Luzerner Seetal southwest of Zurich, is dedicated to raspberries grown on a 7,400-square-metre area in accordance with organic farming standards.
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Light levels drive yield and quality
Jocelyn Widmer presented the findings at this year’s Agrovoltaics World Conference, organised by the Fraunhofer Institute for Solar Energy Systems (ISE). The studies showed that light availability is a critical factor in agricultural yields. The research confirmed a clear trend: the more light plants receive, the more berries they produce. As expected, this effect was more pronounced in strawberries than in raspberries.
The picture was different when it came to fruit weight, however. Each individual berry weighed the same, regardless of light conditions. The researchers also found no correlation with acidity levels. In contrast, sugar content declined as shading from the solar modules increased. “Less light meant less sugar,” says Jocelyn Widmer, highlighting another key quality factor. “As for berry firmness, we observed no impact in raspberries. In strawberries, however, less light resulted in softer fruit, which may be linked to the lower levels of solar radiation.”
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Practical guidelines for agri-PV design
Overall, and as expected, strawberries are much more sensitive to shading than raspberries. “We found a strong relationship between DLI, yield and sugar content in strawberries. For stable yields, strawberries should receive at least 25 moles per day per square metre. Raspberries, on the other hand, tolerate shading better. Some yield reductions are possible, but the system can be optimised. Here, we recommend a DLI of at least 15 moles per day per square metre,” Widmer concludes.
With these light exposure guidelines, agri-PV systems could in future be installed above both strawberry and raspberry crops. One key finding from the researchers: raspberry yields are higher under any type of protective covering – whether solar modules or plastic film roofs – than in open fields without cover. This also holds true for average fruit weight. (su)
