Sun-tracking systems are taking on a growing role in agrivoltaics, especially on larger sites where they make it possible to farm and generate solar power side by side. These trackers typically operate just like those on conventional installations. When it’s time for fieldwork, farmers simply rotate the module tables to give their machinery plenty of room to pass through.
At the same time, the modules create shade on the ground below. The distance between each row of modules plays a crucial role in this effect. Maria König and Hubertus Wiberg from the University of Natural Resources and Life Sciences (BOKU) in Vienna studied how row spacing influences outcomes at a site in Bruck an der Leitha, Lower Austria, southeast of Vienna.
Experimental setup and parameters
The facility features tracker systems with varying row spacings: one section has trackers set six metres apart, another has nine metres between rows, and a third area uses a twelve-metre spacing. For comparison, a fourth plot was left without any solar installation as a reference.
All tracker rows are aligned north–south, so the modules can rotate from east to west during the day to maximise solar yield. The facility has a total capacity of three megawatts. “To assess the impact of the agrivoltaic system, we collected data on several plant parameters, with three repetitions for each management width,” explains Hubertus Wiberg, describing the experimental approach.
Wiberg and König evaluated morphological development using the BBCH scale, which tracks a plant’s growth stage. They also measured stand height across the space between module rows to compare plant height under the modules with less shaded areas. To assess plant growth rate – meaning biomass development – and relative growth rate, or growth efficiency, they collected soil samples every four weeks. The trial covered an entire growing season, from sowing to harvest. Importantly, no nitrogen fertiliser was used on the plots during the experiment. This created standardised conditions for both crops grown at the site: grain sorghum and winter wheat.
Sorghum and wheat – growth trends and plant height
König and Wiberg observed that sorghum developed fastest in the control area without any solar installation. The narrower the spacing between module rows, the slower the sorghum plants grew. However, by harvest time, these differences had disappeared, and the morphological development of the plants was the same regardless of tracker row spacing, matching the results from the control plot. The same trend was seen with winter wheat, although its growth remained more consistent overall, regardless of tracker spacing. On the control plot, the wheat developed at a similar rate.
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The results for plant height told a different story. Sorghum reached its greatest height on the control plot, with plants growing shorter as the tracker rows moved closer together. For winter wheat, the opposite pattern emerged: the closer the module rows, the taller the plants grew. There was no difference in wheat height between the plots with tracker spacing of nine or twelve metres, while the shortest wheat was found on the control plot without a solar installation.
Biomass development and growth efficiency
These findings also influenced biomass development and, ultimately, harvest yields. Sorghum grew fastest on the control plot, while its biomass growth declined as tracker spacing decreased. “When we look at the relative growth rate, namely biomass gain per existing biomass per day, we see that the narrowest spacing between trackers delivers the highest efficiency in biomass growth,” says Hubertus Wiberg.
For winter wheat, the results told a very different story. “We found that not only was peak growth lower on the control plot, but that it also maintained an almost consistently positive growth rate throughout the season,” explains Hubertus Wiberg. “This could indicate that winter wheat on the control plot was unable to reach its full potential.”
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For relative growth rate, the wheat plants performed more or less equally across all sections of the site, with hardly any differences between the areas. “This suggests that overall biomass gain, or the rate of biomass production, was actually better on the agrivoltaic plots than on the control plot,” says Wiberg. According to Hubertus Wiberg, this indicates that winter wheat is better able to adapt to the changing conditions between the solar trackers than sorghum, at least for this region and location.
Shading effects and spatial variation
The researchers also observed that plant development varied within the spaces between tracker rows. A clear pattern emerged: the closer the plants were to the trackers, the less well they developed. This effect became even more pronounced as the module rows were positioned closer together. The researchers therefore suspect that shading, rather than water distribution, is the main factor influencing plant growth. (su)
