The annual drought is no longer just a problem for farmers in Southern Europe. Across Central Europe, too, growers are struggling with too little water, or rain arriving at the wrong time. This year brought a relatively normal summer, yet many farmers in Germany still saw their harvests ruined. Sunshine and dry conditions dominated spring and early summer, while heavy rainfall hit during harvest season.
More water under the modules?
Agri-PV systems are said to offer protection against such uncertainties, with the promise that solar modules help regulate the soil’s water balance. But do they actually deliver on that claim? Initial findings from a study led by Ulrike Feistel, Professor of Engineering Hydrology at Dresden University of Applied Sciences, and hydrologist Stefan Werisch from the Brandis Lysimeter Station, suggest they can. With support from energy supplier EnBW, the team measured changes in soil moisture beneath several solar parks in Saxony and Brandenburg, including an agri-PV system in Pillnitz near Dresden.
The researchers hypothesised that soil beneath solar modules would see greater water recharge compared to uncovered ground. In areas of the solar park without panels, the balance between rainfall and evaporation remains similar to that of open land. Under the modules, however, evaporation is reduced – allowing more moisture to remain in the soil.
On the other hand, rainwater reaches only the ground on the eaves side of the modules, as it flows down the panels and off the lower edge. In a narrow strip behind each row, the same amount of precipitation is received as on uncovered land. But because these areas are shaded from the southern sun, evaporation is reduced, helping to retain more moisture in the soil.
Soil moisture determined
That was the theory – but does it hold up in practice? To find out, the team led by Ulrike Feistel and Stefan Werisch measured soil moisture at a depth of ten centimetres beneath solar parks in Boxberg, Pillnitz and Weesow, comparing the results with nearby reference plots. The findings were clear: while moisture levels in the reference areas rose quickly after rainfall, they also dropped off rapidly. Under the solar modules, by contrast, soil moisture remained more stable. In Boxberg, for example, the reference plot showed a daily moisture loss of 0.99 to 1.88 percent, whereas the ground beneath the modules lost only 0.57 to 1.39 percent per day. This effect was especially pronounced during periods of several dry days in a row.
More water in dry periods
A similar pattern was observed in Pillnitz beneath the elevated agri-PV system. Although less rainfall reached the soil directly under the modules, moisture levels still declined more slowly during dry spells. In April 2020, soil moisture under the system dropped by just 0.35 percent per day, compared to 0.69 percent per day in the reference area.
New study highlights potential of UK agri-PV
In Weesow, measurements were taken in late October and early November 2021. Once again, less water reached the soil directly beneath the modules during rainy days – but evaporation was also significantly lower. Under the solar system, soil moisture decreased by just 0.02 percent per day, compared to 0.27 percent on the reference area.
While the study did not yet assess how reduced evaporation and shading affect crop yields – particularly near the drip edges of the modules – it clearly shows that both photovoltaic and agri-PV systems significantly influence soil moisture. According to the researchers, lower evaporation combined with increased water retention during dry periods could help stabilise the water balance across the entire site.
Researchers at the University of Science and Technology of China in Hefei, Anhui Province, also examined how agri-PV systems affect water evaporation. They set up water basins beneath two different agri-PV configurations, with a third basin placed on an open reference area. Over an extended period, they measured and compared evaporation rates across all three setups. One of the agri-PV systems featured an open design that allowed significant rainfall to reach the ground, while the second was a standard agri-PV system with semi-transparent modules spanning the area.
You can read the results of the research project in China and the complete report in the special on photovoltaics for farmers.
Download the special report here, free of charge. (su)
