The annual drought is not only affecting farmers in Southern Europe. In Central Europe, too, farmers complain about too little water – or water at the wrong time. This year was finally a relatively normal summer. Nevertheless, it ruined the harvests of many farmers in Germany. While there was plenty of sunshine and little rain in the spring and early summer, the rainfall was abundant during harvest time.
More water under the modules?
Agri-PV systems can protect against such imponderables. The promise is that the modules regulate the water balance. But do they really do that? The first signs that photovoltaics can actually ensure a balanced water budget are shown by the results of a study by a team of scientists led by Ulrike Feistel, Professor of Engineering Hydrology at the Dresden University of Applied Sciences, and Stefan Werisch, hydrologist at the Brandis Lysimeter Station.
With the support of the energy supplier EnBW, they measured the change in the soil water balance under various solar parks in Saxony and Brandenburg, including an agri-PV plant in Pillnitz near Dresden.
The research team assumed that compared to soil without solar modules, soil water recharge increases more strongly. In the areas of the solar park not covered by modules, the ratio between precipitation and evaporation remains the same as for an area without solar panels. Under the solar modules, however, evaporation decreases.
On the other hand, the precipitation water only enters the areas on the eaves side of the modules. This is because the rainwater runs over the modules and over the lower edge onto the ground. In a small area behind the module rows, the same amount of precipitation arrives as would fall on a surface without a solar system. Due to the shading against the sun that shines on the surface from the south, evaporation decreases.
Soil moisture determined
So much for the assumption. To investigate whether this is true, the team led by Ulrike Feistel and Stefan Werisch measured the soil moisture at a depth of ten centimetres under the solar parks in Boxberg. Pillnitz and Weesow and compared it with the respective reference areas.
The result is relatively clear: While the soil moisture on the reference area rose very quickly due to precipitation, but then declined rapidly again, the soil moisture under the solar modules remained relatively constant. Specifically, they measured a decrease in soil moisture on the reference area in Boxberg of between 0.99 and 1.88 per cent per day. Under the photovoltaic system, on the other hand, the soil moisture decreased by only 0.57 to 1.39 per cent per day. Above all, several dry days in a row caused a faster decline in soil moisture. During this time, the moisture under the modules remained more stable.
More water in dry periods
A similar effect has also been seen in Pillnitz under the elevated agri-PV system. It is true that less precipitation arrives under the solar system here as well. However, the soil moisture during dry periods only decreased by 0.35 per cent per day during the measurement in April 2020. The reference area, on the other hand, lost 0.69 per cent of soil moisture per day.
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In Weesow, the researchers measured at the end of October and beginning of November 2021. Here, too, it can be seen that less water arrives under the modules during the rainy days. However, less also evaporates. Under the solar system, the daily decrease in soil moisture was actually only 0.02 per cent, while on the reference area the soil lost 0.27 per cent of moisture every day. Evaporation measured Although the investigations have not yet yielded any findings on how the lower evaporation in relation to the shading affects the yields and how the yields develop under the drip edges of the modules. But it has become clear that the photovoltaic and agri-PV systems have a major impact on soil moisture. In addition, the reduced evaporation, together with increased water input into the soil during dry periods, could have a balancing effect on the water balance of the entire area, the researchers summarise their findings.
Researchers from the University of Science and Technology of China in Hefei in Anhui Province have also investigated how water evaporation behaves under agri-PV systems. For this purpose, they placed water basins under two different agri-PV systems. They placed a third water basin on a reference area. They measured evaporation with these basins over a longer period of time and compared them with each other. One of the two agri-PV systems had a rather open design, where the modules let a lot of precipitation through to the ground. The second system was a normal 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. You can download the special here free of charge. (su)
