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Pears under solar panels
© KU Leuven – Departement Biosystemen – Thomas Reher
Researcher

Pears under solar panels

Will local farmers of the future provide us with both food and energy? Bioengineers at KU Leuven are testing agrivoltaics.

5 minutes
02 August 2021

Bierbeek is home to a pear orchard covered with solar panels. The fruit farmer, who studied bioengineering, is collaborating with Leuven researchers on a pilot project: is it possible to harvest pears and electricity at the same time? In the future, local farmers may be able to supply us with both food and energy thanks to ‘agrivoltaics’.

There are many farms with solar panels on the buildings – on the roofs of stables, barns or sheds – to generate electricity for immediate use, for example to provide the power that storage fridges for fruit consume. And there are also a small number of solar parks: large stretches of land, that are used for solar panels.

However, these do not provide a solution to our growing need for sustainable energy, Professor Bram Van de Poel at the Department of Biosystems tells us. “We could cover all roofs with solar panels, but they would not provide enough green energy to supply this country’s total electricity demand. Furthermore, sacrificing our limited and fertile agricultural land for solar parks is not a good idea. We must therefore integrate the production of food and energy, and this is possible using agrivoltaics. Half of the surface area of Belgium is agricultural land, so this can provide a sustainable solution.”

Pears under solar panels
© KU Leuven – Departement Biosystemen – Thomas Reher

Agrivoltaics

Agrivoltaics stands for a combination of agriculture and photovoltaic cells. And the latter are not solar panels on roofs, but above or between fields with crops or fruit trees. The panels are installed on a construction above the fields or stand in rows between the crops in the field. KU Leuven, the Innovation Support Center for Agricultural and Rural Development and a number of farms are cooperating to test various agrivoltaic systems at different test sites.

In Bierbeek, it looks as though there are little roofs above the pear trees. “There were already hail nets in the pear orchard of the fruit company Van der Velpen. We used the constructions for the nets to install the solar panels. These are semi-transparent solar panels. The solar cells in the panels are spaced further apart, so light can get through, which is essential for the pears to grow. These are also bifacial solar panels: double sided panels that absorb light on both sides, both the direct sunlight and the reflection of that light from the ground.”

Pears under solar panels
© KU Leuven – Departement Biosystemen – Thomas Reher

Microclimate

It wasn’t hard to convince fruit farmer Jan Van der Velpen to collaborate: he saw that the solar panels would not only generate electricity, but would also have agricultural benefits. The solar panels protect the trees and ensure a favourable microclimate, Van de Poel explains. “They replace the hail nets. In the spring, they provide protection against night frost, which can damage the blossoms. The night temperature is one or two degrees higher under the panels. During heatwaves in the summer, they provide shade, making it one or two degrees cooler.” The fruit farmer adds that due to the reduction in sun hours, he may be able to leave the pears on the trees longer and stagger the harvest.

What about the light that the trees need? “It is about finding the right balance between a profitable harvest and extra electricity, since we don’t want to sacrifice a lot of the harvest for the additional energy. How much light can you take away from the plants? Studies conducted with lettuce and wheat in France and Asia demonstrate that it is possible to reach 80 to 90 percent of the normal yield. This does require a careful calculation to find the best approach with solar panels.”

Pears under solar panels
© KU Leuven – Departement Biosystemen – Thomas Reher

Calculation tool

At the Electrical Energy Systems and Applications Research Group (ELECTA) on the Technology Campus in Ghent, Professor Jan Cappelle and his team developed a tool precisely to perform these calculations. “You have to take a variety of factors into account. What is the orientation of the field? What crops grow on it? Pear trees grow for many years and cannot be moved. On some fields, there is crop rotation: beets one year, potatoes or corn the next. What is the orientation of the rows of crops? All these factors make a difference for solar panels: how should they be positioned? How many should there be and of what type? We first use our tool to make a 3D simulation of the optimal harvest under the panels before we start building.”

Depending on the plot and the plants, the solution will be different each time. In the pear orchard, the semi-transparent solar panels are mounted above the trees. “On another test field in Grembergen, there are traditional solar panels lined up vertically between rows of beets. These intercept more light, but they are cheaper than semi-transparent solar panels. And they can rotate: you can turn them so that they provide more shade during a heatwave, for example. At our own KU Leuven test farm TRANSfarm in Lovenjoel, standard solar panels will be placed horizontally over the crops, but they will be mobile. We will be able to increase or decrease the distance between the panels, and test the resulting effects.”

It is about finding the right balance between a profitable harvest and extra electricity.

Pear monitoring

The condition of the fruit or the crops under the solar panels is monitored very closely using sensors. “In the pear orchard, for example, we measure the temperature, light, humidity… We monitor whether the blossoms are delayed, the size of the first leaves, and we compare them to a standard orchard. And we are of course waiting for the first harvest: we want to know the yield and quality of the pears under the solar panels. The harvest will only be in late September or early October, but everything looks very good so far.”

The production of electricity with agrivoltaics is a different story. The generated energy is currently only economically worthwhile for personal use. “But farmers could supply electricity to families or companies in the area. Unfortunately, Belgian legislation does not yet permit this. If that obstacle were to be removed, agrivoltaics might really take off. There is great interest among farmers.”

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