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This side and that. (02/2023)

The magic mirror.

Text: Andreas Lorenz-Meyer

The system of using the same piece of land to generate electricity and produce agricultural products is known as agrivoltaics. A graduate of the University of Basel has founded a start-up that offers a novel type of product in precisely this field.

Photovoltaic with dichroic mirrors
Dichroic mirrors (yellow to purple in the picture) let through the wavelengths important for plants and direct the rest to solar panels for electricity production. (Photo: Voltiris SA)

Tomatoes, cucumbers, lettuce: All plants need light to grow, but it’s not the whole spectrum they’re after. The green leaf pigment chlorophyll only absorbs light from a certain range of wavelengths and transforms it into chemical energy. That range aligns approximately with the colors blue, green and red. Yet plants mainly make use of the blue and red wavelengths and reflect green light – which is why we perceive leaves as green.

This fact was the core idea for Jonas Roch’s new start-up. A physicist working with plants? Indeed, for a long time, his work had nothing at all to do with them. As a postdoctoral researcher at the University of Basel, his area of focus was nanophotonics. During his time as a researcher, he participated in an Innovation Office training program in entrepreneurship at the university. And later on, when he was employed at an electricity company, his work focused on energy-related topics.

But then Roch discovered that the two areas – plants and physics – could indeed be combined. He began developing photovoltaic modules specially for greenhouses. These modules allowed light in the color spectrum used by crops to pass through while capturing the unused wavelengths to convert into electricity.

It was this idea that led to his company, Voltiris. Roch founded the start-up in 2022 together with economist Nicolas Weber and mechanical engineer Dominik Blaser. Their product: modules designed to be installed up on the glass roofs of greenhouses, where they would industriously generate electricity without depriving the plants below of the light they need for photosynthesis. The concept works because the modules are equipped with a so-called dichroic mirror. “It’s a very special type of mirror,” says Roch. “It reflects only certain colors of light and allows others to pass straight through.”

From nanostructures to photovoltaics.

Roch had encountered this “magic mirror” back when he was working as a postdoctoral researcher at the University of Basel. In his area of research, nanophotonics, the aim is to study how miniscule structures – one nanometer equates to one-billionth of a meter – interact with light. To research the complex optical properties of such nanostructures, Roch conducted experiments. And those experiments involved dichroic mirrors.

Those mirrors were just like the specially manufactured greenhouse mirrors he uses today. These models redirect the wavelengths of light that the plants don’t need toward solar modules, which, in turn, generate electricity. And the exact wavelengths the mirror filters out can be controlled to suit whatever is being grown in the greenhouse. “Near-infrared is always good,” explains Roch. Near-infrared waves are on the long end of the electromagnetic spectrum directly adjacent to visible light. “Plants don't need near-infrared, and it’s excellent for generating electricity.”

However, even colors inside the visible spectrum are suitable for producing power. For example, it’s possible to filter out green light, which would have been reflected by the plants anyway, without measurably compromising yields. The third part of the spectrum that is useful for photovoltaics lies at the extreme end of the visible spectrum and is known as “far-red” light. It signals to plants whether they should grow taller or wider. Yet it cannot be filtered out for all kinds of crops; cucumber plants need it, for instance. Leafy plants like lettuce or spinach, on the other hand, do just fine without far-red light.

Promising results.

The light filtration technique certainly works – Roch and his colleagues have proven that. From March through November 2022, they conducted a test run with tomatoes and other crops at a greenhouse in Wallis. The results were glowing. The team registered no decline in plant growth and produced considerable quantities of electricity. Yet the experiments in Wallis were carried out with only ten square meters of photovoltaic panels and dwarf plant varieties. That meant that the fruit yields – at five kilograms per square meter – were far removed from those of an industrial greenhouse operation. “A tomato farmer has an annual harvest of 50 kilograms of fruit per square meter,” says Roch. This means that larger tests are needed – and such experiments are currently underway with tomatoes in Bleiswijk, the Netherlands, and with cucumbers in Orléans, France.

The interim results are encouraging. The modules are suitable for industrial farming operations, and they even have certain advantages when it comes to insects, which can cause significant problems in greenhouses. Tiny insects known as thrips, for example, can reduce annual cucumber yields by ten percent. So, how do the modules help? “Insects are attracted to luminescent colors such as yellow and green,” explains Roch. “Because the mirror filters out green light, the number of insects under the modules decreases. That allows farmers to further reduce pesticide application, which is already used at far lower levels in greenhouses.”

In September, Voltiris launched its products on the market. The modules are currently generating electricity in one of Switzerland’s largest tomato greenhouses, and their operating area is being gradually increased to cover 1,000 square meters. Overall, the project is really taking off. Roch has high hopes for his invention. Using these modules, it would be possible to lower operating costs for greenhouse farming. They also help reduce greenhouse gas emissions and promote energy autonomy. The modules provide all the energy needed for the operation.

Moreover, if facilities were to switch from gas as a heat-transfer medium to a heat pump system that makes use of the surplus power, the greenhouse could produce 60 percent of its own electricity completely independently. That would go a long way to preventing situations like last year, in which 40 percent of Dutch greenhouse farmers were forced to temporarily halt operations due to the high price of natural gas.

“Greenhouses are and will continue to be key to food production.” Roch is certain of that. “They are currently the most efficient, water-saving way to grow food crops. They also protect harvests from extreme weather events, thereby increasing food security.” And now, thanks to these modules, greenhouses will improve land use as well, says Roth, “because now we can farm food crops and generate energy using the same amount of space.”

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