Plants are the original solar panels, and they’re very good at converting light into energy – red and blue light, that is.
But what if a solar panel could co-exist with plants, capturing the light that they aren’t using while allowing those red and blue wavelengths through?
That’s what a team led by Silvia Ma Lu of Mälardalen University in Sweden has tested in a new study, using semi-transparent, rose-tinted solar panels that let specific wavelengths of light through to plants growing below, while also capturing solar energy for electricity.
They’ve presented their prototype in the scientific journal Cell Reports Physical Science.
“The basic concept is quite straightforward,” Ma Lu says.
“The solar panels use part of the incoming sunlight to generate renewable electricity while allowing part of the light to pass through to the crops growing underneath.”
Perhaps we could be harvesting sunlight more efficiently by allocating different portions of the solar spectrum to crop growth and electricity generation, she says.
The magenta panels tested in this study, however, are more like a prototype than the final product. Their magenta color filters light in the green spectrum, but at present, that green light energy is not actually captured by the solar cells.
But the experiments suggest this could be a possibility in the future.
Ma Lu and team chose broccoli as their crop of choice to test the magenta cells.
Broccoli, being a cultivar of Brassica oleracea, is the same species as many other commercial crops, including cabbage, cauliflower, kale, Brussels sprouts, and kohlrabi. If broccoli could grow well in the rosy shade of the solar cells, perhaps others in its species will, too.
The researchers set up three different 20-square-meter plots of land to grow the broccoli.
In one plot, the broccoli was grown in full Sun, to provide a comparison point for ‘normal’ growth, sans solar cell.
On the other two plots, the broccoli plants were reared beneath magenta solar cells: on one plot, the cells had 50 percent transparency, while on the other, transparency was 70 percent.
“One of the most interesting findings was how similarly the broccoli performed under the two solar panel systems despite their different transparency levels,” says Ma Lu.
“However, these findings are specific to our experimental conditions and should be validated across additional growing seasons and system configurations.”
The broccoli plants all had similar yields when it came time to harvest, but the ones grown under the magenta panels took 25 days longer to reach that point.
Given that these plants received far less light than those grown in the full Sun, this actually means they were making more efficient use of the available light: 4.5 times greater under the denser, 50-percent-transparency panels, and 2.8 times greater under the 70-percent panels.
It’s not just the magenta tint that alters the light reaching those plants.
The active component of these cells, opaque cadmium telluride (CdTe), is embedded in strips across the glass panel.
It’s this element of the design that truly makes for a solar panel that light can pass through. If the strips are more densely spaced, then the cell can soak up more sunlight to produce electricity – but it also means less light gets through to the plants below.
That raises an interesting question: why do the panels need to be magenta at all, if the strips of CdTe let light through anyway, regardless of the glass color?
Well, they don’t, really. The glass could just as well be neutral.
These magenta panels were actually originally intended for building design, to offer a rosy glow to a building’s interior while also capturing some electricity. They ended up in this experiment purely because they were already commercially available, ripe for repurposing.
The magenta filter lets through two important light spectra for plant growth: red and blue. The leftover green light that the filter blocks isn’t necessarily captured by the solar cell.
As a result, it’s not entirely clear from this study whether it’s the magenta filter into which the CdTe strips are inlaid, or the overall light levels, that affected the broccoli’s growth. And there’s the greenhouse effect of laying glass over a growing vegetable: something this experiment does not account for.
More detailed experiments will be needed to tease out all those variables.
But since these rose-tinted panels are already on the market, Ma Lu says they could readily offer households and community groups a way to make the most out of a small space.
“Configurations similar to our prototype may currently be suitable for smaller-scale applications, such as community gardens, or for integration into greenhouse roofs rather than immediate deployment over large agricultural areas,” she says.
The idea of filtering different spectra of light to allocate the energy to separate goals – growing plants and generating electricity – makes sense in theory. However, it seems there’s still a way to go in creating a solar panel that would be worth it for larger commercial applications.
“From an engineering point of view, this is about getting the best overall use from sunlight and land,” explains power systems engineering expert Ramesh Rayudu, who was not involved in the study.
In terms of energy harvested, in the experiment the magenta cells produced around 224.7 MWh of electricity per hectare over the growing period – about 100 MWh per hectare less than conventional solar panels of similar size.
“Depending on the value of electricity and the price received for broccoli, that lost electricity could require roughly 1.2 to 3.2 tonnes of additional broccoli per hectare to offset its value,” Rayudu says.
“The real question is whether the extra complexity of wavelength-selective solar panels produces enough additional agricultural value to compensate for lower electricity generation. That is the comparison I would like to see next.”
The research has been published in Cell Reports Physical Science.
This article was fact-checked by Fiona MacDonald and edited by Fiona MacDonald. While we pride ourselves on our process, we are only human. If you spot a mistake, please let us know.
