A study shows that even a greenhouse roof could generate profitable electricity: lettuce also thrives without any problems in the “filtered” light that shines through organic solar cells. Since they only use a frequency range that is unimportant for plants, the generation of electricity is not at the expense of yield and quality, according to experiments. This illustrates the potential of the technology for use in energy-intensive greenhouse cultivation, say the researchers.
When you think of solar cells, you usually think of the versions in which the light-absorbing layer is made of opaque silicon. These modules are of course not suitable for installation on greenhouse roofs. Because they would darken the area under cultivation and thus rob the plants of the light for photosynthesis. But there is already an alternative solar technology that could circumvent this problem: In organic solar cells, mixtures of carbon-containing plastics form the light-absorbing layer. The flexible material can be made semi-transparent – it only captures certain wavelengths to generate energy and lets the rest pass.
A special kind of solar cells
With regard to the use of this technology on greenhouse surfaces, a team of researchers from North Carolina State University in Raleigh is taking advantage of the fact that plants do not use the entire light spectrum for photosynthesis: For example, the “plant solar cells” hardly need the green frequency ranges to generate energy. In their previous investigations, the researchers focused primarily on how much energy greenhouses could produce with “plant-friendly” transparent solar cells. They made it clear: Although the energy yield is low compared to conventional solar cells, organic solar cells could make many greenhouses energy-neutral – or even feed electricity into the grid. So far, however, it has remained unclear how these semi-transparent solar cells affect the growth of greenhouse plants. Because it seemed possible that the supposedly unimportant light frequencies also have a certain significance for this.
To answer this question, the researchers examined the effects of different “filter light” on the development of red-leaved lettuce (Lactuca sativa). They exposed all the test plants to the same temperature, water supply and fertilization in test chambers – only the light was varied. The control group got the full light spectrum. The other salads were divided into three test groups: They were exposed to light that shone through different types of filters that, analogous to the semitransparent solar cells, absorbed certain wavelengths. These were frequency ranges that, at least in theory, did not appear to be important for the plants and could therefore be suitable for the development of organic greenhouse solar cells. After development from seed to maturity, all test plants were subjected to various analyzes.
No loss of earnings
“We were even a little surprised that we didn’t notice any significant impairments,” says co-author Heike Sederoff. Accordingly, removing the wavelengths did not affect traits that are important to growers, grocers, and consumers, including the number of leaves, leaf size, and the weight of the heads of lettuce. The plants were just as healthy as the controls and the levels of nutritionally relevant antioxidants in the lettuce remained unchanged, the analyzes showed. “We are now planning to take a closer look at the way in which different wavelengths of light influence the biological processes in salads, tomatoes and other greenhouse plants,” says Sederoff.
But the current results are already promising: “Not only did we not find any significant difference between the controls and the experimental groups, but also no important difference between the various filters,” says co-author Brendan O’Connor. This in turn could allow leeway in the design of the solar cells, say the scientists. “These are good prospects for the future of solar-powered greenhouses,” says co-author Harald Ade. “Getting growers to use this technology would be difficult if there was a loss of productivity. But as it turns out, it only needs to be weighed up whether the investment in the new greenhouse technology can be offset by energy production and savings ”.
Concluding, O’Connor says, “Based on the number of prospects who contacted me about solar-powered greenhouses when we published previous work in this area, there is a lot of demand from growers. We now have enough proof-of-concept prototypes for the solar cells that make it clear that this technology is feasible in principle. The task now is to find a company that recognizes the potential and starts producing on a large scale, ”says the scientist.
Source: North Carolina State University, Article: Cell Reports Physical Science, doi: 10.1016 / j.xcrp.2021.100381