Greenhouses and solar panels both need to be placed in areas with a lot of sunlight – so why not combine the two? Semi-transparent solar cells could potentially be built into the glass roof panels of greenhouses, capturing light at wavelengths that plants don’t use anyway. Now, researchers at North Carolina State University have modeled how this might work, and found that in some climates the cells could produce enough solar energy to make the greenhouse completely self-sufficient.

Organic solar cells (OSCs) have a few advantages over other designs. They still collect energy from sunlight, but can be made more flexible, transparent (or at least semi-transparent) and can be tuned to only absorb certain wavelengths of light. That potentially makes them perfect for greenhouse roofing – they can let most light through for the plants, while harvesting enough to offset a decent chunk of the facility’s energy needs.

“Plants only use some wavelengths of light for photosynthesis, and the idea is to create greenhouses that make energy from that unused light while allowing most of the photosynthetic band of light to pass through,” says Brendan O’Connor, corresponding author of the study. “However, until now it wasn’t clear how much energy a greenhouse could capture if it was using these semitransparent, wavelength selective, organic solar cells.”

A diagram of a greenhouse with organic solar cells built into the glass panels in the roof North Carolina State University

To begin to answer that question, the researchers modeled how much energy would be coming in from a greenhouse with OSCs in the roof, versus the amount of energy it would normally consume. The idea was to find the point where the greenhouse becomes energy neutral – that is, it generates enough energy from the Sun to completely power itself.

For this study, the theoretical greenhouses were modeled on the energy needed to grow tomatoes in three locations with different climates – Arizona, North Carolina and Wisconsin. As a bonus, the OSCs are effective insulators too, helping maintain the right temperature.

The team found that there would be a small hit to the amount of usable light the plants inside would receive, but the benefits would be worth it. In the famously sunny Arizona, for example, a greenhouse with OSCs installed could become energy neutral while blocking just 10 percent of the light the plants need. This shouldn’t negatively affect the plants, the team says. In fact, the energy output could be doubled with just a little more light blocked.

In North Carolina, sunlight is a little more sparse, so a greenhouse would need to block 20 percent of the photosynthetic light to become energy neutral. Chilly Wisconsin winters would be too much to ever achieve neutrality though, but these greenhouses could still generate almost half of their energy needs.

“While the technology does use some of the light plants rely on, we think the impact will be negligible on plant growth – and that the trade-off will make financial sense to growers,” says O’Connor.

This isn’t the first time researchers have looked into putting solar panels on the roofs of greenhouses. Back in 2012 a Spanish team developed a system that would harness surplus summer sunlight for energy while letting in the whole spectrum over winter, when it’s needed.

In 2017 a similar study tested magenta-colored solar cells embedded in greenhouse panels. Its focus was more on the plant-side of the equation, rather than the energy focus of the new study. It found that 80 percent of the plants weren’t affected by the dimmer light, while 20 percent actually grew better than in full sunlight.

The new study was published in the journal Joule.

Source: North Carolina State University