Some greenhouses could become energy neutral with the application of semi-transparent solar cells that utilise energy from the wavelengths of light not used during plant photosynthesis.
This is the claim of engineers, plant biologists and physicists at North Carolina State University (NC State) whose new modelling study estimated energy use for greenhouses growing tomatoes at locations in Arizona, North Carolina and Wisconsin. Their findings are published in Joule.
“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,” said Brendan O’Connor, corresponding author of the study and an associate professor of mechanical and aerospace engineering at NC State.
“We’re able to do this by using organic solar cells, because they allow us to tune the spectrum of light that the solar cell absorbs – so we can focus on using mostly wavelengths of light that plants don’t use. However, until now it wasn’t clear how much energy a greenhouse could capture if it was using these semi-transparent, wavelength selective, organic solar cells.”
Researchers used computational modelling to estimate how much energy a greenhouse could produce with roof-mounted semi-transparent organic solar cells, and whether that would be enough energy to offset the amount of energy required by the greenhouse to operate effectively.
“A lot of the energy use in greenhouses comes from heating and cooling, so our model focused on calculating the energy load needed to maintain the optimal temperature range for tomato growth,” O’Connor said in a statement. “The model also calculated the amount of energy a greenhouse would produce at each location when solar cells were placed on its roof.
The team found a trade-off between the amount of power the solar cells generate and the amount of light in the photosynthetic band that they allow to pass through, so if growers are willing to sacrifice photosynthetic growth, they can generate more power.
The team found also that solar cells used for this analysis are effective insulators, because they reflect infrared light, which helps to keep greenhouses cooler in the summer and warmer in the winter.
The result is that, for many greenhouse operators, the trade-off could be a small one, particularly for greenhouses in warm or temperate climates.
According to NC State, greenhouses in Arizona could become energy neutral while blocking 10 per cent of the photosynthetic band of light. However, if growers are willing to block more photosynthetic light, they could generate twice as much energy as they required to operate the greenhouse. In North Carolina, a greenhouse could become energy neutral while blocking 20 per cent of the photosynthetic light. In Wisconsin, greenhouses couldn’t become energy neutral using the semi-transparent solar cells as keeping the greenhouse warm in winter requires too much energy. However, the solar cells could meet up to 46 per cent of the greenhouse’s energy demand.
“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,” O’Connor said.