Tinted solar panels could allow land to be used to grow crops and generate electricity simultaneously, with financial gains, according to researchers in the UK and Italy. The orange solar panels absorb some wavelengths of light, while allowing those that are best for plant growth to pass through. The team even claim that their setup can produce crops offering superior nutrition.
Agrivoltaics uses land to simultaneously grow crops and produce electricity from solar panels. Usually opaque or neutral semi-transparent solar panels are used. Now, Paolo Bombelli, a biochemist at the University of Cambridge, and his colleagues used orange-tinted, semi-transparent solar panels to see if selective use of different wavelengths of light for plant growth and electricity production could offer additional benefits. The solar panels allow orange and red light to pass through, as these wavelengths are the most suitable for plant growth, while absorbing blue and green light to generate electricity.
The researchers grew basil and spinach in greenhouses in northern Italy with the glass roofs replaced with semi-transparent, orange-tinted solar panels. Even though the yield of both crops was reduced compared with plants grown in standard greenhouses, the agrivoltaic system offered a financial advantage over standard growing conditions, they report in the journal Advanced Energy Materials.
Overall, the spinach and the electricity produced in the agrivoltaic greenhouses were worth about 35% more than a spinach crop grown in a standard greenhouse. While basil and the electricity generated offered financial gains of around 2.5%. This was based on the wholesale global market price of the crops and the local feed-in-tariff for selling electricity to the Italian national grid. According to the researchers, the substantial difference in financial gains occurs because basil sells for about five times the price of spinach. In other words, such an agrivoltaic systems offers more financial reward when used with lower value crops.
The yields of basil grown under the orange-tinted solar panels dropped by 15% compared to those in the standard greenhouses, while spinach yields fell by 26%. However, the researchers noticed some interesting differences between the agrivoltaic and traditionally grown plants. The plants grown beneath the solar panels demonstrated a more efficient photosynthetic use of light, and they produced more tissue above ground and less below ground. This resulted in differences in plant morphology, with the basil producing larger leaves and the spinach longer stems.
Additionally, laboratory tests found that both basil and spinach plants grown under the solar panels contained more protein than those grown in standard greenhouses. The researchers suggest that the changes in morphology, redirection of above and below ground metabolic energy, and the increases in protein could be adaptations to improve photosynthesis under reduced light conditions. They add that the accumulation of more protein is interesting “in view of the need for alternative sustainable protein sources to substitute animal proteins, for example, in plant-based artificial meats”.
More experiments needed
Bombelli told Physics World that the technique might be applicable in locations besides Italy’s Mediterranean climate, depending on the chosen conditions. He says, “It depends on the percentage of the land cover by solar panel and the type of crop chosen”, adding that the only way to know for sure is to conduct additional experimental work. Indeed, the team is now hoping to run a trial in the UK.
Earlier this year Brendan O’Connor and colleagues at North Carolina State University published a modelling study in the journal Joule looking at how much energy could be produced with the addition of the solar cells on greenhouses. Like Bombelli’s work, the study analysed solar panels that harvest energy from the wavelengths of light that plants do not use for photosynthesis.
O’Connor explains: “We found that there are greater opportunities in hot and moderate climates. Yet, the heating energy requirements in colder climates result in a significant cost to greenhouse growers, and offsetting those energy costs is critical. If the solar cells can be designed to minimize losses in plant yield, there should be benefits across different climate zones.”
O’Connor says that the latest study is impressive. “While there were some losses observed in crop biomass, they demonstrate a net economic benefit of the system, which is a very exciting result for the concept,” he explains. O’Connor adds that research on integrating solar cells with greenhouse structures is growing rapidly as “there is a need to reimagine producing food to meet human needs in the most environmentally friendly and sustainable manner possible”.