By Michelle Lotker
By splitting sunlight, NC State researchers hope crops and solar panels can thrive together
Generating electricity from the sun is efficient and renewable. Solar installations, however, can take up a lot of land, and often the best sites overlap with prime farmland.
“Typically, they have a comfortable slope and level,” Ricardo Hernandez, associate professor at NC State, shares. “They typically are very clear, and they’re always full exposure to the sun.”
Ricardo and fellow researchers of the NCSU-CEA (NC State University–Controlled Environment Agriculture) Coalition (of which Ricardo is director) are concerned about keeping that farmland in agricultural production.
“That farmland is needed to produce food, and if we remove the capability of producing food in these farmlands, we have removed our ability to feed the population in the world. So, there is a conflict,” Ricardo explains.
That conflict could be resolved by combining solar energy generation with farming through something called agrivoltaics. Ricardo is working with collaborators at NC State and in Israel to develop new variations in solar panel design that will allow solar energy and crop production to happen on the same land.
“The idea behind it is, ‘Can we use the land for dual purposes? Can we grow crops in the field, in soil, and can we harvest the solar energy to generate electricity?’” Ricardo asks. “Great idea, but there [are] some challenges around it.”
Competing for sunlight
Solar panels and crops in the same field compete for sunlight because solar panels harvest the same light that plants need to grow. The challenge is how to design a way for plants and panels to share the sunlight.
Ricardo is part of a multinational team working to solve this by splitting sunlight between solar panels and the crops around them.
Sunlight can be broken down into different types of radiation based on wavelength, ranging from 320 nanometers all the way up to 3,000 nanometers. And a small portion of those wavelengths (ranging around 380–750 nanometers) is what we call visible light.
“Our eyes can detect that light. A similar range is also [used by] the plants to do photosynthesis,” Ricardo explains. “If you deploy these panels, you’re going to be removing those essential wavelengths, and the plant will actually have a big penalty in yield. We don’t want to penalize the production of food or the production of plant growth, because we already don’t have enough farmland in the world to grow crops. We want to prioritize plant growth. So in order to do that, we’re trying to actually split the sun.”
Splitting the sun
A key component to splitting the sun is a rather ordinary looking, very thick piece of glass that’s actually two sheets of glass glued together.
“This piece of glass has optics in the middle,” Ricardo explains. “It’s a film, and this film is what causes the reflection of absorption of specific wavelengths.”
Glass like this is often used in office buildings, where architects want to reduce the sun’s heat coming into the building without reducing natural light. They use glass designed to let visible light through while reflecting radiation with a longer wavelength, like infrared light.
Ricardo and his team are hoping to use similar glass to let plants get the spectrum of light that they need to grow while harvesting the rest of the sunlight’s radiation for solar energy production.
In a greenhouse on NC State’s campus, Ricardo propped up the sheet of glass with the wavelength-blocking layer and used sensors above and below the glass to show what light it reflects.
The glass blocked about half the total light or solar radiation in the greenhouse and a slightly lower percentage of photosynthetically active radiation, which is what plants use for photosynthesis.
“We want to maximize the amount of light that reaches the plant,” Ricardo explained. “We want to reduce the amount of light that is reflected or absorbed by the optics. There are still some inefficiencies, so that’s an improvement that we can [achieve] through research.”
A spectrum radiometer measures what parts of the spectrum are present and what parts are reflected by the glass. After taking a reading above the glass, Ricardo put the sensor below the glass to take another reading. Instantly, the shape of the rainbow-colored bell curve on the radiometer changed. The peak became sharper because less high and low wavelength light was present below the glass.
“You can see how we actually remove some of those wavelengths that are closer to the infrared,” Ricardo says.
This sensor shows us that the wavelengths of light reflected by the glass are mostly useless to plants, especially the wavelengths that just make them “hotter and thirstier.” But those wavelengths could be harvested for solar energy production.
New panel designs
To use this glass with solar panels, some changes have to be made to the traditional photovoltaic panel setup. In the model they’re testing, Ricardo’s team is orienting a double-sided photovoltaic panel vertically and angling the glass 45 degrees on both sides to bounce sunlight back at the panel while letting light through to crops underneath.
To test how much impact new solar panel designs might have on crops, the team uses mathematical models with information about how lettuce or tomato plants react to things like less light to predict the crops’ yield in different scenarios.
Shade-loving crops like lettuce only need a few more days in the field to have the same yield. But for sun-loving crops like tomatoes, the penalty is much higher, up to a 25%–30% reduction in yield. So there’s still room for improvement.
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