Long the territory of cats, weather vanes, and the occasional fiddler, roofs are growing thick with solar panels. A home or business rooftop is an ideal place to site them because sunlight there is less obstructed by shadows and rooftops are generally unutilized spaces—it’s better for the environment to add panels to an existing structure than to clear new land for a solar farm.
But even panel-covered rooftops may not be as well-utilized as they could be. A new scientific field known as rooftop agrivoltaics asks: What if we also grew crops under them? These wouldn’t be ordinary green roofs, which are typically small gardens, but rather working farms. The panels would provide shade for the plants—actually boosting their yields—as well as for the building, simultaneously reducing cooling costs and generating clean energy for the structure. Urban populations are projected to more than double by the year 2050. As people continue to migrate into metropolises, rooftop agrivoltaics could both feed people and make city life more bearable.
A roof is actually a fairly challenging place for plants to grow. Up there, a plant is exposed to gusty winds and constant bombardment from sunlight since there aren’t any trees around to provide shelter. (Accordingly, hardy succulents are the preferred plants for green roofs.) Yes, plants need light, but not this much. “Plants end up going into what we call photorespiration mode, where it's too bright and sunny for them to efficiently photosynthesize,” says Colorado State University horticulturalist Jennifer Bousselot, who’s studying rooftop agrivoltaics. “They start trying to take oxygen and break it down, rather than carbon dioxide, and so they waste energy.”
By contrast, think about how a forest works: All the plants, except the tallest of trees, are getting some measure of shade. For the plants closest to the forest floor, the light is diffuse, bouncing off surfaces around them. The taller trees surrounding them also make them less exposed to wind and temperature swings than they'd be if they were growing out in the open.
The idea of agrivoltaics is to replicate this forest environment for crops. In Colorado, scientists have been experimenting with terrestrial agrivoltaic gardens and are finding that the plants tend to grow bigger in the shade. That’s likely a physiological response to the need to soak up more light, and it’s great for leafy crops like lettuce because it increases their yields. Pepper plants, too, produce three times as much fruit in agrivoltaic systems as in full sun. As a bonus, the shaded plants require about half the water they do otherwise because there’s less sunlight to cause evaporation.
The same concept would work up on a roof: Solar panels would provide the shade that makes plants happier and less thirsty. Under rooftop panels, Bousselot has found, it’s cooler in the summer and warmer in the winter, and the panels act as windbreaks. The plants wouldn’t have to be food crops to benefit the surrounding landscape—adding native plants to rooftop agrivoltaics, for instance, would provide flowers for local pollinators. Scientists are also playing around with designs for semi-transparent solar panels, which would theoretically work better for species that require less sunlight than out in the open, but not total shade.
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The plants, in turn, boost the efficiency of the panels. Plants sweat, in a sense, giving off water vapor as part of the photosynthetic process. This vapor rises into the panels, cooling them. That’s ideal because solar panels don’t generate as much electricity when they overheat. They produce a current when photons hit atoms and knock out electrons, which are already overexcited at higher temperatures. “The cooling effect is actually good for voltage,” says Carmine Garofalo, operations manager at Occidental Power, which develops rooftop agrivoltaic systems. “The cooler the area, the more efficiently the panel works.” That’s why scientists are also researching how to slap solar panels above canals, where the flowing water can provide cooling.
Bringing down the temperatures of not just buildings, but whole cities, is critical because urban areas can be up to 20 degrees Fahrenheit hotter than surrounding rural areas. This is known as the urban heat island effect. The built environment absorbs the sun’s energy during the day and slowly releases it at night, but the country has enough vegetation to cool things off—just like crops under a solar panel do. Temperatures can even vary wildly across a city: Poorer neighborhoods with more concrete are consistently hotter than richer neighborhoods with more trees, and climate scientists are urging city officials to create more green spaces to mitigate this effect.
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But the challenge is that you can’t just convert any roof to solar agrivoltaics. It has to be flat, for one thing. It might require a significant retrofit to support the extra weight of the soil, plants, and panels. And you’d need to be damn sure it’s waterproofed. “Trying to sell a building owner, or certain stakeholders, on this concept probably isn't going to be the easiest thing,” says Thomas Hickey, research associate at Sandbox Solar, which develops agrivoltaic systems.
Hickey thinks it would be much easier to incorporate rooftop agrivoltaics into new building projects, and that governments—especially in countries that are rapidly urbanizing—can step in to subsidize them, as governments have subsidized solar panels in general. In the long term, such a system would generate both solar power and crops in perpetuity. “You get that energy coming in no matter what,” Hickey adds. “And then you get food, or herbs, or whatever it might be, provided right there in the middle of the city.”
Bousselot has calculated that in Denver County, Colorado, which has 5,000 acres of suitable roof space, rooftop agrivoltaics might produce 5,000 pounds of food per acre. That’s 25 million pounds of food in just one county, if all those roof owners committed to agrivoltaics.
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Growing crops locally like this would have other climate benefits. It would reduce the need to ship produce long distances on carbon-spewing trucks. And that layer of green would save building owners some green by insulating the roof. “In general, you can expect a 10 percent energy savings in a year by adding a green roof,” says Bousselot. “And then your energy budget gets even better if you calculate in the solar panels.”
To be clear, Bousselot’s research is still in its early days. Scientists are still learning which crops do best under agrivoltaics on the ground, and doing the same for rooftops might require finding hardier species. While the panels will provide some shade, it’s still harsh up there. But agrivoltaics could exploit wasted rooftop space all over the world, helping provide clean energy and food for a ballooning urban population. It’s not a bad deal for the plants, either, says Bousselot: “If you can just give them a little bit of a break from that intense sun and temperature, those plants actually just thrive.”
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