By Kathryn Nave, Contributor
Deep in the Sonoran Desert, tendrils crawl up the legs of solar panels, while vines sprawl beneath their shade. It may look like the natural reclamation of abandoned infrastructure, but this is no solar graveyard: This is the University of Arizona’s first agrivoltaics facility, a new approach to energy generation that combines solar panels and agriculture on the same site.
“If you look at the kind of solar projects that were created five years ago, they looked like industrial facilities,” says National Renewable Energy Laboratory (NREL) analyst Jordan Macknick. “They removed the vegetation, sterilized the ground, and put gravel on top. So there was a lot of pushback from people not wanting to see bucolic landscapes transformed into these industrial sites.” But things have changed since then.
By 2030, solar energy sites could occupy as much as 800,000 hectares of land in the United States alone (that’s nearly 2 million acres), and opposition to this mass transformation of vast swathes of countryside into energy production sites is a matter of more than just aesthetic preference.
“…what solar developers are looking for are flat open areas around city centers. In other words—farmland.”
—Jordan Macknick, analyst, National Renewable Energy Laboratory
“If you want to supply the world’s energy, you need large installations [in the region of hundreds of acres in size] ,” explains Macknick. “You can’t put these in the middle of nowhere because it’s expensive to transport electricity. So what solar developers are looking for are flat open areas around city centers. In other words—farmland.”
In 2015, the NREL launched the InSPIRE project, led by Macknick, which now works with 25 sites across the U.S. on developing a new model for solar energy generation in which plants and panels share the same space. Five of these, in Georgia, Oregon, Colorado, and Massachusetts—alongside the Arizona site—are focused on tackling the competition for land between energy and agriculture by exploring how the two can be combined within the same site.
“…if we simply put solar arrays over the plants, we have this really elegant solution where we harvest the sun twice. That means sites that produce food while also producing clean energy at the same time.”
—Greg Barron-Gafford, biogeographer and director of the agrivoltaics project, University of Arizona
“We’ve increasingly been coming up against this food vs. energy mindset,” says Greg Barron-Gafford, University of Arizona biogeographer and director of the agrivoltaics project. “But if we simply put solar arrays over the plants, we have this really elegant solution where we harvest the sun twice. That means sites that produce food while also producing clean energy at the same time.”
Sharing the Sun
You can’t throw any plant under a panel and expect it to flourish. Where many plants love direct sunlight, others, like lettuce, thrive in the shade. Delicate plants, like herbs and tomatoes, will die if the weather turns freezing, but broccoli and cabbage rely on that cold snap to form edible heads.
Being in Arizona, Barron-Gafford wanted crops that would support traditional Southwestern cuisine. For the first planting, he settled on salsa.
“We started with three key ingredients of cilantro, tomato, and peppers,” he says. “It’s a good test because cilantro is a really delicate herb, so it gives a great visual indicator of crop health. Peppers evolved to grow under trees, so they have a history of being in shade deep in the genetic code. And then we picked tomatoes because they’re this really common crop that everyone understands.”
“Plus,” he adds, “it’s really fun to be able to eat your science.”
Still, Barron-Gafford’s initial funding proposal met with some skepticism, particularly from the National Science Foundation, who were concerned about whether gains in energy generation would make up for the impact of reduced light on crop yields.
“Everyone knows that plants need light,” Barron-Gafford says. “But while photosynthesis does initially increase with light level, at a certain point it plateaus out. That means that even if you cut out two thirds of the light, you don’t cut out two thirds of the photosynthesis.”
In the harsh extremes of the Sonoran Desert, where temperatures routinely exceed 100 degrees during the day and sink below freezing overnight, Barron-Gafford had expectations that the decrease in photosynthesis would be more than compensated for by the temperature regulating effects of the panels. Beyond shading crops from the midday heat, these can also reduce heat lost to radiation at night—just as a cloudy evening stays much warmer than a clear one. As well as directly protecting plants from temperature fluctuations, the shade of the panels also reduces their water usage by slowing down moisture evaporation from the soil.
The results were even better than Barron-Gafford had hoped. In a paper published in Nature Sustainability in September 2019, his group reported a three-fold increase in chiltepin peppers and tomato yields under the solar panels compared to the open air plots next door, while their jalapeño peppers maintained the same level of productivity, but with over 50 percent less water usage.
Mutually Assured Production
What Barron-Gafford hadn’t anticipated was how the solar panels would also benefit from the crops beneath them. While the desert might seem like the ideal place to maximize solar energy efficiency, solar panels, just like plants, suffer from the heat. Most are designed to operate at around 77 degrees, with efficiency decreasing by around half a percent for every degree of temperature increase.
Plants, fortunately, have a built-in cooling mechanism. Just as animals sweat to cool down, plants open their pores to let moisture evaporate, reducing the temperature of their surrounding environment. Barron-Gafford’s group found that this process of transpiration reduced the daytime temperature of their overhead solar panels by around nine degrees—amounting to a 4.5 percent increase in the efficiency of energy generation.
“I remember when we went to our first solar conference and there were all these people trying to figure out how to cool solar panels using all of these highly mechanized processes,” he says. “They didn’t realize plants could do all that naturally. ”
Based on their success in the Sonoran desert, the group recently received funding to set up further Argrivoltaics sites in drylands across Kenya and Tanzania. The effectiveness of this synergy, where panels protect crops, which then cool the panels in turn, has also expanded the way Barron-Gafford thinks about agrivoltaics—beyond merely facilitating more efficient sharing of already valuable land.
“I see the biggest opportunities in some of the driest places in the world ” he says. “That includes our own rural Southwest, which already struggles with water, food, and sometimes energy insecurity. But it also includes places like rural Tanzania, which may have never even been connected to the power grid before.”
“The most inhospitable places for agriculture or solar energy alone may turn out to be some of the best sites for agrivoltaics,” concludes Barron-Gafford.