Solar Panels Pair Surprisingly Well With Tomatoes, Peppers and Pollinators

The world already needs more solar power. It's clean, renewable energy, and it's quickly outpacing the job creation and affordability of fossil fuels. But on top of that, a growing field of research suggests it can improve agriculture, too, helping us grow more food and pollinator habitat while also conserving land and water.

Big, utility-scale "solar farms" are one important source of solar power, helping complement smaller, less centralized sources like solar panels on the roofs of buildings. Solar farms take up a lot of space, though — and they thrive in places with many of the same qualities favored by food crops. As one recent study found, the areas with the greatest potential for solar power tend to already be in use as croplands, which makes sense, given the importance of sunlight for both.

"It turns out that 8,000 years ago, farmers found the best places to harvest solar energy on Earth," said Chad Higgins, study co-author and professor of agricultural sciences at Oregon State University, in a statement.

Since crops already occupy many of those places, this might seem to cast solar farms and food farms as rivals for real estate. Yet while it's smart to balance food and energy production, a growing field of research suggests it can also be smart to combine them. Unlike fossil fuels, one of the great things about solar power is that it's clean enough to still use the land for food production, without needing to worry about contamination. And not only can crops and solar panels co-exist on the same land, but when combined in the right ways at the right locations, researchers say each can help the other function more efficiently than it would alone.

This idea — known in the U.S. as "agrivoltaics," a mashup of agriculture and photovoltaics — isn't new, but new research is shedding light on how beneficial it can be. Beyond the benefits of harvesting food and clean energy from the same land, studies suggest solar panels also boost crops' performance — potentially raising yield and reducing water needs — while crops help the panels work more efficiently. This could increase global land productivity by 73%, while generating more food from less water, since some crops under solar panels are up to 328% more water-efficient.

Agrivoltaics won't necessarily work the same for every location or every crop, but we don't need it to. According to Higgins' research, if even less than 1% of existing cropland was converted to an agrivoltaic system, solar power could fulfill global demand for electricity. That still wouldn't be as simple as it sounds, but amid the growing urgency of climate change, energy demand and food insecurity, it's an idea that seems more than ready for its moment in the sun.

Types of agrivoltaic systems

Three different types of agrivoltaic systems: (a) using the space between solar panels for crops, (b) a photovoltaic greenhouse, and (c) a stilt-mounted system. (Illustration: Sekiyama et al. [CC BY 4.0]/Environments)

The basic idea of agrivoltaics dates back at least to 1981, when two German scientists proposed a new kind of photovoltaic power plant "which allows for additional agricultural use of the land involved." It has evolved in the decades since, leading to new twists on the concept that have found success in several countries, including Japan — which has emerged as a global leader in "solar sharing," as the practice is known there — as well as France, Italy and Austria, among others.

There are three general categories of agrivoltaic systems. The original idea placed crops between rows of solar panels, capitalizing on spaces that are otherwise mostly unused (see example "a" in the illustration above). A different tactic, developed in 2004 by Japanese engineer Akira Nagashima, involves solar panels raised on stilts about 3 meters (10 feet) off the ground, creating a pergola-like structure with space below for crops (example "c" above). A third category resembles the stilted method, but places the solar panels on top of a greenhouse (example "b").

It's one thing to plant crops in sunny gaps between solar panels, but sowing them underneath the panels means sunlight is blocked for at least a few hours every day. If the goal is to maximize the efficiency of both the crops and the solar panels, why let one block any sunlight from the other?

Made in the shade

Plants obviously need sunlight, but even they have limits. Once a plant maxes out its ability to use sunlight for photosynthesis, more sunlight can actually impede its productivity. Plants native to dry climates have evolved various ways to deal with excessive solar energy, but as researchers at the University of Arizona point out, many of our agricultural crops are not desert-adapted. To successfully grow them in deserts, we make up for their lack of adaptation with intensive irrigation.

Instead of using all that water, though, we could also mimic some of the natural adaptations used by dry-climate plants. Some deal with their harsh habitats by growing in the shade of other plants, for example, and that's what agrivoltaics advocates are trying imitate by growing crops in the shadows of solar panels.

And that payoff can be substantial, depending on the crops and conditions. According to a September 2019 study published in the journal Nature Sustainability, agrivoltaics systems can improve three important variables that affect plant growth and reproduction: air temperatures, direct sunlight and atmospheric demand for water.

The study's authors created an agrivoltaics research site at Biosphere 2 in Arizona, where they grew chiltepin peppers, jalapeños and cherry tomatoes under a photovoltaic (PV) array. Throughout the summer growing season, they continuously monitored sunlight levels, air temperature and relative humidity using sensors mounted above the soil surface, as well as soil temperature and moisture at a depth of 5 centimeters (2 inches). As a control, they also set up a traditional planting area near the agrivoltaics site, both of which received equal irrigation rates and were tested under two irrigation schedules, either daily or every other day.

Shade from the panels led to cooler daytime temperatures and warmer nighttime temperatures for plants growing below, as well as more moisture available in the air. This affected each crop differently, but all three saw significant benefits.

"We found that many of our food crops do better in the shade of solar panels because they are spared from the direct sun," said lead author Greg Barron-Gafford, a professor of geography and development at the University of Arizona, in a statement. "In fact, total chiltepin fruit production was three times greater under the PV panels in an agrivoltaic system, and tomato production was twice as great!"

Jalapeños produced a similar amount of fruit in both the agrivoltaic and traditional scenarios, but did so with 65% less transpirational water loss in the agrivoltaic setup.

"At the same time, we found that each irrigation event can support crop growth for days, not just hours, as in current agriculture practices," Barron-Gafford said. "This finding suggests we could reduce our water use but still maintain levels of food production." Soil moisture remained about 15% higher in the agrivoltaics system than in the control plot when irrigating every other day.

This echoes other recent research, including a 2018 study published in the journal PLOS One, which tested the environmental effects of solar panels on an unirrigated pasture that often experiences water stress. It found that areas under PV panels were 328% more water-efficient, and also showed a "significant increase in late-season biomass," with 90% more biomass under solar panels than in other areas.

The presence of solar panels might seem like a headache when it's time to harvest crops, but as Barron-Gafford recently told the Ecological Society of America (ESA), the panels can be arranged in a way that lets farmers continue using much of the same equipment. "We raised the panels so that they were about 3 meters (10 feet) off the ground on the low end so that typical tractors could access the site. This is was the first thing that farmers in the area said would have to be in place for them to consider any kind of adoption of an agrivoltaic system."

Of course, the details of agrivoltaics vary widely depending on the crops, the local climate and the specific setup of solar panels. It won't work in every situation, but researchers are busy trying to identify where and how it can work.

A 'win-win-win'

The potential perks for crops alone might make agrivoltaics worthwhile, not to mention the reduced competition for land and demand for water. But there's more. For one thing, research has found that an agrivoltaic system can also increase the efficiency of energy production from the solar panels.

Solar panels are inherently sensitive to temperature, becoming less efficient as they warm up. As Barron-Gafford and his colleagues found in their recent study, cultivating crops reduced the temperature of panels overhead.

"Those overheating solar panels are actually cooled down by the fact that the crops underneath are emitting water through their natural process of transpiration — just like misters on the patio of your favorite restaurant," Barron-Gafford said. "All told, that is a win-win-win in terms of bettering how we grow our food, utilize our precious water resources and produce renewable energy."

Or maybe it's a win-win-win-win? While solar panels and crops cool each other off, they might do the same for people working in the fields. Preliminary data suggest human skin temperature can be about 18 degrees Fahrenheit cooler in an agrivoltaics area than in traditional agriculture, according to research from the University of Arizona. "Climate change is already disrupting food production and farm worker health in Arizona," says agroecologist Gary Nabhan, a co-author of the Nature Sustainability study. "The Southwestern U.S. sees a lot of heat stroke and heat-related death among our farm laborers; this could have a direct impact there, too."

Generating buzz

Aside from all the aforementioned benefits of agrivoltaics — for crops, solar panels, land availability, water supplies and workers — this kind of combination could turn out to be a big deal for bees, too, along with other pollinators.

Insects are responsible for pollinating nearly 75% of all crops grown by humans, and about 80% of all flowering plants, yet they're now fading from habitats worldwide. The plight of honeybees tends to get more attention, but pollinators of all kinds have been declining for years, largely due to a mix of habitat loss, pesticide exposure, invasive species and disease, among other threats. That includes bumblebees and other native bees — some of which are better at pollinating food crops than domesticated honeybees are — as well as beetles, butterflies, moths and wasps.

Lots of valuable crops depend heavily on insect pollination, including most fruits, nuts, berries and other fresh produce. Foods like almonds, chocolate, coffee and vanilla wouldn't be available without insect pollinators, according to the Xerces Society for Invertebrate Conservation, and many dairy products would be limited, too, given the large number of cows that feed on pollinator-dependent plants like alfalfa or clover. Even many crops that don't need insect pollinators — like soy or strawberries, for example — produce higher yields if they're pollinated by insects.

And that's the impetus behind a push for more pollinator habitat on solar farms, especially in agricultural areas where pollinators can play the biggest economic role. This is well-established in the U.K., where a solar company began letting beekeepers set up hives at some of its solar farms in 2010, according to CleanTechnica. The idea spread, and the U.K. now has a "long and well-documented success using pollinator habitat on solar sites," as Minnesota nonprofit Fresh Energy describes it.

The pairing of pollinators and solar power is increasingly popular in the U.S., too, especially after Minnesota enacted the Pollinator Friendly Solar Act in 2016. That law was the first of its kind in the country, establishing science-based standards for how to incorporate pollinator habitat into solar farms. It has since been followed by similar laws in other states, including Maryland, Illinois and Vermont.

Much like crops, wildflowers could help cool off solar panels overhead, while the panels' shade could help wildflowers thrive in hot, dry places without taxing water supplies. But the main beneficiaries would be bees and other pollinators, who should then pass on their good fortune to nearby farmers.

For a 2018 study published in the journal Environmental Science & Technology, researchers at Argonne National Laboratory looked at 2,800 existing and planned utility-scale solar energy (USSE) facilities in the contiguous U.S., finding "the area around solar panels could provide an ideal location for the plants that attract pollinators." These areas are often just filled with gravel or turf grass, they noted, which would be easy to replace with native plants like prairie grasses and wildflowers.

And aside from helping pollinators in general — which would likely be wise even if we couldn't quantify the payoff for humans — the Argonne researchers also looked at how "solar-sited pollinator habitat" might in turn boost local agriculture. Having more pollinators around can increase the productivity of crops, potentially offering farmers a higher yield without using additional resources like water, fertilizer or pesticides.

The researchers found more than 3,500 square kilometers (1,351 square miles, or 865,000 acres) of farmland near existing and planned USSE facilities that could benefit from more pollinator habitat nearby. They looked at three example crops (soybeans, almonds and cranberries) that rely on insect pollinators for their annual crop yield, examining how more solar-sited pollinator habitat might affect them. If all existing and planned solar facilities near these crops included pollinator habitat, and if yields rose by just 1%, crop values could rise by $1.75 million, $4 million and $233,000 for soybeans, almonds and cranberries, respectively, they found.

Enlightening research

Farming in the U.S. has become increasingly difficult lately, due to a mix of factors from droughts and floods to the U.S.-China trade war, which has reduced demand for many American crops. As the Wall Street Journal reports, this is leading some farmers to use their land for harvesting solar power instead of food, either by leasing the land to energy companies or by installing their own panels to cut electricity bills.

"There's been very little profit at the end of the year," says one Wisconsin corn and soybean farmer, who's leasing 322 acres to a solar company for $700 per acre annually, according to the WSJ. "Solar becomes a good way to diversify your income."

Agrivoltaics may not be a quick fix for farmers who are struggling now, but that could change as research reveals more insights, potentially informing government incentives that make it easier to adopt the practice. That's what many researchers are focusing on now, including Barron-Gafford and his colleagues. They're working with the U.S. Energy Department's National Renewable Energy Lab to assess the viability of agrivoltaics beyond the U.S. Southwest, and to examine how regional policies might encourage more novel synergies between agriculture and clean energy.

Still, farmers and solar companies don't necessarily need to wait for more research to capitalize on what we already know. To make money from agrivoltaics right away, Barron-Gafford tells the ESA, it's mostly just a matter of elevating the masts that hold up the solar panels. "That is part of what makes this current work so exciting," he says. "A small change in planning can yield a ton of great benefits!"