In the South Boulder Canal near Denver, CO, USA, turbine blades that resemble the mixers used to stir cake batter in kitchens (
Fig. 1) spin in the rushing water [
1]. Each of the turbines, manufactured by Emrgy of Atlanta, GA, USA, generates up to 25 kW of electricity for the area’s grid [
2]. In an orchard in southern Italy, an arcade of solar panels stands above rows of citron trees, which are valuable for the essential oils in their fruits [
3]. The solar panels not only produce electricity for local homes, but also protect the trees from the sun, reducing their water use by 70% [
3].
These are two examples of a clean energy trend: integrating power-generation with other uses. In the approach called agrivoltaics, for example, farmers and ranchers grow crops or raise livestock around and beneath solar panels (
Fig. 2) [
4]. Experts hope that such add-on strategies can spare farmland, cut the environmental costs of clean energy development, supply extra income to landowners, and furnish other benefits [
5], [
6].
So far, these approaches have made a small contribution to the world’s power supply. The United States boasts about 8 GW of agrivoltaic generating capacity, about 0.7% of the country’s total [
7], [
8]. However, more of these systems are being deployed. Worldwide, agrivoltaic capacity grew from 5 MW in 2012 to 14 GW in 2021 [
5]. And its potential to generate energy is enormous, said Joshua Pearce, professor of electrical and computer engineering at Western University in London, ON, Canada. “Agrivoltaics alone could easily power any of the larger countries.”
Agrivoltaics and similar technologies are under consideration because countries face “a real estate problem,” said Mark Uchanski, associate professor of horticulture and landscape architecture at Colorado State University in Fort Collins, CO, USA. Vast amounts of land will be necessary to switch the world’s economy to clean energy. The United States plans to obtain all its electricity from renewable sources by 2050; to meet that goal, according to calculations by the US Department of Energy [
9], the country will need to convert up to 4.9 × 10
6 hm
2 of land, as much as 0.5% of its total area, to solar power generation. The challenge for the United States and other countries is to reach their renewable energy targets without reducing food production or diminishing natural habitats.
“What makes agrivoltaics appealing is that you are not sacrificing the land,” said Pearce. It can continue to produce crops or livestock—and yields can even be higher with solar arrays. In a 2019 study, for instance, researchers in southern Arizona planted chili peppers, jalapeno peppers, and tomatoes on two plots, one with an overhead solar array [
10]. At the end of the harvest season, the chili pepper plants growing under the solar panels produced about three times more fruit than did the plants growing in the open, and the shaded tomatoes also outperformed their full-sun counterparts [
10]. The jalapeno peppers, however, showed no difference in yield. Depending on the frequency of irrigation, the shaded soil retained 5% to 15% more moisture, suggesting that the panels save water [
10].
Studies also indicate that agrivoltaics can provide significant amounts of power. A 2023 report from the European Union estimates that switching just 1% of agricultural land in the member countries to agrivoltaic generation could provide 944 GW, enough to allow the bloc to reach its solar power goal for 2030 [
11]. A 2023 study by Pearce and colleagues found that agrivoltaics on the same percentage of Canada’s farmland could furnish 28% to 43% of the country’s power, depending on the types of solar voltaic systems installed [
12]. This would be enough to offset all fossil fuel use for electricity generation in Canada.
“We see that agrivoltaics works,” said engineer Jordan Macknick, lead energy-water-land analyst at the US National Renewable Energy Laboratory (NREL) in Golden, CO, USA. What scientists are now trying to figure out is which types of agrivoltaic systems provide the best results at specific locations. For more than a decade, Macknick and colleagues have been researching agrivoltaics and tracking projects across the United States. Their work has identified five factors that influence whether these projects succeed: ① the local climate; ② the types of crops and how they are grown; ③ the configuration of the solar arrays; ④ whether the solar installations mesh with agricultural practices; and ⑤ whether necessary collaborations, such as legal agreements, are in place [
13].
The choice of crops is crucial, but which ones will prosper under or between solar panels is not always clear, Macknick said. For instance, it seems obvious that sun-loving crops such as cucumbers would not fare as well. And the studies so far show that cucumber yield does decrease by between 21% and 58% [
13]. But other research suggests that certain full-sun crops can do better when farmers shift to agrivoltaics, at least in some areas. One study in Germany, for example, found that yields of wheat increased by about 3% when solar panels were installed [
14]. But because agrivoltaic research has just begun, it is too early to draw conclusions about how to optimize agricultural yield and power production, Macknick said. The research so far “is just a drop in the bucket of what we need.”
Several factors can deter farmers and solar developers from deploying agrivoltaic systems. For solar developers, they usually mean higher costs, said Uchanski. Conventional panels are typically less than 1 m above the ground, but some agrivoltaic arrays may need to be 3 m or taller [
5], [
11]. Such arrays require more materials and are more expensive to install [
5]. Farmers may also have to take a “yield penalty,” said Uchanski, although earnings from the panels may make up the loss. Solar installations can also make farmers “feel like they are trying to grow food in an obstacle course,” said Macknick. In addition, agrivoltaic projects may face regulatory or legal obstacles. In some parts of Canada, for example, placing solar panels on farmland is illegal [
15].
Because of such limitations, almost all agrivoltaic systems in the United States are on ranchland or marginal agricultural land, where adding solar panels can provide a much-needed income boost, said Uchanski. Farmers who are otherwise doing well “are not beating down my door” to add agrivoltaics, he said. So far, only about 1% of the land devoted to agrivoltaics is prime agricultural land, he noted, but much more of that type of land will have to be converted if the United States is going to meet its renewable energy goals. “We need to be ready for that,” Uchanski said, with solutions that will allow solar and agriculture to mesh.
Using canals to generate power is another way to reduce the land requirements of renewable energy. One approach involves installing solar panels above canals (they have also been installed above or floating on bodies of still water, such as reservoirs) [
16]. Canal-top solar panel arrays have been operating for a decade in India and are being tested in the United States [
16]. But the water flowing through the canals can also yield electricity. The United States already obtains about of 530 MW of power from relatively small turbines spinning in canals, pipelines, and even water purification plants [
17]. Estimates suggest the nation could add an additional 1.4 GW of generating capacity by installing more such turbines in these sites [
17].
In India, where around 21% of the population lacks electricity, canal-based sources could provide power for a substantial fraction of the many residents living near a major canal and supplement existing grid power. In an analysis published in 2021 [
18], Sabah Usmani, a graduate student at the Massachusetts Institute of Technology in Cambridge, MA, USA, and colleagues evaluated the hydropower and solar power generating potential of the 291 km-long Upper Ganga Canal in northern India that directs water from the Ganges River to agricultural land in the state of Uttar Pradesh [
18]. The researchers considered two sources of hydropower. The first was turbines at locations where the elevation drops sharply. Like the turbines in conventional hydroelectric dams, these turbines generate electricity as water forcefully flows downhill across their blades [
19]. Based on the elevation changes along the different stretches of the canal, Usmani and colleagues estimated that this source could generate 44 MW [
18].
The contribution from the second source, hydrokinetic turbines like those on the South Boulder Canal in Colorado, with blades driven by water flowing at slower rates through more level sections, was much smaller. Using water speed measurements from eight sites, the group’s work showed that hydrokinetic turbines would add 65 kW [
18]. While the researchers described the potential electricity generation from these two sources as “a substantial hydropower contribution in the region,” the 44 MW they could produce is dwarfed by the estimated 1676 MW that could be delivered by canal-top solar panel arrays [
18].
If canal-based power was directed to local villages that lack electricity for most of the day, it could help meet their agricultural and household needs, said Usmani, now a Doctor of Philosophy candidate in climate and health at Columbia University in the City of New York, NY, USA. The group’s calculations suggest that canal hydropower alone could provide 54% of villages on one 58.3 km stretch of the canal with electricity for at least 18 h per day—the government standard for Uttar Pradesh [
18].
Canal- based hydropower is slowly catching on in the United States and other countries. So far, Emrgy has installed turbines in four US states and in New Zealand [
20], and other companies are tapping this energy source as well [
21]. Emrgy touts the fact that its turbines do not require falls, which means they can be located at a greater variety of sites [
22]. However, hydrokinetic turbines still require relatively rapid water flow rates. For instance, Usmani and colleagues found that water flow was fast enough to generate power with these turbines—at least 0.5 m∙s
−1—at only three of the eight measuring sites they analyzed on the Upper Ganga Canal [
18].
In any case, while small hydropower and add-on solar power projects could help meet renewable energy goals, the latter may have the greater potential for impact. In particular, although the optimal mix of crops and solar panel configurations for different areas remains unclear, the agrivoltaics approach is promising and we should encourage experimentation, Macknick said. “We do not have time to wait 40 years for research results.”
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