Look at a solar farm, and it might be easy to see only row upon row of glossy panels.
But there’s much more than meets the eye. Steven Loheide, a professor of civil and environmental engineering at the University of Wisconsin-Madison, is studying what happens underneath those panels, and how it may shape how we design—and use—this land in the future.
He’s focusing on agrivoltaics—when agriculture and sustainable power generation converge at solar farms. For example, crops could grow between or beneath rows of solar panels. “Ultimately, the idea here is to try to have the land do double duty,” Loheide says. “That is, to produce energy that can sustainably meet demand, while also being able to feed a growing global population.”
Loheide is working with UW-Madison colleagues Chris Kucharik and Ankur Desai to study a 15-acre site at the UW-Madison Kegonsa Research Campus south of Madison. The site includes a small-scale solar farm that produces 2.25 megawatts of power and allows the team to learn from an active agrivolatics system.
As a hydrologist, Loheide is especially interested in seeing how solar farms influence the hydrological processes and ecosystems. To that end, the researchers are monitoring the soil moisture within the solar array and comparing it to measurements from outside it. They’re also tracking metrics like groundwater recharge rates and quantifying how the soil’s moisture regime changes beneath solar panels.
Steven Loheide, a professor of civil and environmental engineering at the University of Wisconsin-Madison holds soil samples taken at the agrivoltaics testing site. Researchers are studying how solar farms influence hydrological processes, and are monitoring soil moisture as part of that effort. Submitted photo.
“There are two primary ways solar panels affect hydrologic processes,” says Loheide. “The first is that they’re impermeable, so any water hitting them is redirected toward the ‘drip line,’ where you then have a concentration of water hitting the ground. That can push soil moisture deeper toward the groundwater—or cause more runoff if there’s too much for the ground to take in at once. The second way is that plants growing in a solar panel’s shadow get a lot less sun, and therefore have less energy to transpire. Because of that, they use less water.”
Some panels are stationary—typically facing south in the northern hemisphere—with a fixed tilt that depends on latitude. Others, called sun-tracking panels, rotate to follow the sun as it arcs across the sky during the day. The Kegonsa test site has both types of panels, including some stationary panels that are tall enough so people and larger animals can walk underneath them.
In Wisconsin, where solar energy has blossomed in the half-decade since solar costs have gone down, Loheide says there’s a lot of interest in using solar farms as pastureland. Though there won’t initially be any grazing livestock at the Kegonsa site, the researchers are studying how different types of forage vegetation perform while growing within solar arrays.
Different types of solar panels array configurations affect their surroundings in different ways. Sun-tracking panels, for example, may encourage runoff in one direction or another depending on when rainfall occurs. Plants beneath the moving panels are more likely to get direct sunlight at some point in the day, while shadows beneath fixed panels are more consistent.
Those differences may shape decisions about what sorts of plants are best for any given agrivoltaics project. After all, Loheide says agrivoltaics systems can be flexible and customizable, with no one-size-fits-all answer for every solar farm in every climate. In some instances, an agrivoltaics system could, for example, be used to grow high-value specialty crops, while others could benefit nearby agricultural operations by serving as a refuge for pollinators.
“If you look at where a lot of solar growth has been in the U.S. over the past 15 years, it’s been in drier areas with more sun, like the Southwest,” he says. “Our climate is wetter here, but we get less direct sunlight because we’re further north. The growth of our vegetation is light-limited, while some of those other places are rain-limited. So I expect the plant response we see in Wisconsin and other similarly temperate regions will be different than what’s been found in other areas where this has been studied.”
Kucharik is a professor of plant and agroecosystem sciences in the UW-Madison College of Agricultural and Life Sciences and brings expertise on biophysical modeling of Midwestern cropping systems to the project. Desai is the department chair and a professor of atmospheric and oceanic sciences in the UW-Madison College of Letters and Science; in this project, he will use high-frequency atmospheric measurements to quantify exchange of water, energy and carbon between the solar array and the atmosphere.
Featured image caption: Researchers are testing an agrivoltaics system among solar panel arrays at the UW-Madison Kegonsa Research Campus south of Madison. Submitted photo.