The Golden Gate Bridge in San Francisco, California.
The Golden Gate Bridge in San Francisco, California. Credit: Brett Sayles / Creative Commons

Some of the world’s most iconic bridges — the Golden Gate and Brooklyn Bridge, for example — share a design principle: They’re suspended, using a web of vertical and horizontal cables to string roads and railways across landscapes. 

Now, a renewable equipment provider with experience in the wind industry is applying the engineering concept to solar projects. Oregon-based Rute Foundation Systems is testing a system that rigs panels 9 feet above the ground using steel cables. The company’s design is aimed at a growing market for dual-use solar, which combines projects with agricultural uses like planting row crops or pollinator-friendly flowers. 

“It really started with the idea of using more efficient and modern bridge technology to affect other civil projects,” said David McFeeters-Krone, the company’s business development advisor. “This is typical for engineers. You see a situation that looks inefficient, and then you wonder, ‘Could it be done a better way?’”

Rute engineers devised the technology on the back of a napkin just 18 months ago. It’s newly out of the prototype stage, but the company hopes to deploy it at dual-use projects on cattle ranchlands, where grazing activities could require highly elevated panels. The company piloted the technology at an installation in North Dakota with support from the Department of Energy. Now Rute is tinkering with the design at a public-private innovation and research station in Oregon and working with a team from Oregon State University to study how animals interact with the structure.

The technology aims to make dual-use projects more feasible by reducing their costs. Today, combining solar and agriculture comes with a price premium; installing projects high enough off the ground and with enough space in between rows to grow crops or graze animals means more steel to support the solar panels. Building dual-use projects can increase installed costs by a wide range, anywhere from 7 to 80 cents per watt, according to the National Renewable Energy Laboratory. Rute says its product, called Suntracker, reduces steel costs by about a quarter from more traditional dual-solar configurations.

A rendering of a Rute Suntracker project on a ranch in Central Oregon.
A rendering of a Rute Suntracker project on a ranch in Central Oregon. (Submitted by Rute)

Reducing solar costs is “critically important” in general, but that’s particularly true for dual-use solar, which is still viewed as somewhat experimental, said Dwayne Breger, who directs the clean energy extension at the University of Massachusetts Amherst. 

“The most difficult economic thing for farmers is risk,” said Breger, who previously led the renewable energy division of Massachusetts’ Department of Energy Resources.

Dual-use solar, or agrivoltaics, as it’s sometimes called, is still a relatively small portion of the overall solar market. Jordan Macknick, who heads research on low-impact solar projects at the National Renewable Energy Laboratory, estimates there are about 500 such projects nationwide, making up roughly 10% of large-scale projects installed in the U.S. The lab has studied agrivoltaics since 2015. Their Department of Energy-funded research has shown that such projects, if designed properly, can benefit both landowners and solar companies. 

“We’re very confident any crop you can grow outside of a solar array, you can grow within a solar array,” Macknick said. Perhaps more important now, he said, is whether developers can find farmers who want to offer up land for those types of projects and whether growers can make a living that way. 

Prime farmland — flat, sunny, and connected to roads — has become increasingly enticing for solar developers as clean energy expands. That’s created tensions in communities where agriculture has long been a way of life. Dual-use solar has the potential to become a type of “compromise” between those two land uses, Macknick said. In its next phase of research, the lab is working to expand the list of entities conducting dual-use solar research. The lab has also built resources, like a financial calculator, to help more landowners and solar companies learn about agrivoltaics. 

The field is “rapidly expanding,” according to Macknick. But Rute is focused on a use that’s gotten comparatively little attention. Many agrivoltaic projects now designed for grazing use smaller animals, like sheep, according to tracking from the National Renewable Energy Laboratory. Rute’s strategy would go after untapped ranchland “that no one’s even thought about yet,” said Doug Krause, the company’s chief executive officer. If the technology proves compatible, that would be a significant advantage compared to other solar technologies, which are generally unsuitable with land used for cattle, Macknick said.    

Rute’s system can also be installed on uneven ground. It may cause less disturbance to land than more traditional dual-use setups, said Sujith Ravi, who studies agrivoltaics and directs the environmental science program at Temple University. Scientists are still working to fully understand the benefits and tradeoffs of agrivoltaics projects. Because each site is unique, Ravi said more research is needed to understand how solar and agriculture interact over a wide range of geographies. 

“There’s a lot of potential,” Ravi said. “Research happening all over the world in the context of energy and agriculture colocation is [working] to find what benefits should we target in different locations.”

The company will also need to demonstrate its cables and panels placed high in the sky can withstand strong storms, said Breger. 

Rute got its start about seven years ago, building modular foundations for wind turbines that it says use 50% to 75% less concrete than traditional installations and take just a few days to install. Traditional turbine foundations may require excavation to install and it can take weeks for the cement to cure. 

For now, Rute is shifting its focus to solar and Suntracker. Using tests at its installation in Oregon, Rute has reengineered lighter racks to hold solar panels and shifted how it places the cables to support them, lowering the number of poles required to support the installation. The company is now experimenting with ways to reduce the amount of steel per kilowatt to further reduce costs. It’s also started planning a project with an Oregon rancher that’s expected to begin construction this summer. 

Much like the wider field of agrivoltaics, Breger sees Rute’s solution as innovative, but worthy of more tests.

“We need to have solar. … Solar is going to take up land. And if we can get this dual use out of it, then there’s a lot of appeal to that,” Breger said. “Exactly how that works out and [gets] optimized, there’s still a lot of innovation, a lot of data and science that needs to be collected.”

Emma Foehringer Merchant is a freelance journalist who covers climate change, the environment, and energy. Her work has appeared in the Boston Globe Magazine, Inside Climate News, Greentech Media, Grist, and other publications. She lives in California.