Solar panels on Denver rooftops throw shade to help crops grow
Then a funny thing happened. It turned out the plants under or next to the solar panels, benefitting from the installation's shade, had greater total biomass, moisture content and overwintering survivability.
"We ended up with larger plants," Bousselot said. "It was a happy accident."
That accident — 15 years ago — set the now 46-year-old Colorado State University assistant professor of horticulture on a career researching rooftops, plants and solar. Along the way Bousselot upped the ante by growing not just any plants but crops.
The goal is to combine the building cooling and heating advantages and stormwater management benefits of a green roof with growing food and producing electricity.
"Stacking all of those benefits on relatively small spaces really got me absolutely thrilled, and so that's what we're trying to do," Bousselot said.
And so, on her rooftop farm at CSU's Spur Campus in Denver, Bousselot, who grew up on a Nebraska farm, has grown chili peppers, lettuce, cucumbers, tomatoes, onions and garlic. Carrots, spinach and arugula didn't like it up on the roof — too hot.
While the legendary Hanging Gardens of Babylon, believed to have been constructed around 500 B.C., may be the most famous green roof, modern green roof technology has more modest beginnings in Germany in the 1960s.
The benefits of covering a roof with greenery include doubling the life of a conventional, white roof, cutting the heating and cooling costs for a building and reducing the rate of stormwater runoff up to 65%, according to a report from the federal General Services Administration.
"Research has identified green roofs as one of the best ways to address wet weather flows in urban areas with high-density development," according to the administration, which maintains nearly 2 million square feet of green roofs on government buildings, including the Denver EPA office .
Green roofs can reduce the amount of energy a building uses for cooling in the summer and heating in the winter. A green roof can be as much as 40 degrees cooler than conventional roofs and can cut building energy use by thousands of dollars a year, according to the EPA .
"We are in this kind of green revolution," said Janelle Cameron, marketing manager for Portland, Oregon-based Columbia Green Technologies , a designer and installer of green roofs. "Especially post-pandemic, there has been a demand for more green spaces in cities."
"It is being driven not just by policy but also by people and communities," Cameron said. Six U.S. cities — including Cambridge, Massachusetts, New York, Seattle and Washington, D.C. — have adopted policies promoting green roofs.
In 2018, Denver became the second city, after San Francisco, to adopt a green roof ordinance , mandating every new building, building addition, or roof replacement with a gross floor area of 25,000 square feet or more must include a green roof or a combination of green roof and solar array.
So far, there have been 24 projects in Denver, mainly on the roofs of office and commercial buildings, covering 35,285 square feet with 43,685 planters, according to the city's Department of Community Planning and Development.
These installations usually include succulents, grasses or perennial flowers, needing a base of 4 to 6 inches in which to grow. When solar panels are added, they are close to the growing surface. The beds can act as anchors for the arrays.
Weight matters, especially on existing buildingsTrying to grow crops on a roof under solar panels is a trickier exercise.
First, the growing medium must be 8- to 18-inches deep, with each inch of soil covering a square foot calculated to weigh 8 pounds — accounting for the beds being saturated with rain. So, a 200-square-foot roof with 18-inch-deep planters would have to be able to hold up 28,800 pounds.
"That's why 4-inch-deep is a sweet spot, because the majority of existing building inventory can take that weight," Bousselot said. "They can't take the deeper ones. So structural integrity is always No. 1."
To reduce the load, Bousselot doesn't use soil but a "engineered substrate" designed to be super lightweight. "That's our secret sauce," she said. "Each company has their own specific formula."
Cameron said that using its own lightweight medium and a 6-inch system, her company was able to put a large "meadow" atop the Old Chicago Post Office , a two-block-long Art Deco building built in 1921.
Blending in solar panels just adds to the complexity.
While solar panels on a green roof are close to the ground, panels on an agrivoltaic roof have to be elevated enough for crops to grow and for workers to tend the beds. "The biggest challenge is wind loading," Bousselot said. "The panels act like a giant sail with the wind."
This is a particular problem with the array at the Spur Campus, which is near the National Western Complex and faces the foothills. "The wind comes in a straight line right over the mountains from the west and right under our panels," she said. "It is a constant thing on a windy day."
The ballast from the planting beds was heavy enough to hold down solar arrays, but the substrate was too loose for the mounting poles to stay in place, so they had to be anchored to the roof. What measures need to be taken to avoid solar from flying away will depend on the particular wind loading of each rooftop garden.
The fifth-floor roof of the Aqua building at the campus may not be Babylon, but its 5,000 square feet of beds are fitted with two types of solar panels, beds that are in partial shade and some in full sun. A network of sensors monitors the crops.
Both the plants and the solar panels turn the sun's rays into energy, and surprisingly there is a synergy between them.
Through photosynthesis, plants use the sun to grow and during the summer "essentially plants are constantly trying to cool themselves," Bousselot said. They do that by releasing water, which can be stressful in a hot, dry Colorado summer.
Bousselot's research has found that the substrate remained cooler and more moist under the solar panels and that the shade helped reduce the release of water — evapotranspiration — from the plants.
Being in the shade of the solar panels also protected plants from wind and hail and improved their overwintering survival.
Can solar panels protect the raspberry crop?Next spring, Bousselot will begin tests with a singularly fragile crop — raspberries. "It is one of the highest-value food crops on the Front Range. It also has a very short shelf life, so when you pick it, you have to consume it very quickly."
"It's an ideal crop here, because it's cold hardy, it's drought tolerant," she said. "However, there's lots of things that limit yields on the Front Range, things like hail, winter drying of the buds ... high wind, torrential rain, those kinds of things all limit the yield. They cause the berries to shatter."
The aim is to see what kind of protection the solar arrays provide for the berries.
"As a farm kid myself, I know the sort of tragedies that can occur with weather," Bousselot said. "But if you can have a semiprotected growing environment, and you can have consistent yield, then you can plan more effectively."
And while the solar panels have provided benefits for the crops, the plants have returned the favor.
Solar panels operate most efficiently at temperatures up to 77 degrees, after that for every 1.8 degrees they lose about a half of percent efficiency. The plants' release of water helps cool the panels.
Bousselot said it was equal to about a 7-degree benefit over the course of the year. Research done in Australia found that solar panels above a green roof had an average 4.5% increase in solar output compared to a conventional rooftop solar array. Other studies have found output increases of up to 8%.
In essence, the crops and panels are creating their own microclimate and, what's more, their own ecological niche for the crops attracted bees, ants, worms, butterflies and grasshoppers. This past summer there were too many grasshoppers.
"Everything you find on the ground you find up here, except deer," Bousselot said. "This is a unique environment and not a bad view."
Rooftop agriculture also offers the chance to produce fresh vegetables and fruits right in the city. "Of course, we are not going to feed the entire world, we can be an ambassador for agriculture and add resiliency to the food system," Bousselot said.
This year the CSU rooftop has donated 500 pounds of produce to the nonprofit GrowHaus , which serves 1,000 families in Globeville and Elyria-Swansea with weekly fresh food boxes. The garden provides jalapeño peppers, garlic, cilantro, onions and tomatoes at the request of the predominantly Latino communities.
The ability to forge a link between rooftop farming and surrounding communities is one of the most promising and dynamic developments, said Anastasia Cole Plakias, co-founder of the Brooklyn Grange , which farms 5 acres of rooftops (no solar) in the New York City borough.
Its most recent roof farm is in Brooklyn's Sunset Park where there is a burgeoning Chinese immigrant community. Based on community feedback, they are growing ingredients that are common in Asian cuisine, including bok choy, napa cabbage and bitter melon . The produce is distributed through a health center, food banks and a community coalition.
"There are deep benefits of rooftop farming to provide community solutions," Plakias said. "It is as much about community health as food."
So how much rooftop farming could Denver see?
"There's about 5,000 acres of rooftops on relatively large buildings" in the city, Bousselot said. "So even if we could green 10% that is still 500 acres. That's bigger than the farm I grew up on."
