As Denver pushes to green its rooftops, a CSU professor takes agrivoltaics to new heights
Jennifer Bousselot is a gardener, but her crops aren’t on the ground – they’re on the roof of one of the Colorado State University buildings in downtown Denver.
On the top of the roof are rows of native plants, colorful flowers and ripe vegetables. It’s not exactly a quiet garden scene as cars zoom past the building on Interstate 25. But Bousselot has a vision for it.
“I wanted to use plants in order to provide more ideal conditions for livability in cities,” she said. “So these are about providing additional benefits in places that are often unused.”
Bousselot, an assistant horticulture professor , is conducting research on rooftop agrivoltaics — how plants grow under solar panels. Her fascination started 15 years ago, but it was unintentional.
“I was very frustrated to see that another scientist had put up a solar array at the edge of my research plots,” Bousselot said. “But over the next two or three growing seasons, I saw an incredible response of the plants around that space.”
It’s due to their synergy, she said. Solar panels tend to get too hot on conventional rooftops, which can reach 150 degrees or higher, and that heat reduces their efficiency. Plants help cool them off.
“If you have plants under there, they evaporatively cool their surrounding areas so they can survive,” Bousselot said. “So typically plants under a solar panel get the conditions very close to what ambient conditions are and therefore the panels end up benefiting.”
The same goes for the panels helping the plants.
“In the West, where we have a lot more sun, we're basically providing the same protection as a slightly cloudy day would by putting solar panels — especially semi-transparent panels — over those plants,” she said.
One way to see how a plant is doing is to measure chlorophyll. It's a substance that makes plants green and helps them absorb sunlight to grow. Bousselot and her grad students use a thermometer-like tool to measure it.
“It's called a chlorophyll fluorometer,” she said. “It helps us gauge between various treatments, [shows us] how effective or how healthy that plant is under those conditions.”
Their tests show that plants growing under or near solar panels survived longer and retained more moisture, using less water overall. Leafy greens are especially promising.
“It's not urban versus rural,” she said. “It's really trying to grow plants near people. And most of our population is in urban spaces and not enough of our green [spaces are] in urban spaces.”
Bousselot's agrivoltaics research at CSU takes to new heights the growing interest in "green roofs," the term for those at least partially covered in vegetation. The Latter-Day Saints’ Conference Center in Utah has a green roof that is over 200,000 square feet. In Idaho, there’s one atop St. Luke’s Magic Valley Hospital.
Bousselot hopes the fast-growing region will see many more as cities and architects rethink the utility of rooftops.
“Does the rooftop get used for much besides storage? No,” she said. “So to me, it would be absolutely wonderful to see scenarios where we're contributing to a lot of the things we need as a human society.”
She’s not the only one who feels this way. Back in 2017, Denver voters passed an initiative to mandate green roofs on new and existing buildings over 25,000 square feet. Bousselot was one of the initiative's technical advisors.
“The ordinance sought to achieve important environmental benefits for the city, primarily by reducing the urban heat island impacts and greenhouse gas emissions,” said Amanda Weston, Denver's planning department spokesperson.
But the ordinance was tough to implement, especially on existing buildings.
“If the building can't structurally hold the weight, it's not something that we can even approve and say that that's okay to do because it's not safe,” she said. “When the pen goes to the paper, and everything starts coming out in plans, we're like, ‘Oh, wait a second, that's not exactly feasible.’”
So, in 2018, the city broadened the ordinance and renamed it the Green Buildings Ordinance. It gave building owners more options and flexibility while keeping the sustainable development goals. The city estimates it could create up to 3.5 million more square feet of green space by 2050 compared to the old ordinance and decrease cost requirements by up to 90%.
Among the new ordinance's requirements is that every building — new or existing — needs to install a so-called "cool roof'".
“A cool roof is made of a highly reflective material,” Weston said. “And so it reflects more sunlight and absorbs less heat than like a standard roof would.”
After that, there's a range of compliance options, including paying into a green building fund and purchasing offsite solar power.
“We don't want to just say, here's the rules, deal with it,” Weston said. “We want to give those project teams resources and tools to look at this as an opportunity, an opportunity to design more sustainable buildings.”
Since the ordinance was put in place, 28 buildings have chosen onsite green spaces, according to the city. “Green space” usually means trees and shrubs — not a garden with solar panels.
“It's actually a pretty low percentage of projects that are able to comply with the vegetated roof requirement of the green building ordinance,” said Jeff Stoecklein, a Denver-based landscape architect.
Stoecklein’s goal as a designer is to balance usable outdoor space with vegetation. But the ordinance requires either 60% of the total roof area, or 10% of the gross floor area devoted to green space. That leaves little space to add chairs or a pool, for instance, so clients often choose easier options.
“Sometimes it just takes some slight tweaks in the HVAC or mechanical systems of the project, the architectural systems, to really put them in a place where they're already close enough to be achieving that credit option,” he said. “And so the value of contributing a little bit more makes sense as opposed to translating it all to vegetated roof area.”
Still, Stoecklein worries about long-term success due to the extreme solar exposure in the city.
“Any vegetated assembly requires things like irrigation, long-term maintenance,” he said. “So the viability of these systems is challenged by just where we are in our climate."
In any case, Stoecklein sees the benefit.
“We're becoming much more dense than we were 5, 10, 20 years ago,” he said. “And so it becomes even more important to give, I think, daily access to landscape and outdoor space."
Despite the challenges, Bousselot hopes more people embrace the greening of rooftops.
“If we can bring a little bit of life back into that city, choose to use about the only remaining space in our cities left — rooftops — we can really make an incredible impact,” she said.
This story was produced by the Mountain West News Bureau, a collaboration between Wyoming Public Media, Nevada Public Radio, Boise State Public Radio in Idaho, KUNR in Nevada, the O'Connor Center for the Rocky Mountain West in Montana, KUNC in Colorado, KUNM in New Mexico, with support from affiliate stations across the region. Funding for the Mountain West News Bureau is provided in part by the Corporation for Public Broadcasting.