Rooftop solar panels have been vying for space with the green roof trend, and for a while there it looked like no roof was big enough for the two of them. Well, that was then. The emerging field of agrivoltaics is getting set to jump from fields to rooftops, and the result could be a proliferation of solar powered rooftop microfarms across the country.
Green Roofs & Rooftop Solar
The idea behind a green roof is simple enough. Instead of finishing off a flat rooftop with a synthetic membrane, you can grow a layer of plants. The vegetation cools down buildings in hot weather, and it can also have a ripple effect on the urban environment by trapping CO2 and preventing storm runoff. Advocates also point out that the living layer helps stretch out the lifespan of the underlying membrane.
Green roofs are typically planted with sedum and other low-maintenance, non-native species. More recently, green roofs have also promoted native species and biodiversity. In Minnesota, for example, researchers have found that several species native to the state’s bedrock bluff prairies are a good match for green roofs.
“Plants growing in bedrock bluff prairies are adapted to growing conditions very similar to those found on many green roofs, including thin growing medium and high exposure to wind, sun and drought,” they note.
The researchers caution that native species in their natural environment can send roots deep into the ground in a quest for water. However, the team also takes note of anecdotal evidence that plants adapt to the rooftop environment, and grow their roots horizontally.
Rooftop Solar Loves Green Roofs
The idea of combining solar panels with green roofs has begun to attract interest, and a solar-plus-green roof study in Australia from back in 2021 affirmed something that ground-mounted solar researchers have already noted: vegetation beneath a solar panel creates a cooling effect that raises solar conversion efficiency, as solar panels function more efficiently in cooler temperatures.
The study was primarily aimed at assessing the urban ecosystem services provided by green roofs. The researchers studied two identical roofs with rooftop solar arrays, one with a green roof and one without. The study produced comparative data on the ability of green roofs to cool off buildings, as well as urban biodiversity enhancements and stormwater retention among other findings.
Things got particularly interesting with the addition of solar panels to the mix. On the plant side, the study found that certain plants grew far better in the shade of the solar panels than they do in other environments. That is consistent with findings in the agrivoltaic field, which demonstrate that some crops perform better in the cooling, protective microclimate created by solar panels.
On the solar side, the researchers calculated that solar panels in the rooftop solar-plus-green roof combination outperformed the solar-only panels by a significant margin of 13.1% overall.
The World Green Infrastructure Network took a look at the study and teased out the symbiotic relationship between energy-saving green roofs and the clean energy from rooftop solar arrays.
“Surface temperatures were greatly reduced on the green roof – in some cases by up to 20°C during summer, suggesting a potential reduction in urban heat island effects,” they noted. “Insulation was another benefit of the integrated system, preventing heat transferring inside the building as well as retaining heat in cooler periods.”
“Overall, the integrated green roof solar PV system outperformed the conventional solar PV system, confirming that green infrastructure is among the easiest and most efficient solutions that, through its multiple benefits, can help make our cities more resilient to climate change,” they concluded.
Large-Scale Rooftop Farming Discovers Solar Power
Meanwhile, the business of growing food on rooftops on a large scale has also begun to take hold. The New Jersey blueberry firm Full Blues, for example, has applied its proprietary growing process to a sprawling rooftop in New York City.
Another example is the award-winning firm Brooklyn Grange, which has established a massive rooftop farm on the roof of the Jacob Javits convention center in Manhattan including an orchard and food forest as well as vegetable crops.
The idea of adding solar to the rooftop farm mix began to take shape back in 2015, when the hydroponics firm Gotham Greens constructed a two-acre system of rooftop greenhouses on a soap factory in Chicago. The company has continued to expand its rooftop farming model since then.
All of this activity has laid the groundwork for introducing agrivoltaic technology to rooftop farming.
For those of you new to the topic, agrivoltaics refers to solar arrays that are designed to accommodate large-scale farming. Initial projects typically involved pollinator habitats and grazing lands. Now the field has branched out into food crops for humans as well as ecosystem restoration.
The Colorado solar firm Sandbox Solar has been carving out a name for itself as an agrivoltaics innovator, thanks in part to research funding from the US Department of Agriculture.
One especially interesting project involves a row of greenhouses at the Colorado farm Spring Hill Greens. Sandbox spotted an opportunity to place bifacial solar panels on strips of land between the greenhouses, which was otherwise unusable. The bifacial panels are mounted vertically to catch the ultraviolet light reflected from the greenhouses.
Rooftop Solar & Urban Agriculture
From hothouses and bifacial solar panels, it’s just a short hop over to the rooftop agrivoltiacs trend.
Last year, a team of researchers in China and Japan modeled the capability of rooftop agrivoltaics to resolve the food-energy-water nexus. They assessed that the city of Shenzhen, China, had enough rooftops to grow enough lettuce to supply the city for a year. The demand on water resources for irrigating such a system would be substantial, though. The researchers also assessed that the rooftop solar panels systemwide would only provide 0.2% of the city’s electricity.
The University of Arizona may have some hard data to contribute in the upcoming years. The school has been working on a rooftop solar-plus-farming project since 2017. The rooftop solar installation was completed in 2020 and they got to the planting stage in August of 2021. Sensors and other data collection instruments were also installed in 2021.
So far the focus is on growing native desert perennials and annuals including milkweed and whitethorn acacia. “The seeds were provided by Strategic Habitat Enhancements, a local, women-owned organization that ensured all seeds were sustainably harvested from around the Sonoran Desert,” the school notes.
Bousselot is also working on a rooftop solar project with urban agriculture, located at Colorado State University’s new Hydro building in Denver, so stay tuned for more on that.
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Photo: The new Hydro Building in Denver will serve as a research site for rooftop solar and urban agriculture (courtesy of Colorado State University).
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