As solar panels devoured the winter sun from the roof of North Market on a recent morning, a new set of nearby panels was sending energy right back to the sky.
Equipped with a 3M-manufactured film that reduces temperatures without electricity, these reflective panels can dramatically improve the efficiency of energy-intensive refrigeration systems.
For the North Minneapolis grocery store — the first in Minnesota with a SkyCool Systems installation — that means money saved. For a warming planet, that means greenhouse gas emissions reduced while cooling needs climb.
The world now faces "unavoidable multiple climate hazards" in the coming decades, according to the most recent U.N. climate report. Adapting to a changed climate in which air conditioning will play a bigger role has become more important as efforts to mitigate global warming have fallen short.
SkyCool is pitching the cooling panel systems to grocery stores and other users who consume a great deal of electricity keeping things cold — though the concept could spread to other energy-saving uses.
"Grocery stores run on thin margins — anything over 10% in energy savings is going to be very meaningful," said 3M business developer Billie Pritzker. The panels can provide savings of 15% to 20%, she said.
Air conditioning and refrigeration consume a quarter of the world's electricity and contribute 7% of global greenhouse gas emissions. SkyCool says its panels can save twice as much energy as similar-sized solar panels can produce.
Chief Executive Eli Goldstein knows the assertion raises some eyebrows.
"There's always skepticism around new technologies," said Goldstein, whose company is based in Silicon Valley. "This is a technology that can directly cool the environment. It's a very clear cost-benefit."
A few big-box retailers are trying out the panels as a way to meet ambitious carbon reduction goals. 3M is installing the technology on one of its plants in California. A data center is also piloting the cooling panels.
"There's a lot we can do just by having these very reflective surfaces on rooftops," Goldstein said. "Now how do you integrate it into a product, and who do you sell it to?"
How the technology works
3M's high-tech piece of what feels like plastic — really hundreds of nano-layers of precise thickness that work together to reflect solar energy — is borrowed from the natural world.
"That same structure is in the wings of butterflies and fish scales," 3M scientist Tim Hebrink said.
The type of film used to reflect sunlight and radiate heat back into the atmosphere relies on "passive radiative cooling," another natural phenomenon seen when frost appears on windows even when air temperatures remain above freezing.
"The two key properties are solar reflectivity and thermal emissivity," Hebrink said. "That's what gives us the ability to radiate heat to the sky."
SkyCool arrays work by cooling a fluid that moves below the panels and is pumped through refrigeration systems. The cooling helps reduce the run time of motors in the refrigeration system to reduce electricity consumption. All told, the SkyCool system can reject heat at about 100 watts per square meter, saving between 450 and 550 kilowatt-hours per square meter every year — twice the average production of a solar panel.
The film emits heat in a specific wavelength that allows it to escape into space — a property that could be used to cool urban centers if the film was on enough buildings, research shows.
Arizona State University researchers installed a 3M passive radiative cooling film directly on a bus stop last summer and found it cooled air temperatures in the shelter by as much as 4 degrees.
"Based on these positive outcomes, the city of Tempe is considering incorporation of cooling films in future bus shelter design, bus roofs and other shade infrastructure," the university announced last month.
A team of University of Minnesota researchers has sought funding to simulate how much of the 3M film on downtown Minneapolis and St. Paul rooftops it would take to lower summer daytime temperatures by 2 degrees.
Pritzker said 3M is piloting more ways to apply the film directly — including on shipping container-based housing units and potentially on electric vehicle batteries.
Most uses have been pilot projects so far. In order for the film to make a dent in adapting to or stemming climate change, it needs to sell.
Pilots expanding use
North Market's nonprofit owner, Pillsbury United Communities, saw an easy decision when 3M approached with a novel way to keep energy costs down in the warm months.
"The primary reason we are participating is to help advance this technology," said Vanan Murugesan, chief transformation officer at Pillsbury United Communities. "Refrigeration is one of the largest expenses for a grocery store — they are energy hogs — and we couldn't pass up the opportunity to participate in this pilot and promote this technology."
Depending on the level of energy savings, it could help keep rising food prices in check, he said.
The federal Department of Energy says $300 billion is spent globally on cooling systems annually, and the environmental impact could grow by an order of magnitude in the coming decades as developing countries start using more air conditioning and data centers keep getting built.
"Currently, 1 gigaton of CO2 emissions annually come from electricity needed for cooling applications," according to the department's Advanced Research Projects Agency. "This will grow to greater than 10 gigatons of CO2 emissions per year by 2050 if improved technologies for cooling are not developed."
In 2018, the agency awarded 3M more than $2.7 million to refine the passive radiative cooling film first developed by Stanford University researchers. Last year it gave SkyCool a $3.5 million grant to deploy the technology.
Government support — through funding or policy — may be crucial for widely adopting passive radiative cooling. 3M's smog-canceling roof granules have benefited from a Los Angeles requirement that new homes have environmentally beneficial roofing.
While 3M and SkyCool declined to say how expensive the systems are — each installation has been a custom project — the companies say the average time to recoup the expense through energy savings is two to five years. The payback period for solar panels averages eight years.
"The idea is it can be cheaper than a solar panel and save more energy," Goldstein said. "Everyone gets benefits from that."