It’s below freezing on a rooftop in Cambridge, Massachusetts, but inside a device created by researchers at MIT, the temperature is 220º C. There are no moving parts, compressors, or vacuum pumps involved. Just sunlight and a special aerogel it has taken researchers at MIT 4 years to perfect, aided by grants from the Department of Energy’s ARPA-E program.
According to MIT News, an aerogel is a foam-like material made of silica particles. Silica is the basic ingredient in glass. It is abundant and inexpensive. Aerogels can be a highly efficient and lightweight insulating material but generally block much of the visible light that falls on them, limiting their usefulness as solar heat collectors.
Evelyn Wang, head of the mechanical engineering department at MIT, says that it took a team of researchers four years to devise an aerogel that transmits 95% of visible light while maintaining its high insulating properties. The result is an aerogel so transparent it is nearly invisible.
The key was finding the correct mix of catalysts and silica materials to create the aerogel and then drying it properly. What’s left is a matrix that is mostly air but retains the original mixture’s strength. Drying out the mixture much faster than is normal with conventional aerogels produced a gel with smaller pore spaces between its grains which led to far less light being scattered inside the gel.
Graduate student Lin Zhao describes the basic function of the aerogel layer as “like a greenhouse effect. The material we use to increase the temperature acts like the Earth’s atmosphere does to provide insulation, but this is an extreme example of it.”
Solar collectors using the new aerogel technology could replace conventional solar hot water heaters with a less costly alternative. They could also be used to heat residential and commercial buildings at lower cost than traditional heating equipment. Because the aerogels can create temperatures of 200º C and more, there are numerous potential applications in industry and agriculture as well.
The materials needed are inexpensive. The principal cost of making the aerogel in commercial quantities is the drying process, which requires a specialized device called a critical point dryer to allow for a very precise process that extracts the solvents from the gel while preserving its nanoscale structure.
Professor Wang explains that the drying phase is done batch by batch and is not adaptable to the continuous roll to roll process commonly used in industrial applications. “The key to scaleup is how we can reduce the cost of that process,” she says. A preliminary economic analysis shows the system can be economically viable for some uses, especially in comparison with conventional systems that use glass surfaces bonded to a substrate with a vacuum in between.
Like all laboratory breakthroughs, this one may yet be years away from commercial scale manufacturing. But the promise of virtually free heat from sunlight could be an important adjunct to the quest to decarbonize the built environment. Building heating consumes enormous quantities of fossil fuels. Virtually free zero emissions heat from the sun could be the greatest thing since solar panels.