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Space Cooling

SkyCool Systems can revolutionize the way people cool their homes by utilizing the power of the sky.

Did you ever think you could operate your refrigerator or air conditioner with little or no electricity? Aaswath Raman, assistant professor of Electrical and Systems Engineering at the University of Pennsylvania, has co-invented a technology that uses an untapped source of energy—the sky—for cooling and refrigeration. Raman, who featured in MIT Technology Review’s TR35 (Innovators Under 35) list in 2015, has co-founded SkyCool Systems. The California-based start-up work on prototype systems that use a greener method of cooling, known as radiative sky cooling, for industrial and residential cooling needs.

Excerpts from an interview.

We don’t usually hear of the environmental impact of a household appliance like a refrigerator. Could you give us an idea about how big is the carbon footprint of refrigerators?
For a typical household refrigerator, I’ve seen numbers of around one ton of carbon dioxide per year. This depends on the mix of electricity sources a given customer has access to, of course.

Do you envision a time when something like the emission standards now attached to automobiles becomes the norm with household appliances?

I have observed that improved efficiency has become an attractive proposition for customers across a broad range of segments. On one level, it’s just about saving money. But, improved sustainability and lowered environmental impact of a product are also becoming increasingly important for many consumers. Of course, this long-term trend can be stimulated by policies that favor or incentivize efficiency. Even seemingly straightforward programs like the Energy star program in the U.S. have been very influential in increasing the efficiency of household appliances.

Could you tell us more about your research and how radiative sky cooling works?

We’ve developed a way to improve the efficiency of cooling systems—air conditioners and refrigerators from residential to industrial scales—by harnessing an untapped renewable resource—the cold of space. It sounds a bit like science fiction, but it’s actually a completely natural phenomenon known as radiative sky cooling.

This phenomenon was used by many ancient desert cultures to make ice in the desert at night. In this approach to cooling, a sky-facing surface can emit its heat away as thermal radiation, infrared light we can’t see, to the sky. A fraction of that heat naturally escapes as light to the upper atmosphere, if not space, thereby allowing the sky-facing surface to cool passively below the surrounding air temperature. More recently, it has been a somewhat obscure concept, partly because the cooling effect was only ever observed at night.

We made key breakthroughs in developing this concept as a technology by making this cooling possible during the day as well, for the first time. The key challenge to achieve this cooling during the day is, ironically, the Sun.

The Sun heats everything up and we don’t want that when we want to cool. To do this, we’ve developed specialized optical surfaces that simultaneously send heat away as thermal radiation, while also avoiding getting heated up by sunlight. To the naked eye, it looks like a very good mirror. Remarkably, this, then, enables zero-electricity cooling that also doesn’t require any water—typically, we see cooling of up to 20 degree Celsius below ambient air temperature, but performance varies in differing conditions. This is very unique in the space of cooling technologies, and a radically different way of approach to cooling.

We founded a company called SkyCool Systems in 2016 to commercialize this research. Our product is a panel that uses these surfaces to cool water or other related fluids passively. Think of it as an analogue to a solar water heater, except we cool water by rejecting heat to the sky. The panels will typically serve as a supplemental cooling system, augmenting, for example, commercial-scale refrigeration and air-conditioning systems to make them more efficient. They can also be used directly in some buildings in milder climates.

Are research and innovation in the green sector driven more by private enterprise or by universities and government agencies?

Basic research—the fundamental science and early proof of concept work—is typically done in university settings and government labs. Some companies have invested in doing more fundamental work, but early-stage research remains largely an academic enterprise.

Ideally, if a technology is promising, it can be commercialized to find its way into products or technologies used in the market. This latter phase is typically done in the private sector, whether in a new startup or in an existing company that licenses or purchases the technology.

That being said, the process is unique for every technology and dependent on a range of contextual factors. The private funding environment for clean technologies has evolved over the years, with many start-ups relying on a combination of government grants and private investment to support their early work in commercializing technologies.


Natasa Milas is a freelance writer based in New York City.