Cooling Without Electricity
 
Today, it’s estimated that worldwide electricity costs total roughly $2.5 trillion a year and that cooling systems consume 15% of that electricity. With experts forecasting demand for cooling to grow ten-fold by 2050, that means electricity expenditure for cooling alone could rise to nearly $4 trillion per year. Therefore, improving the efficiency of cooling systems is a critical part of the twenty-first-century energy challenge.

Fortunately, there appears to an extremely clever and cost-effective way of doing just that. Here’s how it works.

All objects give off heat in the form of thermal radiation. But the air around them, mainly because of water molecules, absorbs and radiates back most of that heat. However, a sliver of those emissions in the mid-infrared range, can slip past these compounds, enabling surfaces that emit radiation at those wavelengths to become cooler than the surrounding air. A team of Stanford researchers developed a thin film tuned to radiate infrared heat in exactly this band. Then, in an even bigger advance, they coupled those radiative properties with reflective ones, enabling the materials to throw back nearly all the heat in sunlight. That’s crucial because without this reflective capability, the sun would more than offset the radiative cooling effect during the daytime.

Recently, the team demonstrated that retrofitting radiative panels to an office building could cut its cooling electricity needs by 21 percent in summer. Extrapolated to the expected global electricity demand for cooling in 2050, that amounts to roughly $800 billion a year.

To commercialize this technology, team members Shanhui Fan, Aaswath Raman and Eli Goldstein, founded a company called SkyCool Systems.

The underlying scientific phenomenon called “radiative sky cooling” is a natural process that everyone and everything does, when their molecules release heat. You can witness it for yourself in the heat that comes off a road as it cools after sunset. This phenomenon is particularly noticeable on a cloudless night because, without clouds, the heat we and everything around us radiates can more easily make it through Earth’s atmosphere, all the way to the vast, cold reaches of space.

If you have something that is very cold, like outer space, and you can dissipate heat into it, then you can do cooling without any electricity or work. The heat just flows! For this reason, the amount of heat continuously flowing off the Earth into the universe is enormous.

But on a hot, sunny day, radiative sky cooling doesn’t work that well for the human body or for most other objects. This is because sunlight will warm them more than radiative sky cooling will cool them. To overcome this problem, the SkyCool team created a surface using a multilayer optical film that reflects about 97 percent of the sunlight while simultaneously being able to emit the surface’s thermal energy through the atmosphere.

Without absorbing heat from the sunlight, the radiative sky cooling effect can enable cooling below the air temperature even on a sunny day.

That means we’re no longer limited by what the air temperature is, we’re limited by something much colder: the temperature of outer space.

The first experiments published in 2014 were performed using small wafers of a multilayer optical surface, about 8 inches in diameter, and only showed how the surface itself cooled. Naturally, the next step was to scale up the technology and see how it works as part of a larger cooling system.
 
In their late 2017 paper in Nature Energy, the researchers described a system where panels covered in the specialized optical surfaces sat atop pipes of running water and tested it on the roof of the Packard Building at Stanford    They also applied data from this experiment to a simulation where their panels covered the roof of a two-story commercial office building in Las Vegas ? a hot, dry location where their panels would work best. They calculated how much electricity they could save if, in place of a conventional air-cooled chiller, they used a vapor-compression system with a condenser cooled by their panels. They found that, in the summer months, the panel-cooled system would save 14.3 megawatt-hours of electricity, a 21 percent reduction in the electricity used to cool the building. Over the entire period, the daily electricity savings fluctuated from 18 percent to 50 percent.

Right now, SkyCool Systems is measuring the energy saved when panels are integrated with traditional air conditioning and refrigeration systems at a test facility, and Fan, Goldstein and Raman are optimistic that this technology will find broad applicability in the years to come.

But, according to Nick Fernandez, an energy analyst at the Pacific Northwest National Laboratory, far larger energy savings may be possible for developers who opt to incorporate radiative cooling systems directly into new buildings during the design phase. According to a simulation analysis published in 2015, on which Fernandez was the lead author, if the system were coupled with a hydronic radiant cooling system, a rare but highly-efficient way of cooling buildings that works by circulating water instead of blowing air, the energy savings for heating, cooling, and ventilation could reach nearly 70 percent in ideal climate conditions.

Translated into dollars and cents, that could mean global electricity savings in 2050 totaling around $2.5 trillion a year.

Given this trend, we offer the following forecasts for your consideration.

First, the adoption rate for radiative cooling systems will vary greatly depending on climate and type of construction.

The Pacific Northwest Lab study estimated that if a retrofit rooftop radiator of the type SkyCool is developing could be produced and installed for less than 58 cents per square foot, the energy savings would cover those costs in about five years based on typical savings. Buildings with a large roof area in hot, dry climates are ideal. The southwest United States and the Middle East are obvious targets. The Pacific Northwest and the UK are less appealing near-term targets.
 
Second, SkyCool will not be the only firm developing products designed to address the enormous opportunity in radiative sky cooling systems.

In February 2018, a team of engineers at the University of Colorado, Boulder, published a paper in Science describing a glass-polymer hybrid material that achieved “noon-time radiative cooling power of 93 watts per square meter under direct sunshine.”  According to a university publication, the CU Boulder researchers stressed that they’ve already figured out how to affordably manufacture rolls of the film-like material, “making it a potentially viable large-scale technology for both residential and commercial applications.” Like the Stanford team, the Boulder researchers raised money from ARPA-E, applied for a patent, and formed a company, which is called Radi-Cool.  According to Ronggui Yang, a professor of mechanical engineering, who is a coauthor of the paper and acting CEO of the startup, the CU Boulder scientists are now in talks with potential investors and manufacturers. And,
Third, with trillions in potential savings, this is precisely the kind of “green technology” that will capture the imagination of policy-makers, consumers and investors.

Unlike many so-called, “environmentally-friendly solutions,” radiative sky cooling technology reduces costs, conserves finite resources, cleans up the environment and improves the lives of consumers. It’s the kind of win-win innovation that benefits everyone.

References
1. Eli A. Goldstein, Aaswath P. Raman, and Shanhui Fan. Nature Energy, 2017. Sub-ambi- ent Non-evaporative Fluid Cooling With the Sky. https://www.nature.com/articles/nener-gy2017143

2. James Temple. MIT Technology Review, November 28, 2017. How High-tech Mirrors Can Send Heat Into Space. https://www.technologyreview.com/s/609321/how-high-tech-mirrors-can-send-heat-into-space/

3. James Temple. MIT Technology Review, September 12, 2017. A Material That Throws Heat Into Space Could Soon Reinvent Air Conditioning. https://www.technologyreview.com/s/608840/a-material-that-throws-heat-into-space-could-soon-reinvent-air-conditioning/

4. N. Fernandez, W. Wang, K. Alvine and S. Katipamula. Pacific Northwest National Labora- tory Resport, November 2015. PNNL-24904: Energy Savings Potential of Radiative Cooling Technologies. https://www.pnnl.gov/main/publications/external/technical_reports/PNNL-24904.pdf

5. Chris Cesare. Stanford Report, November 26, 2014. Stanford Engineers Invent High- tech Mirror to Beam Heat Away From Buildings Into Space. https://news.stanford.edu/ news/2014/november/radiative-cooling-mirror-112614.html

6. Trent Knoss. CU Boulder Today, February 9, 2017. Newly Engineered Material Can Cool Roofs, Structures With Zero Energy Consumption. https://www.colorado.edu/today/2017/02/09/newly-engineered-material-can-cool-roofs-structures-zero-energy-con-sumption

7. Yao Zhai, Yaoguang Ma, Sabrina N. David, Dongliang Zhao, Runnan Lou, Gang Tan, Rong- gui Yang, and Xiaobo Yin. Science, March 10, 2017. Scalable-manufactured Randomized Glass-polymer Hybrid Metamaterial For Daytime Radiative Cooling. ?http://science.science-mag.org/content/355/6329/1062.full?