Scientists use radiant cooling to help people chill out during hot and muggy weather

The novel coronavirus has made an enemy of one of humanity’s most reliable sources of comfort: the air conditioner.

Given the airborne nature of the virus, recirculating air — a defining feature of traditional air conditioning — has become a dangerous proposition, with at least one example from China showing possible contagion in an air-conditioned restaurant.

But a new study published in the Proceedings of the National Academy of Sciences shows that it’s possible to keep cool without recirculating air, and to do so outside in Singapore, a hot and humid tropical environment.

The process, known as “radiant cooling,” has taken on new importance in a time when fresh air is practically a health requirement.

The study was originally intended to address another problem posed by air conditioning: energy consumption. With global temperatures rising, along with income in some developing nations, air conditioning is expected to grow by two-thirds over the next decade, according to the International Energy Agency, an intergovernmental organization.

But the cost is high: More units mean greater demand for electricity, to say nothing of the refrigerant most air conditioners use — hydrofluorocarbons, greenhouse gases that can leak from the manufacturing or improper disposal of old units.

There are about 2 billion air conditioning units in the world, more than half of which are in China and the United States, according to the IEA. They consume about 9 percent of total global electricity production, and result in the emissions of 1 billion tons of carbon dioxide.

The study’s authors employed the principle of radiant cooling to see whether subjects could be kept cool on hot, humid, sunny days in Singapore — without a heavy reliance on electricity. They built a “Cold Tube” pavilion, made up of 10 panels, each eight feet tall, containing capillaries full of chilled water, insulated and enclosed in a polyethylene membrane that allows for infrared transfer, but not condensation.

Radiant cooling is something we regularly experience when we stand in front of an open refrigerator, though we might not remember this term from physics class. Basically, in the words of Eric Teitelbaum of Princeton University, one of the study’s authors, “You are the sun; you are radiating the warmth, and it’s the surfaces around you that take the heat away.”

Inside the Cold Tube, panels of capillaries filled with cool water are covered by membranes that don't allow condensation and maintain comfort without cooling or dehumidifying the air. (Lea Ruefenacht)

In the Cold Tube pavilion, those surfaces are panels full of cold water that formed two sides and a ceiling to resemble an open-ended bus shelter. As study participants stood or sat in the pavilion in temperatures above 85 degrees Fahrenheit, with more than 66 percent relative humidity and barely a breeze, they reported feeling “cool” and were comfortable.

To feel the cooling effect, people don’t need to be close to the panels, just surrounded by them, said Adam Rysanek of the University of British Columbia, another researcher on the project.

“In future real-world installations, we will likely rely more on ceiling-based coverage of cold tube panels, which means that sitting or standing will not matter, provided Cold Tubes are sheltering you overhead like an umbrella,” Rysanek said.

Dehumidification, Teitelbaum said, is the main reason air conditioning uses so much electricity. “In tropical locations, dehumidification accounts for 60 percent of air conditioners’ energy budget.”

The Cold Tube pavilion also uses electricity, to keep the water inside its panels cold. But it doesn’t require dehumidification, which means it uses half the energy of air conditioning.

This is not the first system to use radiant cooling — in fact, there are companies, including Interpanel in Germany, that use this principle to provide “thermal comfort" in indoor environments. The Cold Tube’s innovation was to show that this type of cooling is possible in humid, tropical environments.

While the Cold Tube pavillion relies on the same refrigerating hydrofluorocarbons as air conditioners, which have a powerful heat-trapping effect when released into the atmosphere, it could use other coolants, including those with smaller global warming potential, said Forrest Meggers of Princeton University, another of the study’s authors.

In international negotiations about these compounds, tensions have arisen over decisions to phase them out in favor of others that are less-warming but more expensive, at a time when many more people in the developing world are newly able to afford air conditioning. Reducing the use of certain chemical compounds could avoid an additional 1 degree Celsius of warming. An agreement was reached in 2016 to phase out these chemicals, but the Trump administration has yet to ratify it.

Despite the possible solution presented by this study, air conditioning carries a certain social cachet that is important for researchers and policymakers to understand, Rysanek said.

“The cultural attachment between air-conditioning and affluence, development and modernity in Singapore and Southeast Asia was equally strong in our time in Singapore, and still is — the often cited example is that Singapore’s founding father declared air conditioning to be the most important technology of the 20th century,” he said.

“There is a reason why there is an emotional attachment to air conditioning and cooling,” he said, and that residential buildings might not be the best place for an indoor version, both for cultural and architectural reasons. But, at least in Singapore, “there are in fact a number of spaces where this works quite well culturally and with sensitivity to people’s needs,” he said, such as schools built without air conditioning.

Exposure to extreme heat and access to air conditioning is unequal in many countries, including the United States, where Black, Latino and other communities of color are disproportionately affected by rising temperatures. Black urban communities in particular are more likely to suffer from extreme heat, a result of redlining and other racist housing patterns that filled those neighborhoods with pavement and buildings instead of cooling green spaces.

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Many cities set up “cooling centers” in the summer for residents without air conditioning to seek relief. But this summer, in the middle of a pandemic, cooling centers may not be open, or going to one might not be a risk people are willing to take, especially since those who might need a cooling center may also be especially vulnerable to covid-19. Black and Latino communities as well as indigenous groups have been disproportionately affected by the virus as a result of certain underlying health conditions, unequal access to health care, and greater exposure to air pollution, among other factors.

Climate change and the coronavirus pandemic are making air conditioning use more complicated. In the past, Californians might have gone to the mall or movie theater to find some air-conditioned relief from the heat, an impossibility due to the pandemic. Instead, during the recent heat wave, people stayed home with their air conditioners on full blast, leading to the state’s most significant energy shortage in 20 years and a round of temporary blackout orders in parts of the state.

In Louisiana and parts of Texas, excessive heat warnings have followed quickly on the heels of Hurricane Laura, which left hundreds of thousands without power. Without electricity, residents will be at home due to the pandemic, without necessary air conditioning.

Despite the environmental advantages offered by radiant cooling systems such as the Cold Tube pavilion, Rysanek said he is cautious about recommending its use in poor communities because it may be misunderstood as a different or less ideal form of air conditioning.

Still, as the world continues to warm, there will be an inevitable need for a “paradigm shift” when it comes to cooling, the researchers said.

“We’ve been working on trying to fight this ever-increasing AC demand curve, and by 2070, cooling demand is predicted to overtake heating demand,” Teitelbaum said. “There is this weird interface between climate and comfort and controlling our physical environment, and air conditioning is really intertwined in all of those aspects.”

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