The “Cold Tube,” is an an outdoor pavilion made of radiant cooling panels, which provide cooling to passersby without cooling the air. Credit: Photos courtesy of the researchers

New radiant cooling technology keeps people cool outside.

When most people think of cooling, they automatically imagine air conditioning (AC), or cooling the air in a room. But, there is a much more efficient way to cool people, using your body’s radiation.

To demonstrate the effect of radiant cooling, Forrest Meggers, assistant professor of architecture and the Andlinger Center for Energy and the Environment, and a team of researchers built a “Cold Tube,” in Singapore last year. It was an outdoor pavilion lined with novel insulated radiant panels that held cold water pipes inside. Because your body is constantly exchanging radiation with objects around you, and radiation flows from hot to cool surfaces, the participants who walked through the exhibit shed their radiation toward the panels, similar to what would happen if you stood near a freezer. The participants reported feeling cool, despite the air itself having temperature and humidity levels that would ordinarily feel sweltering. The new research showed that people could feel comfortable in hot and humid outdoor environments using only radiant cooling, which could use far less energy than cooling large volumes of air.

The researchers, collaborating with scholars at the University of British Columbia, University of Berkeley, ETH Zurich in Singapore, and the University of Pennsylvania, published their results on August 18, 2020, in the Proceedings of the National Academy of Sciences (PNAS).

Eric Teitelbaum who built the “Cold Tube,” walks viewers through the outdoor pavilion and explains how the radiant cooling system keeps people cool in the summer heat in Singapore. Credit: Video courtesy of Eric Teitelbaum

In this Q&A, Meggers and first author of the paper, Dr. Eric Teitelbaum, now a senior engineer at AIL Research, comment on why this research is so relevant not just in a warming world, but also in a contagious one, where equipping indoor spaces with outdoor levels of air flow is part of the strategy to contain the spread of the COVID-19 virus.

Your recent work published in PNAS showed that people can be kept cool in very hot and humid environments without air conditioning. What is the benefit of cooling surfaces, such as walls and tables, instead of cooling the air? How are the findings relevant to keeping people safe from viruses like COVID-19?

Meggers: Today, around the world, people are trying to achieve higher outdoor air flow indoors to dilute the amount of virus in the air. With air conditioning (AC), dehumidification and cooling occur simultaneously. The benefit of the Cold Tube technology is that it decouples cooling from the providing of fresh air, meaning people can keep their windows open, while maintaining comfort, and without expending massive amounts of energy to cool and dehumidify the air flow.

Teitelbaum: The way buildings are built today, using exclusively AC for cooling, we can’t increase the amount of fresh air we deliver to buildings at will because outdoor air flow is coupled with the amount of air conditioning buildings need to provide. If you want more outdoor air flowing into the building, you also need additional capacity to dehumidify and cool that air. Most systems weren’t built with the capacity to flow the amount of air that, in many cases, is being recommended to dilute indoor air pollutants and prevent the spread of COVID-19. And if they can, it requires massive amounts of energy.

Meggers: In the Cold Tube, occupants were cooled entirely by thermal radiation, which means energy was primarily used for cooling the water inside the walls, not cooling the air. Through most summer conditions, we could cool people using surfaces, while leaving all the windows open. Achieving 100% outdoor air would do a lot to stymie the spread of the virus, and the efficiency benefits scale proportionally with how much outdoor air you want. Air should be only for breathing, not cooling.

How does this help mitigate climate change?

Teitelbaum: This system uses at least 50% less energy than a comparably-sized air conditioner. Letting the air warm up by five degrees while cooling surfaces, can lower energy demand by up to 40% and maintain occupant comfort. Allowing even hotter air temperatures would result in higher energy savings.

What is the biggest misconception about air conditioning?

Meggers: Air conditioning does not equal cooling. It’s a highly engrained method of cooling buildings, but it’s not the only one. Additionally, your window air conditioners are just cooling existing air in the room; they don’t bring new, fresh air into your room. Air conditioning units have become so normalized and integrated into daily life but, in reality, they are huge machines that require a lot of energy, and should not be treated like turning on a light switch. A seemingly simple window unit requires 10-1000x more energy than a ceiling fan, and leaving the AC on is comparable in energy to leaving the light on in 100 rooms.

In terms of urban planning and outdoor air quality, how does this alternative to air conditioning provide dual-benefits and help mitigate the urban heat island effect?

Meggers: Most of my coauthors and I have traveled around Southeast Asia and have seen firsthand how quickly AC units have been deployed at scale. Adding AC window or split units to buildings is done with little contemplation of the effects on surface temperatures, and the climate and heat in a city. The units work by rejecting the heat from the air in a room to the outside. Rejecting heat outside the buildings, along the façade, leads to sidewalks and areas around buildings becoming very hot, and many spaces becoming unusable. Our technology does exactly the opposite; it provides opportunities to regain thermal acceptability in various parts of the city without having to build a huge park. You can install these cooling panels outside in hot and humid environments, and build “cool havens” where people can gather, eat, and play.

Is there anything else you want to talk about in regards to this paper or today’s context for it?

Teitelbaum: It will take more than an 8-month experiment to change the way people think about comfort systems and energy efficiency in the built environment. Much of the C.H.A.O.S lab’s research has focused on expanding the knowledge bubble of thermal radiation’s influence on comfort and efficiency. The Cold Tube experiment created a lot of new knowledge, which is a great academic success. Commercially, while there are companies that manufacture similar technologies, there is still a need to continue to demonstrate and experiment with new concepts to not only further the technology, but also this paradigm shift. The shift away from air conditioning towards more holistic comfort design would help us act as stewards of the planet as well as our own built environments. In many parts of the world, such as Singapore and other tropical areas that are increasingly seeking ways to condition spaces, significantly more energy goes towards dehumidifying the air than just cooling the air. This is one of the places that we believe our comfort paradigm will have the greatest efficiency increases and impact, since no dehumidification is required for people to feel comfortable.

Reference: “Membrane-assisted radiant cooling for expanding thermal comfort zones globally without air conditioning” by Eric Teitelbaum, Kian Wee Chen, Dorit Aviv, Kipp Bradford, Lea Ruefenacht, Denon Sheppard, Megan Teitelbaum, Forrest Meggers, Jovan Pantelic and Adam Rysanek, 18 August 2020, Proceedings of the National Academy of Sciences.
DOI: 10.1073/pnas.2001678117