Princeton University researchers have shown that an often-overlooked cooling technology can enable more ventilation in buildings around the world while substantially decreasing energy costs.
In a study published in a COVID-19 edition of the journal Applied Energy, the researchers showed that radiant systems – which control the temperature of surfaces around people instead of the air – are an alternative to air conditioning. Radiant cooling panels have been around for over a century, but have been of limited use due to problems associated with condensation building up on cool panels.
The study, led by Forrest Meggers, assistant professor of architecture and the Andlinger Center for Energy and the Environment, found that encapsulating the radiant panels in a special membrane could make the technology useful in every major climate zone, at least for part of the year, and could reduce energy costs up to 25%. The researchers said that surrounding people with cool surfaces allows building occupants to feel comfortable even when the temperature of the air around them is unusually high.
Such radiant cooling could allow buildings to bring in large amounts of fresh air without the massive expenditure of energy needed to cool that air. The innovation could be particularly relevant today as public health guidelines call for high ventilation rates to combat COVID-19, an airborne virus easily transmitted in indoor spaces.
Cooling surfaces, not air
Radiant systems rely on radiant heat transfer, or radiation, the principle that heat constantly flows from warm to cooler surfaces. The effects of radiant heating account for 50% of how people feel in a space, with the other half being air temperature, the researchers said. Radiant heat transfer is the reason that people feel warm near a bonfire on a winter night, or cool near a window on a frigid day even when the room temperature is normal.
Meggers took advantage of this effect by creating systems of small cold-water pipes enclosed within a transparent membrane, which are then placed over walls and ceilings.
Meggers has spent years studying radiant systems and how they can help make buildings more efficient, but the COVID-19 pandemic brought into sharp focus the additional benefits of cooling surfaces rather than air. “We realized we could double the efficiency while better mitigating COVID-19 risks,” said Meggers.
Reviving the classroom globally
Meggers said that schools retrofitted with radiant panels could increase ventilation simply by opening windows and doors and moving air with fans without incurring big air conditioning costs.
“Although our system requires a ceiling space reconfiguration, it does not require duct work or envelope retrofits, which means it can be widely implemented in existing spaces,” said Kian Wee Chen, a former Andlinger Center Distinguished Postdoctoral Fellow and a current postdoctoral researcher in Meggers’ lab.
Chen said classrooms are good places to test radiant systems because, unlike high-rise office buildings, they usually have windows that open and because of the potential health risk of having many people in one room. Chen said the retrofits would require plumbing work due to the water tubes in the system, but the task would not be prohibitively labor intensive or technically demanding.
In a 2020 paper published in the journal Building Research and Information, Chen showed that radiant cooling systems could enable 100% of the air flowing into the classroom to be fresh air from the outdoors, compared to recirculated air, in Singapore, one of the hottest, most humid climates in the world. Chen and team found that radiant systems can be effective for high-humidity environments assuming there are fans and ways to move the air, even without conventional air conditioning.
To follow up that study, researchers in Meggers' lab wanted to explore exactly where this technology could be useful among all the climates worldwide. They designed models that incorporated weather conditions, such as humidity, wind, dew point and others, and found that, in the 60 cities they studied, the system could be used for much of the year in most buildings. According to their model, buildings would see an average annual energy cost savings of 10% to 25% against their baseline, and even more so against the operational upticks of the pandemic.
The researchers found the technology would yield the most energy and cost savings in places like Singapore, Rio de Janeiro, and Miami because existing air conditioning systems use lots of energy to dehumidify the air before sending it into buildings, which radiant systems do not require. In those places, air conditioners could be kept off and radiant systems run in their place for 70% of the year, the results showed. In these and other parts of the world, mechanical heating or cooling systems would be needed when temperature conditions were too extreme to be met by radiant systems. The two systems could operate together, in a mixed mode of radiant and air-based comfort control, or with air conditioning or heating used only as backup. Both of those options require far less energy to heat or cool than if relying entirely on conditioning the air for comfort, the researchers concluded.
The researchers said there are several reasons why this technology has not been widely adopted. One is condensation, which Meggers' group addressed with its encapsulation membrane that separates the cool piping from the humid air.
But, according to the researchers, there is also a larger, cultural issue at play: air conditioning is ingrained in American life and buildings. Air conditioning was a sign of wealth in the 1950s and also two decades later when central systems replaced window units. Buildings have now been designed to be air tight and only in contact with outdoor air through air conditioning systems, leaving few tools and design norms for alternatives.
Chen said the global COVID-19 pandemic show a need to reevaluate HVAC systems and an opportunity to create lasting change, both among the American public and in the design industry.
Designing for the future
"If architects assume an air-tight building envelope with an HVAC system, detailing would be very different from a building that allows for natural ventilation," said Dorit Aviv, an assistant professor of architecture at the University of Pennsylvania’s Weitzman School of Design and a co-author on the latest paper. "But, if you think about it, buildings were designed in ways that encourage natural ventilation for centuries before air conditioning." Aviv completed her Ph.D. in Meggers’ lab in 2020.
As a next step, Meggers’ group is developing the equivalent of a thermostat for buildings that rely on radiant systems instead of air conditioning. They also envision vents and filters that bring in large volumes of air while controlling allergens, dust and pollen. Their goal is to
test more deployments and provide guidelines for construction and use of the systems, including retrofitting classrooms that need extra ventilation to operate safely during the pandemic.
Other authors of the study include: Eric Teitelbaum, Denon Sheppard, Jovan Pantelic, and Adam Rysanek.