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A data-driven approach to cooling

Think about the temperature of the room you’re sitting in. Is it too hot? Too cold? Or is it Goldilocks style: just right?

This perception of a room’s temperature and whether it’s “just right” is called thermal comfort. “Everybody can understand thermal comfort,” says Rishee Jain, an assistant professor of civil and environmental engineering at Stanford. “Being uncomfortable on a very hot and humid day is something pretty much everybody in the world can empathize with.”

More than discomfort, excessive heat has very real health consequences: When the body can no longer manage high levels of heat and humidity, it begins to overheat, leaving people vulnerable to heat-related illnesses like heat stroke, which can be fatal. The process, known as heat stress, already impacts hundreds of millions of people globally, particularly those living in the global south and tropical regions.

Rising temperatures mean rising rates of heat stress, with some researchers estimating that 1.2 billion people will be impacted by extreme heat and humidity by 2100. This means more incidents of heat-related illnesses, but also further degradation of the environment, including more droughts and wildfires, lower air quality and decreased agricultural production. Urban areas, where grass and forest have been replaced by pavements and buildings that absorb heat, are particularly vulnerable to rising temperatures. As the world warms and more people move to urban areas – an estimated 7 out of 10 people will live in cities by 2050 – how to keep the world cool is a major issue.

From his lab at Stanford, Jain is working with collaborators across three continents to find universal solutions for this issue that can be adopted as a global policy, with a special focus on informal settlements in urban areas. The biggest challenge, he says, is finding solutions for extreme heat that don’t mortgage the future. Air conditioning, for example, is great at cooling buildings, but it accelerates climate change. So he and his lab are instead creating more reliable models of urban buildings that they are using to find feasible retrofits that can help cool homes and protect people without contributing to climate change.

The importance of being contextualized

When engineers first began modeling buildings, they would model them as if they were sitting in a field. But this, Jain points out, doesn’t represent how we actually build, especially in cities. “If you model the building in the middle of a field, you’d be missing a lot of things,” says Jain, such as the way closely packed buildings create their own microclimates.

All of these missing elements influence a building’s temperature: Shade provided by a neighboring skyscraper cools it, while sunlight reflected on it by the glass office building across the street raises the temperature. Maintaining a building at a temperature that provides comfort is energy expensive, eating up between 30 and 40 percent of a building’s total energy usage.

To really understand how buildings in cities use energy and to find energy-efficient solutions for thermal comfort, it’s vital to not model them in a vacuum, but rather in their urban context. To do this, engineers developed urban building energy models (UBEMs) that represent entire cities. Jain’s research uses these UBEMs to help cities understand their energy usage and develop plans to achieve their decarbonization goals, including determining which buildings should be targeted for retrofits and energy-efficiency policies.

So far, UBEMs have primarily been used to model cities in North America and Europe. Urbanization, however, is a global trend, and much of it is happening in the global south. As people move from rural areas to cities in search of opportunities, many end up living in informal settlements, where they are more vulnerable to heat stress. So in 2016 Jain began talking about the challenges of applying UBEMs to informal settlements in the global south with Ronita Bardhan, an urban engineer now at Cambridge University in the United Kingdom.

One of the main challenges is that these models are data hungry; to build a reliable model you need to know what the building looks like, what materials it’s built from, how it heats and cools throughout the day, as well as hundreds of other inputs. But data is scarce, particularly in the global south. “If it’s a challenge in San Francisco,” says Jain, “how are we going to do it in Mumbai and other parts of the world?”

The solution was on-the-ground work with sensors in Dharavi, the largest informal settlement in Asia, located in Mumbai, India. Working with the nonprofits Dost Education and CORP India, Jain and Bardhan held workshops with members of the community to explain the reasons for the research and the impacts it could have. “And then we solicited volunteers who were willing to allow us to install these sensors in their space and collect some data,” Jain says.

For two weeks during the summer month of August in 2016, these sensors collected around-the-clock information on heat and humidity inside homes in Dharavi. The fieldwork also revealed that the way people in Dharavi use and occupy their homes was dramatically different from previous UBEMs. Homes are multi-generational and multi-use. “The space is always occupied,” says Jain. “So this notion that in the hottest part of the day no one’s home was quickly broken.” In reality, young children and the elderly are often home during these hours.

With the data collected, Jain and Bardhan created UBEMs of an archetypal home in a Dharavi neighborhood. They also simulated how proposed redevelopments would impact thermal comfort in these dwellings. Dharavi is slated for future redevelopment, which would demolish the current low-rise buildings and replace them with high-rise social housing. Though there are a number of advantages to high-rise housing, Jain and Bardhan found that thermal comfort was not one of them. The work, Jain says, could help guide building design for more sustainable redevelopment.

From Dharavi to the world

For Jain, the paths his research has taken have been influenced by conversations with other researchers. It was through talks with Bardhan that the work to develop UBEMs in Dharavi came about. It was having coffee with Narasimha Rao, now an associate professor at Yale University’s School of Environment, during a conference in 2018 that Jain realized the work done in Dharavi could be extrapolated to other informal settlements and even drive global policy.

Dharavi, which is home to about 1 million people, is not an outlier. Globally, 1 in 8 people live in informal settlements; in the last few years, the number of people living in these settlements has steadily increased. Jain, working with Alex Nutkiewicz, PhD ’21, who was a graduate student in his lab, chose 17 cities in Brazil, India, Indonesia, Kenya and South Africa and got to work.

Together with Alessio Mastrucci of the International Institute for Applied Systems Analysis (IIASA) they built upward of 150,000 unique models of homes in informal settlements, developing a dataset that they could then use to probe different design solutions for extreme heat. “What would happen is we chose a concrete roof versus corrugated metal,” says Nutkiewicz, giving one example. “How would that impact the future energy and thermal performance of these buildings?”

As was done in Dharavi, Jain and Nutkiewicz explored how future redevelopment in the informal settlements would impact thermal comfort and heat stress, but they also looked at the impact of tangible retrofits that could be put in place more easily. For example, what happens if windows are kept open or overhangs are added above windows? With the models they built, the researchers wanted to “figure out if there were any cross-city solutions that could be deployed in many cities around the world,” says Nutkiewicz.

Of all the retrofits they explored, one rose to the top: cool roofs. Painting the roofs of buildings with a white, highly reflective paint, so that roofs bounce off sunlight instead of absorbing it, could reduce heat stress incidents by 91 percent, according to their simulations. “We knew cool roofs could have an impact,” says Jain, “but we didn’t think it would be so effective at mitigating that high end of the spectrum.”

Unlike large-scale redevelopment, cool roofs and other retrofits are solutions that can be adopted now. “This indicated that you don’t hold out for a better solution and let people suffer today,” says Jain. Another benefit of cool roofs is that they are an adaptable solution, with the potential to mitigate heat stress in informal settlements across the globe. This universal nature makes cool roofs more likely, Jain explains, to be adopted as a policy by a nongovernmental organization and rolled out.

The promise of an adaptable solution for extreme heat that could then drive global policy is, for Jain, one of the motivations of his focus on informal settlements. Policy is not untrod territory for Jain. He has been interested in it since his PhD studies, where he was part of an integrated graduate program between engineering and urban planning; “planners are much more used to engaging with policymakers,” he explains.

“For me, policy provides some grounding and constraints to what is actually doable,” says Jain. And he’s not alone; policy is also a huge interest of his students. “Many of them realized that simply building a good technical analysis wasn’t going to get the job done,” says Jain. “I can really say that this generation of engineers understands the impact policy could have.”



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