Why surfaces heat up | Location matters | An elegant solution, if used with care
When heat waves hit, people start looking for anything that might lower the temperature. One solution right beneath our feet is PAVEMENT! People can feel how hot the soles of their shoes can get when walking on dark pavement or asphalt. A hot street hot to touch and it raises the surrounding air temperature.
Research shows that building lighter-colored, more reflective roads has the potential to lower air temperatures by more than 2.5° Fahrenheit (F) (1.4° Celcius (C)) and, in the process, reduce the frequency of heat waves by 41% across U.S. cities. But reflective surfaces have to be used strategically – the wrong placement can actually heat up nearby buildings instead of cooling things down. Researchers in Massachusetts Institute of Technology (MIT)’s Concrete Sustainability Hub (CSHub) have been modeling these surfaces and determining the right balance for lowering the heat and helping cities reduce their greenhouse gas emissions. Here’s how reflective pavement works and what cities need to think about.
Why surfaces heat up
[Source Image: raven0246/Blendswap] Article “How to dramatically lower city temperatures:
www.fastcompany.com/90648731/how-to-dramatically-lower-city-temperatures
All surfaces, depending on the amount of radiation they absorb or reflect, can affect air temperatures in cities. In urban areas, about 40% of the land is paved, and that pavement absorbs solar radiation. The absorbed heat in the pavement mass is released gradually, warming the surrounding environment. This can exacerbate urban heat islands and worsen the effects of heat waves.
Heat islands: Structures—buildings, roads, and other infrastructure—absorb and re-emit the sun’s heat. In urban areas, these structures are highly concentrated and greenery is limited, so the areas become “islands” of higher temperatures relative to outlying areas—pockets of heat referred to as “heat islands”—part of the reason cities are regularly a few degrees warmer in summer than nearby rural areas and leafy suburbs.
• Reflective materials on pavement can prevent that heat from building up and help counteract climate change by reflecting solar radiation back to the top of the atmosphere.
• White roofs can have the same effect.
• Albedo—a measure used to estimate a pavement’s reflectivity—refers to the proportion of light reflected by a surface. The lower a surface’s albedo, the more light it absorbs and, consequentially, the more heat it traps. Typically, the darker the surface, the lower the albedo. Conventional pavements such as asphalt have a low albedo of around 0.05-0.1, meaning they reflect only 5% to 10% of the light they receive and absorb as much as 95%. Lighter pavements like concrete, pavements with brighter additives, reflective aggregates, and light-reflective surface coatings can triple the albedo, sending more radiation back into space.
Though the benefits of reflective pavements can vary across the nation’s 4 million miles of roads, they are, on the whole, immense.
An MIT CSHub model estimated that an increase in pavement albedo on all U.S. roads could lower energy use for cooling and reduce greenhouse gas emissions equivalent to 4 million cars driven for [an entire] year.
These roads can also save money when materials are locally sourced, such as: light-colored binders, aggregates, crushed stone, gravel, or other hard materials in concrete.
Location matters
Not all paved areas are ideal for cool roads. Within cities, and even within urban neighborhoods, the benefits differ. When brighter pavements reflect radiation onto buildings – called incident radiation – they can warm nearby buildings in the summer, actually increasing the demand for air conditioning. That’s why attention to location matters.
Consider the differences between Boston and Phoenix
Click to enlarge image of Phoenix, Arizona
Boston’s dense downtown of narrow streets has tall buildings that block light from directly hitting the pavement most hours of the day. Reflective pavement won’t help or harm much there. But Boston’s unobstructed freeways and its suburbs would see a net benefit from reflecting a large fraction of incoming sunlight to the top of the atmosphere. Using models, we found that doubling the traditional albedo of the city’s roads could cut peak summer temperatures by 1° to 2.7° F (0.3° to 1.7° C).
Phoenix could reduce its summer temperatures even more – by 2.5° to 3.6° F (1.4° to 2.1° C) – but the effects in some parts of its downtown are complicated. In a few low, sparse downtown neighborhoods, we found that reflective pavement could raise the demand for cooling because of increased incident radiation on the buildings.
In Los Angeles, where the city has been experimenting with a cooler coating over asphalt, researchers found another effect to consider. When the coating was used in areas where people walk, the ground itself was as much as 11° F (6.1° C) cooler, but a few feet off the ground, the temperature rose as the sun’s rays were reflected. The results suggest such coatings might be better for roads than for sidewalks or playgrounds.
An elegant solution, if used with care
Cities will need to consider all of these effects.
Reflective pavements are an elegant solution that can transform something we use every day to reduce urban warming. The full lifecycle emissions of roads, including the materials used in them, have to be factored in. But as cities consider ways to combat the effects of climate change, the researchers at MIT CSHub believe strategically optimizing pavement is a smart option that can make urban cores more livable.
Hessam AzariJafari, Postdoctoral Associate in Engineering, MIT and
Randolph E. Kirchain, Co-Director, MIT Concrete Sustainability Hub, MIT
For more information, please go to the ARTICLE from “The Conversation” titled “Lighter pavement really does cool cities when it’s done right”: https://theconversation.com/lighter-pavement-really-does-cool-cities-when-its-done-right-162918
HOME PAGE PHOTO: Montage images from cover of EPA “Reducing Urban Heat Islands: Compendium of Strategies-Cool Pavements”: www.epa.gov/sites/production/files/2017-05/documents/reducing_urban_heat_islands_ch_5.pdf
WEB PAGES (10) (also accessible in article-link above):
Click image to go to MIT CSHub PDF
MIT CSHub: “Mitigating Climate Change with Reflective Pavements CSHub Topic Summary”: Click on image above, or go to: https://cshub.mit.edu/sites/default/files/images/Albedo%201113_0.pdf
MIT’s CSHub: https://cshub.mit.edu and Researchers: https://cshub.mit.edu/hessam-azarijafari-0
EPA “Reducing Urban Heat Islands: Compendium of Strategies-Cool Pavements”—describes the causes and impacts of summertime urban heat islands and promotes strategies for lowering temperatures in U.S. communities: www.epa.gov/sites/production/files/2017-05/documents/reducing_urban_heat_islands_ch_5.pdf
U.S. Environmental Protection Agency (EPA) “Heat Islands”: www.epa.gov/heatislands/learn-about-heat-islands
Nature: Paper: “Strong contributions of local background climate to urban heat islands”: www.nature.com/articles/nature13462
The Conversation article: “Green or white? Planted or painted roofs can cool buildings”: https://theconversation.com/green-or-white-planted-or-painted-roofs-can-cool-buildings-25352
NSIDC‘s “Thermodynamics: Albedo”—Albedo is a non-dimensional, unitless quantity that indicates how well a surface reflects solar energy… : https://nsidc.org/cryosphere/seaice/processes/albedo.html
L.A. Times article: “L.A. takes climate change fight to the streets by pouring cooler pavement”: www.latimes.com/local/lanow/la-me-cool-pavement-climate-change-20190425-story.html
The Conversation Article: “Bendable concrete and other CO2-infused cement mixes could dramatically cut global emissions”: https://theconversation.com/bendable-concrete-and-other-co2-infused-cement-mixes-could-dramatically-cut-global-emissions-152544
UCLA Newsroom article: “On-the-ground guidance for L.A.’s far-reaching climate strategy: University researchers and a robot named MaRTy complete first on-site test of city’s Cool Streets program”: https://newsroom.ucla.edu/releases/on-the-ground-guidance-for-cool-streets
Fast Company article “How to dramatically lower city temperatures”: www.fastcompany.com/90648731/how-to-dramatically-lower-city-temperatures