Researchers at the US Department of Energy used the rise in extreme heat events as an opportunity to find unique solutions to help people beat the scorching heat and keep their homes cool. Three different types of roofing material – solar panels, green roofs, and cool roofs – were put up to the test to determine which ones lowered temperatures and air conditioning consumption during heatwaves. The results are explored in this article.
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Summer is a time to relax, spend time with friends, travelling and making memories. For many, it is also a time to stay indoors because it is too hot outside. At least the latter has become more commonplace over the past decade. In the US alone, heatwaves have increased 200% from an average of two per year in the 1960s to six per year in recent years, lasting an average of four days. The heatwave season now lasts over 70 days, meaning that, on any given summer day, there is a 34% chance the thermometer will be reading higher than normal.
As temperatures rise, air conditioning (AC) gets cranked up as well. A rush of cold air is a welcome reprieve from sweaty and sticky conditions outside.
And yet, the very same technology that is helping the world cool off is also heating it up. Air conditioning alone is responsible for 10% of total energy consumption worldwide and this number is predicted to grow dramatically into the future. The higher the temperature, the more the AC runs. The more the AC runs, the more energy is used. The more energy used, the greater the emissions. The greater the emissions, the higher the temperature. And repeat.
Unfortunately, vacuuming carbon dioxide (CO2) out of the sky will not become feasible on a large-scale for several decades, and 100% clean, renewable energy has its own list of hurdles to clear before it can completely replace fossil fuels. So, how can this cycle be broken?
Researchers at the US Department of Energy’s (DOE) Argonne National Laboratory may have found an answer while studying the potential for green roofs, solar panels, and cool roofs to reduce the temperature and AC consumption in a building. PV solar panels and green roofs may get all of the publicity, but a cool roof proved to be most effective during a heat event, lowering temperatures by 1.5C and AC consumption by 16.6%.
Researchers at the Argonne National Laboratory’s Center for Climate Resilience and Decision Science (CCRDS), which was responsible for the study, led by its director Rao Kotamarthi, focused on the Chicago Metropolitan Area (CMA), which endured a devastating heatwave in 1995, and saw the heat index reach 120F (48.8C) just this past summer. They created data models to represent the three-dimensional heat transfer properties of the diverse cityscape, including shading from trees, building height, and urbanisation density. These models measured how a mass deployment of each of the roof types impacted a building’s temperature and need for air conditioning using weather data collected between 21 August and 27 August 2021, during which a one-day heat event occurred.
Why Focus on Roofs?
A building can gain heat from a few different sources: the activity inside, the temperature outside, and the radiation that arrives via the rays of the sun. While TVs, ovens, warm bodies, and whether or not it is a hot day certainly create a need for AC, the sun’s radiation turns up the temperature more than anything else.
Cooling down a space can be accomplished in many ways, including larger and more efficient air conditioners. However, as previously mentioned, the energy demand from AC units is projected to keep growing at a fast rate in the coming years as the climate crisis intensifies. So, instead of pumping hot air out, it makes more sense to never let a building heat up in the first place.
This goes to show that the ability for a roof to reflect solar radiation back into the environment and away from the building’s residents is integral in making sure it does not get too hot indoors.
A cool roof is designed to reflect more sunlight than a conventional roof, absorbing less solar energy. The reflective characteristics are increased from a mixture of colour choice, like white, and material choice, like ceramic tile, metal, or specialty shingles. The end result is a roof that can reflect up to 80% (an albedo of 0.8) of the solar radiation it is bombarded with.
In the study, researchers measured roofs covered by solar panels (0.2 albedo), green roofs (0.25 albedo), and cool roofs (0.8 albedo) as well as a control scenario for a standard roof (0.2 albedo).
The Results
Throughout the course of a day, each roof showed an ability to reduce its surface temperature relative to the control scenario, peaking in the late afternoon when outside temperatures began to reach their highest point. The cool roof was the most effective in this measure, lowering temperatures by 1.5C compared to 1.2C for green roofs, and 0.6C for solar panels.
As day shifts to night, some of the radiative energy absorbed by a roof can begin to flow into the building, raising the indoor temperature. Cool roofs were the only solution that did not exhibit nighttime warming because they reflect rather than absorb solar energy.
Surface temperature difference of cool roof, green roof, and solar panel roof relative to control. Image: Kotamarthi et. al, 2021.
The effectiveness of a cool roof also demonstrated an inverse relationship with the urbanisation of the city, meaning that, as the city became more dense with skyscrapers, roads, and traffic (all factors that increase the Urban Heat Island effect), the cooler a building was kept. Currently around 55% of the world population lives in urban areas, and by 2050 that number is projected to reach 6.6 billion people, equivalent to 66% of the world’s population.
All of this avoided temperature gain is going to alleviate some of the burden on an AC unit because it will be staving off less heat. And the less cooling an air conditioner needs to do, the less energy it will consume.
The cool roof also made the largest difference by reducing overall AC energy consumption by 16.6% across the Chicago Metropolitan Area, followed by green roofs at 14.0%, and solar panels at 7.6% (excluding any energy created by the solar panel itself). While 16.6% may not seem like a jaw-dropping number, every kilowatt-hour of energy saved will help protect electrical grids from a dramatic rise in demand due to both increased human consumption, and mass adoption of technologies like EVs.
Daytime air conditioning consumption. Image: Kotamarthi et. al, 2021.
Are Cool Roofs the Future?
Cool roofs are a simple solution that has proven its effectiveness time and time again across the globe, so why are they not more popular?
A silver bullet, save-the-day technological breakthrough is exciting to dream of, instilling hope, and drawing the attention (and dollars) of many. Changing the material on a roof? Much less so. But in situations where a small reduction in emissions here and a better use of electricity there begin to add up, the simple answer is often the best answer. Because the reality of it is that the vast majority of progress towards net-zero will be made by limiting the emissions currently being produced rather than pushing this problem off into the future.
The research team plans to continue their work, conducting more experiments across wider ranges of cityscapes, and including seasonal data. As more evidence mounts in its favour, one can look to pioneers like the state of Telangana in India which recently announced a Cool Roof Policy that aims to bring 300 square kilometres of heat-reflecting roofs to the state by 2028 saving 600Gwh per year of electricity.
Will it be enough for cool roofs hit the mainstream?
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