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Urban heat islands, created by human activity concentrations, are a causal factor in climate change and Earth’s increasing temperature. Retro-reflective materials could be used to mitigate the temperature rises in these regions by reflecting solar energy without diffusion.
As awareness of global warming increases and the need to tackle climate change becomes more urgent, we are all becoming quite familiar with the strain that our modern lifestyles exert on the planet. Yet, while our undeniable contribution to the presence of greenhouse molecules -particularly carbon- in the atmosphere is a familiar subject of discussion, there is less focus on the effect of heat emitted by increasing urbanization on the climate.
What are Urban Heat Islands?
Urban heat islands are metropolitan areas that are significantly warmer than surrounding rural regions due to human activities that occur within them. This temperature difference is more marked at night and during summer and winter.
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How Do Urban Heat Islands Affect the Climate?
The true effect of urban heat islands on global warming is currently not fully known. Some studies suggest that the effect is marginal. Others say that the difference caused by the increase in temperature in these areas has a measurable impact on climate change. These areas also carry some other considerable consequences for the environment. Raised temperatures can lead to an overdependence on air conditioning and refrigeration driving up energy consumption in urban areas.
In addition to an effect on global warming, urban heat islands can influence the natural world. These include changing weather patterns, affecting local water quality, damaging natural habitats, and affecting animals’ behavioral patterns.
Mitigation Strategies to Help Tackle Heat Islands
Many mitigation strategies have been suggested to tackle these pockets of increased heat due to human activity, including straightforward techniques such as painting roofs white and the use of lighter color concrete to increase albedo — the reflection of solar energy — and the planting of vegetation.
An example of such a program is Los Angeles’ hypothetical ‘cool communities’ project. City planners calculate that planting 10 million trees and replacing the roofs of five million trees at a US$1 billion US cost could reduce temperatures by 3 ⁰C and save an annual US$170 million in energy costs. But are the materials suggested in projects such as this the most effective for the task at hand?
Retro-Reflective Materials for Urban Heat Islands
Researchers from the Toyohashi University of Technology, in collaboration with Osaka City University, have devised new models to test the effectiveness of retro-reflective materials as an alternative to current strategies.
Something to Reflect On
The team’s research, published in the journal Energy and Buildings, lays out two analytical models to reduce urban heat islands’ contribution to global warming. The key to the team’s models lies in testing revolutionary retro-reflective (RR) materials to create building envelopes.
Current models of urban heat island mitigation suggest diffuse highly reflective (DHR) materials — such as highly reflective paints — on rooftops to increase albedo and reflect solar energy to the sky. However, this system only works if there are no taller buildings around.
When deployed on a rooftop dwarfed by taller buildings, DHRs reflect it at these neighboring buildings and even at the surrounding ground and roadways rather than reflecting solar energy to the sky. This makes them hotter and has the ultimate effect of limiting their mitigating power. This obvious drawback has led researchers to consider the possibility of protecting these buildings with retro-reflective materials (RR) instead of DHRs.
RRs reflect incident energy towards its source regardless of the direction of incidence. Therefore, replacing DHRs with RRs could partially eliminate reflected energy reaching neighboring buildings and roads.
Why Are Retro-Reflective Materials Not Used Commercially?
Retro-reflective materials are still very much in the research stage of their existence. There are yet to be any practical examples of their use. This means that predicting their reflectional properties and modeling their use has become a pressing research topic.
This is where the Toyohashi University of Technology team comes in.
The Future of Retro-Reflective Materials
The team used measured data collected from a series of optical experiments to develop two new predictive models. The researchers used these models to evaluate the reflective characteristics of three different forms of RR plating.
The first step for the team was to synthesize these RR plate samples. From here, they could perform optical experimentation to take an optical measurement on these materials and apply this collected data to new analytical models.
The main goal for the future will be the perfection of these models. However, a positive sign for this endeavor has emerged from the fact that one of the team's new calculations delivers much more favorable results regarding the reflection directional characteristics of the RR materials tested than the optical experiment alone yielded.
Researchers will now also seek to develop a three-dimensional optical system and model for assessing the reflective directional performance of RRs in three dimensions. While research into the most effective form of RR materials continues, scientists are also evaluating the cost of using RRs as an urban heat island mitigation technique.
A 2017 guide presents possible costings for civil engineers, architects, and other professionals working in the field of cost-effective energy-efficient building retrofitting. The authors of this guide point out that retrofitting buildings with RR plating cannot just be energy efficient but also cost-effective.
This unification of economy and ecology will require integrating current and future building techniques with a deeper understanding of the optical and reflective characteristics of materials such as retro-reflective plates.
References and Further Reading
Yuan. J., Emura. K., Farnham. C. (2020) Analytical model to evaluate the reflective directional characteristics of retro-reflective materials. Energy and Buildings. Vol. 223, 110169, DOI:10.1016/j.enbuild.2020.110169
Pacheco-Torgal. F., Granqvist. C-G., Petter Jelle. B., Bianco. N., Kurnitski. J. (2017) Cost-Effective Energy Efficient Building Retrofitting. Materials, Technologies, Optimization and Case Studies. Elsevier. https://www.sciencedirect.com/book/9780081011287/cost-effective-energy-efficient-building-retrofitting#book-description