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Research reveals effectiveness of urban heat-reducing technologies

February 10, 2014

Life in a warming world is going to require human ingenuity to adapt to the new realities of Earth. Greenhouse-gas-induced warming and megapolitan expansion are both significant drivers of our warming planet. Researchers are now assessing adaptation technologies that could help us acclimate to these changing realities.

But how well these adaptation technologies – such as cool roofs, green roofs and hybrids of the two – perform year-round, and how this performance varies with place remain uncertain. aerial view of a suburban Phoenix neighborhood Download Full Image

Now a team of researchers, led by Matei Georgescu, an assistant professor in Arizona State University's School of Geographical Sciences and Urban Planning and a senior sustainability scientist in the Global Institute of Sustainability, has begun exploring the relative effectiveness of some of the most common adaptation technologies aimed at reducing warming from urban expansion.

The work showed that end-of-century urban expansion within the United States alone, separate from greenhouse-gas-induced climate change, can raise near surface temperatures by up to 3 degrees Celsius (nearly 6 degrees Fahrenheit) for some megapolitan areas. Results of the new study indicate that the performance of urban adaptation technologies can counteract this increase in temperature, but also varies seasonally and is geographically dependent.

In the paper, “Urban adaptation can roll back warming of emerging megapolitan regions,” published in the online Early Edition of the Proceedings of the National Academy of Sciences, Georgescu, Philip Morefield, Britta Bierwagen and Christopher Weaver – all of the U.S. Environmental Protection Agency – examined how these technologies fare across different geographies and climates of the United States (

“This is the first time all of these approaches have been examined across various climates and geographies,” said Georgescu. “We looked at each adaptation strategy and their impacts across all seasons, and we quantified consequences that extend to hydrology (rainfall), climate and energy. We found geography matters,” he said.

Specifically, what works in California’s Central Valley, such as cool roofs, does not necessarily provide the same benefits to other regions of the country, like Florida, Georgescu explained. Assessing consequences that extend beyond near surface temperatures, such as rainfall and energy demand, reveals important tradeoffs that are oftentimes unaccounted for.

Cool roofs are a good example. In an effort to reflect incoming solar radiation, and therefore cool buildings and lessen energy demand during summer, painting one’s roof white has been proposed as an effective strategy. Cool roofs have been found to be particularly effective for certain areas during summertime.

However, during winter, these same urban adaptation strategies, when deployed in northerly locations, further cool the environment, and consequently require additional heating to maintain comfort levels. This is an important seasonal contrast between cool roofs (i.e. highly reflective) and green roofs (i.e. highly transpiring). While green roofs do not cool the environment as much during summer, they also do not compromise summertime energy savings with additional energy demand during winter.

“The energy savings gained during the summer season, for some regions, is nearly entirely lost during the winter season,” Georgescu said.

In Florida, and to a lesser extent southwestern states, there is a very different effect caused by cool roofs.

“In Florida, our simulations indicate a significant reduction in precipitation," he said. "The deployment of cool roofs results in a 2 to 4 millimeter per day reduction in rainfall, a considerable amount (nearly 50 percent) that will have implications for water availability, reduced stream flow and negative consequences for ecosystems. For Florida, cool roofs may not be the optimal way to battle the urban heat island because of these unintended consequences.”

Georgescu said the researchers did not intend to rate urban adaptation technologies as much as to shed light on each technology’s advantages and disadvantages.

“We simply wanted to get all of the technologies on a level playing field and draw out the issues associated with each one, across place and across time.”

Overall, the researchers suggest that judicious planning and design choices should be considered in trying to counteract rising temperatures caused by urban sprawl and greenhouse gases. They add that “urban-induced climate change depends on specific geographic factors that must be assessed when choosing optimal approaches, as opposed to one-size-fits-all solutions.”

Including all expansion scenarios, adaptation strategies and ensemble members, nearly 150 years of regional climate modeling experiments encompassing the continental United States were carried out on ASU's Advanced Computing Center (A2C2).

This research was supported by the National Science Foundation under a Water Sustainability and Climate grant with Georgescu as principal investigator.

Associate Director, Media Relations & Strategic Communications


ASU engineers to lead national solar energy technology projects

February 10, 2014

Arizona State University engineers will lead two multi-university/industry research teams in support of a new U.S. Department of Energy program to develop technologies that use the full spectrum of sunlight to produce inexpensive power during both day and night.

The department's Advanced Research Projects Agency – Energy (ARPA-E) recently announced allocation of $30 million in funding for 12 projects selected to conduct research for its Full-Spectrum Optimized Conversion and Utilization of Sunlight (FOCUS) program. Stephen Goodnick and Zachary Holman Download Full Image

Stephen Goodnick will lead the project High-Temperature Topping Cells from LED (Light-Emitting Diode) Materials, which has been allocated $3.9 million.

Goodnick is a professor in the School of Electrical, Computer and Energy Engineering, one of ASU’s Ira A. Fulton Schools of Engineering, and deputy director of ASU LightWorks, a strategic framework for light-inspired research.

Zachary Holman will lead the project Solar Concentrating Photovoltaic Mirrors, which has been allotted $2.6 million.

Holman is an assistant professor in the School of Electrical, Computer and Energy Engineering.

ARPA-E selects energy technology development projects based on their potential to enhance the nation’s economic and energy security. The projects promise to help reduce imports of energy from foreign sources, reduce energy-related emissions – including greenhouse gases – improve energy efficiency in all economic sectors and ensure the United States maintains a technological lead in developing and deploying advanced energy technologies.

Goodnick’s FOCUS project will develop a photovoltaic device that operates effectively at more than 400 degrees Centigrade (more than 750 degrees Fahrenheit) as the key component of a hybrid concentrating solar thermal power (CSP) system that provides overall higher sunlight-into-electricity conversion efficiency than either a stand-alone photovoltaic system or current CSP systems. It is also to provide a lower dollar-per-watt cost.

The material technology used in the photovoltaic device has already demonstrated its reliability and performance in solid-state lighting applications and should be rapidly applicable to solar systems, Goodnick says.

ASU’s research partners on the project are professor Alan Doolittle at the Georgia Institute of Technology, Soitec, one of the world’s leading providers of concentrator photovoltaic systems and a manufacturer of semiconductor materials for electronic and energy industries, and AREVA Solar, a leader in providing concentrated solar power to a global customer base.

The project team includes ASU professors Christiana Honsberg and Dragica Vasileska, and assistant professor Srabanti Chowdhury, faculty members in the School of Electrical, Computer and Energy Engineering, along with Fernando Ponce, a professor in the Department of Physics in ASU’s College of Liberal Arts and Sciences. Honsberg is director of the Quantum Energy and Sustainable Solar Technologies (QESST) Engineering Research Center at ASU, which is supported by the Department of Energy and the National Science Foundation.

Holman’s FOCUS project will incorporate photovoltaic cells into large reflectors used by solar power plants to generate heat – and subsequently electricity – from the concentrated sunlight.

The process is designed to improve the efficiency of how solar thermal power plants generate electricity, promising a significant increase in the daytime output of energy while also being able to store solar energy for power generation at night.

Holman says the photovoltaic cells he will use will replace the traditional silver mirrored surface of the large parabolic troughs now used in solar power plants such as the Solana plant in Gila Bend, Ariz.

The cells will absorb visible light and efficiently convert it to electricity, while ultraviolet and infrared light, which would be wasted if it were collected in photovoltaic cells, will instead be reflected onto a black tube at the trough focus. A fluid in the tube will carry the generated heat to be either converted to electricity with a steam turbine or stored for later conversion, he explains.

His project team includes researchers at the University of Arizona, along with Mariana Bertoni, an assistant professor in the School of Electrical, Computer and Energy Engineering.

Read more: Energy Secretary Moniz Announces New ARPA-E Solar Projects

Joe Kullman

Science writer, Ira A. Fulton Schools of Engineering