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Beat the Heat in Arizona: Why Passive Daytime Radiative Cooling Thrives in the Desert

Arizona’s southern desert regions, particularly the cities of Tucson and Phoenix, are known for their extreme heat. Its arid landscapes and scorching summers present a unique set of challenges when it comes to staying cool. But within these challenges lies an opportunity for innovation. In this blog post, we will explore why Arizona, specifically the southern Arizona desert, is the perfect climate for passive daytime radiative cooling (PDRC). We will also delve into the reasons why Tucson and Phoenix are warming faster than other cities, the specific advantages of PDRC in Arizona, and the political climate that embraces solutions to combat the heat.

Arizona: An Ideal Climate for Passive Daytime Radiative Cooling

The southern Arizona desert, with its abundant sunlight and clear skies, provides the perfect environment for passive daytime radiative cooling. Let’s look at a few of Arizona’s advantages.

Abundant Sunlight

Southern Arizona enjoys an abundance of sunlight throughout the year. With more than 300 sunny days annually, passive daytime radiative cooling materials in Arizona reflect more solar energy to space than almost any other region of the world. 

In other words, changes in albedo (the reflectivity of Earth’s surface) in Southern Arizona have larger effects on Earth’s energy balance than changes in the rest of the United States and most of the globe. If passive daytime radiative cooling materials are installed on existing highly absorptive materials like dark roofs, effective increases in albedo are even further magnified. 

A Dry Climate

Low relative humidity in Arizona’s desert air allows for highly efficient radiative cooling. Dry air allows more mid-IR heat emitted from the surface of PDRC materials through the atmospheric window, and eventually into space. For a refresher on the atmospheric window and general principles of passive daytime radiative cooling, check out our in-depth article: The Science of Passive Daytime Radiative Cooling

Clear Skies

Think of how deserts cool quickly at night when clear skies allow sand to release heat to the sky above. This same principle also works in favor of daytime radiative cooling materials. Clouds act as blankets that trap and reflect mid-IR heat back to the ground. Without them, PDRC materials can reach peak cooling efficiency, even under noontime sun.

Extreme Temperatures

The extreme summer temperatures in Arizona provide a strong motivation for developing effective cooling solutions. In addition, passive daytime radiative cooling, unlike air conditioning, actually becomes more efficient at higher temperatures. This is due to Stephan Boltzmann’s law, which states that the power of an emitter increases to the fourth power of the emitter’s temperature. In Arizona’s extreme heat, PDRC becomes even more effective at keeping spaces cool.

PDRC-Friendly Architecture

Without the need to build homes and commercial buildings to handle heavy snow loads, Southern Arizona architecture tends to favor flat and low-slope roofs. In addition, short trees native to the desert leave roofs unshaded. PDRC materials, similar to photovoltaic panels, perform best with an unobstructed view of the sky. With unshaded low-slope surfaces, Southern Arizona roofs are ideal for maximizing the benefit of passive daytime radiative cooling. 

In addition, the distinctive adobe-style architecture prevalent in the region, with thick, thermal mass walls and strategic shading elements, provides a natural synergy with the cooling mechanisms inherent in PDRC. The widespread use of light-colored materials in Arizona construction works alongside the reflective properties of PDRC, minimizing solar heat absorption and further facilitating the radiative cooling process.

Finally, the prevalence of courtyards and outdoor living spaces in Southern Arizona architecture allows for the seamless integration of outdoor PDRC systems. By incorporating radiative cooling elements into these open areas, architects can enhance the overall cooling efficiency of both indoor and outdoor spaces while maintaining a harmonious connection with the surrounding environment.

Reduced Water Usage

Arizona’s desert climate is also well-suited for another form of low-energy cooling: evaporative, or “swamp” cooling. Swamp coolers pass water over a membrane behind a high-speed fan. The evaporating water pulls heat from the passing air, allowing a low-energy and relatively low-cost way to cool indoor and outdoor spaces. 

Most Arizonans have a love-hate relationship with swamp coolers. While they work great in dry conditions, their efficacy drops to near zero in the humid conditions common during Arizona’s late summer monsoons. They also cannot be run in parallel with a traditional air conditioner, as they humidify the air and work against a running A/C system. Swamp coolers also use significant quantities of water, with many whole-home systems using 150 or more gallons per day. In a region where long-term water access and water rights make the front page of the newspaper more than any other issue, large-scale evaporative cooling is an unsustainable cooling technology for Arizona.

When used in conjunction with existing evaporative cooling systems or to replace evaporative cooling, PDRC keeps air less humid and more comfortable while reducing water demand. In a region where water is a precious resource, this advantage cannot be overstated.

Energy Savings, Especially During Peak Hours

In Arizona, electricity demand peaks during the hottest hours of the day due to heavy use of power-hungry air conditioners. These peak hours come with the highest energy costs and the dirtiest power generation, as power companies must fire up fossil fuel “peaker plants” to meet demand. PDRC reduces the need for energy-intensive air conditioning systems during this critical time of day, resulting in a cleaner, cheaper, and more efficient power grid for all Arizonans.

Sustainability and Environmental Benefits

PDRC aligns with Arizona’s growing concerns around environmental sustainability. With its ability to reduce energy consumption, save water, and lower carbon emissions, PDRC addresses key sustainability issues that resonate with the state’s population.

Warming Trends in Tucson and Phoenix

Tucson and Phoenix have garnered attention for their rapid warming rates, which exceed the global average. Understanding the causes of this accelerated warming is crucial for implementing effective cooling strategies. Some of the factors contributing to the increased temperatures in these cities include:

Urban Heat Island Effect

Both Tucson and Phoenix have experienced substantial urbanization, which has led to the creation of urban heat islands. These areas are characterized by higher temperatures due to the concentration of buildings, concrete, and asphalt, which absorb and radiate heat. The urban heat island effect intensifies heat in cities, making cooling strategies even more critical.

Climate Change

The overarching issue of climate change exacerbates the rising temperatures in Arizona. Increasing greenhouse gas emissions have led to a warmer global climate, and Arizona is no exception.

Declining Vegetation

As urban areas expand, natural vegetation diminishes. The loss of vegetation contributes to higher temperatures as trees and plants provide both shade and evaporative cooling. Attempts to revegetate Phoenix and Tucson are underway and are already showing success, but PDRC represents a new way to cool areas that cannot be easily planted, like vehicles and commercial buildings.

Arizona’s Public Willingness for Solutions

Arizona’s political climate and public attitude toward climate change and extreme heat are evolving. As the effects of climate change become increasingly apparent, there is growing recognition of the need for action at state and local levels. Here are some key points to consider:

State-Level Initiatives

Arizona has been taking steps to address climate change and its associated challenges. State-level initiatives and policies are being developed to mitigate the impact of warming trends, including exploring innovative cooling solutions. In the wake of a deadly 2023 heatwave, Governor Katie Hobbs signed an executive order to better coordinate future heat wave response and set up state-run cooling centers.

City-Level Initiatives

Phoenix’s Office of Heat Response and Mitigation, founded in 2021, is the first publicly funded city-level organization dealing specifically with urban heat. Similarly, Tucson has tasked the Climate Action Team, led by the city’s Chief Resilience Officer, with overseeing the city’s response to heat. 

Community Engagement

Local communities and organizations in Arizona are actively engaging in discussions and initiatives related to climate change and heat resilience. There is a strong sense of community and a willingness to adopt innovative solutions.

Public Awareness

Public awareness of the impacts of climate change and extreme heat is on the rise. Arizonans are increasingly concerned about the effects of warming temperatures and are seeking sustainable ways to adapt to and mitigate these challenges.

Collaborative Efforts

There is a growing spirit of collaboration between government agencies, research institutions, and private sector stakeholders in Arizona. This collaborative approach is essential for finding and implementing effective cooling solutions like PDRC.

Conclusion

Arizona’s southern desert region, with its harsh and unforgiving climate, is an ideal location for passive daytime radiative cooling. The state’s extreme temperatures, abundant sunlight, and the urgent need for cooling solutions make it a prime candidate for the application of this innovative technology.

By harnessing the advantages of PDRC, such as reduced water usage, energy savings during peak hours, and high-temperature efficiency, Arizona can tackle its warming challenges head-on. Moreover, the state’s evolving political climate and the willingness of its population to address climate change and extreme heat present a promising environment for the adoption of PDRC and other sustainable cooling solutions. As Arizona embraces these innovations, it can take significant strides toward beating the heat and creating a more comfortable, sustainable future in the desert. With the need to confront climate change and urban heat islands earlier than other regions, Arizona stands as a model for the global community.

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Chris Hiller
Chris Hiller
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