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ABSTRACTTo understand the transportation sector’s role on influencing and mitigating heat in cities, this research quantifies added heat from pavement infrastructure and vehicle travel in the hot and automobile dependent metropolitan Phoenix, Arizona. Construction of a one-dimensional heat transfer model for local weather conditions and pavement design is combined with vehicle travel densities to simulate spatiotemporal sensible heat flux magnitudes. In metro Phoenix, sensible heat from pavements and vehicles is comprised of 67% from roadways, 29% from parking, and 3.9% vehicles. Concrete and asphalt pavement emit 15% and 37% more sensible heat compared to the bare ground, respectively. Added sensible heat from pavement peaks during summer afternoons when heat emissions relative to the ground are 26% and 46% greater for concrete and asphalt. Results indicate pavement infrastructure contributes significantly to Phoenix’s urban heat balance, and areas surrounding busy vehicle corridors may be undesirable for outdoor activities during summer rush hours.

Photovoltaic (PV) panels are commonly used for on-site generation of electricity in urban environments, specifically on rooftops. However, their implementation on rooftops poses potential (positive and negative) impacts on the heating and cooling energy demand of buildings, and on the surrounding urban climate. The adverse consequences can be compounded if PV is installed on top of an otherwise highly reflective (“white”) rooftop. This study investigates these impacts on a test building in Tempe, AZ, by directly measuring the temperature of all involved surfaces. These measurements are supplemented by whole-building energy simulations to model the energy implications for archetypical residential and retail buildings. This includes calculations of the ratio of the energy demand penalty to electricity generation as well as the net sensible heat flux to the ambient environment. Results indicate that the summertime cooling energy penalty due to blockage of outgoing longwave radiation can be 4.9—11.2% of the PV electricity generation. The addition of PV to the white roof resulted in a small decrease in the computed sensible heat flux at night, but a daytime increase in sensible flux by more than a factor of 10 (from less than 25 W/m2 for the white roof alone, to more than 250 W/m2 when PV is added to the roof). This study highlights the potential unintended consequences of rooftop PV under certain conditions and provides a broader perspective for building designers and urban planners.

In many US cities, indoor exposure to heat continues to be the underlying cause of a considerable fraction (up to 80% during extreme events) of heat-related mortality and morbidity, even in locations where most citizens have air conditioning (AC). Nevertheless, the existing literature on indoor exposure to heat often regards AC as a binary variable and assumes that its presence inevitably results in a safe thermal environment. This is also reflected in heat vulnerability assessments that assign a binary attribute to AC. In this study, we used thermal simulation of buildings to investigate overheating in residential buildings in three US cities (Houston, Phoenix, and Los Angeles) and focused on scenarios where an AC system is present; yet not fully functional. Moreover, we identified the role of key building characteristics and investigated the sensitivity of indoor environment to the ambient temperature. Our results show that energy poverty and/or faulty systems can expose a considerable fraction of AC-owning elderly in Phoenix and Houston to excess heat for more than 50% of summer. This highlights the need to reevaluate AC as the primary protective factor against heat and introduces several implications that need to be considered in heat vulnerability assessments.