Urban Climate 29 (2019) 100495 Contents lists available at ScienceDirect Urban Climate journal homepage: www.elsevier.com/locate/uclim Potential energy and climate benefts of super-cool materials as a rooftop strategy T ⁎ Amir Baniassadia, David J. Sailorb, , George A. Ban-Weissc a School of Sustainable Engineering and the Built Environment, Arizona State University, Tempe, AZ, United States of America b School of Geographical Sciences and Urban Planning, Arizona State University, Tempe, AZ, United States of America c Department of Civil and Environmental Engineering, University of Southern California, Los Angeles, CA, United States of America ARTICLE INFO ABSTRACT Keywords: For decades, refective rooftops have been used and advocated as cost-efective measures to Refective roofs mitigate the urban heat and reduce building cooling loads. However, their efectiveness has al- White roofs ways been limited by shortwave refectivity and long-wave emissivity of commercially available Urban heat mitigation technologies. Recent advances in coating materials with engineered spectral properties have Building energy consumption resulted in inexpensive “super-cool” technologies that can be applied to most surfaces and have Sensible heat fux albedo and emissivity values greater than 0.96 and 0.97, respectively. This study is an efort to Passive daytime radiative cooling quantify the potential benefts of applying the newly developed materials on building rooftops. To do so, we conducted whole-building energy simulations of archetypical residential and commercial buildings to calculate rooftop surface temperature, sensible heat fux to the ambient, cooling energy saving, and heating energy penalty in 8 U.S. cities with urban heat mitigation plans that include use of high albedo materials. Our results suggest that in all climates, the surface temperature of the super-cool rooftop remains below the ambient air temperature throughout the year, resulting in a negative average daily sensible heat fux of 30–40 W.m−2. In addition, we found that the new technology can double the cooling energy saving (and heating energy penalty) compared to typical white roofs. 1. Introduction Urban areas tend to experience higher ambient temperatures than their surroundings, a phenomenon commonly referred to as “Urban Heat Islands (UHI)” (Landsberg, 1981). Given the numerous negative impacts of urban heat, there is a large body of research on mitigation strategies that can reduce its negative impacts on energy and water demand (Touchaei and Akbari, 2015), air quality (Epstein et al., 2017), public health (Jandaghian and Akbari, 2018), and an overall reduction in quality of life for citizens. Refective surfaces (mostly applied on exterior surfaces of buildings) and urban vegetation are the two most studied methods of fghting urban heat. By reducing the absorbed solar radiation, the former has the potential to cool the near surface air (Morini et al., 2018). Because of their practicality, refective rooftops are the most studied (and applied) type of refective surfaces in the urban context (Aleksandrowicz et al., 2017), with a literature that dates back to early 90's (Rosenfeld et al., 1995). In addition, through building energy codes or other initiatives, many states around the world (especially, in mid-latitudes) already have established policies to promote (or mandate) the use of refective rooftops. For example, commercial buildings in many U.S. cities are required to implement white roofs (EPA, 2018). ⁎ Corresponding author. E-mail address: [email protected] (D.J. Sailor). https://doi.org/10.1016/j.uclim.2019.100495 Received 21 January 2019; Received in revised form 9 May 2019; Accepted 27 June 2019 2212-0955/ © 2019 Elsevier B.V. All rights reserved. A. Baniassadi, et al. Urban Climate 29 (2019) 100495 Studies on the efect of refective rooftops encompass a large range of scales. At the broadest level, Zhang et al. (2016) studied the potential impact of widespread cool roof implementation on continental and global scales and reported statistically signifcant (−0.0021 K) reductions in global mean temperature. At regional or metropolis-wide scales, examples include the work of Sailor (1995) and Vahmani et al. (2016) whose simulations suggest that wide adaptation of cool surfaces in the Los Angeles metro area can reduce daytime urban heat by 1 to 1.5 K in summer. Similarly, Li et al. (2014) showed that if 95% of rooftops in Washington- Baltimore area implement cool roofs, a 0.5 K reduction in near-surface UHI can be expected during a summertime heatwave. Based on a sensitivity analysis, Li and Norford (2016) report that in Singapore, city-wide deployment of cool roofs can reduce near-surface air temperature by up to 2 K. After reviewing the peer-reviewed literature, Santamouris (2014) concluded that the decrease in peak ambient temperature across all reliable studies is around 0.9 K per 0.1 increase in average surface albedo. At fner scales (e.g., neighborhood or a single street canyon), recent examples of studies include the work by Botham-Myint et al. (2015) who quantifed the variation of pedestrian-level efects of white roofs with respect to the arrangement and heights of buildings. Their computational fuid dynamics simulations showed that white roofs can potentially cause a near-surface air temperature reduction of 0.75 K. In another study, Taleghani et al. (2016) used micrometeorological simulations to show that, compared to cool pavements, cool roofs have a relatively low impact on thermal comfort of nearby pedestrians. In addition to the mentioned scales, there is another line of investigation that focuses on impacts at the building level. Specifcally, through measurements or whole-building energy simulations, researchers study the impacts of refective rooftops on the energy balance of buildings, and the consequent change in energy demand, thermal comfort, and heat fux into the ambient. For example, Baniassadi et al. (2018b) used EnergyPlus, a validated whole-building energy simulator, to study direct and indirect benefts of cool roofs for energy efciency and thermal comfort in residential buildings. Their work suggested that depending on the construction quality, the direct energy consumption beneft of shifting from a typical dark roof (albedo = 0.2) to a cool roof (albedo = 0.6) for a residential building in California could be up to 30%. Scherba et al. (2011) also used EnergyPlus in conjunction with a set of experimental measurements and reported that depending on the climate, a white roof can reduce total daily fux to the ambient by 75–80% compared to the baseline dark roof. While this summary covers few examples of a much larger body of work, the magnitude of benefts (regardless of the reported metric) in all previous studies is limited by the properties of commercially available products (Mastrapostoli et al., 2016). However, recent developments in Passive Daytime Radiative Cooling (PDRC) technology show the potential of a substantially diferent type of cool-roofs (Santamouris and Feng, 2018). The very recent development of an inexpensive and practical method for producing hierarchically porous poly (vinylidene fuoride-co-hexafuoropropene) coatings is one promising example. In particular, the work by Mandal et al. (2018) resulted in a coating with a substrate-independent albedo of 0.96 and emissivity of 0.97. Notably, they observed a surface temperature drop of up to 6 K below ambient air temperatures under solar intensity of 890 W/m2. This was only achievable through the extremely high albedo combined with the very high emissivity inside the atmospheric window that allow for surface cooling even in the afternoon hours. More importantly, while Mandal et al. did not scope the benefts of this coating as a rooftop strategy, they reported that the coating can be easily applied to any substrate (including rooftops). While this is the most recent and promising example, other research groups (Bhatia et al., 2018; Gentle and Smith, 2010; Gentle and Smith, 2015; Raman et al., 2014; Zhai et al., 2017) have also been working on this topic over the past few years, and achieved similar properies. Although, most are substrate-dependent and expensive solutions. There are several studies on applications of PDRC as a “free” source of cooling in buildings. Most of these studies consider the use of these materials in rooftop heat exchangers integrated within hydronic cooling and heating loops (Goldstein et al., 2017; Wang et al., 2018). For example, Fernandez et al. (2015) simulated this hybrid system (based on the coating proposed by (Raman et al., 2014)) in an ofce building in 5 US cities and reported 45–68% savings in cooling electricity demand. If this technology shows the same sub-ambient temperature performance on rooftops, it can result in a major paradigm shift in how refective rooftops interact with urban environments and individual buildings. While commonly referred to as “cool-roofs”, in reality, typical refective rooftops only reduce the daytime heat fux to the ambient (and into buildings) as opposed to actually cooling them. Hence, while they mitigate urban heat and reduce building cooling demand in comparison to dark roofs, they do not result in an actual cooling efect. In contrast, if the extremely high albedo and emissivity of the mentioned technology results in consistent sub-ambient surface temperatures, it can be considered as an actual urban cooling strategy. In addition, instead of simply reducing building heat gains, super cool roofs can passively cool the building interiors and thus, reduce the demand for mechanical cooling. To study these potential benefts, we used simulations at the building level to scope the benefts of the newly developed coating as a potential rooftop strategy (henceforth, super cool roofs). Through whole-building energy simulations, we compared roof surface temperatures and heat fuxes from super cool roofs in residential and commercial building archetypes to those of typical white (albedo of 0.7 and emissivity of 0.9) and dark (albedo of 0.2 and emissivity of 0.9) roofs.