Citywide Impacts of Cool Roof and Rooftop Solar Photovoltaic Deployment on Near-Surface Air Temperature and Cooling Energy Demand

Boundary-Layer Meteorol (2016) 161:203–221 DOI 10.1007/s10546-016-0160-y RESEARCH ARTICLE Citywide Impacts of Cool Roof and Rooftop Solar Photovoltaic Deployment on Near-Surface Air Temperature and Cooling Energy Demand F. Salamanca1,4 · M. Georgescu2,4 · A. Mahalov1,3,4 · M. Moustaoui1,4 · A. Martilli5 Received: 29 June 2015 / Accepted: 5 April 2016 / Published online: 21 April 2016 © The Author(s) 2016. This article is published with open access at Springerlink.com Abstract Assessment of mitigation strategies that combat global warming, urban heat islands (UHIs), and urban energy demand can be crucial for urban planners and energy providers, especially for hot, semi-arid urban environments where summertime cooling demands are excessive. Within this context, summertime regional impacts of cool roof and rooftop solar photovoltaic deployment on near-surface air temperature and cooling energy demand are examined for the two major USA cities of Arizona: Phoenix and Tucson. A detailed physics-based parametrization of solar photovoltaic panels is developed and implemented in a multilayer building energy model that is fully coupled to the Weather Research and Forecasting mesoscale numerical model. We conduct a suite of sensitivity experiments (with different coverage rates of cool roof and rooftop solar photovoltaic deployment) for a 10-day clear-sky extreme heat period over the Phoenix and Tucson metropolitan areas at high spatial resolution (1-km horizontal grid spacing). Results show that deployment of cool roofs and rooftop solar photovoltaic panels reduce near-surface air temperature across the diurnal cycle and decrease daily citywide cooling energy demand. During the day, cool roofs are more effective at cooling than rooftop solar photovoltaic systems, but during the night, solar panels are more efficient at reducing the UHI effect. For the maximum coverage rate deployment, cool roofs reduced daily citywide cooling energy demand by 13–14 %, while rooftop solar photovoltaic panels by 8–11 % (without considering the additional savings derived Electronic supplementary material The online version of this article (doi:10.1007/s10546-016-0160-y) contains supplementary material, which is available to authorized users. B F. Salamanca 1 School of Mathematical and Statistical Sciences, Arizona State University, PO Box 871804, Tempe, AZ 85287-1804, USA 2 School of Geographical Sciences and Urban Planning, Arizona State University, Tempe, AZ, USA 3 School of Life Sciences, Arizona State University, Tempe, AZ, USA 4 Julie Ann Wrigley Global Institute of Sustainability, Arizona State University, Tempe, AZ, USA 5 Research Center for Energy, Environment, and Technology (CIEMAT), Madrid, Spain 123 204 F. Salamanca et al. from their electricity production). The results presented here demonstrate that deployment of both roofing technologies have multiple benefits for the urban environment, while solar photovoltaic panels add additional value because they reduce the dependence on fossil fuel consumption for electricity generation. Keywords Cooling energy demand · Cool roofs · Rooftop solar photovoltaic panels · Urban climate modelling List of Symbols αPV Tair ε f f PV downwelling LWSky downwelling LWPV downwelling LWRoof downwelling SWSky downwelling SWRoof E PV εPV f PV σ H TPV upwelling LWPV upwelling LWRoof Albedo of the upward face of the solar photovoltaic panels Air temperature (K) above roofs Conversion efficiency of the solar photovoltaic panels Downwelling longwave radiation (W m−2 ) from the sky Downwelling longwave radiation (W m−2 ) emitted by the downward face of the solar photovoltaic panels Downwelling longwave radiation (W m−2 ) reaching a roof covered with solar panels Downwelling shortwave radiation (W m−2 ) from the sky Downwelling shortwave radiation (W m−2 ) reaching a roof covered with solar panels Electricity production (W m−2 ) of the solar photovoltaic panels Emissivity of the upward face of the solar photovoltaic panels Fraction of the roof covered by the solar panels Stefan-Boltzmann constant (W m−2 K−4 ) Sensible heat flux (W m−2 ) from the solar photovoltaic panels to the atmosphere Temperature (K) of the upward face of the solar photovoltaic panels Upwelling longwave radiation (W m−2 ) emitted by the upward face of the solar photovoltaic panels Upwelling longwave radiation (W m−2 ) emitted by a roof covered with solar panels 1 Introduction Many studies reveal that the large-scale deployment of roofing technologies is an effective means of reducing energy consumption (e.g., Akbari et al. 2009; Oleson et al. 2010; Menon et al. 2010; Salamanca et al. 2012a; Cotana et al. 2014; Georgescu et al. 2014). Cool roofs, by virtue of increased reflectivities, absorb less incoming shortwave radiation than dark roofs, thereby promoting a lower skin temperature. As a result, cool roofs reduce heat transfer into the urban environment and into buildings, decreasing near-surface air temperature and cooling energy demand. In wintertime, the potential penalty associated with cool roofs is in general outweighed by the summer benefit and can be annulled if the roofs are typically covered with snow during the cold season (Bretz and Akbari 1997). However, energy savings are more limited for areas that do not have extensive wintertime snow pack, such as the Mid-Atlantic states of the USA (e.g., Georgescu et al. 2014). Various studies have documented the direct 123 Citywide Impacts of Cool Roof and Rooftop Solar Photovoltaic... 205 benefits of large-scale cool roof deployment in urban areas. Performing continental-scale simulations with a regional climate model, Millstein and Menon (2011) reported that nationwide large-scale cool roof deployment for the USA reduce the summertime air temperature by 0.1–0.5 ◦ C in most urban locations. Similarly, Georgescu et al. (2012, 2013) reported that summertime statewide warming due to projected urban expansion for Arizona could be reduced by about 50 % with the complete integration of highly reflective cool roofs. More recently, Li et al. (2014) have evaluated regional impacts of cool and green roof deployment for the Baltimore-Washington (USA) metropolitan area during an extreme heat event. Green roofs can more efficiently partition available energy into latent heat and reduce sensible heat transmission into the urban environment. Although regional differences exist between cool and green roof technologies (e.g., Georgescu et al. 2014), and their assessment must extend beyond the examination of near-surface temperature impacts, this recent work has demonstrated that green roofs (assuming abundant soil moisture) can be nearly as effective as cool roofs at reducing near-surface air temperature. Roofing technologies, however, are not limited to cool and green roofs. Recent investigations have documented indirect benefits—in addition to the reduction of greenhouse gas emissions—derived from the use of solar p (...truncated)