Optimizing Passive Daytime Radiative Cooling Technologies for Building Energy Savings and Urban Heat Mitigation

Recently, Passive Daytime Radiative Cooling (PDRC) materials and coatings have received a great deal of attention in the urban climate and building science communities. These PDRC materials can be used as roof coatings in hot climate cities to provide ultra-high values of thermal emissivity and solar reflectivity (albedo). As a result, the rooftop surface temperature can be reduced below ambient air temperatures for all hours of the day, even on the hottest day of the year. Thus, in addition to their ability to reduce building cooling loads, PDRC-coated roofs can act as a heat sink to cool the urban environment through sensible transfer of heat from the air to the roof, followed by long-wave emission of energy out of the urban system.

Developers of PDRC coatings need further guidance, however, in terms of how best to optimize coating radiative properties for material cost and thermal performance with respect to the combined effects of desirable urban cooling and air conditioning saving in summer, and the potential undesirable effects in winter.

To address this need we conducted a parametric modeling study in which we vary rooftop albedo and emissivity across a reasonable range of values representing current roofing materials and high performance PDRC coatings (albedo values from 0.2 to 0.96 and emissivity values from 0.9 to 0.98). For this analysis we have focused on four commercial building archetypes in the hot arid climate of Phoenix, Arizona, USA and in the hot humid climate of Atlanta, Georgia, USA.

The results indicate that replacing a conventional roof (albedo=0.2 and emissivity=0.9) with a “super-cool” roof coating (albedo=0.96 and emissivity=0.98) can reduce summertime average daytime roof surface temperatures by as much as 27˚C for Phoenix and 29 ˚C for Atlanta. The conventional roof acts as a source of sensible heating for the urban airshed, resulting in a summertime average daytime roof heat flux of 250 and 200 W/m² for Phoenix and Atlanta, respectively. However, the super-cool roof acts as a heat sink, removing on average 44 and 27 W/m² (for Phoenix and Atlanta) from the air adjacent to the roof during a hot summer day.

In winter, the super-cool coating reduces daytime average roof surface temperatures by as much as 13˚C and 17˚C for Phoenix and Atlanta, respectively. The cooler roof temperatures in winter result in a heating penalty for the buildings and a corresponding undesirable (albeit small) cooling of the urban airshed. This presentation will explore results of the parametric runs (21 cases in total for each city) in some detail and examine the relative importance of emissivity vs. albedo in the design of PDRC coatings. We will also discuss implications for application of PDRC across climate zones.