Summer 2021 – Passive Radiative Heat Pump Surfaces for Urban Cooling: From Laboratory to Field Testing

Project Director David Sailor      

In this partnership with the City of Tempe and 3M, the Zimin-Institute-funded team installed 3M-supplied radiative cooling thin films on the rooftops of transit stops for a bus route in Tempe and found their thermal performance to be quite promising.  The films exhibit high solar reflectance of ~94% (vs. 45% typical surface) and a high thermal emittance of ~93% (vs.  70% typical), such that surfaces on which they are installed can remain substantially below the urban ambient temperature.  Comprehensive studies over a range of summer days and times-of-day cool revealed that the film surface is 0.63 °C (1.1 °F) cooler than ambient air, while conventional surfaces are 3.6 °C (6.4 °F) hotter than ambient air.  Bus riders under the cool film shelter experience mean radiant temperatures (MRT) that are ~ 1-2 °C (~2-4 °F) cooler than under control shelters without the films.  Based on these positive outcomes, the City of Tempe is considering incorporation of cooling films in future bus shelter design, bus roofs and other shade infrastructure.  In addition, the results are being shared with other cities on the Phoenix metropolitan area to assess their interest.