The Effects of Roof Reflectance on Air Temperatures Surrounding a Rooftop Condensing Unit
Recent anecdotal information suggests that “cool” roofs might produce significant energy savings by reducing the temperature of air entering rooftop air-conditioner (RTU) condensers. Unfortunately, measurements to support this claim are not well documented. To overcome this problem, we carried out a set of six rigorous field experiments to determine the effects of roof reflectance on the heating of condenser inlet air by the roof, and to assess the effects of condenser fan operation on the potential recirculation of hot discharge air from the condenser. The experiments involved combinations of two roof conditions (a “hot” roof and then the same roof with a “cool” coating) and three RTU operation modes (condenser fan and compressor both operating, condenser fan operating without the compressor, and condenser fan and compressor both not operating). For each case, we continuously measured outdoor air temperature at 26 locations near and far from the RTU, as well as roof surface temperatures at two locations (one near and one far from the RTU), wind speed and direction, and solar radiation. With a “hot” roof and the compressor and condenser fan both operating, the air temperature at the condenser inlet was only 0.3ºC warmer on average during peak solar radiation times compared to a reference located far from the RTU. Applying a “cool” roof coating around the RTU eliminated this small temperature rise. The temperature rise was not significantly different when the condenser fan operated without the compressor, which suggests that hot air discharged by the condensing unit was not recirculated. Based on published relationships for cooling capacity and system power versus condenser inlet air temperature (normalized, respectively, by the capacity and power at the ARI outdoor temperature rating point of 35ºC), the 0.3ºC reduction in inlet air temperature associated with installing a cool roof corresponds to a decrease in RTU energy consumption of about 0.3 to 0.6% and an increase in EER of about 0.6 to 0.7%. Energy codes such as ASHRAE Standards 90.1 and 90.2 and California Title 24 already include energy saving credits related to cool roofs. Additional energy savings from reducing condenser inlet air temperature by installing a cool roof would only slightly increase these credits.