Monitoring the energy-use effects of cool roofs on California commercial buildings

Abstract

Solar-reflective roofs stay cooler in the sun than solar-absorptive roofs. Such "cool" roofs achieve lower surface temperatures that reduce heat conduction into the building and the building's cooling load.The California Energy Commission has funded research in which Lawrence Berkeley National Laboratory (LBNL) has measured the electricity use and peak demand in commercial buildings to document savings from implementing the Commission's Cool Roofs program. The study seeks to determine the savings achieved by cool roofs by monitoring the energy use of a carefully selected assortment of buildings participating in the Cool Roofs program. Measurements were needed because the peak savings resulting from the application of cool roofs on different types of buildings in the diverse California climate zones have not been well characterized to date. Only a few occupancy categories (e.g., office and retail buildings) have been monitored before this, and those were done under a limited number of climatic conditions. To help rectify this situation, LBNL was tasked to select the buildings to be monitored, measure roof performance before and after replacing a hot roof by a cool roof, and document both energy and peak demand savings resulting from installation of cool roofs.We monitored the effects of cool roofs on energy use and environmental parameters in six California buildings at three different sites: a retail store in Sacramento; an elementary school in San Marcos (near San Diego); and a 4-building cold storage facility in Reedley (near Fresno). The latter included a cold storage building, a conditioning and fruit-palletizing area, a conditioned packing area, and two unconditioned packing areas (counted as one building).Results showed that installing a cool roof reduced the maximum roof surface temperature of each building by 60-75ºF.In the retail store building in Sacramento, applying a cool coating reduced the maximum roof surface temperature elevation by 60-65ºF, and reduced the underroof temperature in the conditioned area by as much as 40-50ºF. For the period of 08/08/2002 to 09/30/2002, the estimated savings in average air conditioning energy use was about 83 kWh/day (52%; 6.7 Wh/ft 2 /day). On hot days when the afternoon temperature exceeded 100°F, the measured savings in average peak demand for peak hours (hours 12-17) was about 12-13 kW (about 1 W per square foot of conditioned area). • • •In the school building in San Marcos, the reductions in the maximum roof surface temperature and under-roof temperature were about the same as for the retail store. For the period of 07/08/2002 to 08/20/2002, the estimated savings in average air conditioning energy use was about 22-26 kWh/day (17-18%; 3.9-4.5 Wh/ft 2 /day). On hot days when the afternoon temperature exceeded 90°F, the measured savings in average peak demand for hours 10-16 were about 3.1 kW (about 0.5 W per square foot of conditioned area).In the cold storage facility in Reedley, coating reduced the maximum roof surface temperature elevation by 65-75ºF...

Cool roof technology in London: An experimental and modelling study

“…Heat transfer to the internal space by conduction is therefore reduced; the magnitude of the reduction will be determined by: A number of experimental and computational studies have been carried out to demonstrate the energy benefits of cool roofs in reducing energy demand in buildings in cooling dominated climates. A number of papers have been published for residential buildings [1,2,3,4,5,6], retail stores [7], and other commercial buildings [8,9]. Work has also been carried out outside the US.…”

“…Furthermore, they concluded that the two most influential factors for the performance of roof reflective coatings are the U-value of the roof and the climatic conditions. Akbari et al [8] conducted experimental and numerical studies for 16 Californian climate zones, and reported energy savings of about 4.5-7.4 kWh/m 2 of conditioned roof area per year. They also differentiated between the investigated buildings, signifying the importance of building operation on the performance of roof reflective coatings.…”

Study on the Application of Cool Paintings for the Passive Cooling of Existing Buildings in Mediterranean Climates

“…Furthermore, Akbari et al [11] have monitored six commercial buildings in three different sites in California (USA) and demonstrated that the use of the cool roof technology is very effective in a hot-humid climate; an average reduction of 30 ∘ C is measured on the daily peak of the outer surface temperature of the roof, while the mean savings for air conditioning range from 42 Wh⋅m −2 ⋅day −1 to 81 Wh⋅m −2 ⋅day −1 , depending on the local weather conditions of the three sites considered and on the main features of the buildings monitored.…”

Energy Efficiency of Ventilated Façades: Residential Buildings, Alicante, Spain

“…The construction characteristics are shown in Table 2. For the study of the thermal behaviour of these two selected façades, the monitoring of each of the façades is carried out [4], [5]. The installed equipment consists of sensors of temperature, humidity, air speed and solar radiation.…”