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Research Papers

Measuring the Effect of Vegetated Roofs on the Performance of Photovoltaic Panels in a Combined System

[+] Author and Article Information
Hamid Ogaili

Mechanical and Materials
Engineering Department,
Portland State University,
Portland, OR 97207

David J. Sailor

School of Geographical Sciences and
Urban Planning,
Arizona State University,
Tempe, AZ 85287
e-mail: David.Sailor@asu.edu

1Corresponding author.

Contributed by the Solar Energy Division of ASME for publication in the JOURNAL OF SOLAR ENERGY ENGINEERING: INCLUDING WIND ENERGY AND BUILDING ENERGY CONSERVATION. Manuscript received November 13, 2015; final manuscript received August 19, 2016; published online October 3, 2016. Assoc. Editor: Carlos F. M. Coimbra.

J. Sol. Energy Eng 138(6), 061009 (Oct 03, 2016) (8 pages) Paper No: SOL-15-1380; doi: 10.1115/1.4034743 History: Received November 13, 2015; Revised August 19, 2016

Experiments were conducted in summer using two identical photovoltaic (PV) panels at two heights using three roofing types: white, black, and green (vegetated). For experiments at an 18 cm height, the mean power output of the PV-green roof system was 1.2% and 0.8% higher than the PV-black and PV-white roofs, respectively. At a 24 cm height, the benefit of the green roof was slightly diminished with power output for the PV above a green roof being 1.0% and 0.7% higher than the black and white roof experiments, respectively. These results were consistent with measured variations in mean panel surface temperatures; the green roof systems were generally cooler by 1.5–3 °C. A unique aspect of this research is the investigation into the effect of vegetation on the convective cooling of the PV panels. Panel heat transfer coefficients for the PV-green roof were 10–20% higher than for the white and black roof configurations, suggesting a mixing benefit associated with the roughness of the plant canopy. While the best PV performance was obtained by locating PV above a green roof, the relative benefits diminish with distance between the PV and the roof.

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Figures

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Fig. 1

Layout of the experiments illustrating the configuration in which one panel (A) was exposed to a green roof and the other panel (C) was exposed to a black roof. The panel between A and C straddled both surfaces and was not monitored.

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Fig. 2

Cross-sectional view of the PV panel above the green roof showing the positions of the sensors

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Fig. 3

Comparison of panel surface (a) heat transfer coefficients, (b) temperatures, and (c) power output during the green–black roof experiment at a height spacing of 18 cm

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Fig. 4

Comparison of panel surface (a) heat transfer coefficients, (b) temperatures, and (c) power output during the green–white roof experiment at a height spacing of 18 cm

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Fig. 5

Comparison of panel surface (a) heat transfer coefficients, (b) temperatures, and (c) power output during the green–black roof experiment at a height spacing of 24 cm

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Fig. 6

Comparison of panel surface (a) heat transfer coefficients, (b) temperatures, and (c) power output during the green–white roof experiment at a height spacing of 24 cm

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Fig. 7

Results for the (a) heat transfer coefficients, (b) temperatures, and (c) power output of the PV panel above the green roof at 18 cm and 24 cm heights

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Fig. 8

Results for the (a) heat transfer coefficients, (b) temperatures, and (c) power output of the PV panel above the black roof at 18 cm and 24 cm heights

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