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Research Papers

Convective Heat Transfer Coefficients in a Building-Integrated Photovoltaic/Thermal System

[+] Author and Article Information
Luis M. Candanedo, Andreas Athienitis, Kwang-Wook Park

Department of Building, Civil and Environmental Engineering, Concordia University, Room EV16.117, 1455 Maisonneuve West, Montréal, QC, H3G 1M8, Canadalm_canda@encs.concordia.ca

J. Sol. Energy Eng 133(2), 021002 (Mar 22, 2011) (14 pages) doi:10.1115/1.4003145 History: Received October 14, 2009; Revised July 20, 2010; Published March 22, 2011; Online March 22, 2011

This paper presents an experimental study for the development of convective heat transfer correlations for an open loop air-based building-integrated photovoltaic/thermal (BIPV/T) system. The BIPV/T system absorbs solar energy on the top surface, which includes the photovoltaic panels and generates electricity while also heating air drawn by a variable speed fan through a channel formed by the top roof surface with the photovoltaic modules and an insulated attic layer. The BIPV/T system channel has a length/hydraulic diameter ratio of 38, which is representative of a BIPV/T roof system for 30–45 deg tilt angles. Because of the heating asymmetry in the BIPV/T channel, two average Nusselt number correlations are reported as a function of Reynolds number: one for the top heated surface and the other for the bottom surface. For the top heated surface, the Nusselt number is in the range of 6–48 for Reynolds numbers ranging from 250 to 7500. For the bottom insulated surface, the Nusselt number is in the range of 22–68 for Reynolds numbers ranging from 800 to 7060. This paper presents correlations for the average Nusselt number as a function of Reynolds number; this correlation is considered adequate for the design of BIPV/T systems where forced convection dominates. Local Nusselt number distributions are also presented for laminar and turbulent flow conditions.

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Copyright © 2011 by American Society of Mechanical Engineers
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Figures

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Figure 5

(a) Experimental BIPV/T setup replicating one strip of the BIPV/T system of the EcoTerra™ house with and without amorphous PV modules attached; (b) schematic of the setup; (c) cross section details of BIPV/T

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Figure 6

Transient response of the bulk air temperature for constant volumetric flow rate of 0.021 m3/s and incident total solar radiation of 815 W/m2 (the arrow shows increasing time)

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Figure 7

Bulk air temperature for different Reynolds numbers compared with exponential correlation fits (points show experimental measurements and solid lines show the exponential fit)

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Figure 8

Local Nusselt number for the top surface versus nondimensional distance from the entrance

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Figure 9

Nusselt numbers for the top surface versus Reynolds number. The data are compared with the correlation given by Eq. 18. (The uncertainties of each of the data points are shown by the vertical line segments.)

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Figure 10

Nusselt numbers for the bottom surface versus Reynolds number. The data are compared with the correlation in Eq. 19. The uncertainties of each of the data points are shown by the vertical bars.

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Figure 11

Nusselt numbers for the top surface versus Reynolds number comparison for 45 deg and 30 deg tilt angles

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Figure 12

Comparison of the top channel surface Nusselt number correlation with Dittus–Boelter, Gnielinski, Martinelli, Malik, and Mercer correlations

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Figure 13

Comparison of Eqs. 1,2 with the experimental data (Pr=0.71). All the experimental points are above the limits established by the equations.

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Figure 1

Schematic of a typical air-based open loop BIPV/T system (1)

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Figure 2

BIPV/T thermal network model showing the interior convective heat transfer coefficients hct and hcb(1) (the configuration shown corresponds to an experimental prototype studied in this paper)

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Figure 3

Photo of the wood framing structure employed to support the PV modules in the BIPV/T system of Northern Light Canadian Solar Decathlon 2005 house (13)

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Figure 4

Photograph of the BIPV/T roof in the EcoTerra™ house (the amorphous PV modules are attached to a metal roof skin on vertical and horizontal wood framing that also creates the flow channel). The roof has a length of 5.8 m in the flow direction shown by the arrows.

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