Numerical modelling of forced convective heat transfer from the inclined windward roof of an isolated low-rise building with application to photovoltaic/thermal systems Panagiota Karava a, * , Chowdhury Mohammad Jubayer b , Eric Savory b a School of Civil Engineering and Division of Construction Engineering and Management, Purdue University, 550 Stadium Mall Dr., CIVL 1227, West Lafayette, 47907 IN, USA b Department of Mechanical and Materials Engineering, University of Western Ontario, London, ON N6A 5B9, Canada article info Article history: Received 17 January 2011 Accepted 23 February 2011 Available online 21 March 2011 Keywords: Photovoltaicethermal systems Building, convective heat transfer coefficient Computational fluid dynamics RANS Atmospheric boundary layer abstract The present work evaluates forced convective heat transfer from the inclined windward roof of an isolated low-rise building, with application to building-integrated Photovoltaic/Thermal (PV/T) systems. High resolution, 3-D, steady, Reynolds-Averaged NaviereStokes (RANS) Computational Fluid Dynamics (CFD) simulations of the wind flow field near the roof of a building with plan dimensions of 4.2 m by 6 m, a 3 m eaves height and a 30  roof slope, are conducted, with the results validated by experimental data from a 1:50 scale model tested in a boundary layer wind tunnel. The heat transfer model is validated using the boundary layer correlation for an isothermal horizontal flat plate in uniform flow. Full-scale simulations, with the same building geometry and numerical model, for Reynolds numbers (Re) from 2.2  10 5 to 7.7  10 5 based on wind speed at eaves height and sloped roof length, are also carried out, for four roughness length (z 0 ) values representing open and suburban terrain. From these, dimensionless correlations for the exterior convective heat transfer coefficient (CHTC), expressed as Nusselt number (Nu), for the windward roof are developed which include the Re and the incident turbulence intensity at eaves height. The windward roof average CHTC is dependent upon the presence and extent of the leading edge separated flow region. This region reduces in size as both Re and eaves height turbulence intensity are increased, such that at high turbulence levels it disappears and the value of the exponent on the Re term in the Nu correlation approaches that for a flat plate turbulent boundary layer. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction A building-integrated Photovoltaic/Thermal system (BIPV/T) consists of a Photovoltaic array installed as an essential component of the building envelope (typically a façade or a roof). In this system, the circulation of a fluid (usually air) in a channel underneath the PV panels, permits recovery of a significant portion of the incident solar radiation as thermal energy [e.g. 1e3]. Thus, BIPV/T systems produce both electricity and heat. On the other hand, recovering heat from the Photovoltaic panel cools the panel thereby improving its electricity generation efficiency. The energy balance parameters of a BIPV/T system (mounted on the roof of a house) are shown schematically in Fig. 1 and the relevant equation for the PV panel is: fG ¼ E þ Q c in þ Q cout þ Q r in þ Q rout (1) where, fG is solar irradiance absorbed by the PV panel, E is electrical power flux produced, Q c in is rate of heat removed by convection to the air in the channel underneath the panel per unit area (p.u.a.), Q cout is rate of convective heat removed by wind flow over the panel p.u.a., Q rin is rate of heat removed by radiation heat transfer from the underside of the panel p.u.a. and Q cout is rate of radiation heat loss p.u.a from its top surface. Monitoring data for the thermal perfor- mance of a roof-mounted PV/T system in a house in Eastman, Quebec, Canada show that depending on ambient temperature, solar energy flux and wind velocity, 30e50% of the absorbed solar radiation is removed by convective heat transfer [1e3]. Similar results for other building envelope energy systems are reported by Clear et al. [4] and Palyvos [5]. Thus, accurate prediction of the exterior convective heat transfer coefficient (CHTC) is essential for evaluating the overall performance of BIPV/T systems. The exterior CHTC relates the heat flux normal to the PV panel to the difference between the surface temperature of the PV and a reference temperature, which is generally the temperature of the outside environment: h ¼ q PV T PV  T ref (2) * Corresponding author. Tel.: þ1 765 494 4573; fax: þ1 765 494 0644. E-mail address: pkarava@purdue.edu (P. Karava). Contents lists available at ScienceDirect Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng 1359-4311/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2011.02.042 Applied Thermal Engineering 31 (2011) 1950e1963