Estimation of flow and heat transfer for the design of PV cooling ducts

A practical and convenient procedure is set out, for determining the flow and convective heat transfer in inclined cooling ducts, as used in PV installations. This covers the critical case of free convection, induced only by the buoyancy due to heat input into the duct, when temperatures are highest, those of forced convection induced by wind pressure differences between the duct ends, and the mixed-flow conditions when both influences occur together. The flow regime may be laminar or turbulent. Conditions usually fall in the entrance region of ducts, where temperature and velocity distributions are far from the fully-developed state, and wall friction and heat transfer are enhanced. The main object of the procedure is to generate the set of heat transfer coefficients for the duct walls, required for insertion into the overall thermal model of the PV installation and the underlying building. The effects of the major influences in this area are illustrated by generalised solutions, given in dimensionless forms, for a standard case chosen to approximate to many real situations: the parallel-plate duct with heat input from one side. This shows graphically how the operating conditions and the proportions of the duct affect the flow and heal transfer characteristics, directing attention to situations in which optimum duct designs may be sought. (C) 2000 Elsevier Science Ltd. All rights reserved.